need some help with heat treating theory and practice 6

[t2photo]

Hi every one i am relativly new to this sight and to heat treating my question to all of you is this... Can you reverse the heat treating process I am heat treating parts made from 4140 steel using a oven some parts that i do come out just a tad softer than the requirements that are in place for the parts we are making (i.e a part come out with a ROC of 30 when i was shooting for 46-50) the part is hard than before but is there a way to eather get it to the higher mark or take it back down to a "pre-heattreat" condition any info you guys have will be greatly appreciated thanks

 

[TVP]

Yes, you can re-heat the parts to the austenitizing temperature (~ 845-870 C), oil quench, and then temper at ~ 400 C in order to achieve 46-48 HRC. Re-heating and re-quenching may create distortion, cracking, etc., but from a microstructure standpoint it can definitely be done.

 

[mfgenggear]

seems the parts where tempered wrong

In addition I would also recommend a sub-crittical anneal if the parts are critical & and the geometry complex. Then preheat & then reharden

this will lessen the distortion & prevent cracking

 

[unclesyd]

If the 4140 parts has any complexity like a lot of through holes and threaded holes near the edge I would normalize at 1600F, air cool the parts prior to reheat treating them as per the post by TVP.

 

[mfgenggear]

Unclesyd

I don't aggree with Normalize. although not saying it will not work. It's My personnel prefence
sub critical anneal has been working great.

 

[unclesyd]

Either process will help mediate some distortion and or cracking.

I have always used the normalizing path as this was the way I was taught and experienced on thousands of 4140 parts. As stated sub-critical anneal or normalizing will help but not eliminate.

I like the idea of a little preheat during the hardening process. I've hardly used preheat except for large or extremely complex parts to minimize time at temperature at in the austenizing temperature.

 

[swall]

t2photo--you should start by telling us exactly what your heat process was.

 

[t2photo]

first of all thanks for the tips and comments they are greatly appreceated, secondly our process for heat treating are as such 1)parts are placed in stanless steel bags and sealed then we put them in the oven at 1550'f for one hour then the bags are cut open and oil quenched, after they are cooled down and washed they are then put back in for two hours at 550'f for two hours and then air cooled

 

[redpicker]

That helps. There are several possible reasons your parts are soft.

1) surface decarburization.
Your stainless steel bags may not have been effective on some of the parts and they may have experienced some decarburization. If you can grind the hardness indentation out and re-check the hardness it will tell you if this is the case. If the hardness goes up to the proper range after grinding, then this is your problem. If so, you will have to determine if a low surface hardness is detrimental to the part.

2) The parts cooled too much during the cutting of the stainless bags and quenching. The parts have to hit the oil while they are still red-hot (above around 1350F) when you hit the quench.

3) The parts did not cool rapidly enough during the quench. They were hot enough, but there wasn't enough agitation of the oil or some other problem slowed their cooling rate.

4) The parts never got hot enough in the first place. Since the parts are in bags, you can't tell the temperature of the part, only the temperature of the bag.

With problem 1, you would have to perform a carbon restoration operation to salvage the parts. With the other three problems, a re-treatment, going through all the steps, should lead to acceptable results.

When you seal the bags, do you wrap the parts in paper or some other carbon-containing material to consume any oxygen that is inside the bag? That might help prevent decarburization.

rp