Hardness after PWHT dropped

[LahaneD]

One of my project pipe spool required PWHT & production hardness test.The material for Pipe spool was A105/A106 GrB (sch STD)& NACE MR0175 was specified on spool drawing.
The base metal hardness for A105 was 155-160BHN & for pipe A106 GrB was 135-138 BHN before fabrication.
After welding & PWHT(stress relieving at 620 deg C for 2hrs)the hardness for A 105 was 111-120 BHN & that of for A 106 GrB was 102-108 BHN.The weld hardness was between 175-180 BHN.
Now the customer is not accepting the material by saying that UTS for A106 GrB may have dropped below to its min requirement.

What is the minimum acceptable base metal hardness for above two material after PWHT?

how to handle this situation. I am confused????
A quick reply is appreciated.

 

[metengr]

LahaneD;
Where was the location for the post weld hardness testing?
IF it was located next to the weld region, you will see local softening of the base material because of the heat from welding. This is a typical response for this material. IF the hardness testing was performed remote from the weld region, something else happened to cause softening. Another factor could be surface preparation that could cause lower readings.

Normally, PWHT is performed at a temperature that should not affect bulk properties.

What I would do is to have a weld coupon of the same material used in production and exposed to PWHT and have tensile specimens removed. The results of the tensile tests should decide the fate of this material, and not necessarily hardness testing.

 

[LahaneD]

The PWHT was done for weld joints only (localized).The hardness reading was taken very close to the weld on base metal.

 

[kclim]

How were the hardness measurements taken? With an actual Brinell indenter or were they converted? Sometimes the conversion error can be significant. In particular, the specific case of a rebound tester on thin sections tends to lead to lower than expected hardness values.

It's a longshot, but thought I'd throw it out there.

Tend to agree with meteng, in that you should fall back on tensiles because thats what the mechanical requirements in the standards refer to.

 

[stanweld]

I've seen some very low carbon equivalents for the involved materials and I would not doubt the potential for lowered hardness after 2 hrs PWHT, especially in the HAZ and the materials were thin wall.

 

[LahaneD]

The hardness reading were taken with portable hardness tester (rebound type).The value are not converted.
But is my stand valid that i am not violating the code requirement as code only gives max hardness limit & not minimum.

 

[stanweld]

The rebound hardness values are highly suspect. They are more than likely to lead to a low BHN value in the conversion process. I would not trust their validity at all.

 

[TomEun]

Unless you proved the actual mechanical properties from the heated material, the two materials after PWHT may not be acceptable because the UTS may be dropt below the SMTS (specified minimum tensile strength, 70 ksi for A1-5 and 60 ksi for A106-B). ASTM A105 (2009), 7.3.4 indicates the hardness of all forgings so tested shall be 137 to 187 HB inclusive while A106-B does not have the hardness limitation. However normally minimum 120-125 BHN is required for the strength, UTS = 60ksi.and minimum 135-145 BHN is required for the strength, UTS = 70ksi. Please find API 579 (2007), Table F.1.
I am sure all pressure components had strength-calculated with the allowable stresses based on the SMTS before the fabrication.

Thomas Eun

 

[metengr]

Unless a specific hardness range is provided within a material specification, I would not use hardness to approximate tensile strength. There are procedure techniques associated with hardness testing that can effect results, period. Hardness testing should be used as a quality check regarding post fabrication heat treatment effectiveness. The only sure thing are tensile test results to evaluate mechanical properties, as I mentioned above.

The reason most material specifications for ASME Code applications do not have a minimum hardness value is that the concern is for lack of ductility post fabrication. This is why a maximum hardness would be specified in most cases.

 

[LahaneD]

so metengr, I am not violating the code requirements here. so i will not accept the rejection in above case unless it is proved by customer that i have violated the code requirement, right?

 

[metengr]

LahaneD;
I reviewed SA 105. The only mention of hardness testing in this specification occurs when a subsize tensile specimen cannot be obtained from a forging that is too small to verify mechanical properties or as in 9.5 a quality check for multiple forgings.

With that said, the maximum hardness listed by specification in Table 3 of SA 105 is 187 BHN. For SA 106, there is no hardness testing.

The way I see it, you have not in any way violated code requirements as a result of fabrication based on information you provided. If the client has a concern with hardness testing because NACE MR0175, you are only dealing with maximum hardness to ensure adequate tempering after welding/fabrication. You will need to redo the hardness testing following ASTM A 370.
If the client still believes the mechanical properties have been compromised because of fabrication, take some of this material and simulate production welding and have a lab perform tensile tests as though you were qualifying a weld procedure. If the post weld mechanical test results meet Table 3 of SA 105 and SA 106, the mechanical properties have not been compromised. The hardness testing may also need to be redone to verify compliance with NACE MR0175.

 

[LahaneD]

metengr,
The design code here is ASME B31.3.where the minimum allowable stress at 100F is 20 ksi for A106 GrB.

Does it help in any way to support my lower hardness in above case.
have I violated ASME B 31.3 code requirement in above case?

 

[metengr]

LahaneD;
In ASME B31.3, after PWHT, there is no hardness requirement. So, no you have not violated code requirements.

 

[LahaneD]

metengr
I have done simulation on same heat pipe & results are good after PWHT.The tensile sample did pass the test.

Thanks for your help

 

[metengr]

Excellent. Sometimes a little extra work pays off big.