Welding A335-P91 Pipe, A182-F91 Forgings 1

[jgbf]

I am working on a ASME Section IX procedure qualification using A335 Grade P91 pipe. I am planning to use this Procedure Qualification (PQR) as the basis for a Welding Procedure Specification (WPS) for a seal welding A182 Grade F91 valve body forging to valve bonnet forging. However, I am having difficulty in obtaining the final hardness that we are looking for. Our product engineer tells me that we need to be Max hardness of 238HB to comply with API 602 and the Max material hardness according to ASTM A182 Grade F91. My final hardness by Vicker’s method after PWHT is right around 238 with weld and HAZ usually 232HB to 243HB.

We are using a GTAW for all passes. Our filler metal wire meets the proper XFactor and the composition of Mn+Ni is 1.29%. Preheat is around 420?F with MAX Interpass Temp is 482?F. We are allowing the weld to cool to just under 200?F and then I am heating (wrapping with ceramic blanket and insulation) rapidly to 600?F and then 175?/Hr from there to 1410?F. We then hold at 1410?F for 4 hours and then control cool to 600?F and then open insulation and let cool in air until room temperature.

I have tried a number of resources from so local metallurgists to some field welding and heat treating professionals and they all believe that our methods are sound and we should be getting the results that we need. I am at a loss and would appreciate any direction anyone can offer.

 

[metengr]

The maximum hardness for Grade 91 under the ASTM A 182 specification is 248 BHN. I don't have the API specification for valves but what is the method cited for hardness testing (for example in ASTM A 182, hardness testing shall be performed in accordance with ASTM A 370)? For forgings, any location is permitted in accordance with 7.6 of A 182.

 

[jgbf]

The API requirment brings the MAX hardness to 238HB. Could my difficulty be that I am trying to use A335 Grade P91 pipe? Do you think that I should get some A182 F91 bar and machine a coupon from that for qulaification?

The testing lab is using a Vicker's hardness test, test method ASTM E92. When I read A335 it says that hardness testing can be done by any of the three methods. And you are right that under A182 hardness should be per A370.

I'm still scratching my head. Do you think that trying to qualify by welding on forged bar would get me the results that I am looking for?

 

[metengr]

jgbf;
After final review, I would slightly increase your PWHT temperature from 1410 to 1425 deg F based on your Ni+Mn content being less than 1.5 mass % content. With all things being equal, the critical factor controlling final hardness is really tempering or PWHT temperature. You do have some wiggle room regarding PWHT temperature - the max by Code is 1450 deg F for Ni+Mn less than or equal to 1.5

 

[jgbf]

Thanks. I will try that and post the results.

 

[ulyssess]

jgbf
How did you the conversion from Vickers to Brinell? ASTM A370? ISO 18265?
Where exactly Vickers hardness was carried out?
If you compare Brinell hardness and Vickers hardness, both measured in the coarse-grain part of the HAZ, you will not obtain comparable values. With the large Brinell indentation of several millimeters, you will not measure the max. hardness of the coarse-grain-zone (like HV), but an average over different structures incl. weld metal. The usual conversion tables between Vickers and Brinell hardness cannot be used for inhomogenous structures.

 

[stanweld]

jgbf,
Your dilemma illustrates one of the pitfalls of using P91 in petrochemical service where maximum material and weld hardness is limited in lieu of the more traditional P9 alloy steel. Sometimes the benefits of P91's higher design allowables may be outweighed by its peculiar manufacturing and weldability issues relative to those of P9.

P91 alloy steel was created primarily for use in high temperature steam service.