SABP-A-022

Best Practice SABP-A-022

25 August 2008

Stainless Steel Fabrication, Testing and Installation Document Responsibility: Materials & Corrosion Control Standards Committee

Saudi Aramco DeskTop Standards Table of Contents 1 2 3 4 5 6 7 8 9 10

11 12 13 14

Previous Issue: New

Scope and Purpose............................................ 2 Conflicts and Deviations..................................... 2 References......................................................... 2 Definitions and Abbreviations............................. 5 Introduction to Stainless Steels.......................... 6 Classes of Stainless Steel.................................. 6 Composition, Corrosion Resistance and Sensitization..................................... 9 Materials Selection........................................... 11 Castings .......................................................... 12 Fabrication....................................................... 13 10.1 Welding and Corrosion Resistance....... 13 10.2 Welding of Stainless Steels................... 14 10.3 Storage.................................................. 32 10.4 Shop Fabrication................................... 32 10.5 Pickling, Passivation & Iron Removal.... 33 10.6 Field Fabrication.................................... 34 Hydrostatic Testing.......................................... 36 Microbiologically Influenced Corrosion (MIC)... 37 Coating Stainless Steel.................................... 38 Summary.......................................................... 39

Next Planned Update: TBD

Primary contacts: Lobley, Graham R. on 966-3-8746678; Niemeyer, Dennis C. on 966-3-8736700 Copyright©Saudi Aramco 2008. All rights reserved.

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Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication, Testing and Installation

1

Scope and Purpose Stainless steels are more expensive than carbon and low alloy steels and are normally selected for their superior corrosion or heat resistance. For the austenitic stainless steels, good low temperature toughness is also a key mechanical property. However, improper fabrication and testing procedures can seriously degrade corrosion resistance and directly lead to premature failures. This SABP provides guidelines to optimize performance of stainless steels, particularly austenitic, duplex and martensitic grades. It identifies specific procedures and specifications to be followed, during manufacturing, fabrication and commissioning, in order to achieve optimum performance of stainless steel equipment and piping. This SABP is based on current industry experiences and is intended for internal Saudi Aramco operations and maintenance applications. It also provides guidance for inspection in fabrication facilities.

2

Conflicts and Deviations If there is a conflict between this Best Practice and a Saudi Aramco standard then the Standard shall govern. If there is a conflict between this Best Practice and an approved welding procedure then the approved welding procedure shall govern. If there is a conflict between this Best Practice and other standards and specifications, please contact the Coordinator of ME&CCD/CSD.

3

References 3.1

Saudi Aramco References Saudi Aramco Engineering Standards SAES-A-007

Hydrostatic Testing Fluids and Lay-Up Procedures

SAES-G-005

Centrifugal Pumps

SAES-H-001

Coating Selection & Application Requirements for Industrial Plants and Equipment

SAES-L-132

Material Selection for Piping Systems

SAES-W-010

Welding Requirements for Pressure Vessels

SAES-W-011

Welding Requirements for On-Plot Piping

SAES-W-014

Weld Overlays and welding of Clad Materials

SAES-W-016

Welding of Special Corrosion Resistant Materials Page 2 of 40


Saudi Aramco Standard Drawing AD-036821

Material Guide for Centrifugal Pumps

Saudi Aramco Best Practice SABP-A-001

Polythionic Acid SCC Mitigation - Materials Selection and Effective Protection of Austenitic Stainless Steels and other Austenitic Alloys

Saudi Aramco Technical Alert ALERT-93-011 3.2

Technical Alert Number 11, Avesta 254SMO Stainless, issued 11/21/93

Industry Codes and Standards American Petroleum Institute API TR 938-C

Use of Duplex Stainless Steels in the Oil Refining Industry-First Edition, 2005

American Society for Testing and Materials ASTM A380

Standard Practice for Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment, and Systems

ASTM A967

Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts

ASTM C871

Standard Test Methods for Chemical Analysis of Thermal Insulation Materials for Leachable Chloride, Fluoride, Silicate, and Sodium Ions

American Welding Society AWS D10.18:2008

Guide for Welding Ferritic/Austenitic Duplex Stainless Steel Piping and Tubing

AWS D18.2:1999

Guide to Weld Discoloration Levels on Inside of Austenitic Stainless Steel Tube

International Standards Organization ISO 15156-3

Petroleum, Petrochemical and Natural Gas Industries Materials for Use in H2S-Containing Environments in Oil and Gas Production Part 3: Cracking-Resistant CRAs (CorrosionResistant Alloys) and Other Alloys

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National Association of Corrosion Engineers NACE MR103-2007

Materials Resistant to Sulfide Stress Cracking in Corrosive Petroleum Refining Environments

NACE RP0198

The Control of Corrosion Under Thermal Insulation and Fireproofing Materials - A Systems Approach

New Zealand Stainless Steel Development Association NZSSDA 3.3

Code of Practice for the Fabrication of Stainless Steel Plant & Equipment, 2001

Publications Optimising Stainless Steel Piping Fabrication Practice, Dr. Liane Smith, North Scottish Branch of the Welding & Joining Society, Aberdeen, 26th May 2005 NiDI Publication 11003: Nickel Stainless Steels for Marine Environments, Natural Waters and Brines (1987) Guidelines for Successful Use of Stainless Steel in Potable Water Treatment. Plants (PWTP), R. E. Avery, S. Lamb, A. H. Tuthill, Nickel Development Institute, April 1998 Fabricating Stainless Steels for the Water Industry Nickel Development Institute Reference Book 11026, C. Powell, D. Jordan, October 2005 Microbiologically Influenced Corrosion – Case Studies in Australasia, ACCA 2007, Paper 21, L. H. Boulton Influence of Sulfate-Reducing Bacteria on Corrosion of 2205-Type Duplex Stainless Steel in Chloride Medium, ACCA 2007, Paper 72, P. J. Antony, R.K. Singh Raman, et. al. “Welding of Stainless Steels and other Joining Methods,” A Designers’ Handbook Series No. 9002, NiDI, Nickel Development Institute American Welding Society, “AWS Welding Handbook, Eighth Edition Volume 4, Materials and Applications Part 2”, 1998 ASM Handbook, Volume 6, “Welding, Brazing, and Soldering,” 1993 Stainless Steels, Properties, How to Weld Them, Where to Use Them, Kotecki, D. and Armao, F., 2003, The Lincoln Electric Company Welding of Austenitic Stainless Steels - a Guide to Best Practice, TWI Members Website

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4

Definitions and Abbreviations API

American Petroleum Institute

ASSDA

Australian Stainless Steel Development Association

ASTM

American Society for Testing & Materials

CRA

Corrosion Resistant Alloy

CUI

Corrosion under Insulation

DSS

Duplex Stainless steel

Free Iron

Surface contamination of SS with carbon or ferritic steels

HAZ

Heat Affected Zone

ISO

International Standards Organization

MIC

Microbiologically Influenced Corrosion

NACE

National Association of Corrosion Engineers

NZSSDA

New Zealand Stainless Steel Development Association

PASCC

SCC which can occur rapidly under refinery shutdown or T&I conditions. Cracking is due to sulfur acids forming from sulfide scale, air and moisture acting on sensitized austenitic stainless steels.

Phase Balance Percentage of delta ferrite in duplex stainless steel PMI

Positive Material Identification is a chemical analysis that ideally includes all alloying elements, to determine conformance to specified alloy composition. It can be used to check all parts, including weld consumables and fabricated components.

PWHT

Postweld heat treated: Solution Anneal and rapid cool

SCC

Stress Corrosion Cracking

SDSS

Superduplex Stainless Steel

Sensitization

Formation of chromium carbides along grain boundaries which causes loss in corrosion resistance. Sensitization is dependent on the composition (see sections 9 and 10.2).

Stabilization

Stabilization is an alloying method of preventing sensitization. Stabilized grades with Ti (grade 321) or Nb (grade 347) are commonly used. Ti and Nb additions preferentially combine with the carbon, preventing the depletion in chromium.

