4140 Heat Treatment Options for Special Application

I build structural beams that support ladles of steel that weigh 200k-1000k pounds. Because of their proximity to the hot ladles, the beams live in an environment that can be as low as -10ºF during mill shut-downs in the winter; and as high as 600-700ºF while up and running in the summer.

Of course, the material is never subjected to rapid temperature changes.

The beams themselves are generally between 30 and 50" long, and are usually about 12"x12" square in cross section. I machine pockets and a few other features into the beams to house other peripherals, but I don't remove massive amounts of material.

Traditionally, I have chosen 4140 pre-hard for the beams and have them stress relieved after machining.

The trouble is after the beams are put into service, over the course of several years, they warp and require re-machining. They seem to remain dimensionally stable after the rework is completed.

What heat treatment options are available for me to use that will "pre-warp" the beams, so that I can "re-machine" them before ever delivering them to my customer?

Guessing here

The load is stressing the beam and the long 600F soaks are relieving that stress - putting a bend in the beam

It is a low stress, about 13,000 psi if 12" square, 50" long, with 600K load centered on length

4340 is much deeper hardening, you might want to see if the change is worth the effort

You do know that on a 12 X 12 section, the effect of "heat treat" is not very deep with 4140

Mass effect charts on 4142 and 4340

Thanks for the reply, however I'm not sure what benefit would come from 4340's deeper hardening?

What's interesting in this application, is that once the beams warp the first time, they can be remachined straight and don't warp again.

Why bother with prehardening???. Seems pointless as at 600f your hardness slowly withers away. In time they become annealed .

Exposure to 600f 24hrs a day surely cancels out any tempering.

In this application I don't see any advantage to hardening.

If I'm wrong , feel free to correct me.....

I would think that the first stress relief process is not hot enough, or long enough to relieve the stress. Repeated 600 soaks then slowly finish the job that the first process did not.

If a second SR after a second re-machining, holds without warping, that proves that the material is not bending under a Hot load, but moving due to internal stresses, which have now being mitigated.

Toms Wheels said:

I would think that the first stress relief process is not hot enough, or long enough to relieve the stress. Repeated 600 soaks then slowly finish the job that the first process did not.

If a second SR after a second re-machining, holds without warping, that proves that the material is not bending under a Hot load, but moving due to internal stresses, which have now being mitigated.

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This is more or less what I was thinking. However, I do NOT do a stress relief process after the rework machining. They get warped when in service, I remachine them and put them directly back into service - and they don't warp again.

Rigor said:

Why bother with prehardening???. Seems pointless as at 600f your hardness slowly withers away. In time they become annealed .

Exposure to 600f 24hrs a day surely cancels out any tempering.

In this application I don't see any advantage to hardening.

If I'm wrong , feel free to correct me.....

Click to expand...

I used to hold the same belief; that heat treated steel being operated at tempering temperatures for extended periods would temper away any hardness gained from heat treatment.

However - I also make 2.25" bolts out of 4340, quenched and tempered to ~45Rc that are subjected to the same 600-700º environment that don't lose much hardness at all. I've had them come back after 10 years and they still ping ~40-42Rc.

I can't explain why, but my experience tells me they just don't lose their hardness as I would expect!

At first glance, there are a few ways this could happen:

1) rolling/machining/cooling stresses relaxing

2) bending stresses relaxing. Given that deformation doesn't occur after the first remachine, I would discount this cause.

3) Change in material structure due to further tempering. Without knowing the tempering temperature, it's hard to say how likely this cause is. The usual exposure limit for steel is 100F below the tempering temperature, but this is for a time exposure no longer than 1 hour/inch.

4) temperature induced lowering of yield strength leads to localized yielding at a stress concentration. Not the most likely since since you don't machine away that much material, so it's mostly still a 12" square bar.

As John Oder pointed out, the heat treatment of 12" square 4140 isn't going to have much of an effect other than close to the surface. 4140 Just doesn't harden well on sections thicker than 1". Unless there's a reason for needing a hard-ish surface (like wear resistance), the heat treat probably isn't doing much for you.

If your stresses are low enough, you might consider using annealed. This should minimize any stresses in the material, and it won't have a temper to ruin.

Unfortunately for you, most of the easily available information on temperature effects on carbon/alloy steels is based on short exposure times. After 30 minutes at 600F, you'll lose about 20% of your yield strength, but I don't know of numbers for a longer soak.

turbo54 said:

I used to hold the same belief; that heat treated steel being operated at tempering temperatures for extended periods would temper away any hardness gained from heat treatment.

However - I also make 2.25" bolts out of 4340, quenched and tempered to ~45Rc that are subjected to the same 600-700º environment that don't lose much hardness at all. I've had them come back after 10 years and they still ping ~40-42Rc.

I can't explain why, but my experience tells me they just don't lose their hardness as I would expect!

Click to expand...

Thanks, that's a handy piece of information to have in the back of my head for someday in the future.

How does the tempering temperature of your 4340 compare to that of the 4140? If lower, that would tend to eliminate my cause 3 as a likely option.

Some remarks:

4140 is great stuff but its strength declines at temps above 600F. Maybe another having higher hot strength.

Also while the whole beam may not exceed the safe temperature the part exposed to the radiance from the ladel may rise well above the figure you cited..

Lot of mass to the beam. Enough to allow a significant gradient to develop.

Have you considered a heat shield of some kind?

So....why 4140 to begin with. Plain old mild steel no good???

After all according to your post you point it out as a load bearing member.

johnoder is right when he questions how deep is the tempering of 4140.

Could you pre-camber the beams so that after the initial deformation they would be straight?

Well... the part about not deforming anymore after a tune up may have knocked off my first instinct that it’s creeping due to elevated temps while under strain. This is a failure mode of slow plastic flow of the material (with normal steel it shows up around 30-35% of the melting temps). Another (unlikely) choice would be you somehow reached the .2% offset yield point & the material is somewhere on the uphill strain curve but still working… You usually don’t want to be in that place.

Prehard 4140 to me means tempered around 1200°F resulting in 29Rc hardness & 125Kpsi strength. ASTM A193 Grade B7 capscrews are 4140 tempered at 1200°F and considered safe up to 1000°F (they are known to creep under too much strain when hot though). I’m also thinking that true Q&T’s to a lower temp (to increase strength) on 1200lb+ to 2000lb+ parts is likely more expensive than your time to make them right again. With billets this size the heat treater WILL water quench the lump and interrupt the quench a bit over the Ms start temp so the section will deeper harden compared to an oil quench. Unless you really get over the original temper in heat they really don’t temper back any more, it’s something else happening.

The 4340 gives roughly a 15% bump in strength at the same temper FWIW, but it droops pretty similar to 4140 when hot.

If fighting strength droop or creep strength really is a worry, then people usually go to more chrome, maybe more moly, maybe some V or W and usually less carbon (if it’s steel) & I’ll attach 1&1/4Cr_1/2Mo_1/4V very good to 1000°F. After that it’s usually H11 tool steel & after that it gets real expensive.

Good luck
Matt