Cracking problem

[Throwingsteel]

hello all, I am having a problem of cracking in 4140. I am induction hardening and tempering. I am having small crack develop along the parting line showing up under magnaflux. The draw temperature is 550 for 2 hours. Is temper embrittlement any issue? I am racking my brain on why we are seeing cracks.

 

[EngJW]

Check with the guys in the Metal and Metallurgy forum

 

[Metalguy]

550 F or C? How long after you ind. harden do you temper? How deep are you hardening, and what Rc do you get?

 

[Throwingsteel]

550 F The Time always depends on the 3rd shift operator can be as long as 4-5 hours before the first part hardened gets to the draw oven. About 50 Rc after draw. Thanks for your help guys

 

[thundair]

The best we could get out of 4140 is a R/c 42 and then we went to nitride to get up to 60 c I have gotten 4340 up to 48 C but that is the best for CM

Regards

 

[Metalguy]

4140 should quench to the mid Rc50's if it's not too thick.

If you can, try to get your parts into the tempering step ASAP. They should still be warm, not room temp. When this is not possible they should be reheated to 375F min. ASAP after quenching.

 

[tbuelna]

Throwingsteel,

You mention a "parting line". Is this part forged?

If the stock material is hot rolled or hot forged, it is standard practice to remove the outer 1/8" (or so) of hot rolled bar stock, or to 100% machine hot forged blanks. This is because the hot working process can contaminate the outer layer of material.

Also, induction hardening is usually a localized process. That is the whole part is not subject to the heating/quenching process. Personally, I would not use induction hardening (or similar processes like flame hardening) unless I was concerned about distortion during quench. The transition area between the heat treated area and the non-heat-treated area can cause problems.

Finally, trying to get a TS of over 250 ksi (Rc 50) with 4140 is really pushing the limits for that alloy. You might want to consider another material.

Good luck.

 

[Throwingsteel]

First off thanks guys for everybodys responses, Everybody has great input. The part is forged, it is rough machined before induction hardening. The part is a crankshaft so induction hardening of the main journals is the process used. We are only seeing the cracks directly in the middle of the part along the parting line.

 

[swall]

Have you mag particle inspected any cranks after rough machining? If the trim dies for the forging are in poor shape, the parting line could be opening up slightly when the forging flash is trimmed. Also, how prevalent is the cracking? Is it just sporadic or on every piece?

 

[Throwingsteel]

we have mag'd some after rough machining and saw some evidence of inclusions but not any cracks. It is sporadic on about 20-25 % of the parts. Thanks for the info on the trim dies.

 

[swall]

Throwingsteel--I was going to bring up inclusions next, but you were ahead of the curve. Do you have any control over the steel source or is it all in the hands of the forging vendor? Inclusion content is typically a steel spec issue and a melting/degassing process issue and these require that the steel user work closely with the steel source to control inclusion related problems.

 

[Throwingsteel]

there are some evidence of sulfide inclusions that we may be opening up at heat treat. Can we do anything to lessen the effects of these inclusions when he induction harden and quench?

 

[turbocohen]

Ask your metal supplier about the degassing processes they use and which gas pressures they purge their foundry with. Any good metal supplier should be able to find out but if they cant tell you then find another supplier.

Take a sample and grind down the parting line and shot peen it before hardening to relieve internal stresses then see if it still cracks after tempering.

 

[inductionexpert]

During the Magneflux process, the dye can stay in a forging parting line and look like a crack if not cleaned really good. Have you cut the part to see if the crack is really under the parting line? This has fooled me in the past.

Fred Specht
Ajax TOCCO
847606-9462

 

[Throwingsteel]

Hey Fred, yeah these are opening up to where you can see it visually, it has fooled me before too but i know now what to look for, we did cut one up and cracks are present

 

[swall]

The ideal solution here would be a tweak of the induction hardening process to deal with these forgings. But, I think you should first find out if they are cracking during the heat cycle. So, I would run several samples though with the quench turned off and mag particle them. If cracks show up, you are out of luck. But, if there are no cracks, I would try to "soften up" the induction hardening cycle. I would try cutting back on the heat time, power factor and quench severity to see if this helps. You could also try adding a quench delay if you don't already have one.

 

[Throwingsteel]

thanks swall, that is what i have been trying to do here, i have increased the quench temp to "soften" the quench so far this has not helped. There is only a second delay after heat treat to quench, should i lenghthen that time?

 

[Throwingsteel]

The bands of segregation and where the cracks are forming have high Rc harness 57-58 after draw, could this lead to the grinder causing the cracks? I want to increase draw time to get the low end of the spec for hardness.

 

[inductionexpert]

What is the temp of quench? Straight water or polymer added, at what %. Suggest raising polymer percentage will reduce cracking more than just temp.

 

[Throwingsteel]

it is polymer added 22% which is considered very mild.