SSC

Sulfide Stress Cracking

SS

Stainless Steel Page 5 of 40


SSS 5

Super Austenitic Stainless Steel

Introduction to Stainless Steels Stainless steels are selected for service in oil, gas and downstream systems because of their high level of corrosion resistance in typical producing and refinery environments. The protective thin chromium-rich passive film forms spontaneously in the presence of oxygen. Stainless steel grades are specified for ambient, cryogenic and high temperature services. The stainless steels addressed in this best practice (BP) are primarily the 300-series austenitic stainless steels and the duplex stainless steels. Unfortunately, these materials are not totally resistant to corrosion in other environmental conditions. In particular, they are sensitive to pitting and crevice corrosion in aerated water environments with chloride ions present at certain temperature conditions. Stainless steel piping systems are typically hydrotested with water of varying quality from potable water through to raw water with or without various chemicals present. Depending upon the duration of exposure, any of these environments may result in pitting attack. The resistance to pitting is strongly affected by the stability of the protective passive film on the stainless steel, which is deleteriously affected in various ways by welding. Microbes introduced in the hydrotest water can promote MIC as another form of pitting. This usually occurs at weak points such as heat tint oxides, that are associated with welding cycles. Two approaches are therefore used for preventing pitting attack of stainless steels during hydrotesting and prior to service. The fabricator should make every effort to optimize the quality of the weld and minimize the reduction of passive film quality. Also, the commissioning procedures should be optimized to minimize the corrosivity of the environment for the hydrotest period. The purpose of this BP is to provide guidance on optimum practice at every step in order to give the very best confidence in the quality of fabrications, castings and other products.

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Classes of Stainless Steel Stainless steels are broadly defined as steels that contain at least 11% Cr. On the basis of microstructure, five major families of stainless steels are recognized: ferritic, martensitic, austenitic, duplex and precipitation-hardenable (PH). The simplified Alloy Tree (Figure 1) shows the relationship of these types of stainless, building from a base case of 405 ferritic stainless steel (composition 11.5 – 14.5% Cr, ≤0.08% C). Adding C gives a martensitic structure, adding more Cr and > 8%Ni gives an austenitic microstructure, adding less Ni and some Mo gives a duplex structure, Page 6 of 40


finally, adding Cr, Ni and Cu gives a precipitation hardening stainless steel. Austenitic steels include 316 with Mo to increase pitting resistance, low C and weld stabilized grades (with Ti [321] or Cb [347]). Higher %C and heat resisting grades are specified for the highest temperatures, where creep and oxidation are the principal damage mechanisms (examples 304H and 310). Ferritic stainless steels are named because their body-centered-cubic (bcc) crystal

structure is the same as iron at room temperature. Their Cr content usually range from 11 to 18%. They are magnetic, have moderate corrosion resistance and are not susceptible to SCC. Generally, ferritic stainless steels do not have particularly high strength. Their annealed yield strengths range from 275 to 350 MPa (40 to 50 ksi), and their poor toughness limits their fabricability and the usable section size Martensitic stainless steels are similar to Fe-C alloys that are austenitized, hardened

by quenching, and then tempered for increased ductility and toughness. These alloys are strongly magnetic and their heat-treated structure is body-centered tetragonal. In the annealed condition, they have a tensile yield strength of about 275 MPa (40 ksi) and are generally machined, cold formed, and cold worked in this condition. Austenitic stainless steels comprise the largest stainless family, in terms of number of

alloys and usage. Like the ferritic alloys, they cannot be hardened by heat treatment. Austenitic alloys are nonmagnetic in the solution annealed condition. Castings, welds and cold-worked austenitic materials can show appreciable magnetism, due to the presence of second phases such as delta ferrite or martensite. The austenite structure is face-centered-cubic (fcc), like the high-temperature (900 to 1400°C) austenite form of iron. They possess excellent ductility, formability, and toughness, even at cryogenic temperatures. In addition, they can be substantially hardened by cold work.

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Ferritic 405 base: 11 – 12% Cr < 0.03 %C

Martensitic 410 base: 12 %Cr, 0.1 - 0.2%C

Austenitic 304 base: 18 %Cr, 8% Ni, 0.03 -0.08%C

Duplex 2205 base: 22%Cr, 5%Ni, 3%Mo, N

13 Cr 13%Cr, 6%Ni, Mo A486 CA6NM

316 (Mo-alloyed) Stabilized (Ti or Cb) or low C (≤ 0.03) Heat Resistant (310)

Superduplex PREN ⋝ 40 2507 & UNS S32760 2205 base: 22%Cr, 5%Ni, 3%Mo, N

& high C grades Austenitic Superaustenitic 304 base: 18PREN %Cr, 8% ⋝ 40Ni, 0.03 UNS-0.08%C S31254 0.03 -0.08%C

Precipitation Hardening 15-5 PH 17-4 PH

Figure 1 – Stainless Steel Alloy Tree Austenitic stainless steels of the 300 series are the most commonly used, such as 304, 316, 321 and 347 grades (see Table 1). Higher Mn-containing alloys, with Mn exceeding 4%, are covered under 200 series stainless steel specifications. Examples used in rotating equipment include UNS S20910 (Nitronic 50) and UNS S21800 (Nitronic 60) for shafts, wear rings, etc. The higher Mn levels greatly improve resistance to wear including galling (a common problem with 300 series steels). Duplex stainless steels are Cr-Ni-Mo alloys that contain a balanced mixture of

austenite and ferrite, and are therefore significantly magnetic. Duplex stainless steels combine the optimum properties of austenitic and ferritic types. Typically, they contain 18 - 26% Cr plus 4.5 to 6.5% Ni. Their duplex structure results in improved SCC resistance, compared with the austenitic stainless steels and improved toughness and ductility, compared with the ferritic stainless steels. They can have yield strengths ranging from 550 to 690 MPa (80 to 100 ksi) in the annealed condition, which is approximately twice the strength level of either phase alone. Typical applications include handling chlorinated seawater and for some heat exchanger tubing. Temperature limits typically range from -50 to +300ºC. Duplex stainless steels have been utilized for seawater applications, such as in Seawater Treatment Plants and some refinery heat exchanger tubing. Applications have been limited by two factors: (1) weldability concerns due to formation of deleterious

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intermetallic phases leading to loss of toughness and corrosion resistance, especially with the superduplex grades; (2) low-to-moderate resistance to SSC. PH stainless steels may be martensitic, semi-austenitic or austenitic. They combine

the heat treatability of normal martensitic grades with the corrosion resistance of austenitics. They are available in bar form for the production of heavy duty engineering components. Various alloying elements, such as Al, Ti, Nb or Cu, are used to achieve age hardening. They generally form intermetallic compounds, but in S17400 (17-4PH), fine Cu precipitates are formed. Typical uses of 17-4PH include valve stems, drive shafts and control valve parts. “L”, Dual Certified and Stabilized Grades

Types 304L or 316L are the standard grades of stainless steel used for welded applications. Type 316L with 2 - 3% Mo is more resistant to pitting and crevice corrosion and is preferred over Type 304L for more severe services. The low C "L" grade which has a maximum of 0.03% C is required for welded structures to minimize risk of sensitization. Stabilized grades 321 and 347 are also specified for welded structures. Where mechanical strength is important for design purposes, the slightly lower tensile and yield strengths of the "L" grades should be recognized. It is increasingly common to encounter Dual Certified Type 304/304L and Type 316/316L stainless steel in warehouse stock. Dual certification means that it meets the 0.03% C maximum requirement for the "L" grades and also meets the higher mechanical properties of the regular grades. 7

Composition, Corrosion Resistance and Sensitization The resistance of stainless steels to pitting and crevice corrosion in aerated waters is strongly related to the chemical composition. Steels with higher levels of chromium (Cr), molybdenum (Mo) and nitrogen (N) are more resistant. The resistance to pitting is related to the composition using the empirical Pitting Resistance Equivalent Number (PREN) as follows: PREN = %Cr + 3.3 x %Mo + 16 x %N Generally, stainless steels with a PREN value above 40 are resistant to pitting corrosion in ambient temperature seawater. These steels are sometimes referred to as superstainless steels (SSSs). Superstainless cast and wrought grades are specified with PREN ≥ 40 in SAES-G-005, Standard Drawing AD-036821 for highly corrosive services, such as for pumps handling produced brine water, with high TDS and Cl levels.

