Post Weld Heat Treatment of 4130 1

[TexasRotorHead]

I currently have a welded structure made of various wall thickness 4130 square tubing. The tubing is all 1x1 inch with wall thicknesses ranging from .035 to .125 inches.

The tubes have varying lengths between 4 to 18 inches and are interconnected.

I do not know exactly how the welds were made, but I do know that the welder is very competent (Im sorry I cant give more information beyond that).

I have recently become concerned with potential issues with heat treatments for strength. We are planning on getting the structure heat treated by quenching and tempering to a temperature to achieve approximately 1000 MPa yield.

My main concern is that there could be problems during the quench process that could potentially damage the welds.

Do you guys have any advise to give as to the best course of action to take from this point?

I appreciate your input.

 

[desertfox]

Hi TexasRotorHead

Have a look at this site:-

http://www.netwelding.com/Heat_Treated_4130.htm

it may help

desertfox

 

[metengr]

Are you heat treating to achieve original strength or for post weld heat treatment? If you're post weld heat treating because of welding, I would not recommend it because of concerns with distortion. The 4130 has decent weldability and with your stated thicknesses, I don't see a problem with leaving the structure as welded.

 

[swall]

You might consider heat treating in a vacuum furnace and using intensive gas quenching to minimize distortion.

 

[arunmrao]

Though I agree to metengr's observation, have you considered vibratory stress relief methods?

 

[TVP]

If the 4130 tubing was originally in the normalized condition (typical of aircraft grade tubing), then quenching and tempering will be needed to achieve a yield strength of ~ 1000 MPa. Distortion will definitely be a convern. Definitely don't use water as the quenchant, even if it is a water + polymer mixture. Even oil quenching will likely distort an assembly of welded thin wall tubes. I like swall's recommendation of vacuum heat treating with high pressure gas quenching, which has become a more popular method when distortion must be minimized.

 

[TexasRotorHead]

Swall, do you have any more information about vacuum heat treatments with gas quenching?

The tubes were all normalized prior to welding.

 

[desertfox]

From the earlier link I posted there is another link:-

http://www.netwelding.com/Welding 4130.htm

From the above link I found this information regarding post heat treatment.:-


If the part will be heat-treated after welding to achieve very high strength, a matching chemistry filler metal to the 4130 should be employed. Because of the relatively high carbon, a minimum of 200 degrees C, (400 degrees F) preheat and very slow cooling after welding should be used to avoid cracking. After welding, the part can be heated to 870 degrees C (1600 degrees F), quenched in oil or water then tempered back to say 370 degrees C (700 degrees F). A complex cycle, but this will result in a tensile strength of approximately 1380 MPa (200,000 psi). Since the weld is the same chemistry as the base material, it and the heat-affected zone will have the similar properties as the base material when heat-treated. All critical welds of this type should be inspected for internal soundness to assure they are free from cracks.

 

[TexasRotorHead]

I spoke to the welder and the weld filler was all 4130, so that is really good news.

Here is a picture of the frame. There are 2 separate pieces. One piece slides on the other.

This should give you a better idea of what we are working with.

Thanks

 

[metengr]

Looking at this frame makes me think a quench and temper heat treatment is going to result in distortion problems. You can try the heat treatment with the proposed method above, but I have serious doubts on the outcome. It would have been much better to have specified the 4130 structural tubing heat treated and conduct the welding with no post weld thermal treatment.

 

[TVP]

I agree with metengr, this type of welded structure will be extremely prone to distortion, and probably quench cracking in some of the welds. If it absolutely must be heat treated to increase the material strength, then I would definitely look at high pressure gas quenching. There are a number of good heat treaters that are capable of this process, including Bodycote, which has many locations across the US. Here is a link for more information on the process:

http://www.google.com/search?hl=en&...ure+gas+quenching&aq=f&aqi=&aql=&oq=&gs_rfai=

Perhaps a well-controlled marquenching setup could produce acceptable results, but I not personally familiar with any heat treat vendors that would process such a large part in this manner. Vac Aero is one supplier that could do this type of work, as they process complicated aerospace landing gear components, but I have no idea about cost, etc. Here is a link to their website.

http://vacaero.com/Ontario-Plant-No.1.html

 

[swall]

After seeing the piece, I agree that it would be prone to distortion as metengr and TVP have suggested.A first step would be to stress relieve it at 1200F prior to any heat treatment. This would need to be accomplished in a fixture to control the warpage.

 

[swall]

An alternative procedure would be to stress relieve and let it warp. Then, pull it straight with clamps, tie rods, chains, etc. and perform a second relief in the clamped up state.

 

[TexasRotorHead]

So I went and sat down with a local heat treating company and they suggested either a gas quenching or possibly a salt bath heat treatment.

Are any of you familiar with the IntensiQuench process?

From what I have seen, there are a few types of quenching that minimize distortion:

Gas quenching from a vacuum furnace (8+ bar)
Salt Bath Heat treating followed by oil quench
Intensiquench?

There is also austempering. Any thoughts on that?