Suggestions on Re-heat Treating a Q&T CrMo. 2

[tc7]

I have some fairly extensive weld repairs on a heavy 4130 valve bonnet that has been previously heat treated to ~HRC 28-30. The max thickness of this item is approximately 3” thick. I do not know what the hardness values may be at the center.

The nature of the weld repairs will include crack repairs and surface build-up and have purchased a high quality 4130 mig wire. Temper beading is out of the question – I will weld and then re-Q&T. What would the advice be to accomplish this?

I had planned to post weld anneal as per times and temps in AMS 2759/1 ( i.e., soak at 1550 deg F for 3 hours in an (air oven)) then air cooling).

Next, Q&T operation would need to be heated at austenizing temp or above(?) of 1575 deg F for three hours and water quench. Then temper at 1100 deg F for 4 hours.


Questions:
1. Is annealing a good first step or should it be normalized? and why?
2. what is the benefit of water quench over oil quench? Or vice versa.
3. What value does a “post tempering stress relief “ have? I never heard of this before I read the AMS spec.
4. according to AMS 2759/1, tempering temperature for oil quenched is slightly lower than for tempering temps for water quenching – why?

Thanks for any advice.

 

[metengr]

tc7;
I would not recommend a re-heat treatment of this bonnet on your own because if not done correctly you will damage (distort or crack) it beyond repair. If you intend to reheat heat treat the bonnet after weld repairs you must discuss this with a reputable heat treatment company that can handle this "heavy" bonnet. These are all questions that must be answered by the organization that will heat treat the bonnet. I can't think of anything else to ask other than below....

I don't see why elevated preheat and temper bead will not work.

 

[tc7]

temper bead is not in my thought because the most extensive weld will be for the buildup surfacing that I have to do in inside bores and on flange surfaces that will be machined later. The required weld buildup is less than 1/16 which will be machined later to restore all the precision surfaces so we will only be applying a single layer - not sufficient for a TB operation.

Given these contstraints, what is the difference between annealing or normalizing before we re-Q&T? and what value does a post temper stress relief have?

 

[TVP]

tc7,

The microstructure produced by normalizing (fine ferrite + pearlite) is more easily austenitized than one produced by annealing (coarse ferrite + pearlite), because it takes more time to uniformly re-distribute the carbon into the austenite upon transformation. So the answers to your questions are as follows:

1. Normalizing is better.

2. Water quenching allows faster heat transfer and therefore more martensite formation and higher hardness and hardenability. However, this is only suitable for parts with simple geometry and uniform quenching. A previously fabricated valve bonnet will be at severe risk for cracking and distortion if water quenched. In fact, re-austenitizing and oil quenching a previously fabricated valve bonnet will also create a significant risk for cracking and distortion. Unless your organization or your heat treat vendor has experience doing this, I would strongly encourage you to consider an alternative. I doubt that the surface that mates with the body will be sufficiently flat after quenching for proper use.

3. If machining, forming, etc. is performed after tempering, then a stress relief operation may be desirable or necessary.

4. If the part is somewhat harder after water quenching than with oil quenching, then tempering it at a higher temperature will produce the same hardness as the part quenched in oil.

 

[tc7]

Thanks TVP-

Distortions don't particulary bother me since the precision and mating surfaces will be machined after the weld surfacing anyway.

Would there be any value in normalizing the piece BEFORE welding takes place, then perform the Q&T operations?

 

[TVP]

The parts might have a reduced tendency to crack if they were normalized prior to welding, but the most important aspect will be to pre-heat the weld area.

 

[stanweld]

How thick are the weld repairs? We just qualified a similar procedure with preheat of 400F and no PWHT. Weld metal hardenss of 34-36 Rc. HAZ hardenss of 26-28 Rc.

 

[metengr]

temper bead is not in my thought because the most extensive weld will be for the buildup surfacing that I have to do in inside bores and on flange surfaces that will be machined later. The required weld buildup is less than 1/16 which will be machined later to restore all the precision surfaces so we will only be applying a single layer - not sufficient for a TB operation.