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PREN data relates bulk chemistry to corrosion performance, but this is only a ranking guideline to compare different alloys. ISO 15156-3 includes PREN ranges for stainless steels in the austenitic and duplex alloy classes. The PREN does not factor alloy microstructure, microsegregation or surface condition, all of which strongly influence alloy performance in practice. Thermal history, including original heat treatment and welding, are major factors influencing ultimate performance. Consequently, SSS casting alloys must be in the solution annealed condition after weld repairs. It is important to ensure that the grades of stainless steels which are used in piping systems are not mixed up. A section of piping with lower resistance to pitting attack, as a consequence of its lower PREN value, may be more likely to initiate pitting when connected to higher alloyed stainless steel. Weld metal must have a pitting resistance equivalent to or better than that of the parent

metal. This means that even in the root region, with some dilution of the filler metal composition by the parent metal, the pitting resistance will still be acceptable. For this reason, superstainless steels such as UNS S31254 are fusion welded using overalloyed filler metal (Inconel 625). ALERT-93-011 (Saudi Aramco Technical Alert Number 11) provides additional information and advice concerning seawater service, as follows: 

Corrosion damage initiated in the root pass of the UNS S31254 butt-welds.



Field and laboratory investigations indicate the most likely cause of failure was insufficient (or no) addition of the recommended Inconel 625 filler material during welding of the root pass. Table 1 – Some Commonly Used Stainless Steel Grades

11 Cr-C≤0.08 13 Cr-C≤0.08 12 Cr 13Cr-4Ni-0.5Mo 13Cr-4Ni-0.5Mo 18Cr-9Ni 18Cr-8Ni-2.5Mo 18Cr-8Ni-3.5Mo 18Cr-10Ni-Ti 18Cr-10Ni-Nb 25Cr-20Ni

Example Specification (ASTM/SAE) A240 A240 A240-410 A487 CA6NM A276 A240 S30400 A240 S31600 A240 S31700 A240 S32100 A240 S34700 A240 S31000

S40500 S41008 S41000 J91540 S41500 S30400 S31600 S31700 S32100 S34700 S31000

10.5 – 12.5 11.5 – 14.5 11.5 – 14.0 12.8 – 17.3 12.8 – 17.3 18 - 20 23 - 28 28 - 33 17 - 20 17 - 19 24 - 26

200 series

22Cr-12.5Ni-5Mn-2.5Mo-0.3N 17Cr-8.5Ni-8Mn-4Si-0.15N

A240 XM-19 A240 S21800

S20910 S21800

29 -38 17.3 - 20.9

Superaustenitic

20Cr-18Ni-6Mo-N (254SMO) 21Cr-24Ni-6Mo-N (AL-6XN)

A240-S31254 B688

S31254 N08367

42 - 45 43 – 49

Material Ferritic – 400 series Martensitic – 400 series Austenitic 300 series

Nominal Composition

UNS # / Grade

PREN

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Material Other grades Duplex

Nominal Composition 20Cr-35Ni-2.5Mo-3.5Cu 21Cr-25Ni-4.5Mo-1.5Cu 22Cr-5Ni-3.5Mo-N

Example Specification (ASTM/SAE) B464 B673 A182-F51 A890 Gr 4A

Superduplex Precipitation Hardening

UNS # / Grade

PREN

N08020 (20Cb-3) N08904 (904L) S31803 S32205 J92205

26 - 31 32 - 40 31 - 38

25Cr-7Ni-3.5Mo-N-W 25Cr-7Ni-3.5Mo-N-W 25Cr-7Ni-4Mo-0.3N

A276/A182-F55

A890 Gr 6A A240-S32750

S32760 J93380 S32750

40 - 46 39 - 46 38 - 44

17Cr-4Ni-3Cu

A564 Type 630

S17400

15 – 17.5

Note: The international UNS numbering system uses a letter prefix and numbers to designate specific alloy grades. In this system: S = a regular stainless steel, N = a Ni-based alloy (including some higher grade stainless steels) and J = a casting specification.



Autogenous welds in UNS S31254 (without Inconel 625 filler) lack sufficient corrosion resistance to withstand seawater - particularly chlorinated seawater. Therefore, welding must be performed with the continuous addition of filler.

Sensitization is the formation of chromium carbides along grain boundaries that causes loss in corrosion resistance. The degree of sensitization is dependent on the composition of the steel and the time it spends in the temperature range 370°C to 815°C (700°F to 1500°F). If a steel has been sensitized, corrosion resistance may be recovered only by the use of a full anneal (and suitably rapid quench) to dissolve the carbides. Use of low carbon or stabilized grades are methods of preventing sensitization. The (L grade) low carbon level limits the amounts of chromium carbides that form, thereby limiting chromium depletion at the grain boundaries. Sensitization can occur in the HAZ adjacent to the weldment. Sensitization gives rise to corrosion in the HAZ and is a prerequisite for polythionic acid stress corrosion cracking. 8

Materials Selection Guidelines and standards that cover selection and specifications of stainless steels include international standards, company standards and technical guidelines. ISO 15156-3 presents materials selection for CRAs for sour chloride containing environments for upstream applications. Sour corrosion issues are different for refinery applications and NACE MR103 presents materials selection for CRAs for sour refinery environments. SABP-A-001 (Polythionic Acid SCC Mitigation - Materials Selection and Effective Protection of Austenitic Stainless Steels and other Austenitic Alloys) provides practical advice on mitigation of PASCC in refinery environments. This

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damage mechanism may also affect burner tips and other combustion equipment which are fired on sour hydrocarbons. For sour services, UNS S41000 stainless steel (410 stainless steel) and other martensitic grades must be quenched and double tempered to a maximum allowable hardness level of 22 HRC. PWHT is also required, since this material is strongly air-hardenable. Austenitic stainless steel grades offer a combination of good mechanical properties and material performance at both low (cryogenic) and high temperatures (heat resisting grades), as well as for a range of corrosive environments. Duplex stainless are attractive since they offer improved mechanical properties compared to austenitics and better corrosion resistance in saline environments. Duplex limitations include lower subzero impact toughness (-50ºC limit) and generally lower tolerance to sour environments. API TR 938-C (Use of Duplex Stainless Steels in the Oil Refining Industry-First Edition, 2005) provides advice on duplex steels in refining. Wrought S17400 stainless steel is permitted for sour service but must be carefully processed to prevent SSC. ISO 15156-3 specifies two different acceptable heat treatments for S17400. The maximum hardness level is 33 HRC for both conditions. Solid austenitic stainless steels are used for cryogenic and other applications, but should normally be avoided for services above 60ºC, when internal stainless steel cladding is recommended. Above 60ºC, there is an increased risk of either process-side or external chloride SCC. 9

Castings The chemical composition of cast stainless grades is adjusted to increase fluidity and minimize defects such as porosity and hot tears. Sensitization can occur in the HAZ of weld repaired areas of higher carbon austenitic grades especially, lowering corrosion resistance by intergranular attack, which can also promote PASCC. ASTM A744 requires that castings shall be PWHT (solution anneal and rapid cool) after all major weld repairs and after those minor weld repairs involving either of the following conditions: (1) welding on a wetted surface, or (2) welding that heats a wetted surface to or above 800°F [425°C]. PWHT is omitted from ASTM A743 but is available as a Supplementary Requirement. ASTM A744 is preferred if non-L grade steels are specified or allowed. Foundries often make “minor” weld repairs without PWHT and this practice is generally satisfactory if L (lower carbon) grades are used. PWHT (solution anneal and rapid cool) should be considered for duplex stainless steel castings that have been weld repaired. Solution treatment requirements are provided in ASTM A890.

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10

Fabrication 10.1

Welding and Corrosion Resistance When stainless steel piping or components are manufactured they are normally subjected to a final high temperature solution heat treatment (typically at 1050ºC) followed by water quenching. This leaves the material in the solution annealed condition which is the optimum for corrosion resistance. This heat treatment gives a poor quality high temperature oxide film on the surface which is quite thick, porous and cracked. During manufacturing, this oxide film is normally removed by pickling and passivating the surface in oxidizing acids. The oxide film which is formed at low temperature, either in the passivating solution or just by exposure to air, is thin and dense, giving good protection to the stainless steel. The as-delivered piping is therefore in an optimum condition, with a solution heat treated microstructure and a well-passivated surface oxide finish. When welding is carried out, the metal close to the weld is reheated to high temperatures, from the melting point at the fusion line to lower temperatures through the HAZ. An unprotected HAZ which is exposed to air during welding will therefore show a high level of oxidation. Characteristically, it would be black in color and in the worst condition may be visibly thick and porous. This oxide is far more easily broken down than the original passivated surface and there is a zone of metal under this oxide which is lower in chromium content relative to the bulk metal. Therefore, the HAZ has to be protected from re-oxidising at high temperature during the welding process. This is done by shielding around the weld using an inert gas to exclude air, thus preventing oxidation whilst the weld metal and HAZ is hot. Once the weld is cooler, the HAZ will re-oxidise slightly at lower temperatures but this oxide film is a more protective film than the high temperature oxide film. This „heat tint‟ oxide formed at the HAZ at lower temperatures is so thin that it forms interference colors, forming characteristic blue or rainbow colors. AWS D18.2:1999 addresses factors affecting weld discoloration inside a 316L austenitic stainless steel tube. It presents a color guide (Figure 2) relating degree of discoloration to oxygen content in the backing shielding gas. The heat tint oxide can be identified by a number corresponding to the oxygen level in the shielding gas. A straw-to-yellow heat tint (chart no. 3) is considered desirable but a blue tint up to chart no.6 is acceptable for standard applications. Heat tint exceeding no. 6 is normally removed by grinding, pickling and passivating the surface in oxidizing acids. Where welds are accessible, excessive heat tint can be removed by pickling and passivating the surface using

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standard products. Surfaces should be fully rinsed with clean water to ensure removal of any residual acids on the surface.

