SABP-W-001 Welding of Cast Iron

Best Practice SABP-W-001

31 March 2008

Best Practice for the Welding of Cast Iron Document Responsibility: Welding Standards Committee

Saudi Aramco DeskTop Standards Table of Contents 1 2 3 4 5

Introduction..................................................... 2 Conflicts and Deviations................................. 2 References..................................................... 2 Types of Cast Irons and Weldability.............. 3 Welding General............................................. 6

Attachment 1....................................................... 10 Attachment 2....................................................... 11

Previous Issue: New

Next Planned Update: TBD Page 1 of 15

Primary contact: Niemeyer, Dennis Charles on 966-3-8736700 Copyright©Saudi Aramco 2008. All rights reserved.

Document Responsibility: Welding Standards Committee Issue Date: 31 March 2008 Next Planned Update: TBD

1

SABP-W-001 Best Practice for the Welding of Cast Iron

Introduction 1.1

Scope and Purpose Within Saudi Aramco operations sometimes there is a need to weld cast iron materials. This is normally for the repair of cast iron components. It is not unusual to have broken cast iron parts given its brittle nature. Cast irons can be difficult to weld and there are some types of cast iron that are impossible to weld. Cast iron is not like the normal carbon or low alloy steels that we are used to welding. The welding of cast irons is not covered by our normal welding procedures for pressure vessels or piping. This Best Practice is intended to give guidance for determining which cast irons can be welded and how to weld them.

1.2

Disclaimer The use of this Best Practice does not relieve the engineer from his responsibility or duty to confirm and verify the accuracy of any information presented herein. The use of this information or material does not guarantee results that will satisfy any applicable requirements of Saudi Aramco Standards. CSD assumes no responsibility or liability whatsoever for the misuse of the information in this document. This Best practice is intended as guidelines and shall not be considered as replacement for the Mandatory Saudi Aramco Engineering Requirements. Saudi Aramco® is a registered trademark of the Saudi Arabian Oil Company. Copyright, Saudi Aramco, 2008.

2

Conflicts and Deviations In the event of a conflict between this Best Practice and other Mandatory Saudi Aramco Engineering Requirement, the Mandatory Saudi Aramco Engineering Requirement shall govern.

3

References This Best Practice is based on the latest edition of the references below, unless otherwise noted. 1.

American Welding Society WELDING HANDBOOK, 8th Edition, Volume 4, Materials and Applications - Part 2

2.

ANSI/AWS D11.2-89 (R2006) Guide for Welding Cast Irons

3.

American Welding Society, Welding of Cast Iron, A Selection of Papers Page 2 of 15


4


4.

American Society for Metals, Metals Handbook, 9th Edition, Volume 6, Welding, Brazing and Soldering

5.

American Society for Metals, Metals Handbook, 10th Edition, Volume 1, Properties and Selection: Irons, Steels and High-Performance Alloys

6.

The Lincoln Electric Company, the Procedure Handbook of Arc Welding

7.

The James F. Lincoln Arc Welding Foundation, Metals and How to Weld Them

8.

ESAB, Online Handbook, Welding Cast Iron

9.

Kiser, Samuel D. and Northey, Michael, Welding Cast Iron, Canadian Welding Association Journal, Fall 2005

Types of Cast Irons and Weldability Cast irons are the most complex alloy family used in industry. They have much higher carbon, silicon and manganese alloying than the structural steels that we are used to seeing. See the table below for a general chemistry. General Cast Iron Chemistry carbon

1.7 to 4.5%

silicon

0.5 to 3.0%

manganese

0.2 to 1.3%

phosphorus

0.8% maximum

sulfur

0.2% maximum

Molybdenum, nickel, chromium, cerium, copper for specific properties

The metallurgical structure of cast irons is also much different that structural steels. Cast irons basically have a matrix with a carbon-rich phase distributed throughout the matrix. The carbon rich phase is either in the form of Graphite (pure carbon) or Fe3C (cementite). Both of these are brittle phases. The mechanical properties and weldability of the cast iron are largely determined by the matrix. Cast iron is a general term that applies to a wide range of cast materials. In general, there are four categories of cast iron: white iron, gray iron, malleable iron, and ductile (or nodular) iron. 4.1

