Asking for the right heat treatment

[Zandor20]

I am fabricating a rear sway bar for a race car. I was planning on using 4130 steel tubing, but have the option of using 4140 or 4340 if that would be more appropriate.

Based on my stress calculations for a tube under torsion, I need to have a yield point of 167,500 psi. 4130 in the normalized condition has a yield point of 75,000 psi.

What steel would work the best for this application, which is a tube in pure torsion under cyclic loading, and needs to remain ductile to avoid fatigue issues.

What do I specify to the heat treatment company to obtain the required yield point.

 

[unclesyd]

It will take 4340 to get to your desired strength level. You can't quite make it with 4140 or less so with 4130.
The heat treatment for 4340 is:

Oil quench from 1475°F
Temper two times at 1000°F

This should give you a Y.S. of 166,000-169,000 psi with no problem. As you have a tube you could get a Y.S. of 163,000 psi if you temper at 1100°F with a lot better properties.

 

[Metalguy]

Not to quibble with our current top expert (<g>), but 4140 will certainly go to the YS that is apparently required. After quenching from 1,550 F, temper at 950 F-double tempering is almost always preferred.

Where 4340 has its advantage over 4140 is in thick sections, something over ~1 1/2-2&quot; thick (solid).

 

[Zandor20]

I am using tubing with wall thicknesses of 0.083&quot; to 0.120&quot;, i.e. not very thick.

Does this affect what you guys have discussed?

Thanks for all the help.

 

[Carburize]

Generally speaking you need to use the lowest alloy grade which will give you the mechanical properties you need. In this case it looks like 4130 or 4140. As the alloy content increases problems such as quench cracks in thin sections and more difficult welding will arise.

 

[Metalguy]

A thin-wall tube in either 4140 or 4340 will be difficult to heat treat in a one-of situation. For fatigue crack resistance your heta treater would have to avoid decarb. It can be done but here's what I'd do-assuming you really require all that strength. See if you can buy some tubing in 17-4 or 17-7 PH. These are easy-to-harden stainless steels, requiring only a low-temp (~1000-1100 deg F) heat treatment AFTER you form the tubing into the shape you need. No decarb. worries.

Just buy it in the SA (solution annealed) condition. After you do the low temp. HT be sure to shot-peen the OD surface. MIC (Metal Improvement Co.) is one company with shops all over that can do this work-there are probably others.

 

[Zandor20]

After talking with a couple of aftermarket sway bar manufacturers, they are using 4130 and 4140 for their hollow bars HT to a rockwell of 37-40, and their solid bars are 4340 HT to rockwell 40-44. They did not know what kind of mechanical properties their bars had.

What kind of properties are they achieving with these numbers, and what do you expect they used for the heat treatment process.

They are assuming a 5 degree max twist on pavement, where I am on dirt and expect to be in the range of 7 to 10 degrees of twist at one end.

I could drop down from a 1.25&quot; dia. bar to maybe a solid or 0.25&quot; wall 1&quot; dia. bar, which would lower my max stress.

 

[Metalguy]

4130 is usually water quenched from 1575 deg F., while the others are oil quenched from our earlier messages. The tempering ranges from ~900 deg F up to 1000+. The higher the tempering temp. the lower the strength. Where fatigue is concerned, forget about YS and just look at the UTS. For high-cycle (say over 1 million stress/load reversals), you can figure on 50% of the UTS for fatigue strength--IF the bar doesn't have corrosion and sharp tool marks, etc. Shot peening will raise the allowable stresses considerably.

Now, the big shops probably use a heat treater who is able to avoid decarb.-probably by using a special furnace with a protective atmosphere.

But for your application I'd see about the avail. of either the 17-4 or 17-7. Costs more up front, but avoids HT problems later.

 

[Metalguy]

Following up, the UTS for Rc 37 is ~160-170 KSI, while Rc44 is over 200 KSI.

Don't use a solid bar if weight is a problem-most of the strength is near the OD. If weight isn't a concern, then solid is easier to find and work with.

 

[Zandor20]

After calling a few steel suppliers, 4140 in tubing of the thin walled variety are difficult to obtain, or require a large order.

I can buy 4130 tubing all day in any length and size.

The local heat treat company has some concerns about water quenching a thin walled tube of 4130, and wants to try and oil quench it.

What kind of results can I obtain from this? It looks like I am going to have to just use 4130, and hope for the best.

As a clarification to Metalguy, I am welding steel arms to the ends of the tube, not bending the bar into a shape as you mentioned that I could do with 17-4 or 17-7 PH.

Thanks again.

 

[unclesyd]

I don't have any data on oil quenched 4130. I don't have the TTT curves for it either. I don’t think it will cool fast enough to give you the strength you need.
4140 thin wall tubing used to readily available from the suppliers to the air craft industry. I guess the race car builders switching from 4140 to 4130 for better welding qualities lessened the demand.

I still like 4340 as a shaft material, torsional or otherwise. We had much better luck with than 4140 as shaft material. It welds better than 4140 but not as good as 4130. Is the thin wall tube available in 4340?
We found 17/4 PH needed to treated to at least H1150 to have any endurance as shafting material, especially if there were any stress risers. We had hundreds of 17/4 PH shafts. The old ARMCO Research Group finally admitted this after several years of probing. I haven’t had as much experience with 17-7 PH except as a spring material.

 

[Metalguy]

If your heat treater can get a good oil quench on 4130 tubing, it probably will harden OK. A solid 1&quot; dia. round will harden right to the center in water, so I would assume your thin tubing will cool fast enough in oil to be OK. BUT, there are some conditions: The tube must be quenched very quickly after leaving the furnace-within ~10 sec. or so. What counts is the amount of time it spends around 1200 deg F, which is a dull cherry red. It only takes a few seconds at ~1200 for 4130 to transform from the high-temp austenite to soft pearlite-one of the reasons it welds so well.

Also, different heats of 4130 will give widely different response to HT-at least the quenching part. What you want is to find a supplier who can give you a tube that has the Cr, Mo and Mn all near the high side of the limits. Also ask for AQ (aircraft quality) which will help your fatigue life. Finally, how does your heat treater plan to avoid. decarb?

 

[unclesyd]

You might want to try these people for you tubing.

http://www.factorysteel.com