Figure 2 – Weld Discoloration and Heat Tint Numbers Scanning electron microscopy examination of heavily heat tinted surfaces has revealed a craze-cracked oxide surface that is enriched in Cr (Figure 3). Stainless steel lines have failed prematurely in service at heavily heat-tinted HAZ locations, which was attributed to the heat tint plus poor quality hydrotest and contributory MIC factors. The craze-cracked surface topography may promote bacterial colonization. 10.2

Welding of Stainless Steels Introduction The purpose of this section is to give information on the correct welding of stainless steels. It is intended for internal Saudi Aramco operations and maintenance applications. It also provides guidance for inspection in fabrication facilities. This section is divided into two parts. The first part gives information and techniques that are pertinent to the general classifications of stainless steels. The second part gives consumables and techniques that are specific to individual grades of stainless steel. This best practice gives general information. If there is a conflict between this Best Practice and an approved welding procedure then the approved welding procedure shall govern. All conflicts shall be brought to the attention of CSD. 10.2.1 Welding Processes The welding of stainless steels may be carried out using almost any welding process including all arc welding (Shielded Metal Arc, Gas Tungsten Arc, Gas Metal Arc, Flux-cored Arc, Submerged Arc and Plasma Arc), friction, resistance, laser and electron-beam welding techniques. Stainless steels should not be welded with the oxy-fuel Page 14 of 40


welding processes. Within Saudi Aramco we only use two welding processes: Shielded Metal Arc (SMAW) and Gas Tungsten Arc (GTAW). Fabricators and contractors may use a wider range of welding processes depending on their needs. The welding consumables for the commonly found stainless steels are given in Table 2. The GTAW process is always used for root passes on butt welds and for butt welds on pipes smaller than 2" diameter. The GTAW process can also be used for socket welds. SMAW may be used for the fill passes on butt welds in pipes larger than 2" diameter and all socket welds. SMAW Welding

The SMAW welding electrodes for stainless steels are given in AWS A5.4 specification. Generally, the number of the SMAW welding electrode is equivalent to the type of stainless steel. For example E316L electrode is for type 316L stainless steel. SMAW electrodes for stainless steels come with four usability classifications. These are indicated by the two numbers following the alloy designation -15, -16. -17 and -26. For example, E316L-16. The significance of these usability classifications are as follows: -15. This is a lime (also called basic) coated electrode. These electrodes are usable with DCEP (electrode positive) only. Electrode sizes 5⁄32 in. [4.0 mm] and smaller may be used in all positions of welding. These electrodes tend to weld easier out of position but are not as smooth. They give superior impact properties at cryogenic temperatures. -16. This is a lime - titania (also called basic–rutile) covered electrode. Electrode sizes 5⁄32 in. [4.0 mm] and smaller may be used in all positions of welding. This produces a smoother weld bead appearance than the -15 but is more difficult to weld out-ofposition and a smaller diameter should be selected. -17. This is a lime - silica - titania covered electrode (also called Acid– Rutile). This electrode produces a smooth, concave weld bead. It is generally is not as weldable out-of-position as the -16. -26. This designation is for those electrodes that are designed for flat and horizontal fillet welding and that have limited out of position characteristics. Electrodes with the -26 designation are recommended for welding only in the flat and horizontal fillet positions.

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Electrode care, storage and conditioning

Although hydrogen cracking is not a major problem in austenitic steels, electrodes should not be exposed to humid environments. It may be possible to recover moist electrodes by baking, however, this is not a general rule and the electrode supplier should be contacted for advice on a case by case basis. It is very important to keep electrodes for dissimilar metal welds in a low hydrogen condition. Failure to do this can result in hydrogen cracking on the non-austenitic side of the joint. Dissimilar metal welds are not permitted in sour service exposure. Dissimilar metal welds in hydrocarbon service must be made with a nickel based consumable. SAW Welding

The high heat input from SAW leads to larger weld beads and slow cooling rates. This may lead to problems with hot cracking. Hot cracking is caused by the segregation of minor elements to the liquid phase during freezing. One of the elements that can cause this phenomenon is silicon and, due to the slow cooling rates from SAW, silicon pickup from the slag can be quite significant. Ferrite control in submerged arc welds is therefore very important, but this may be problematic due to the difficulty of control over dilution in the weld. Base metal dilution during SAW can vary between 10 and 75%, more than any other arc welding process. Careful control must be maintained over the arc during welding as small variations in penetration can greatly affect the dilution levels. Electrode wire is readily available from a number of different suppliers. Flux for SAW is usually designed to be either basic or neutral with respect to the steel; however, no AWS specification exists to cover these fluxes. Neutral fluxes give sound weld deposits and permit some oxidation and loss of alloy to the flux. Basic fluxes contain additions of alloying elements that are imparted to the weld when molten. Fluxes can be susceptible to pickup of moisture and should be baked as per the manufacturer's instructions prior to use. Moisture in the flux can lead to defects such as wormholes and porosity. GTAW Welding

GTAW (Gas Tungsten Arc Welding also referred to as TIG) is similar to Gas Metal Arc Welding (GMAW) in that the arc is protected by an inert gas. An arc is established between a tungsten electrode and the work piece. The filler material is added separately into the molten weld puddle. Since the filler material does not form part of the arc, there will be extremely low loss of alloying elements during welding. All grades of weldable stainless steel may be welded using GTAW welding, giving Page 16 of 40


a high quality weld. GTA welding has extremely wide applicability and can be used to weld in all positions. It may be used to weld any thickness of material; but is most suited to thin sections. Only inert gases are used during GTA welding. Argon is primarily used for both shielding the arc and back-purging. Helium shielding provides deeper penetration, which is useful for thicker sections. Contact between the electrode and the weld pool is to be avoided to avoid tungsten contamination. Back-purging to protect the root of the weld is important when GTA welding stainless steels to control oxidation of the weld area. Argon is typically used for this purpose. FCAW Welding

Electrodes for FCAW are designated according to AWS specification A5.22. This designation takes the form EXXXTY-Z wherein XXX denotes the chemical specification of the steel (e.g., 308), Y denotes the applicable positions for welding (1 for all position and 0 for flat or horizontal only) and Z the shielding gas (1 for CO2; 3 for self-shielding, (i.e., no gas); 4 for 75 - 80% Ar in CO2 , and G for unspecified). It is not unusual for bismuth to be added to the flux of flux cored arc welds; this facilitates ease of flux removal after welding. However, bismuth additions should not be employed for components which will operate at high temperatures or require a postweld heat treatment. This is due to the link between bismuth and reheat cracking susceptibility. Saudi Aramco does not approve the use of self-shielded (No shielding gas) FCAW welding for stainless steel. Electroslag Welding

Electroslag is frequently used for overlay welding in fabrication shops. It has very low dilution and can be used for single or multi-pass overlays. Austenitic Stainless Steel Properties Affecting Welding

The coefficient of thermal expansion for the austenitic stainless steels is almost 40% greater than that of carbon steel. This means that distortion is generally more of a problem with austenitic stainless steels and must be considered during welding. The thermal and electrical conductivity of austenitic stainless steel is lower than that of carbon steel. Less welding heat is required to make a weld because the heat is not conducted away from a joint as rapidly as in carbon steel. This also makes the weld puddle more fluid and difficult to control.

