Heat treatment and tempering

**DirkB** · 03-01-2015, 14:16

Hey guys, hoping you can help me with this... I am strugling to figure out the process that is followed and how they differ. Did a lot of reading and watched a lot of Youtube videos (some very useless ones I might mention).

Ok, this is what I understand so far... Heat treatment is the process of hardeining the steel. The metal is heated to a point where it loses its magnetic qualities (easy way to check according to most sources) and then quenching it in water or oil. Most guys prefer oil. At this point the steel is quite brittle. (Please correct if I am wrong in my understanding of this)

Tempering as I understand it is the process of strengthening the steel soon after heat treatment which is done at a lower temperature (about half that of heat treatment). What I can't figure out about this process (which none of the countless sources I checked showed) is the cooling process for this step. Is it also quenched in oil or water or is it left to cool to room temperature? Wouldn't the second option make it too soft (like annealing)?

I appreciate your input!

**Captain Hook** · 03-01-2015, 14:39

It's a multi stage process. Depending on how you have made your blade. Let's use forging as the example.

Once you have forged and ground the blade to final dimensions you would then anneal the steel by heating it up to bright red yellow and allowing it to cool very slowly. This stage could be done a few times. This relieves the stresses built up during the forging. It re aligns the crystal structure of the steel. But this leaves the steel, very soft.

Once you have annealed the blade you would then quench it. Kevin Stuart heats the blade to a red yellow that leaves the blade without any magnetic properties. Once it is at the correct temperature. The red hot steel is plunged into a quenching oil or water solution depending on the steel used. This makes the blade very hard, but as now induced an element of brittleness. Much like glass.

The final stage would be tempering the quenched steel. Amateur knives makers do this in a home oven. They bake the steel till its colour is ever so slightly straw yellow. This softens the steel ever so slightly, removing the brittleness that could make a blade snap or shatter.

**Ds J** · 03-01-2015, 19:25

Hardening is done by quenching in liquid for most steels; tempering is done by slowly baking the steel at a lower temperature.

The most effective way of doing both is to do it with a controlled oven and someone who can read the heat treatment tables correctly. Then the two processes are done within a few degrees of the correct temperatures at which the steel obtains its optimum performance hardness and flexibility as is predicted by its chemical constitution. Each steel is composed differently, therefore I am pretty sure that this way is better than simply heating and quenching on the colour of the steel.

Etienne Vorster in Pretoria/Brits did my last knife and I was more than satisfied. His number is zero seven six three six eight two zero three one.
He also does something with cold treatment which brings out the toughness of the steel as well.

**PistolBoy** · 03-01-2015, 21:52

You can cryogenically freeze metal to align the magnetic polarity. Making it quite strong. This will also change the chrytaline structure.

**DirkB** · 04-01-2015, 10:32

Thanks for the info guys!

**Desert Rat** · 05-01-2015, 07:33

When I did my Knife making course with Thinus Herbst we heat to about 1300 deg c (If I remember correctly) then quench in "Quenching" oil. We then put in an oven at about 100deg for 45 mins...this gave us a hardness of about 58 Rockwell. We learned as well to harden the blank before grinding ....I know this is arse about face compared to how others do it but according to Thinus this reduces any warping when hardening a blade. IE: if the blade is pre ground before hardening the blade is obviously at various thicknesses which when heated and quenched can bring in stresses which would warp the blade. The belts wear marginally more grinding a hardened steel. I use this method now and have god results. Bohler steel in Isando also harden knives for the public I normally take about 10 blanks at a time and it costs about R15 a blank to harden

**bcs** · 05-01-2015, 23:37

I was just about to suggest Bohler as well. I made some blades out of N690 and they hardened / tempered them very nicely. No problems at all. The longest blade was 40cm + and it had NO warpage. Leave it to the pros if you can.

**Chops** · 06-01-2015, 10:01

Cryogenic treatment (actually any heat treatment) has got sweet blue f-all to do with 'magnetic alignment'. I want to find the dimwitt that started the magnetic alignment story and kick him in the fork.
Cryo treatment transforms any remaining austenite to martensite, which could in some applications mean better toughness at the same hardness levels.
As to the question of quenching after tempering it is really only advisable if you are doing higher tempering temperatures (above 500 degrees C). The primary reason for this is to cool the steel quickly through the range of 250 - 500 degrees C, this is the temperature range in which some steels (especially plain carbon and chromium containing steels) suffers from temper embrittlement (old blacksmiths called it blue brittleness). Here you have a soft steel that is also extermely brittle.
Another application where quenching after tempering is advisable is if you are doing differential tempering (i.e. tempering the spine for toughness and the cutting edge for hardness), this is to stop the cutting edge from tempering back too much.
Using a magnet to test the for the correct temperature is actually very accurate, as this indicates whether the phase change to austenite has been reached. Problem with using only the readout on the HT oven/furnace is that a thermocouple might be defective and the temperature distribution in the furnace is normally not as even as most people think, plus the parts packing in the furnace can influence the distribution as well.
Yes I have heat treated parts from 20gram up to about 22Ton for close to 20 years.

**DirkB** · 06-01-2015, 11:35

Thank you guys! I'm just fooling around a little bit, want to make my own knife just for kicks. I appreciate your input!

**Ds J** · 06-01-2015, 12:08

Originally Posted by DirkB

Thank you guys! I'm just fooling around a little bit, want to make my own knife just for kicks. I appreciate your input!

The easy way is stock removal - take a piece of steel and grind away the excess steel untill the knife emerges from the billet.

The fun way is forging - take a piece of steel, heat it and hammer it into the desired shape. It takes time, effort and some boo-boo's until you get there, but it is a fulfilling experience, especially for someone who appreciate skills.

Whatever you do - PLEASE don't use an actual grinder??? It heats up the steel too much.