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NiftyNev
02-28-2017, 04:18 AM
Say you had some 4140 that had to be zinc plated. After plating it needs to be heat treated to prevent hydrogen embrittlement. Now these are some pins about 1" diameter and about 4" length. What besides the plating cracking (which it did) happens to the 4140, if it it not allowed to cool completely and is quenched in water? Not sure how hot they were, but hot enough not to handle with bare hands.

wierdscience
02-28-2017, 09:06 AM
Those would only be drawing temps to not burn the zinc off right?? If that's the case,it probably wouldn't hurt anything except the zinc.

becksmachine
02-28-2017, 01:48 PM
Say you had some 4140 that had to be zinc plated. After plating it needs to be heat treated to prevent hydrogen embrittlement. Now these are some pins about 1" diameter and about 4" length. What besides the plating cracking (which it did) happens to the 4140, if it it not allowed to cool completely and is quenched in water? Not sure how hot they were, but hot enough not to handle with bare hands.

A little confusing? Are you saying that they were taken out of the quench before they had cooled completely? And then finished cooling in air and the plating cracked?

In any event, "hot enough not to handle with bare hands" would mean to me to be less than 400 and I can't imagine that kind of temperature having any effect.

Looking at tempering specifications for 4140 I did find this from All Metals and Forge Group; "Tempering should not be carried out between 400 790 F (200 420 C) to avoid the danger of embrittlement."

Dave

rickyb
02-28-2017, 10:19 PM
Say you had some 4140 that had to be zinc plated. After plating it needs to be heat treated to prevent hydrogen embrittlement. Now these are some pins about 1" diameter and about 4" length. What besides the plating cracking (which it did) happens to the 4140, if it it not allowed to cool completely and is quenched in water? Not sure how hot they were, but hot enough not to handle with bare hands.

Not really sure of what you are saying but I think I can bracket this for you. 4140 is quenched then tempered. Quench is always the same. Final hardness is determined by the tempering temperature. 350 degrees F is a quasi temper and give a final hardness around Rc60. Very brittle but nice and hard. Temper at 1000 degrees and you will end up around Rc30-35. Relatively soft but very tough. Both tempers are time dependent.

An interesting thing for this material is that you can reheat it to 50 degrees below its original tempering temp without any significant reduction in hardness. You might say the material has a memory.

Hydrogen from plating will migrate out of the iron matrix after about an hour at 400 degrees F. So if your original tempering temp was 450+, "no worries mate". Nothing will happen to the 4140.

NiftyNev
03-01-2017, 04:54 AM
Hydrogen from plating will migrate out of the iron matrix after about an hour at 400 degrees F. So if your original tempering temp was 450+, "no worries mate". Nothing will happen to the 4140.

I think they are only heated to a temp of 250 deg F for about two hours. I am not sure of the correct time or temperature that things like this require and just going on what I have seen done. Was just wondering if not allowed to cool (in the oven) slowly that quenching may cause them to bend out of shape. The plating could always be redone but if they bend or distort in other ways then that is a problem.

rickyb
03-01-2017, 09:27 PM
I think they are only heated to a temp of 250 deg F for about two hours. I am not sure of the correct time or temperature that things like this require and just going on what I have seen done. Was just wondering if not allowed to cool (in the oven) slowly that quenching may cause them to bend out of shape. The plating could always be redone but if they bend or distort in other ways then that is a problem.

Distortion is not likely as you are technically not quenching. The temp is so low that no amount of stress relief is happening either so there is no reason for the metal to distort.
Distortion is born in the quench of uneven martensite transformation.

High internal stresses form and vary throughout the part as a result of inadequacies in the cooling and part geometry. Afterward the uneven stresses do their best to reach equilibrium by twisting and bending the part. Tempering relaxes the material overall but does not equalize stress distribution. Machining changes the equilibrium of the twisted and bent part.

To conclude, you are not quenching from an austenizing temp and you are not machining so there should be no distortion.