Page 17 of 40


Purging and Shielding Gas

A chromium oxide layer forms when stainless steel is exposed to air and normally acts as a thin passive layer. This layer becomes thicker at higher temperatures. This chromium oxide can adversely affect the corrosion resistance of the weld and heat affected zone. The chromium oxide has a very high melting point (2435°C). This is much higher than the stainless steel base or weld material which is nominally 1450°C. It will interfere with the fusion of the welding process and cause an unacceptable weld profile. For these reasons the weld and Heat Affected Zone (HAZ) must be protected from the air by an inert gas. This gas protection is known as a shielding gas and a purge gas. Areas that have been exposed to high temperatures but did not have, or had insufficient, gas shielding will form a 'heat tint‟. This heat-tinted area will have comparatively poor corrosion resistance when compared with standard plate due to the heat tint oxide giving little protection to the base metal underneath, along with the base material being depleted of chromium immediately under the tint. The corrosion resistance of the component can only be restored by removing the oxide and chromium depleted layer. A three-step process of grinding, pickling and rinsing is best. (section 10.5) Pickling pastes or baths should be used at such strength and duration so as to remove fully the oxide and chromium depleted layer, but not stain or unduly corrode the surface. Rinsing solutions should, preferably, be de-mineralized water. Argon is normally used as the shielding gas on the torch for the GTA welding process. Argon/Helium and helium gas can also be used but are not common. The SMA welding process does not require a shielding gas because it generates its own protective shielding from the flux during welding. For groove welds in pipe the back side of the weld must be protected by a purging gas. Argon is normally used as the purging gas. Safety Caution

Argon is heavier than air. Though it is not toxic it can cause death by asphyxiation. Always be aware of this when purging in tanks, vessels or other areas where argon can accumulate. Never enter a pipe if it has a purge running. For the purge to be effective it must reduce the oxygen content on the back-side of the weld to less than 0.05% (500 ppm). Purge dams must Page 18 of 40


be installed to contain the purge gas. See the sketch below. There are many types of purging dams (i.e., cardboard, plastic, wood, balloons, and water soluble paper.) There are commercially available purging devices which are recommended because they seal well and reduce the purging volume. It is critical that the purge dam is tight and can retain the purging gas. It will not be possible to achieve a good purge if the purge dam is porous or leaks. The purging volume should be kept as small as possible.

Figure 3 – General Purge Dam Configuration Water soluble paper is porous and several layers of it must be glued together to make it gas tight. Water soluble paper must be installed with water soluble tape and water soluble glue. Water soluble dams should be the last choice for purging dam material. If a purge oxygen monitor is not available then the following table can be used as a guideline for minimum purging time. This table is only applicable if there is no leakage of gas through the purge dam. If this table is used, then the inside of the first portion of root welded must be examined through the root gap to make sure that there is no excessive oxidation. The flow rate shown for this table is 50 CFH (22.5 L/H). Once the purge has been established the flow rate should be reduced to 10CFH or just enough to maintain the purge level. If there is excessive purge flow it

Page 19 of 40


can buildup pressure on the inside of the pipe and cause the root to be defective or unweldable. Purge Times for Stainless Steel Pipe Purging time Minute

Diameter 2 and less

0.5

4

1

6

2

8

3

10

4

12

6

16

10

18

12

24

22

Minimum purging time based on pipe size and 6 volume changes of gas

The above table assumes use of argon gas at a flow rate of 50 CFH (22.5 L/H). Listed times are for each 300 mm of pipe length to be purged (multiply by actual length). Use the above values for 300 mm for any shorter length. Preheat, Interpass Temperature, Heat Input and Post Weld Heat Treatment (PWHT)

The following section provides general information on the preheat, interpass temperature, heat input and PWHT for stainless steel materials. The governing code, Saudi Aramco Standards and approved welding procedure must be followed. Austenitic, Super-Austenitic, Duplex and Precipitation Hardening Stainless

Austenitic, super-austenitic, duplex and precipitation hardening stainless steels are susceptible to sensitization and secondary phase embrittlement when exposed to temperatures in the range of 450°C to 850°C. For this reason preheat is not used for these materials. They can be heated to remove moisture prior to welding if required. To limit sensitization and embrittlement, the maximum interpass temperature is restricted for these materials. For types 304L and 316L Page 20 of 40


the interpass temperature shall not exceed 177°C. For all other stainless steels in this group the interpass temperature shall not exceed 100°C. The heat input for these materials must also be kept as low as possible. For the duplex materials there is a heat input range that is maintained in order to obtain the “duplex” microstructure in the weld. This range will be indicated on the welding procedure. GTA Welding without the addition of filler material is expressly prohibited for this material. This can reduce the corrosion resistance and lead to cracking. Sometimes welders will want to weld without filler consumable to smooth a rough weld profile. This should never be done. Filler material must always be added or the area should be ground. Post weld heat treatment (PWHT) is not normally recommended for these materials because it can cause sensitization. However, the “L” grades and types 321 and 347 will not be sensitized by a normal PWHT. If heat treatment must be performed it is often specified as a Solution Anneal heat treatment. This treatment involves heating the material to a temperature of approximately 1000°C to 1200°C. This relieves the stresses from forming and welding and is above the temperature at which sensitization occurs. The material is then quickly cooled through the sensitization range. 1. Ferritic Stainless Steels

Preheat must be used for the ferritic stainless steels. This is because these materials can be hardened due to the fast cooling from the welding temperature. 250°C preheat should be used. The Interpass temperature is restricted to 315°C. The heat input is not a serious concern for these materials. PWHT is not normally required for these materials unless the thickness exceeds 38 mm. 2. Martensitic Stainless Steels

These materials will readily form martensite when air cooled. The preheat temperature range is from 250°C to 450°C. After welding they should be slow cooled to 120°C (martensitic transformation temperature) and then tempered at 750°C. The Interpass temperature is restricted to 315°C. The heat input is not a serious concern for these materials. Contamination

As discussed further in section 10.4, it is important to avoid contamination of the stainless steel with particles of carbon steel or elements which can adversely affect the corrosion resistance or cause Page 21 of 40


cracking. Stainless steel wire brushes must always be used. All grinding discs must be halogen, sulfur and iron free as indicated by a statement “for stainless steel.” Brushes and grinding wheels used on stainless steels should either be new or only previously used on similar stainless steels. Brushes and grinding disks that have been used on carbon steel must never be used on stainless steel. Stainless steel must be fabricated in a separate shop from carbon steel. One of the greatest sources of carbon steel contamination is through grinding dust from the carbon steel. This can be embedded into the stainless steel by walking on the stainless steel with contaminated shoes, contaminated lifting equipment and contaminated tools. Other contamination can be from grease, paint and dirt. These can cause significant pickup of carbon and other elements which may affect the corrosion resistance. Before any welding operation, it is essential to ensure that all of the components are clean. If a multipass weld is being produced, all slag and surface oxide must be removed between passes. Copper, zinc, tin, lead and other metals may be picked up from fabrication tools and can lead to cracking. These elements are strongly linked to hot cracking due to the formation of low melting point alloys. Ferrite In Weld Deposits

One of the problems associated with welding of austenitic stainless steels is that of solidification cracking. This problem is often referred to as 'hot cracking' since it occurs before the weldment has cooled. The presence of ferrite in the weld reduces the susceptibility of the metal to hot cracking. This ferrite is referred to as “Delta ferrite” and can be observed in the as-deposited microstructure of austenitic stainless steels welds. This is a product of the solidification and transformation sequence experienced at elevated temperature. For certain alloys it is not essential to have ferrite in the weld deposit but normally 3 – 7FN ferrite will prevent cracking. Cracking is more of a problem when the welds are restrained or the joints are large. Ferrite increases the weld strength level. Ferrite may have a detrimental effect on corrosion resistance in some environments. It also is generally regarded as detrimental to toughness in cryogenic service. In high-temperature service delta ferrite can transform into the brittle sigma phase. Materials with a ferrite number greater than 10 will be susceptible to the formation of excessive quantities of sigma phase and should not be used for elevated temperature service (i.e., at temperatures exceeding 650°C). Ferrite can be measured on a relative scale by means of various magnetic instruments. Ferrite percentage can also be determined by Page 22 of 40


metallographic examination. The amount of ferrite in austenitic stainless steel welds can also be predicted from the chemical composition of the weld deposit using one of several constitution diagrams. These diagrams use nickel and chrome equivalence empirical relationships. These diagrams are known as the Schaeffler, DeLong, and the Welding Research Council (WRC). The WRC diagram should be used for ferrite predictions and gives a ferrite number. Ferrite can be expressed as a “percent ferrite” or a ferrite number (FN). The relationship between the percent ferrite and ferrite number is complex. It is based on the cooling rate and alloy interaction. TWI has weld simulation software online that can predict the percentage ferrite http://www.twi.co.uk/j32k/protected/toolkits/Ferrite/IntroInstructions.html. Two rule-of-thumb conversions from FN to percentage are: 1.

