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 welding A182 Grade F91 forgings. 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 420F with MAX Interpass Temp is 482F. We are allowing the weld to cool to just under 200F and then I am heating (wrapping with ceramic blanket and insulation) rapidly to 600F and then 175/Hr from there to 1410F. We then hold at 1410F for 4 hours and then control cool to 600F 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 are methods are sound and we should be getting the results that we need. I am at a loss and would appreciate any direction that can be offered.
In general you will play hell trying to consistently achieve <238 for welds of Grade 91/B9. There is nothing I see that is wrong with your fabrication regime. You have done your homework. You are just asking of the material a requirement it will not give you on a consistent basis. Extended PWHT times may help but is still not a cure all since secondary precipitations kick in that can actually increase hardness. Even if you find a balance for a particular heat once you change the chemistry the rules can change just enough for you to fall outside your requirements again. in fact, at 4 hours they are already kicking in. Most people that deal with Grade 91 on a regular basis impose hardness maximums around 280. So this should give you an idea of what you are facing. But let me ask, I do not have a copy of API 602 but it appears to be a product standard. To me that means that it is not applicable to fabrication utilizing the product. For example, when I fabricate with SA-333 Gr 6 pipe I do not have to comply with the toughness requirements at -50degF. I think you'll find most Grade 91 valves fall in the 190 to 220 range. But welds just won't get there on a regular basis.
API 602 only applies to the manufacture of the valve assembly, and does not apply to welds made for fabrication or installation. A base metal hardness of 190-250 HB and weld metal/HAZ hardness of 190-280 HB is reasonable for P91/F91, as Jeff mentioned. Consult the Vallourec-Mannesman website for "The P91 Book" for additional info. Your product engineer is holding you to an unreasonable standard for the weld deposit.
Also, how are you converting HV to HB for P91? 238 HV is around 228 HB on Grade 91.
Another good .pdf article with a metalurgy primer for P91 steel, but disregard the advertisement between the article contents since it's from an industry magazine:
API 602 is the specification for valves. The hardness criteria for this spec should not apply to your situation. What's the design code for the Piping being fabricated?.
I have summarized below some of the requirements on PWHT and Acceptable hardness values on weld and HAZ for Grade F91 (P5B-Gr-2) steel.
-ASME Sec-I Code- Power Boilers- Recomemnded PWHT temperature range 1350-1425 F- No restriction on Weld and HAZ hardness after PWHT. However the specific PWHT specified are :-If Ni + Mn < 1.50% but ≥ 1.0%, the maximum PWHT temperature is 1,450°F (790°C).If Ni + Mn < 1.0%, the maximum PWHT temperature is 1,470°F (800°C).
-ASME B-31.1 Code- Power Piping-Recomemnded PWHT temperature range 1300-1400 F- No restriction on Weld and HAZ hardness after PWHT.
-ASME B-31.3 Code- Process Piping-Recomemnded PWHT temperature range 1300-1400 F-Max Weld and HAZ hardness after PWHT, 241 HB(255 HV)(23HRC).
Max Allowable Hardness for Basemetals:-
A-335 P91 Piping-190-250 HBW(196-265HV or 91HRB to 25 HRC). A-182 F91 Forging - 248 BHN (260HV or 24 HRC)
As a good engineering practice, if the hardness and other design requirement/s after welding and PWHT are known, procurement of basemetals and welding consumable becomes lot easier. In the same token WPS may be qualified with choosing PWHT temperatures which would give the desired hardness values.
Generally ASME Codes and relevant industry specifications e.g NACE ,API are very clear about hardness requirements. I have found that API 938B-01-Use of 9Cr-1Mo-V (Grade 91) Steel in the Oil Refining Industry is a very well written spec on various fabrication aspects of Grade-91 steel.
The approaches taken from your end for welding of this alloy is quite sensible. Please ensure that final PWHT tempereature does not exceed the individual tempering temperature of the componenets (piping or forging). This may cause operational issues (long term creep ductility issues.
pgoswami, thanks for the great info you posted on this issue, but I disagree with one aspect of what you posted. ASME is not clear about hardness values for P91 material. Those of us that are in the power industry know all to well that other then B31.3 and ASME Sec II Part A giving max. values, ASME is silent about hardness values for P91 material. I have heard that the ASME Sec II code committee is discussing putting a table in ASME Sec II Part A for min. and max. hardness values for chrome material but has not to date. It has been generally accepted that 190 HB min. and 248 HB max are acceptable ranges. Min. ultamate tensile for P91 is 85000. That eqautes to about 174 HB.
Just to add to your reply Marty, Here's a .pdf from Sperko Engineering - our friend Walter which explains what you're referring to...
Forum
