Minimun Hardness T-91

[t1t2t3]

My problem is opposite to the raised one for bob74r. For specification of the client the minimum hardness required after PWHT is 200HBN, nevertheless the values are around 170HBN for welds of T-91 (P5B, now P15 E). In no specification I find (including ASTM) minimun values of hardness after PWHT for this material.
Some suggestion? Thank you

 

[stanweld]

There are no materials specs specifying min hardness. Your client's specification trumps others. The hardness obtained must be furthewr scutenized. Is it correct? How was hardness measured? If verified that the 170 BHN is correct, there is near certainty that the P/T91 material has been improperly heat treated and will not meet long term service. In the thouisands of hardness tests that I have performed, hardness of 170 and below revealed microstructures predominantly of ferrite with no evidence of tempered martensite.

 

[Mr168]

We take second BHN values on anything below 190 to verify accuracy, and will take a second look at anything below that. 200 BHN is a perfectly acceptable client specification, and is the number I see more and more owners leaning towards.

I would review your postweld heat treat charts first to verify correct PWHT, and pay particular attention to your ramp rates.

I will say though, that every single P/T91 weld on our current projects that were in the 170/180 BHN range for initial hardness testing came back over 200 BHN after additional surface preparation and a second reading from the telebrineller. Inadequate cleaning of the scale and removal of the decarburized surface can lead to consistently low Brinell values.

 

[metengr]

With proper surface preparation, portable hardness testing qualified by procedure for Grade 91 material and based on my experience with Grade 91 material, I would use 190 BHN as the minimum for acceptance. Anything softer (lower hardness) implies improper thermal treatment which really means the creep rupture strength can be much lower than you realize.

 

[Guest102023]

Metengr, I would like to know your preferred hardware for portable hardness testing.

 

[stanweld]

FYI, the proposed 2010 ASTM A-335 specifies a hardness range for P91 of 190 to 250 BHN/196 to 265 HV.

We have found the MIC 20 hardness tester (in proper working order) most effective for tube welds. As metengr states, proper surface preparation is imperative.

 

[Mr168]

Interestingly enough, we have found that all of our subcontractors using the MIC equipment have had considerable issues with repeatability. Consequently, they have gone to the clamp style pin brineller for smallbore pipe/tubes.

 

[stanweld]

That would be another excellent method.