For low percentages of ferrite (less than 10) the FN is the same as the percentage.

2.

For higher percentages (duplex) 0.70(FN) = % ferrite

Measurement of ferrite content in DSS, however, is currently performed with a magnetic measuring technique and is more commonly referred to in terms of percentage. SAES-W-016 gives ferrite measuring requirements for welding procedure qualification and production and special requirements for duplex materials. SAES-W-014 gives ferrite requirements for overlays.

Page 23 of 40


Figure 4 – WRC Ferrite Number Weld Overlay Cladding

Weld overlay cladding is the process of depositing a layer (or layers) of austenitic stainless steel (or other corrosion or wear resistant material) onto the surface of a base material (such as mild steel). This layered component can provide corrosion resistance and/or wear resistance without the expense of producing it entirely from a higher alloy material. Submerged arc welding is by far the most common method of cladding with austenitic stainless steel. SAW is ideally suited to the task since relatively large areas may be deposited with high deposition rates. Electroslag welding is often frequently used. Less frequently are GTAW and GMAW and FCAW welding processes. There is a variety of different equipment available, employing either strip or between one and six wires. Dilution of the weld pool with molten base material may be a problem during cladding since the underlying materials may contain elements detrimental to the performance of the stainless steel (e.g., carbon). In these instances, dilution could be minimized but this may be difficult since it is very sensitive to arc length and voltage. Alternatively, multiple layers can be deposited so that dilution with the parent metal is reduced in each progressive pass. Multiple layer overlays are preferred and single layer overlays require special approval. Joining of Clad Materials

There are two techniques for joining of carbon steel which is clad with Page 24 of 40


stainless steel: Single-sided and double-sided: 1.

Single-sided When welding clad materials from one side (for example a small diameter pipe joint) the entire weld will be made with a consumable that is compatible with the dissimilar metal welding of the base material and the CRA.

Figure 5 – Single-sided Cladding 2.

Double-sided The entire weld is made from both sides (See Saudi Aramco Drawing AB-036367). The stainless steel must be “stripped back” by grinding or machining to no closer than 10mm from the edge of the carbon steel weld. The “stripped back” area shall be checked with a copper sulfate solution to verify that all of the stainless steel has been removed. Copper sulfate solution preparation: 4 gm copper sulfate pentahydrate CuSO4.5H20 (use reagent grade). 250 ml of distilled water. 1 ml sulfuric acid H2SO4 (specific gravity 1.84). The solution is swabbed on the part, and left to stand for 6 minutes minimum, the part should be carefully rinsed and dried. If copper deposit is observed, then free iron is on the surface of part. (Reference ASTM A967) The carbon steel shall be completely welded and NDE inspected prior to welding the cladding portion.

Page 25 of 40


Figure 6 – Double-sided Cladding Caution: Never weld carbon steel or low-alloy steel into high alloy, stainless steel or nickel base material. Never weld stainless steel electrode into nickel base material.

Welding of Duplex and Superduplex

Duplex stainless steels (DSS) are broadly divided into standard and superduplex stainless steels. These materials are characterized by high strength, good corrosion resistance, good resistance to chloride stress corrosion cracking and relatively good notch toughness. Duplex stainless steels (DSS) contain 35 to 65% ferrite and the remainder is austenite. The percentage of ferrite is also called the phase balance. Duplex stainless steels that are currently produced tend to have slightly more austenite that ferrite. The table below gives an approximate comparison between the chemistry of the Duplex and the Superduplex.

Figure 7 – Duplex Microstructure, magnification approximately 200X

Page 26 of 40


Table 3 – Typical Chemistry of Duplex and Superduplex Stainless Steels Cr (%)

Ni (%)

N (%)

Mo (%)

Fe

Standard Duplex Chemistry

22

5

0.12

3.2

Balance

Superduplex Chemistry

25

7

0.25

3.5

Balance

Besides the phase balance, there is a second major concern with duplex stainless steels. Intermetallic phases (Sigma and chi) form at the temperature range of 540 to 950°C. These phases can significantly reduce the toughness and corrosion resistance of these materials. These phases can be eliminated and the original properties restored if the material receives a solution anneal heat treatment at 1040°C and is rapidly cooled through the critical temperature range. The addition of nitrogen significantly delays formation of these phases. Therefore, it is critical that sufficient nitrogen be present in these alloys. The characteristics of welding duplex stainless steel are very similar to welding 300 series stainless steels. The same welding processes, joint details and techniques are generally followed. Some of the differences are noted below. There is an increased emphasis on heat input control. Both the upper and lower ends of the heat input range must be established on the welding procedures in order to achieve the proper phase balance. Welding at lower heat inputs promotes higher ferrite levels; whereas welding at higher heat input levels promotes lower ferrite levels. The heat input is based on the following formula. Generally, the range on heat input for duplex stainless steel is 0.5 to 2.5 KJ/mm but the heat input restrictions on the welding procedure must be followed.

HeatInput 

amps * volts * 60 TravelSpeed

Preheating is not recommended with duplex stainless steels except to dry the surface or when the temperature is below 5°C [40°F], or when welding heavy sections under restraint. The maximum interpass temperature for duplex stainless steels is 100°C per the Saudi Aramco specification SAES-W-016. The low interpass temperature reduces the amount of sigma formation in the HAZ.

Page 27 of 40


Autogenous welding (welding without filler material) should not be performed on duplex stainless steels. DSS welds tend to be higher in ferrite than the base metal being welded. There are near matching composition proprietary filler metals for many of the standard and super DSSs. Suggested filler metals are shown in Table 2. It is also advisable to consult the alloy producer of proprietary alloys for their filler metal recommendations and filler metal availability. Nickel alloy filler metals have also been used to weld both the standard and super DSSs. One such nickel alloy filler metal for which data is available is AWS A5.14 ERNiCrMo-3. Since the nickel alloy welds are fully austenitic, there is no concern regarding obtaining a balance of austenite and ferrite. This material meets the strength, corrosion resistance and impact resistance of the base materials. Dissimilar metal welds between DSS and austenitic stainless steels such as Type 304 or 316 or for welding to carbon steel should be made with ENiCrMo-3 or ERNiCrMo-3. Because of the ferrite present in the duplex stainless steels there is a possibility of delayed hydrogen cracking. For this reason all welding electrodes must be handled to insure that they are low hydrogen. All surfaces to be welded must be free from contaminants that could cause hydrogen pickup. Dissimilar Metal Joints

Dissimilar metal weld joints are welds between stainless steel and another type of alloy. This is frequently stainless steel welded to carbon steel or a low alloy steel. Consideration must be given to the possibility of galvanic corrosion in this type of joint. It is very important to keep electrodes for dissimilar metal welds in a low hydrogen condition. Failure to do this can result in hydrogen cracking on the non-austenitic side of the joint. Dissimilar metal welds are not permitted in sour service exposure. Dissimilar metal welds in hydrocarbon service must be made with a nickel base consumable, such as E/ERNiCrMo-3. Austenitic stainless steel (i.e., type 309) may only be used for dissimilar metal welds in non-hydrocarbon, non-sour service (i.e. water, air or steam) or external attachments. Sensitization

Sensitization occurs in the HAZ adjacent to the weldment and may be minimized by using either low carbon or stabilized grades of stainless Page 28 of 40


steel. If a steel has been sensitized, corrosion resistance may be recovered only by the use of a full anneal (and suitably rapid quench) to dissolve the carbides. Some degree of corrosion resistance may be recovered by a stabilization anneal, typically in the region of 900°C. Sensitization gives rise to corrosion in the HAZ and is a prerequisite for polythionic acid stress corrosion cracking. Low carbon grades or grades with Ti or Nb stabilization can prevent or reduce sensitization. Embrittling Phases

Another phenomenon similar to sensitization is the precipitation of Chi and Sigma intermetallic phases on the grain boundaries in the HAZ. The precipitation of any secondary, chromium-rich phase will deplete the local area of chromium. The classic sigma-phase is nominally FeCr composition, but it can have a more complex, variable composition. High molybdenum high strength stainless steels can contain the Chi phase (Fe36Cr12Mo10). The presence of this phase, which normally occurs at grain boundaries, depletes the chromium content leading to intergranular corrosion. This may cause alloy embrittlement during long term use. The presence of such phase has proven to be highly sensitive to alloy processing parameters such as the cooling rate after a final heat treatment. Both and phases can be easily formed by the decomposition of ferrite, at 540-950°C for phase and 650-950°C for phase. If these precipitates form a continuous network there can be a corresponding reduction in ductility, toughness and corrosion resistance. Recent alloy developments have included the addition of significant amounts of nitrogen to high alloy stainless steels, this acts to retard the nucleation of both and phases, meaning that thick plate can be successfully multi-pass welded. Microsegregation