White Iron White iron is a cast iron that does not contain any graphite. The carbon combines with iron, to form cementite, Fe3C. This is a hard and brittle component and is distributed throughout the casting matrix. This makes the

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overall material brittle. It is called white iron because the fracture surfaces when broken appear white. All types of White Iron are considered unweldable. 4.2

Gray Iron Gray cast iron gets its name from the color of the fracture surface. The microstructure of gray iron is uncombined carbon as interconnected graphite flakes in a matrix of ferrite or pearlite. The matrix will determine the ductility of the material. Gray iron castings are the most common form of cast iron and there are many classes or grades depending on the matrix, chemistry and details of the graphite flakes. Because of the wide range of microstructures in the Gray Iron range there is a wide range of weldability. Gray cast iron generally has better weldability than white Iron but not as good as cast irons that have the graphite in round nodules as with the malleable Iron and Ductile Iron.

4.3

Malleable Iron Malleable iron is heat-treated white iron in which the Fe3C (cementite) are converted to graphite nodules when annealed at a temperature above 870°C for periods greater than 6 h. Irregularly shaped graphite nodules are precipitated and grow in the solid iron. The malleable iron produced by annealing is either ferritic or pearlitic malleable iron depending on the annealing cycle. Whiteheart and Blackheart are forms of malleable iron. Malleable iron has good weldability.

4.4

Ductile (Nodular) Iron Ductile and gray iron are similar with respect to carbon and silicon contents and in terms of general foundry practice for the production of iron castings. Ductile iron because of its lower sulfur and cleaner conditions and additional alloying has graphite that forms into spheres during solidification. Ductile iron is also referred to as spheroidal-graphite cast iron. Ductile has higher strength and ductility than gray or malleable iron. Ductile iron has the best weldability of the cast irons.

4.6

Other Types of Cast Iron After the four basic grades of cast irons and the sub variations of these there are a large number of other specialty grades of cast irons for corrosion, high temperature and abrasion resistance. There is compacted graphite which is similar to both gray and ductile iron. There are also alloy cast irons, some of which can be stainless steel. The repair of these is not covered in this best practice.

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4.7


How to Identify the Type of Cast Iron The welding and brazing procedures that are given in this best practice are "general" and are designed to work for a wide range of "weldable" grades of cast iron. The easiest way to identify the type of cast iron is from the drawing or manufacturer furnished information (see Tables 1 and 2). If this is not available it is only possible to identify the type of cast iron by sending a sample to the lab. Both the chemistry and microstructure must be checked. Hardness can be used to give an indication as to the ability to weld the material. White cast irons will typically be over 400 Brinell hardness. Gray cast irons will typically have hardness less than 300 Brinell. Table 1 – Classification of Cast Iron by Commercial Designation, Microstructure, and Fracture Commercial designation

Carbon-rich phase

Matrix

(a)

Fracture

Final structure after

Gray iron

Lamellar graphite

P

Gray

Solidification Solidification or heat treatment

Ductile iron

Spheroidal graphite

F, P, A

Silver-gray

Compacted graphite iron

Compacted vermicular graphite

F, P

Gray

Solidification

White iron

Fe3C

P, M

White

Solidification and (b) heat treatment

Mottled iron

Lamellar Gr + Fe3C

P

Mottled

Solidification

Malleable iron Austempered ductile iron

Temper graphite

F, P

Silver-gray

Heat treatment

Spheroidal graphite

At

Silver-gray

Heat treatment

Notes: (a)

F, ferrite; P, pearlite; A, austenite; M, martensite; At, austempered (bainite).

(b)

White irons are not usually heat treated, except for stress relief and to continue austenite transformation.