Incorrect, tc7. You can use two layers with elevated preheat and machine off the first layer as a sacrificial layer. I would not re-heat treat this bonnet based on the reasons above, it would be a waste of money, effort and time.

 

[redpicker]

You need to check to see if there any standards or specifications that you need to be meeting. Valve bonnets are pressure containing parts and are generally manufactured to strict specification requirements. In most of the specifications I have seen, there are very specific requriements for weld repairs, with specific requirements for the WPS and PQR.

I can tell you that, in general, 4130 valve bonnets are water quenched and tempered in the orignial manufacturing, and they are qualified through tensile and impact testing after heat treatment. If you are going to re-quench and temper the bonnet after weld repair, you will probably have to re-run mechanical tests if you need to qualify to the original specifications. You may be better off by just performing the weld repair and including an 1100F post heat, but you would need to read the applicable specifications to be sure.

rp

 

[tc7]

all-

thanks for your interest.
I am qualifying a procedure and will be using an actual piece for the pqr and will do all the associated mechanicals.

Stanweld-
I find it very interesting that HAZ hardness is so low as compared to the weld metal. I wonder what explains the weld metal hardness especially since you used NO pwht. (no pwht is scary to me with this alloy!).

metengr-
I appreciate your concern and you certainly have me thinking about about the sacrificial tempering layer. I had the idea that the temper bead theory needed a series of overlaying beads to provide adequate tempering. And therein is my first concern about temper beading - since I am working on an alloy already in the Q&T condition, I ultimately have to restore that Q&T condition. Isn't both the welding heat and tempering bead likely to setup hardness variations that are both higher and lower than the intended Q&T condition? I don't see any way around my post weld normalizing then re-Q&T plan to ensure I end up with the final hardness that the specifications require.

 

[stanweld]

The weld metal specified by the Engineer was closer to 4330, also interstitial hardening. If you qualify on 1.5" thick, I expect HAZ hardness to be aroundn Rc 30 - 34 with appropriate preheat and PWHT. I have qualified procedures on 4130 previously with "B2L" and E7018 electrodes without PWHT and had silmilar hardness results in the HAZ. I would recommend temper bead welding (per metengr) for your repair if you do not post weld heat treat.

 

[metengr]

tc7;
I would run a temper bead coupon if you have enough material and follow ASME Section IX requirements using QW-290. What harm is there? Also, by conducting hardness testing of the temper bead PQR you have all the information you need to make a final decision and present your results to a customer.

You may find that temper bead may be your best option (2 pass and machine the 2nd pass as discussed above) or weld repair w/PWHT versus complete re-heat treatment that may result in more weld repairs, if not done correctly.

 

[tc7]

Metengr-

How do you answer the question that welding heat IS going to alter the present Q&T condition around the weld and HAZ and temper beading isn't going to restore that?

If the temper bead hardness traverse shows I'm still in the acceptable range then great! but if it doesn't I'll still have another pqr to do.


I suppose I could cut the one bonnet in half and do both a TB procedure and re-Q&T procedure at the same time. My PQR costs will be double but there may be a payback if I get lucky and we can avoid re-Q&T the production lot. But until you help me understand the above question, it is going to depend on luck.

 

[metengr]

tc7:
The temper bead concept was primarily developed for exactly the type of weld repair you intend to perform - heavy wall thickness base metals and relatively thin weld build-up.

Will the heat from welding locally alter the base metal? Of course it will but it can be managed by elevated preheat, stringer bead sequencing and proper bead overlap and depositing an extra layer of weld metal to temper the pass that will be finish machined. You may still have a very narrow band where the microhardness tests may fall slightly below the desired range. However, this band would be very narrow and in relation to the bulk mechanical properties it would not be significant. I believe it is worth trying before re-heat treatment.

 

[tc7]

Your argument makes a lot of sense. I will go for the temper bead approach and update this thread with results in a future post. Thankyou.