[TheForge] Tempering and hardness

[Bruce Freeman]

I am no expert either, but I've also done some reading on these
subjects.  You are correct about carbon steels and some alloy steels.
You harden them (typically after annealing) by heating them to red and
quenching rapidly.  You temper the glass-hard steel by heating to a
much lower temperature (as indicated coincidentally by the temper
colors) to reduce the hardness and increase the toughness. AFAIK,
steel is the only material that can be hardened this way.

Someone else on this forum will have to elaborate on the metallurgy.

SS is steel in that it is mostly iron, but it has so much chromium and
nickel (and other elements?) that its behavior can be quite different
from carbon steel.  There are more than one kind of SS.  I understand
some SS's can be hardened and tempered, but I'm not sure of that.
IIRC, the 300-series can only be work-hardened.

But if you weren't forging SS, then SS was pretty much irrelevant to
the question. He might as well have spoken about copper or brass,
which you anneal by heating to red and plunging into water, and harden
by work-hardening (e.g., hammering or drawing through a die).  Try
THAT with tool steel and see how "soft" it gets!

The effect of hardening and tempering, along with grain growth, can be
demonstrated fairly easily with a test piece, if you want to go to the
trouble.  I would have told this know-it-all to go fire up a forge and
try it himself if he didn't believe me.

On Mon, Dec 5, 2011 at 9:59 PM, Paul Novorolsky <crosspein at sbcglobal.net> wrote:
> During a public demonstration last weekend, I was explaining to someone
> about hardening and tempering in pretty general terms. When I said that
> tempering sacrifices some hardness for toughness, a spectator told me I
> was wrong. He pretty much said "No it doesn't, tempering makes it harder".
>
> I disagreed assuming he had hardness and toughness confused, stating
> that the heat treating charts for common steels show a reduction in
> hardness at various tempering levels. He continued to tell me I was
> wrong, because he had bought 100's of tons of "full hard stainless
> steel" and asked how I explain that material. I told him I had no
> knowledge of it, but tool steels will lose some hardness during
> tempering. He repeated that I was wrong. At that point I was agitated,
> and told him to do some research and he'll see that tempering does
> result in loss of hardness, at which time his wife seemed annoyed with
> him and dragged him out of our shop. (amusing in itself, as he was a big
> man)
>
> While I could have handled that a little better, probably by asking for
> a detailed explanation of the process used for "full hard stainless",
> which I'm sure he had no clue.
>
> Anyway, I'm looking for more education. When I got home, I did some
> research, but I'm still short on details. If I understood what I read,
> "full hard stainless" is tempered, and then maybe cold rolled to work
> harden back hardness. I'm not really looking for tremendous detail here,
> just an explanation "in blacksmith terms" about what this stuff is.
>
> I also came across a statement in one search that popped up that said
> that some alloys will become harder by tempering. Being an internet
> source and therefore dubious in my eyes, I tried unsuccessfully to find
> additional sources to provide some verification. (and then lost the link
> that had that statement in the first place). So question 2 is "are there
> steels that become harder by tempering?"
>
> It's been awhile since I read anything on metallurgy, and since I tend
> to use macro processes for my heat treating, I tend to forget a lot of
> the details, but I'm hoping there are a few bits of knowledge, or maybe
> rumors to be had from the list. Any help?
>
> **Paul
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-- 
Bruce
NJ