PWHT Precautions to Avoid Hydrogen Cracking 2

[tc7]

The AWS Welding Handbook, Volume 1, page 107 mentions that "Steels most susceptible to delayed cracking are those that transform to martensite during the cooling portion of the weld thermal cycle". What steels will form martensite? I thought that all mild steels and alloys did.

This question comes about when we needed to deal with a long time lag in between final welds and post weld stress relief. When should we be keeping the weldments warm to, say, preheat temp until such time as the ovens are available for PWHT?

And should these steels which form martensite undego a hydrogen bakeout instead of a simple 1-hour per inch stress relief?

I should add that we take all the typical low hydrogen welding precautions that we know of (lo-hy rod, proper preheat, top quality rod, high purity gases when TIGing, etc.)

Thankyou

 

[metengr]

tc7;
No, not all steels form martensite upon cooling from welding or thermal treatment. The main factor that drives martensite formation is chemical composition, in addition to cooling rate (quenching). In welding, the carbon equivalent (CE) is the controlling factor, which obviously relates to the alloy content of the steel.

Low alloy steels can form martensite if the preheat is inadequate and the CE is above 0.45. So, increased preheat will help to retard cooling rate and to avoid the formation of martensite in the base metal heat affected zone. Low hydrogen electrodes should be a given for welding any steel that has a CE above 0.45.

When should we be keeping the weldments warm to, say, preheat temp until such time as the ovens are available for PWHT?

No need for maintaining preheat after welding or a post weld bake after welding, if you used low hydrogen electrodes, prior to PWHT.

And should these steels which form martensite undego a hydrogen bakeout instead of a simple 1-hour per inch stress relief?

I would only use a post weld bake out for the high alloy martensitic steels, like (5Cr or Grade 91) before delayed PWHT, again if you did not use low hydrogen electrodes or the completed weldment will be exposed to moisture before PWHT. You should always perform a PWHT if required by Code unless alternative welding methods are used (temper bead). In this case, low hydrogen electrodes are a must OR you must perform a post weld bake.

 

[strider6]

tc7

To know if a certain materials, alloy will form martensite on cooling, you have to look at composition and transformation diagrams (TTT, CCT) for that specific materials.


this articles i've found can give you some help:

http://www.key-to-steel.com/Articles/Art51.htm

http://www.key-to-steel.com/Articles/Art17.htm

hope this help

Strider

http://www.corrosionist.com