# Grain of heat treated O1 Tool Steel



## ForeverLearning (Apr 2, 2020)

Hi all,

Doing some practice pieces on my forge as I need to understand temperature in the forge better. 

I have heat treated some O1, brought it to orange/yellow which I think is far too hot (I'll check or if someone could confirm that would be great)

I tempered at 250 Celsius for 1 hour which according to the manufacturing should achieve 60/61 HRC

As a visual check do you think this grain is acceptable? 

The piece would not deform when taking blows from a hammer, it would return to true. Had to put a notch in with the angle grinder, strook it a couple times and it 'pinged' when snapping. 

What are further checks I can do? 

Any advice or comments is much appreciated!


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## milkbaby (Apr 2, 2020)

Break it without tempering when it's at the most brittle state.

If you find an old file made of high carbon steel and break it, you'll see what the grain should look like: very even and "creamy". Below is a pic of a file that was overheated then normalized (I'm assuming using descending heats: above critical around 1600F, air cool to black, just above critical 1500F, air cool, then just below about 1400F, air cool or slow cool to anneal) so you can see the effect on the grain size.






Don't go strictly by color because it's very difficult to judge and will look different in different lighting conditions and when you've been staring into the forge for a while versus just peeking at it now and then. You can use a magnet to check when the steel goes non-magnetic and then austenitizing temp will be about just "half a shade to a shade hotter". That's the best one can do without strict temperature control IMHO.


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## Beau Nidle (Apr 2, 2020)

If you're heat treating in a forge I'd say get a magnet, rather than going strictly on colour. Get it to non magnetic, then put it back in for a few passes, then quench. It doesn't look that bad though. Normally I'd say orange is a forging temperature, and going into yellow is forge welding. Also it depends on the ambient light at the time, how you perceive colour...What I consider orange might not be the same as you!

When we get done with this lockdown you can come round and we'll play with some stuff in my oven and compare to forge heat treats!


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## ForeverLearning (Apr 2, 2020)

Those replies are extremely helpful thank you:

@milkbaby that is exactly what I was looking for. Could you explain why not to temper? Are the temperatures too cold during tempering to effect grain size?

@Beau Nidle Paul that sounds like a plan!

I did use a magnet, I pushed a screw through a 20kg magnet and held it with an adjustable spanner clamped to my workhorse. Problem being I think I took my eye off and it let it get too hot. Plus pulling out a orange/yellow hot piece of metal and having a 20kg magnet pull at it is a bit intimidating - I just imagined it grabbing the blade and me holding the newly torn tang in my grips.


I will normalise 3 times to black in air then heat treat
Controlling forge temperature better is something I need to read in to for sure, I melted Mokume gane twice because I cannot control it too well

A DIY heat treat oven is planned and costed but I should really use the forge for a bit before that, only done a handful in it.


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## milkbaby (Apr 2, 2020)

You don't need to temper because it won't change the grain size and the steel is easiest to break (most brittle) at it's hardest as quenched.


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## ForeverLearning (Apr 3, 2020)

milkbaby said:


> You don't need to temper because it won't change the grain size and the steel is easiest to break (most brittle) at it's hardest as quenched.



Guessed as much, didn't think of it at the time. Hence it being able to take some pretty hefty blows with a hammer! Thanks!


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## Beau Nidle (Apr 3, 2020)

I use a magnet on an extendable shaft if I'm heat treating in the forge. It's only about a 1kg pull, plenty to know if it's magnetic or not. Think I got it from RS components.


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## ForeverLearning (Apr 3, 2020)

Ordered this beauty, should do the job...


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## BT11 (Apr 5, 2020)

Ditch the magnet, ditch the colour scale. 
Buy yourself a a decent digital thermometer with some high temperature stainless steel braided thermocouples. Drill a hole in the side of your forge and stick the thermocouple in where your blade will be. Hit the exact temperatures needed for your steels.


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