Welding of P91

A Presenta Presentation tion on Welding procedure of SA 335 P91 Material

Presented Pre sented by Siddhartha Sankar Bharali Assistant Engineer Emp Code : M6B194

Siddhartha Sankar Bharali

WELDING WELD ING BASE MA MATERI TERIALS ALS P.No

Group No.

Common Name

Tubes

Pipes

P1

1

Carbon Steels

SA192

SA106GRB

P1

2

Carbon Steels

SA210GRC

SA106GRC

P3

1

Carbon-Moly Steels

SA209T1

SA335P1

P3

1

½Cr½Mo Steels

SA213T2

SA335P2

1Cr½Mo/ 1¼Cr½Mo Steels

SA213T12/ SA213T11

SA335P12/ SA335P11

P4

P5A

1

2¼Cr1Mo Steels

SA213T22

SA335P22

P5B

2

9Cr1MoV Steels

SA213T91

SA335P91


HIGH TEMPE TEMPERA RATURE TURE STEAM PIPING MATERIAL BEFORE P91  Material

used for high temperature piping before the use of P91 was X20.

Composition of P91

9 Cr 1MoV


Composition of X20

12 Cr 1Mo 1/4V

BREIF HISTORY OF X20 

X20 was introduced in the 1950s in Germany and used in steam lines operating at temperatures of 530 0C and higher for power generating sets of 150MW and more.



X20 material was first used in India for high-temperature steam piping around 1970.



The use of X20 allowed wall thickness reductions of about 50%. These reductions resulted in easier handling; less energy needed for preheating and postweld heat treatment , faster start-up and shutdown of the unit.



However, two factors limited its use:

- The extreme care needed for its fabrication and welding. - Its noninclusion in the American Society of Mechanical Engineers (ASME). Siddhartha Sankar Bharali

INTRODUCTION OF A NEW MATERIAL:P91 

U.S. had been trying to develop a new material since the middle 1970s to bridge the gap between ferritic P22 and austenitic steels with respect to creep rupture strength for hightemperature service from 540 to 600 0C.



As a result of these developmental efforts, a new material, designated P91, was introduced in the U.S. in the 1980s by Oak Ridge National Laboratory.



It has very high strength at elevated temperatures and good fabrication properties. These features have made P91 the material of choice for high-temperature steam piping.


P91 DETAILS P NO.

COMPOSITION

P 5A GR 2


9 Cr 1 MoV

FILLER USED

90 S- B9

ELECTRODE

E9018- B9

SA 335 P91 welding steps Edge preparation & fit up

GTAW

SMAW

Cooling

Purging

Preheating

PWHT

Soaking


EDGE PREPARATION & FIT-UP  Cutting

of P-91 material shall be done by band saw / hacksaw / machining / grinding only. Edge preparation (EP) shall be done only by machining.

 The

weld fit-up shall be carried out properly to ensure proper alignment and root gap . Neither tack welds nor bridge piece shall be used to secure alignment. Partial root weld of minimum 20mm length by GTAW and fit-up by a clamping arrangement is recommended. Use of site manufactured clamps for fit up is acceptable .


The fit-up shall be as per drawing. Root gap shall be 2 to 4 mm; root mismatch shall be within 1-mm. Siddhartha Sankar Bharali

ELECTRODE NOMENCLATURE  







Prefix ‘E’ : Arc Welding Electrode First two or three digits : Strenght in ksi For example : E-90xx is 90ksi Second last digit: Welding position For example : E-xx1x = All positions : E-xx2x = Horizontal and Flat : E-xx3x = Flat Last digit : E-xxx1: Type of Coating and other Characteristics For example : E-xxx3 = Rutile Coated : E-xxx8 = Basic Coated (Low hydrogen) Suffix ‘A-1,B-2,B-3 etc.’ : Alloying Element For example : E-xxxx A-1 = 0.5Mo : E-xxxx B-2 = 1.25Cr & 0.5Mo : E-xxxx B-3 = 2.25Cr & 1.0Mo


PURGING Reason for purging 

It is generally carried out on the backside of the weld to eliminate oxygen and other atmospheric gases from mixing with hot metal.



Oxygen have a stronge tendency to combine with hot metal which will lead to the formation of thin films of oxide layers on the weld surface. It prevents the joining of two pieces by welding.



Argon is commonly used as purging gas.


WHY ARGON ?  Argon

is chemically inert, heavy, monatomic gas available in large quantity at reasonable cost.

 Argon

is used for shielding and backing purpose.


ARRANGEMENT FOR PURGING


Purging Details 

Purging should be start from inside of the pipe when root temperature reaches 2200C .



Adequate amount of Argon should be purged in the root area.



Minimum pre-flushing time for purging before the start of welding is 5 min irrespective of pipe thickness.



The min flow rate to be maintained during purging is 10-26 litres/min . For shielding during GTAW welding, flow rate is 814 litres/min.


PREHEATING  Prior

to the start of preheating the surface should be clean from oil, grease and dirt.

 Preheating temperature

should be maintained at 2200C.


