Hardness of P91 weld problem

Date 04-24-2005 23:57

Your question is a little vague, but if I understand you correctly, your P91 weld is harder than expected, and you are trying to figure out why? If that is the case, one place to look is the composition of your filler metal vs the PWHT temperature. If the manufacturer adds too much Ni, or Mn in an effort to improve toughness, the lower critical temperature can fall to within the PWHT range. If this is the case, you need to find a filler metal with a higher critical temperature, or PWHT at a lower temperature.

By snakejhj (*)

Date 04-25-2005 02:40

Dear GRoberts
I am sorry for the not clear question.Because my English is bad.I am chinese.
your understand is right.the welding process is:preheat tem. :220-300 degree clesius,intermiddle treatment tem. 300degree clesius and holding 1 hour.PWHT ten. :720-760degree clesius, holding time is 1 hour per inch. When this finshed, the hardness is less than 275HB, and now, after a few week, some hardness over than 300HB. I want to know WHY?
another:we used GTAW ER90s-B9 (9CrMoV-N) for root pass. FCAW E91T1-B9 (supercore F91) for filler and cap. the welding material factory is METRODE

By MBSims

Date 04-26-2005 01:50

I agree with Groberts. You will need to estimate the lower critical transformation temperature for the filler metal. I know there are empirical equations for this, but don't have acccess to them at the moment.

I am surprised that the hardness climbed after aging for 1 week. I did not think that P91 was age hardenable. Was the weld allowed to cool to ambient temperature after welding? P91 is a martensitic alloy and relies on the PWHT to temper the martensite. If the weld is not allowed to cool to the martensite finish temperature prior to PWHT, then martensite will continue to be formed upon cooling after PWHT. The martensite formed after PWHT will not be tempered and will produce high hardness. The target hardness should be 235 HB after welding and PWHT are complete.

Additional info:

http://www.btwusa.com/html/tips_page_1.html#HRP91

By GRoberts (***)

Date 04-26-2005 23:39

I don't think there is any way that P91 will get harder after PWHT. (assuming that the first time you checked hardness, the weld was down to room temperature.) I would look at your hardness testing practices. Were you in a decarburized zone the first time, how do the locations compare with eachother? What type of hardness testing equipment are you using, etc...