Hardness Test Requirement for P91 1

[carlomaderazo]

Can somebody tell me where can I find in ASME I the minimum and maximum hardness test requirement of P91 material? As our client require this testing though it is not in the contract..
Thanks..

 

[metengr]

You will find the maximum hardness of 250 BHN is stated in ASME SA 335, for Grade 91, seamless, wrought pipe as supplied from the mill.

For butt welded pipe, the hardness specification is not listed in ASME B&PV Code. However, for hardness testing of girth welds, it is recommended to not fall below a hardness of 200 BHN for the base metal weld HAZ, and weld metal.

One suggestion, hardness testing for Grade 91 material requires a qualified procedure and experienced personnel. I have seen many portable hardness test results incorrectly reported because of a lack of procedure. You need to carefully prepare the surface for hardness testing to remove surface effects like scale and possible decarburization. I would recommend you hire a competent metallurgical lab that can perform field hardness testing using a qualified procedure.

 

[stanweld]

Agree with metengr regarding minimum values and the need for qualified procedures and personnel; unfortunately, we see hardness in as received SA-335 P91 pipe lower than 200 BHN but so far never less than 190 BHN. As a result we require hardness after final PWHT to be not less than 190 BHN. Through extensive testing and microscopic examination, we have found that hardnesses below 190 BHN may or may not have the desired microstructure. The requisite microstructure was never seen with hardness below 174 BHN.

Also note that hardness values above 200 BHN may not be indicative of a correct microstructure.

 

[carlomaderazo]

Thanks for your reply.But where in ASME-I can I found this requirement? In which section or paragraph this topic written? I need to show a proof to our management if this is mandatory.

 

[stanweld]

ASME I does not provide requirements for minimum hardness on P91 alloy materials either directly or indirectly through the SA-335, SA-234, SA-182, etc. materials specifications.

Minimum requirements are Engineering decisions.

 

[carlomaderazo]

Stanweld, so it means i can ignore the requirement of the client as the hardness test is not include in the contract? How about ASME B31.1, is there any minimum requirement?

 

[davefitz]

There is an ASTM spec that mandates min UTS, and there is a correlation between UTS and hardness,; according to a recent paper at the EPRI Florida conference, the hardness must be above 185 BHN ( neccesary but not sufficient) to have a UTS above code minimum.

However, a hardness above 185 BHN does not guarantee adequate UTS, and also does not guarantee code assumed high temperature creep strength. Also, the error in measuring hardness implies some margin should be added to the 185.

If the client provided the hardness test as part of the orignal equipment spec and you did not boject , then it is a contract mandate, regardless of code . Also, the ASTM specs for P91 specifically suggest that add'l QC be added above and beyond the ASTM min mandatory requirements.

 

[stanweld]

carlomaderazo:
ASME B31.1 does not require a minimum hardness; however,as davefitz so rightly states, you cannot ignore your Client's requirements. If the Client did not require hardness testing minimums at bid, you need to provide the additional costs for compliance to the proposed Contract Change.

I fear that you do not fully comprehend the problems associated with P91 alloy pipe. The material's high temperature properties are dependent on correct thermal processing to produce the appropriate microstructure. Hardness testing is meerly one method to provide an indication that the appropriate microstructure has been obtained.

If you have hardness values that indicate improper manufacture, that material should be thermally reprocessed or subject to further examination. Premature failure that results in injury or loss of life could prove quite costly to your company should you ignore the evidence, especially with regard to due dilegence and the mounting information available from EPRI and other organizations.