Table 2 – Types of Cast Iron by Code Specifications White Iron ASTM A532 Gray Iron ASTM A48 ASTM A74 ASTM A126 ASTM A159 ASTM 319 SAE J431 Page 5 of 15



For some specifications the Grade or class can indicate the tensile strength in Ksi Ductile Iron SAE J434C ASTM A536 ASTM A716 ASTM A716 ASTM A746 For some specifications the Grade or class can indicate the "tensile strength-Yield strength – Elongation" in Ksi Maleable Iron ASTM A47 ASTM A197 ASTM A220 ASTM A602 SAE J158a

5

Welding General There are a wide range of cast irons with varying weldability. For the weldable cast irons there are two techniques that can be used. There is a "High Temperature" and a "Low Temperature" technique. With the "high temperature" technique, the hard constituents form in the HAZ of the cast iron. These are then tempered by a post weld heat treatment. With the "Low Temperature" technique the amount of the hard phases are limited by maintaining a low temperature. 5.1

High Temperature This is the preferred method of repair and should be used in all cases unless it is completely impossible to perform the heat treatment of the entire piece. Because of casting size, it may not be possible to preheat and perform the PWHT of the entire casting. In these cases the "Low Temperature" method must be used. Weld using a low current, to minimize admixture, and residual stresses. Preheat the casting to 300°C to 650°C. Don't heat over 650°C degrees since that will put the material into the critical temperature range and cracking can occur. Preheat the part slowly and uniformly. It is welded in a narrow temperature range. After welding raise the part to the PWHT temperature without allowing it to cool to below the preheat temperature. Perform the PWHT and allow the part to slowly cool. The purpose of this is to allow the hard constituents to form and then temper them before cooling. Peening of weld beads during welding can be helpful in reducing the stresses and preventing cracking. Wrapping the casting in an insulating blanket, or Page 6 of 15



burying it in dry sand, will help slow cooling rates, and reduce cracking tendencies. 5.2

Low Temperature The size of the casting, or other circumstances, may require that the repair be made without preheat. For this method the casting is preheated to a low temperature (50°C minimum) for the entire welding operation. The low temperature is used to prevent the formation of the hard microstructure phases. Preheat may be needed in areas that are not to be welded to but the weld into compression after welding; see the discussion of preheat below. Each weld bead must cool to about 95°C before subsequent deposits are made. Never heat the casting so hot that you cannot place your bare hand on it. Make short, approximately 1" long welds. Peening after welding is important with this technique. Allow the weld and the casting to cool. Do not accelerate the rate of cooling with water or compressed air. It may be possible to weld in another area of the casting while the previous weld cools. All craters should be filled. Whenever possible, the beads should be deposited in the same direction, and it is preferred that the ends of parallel beads not line up with each other. Sealing Cracks because of the nature of cast iron, tiny cracks tend to appear next to the weld even when good procedures are followed. If the casting must be water tight, this can be a problem. However, leaking can usually be eliminated with some sort of sealing compound or they may rust shut very soon after being returned to service.

5.3

Welding Electrode Selection The welding will be by the SMAW process. The first choice in the welding electrode is ENiFe-CI. The second choice electrode is ENi-CI. The electrode size will normally be 2.4 mm or 3.2 mm.

5.4

Arc Welding Joint Designs The root opening should be sufficient to permit complete fusion between the root faces and the backing, if used. When practical, thick sections should be welded from both sides using either a double-V-groove or double-U-groove preparation. When repairing cracked castings, a hole can be drilled at the end of each crack or a small transverse weld can be applied to prevent propagation. In actual practice, the exact end of the crack is difficult to locate for placement of a drilled hole; whereas precise placement is less critical for a small cross weld near the visible end of the crack. Sufficient cast iron should be removed to eliminate the crack and provide for manipulation of the welding electrode during repair welding. For V-groove welds, an included angle of 60 to 80 degrees is suitable. Page 7 of 15


5.5

5.6

5.7


Base Metal Preparation 5.5.1

The surfaces adjacent to surfaces to be welded should be free of foreign material. The as-cast surfaces should be removed by grinding. It is essential that the casting skin and any ground surfaces be wiped with mineral spirits to remove residual surface graphite prior to welding. Residual graphite inhibits wetting and must be removed to ensure complete joining of parts.

