Recommended Hardness for A-335 P91 Material 1

[spayette]

For the weld and HAZ of ASTM A-335 P91 material, ASME B31.3 recommends a maximum hardness of 241 BHN. However, NACE MR0103 appears to allow a maximum of 248 BHN for the base metal, weld, and HAZ.

For seamless fired heater tubes in hydrocarbon service, which contain circumferential welds, does anyone have a recommendation as to which hardness level would be the most appropriate for the weld and HAZ areas.

Design and fabrication are per B31.3 and owner is proposing that para. 331.2.2 be invoked allowing use of alternate hardness requirements.

 

[metengr]

spayette;
Between 190-245 BHN. I would be more concerned about having lower than 190 BHN for the weld and the base metal HAZ.

 

[stanweld]

Why are yoou using P91 when P9 is the more often used material for this service?

 

[spayette]

metengr
Thanks very much for your reply. Based on past experience it doesn't appear that hardnesses below 190 will occur. However, I reviewed previous NDE reports and noted that approximately 50% of the post PWHT readings for the weld metal exceeded 250 BHN. The highest was 270 BHN. Base metal and HAZ hardnesses were lower.

The heater is composed of 4.5" OD tubes opperating at a metal temperature of about 1050 to 1100 F. Design metal temperature is 1200 F.

Do you see an issue if hardness exceeds 248 BHN?

 

[spayette]

stanweld,

Thanks for your response. P9 material would not provide the required elastic and rupture strengths at the required design conditions and thickness. In essence, it was not a cost effective choice.

 

[metengr]

spayette;

However, I reviewed previous NDE reports and noted that approximately 50% of the post PWHT readings for the weld metal exceeded 250 BHN. The highest was 270 BHN.

This does not surprise me because carbon content will have a direct influence on hardness of the weld metal deposit, before and after PWHT. I would not have any problems with 270 BHN for weld metal. In fact, one of the reputable filler metal suppliers that formulates E9015- B9 specifies in there literature that the expected hardness range is 200-275 BHN.

Keep in mind that hardness testing is only a sorting method and does not provide the total heat treatment condition of the P91 weld and base metal heat affected zone (HAZ). If necessary, surface replication should be used to evaluate the microstructure of the weld and HAZ to corroborate hardness test results.

 

[stanweld]

Hardness over 248 BHN could be a problem based on your service environment (hydrocarbon constituents), although duubtful based on service temperature (creep range). What is your PWHT heating cycle and what were the actual ranges which provided the high hardness? I have never had a problem with weld metal hardnes exceeding 248 BHN when heating to 1400 F for 2 hrs minimum with controlled heating and cooling rates per ASME B31.1. I have seen many cases of higher hardnes with shorter hold times or lower temperatures (actual < 1375 F) or a combination of both.

 

[spayette]

Stanweld,
Thanks for your input. PWHT temperature was 1400F +-25F with minimum 2 hour hold time. Ramp up was 50F per hour above 300F. Ramp down was 300F per inch per hour from PWHT temperature to 300F.

 

[stanweld]

spayette,
I should have added a caveat to my "never" with the statement of "provided a minimum preheat of 400F was maintained by electric resistance heating coils and the weld was not fully made with the GTAW process." I have seen many high hardness welds in smaller bore pipe welded fully out with GTAW and using torch preheating. By the time of arc initiation the actual preheat temperature was closer to 250 F than 400 F. As welded hardness was nearly 450 BHN and it took two PWHT cycles or 3 to 4 hours hold to lower the hardness to below 250 BHN.

 

[xgiorg]

Somebody knows the hardness require for a sa-335 P9? I have to verify the hardness of this material previous and post PWHT

 

[athomas236]

thread330-217600

may be helpful, if you have not seen it already.

 

[stanweld]

Per B31.3, the maximumum hardness for welds in P9 is 241 BHN. There is no hardness requirement in SA-335 for Grade P9. P9 is furnished in either the fully annealed or normalized and tempered heat treated condition.

 

[metengr]

Grade 9 material has been around for 50 years and does not develop the necessary precipitates that result in resistance to softening during tempering and creep strength, as Grade 91 material (enhanced creep strength ferritic steels). In fact, Grade 9 material was developed for oxidation resistance, not necessarily creep strength. It is a work horse material and other than leaving it in the as-quenched condition, there is not much one can do to adversely affect properties.

So, this is the reason for not having to specify an acceptable hardness rangein the material specification.

 

[xgiorg]

Thanks for the information, stanweld and metengr your info is helpfull for me. I saw in the p.64 of B31.3 the maximum hardness for heat treatment.