To Forge or not to Forge... that is the question of CPM steel

D

dirc

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Jan 31, 2018
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I'm very interested to know any smiths experience on this.

If you were using the CPM line of steels, like 3v, or any powder based bar stock...
and you wanted to try to get the most toughness out of your blade,
and since you can get perfect thickness bar stock easily,
does it make sense to heat it and shape it with a hammer a dozen or more cycles?

(IE how much impurity / oxygen etc would that introduce to an otherwise very clean & pure steel matrix?)

Would it be better to just grind CPM bar stock to final shape, and then straight to HT/quenching/temper? Would it maximize toughness and purity of the CPM grain structure?

Or am I really making a mountain of a mole hill?
 
The biggest risk to forging is the narrow range of temperature those steels can typically be forged in, and then to get their condition back into some semblance of desirable takes much longer and more energy than typical "forging steels." There's no advantage to be gained from forging it (unless laminating) and much risk in creating stress fractures from forging outside the correct temperature range.

Low reward+high risk=no one does it outside of satisfying curiosity.

Maybe I'm not understanding the question but are you implying that you believe a forged blade is tougher than one ground from bar?

ETA The one time I tried laminating 410 around 3v the welds set and it promply peeled itself apart like there was a baby xenomorph in it's chest trying to get out.
 
dirc said:
I'm very interested to know any smiths experience on this.

If you were using the CPM line of steels, like 3v, or any powder based bar stock...
and you wanted to try to get the most toughness out of your blade,
and since you can get perfect thickness bar stock easily,
does it make sense to heat it and shape it with a hammer a dozen or more cycles?

(IE how much impurity / oxygen etc would that introduce to an otherwise very clean & pure steel matrix?)

Would it be better to just grind CPM bar stock to final shape, and then straight to HT/quenching/temper? Would it maximize toughness and purity of the CPM grain structure?

Or am I really making a mountain of a mole hill?
Click to expand...
What I can say to you is these CPM Steels were designed to be, Air Hardening Steels! I profile & then I bevel grind these after PAUL Bos at Buck knives has done the HT! Save yourself some headache .. Forge Carbon Steels!!
 
It is not designed to be forged. It is designed to be machined and ground.

I occasionally will heat to 2000-2100°F and put a curve in a bar for a desired knife shape, but would not ever forge it. Realize that when you do heat it up to shape it it will air hardening when it cools. Annealing these steels isn't simple.
 
Thanks for the feedback:

kuraki - "are you implying that you believe a forged blade is tougher than one ground from bar?" --> no, in fact, the opposite, especially if you're using CPM or powder steel

rhino - that seems like a lot of much harder work at the grinder. I'm interested to know why you don't HT after profiling and grinding?? Wouldn't the job be 3x easier that way?

Stacey - yes, thanks, I was expecting an answer like that... do you HT before or after?
 
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Grinding after heat treat is harder to remove material but can resolve some other problems that often occur during heat treat that can't be as easily remedied if there's not enough material left. Especially thinner blades.
 
On thicker blades, I profile and do the basic bevel pre-HT and then finish after. On thinner blades, I only profile before HT. The bevels are ground hard. This helps keep warp down, and reduces redundant grinding procedures. It sounds like more work, but considering you would do it twice normally, it is less total work .... and less chances of a screw-up.
 
CPM steels are dirtier than most conventional steels. The difference is in the size of the impurities.

High alloy steels are more prone to decarb than simple steels and the chrome oxide is very hard on belts.

You won’t gain any toughness from forging cpm steels. There is some benefit from careful forging of conventional high alloy steels because it breaks up primary carbides into smaller ones and produces a slightly tougher steel. The carbides are already 10X smaller in cpm steels and they may even grow with cycle annealing.

The only reason to forge cpm steels is that you’ll have a bigger story to tell the customer.

Hoss
 
Hey Devin, I thought CPM steels were supposed to be cleaner than cast steels. Would you explain to me, please.
Tim
 
tim37a said:
Hey Devin, I thought CPM steels were supposed to be cleaner than cast steels. Would you explain to me, please.
Tim
Click to expand...

I'm just postulating and don't know definitively but...

If Clean/Dirty means inclusion of impurities and not errant alloying - just things that don't belong in steel, the CPM process is bound to be "dirtier" because of the physical nature of the process. It's essentially cannister welding powdered steel, just to a much finer degree than what we typically do at home. In a slag furnace, you have a molten crucible of metal that is tapped from the bottom and those impurities are slagged off the top. In a cannister they're sealed in at whatever level was contained in the powder.
 
CPM steels have a greater percentage of impurity but the size of the inclusion is smaller.

There are several processes for producing conventional steels that are very clean. ESR, VIM, VAR etc.

Hoss
 
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