5.5.2

Castings that have been in service are often impregnated with oil or grease that can be removed by using solvents or steam cleaning. Where possible, the casting should be heated uniformly to about 370°C for 30 minutes using an oxy-fuel gas torch. When the alternative of localized degassing is preferred, heat the weld area by depositing a weld pass (usually very porous) and then removing it by grinding. This welding and removal operation is repeated until the weld metal is sound. Then the weld may be completed as specified in the welding procedure.

Welding Details 5.6.1

To minimize welding stresses, stringer beads should be used. Their length should be 75 mm maximum. These passes should be alternated along the area to be welded and not in a continuous pass as in normal welding. Passes should be as small as possible without weaving. Heat input should be as low as possible. It is best to use a larger electrode at a lower current setting. The cast iron surface should be buttered prior to filling the groove. (Attachment 2)

5.6.2

Melting more of the casting than is necessary should be avoided.

5.6.3

The arc should be struck in the weld groove -never on the casting. Arc length should be kept as short as possible. Welding current needs to be reduced about 25% for the vertical position.

5.6.4

PEENING should be used to reduce the shrinkage stresses in the weld. See the following section on peening.

Peening Peening is mechanical working of the weld bead using impact blows to relieve stresses and prevent cracking. Ball peen hammers generally are used or round nosed punches. Do not use a chisel or other sharp tool. Each weld pass induces shrinkage stresses that approach the ultimate tensile strength of the base material. Proper peening relieves those stresses until the next pass is made. Each weld bead should be peened before the metal cools to below about 350°C. Page 8 of 15



Peening must be performed correctly to stretch the hot weld metal deposit. Too little peening will not adequately relieve shrinkage stresses but severe peening may cause cracking.

31 March 2008

Revision Summary New Saudi Aramco Best Practice.

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ATTACHMENT 1 EXAMPLE OF USING PREHEAT TO REDUCE THE STRESSES ON THE WELD AREA WHEN USING THE LOW HEAT INPUT METHOD OF WELD REPAIR

An Item has a crack as indicated in the sketch.

Heat the appropriate areas to 300°C. This will cause the crack to open slightly.

Make the weld using the Low Heat welding Procedure, LCI

Allow the weld and the preheated areas to cool slowly together. This will reduce the tensile stresses on the weld and chances of cracking

.

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ATTACHMENT 2 USING THE BUTTERING TECHNIQUE TO MINIMIZE DILUTION OF THE WELD AND BASE METAL

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SAUDI ARAMCO WELDING PROCEDURE SPECIFICATION WPS No.: HCI Revision No. 0 Page 1 of 2 By: DCN SCOPE: High preheat repair of cast iron with SMAW Welding Processes: SMAW

PQR: None

BASE METAL: Gray, nodular or ductile cast iron

Filler Material Spec. No. (SFA): AWS A5.15

Material Specification (typical)

AWS No. (Class.): ENiFe-CI or ENi-CI Size of Filler Metals: 2.4 and 3.2mm

BM Thickness Range: NA

Max. Deposit Thickness: as required

Pipe Diameter Range: NA

SAW Electrode-Flux (Class.): NA Flux Trade Name: NA SAMS S/N:

PREHEAT Minimum Preheat Temp.: 300°C to 650°C (see note 1) Maximum Interpass Temp. 750°C

Shielding Gas (Type) NA

POST WELD HEAT TREATMENT: Required (see Note 2) Temperature Range: 620 – 675°C Time Range: 1 hour per 25mm of thickness Heating Rate Cooling Rate: 55°C per hour maximum

