Re heat treating mild steel

[ealanm]

I'm helping someone restore a vintage dirt track racing car that was damaged in a fire. The car has many components made from mild steel (probably in the 1010-1020 range) that have become soft from being in the fire. I'm confused about how to re heat-treat these.

Presumably, the original steel was pearlite. If I understand the process correctly, pearlite needs time to form during the cooling process. But how much time? I seems to me that the parts would have cooled fairly slowly in the fire, much slower than, say, a quenched tempering process. Should we re-heat the parts to a bright cherry red and then put them in some kind of insulating powder to slow the cooling process? What would be the best material for that?

Or am I thinking in the wrong direction and we should quench them to make them harder. I'm pretty sure we're aiming for pearlite, not martensite, but most of the examples of home heat treating I can find seem to be focused on quenching.

I'm an engineer, so don't be shy about details. But metallurgy is not at all my specialty. I only ever knew the small amount I learned in school, almost all of which I've forgotten.

All help appreciated!

[Magicniner]

The hardness in mild steel automotive panels is work hardening created when they are pressed into shape, there is no heat treat process to put it back.

[ealanm]

Thanks, Magicniner.

These are actually hand-made brackets and such, fabricated from mild steel, not automotive panels or other mass-produced parts.

[ealanm]

After reading some more about this I'm thinking maybe what I need is a two-step process: heating to above critical temperature and then quenching, for hardness; and then heating to below the critical temperature and air cooling, for temper.

[johnfreese]

Roll cages are usually made from 4130 alloy tubing. The tubing is not heat treated but has gained strength due to work hardening in the cold drawing process. Having been through a fire its properties are near the annealed state, the strengthening due to work hardening is lost. Similarly the low carbon tubing, if cold drawn, would have been strengthened by cold working. Having been annealed in the fire that extra strength has been lost. I do not know of any way to restore their mechanical properties in situ. Components made from hot rolled steel or pipe are not likely to have lost any strength.

If the parts can be separated from the car they can probably be restored to their original properties by heat treatment. I would go to professional heat treat shop to get it done. That assumes you know what the original material properties were.

[ealanm]

Thanks, johnfreese.

The car is a 50s sprint car so it has no roll cage. All critical load bearing components will be replaced with new parts or parts rebuilt from new material to the original specification, including the frame. The only parts I'm concerned about restoring with heat treating are brackets and other small, removable parts that can easily be handled by whatever process is required.

It's certainly an option to send the parts out for professional heat treating. But the person doing the work is keen to try it themselves. They're also operating on a tight budget for what is a pretty major restoration project. [Addendum: The person doing the restoration is a certified aircraft welder and so not inexperienced in working with steel and other metals, and fairly knowledgeable. Just not about heat treating, per se.]

[NP317]

Ouch!!!
RussN

[John Evans]

I would suspect most if not all of the brackets were made from 1018 mild steel,and the only heat treatment done to that steel is case hardening to my knowledge. Unless that car was built by Kurtis etc. I would doubt any heat treating was done to those type of parts,no real need and if needed mild steel would not be used. Neat looking car though, What engine? 6 cyl GMC?,Ranger aircraft engine?

[Steggy]

What engine? 6 cyl GMC?,Ranger aircraft engine?

The engine compartment seems kind of small for a Ranger. Also, the final drive would be complicated by the fact the Ranger was an inverted crankshaft design. Judging by the position of the carbs and given the period of time during which the car was built, I'd suspect a 270 or 302 CI GMC inline six. Quite a few of those engines ended up in sprint cars and occasionally on the drag strip. The nearest Chevrolet equivalent was 261 CI, but didn't have as beefy a lower end as the GMC powerplants.

Fun fact: the very first engine I blew up was a 261 CI Chevy "Stovebolt." The crank fractured right at the rear main journal and rods and stuff went everywhere.

[Harold_V]

After reading some more about this I'm thinking maybe what I need is a two-step process: heating to above critical temperature and then quenching, for hardness; and then heating to below the critical temperature and air cooling, for temper.

Unfortunately, that doesn't work for mild steel, as the carbon content is too low.

H

[ealanm]

What engine? 6 cyl GMC?,Ranger aircraft engine?

Yes, it was a Ranger. The magnesium block made quite a mess. The replacement Ranger is nearly finished.

[Magicniner]

The hardness in mild steel automotive panels is work hardening created when they are pressed into shape, there is no heat treat process to put it back.

Hmm, was this too complex to impart the information?

Mild Steel does not have the Carbon content to allow heat treatment other than annealing when it is work hardened.

To harden your parts squash them flat, anneal them and then use the original press forms to achieve the required shape and hardness.