4340 vs 4145 - Induction heating response

[dbooker630]

I have a shaft requiring induction hardening along most of its length, which is accomplished by a single shot coil. There are two versions of the shaft, one made from 4340H and the other from 4145 steel. The shafts are quench-and-tempered to 45HRC prior to IH.

With the 4340 I get the desired IH pattern length and depth. However with the 4145 the pattern does not extend as long or is as deep. I use the same heat time, power, frequency, quench temp, etc.

Beyond any chemistry or hardenability issues can anyone think of any other physical reasons or properties that would explain the response difference? Out of the furnace the 4145 responds as good as the 4340 but something is very different with induction. Thanks!

 

[CoryPad]

What is the final hardness target?

Maybe the 4145 material can achieve a "global" 45 HRC hardness during large-scale austenitizing + quenching + tempering, but maybe it can't achieve high hardness (say 60 HRC) during induction treatment because the "local" quench rate is not fast enough for this low alloy material. Maybe the higher alloy content of 4340 is necessary to delay ferrite/pearlite/bainite formation for your particular application.

Can you section parts in the as-quenched condition? Maybe check microhardness?

Maybe this answer is not beyond "chemistry or hardenability issues", but I don't know how to think about it beyond those issues.

Actually, what about decarburization? Maybe the higher alloy content of 4340 is just a little better to prevent some decarburization during IH? That seems like a low-probability occurrence.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dbooker630]

Cory,

Thought I would try thinking out of the box for a moment...thanks for pulling me back in!

I will take a look at the respective cooling curves for these materials. The 4145 shaft when heating appears to be hot enough in the problem areas to get at least some transformation upon quenching. I may need to try a lower polymer concentration for a more severe quench.

 

[redpicker]

Can you say what sizes you are processing?

I can't believe that 4145 would not have adequate hardenability. The hardenability of 4145 is pretty high and it is often modified to high-side chemistries (particularly for large OD bars). What was the original heat treatment of the shafts? Is the original heat treatment different for the different materials?

With the 4145, you have a higher carbon content, which should lower the AC3, making it easier to induction harden, not more difficult. I would hesitate using a lower concentration of polymer as 4145 is more crack sensitive than 4340.

The only thing I can come up with is that, at these hardnesses, perhaps the 4145 has a different magnetic permeability. Is it possible the 4145 is actually harder than the 4340?

I know, more questions than answers, but I deal with 4145 as well as induction hardening and this interests me.

rp

 

[dbooker630]

Now I'm out of the box again...

The shaft diameters range from 2" to 3" on one end, with a 4" spline at the center, then back to 2.25". All hardened at once using a single shot coil. Yes, I know it's crazy but it does work with 4340. It is at the extremes that I'm having the pattern issues.

It is quite possible that the pretreat hardness is higher than the 4340. I had to go nearly 200F higher with the 4145 to lower the hardness after draw to be similar to the 4340, but I think it is a few HRC points higher. Both grades are quenched in 225F oil.

 

[swall]

Dbooker630--As you know, a Q&T structure is the optimum one going into induction hardening. I would hypothesize that with the 4340 you are getter a "better" martensitic structure than with the lower hardenability 4145, so the 4340 response during IH is optimized. To get the 4145 to have the same response, you will either have to change your IH parameters or see if you can develop a better martensitic structure during the original Q&T.

 

[TVP]

db630,

Do you subscribe to ASM's Heat Treat Progress magazine? If not, there is a regular article written by Valery Rudnev from Inductoheat called "Professor Induction" that is excellent. He describes a number of the more detailed issues regarding induction hardening, such as rate effects, increased resistivity with increased temperature, etc. Perhaps some of his insight would be helpful for your "outside-the-box" thinking. Let me know if you can't find back issues of HTP (2006-2007 time frame).

 

[dbooker630]

TVP,

I read HTP cover-to-cover every month. I'll give the Professor a second look. Thanks!

 

[TVP]

May-June 2007, July 2007 both had some good info.