Pitting of the weldment, independent of any precipitation, may occur where micro-segregation and coring within dendrites is particularly severe. This can often be the case if a matching filler, autogenous weld or high molybdenum stainless steel (4-6% Mo) is used or if there are large surface-lying dendrites. This is the reason that high molybdenum grades must be welded with nickel base fillers. Crevice Corrosion

Crevice corrosion is another frequent cause of failure in weldments. Possibly the most frequent cause of crevice corrosion are solidification cracks and microfissures. Smaller microfissures are often invisible to the Page 29 of 40


naked eye and therefore microfissure crevice corrosion is often mistaken for simple weldment pitting. As previously stated, solidification cracks may be avoided by proper control of weldment delta ferrite content. Any welding defect that causes a crevice or confined feature can also lead to crevice corrosion. Such defects include flux layers or inclusions, pores, and weld start/stop cracks and craters. Moreover, poorly adherent weld spatter may also create crevices. Bad joint design and/or poor welding practice can also cause crevice corrosion; for example, non-removable backing rings are to be avoided or, if essential, should be fully consumed during welding. Failure to do this may result in crevices being formed between the component and the backing ring. Microfissuring and crevice corrosion in high molybdenum alloys (4-6%) is best avoided by the use of nickel base fillers. 10.2.2 Fume and Welding Safety Normal safety considerations should be taken when welding austenitic stainless steels. Austenitic stainless steels also generally contain nickel and this may lead to sensitization of the skin. Sensitization may be as minor as mild irritation, or as major as gross swelling. Particular care should be taken to minimize exposure to weld fumes. Fumes from the welding of stainless steels may also contain significant quantities of nickel and hexavalent and trivalent chromium. Chromium is toxic by inhalation; therefore, welding should only be carried out using proper fume extraction. Chromium and nickel are both known carcinogens and typically have maximum exposure limits as specified in the MSSG. It should be noted that MIG and TIG welding can also give rise to ozone; this also necessitates suitable ventilation. Table 2 – SMAW And GTAW Consumable Selection, Preheat and PWHT Austenitic Stainless Steels Parent Material Type

Filler Material SMAW Electrode

GTAW Bare Wire

Preheat

PWHT

304 and 304L

E304L

ER304L

20° and dry

none

304H

E304H

ER304H

20° and dry

none

316 and 316L

E316L

ER316L

20° and dry

none

317L

E317L

ER317L

20° and dry

none

310

E310

ER310L

20° and dry

none

321

E347

ER347

20° and dry

none

330

E330

ER330

20° and dry

none

Comments

ENiCrFe-3 and ERNiCr-3 can also be used Page 30 of 40

Document Responsibility: Materials and Corrosion Control Standards Committee SABP-A-022 Issue Date: 25 August 2008 Next Planned Update: TBD Stainless Steel Fabrication, Testing and Installation Austenitic Stainless Steels Parent Material Type 347


GTAW Bare Wire

Preheat

PWHT

Comments

E347

ER347

20° and dry

none

ENiCrFe-3 and ERNiCr-3 can also be used

Martensitic Stainless Steels Filler Material Parent Material Type

SMAW Electrode

MIG, TIG, SAW Bare Wire

Preheat

PWHT

410

E410

ER410

250°C

required

430

E430

ER430

250°C

required

Comments

Ferritic Stainless Steels Parent Material Type


GTAW Bare Wire

Preheat

405

E309

ER309

250°C

410S

E309

ER309

250°C

PWHT

Comments

Not normally required Not normally required

Superaustenitic, Precipitation Hardening Parent Material Type


GTAW Bare Wire

Preheat

PWHT

Comments

904L

E385

ER385

20° and dry

none

E/ER NiCrMo-3 may be used

254SMO

ENiCrMo-3

ERNiCrMo-3

20° and dry

none

Alloy 20

ENiCrMo-3

ERNiCrMo-3

20° and dry

none

17-4 PH

ENiCrMo-3

ERNiCrMo-3

20° and dry

none

Duplex and Super Duplex Parent Material Type S31803 S32205 J92205 S32550 S32750 S32760 CD-4MCu (cast)


GTAW Bare Wire

Preheat

PWHT

E2209

ER2209

20° and dry

none

E2595

ER2594

20° and dry

none

Comments

E/ERNiCrMo-3 may be used

Page 31 of 40


10.3

Storage Storage areas must be clean, dry and well ventilated. Absorbent wrappings such as interleaving paper, cardboard and timber should be kept dry to prevent surface staining. Plates and sheets should be stored vertically in racks and not be dragged out of the racks or over one another. Racks should be protected to prevent iron contamination. Outdoor storage of stainless steels adjacent to carbon steels should be avoided (Figure 9).

Figure 8 – Microcracked Surface at Heat Tint Oxide Films on 316L Stainless Steel, x200. 10.4

Figure 9 – Unsuitable Materials Separation and Outdoor Storage

Shop Fabrication The number one problem with unsuccessful fabrication of stainless steel is surface contamination. The shop should have a separate area where only stainless steel is fabricated. This will prevent the cross-contamination of the stainless steel surface with iron particles. NiDI publication 11003 highlights that: “experience has shown that (surface) cleanliness and weld quality are far more critical to successful performance than Cl-ion concentration”. Some suggested methods for the removal of surface contamination and defects are presented in Table 1. NZSSDA (Code of Practice for the Fabrication of Stainless Steel Plant & Equipment, 2001) stresses that the fabricator must ensure that the fabrication is clearly identified and protected from damage and contamination. “Mild steel lifting forks, hooks, chains and wire ropes shall be kept from coming into contact with stainless steel equipment”. Plastic-coated or SS weld overlayed parts should be used for handling SS. Cleaners which can be used include stainless steel wool and stainless steel wire brushes, provided that they have not previously been used on non-stainless steels. Page 32 of 40


Figure 10 shows an unacceptable example of poor material segregation, where carbon and stainless steels are being processed together. Figure 11 shows an example rust spotting caused by iron contamination.

Figure 10 – Poor Shop Material Segregation 10.5

Figure 11 – Iron Contamination Caused by Operator Grinding Carbon Steel Nearby

Pickling, Passivation and Iron Removal Chemical passivation is normally only used to remove “free iron”, high temperature oxides and other surface contamination arising from processing and handling. Pickling treatments provided in ASTM A380 include the following:  Typical austenitic stainless steel pickle solution: 10% HNO3 + 2% HF at 50ºC  Removes oxide film plus 25 – 40 μm of surface  Effectively removes welding heat tint  Removes embedded iron particles  Can improve corrosion resistance of ground, wire brushed and blasted surfaces by removing surface contamination and exposed impurities in the metal, such as sulphides  Exposes a new, clean surface which can then be passivated  Air passivate or be chemically passivated Pickling is normally performed during manufacturing processes, such as following solution heat treatment of welded stainless pipe. The parts are picked by immersion in a bath and thoroughly rinsed and blown air dried afterwards. Page 33 of 40


Pickling can also be performed after shop or site welding operations, using proprietary pickle pastes or swabbing followed by thorough rinsing in clean water. Chemical passivation, using nitric acid treatments, etc., is also normally used during manufacturing after pickling treatments. Pickling with nitric-hydrofluoric acid removes free iron and a thin surface layer of metal that may contain surface defects. The metal surface is then passive and in the most corrosion resistant state. If a nitric-hydrofluoric acid pickle is not practical, the free iron can be removed mechanically. Acceptable methods include the use of medium to fine-grit abrasives such as clean flapper wheels, flexible disks or blasting with clean abrasives such as glass beads, garnet or walnut shells. Free iron and heat tint can also be removed by a hand-held electropolishing probe. The “Rust Bloom” water wetting and drying procedure described in ASTM A380 para 7.2.5.1 is an effective test to check for the removal of free iron. The procedure calls for wetting the surface with distilled or deionized water or fresh water followed by drying. Formation of rust stains may be accelerated by periodically wetting the surface with preferably distilled or deionized water or clean, fresh, potable tap water. The wet-dry cycles should be such that the sample remains dry for a total of 8 h in a 24-h test period. After completion of this test, the surface should show no evidence of rust stains or other corrosion products. Free Iron Contamination is detected using a ferroxyl test. A solution of nitric acid and potassium ferricyanide is sprayed onto the surface and free iron contamination is disclosed by the development of a dark blue color within 30 s. The solution should be removed after a few minutes with a damp cloth or water spray. ASTM A967 Practice E (Potassium Ferricyanide–Nitric Acid Test) provides further details of this type of test, which is recommended for application only on 200 and 300 series stainless steels. SAES-W-014 recommends that only stainless steel brushes, ceramic (glass) beads, iron-free grit, or stainless steel grit shall be used to mechanically clean the weld overlay surfaces. 10.6