Flow Rate NA Gas Backing (Type) NA Gas Backing Flow Rate NA

POSITION Groove: All Welding Progression: All

TECHNIQUE & ELECTRICAL CHARACTERISTICS Tungsten Electrode Size and Type: NA

Orifice or Gas Cup Size: NA

String or Weave Bead Stringer Method of Backgouging: grinding or gouging Initial & Interpass Cleaning (Brushing, Grinding, etc.) See note 4

Multiple or Single Passes: multiple Contact Tube to Work Distance: NA

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SAUDI ARAMCO WELDING PROCEDURE SPECIFICATION

JOINT SKETCH

WPS No.: HCI Revision No. 0 Page 2 of 2 Bevel angle 60° Cracks will be ground to about ½ of their depth, welded and backgrind and weld the back side

LAYE R

PROCESS

FILLER METAL

SIZE (mm)

POL.

VOLT

AMP

COMMENT

All

SMAW

ENiFe-CI

2.4

DCEP

21 - 23

60 - 100

All

SMAW

ENiFe-CI

3.2

DCEP

22 - 24

80 - 150

Passes are to be kept as small as possible

NOTES: 1. The preheat temperature may be selected based on the type and chemical composition of the cast iron. If this is unknown, then the preheat will be 400°C minimum. The entire casting must be heated uniformly to the preheat temperature. 2. PWHT shall be performed immediately after the completion of welding. The casting shall not cool to below the preheat temperature prior to PWHT. 3. The area of the casting shall be cleaned of surface and absorbed oil and contaminants. See the best practice for cast iron welding. 4. Each weld pass will be peened. See the best practice for cast iron welding.

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SAUDI ARAMCO WELDING PROCEDURE SPECIFICATION WPS No.: LCI Revision No. 0 Page 1 of 2 By: DCN SCOPE: Low preheat repair of cast iron with SMAW Welding Processes: SMAW

PQR: None

BASE METAL: Gray, nodular or ductile cast iron

Filler Material Spec. No. (SFA): AWS A5.15

Material Specification (typical)

AWS No. (Class.): ENiFe-CI or ENi-CI Size of Filler Metals: 2.4 and 3.2mm

BM Thickness Range: NA

Max. Deposit Thickness: as required

Pipe Diameter Range: NA

SAW Electrode-Flux (Class.): NA Flux Trade Name: NA SAMS S/N:

PREHEAT Minimum Preheat Temp.: 50°C (see note 1) Maximum Interpass Temp. 95°C

POST WELD HEAT TREATMENT: None Temperature Range: NA Time Range: NA

Shielding Gas (Type) NA

Heating Rate Cooling Rate: NA

Flow Rate NA Gas Backing (Type) NA Gas Backing Flow Rate NA

POSITION Groove: All Welding Progression: All

TECHNIQUE & ELECTRICAL CHARACTERISTICS Tungsten Electrode Size and Type: NA

Orifice or Gas Cup Size: NA

String or Weave Bead Stringer Method of Backgouging: grinding or gouging Initial & Interpass Cleaning (Brushing, Grinding, etc.) See note 4

Multiple or Single Passes: multiple Contact Tube to Work Distance: NA

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SAUDI ARAMCO WELDING PROCEDURE SPECIFICATION

JOINT SKETCH


WPS No.: LCI Revision No. 0 Page 15 of 15 Bevel angle 60° Cracks will be ground to about ½ of their depth, welded and backgrind and weld the back side

LAYER

PROCESS

FILLER METAL

SIZE (mm)

POL.

VOLT

AMP

COMMENT

All

SMAW

ENiFe-CI

2.4

DCEP

21 - 23

60 - 100

All

SMAW

ENiFe-CI

3.2

DCEP

22 - 24

80 - 150

Passes are to be kept as small as possible

NOTES: 5. If there is evidence of cracking then this procedure cannot be used and the procedure HCI must be used. 6. The area of the casting shall be cleaned of surface and absorbed oil and contaminants. See the best practice for cast iron welding. 7. Each weld pass will be peened. See the best practice for cast iron welding.

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SABP-W-001 Welding of Cast Iron

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