Can Stress Relief of AISI 4130 Cause Temper Embrittlement 1

[tc7]

I am reviewing tensile test results which were part of an old PQR performed on thin wall (.157" wall thk) Cr-Mo pipe GTAW butt welds. The tensiles broke in the base metal at 92ksi and the pipe spec (AMS-T-6736) requires a tensile strength of 95ksi. However ASME Sect IX QW-153.1(d) allows that a break in base material may occur at 5% less than the specified tensile strength. So the results support that the procedure is legitimately qualified.

My question is why did these breaks occur at less then the base material strength? PWHT SR was performed at 1200 deg F, I don't know for how long. Could PWHT SR over temper this alloy or could it cause temper embrittlement?

Thankyou.

 

[TVP]

The answer is no to temper embrittlement. Overtempering may occur at such a high temperature. That kind of stress relief is usually used prior to subsequent quench and temper heat treatment, not to produce a specified tensile strength. Do you know what filler metal was used? FWIW, there is a lot of info related to post-weld heat treatment of 4130, so Google search Eng-Tips as a starting point.

 

[metengr]

I absolutely agree with TVP regarding the potential for temper embrittlement. However, there is not enough information provided regarding other possibilities to explain the measurable drop in UTS from the original specification.
For example, the 1200 deg F PWHT is not that excessive, and in most cases would be required for a P-No 4 type material after welding. I have said this on numerous occasions that applications for ASME B&PV Code rely on establishing minimum ductility and minimum strength to be met for service by using bend and tensile testing. In reference to ASME Sect IX QW-153.1(d) this is intentional and is for the case where the original material could have been tempered at a lower temperature, but still meeting the minimum tempering temperature requirement by specification OR as with most materials there is a scatter band in mechanical properties. Thus, during PWHT it is very possible the base metal UTS can be slightly altered from original heat treatment, but as long as it meets the necessary requirement based on what was mentioned above, it is acceptable.

One last comment, temper embrittlement will normally manifest itself by showing a slight increase in tensile strength but significant drop in toughness within a certain temperature range of which you were well above for this alloy.

 

[tc7]

Met, TVP-

I failed to make clear in my original post that the pipe was in normalized condition prior to weld and it was not Q&Ted afterwords, only SR'ed.

Incidently, since TVP asked, the welding rod used was ER90S-B3

Met-
You have gone to the very heart of my concern when you say, "...during PWHT it is very possible the base metal UTS can be slightly altered from original heat treatment..."

I am trying to understand what this stress relief operation may have done to "alter" the UTS.


Thanks.

 

[metengr]

tc7;
Your PWHT at 1200 deg F provided tempering to the normalized condition, which resulted in the slight drop in UTS.

 

[tc7]

Met-
Knowing that the material is normalized I would think that all of the mill forming stresses have been erased. And we should not expect to see the existence of any martensite - correct? so I wonder what are we tempering that drops the UTS?

 

[metengr]

tc7;
PWHT does two things for weld metal and for the base metal heat affected zone - reduces hardness and residual stresses from welding. For the base material, a normalized heat treatment from the mill means air cool, and this strength level can be altered with exposure to elevated temperature, as in the case of PWHT or stres relief. Either thermal treatment must be carefully considered and the effect on bulk properties.

 

[metengr]

tc7;
Forgot to add that the microstructure from just plain normalizing will be predominantly ferrite, fine pearlite and possibly regions of bainite for this alloy (depending on section thickness). Exposure to these phase constituents at 1200 deg F will modify the carbide size/spacing from original resulting in the same effects as a normalized and temper heat treatment.

 

[tc7]

One last question Met-

Curious about this mention, "...1200 deg F will modify the carbide size/spacing..."

What is happening to modifyy spacing?
is carbon going back into solution?
or is grain growth occurring?

 

[metengr]

tc7;
In a nutshell, what is happening is that the fine layers of carbides within the ferrite phase are coalescing upon exposure to 1200 deg F resulting in larger carbides and altering the spacing between carbides (on a microscopic scale). It is the size and distribution of these carbides that effects strength. So, if you alter the carbide size and distribution as with tempering bainite or martensite or fine pearlite (as in this case), you will in effect lower the strength. There is a whole lot more to this subject and I tried to provide you only a very high level view (looking from the moon to the earth) .