Field Fabrication Shop fabrication is normally better controlled and potentially much more serious field fabrication errors could be overlooked. Some contractors apparently lack knowledge and/or experience of appropriate handling techniques for stainless and other CRAs. Storage racks, forklift truck forks and handling tools, etc., must be coated with suitable materials such as plastic, rubber or weld overlayed with SS. Alternative Page 34 of 40


lifting equipment materials to those used in carbon steel fabrications shops are also recommended. Fabric or rope slings should be used rather than steel chains. Conveyor tables should be designed and operated to avoid damage and contamination. Where hardened ferrous tools (such as roll, presses and angle rolls) must be used, they must be completely cleaned (solvent/steam cleaned). Heavy paper sheets are sometimes used to prevent direct contact between tool and stainless steel. Packaging materials and methods used must help prevent surface damage. Carbon steel strapping must not be allowed to come into contact with the stainless steel surfaces. If used, wooden bearers should be inserted between the carbon steel strapping and the stainless steel surfaces. Table 4 – Suggested Removal Methods for Various Surface Defects and Contamination

The following photographs (Figures 12 to 15) show improper handling of SS and are from an actual jobsite. Contractors seemed totally unaware of appropriate techniques for good fabrication practice for stainless steels. Examples included: contractors off loading stainless steel pipe with carbon steel hooks, using carbon steel wedges to support stainless steel during tank fabrication, using wire brushes and grinding wheels that were first used on carbon steel, using carbon steel lifting brackets to place shell plates, not covering the storage facilities so the stainless steel does not rest on carbon steel and other items. Page 35 of 40


11

Hydrostatic Testing SAES-A-007 paragraph 6 limits the chloride content of hydrotest water to 50 ppm to minimize the risk of chloride pitting or SCC during startup. Hydrotest water should also be verified as low chloride and low SRB bacteria count, or be biocide treated. For example, use Kitagawa or Draeger tubes or similar field techniques, to verify chloride level on-site (Figure 16). The key actions are summarized: 

Hydrotesting should be carried out with clean treated water



Do not use untreated raw water, seawater, or contaminated recycled water



Drain water from inside plant promptly after hydrotesting



After draining hydrotest water, check that there are no areas of ponding (stagnant water) – if necessary, wipe / mop to dry

Figure 12 – Carbon Steel Wedges on Stainless Steel Tank Bottom

Figure 13 – Carbon Steel Fitting Plates Used to Fit Stainless Steel Shell Plates

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Figure 14 – Carbon Steel “Key Nut” on Stainless Tank

Figure 15 – Carbon steel Fixturing (Wedges and Key Shims) on Stainless Tank

Figure 16 – Kitagawa Chloride Test Tube # 201SB 12

Microbiologically Influenced Corrosion (MIC) MIC is a fairly common problem with stainless steels, including duplex stainless grades. Failures can occur rapidly in microbially contaminated waters containing relatively low chlorides (<100 ppm). These failures can occur too rapidly (in terms of weeks to months) to be attributed to conventional chloride crevice corrosion. The morphology of MIC failed components is usually consistent with localized corrosion and pitting at or adjacent to austenitic stainless welds (Figures 17, 18). According to the ASSDA, MIC can be avoided as follows: 

Ideally, ensure all internal welds are polished smooth – bacteria can form colonies on rough weld surfaces. Though this may be impractical in many engineering applications, weld spatter should definitely be removed or controlled (Table 4).



Remove heat tints, as these favor bacterial colonization



Use only treated water for hydrotesting and for cooling water: microorganisms cannot live in treated (chlorinated) water



Introduce a biocide such as chlorine into the water system Page 37 of 40




Do not leave stainless equipment, such as tanks, pipes and vessels, filled with stagnant water

Note frequently there is confusion between the “chloride” that causes stress corrosion cracking and the “chlorine” found in the biocides used to treat water. “Chloride” is the ion of Chlorine (Cl-) that is present in water from dissolving NaCl, MgCl2, or other salts. The “chlorine” that is present in water from some of the biocides is HOCl and OCl-. These forms of “chlorine” are sometimes referred to as “free chlorine.” They do not break down or change into the harmful “chloride” ion that causes stress corrosion cracking.

Figure 17 – Stainless steel water tank after 8 months, showing preferential corrosion of welds by MIC, where heat tints were not removed.

Figure 18 – Microsection through a SS tank weld showing undercut pitting due to MIC in weld filler metal. 316L plate and filler metal were damaged in 6 weeks due to MIC in raw water

Remedial measures include: drain promptly and completely after hydrostatic testing, or circulate water for about one hour daily if the water cannot be drained. Regarding draining and drying, valves, for example, should not be overlooked and must be drained fully. 13

Coating Stainless Steel 13.1

Coating Under Insulation To mitigate CUI, stainless steels operating at low and high temperatures ranges are insulated and normally the SS is coated. For SSs, CUI damage can potentially occur as SCC, pitting and crevice corrosion. Insulation is normally encased in aluminum sheet wrapping. Causes of CUI-related failures can

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include moisture entrapment under insulation, chloride contamination of insulation and damaged or non-weatherproof casings. NACE RP0198 (The Control of Corrosion Under Thermal Insulation and Fireproofing Materials - A Systems Approach) provides practical guidelines for managing CUI. It is also very important to specify and install low leachable chloride insulation materials for SSs (conforming to ASTM C871). External coating selection shall be reviewed and approved by the CSD Coatings Team. 13.2

Coating above Grade Piping Exposed to Atmospheric Conditions Exposure to chloride contamination due to washing down with saline or raw water and proximity to seawater breeze or splash can cause significant damage to plant piping and equipment made of stainless steel. To prevent chloride induced stress corrosion cracking caused, protective coating shall be used. Select halide free coating materials which need minimum surface preparation either by solvent cleaning or by abrasive blasting using aluminum oxide abrasive without causing damage to the passivation layer.

13.3

Coating Buried Pipes All buried stainless steel pipes shall be coated to prevent adverse interference with the cathodic protection system. In addition, stainless steel pipes are susceptible chloride stress corrosion cracking due to exposure to soil chloride contamination and soil stress, and hence, halide free external coating shall be applied on stainless steel pipes after appropriate surface preparation with solvent cleaning and/or abrasive blasting using aluminum oxide abrasive.

14

Summary The flowchart (Figure 19) summarizes five principal process steps for the specification, fabrication, hyrotesting and protection of stainless steels. The controls associated with each step should be carefully followed and documented where required, to ensure an optimum performance and reliable installation using stainless steels. The controls should be used as a checklist to document the use of correct materials and proper storage, fabrication, hydrotesting and insulation and external coating (where appropriate).

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Stainless Steel Selection and Specification

 Consider service, PREN  Review heat treatment of wrought and cast parts  Perform PMI

Storage and Segregation

 Store SSs separately in dry conditions  Avoid free iron contamination  Use non-chlorinated solvents and chloride-free marking inks

Fabrication and Assembly – Shop and Field

 Control or remove welding heat tints  Check for and remove any iron contamination  Check appropriate handling, tools and equipment  Welding procedure control – PMI of consumables & fabricated parts

Hydrotest Procedure and Documentation

Protection Insulation and External Coating Process Step

 Verify hydrotest water chloride (≤ 50 ppm) and low SRBs (SAES-A-007)  Completely drain and dry water after test  Don’t leave trapped stagnant water

 Verify low leachable chloride insulation  External coating holiday check  Water tightness of sheathing

Control

Figure 19 – Flowchart summarizing principal process steps and associated controls in the specification, fabrication, hydrotesting and protection of stainless steels

25 August 2008

Revision Summary New Saudi Aramco Best Practice.

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SABP-A-022

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