PWHT temperature for Carbon steel with Nb and additions

[Johnae]

The PWHT temperature specified for carbon steels ranges from 1100 F to 1200 F per ASME VIII. Some users have reported it necessary to use the 1175 F or higher PWHT temperature when the steel contains intentional additions of microalloying elements, such as Nb and V, in order to achieve HAZ hardness below 248 HV. I am talking about Carbon steel in wet H2S service.

Could any body here give me more information about this? Any papers recommended?

 

[metengr]

The PWHT temperature requirements established for ASME B&PV Code were never intended to temper the weld and base metal heat affected zone to some maximum hardness. This is above and beyond the scope of ASME B&PV Code.

The 1100-1200 deg F band you stated for P-No1 material is not entirely correct. One can PWHT P-No 1 material between a minimum of 1100 deg F and below the lower critical transformation temperature (usually I select an upper limit of 50 deg F below the lower critical transformation temperature). So, run a few coupons of the microalloy steel and optimize your PWHT temperature in the range I described above to achieve the hardness you desire.

 

[Johnae]

The 1100-1200 deg F band that I stated comes from ASME code. If It is not entirely correct what codes or papers I should refer since I cannot do any test?

 

[metengr]

Johnae;
The ASME Code requires only a minimum temperature of 1100 deg F for P-No1 material. The maximum temperature permitted is the lower critical (about 1330 deg F).

What exactly is the material of interest? I might be able to look up the ASME material specification tempering temperature, which would be an excellent guide for optimizing your PWHT.

 

[weldtek]

metengr,
Would it not be a fair statement to say that if a PWHT temp is selected over 1200 F, caution is in order because it's quite possible to reduce tensile strength below that expected from the material spec. I've seen a tensile strength drop of several thousand lbs / sq in after a PWHT of 1250 F on SA 516-70. Just a thought

 

[stanweld]

Many years ago, we specified a 1200F +/- 25 F as PWHT for microalloyed Nb + V steels to lower hardness in the HAZ. Above 1250F todays low carbon content carbon/manganese welding filler metals drastically loose strength. I've recorded tensile strength as low as 62 Ksi for E7018 (typical carbon content of 0.05%).

 

[metengr]

Johnae;

Would it not be a fair statement to say that if a PWHT temp is selected over 1200 F, caution is in order because it's quite possible to reduce tensile strength below that expected from the material spec. I've seen a tensile strength drop of several thousand lbs / sq in after a PWHT of 1250 F on SA 516-70

I am glad you mentioned the drop in stregnth because most do not understand that the allowable stress values are minimum stregnth property requirements not maximum. So, even though you have a drop in base metal stregnth, as long as it exceeds the minimum stregnth requirements in the material specification you are fine. The ASME Code does not give credit or partial credit for higher stregnth at lower PWHT temperature.

Regarding weld metal stregnth and toughness, yes, caution is required as stanweld pointed out. However, one can compensate by using a different filler metal, so I don't see any real obstacles here.

 

[weldtek]

Agreed, I've experienced a situation where 516-70, which started out at 73,000 UTS ended up in the 68,000 range after a PWHT of several hours at 1250F. As to welding materials undergoing PWHT of 1250F, fillers like EM14K will be ok but an EM12K probably won't. Use caution.

 

[Johnae]

Yes ,I am talking about SA516-70. Some users report if PWHT temperature is higher than 1175F, the tensile strength and toughness will decrease a lot. I want to persuade people to use 1175 or higher temperature for PWHT, but I need more information and evidence.

 

[Johnae]

Again, it is in wet H2S service and with microalloy elements Nb and V.

 

[weldtek]

Johnae,
A couple of things you could do;
1. Contact the metallurgists at the mill you plan to use and see if they can make recommendations. Some of the domestic mills provide excellent technical support.
2. Run some sample plates from the same heat of material putting them through various PWHT cycles. This will give you some idea of what you can expect. Once again, I can tell you that in my experience a PWHT cycle of 1250 F for two hours caused a 3-5k drop in the tensile strength and corresponding hardness values.