WELD PREHEAT & PWHT FOR PIPES(OD>102mm) P No. of Material

P1 Gr 1 & SA106 Gr C

Thickness (mm)

P4 Gr 1

Preheat °C

PWHT °C

≤ 19

NIL

NIL

> 19≤ 25

NIL

595- 625

> 25 ≤ 75

150

595- 625

150

595- 625

≤ 19

Nil

Nil

> 9≤ 19

Nil

620-650

> 19

150

620-650

≤ 75

125

640- 670

> 75

150

640- 670

150

680- 750

> 75 P1 Gr 2 & SA 106 Gr C

Butt Weld

P5 A


Contd. P No. of Material

Thickness (mm)

Butt Weld Preheat °C

PWHT °C

P5B Gr2 + P5B Gr2

All

220

750-770

P5B Gr2 + P5 A

All

220

730- 760


Preheating Methods 3 common methods of preheating:  Electrical

resistance heating.

 Induction

heating.

 Oxy-

acetylene heating.


GTAW WELDING PROCESS


TIG WELGING BENIFITS 

Superior weld quality.



Precise control of welding variables.



Free of spatter.



Low Distortion.



Weld can be made with or without filler material.


SMAW WELDING PROCESS


POST WELD HEAT TREATMENT (PWHT) 

The method shall be by locally heating a circumferential band including the entire weld and adjacent area of base metal, by induction or electrical resistance heating.


DETAILS OF PWHT  PWHT temperature for P91 :

(760±10)0 C

2.5 minutes/ mm of weld thickness subjected to minimum of two hours.

 Soaking time :


HEATING & COOLING RATES Thickness of Material

Maximum Heating Rate Above 300°C

Maximum Cooling Rate Upto 300°C

≤25 mm

220°C/hour

220°C/hour

> 25 ≤ 50 mm

110°C/hour

110°C/hour

> 50 ≤ 75 mm

75°C/hour

75°C/hour

> 75mm


55°C/hour

55°C/hour

GRAPHICAL REPRESENTATION OF SA 335 P91 WELDING PROCRESS


OPERATION

SL NO

TEMPERATURE

RATE OF COOLING/ HEATING

1

PREHEAT

220

2

WELDING BY GTAW/ SMAW

220-350

3

COOLING

80-100

4

HOLDING AT 80-100 FOR MIN 1 HR . HOLDING SHALL CONTINUE TILL THE START OF PWHT

5

HEATING TO PWHT

6

SOAKING at PWHT

760 +/- 10 at 2MIN / mm

7

COOLING

COOLING TO 350

8

COOLING

COOLING TO ROOM TEMPERATURE


760 +/- 10

100

100

PREVENTIVE MEASURES DURING POWER FAILURE No interruption is allowed during welding & PWHT. Following alternative arrangements should be made available:  Diesel generator for power supply.  Gas burner arrangement for maintaining temperature.


PREVENTIVE MEASURES OF POWER FAILURE DURING PREHEATING


PREVENTIVE MEASURES OF POWER FAILURE DURING GTAW/SM AW 

Gas burner arrangement is used to maintain the temperature at 80 to 100°C up to a length of 50 mm on either side from weld centre line along the complete circumference of the pipe.



Root welding shall be continued after power is restored and preheating temperature is raised to 220°C.



PREVENTIVE MEASURES OF POWER FAILURE DURING PWHT HEATING CYCLE During post weld heat treatment; The following shall be followed: 

During heating cycle : the whole operation to be repeated from the beginning.



During soaking : Heat treat (soak) subsequently for the entire duration. (complete period)



During cooling (above 350°C ) : Reheat to soaking temperature and cool at the required rate.

* Temp should not be allowed to fall below 80 to 100°C. Gas burner arrangement shall be used to maintain the temperature. Siddhartha Sankar Bharali


PREVENTIVE MEASURES OF POWER FAILURE DURING PWH T SOAKI NG CYCLE


Do’s and Don’t’s during P91 welding Do’s

 Cutting

by Band saw/Hack saw/Machining .

 Pipes

Edge Preparation by machining. Machining shall be done without excessive pressure to prevent heating up of pipe.

 Adequate Argon

Gas should be ensured for complete purging of air inside the pipe before starting GTAW root welding.

 Preheating

welding. Siddhartha Sankar Bharali

to 220°C minimum before GTAW root

Contd.  Argon

purging should be continued until the GTAW root welding followed by minimum two filler passes of SMAW, is completed.

 GTAW

wires are kept in absolutely clean condition and free from oil , rust, etc.

 SMAW

electrodes are baked to proper temperature.

 Interpass  Argon

temperature should be less than 350°C.

purging arrangements should be removed after welding.


Contd. Don'ts  No

Tack welding or Bridge piece welding is permitted.

 Argon

purging is continued till the completion of GTAW root welding and two layers of SMAW.

 Oxy-Acetylene

flame cutting is avoided.

 Do

not allow moisture, rain, water, cold wind, cold draft etc. to come in contact with the weld zone or heating zone during the entire cycle from preheat to PWHT.

 Use

unidentified TIG wires or electrodes is not permitted.


Welding of P91

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