Trouble with 4V Heat Treat

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PMQ

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Feb 17, 2020
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Hi all, I am having some trouble with heat treating CPM 4V

This is the recipe gave my HT guy:
Aust: 1886F (1030c) for 2hours
Temper: 340f (170c) 3 times, 2 hours each.

The HRC came out to be 45, which is insanely low. I told him to do it again, maybe be made a mistake some where, so he did it again:

Aust: 1886F for 2h
Temp: 340F 3x2h

HRC: 50.

This is really strange. I bought this CPM4V from NJSB, one of their first batch of 4V. The end of the steel bar has 3 colors that match NJSB website. (Thickness is 5/16)

Is anyone facing this issue?
 
Did the heat treater use foil or a vacuum furnace to prevent oxidation?

2 hours is too long to austenitize.

Hoss
 
DevinT said:
Did the heat treater use foil or a vacuum furnace to prevent oxidation?

2 hours is too long to austenitize.

Hoss
Click to expand...
He used Vacuum furnace.

I’m not sure if 2 hours is only the soak time or also include ramping up time. But the blade is very thick (5/16) and the aust temp 1886F is not too high over what Dr Larrin Thomas recommend 1875F

I tested with a file and the file easily cut the blade.
 
First, 30 minutes at 1875-1886° is all you want or need. 2 hours is going to cause trouble.
Second, even with a vacuum oven, there will be lots of decarb after 2 hours. It may take a lot of grinding to remove it. The blade may be significantly smaller once you go past the decarb.
Third, how was the hardness tested?
 
PMQ said:
He used Vacuum furnace.

I’m not sure if 2 hours is only the soak time or also include ramping up time. But the blade is very thick (5/16) and the aust temp 1886F is not too high over what Dr Larrin Thomas recommend 1875F

I tested with a file and the file easily cut the blade.
Click to expand...
Yeah, something odd going on here.

Larrin is my oldest son.

Hoss
 
Stacy E. Apelt - Bladesmith said:
First, 30 minutes at 1875-1886° is all you want or need. 2 hours is going to cause trouble.
Second, even with a vacuum oven, there will be lots of decarb after 2 hours. It may take a lot of grinding to remove it. The blade may be significantly smaller once you go past the decarb.
Third, how was the hardness tested?
Click to expand...
Are you sure about that ? How can that happen in a vacuum ? My friend use such a furnace, he uses it to HT parts made on CNC. Large parts, some weighing more than three kilograms, that need longer soak . If by chance I happen to be there when some parts are being hardened, I will take a picture for you so you can see what they look like at the end. There is no decarburization, my friend, nor should there be. That is why these furnaces were invented :thumbsup:
 
Not wishing to argue the point I will simply say that the depth of decarb is much less an issue on a 3000g part than it is on a 50g knife blade.
Also, the blade has to come out of the furnace sometime. When it does it is exposed to the air. Depending on how soon it was quenched to below 1200°F there can be decarb formed. Not saying that is the cause, just that it may be a possibility.
And, there is always the possibility there may not have been a full vacuum in the furnace.
 
If there was still scale on the knife there would be decarb going in to the furnace
 
Stacy E. Apelt - Bladesmith said:
Third, how was the hardness tested?
Click to expand...
The HT guy used a pretty standard hardness testing machine, the one that drop with a "clang" sound, not a handheld one.

When he got 45 HRC, I took out my Leatherman Wave and used the wood file, yes, wood file, not diamond file, 2 strokes removes the scale and 2 strokes cuts into the steel like butter. I think it somehow got softer after the HT.

The HT guy took out a piece of SKD61 (Japanese steel with 0.4% C) and D2, he used a angle grinder and compare the sparks, then compare to my sparks. It is closer to the SKD61.

Then I ask him if he has HT anything at 1886F for 2h, he said that he HT big D2 dies everyday, 1886F for 6 hours (not including the ramp), then temper, they come out 60-61 HRC. So I don't think the furnace has a problem.

So if decarb is the culprit, then why is only 4V affected and not D2?
 
I'm curious as to why a knife of any steel type would require 2 hours at austenitizing temperature? I thought the phase transformation was pretty quick, so it doesn't make sense to me to hold it for much longer than it takes to get your austenite?
 
Lorien said:
I'm curious as to why a knife of any steel type would require 2 hours at austenitizing temperature? I thought the phase transformation was pretty quick, so it doesn't make sense to me to hold it for much longer than it takes to get your austenite?
Click to expand...
I believe that the heat treater is a tool and die heat treater and not a blade guy. Different animal.

Hoss
 
Lorien said:
I'm curious as to why a knife of any steel type would require 2 hours at austenitizing temperature? I thought the phase transformation was pretty quick, so it doesn't make sense to me to hold it for much longer than it takes to get your austenite?
Click to expand...
Maybe it was kept for so long because there were some other big size parts for hardening inside ?

I'm more curious how it is quenched .With some other part inside furnace with inert gas or it was pulled out and quenched in oil ?
 
Natlek said:
I'm more curious how it is quenched .With some other part inside furnace with inert gas or it was pulled out and quenched in oil ?
Click to expand...
I believe my HT guy use cooled inert gas, but he also has the option to pull out and quench in oil
 
im just a knifemaker over here but NOTHING gets held at temp for 2-3 hours.. the "formulas "are based off the type of metal first. then the "Hold Time" is based of the thickness of said type of metal.
so if the "Tool &Die" guys are using 3/4 thick pieces of D2 they will have to hold it at temp much longer than a blade thickness.. also the foil requirement is not so important ,, how was it quenched??
everything could change the outcome of the steel... slowly cooling it may have annealed it over too much time..
 
Lot of folks here seem to be getting way too much tunnel vision on the soak time. It's 4V not 1095, So the primary carbides are very stable and don't all fully or rapidly dissolve even at austenitizing temperatures all the way up to near melting temps and even as steel liquid there are still some carbides still floating around until the liquid steel is hot enough to dissolve them.

So, grains will be fairly pinned by carbides limiting the growth and the carbon is not readily unlocked from the carbides to put in solution like the cementite in 1095 which rapidly dissolves at austenitizing.

Also, temperature is more significant than time. 1875f for 2 hours will not equal the effects of 2000f at 15 min etc. It's still 1875f, just closer to equilibrium.


We don't have a picture of the knife he is talking about. However, it's safe to assume unless proven otherwise he didn't spend the extra money to get the mill scale removed, nor did he remove it before heat treatment. Sounds like he is using a leeb tester on the mill scale. Also using a file on the mill scale.


The scale should be removed before heat treatment and also can/needs to be removed after for proper assessment of the steels hardness from HT.

Finish the surface up to 120 grit and use a Rockwell hardness tester, scale C.


Needs to Rule out
-surface condition and preparation
-hardness testing method
-quench cooling rate
-austenitizing temperatures
 
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