Effect of Niobium on welding and CTOD 5

[metalguesser]

Hi folks!

I have a question regarding the effect of Niobium (or Columbium) on welding. We have a requirement for forging a modified SA182 F22 grade (with 2.0% Cr max) and achieve an Yield stress of 80 ksi (min). The application is a sour, under water service and welding is involved. With this background, the move to enhance Nb/Cb percentage to improve chances of achieving mechanicals has been viewed as detrimental (with a welding perspective).

Our experience with Q & T SA182 F22 is consistent with an YS of 77ksi at best and with a Cr content around 2.25%. What happens if we increased Nb% to 0.06 or 0.08% to help achieve the required mechanical strength, with a welding perspective? (By the way does anybody know; what is the contribution of Nb% to the Jominy/Grossman hardenability equation?)

The modified spec requires V% to be under 0.01%, CE around 0.45%, J Factor value = 150 max. To top it all the material has to meet Crack Tip Open Displacement (CTOD) requirement as well. We have no experience with CTOD. We are a bit overwhelmed.

Any suggestions, guidance will be greatly appreciated.

Thanks in advance,

Cheers

 

[metengr]

Nb/Cb additions to base metals are for grain refinement, resulting in higher strength and improved toughness. I don't see how this would adversely impact weldability, at this point in time. The concern with the J-factor is for susceptibility to temper embrittlement, and again I don't see any correlation with the addition of Nb/Cb.

 

[metalguesser]

Thanks Metengr .

I provided the J-Factor value to give a complete picture of the specification we are working to accompish. My idea of enhancing Nb% was to achieve 80 ksi YS and improve chances on passing CTOD. Customer and their welding sub cotactor feel that it will be detrimental to welding. I have no clue how they arrived at this inference.

Thanks again,

Regards

 

[Greenleader]

Be sure the Nb/Cb is free of oxide. It can be a bugger in welds.

 

[SJones]

You could possibly hit a couple of problems here dependent upon the welding process and heat input:

1) the niobium could push up the HAZ hardness causing you much pain in meeting ISO 15156 requirements

2) Niobium has a tendency to elevate the CTOD transition temperature and there could be a possibility of struggling with the CTOD requirements (you don't quote a temperature or acceptance criterion)

Dolby R E: 'The effect of vanadium and niobium on weld metal properties', Proc of Steel for Linepipe and Pipeline Fittings, The Metals Society, London, October 1981.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[metalguesser]

Thanks a bunch SJones!

The post was really helpful. The acceptance criterion for CTOD is 0.049" (1.25mm) at -10deg. C (14deg F). Hope it does not pose a problem passing the test with Nb at 0.08% max.

Regards

 

[metengr]

SJones
In the reference you provided, I agree that vanadium will have a significant effect on weldability because of the tendency to form nitrides/carbides that resist tempering AND can result in increased reheat cracking susceptibility in comparison to Nb/Cb additions. For moderate to low heat inputs, and with vanadium held at or below 0.01% as per the modified specification posted above, the Cb/Nb additions should still play a significant role in grain refinement for increased toughness. Granted, higher heat inputs from SAW or electroslag welding can result in dissolution of carbides/nitrides forming a coarse-grained region in the weld fusion zone that is low toughness. Do you have specific reference regarding deliberate columbium additions for weld metal or are you referring to base metal micro-alloy additions, as well?

Source; Welding Metallurgy, by Kou

For HSLA steels that contain niobium and vanadium, the weld metal and the HAZ will exhibit low notch toughness at high heat inputs, such as those used in the electroslag and submerged arc welding processes. This is because the high heat input increases the extent of coarse-grained HAZ. The associated low cooling rate in the HAZ reduces the scope for grain refinement and increases the likelihood of the precipitation of niobium and vanadium carbides, leading to embrittlement of the coarse-grained HAZ. The high heat input and the resulting greater dilution also brings the precipitation-hardening problem of niobium and vanadium carbides to the weld metal.

 

[SJones]

Parent material additions which is what I believed the thread originator was referring to.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[metalguesser]

SJones is right in his belief that I was referring to parent metal additions. We are seeking to get the Nb% raised from 0.06 max to 0.08% max and Cr% from 1.65% max to 2.25% max by our customer. V% has been reduced to 0.008% max. Our main objective is to get thro' CTOD tests with a reasonable success and at the same time achieve 80 ksi YS (for now) and hopefully get past any possible weldability tests (carried out by the welding contractor) in the next stage.

Sincerely thank Sjones, Metengr and Greenleader for sharing their thoughts on this bearbug issue.

Regards

 

[stanweld]

The effect of Nb to lower the CTOD transformation temperature is achieved with a normalizing heat treatment. Tempering afterward causes precipitation of Niobium carbides and carbonitrides which tend to raise the CTOD transition temp.; similarly so for the HAZ when subject to PWHT.

 

[metalguesser]

Hi All!

Can somebody tell me what is the contribution of Niobium/Columbium to the Jominy hardenability equation. For instance the Carbon contribution is;
C=2*(SQRT(0.024*C%)).

I haven't found the equation for Nb/Cb. Any help??

Thanks in advance,

Cheers

 

[TVP]

Niobium does not contribute to hardenability, so there are no coefficients in the Jominy equation. It provides secondary hardening during tempering (precipitation hardening), as well as refining the original austenitic grain size and lowering the austenite to ferrite transition temperature (Ar3).