Welding of P91

A Presentation on Welding procedure of SA 335 P91 Material

Presented by

Siddhartha Sankar Bharali Assistant Engineer

Emp Code : M6B194

1Siddhartha Sankar Bharali M6B194

WELDING BASE MATERIALS


HIGH TEMPERATURE STEAM PIPING MATERIAL BEFORE P91Material used for high

temperature piping before the use of P91 was X20.

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Composition of P91 Composition of X20

9 Cr 1MoV 12 Cr 1Mo 1/4V

Siddhartha Sankar Bharali M6B194

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).


INTRODUCTION OF A NEW MATERIAL:P91U.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 high-temperature 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 FILLER USED ELECTRODE

P 5A GR 2 9 Cr 1 MoV 90 S- B9 E9018- B9


SA 335 P91 welding steps


EDGE PREPARATION & FIT-UP

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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 .


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


ELECTRODE NOMENCLATUREPrefix ‘E’ : Arc Welding ElectrodeFirst two or three digits : Strenght in ksi For example : E-90xx is 90ksiSecond last digit: Welding position For example : E-xx1x = All positions : E-xx2x = Horizontal and Flat : E-xx3x = FlatLast 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


PURGINGReason 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 8- 14 litres/min.


PREHEATINGPrior 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)

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P No. ofMaterial

Thickness(mm)

Butt Weld

Preheat °C PWHT °C

P1 Gr 1 & SA106Gr C

≤ 19 NIL NIL

> 19≤ 25 NIL 595- 625

> 25 ≤ 75 150 595- 625

> 75 150 595- 625

P1 Gr 2 &SA 106Gr C

≤ 19 Nil Nil

> 9≤ 19 Nil 620-650

> 19 150 620-650

P4 Gr 1 ≤ 75 125 640- 670

> 75 150 640- 670

P5 A 150 680- 750


Contd.


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 PWHTPWHT temperature for P91 : (760±10)0 C Soaking time : 2.5 minutes/ mm of weld thickness

subjected to minimum of two hours.


HEATING & COOLING RATES


GRAPHICAL REPRESENTATION OF SA 335 P91 WELDING PROCRESS


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


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.

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PREVENTIVE MEASURES OF POWER FAILURE DURING GTAW/SMAW


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.

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PREVENTIVE MEASURES OF POWER FAILURE DURING PWHT HEATING CYCLE


PREVENTIVE MEASURES OF POWER FAILURE DURING PWHT SOAKING CYCLE


Do’s and Don’t’s during P91 welding

Do’sCutting 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 to 220°C minimum before GTAW root welding.


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 temperature should be less than 350°C.Argon purging arrangements should be removed after

welding.


Contd.Don'tsNo 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.


THANK YOU


Welding of P91

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