Need help understanding heat treatment of Sandvik 20AP steel. Making a broach.

[Rolf]

I' trying to make a 5,2mm square broach out of Sandvik 20 AP steel. This is the only drill rod I can find in Norway.  It's got 1% carbon.
Hardening is easy. I coat the broach in anti-scale from Brownell, soak it at 810 celsius for 30minutes, qwench  in cold water. Hard as glass.
I temperd the first broach at 300 celsius for 30 minutes.  It broke after broaching 1,5 holes.
The second broach, I temperd at 300 celsius for 45 minutes. It broke after broaching 4,5 holes.
The broaches don't break while cutting the the hole. They beak when tapping them gently out of the hole with a brass hammer.

I've started on a third broach and need advise on how to temper it. Should I stay with 300 celsius and lengthen the time to 1 hour or more?
Or should I increase the heat to 400 celsius?

Here is the link to the data sheet for Sandvik 20 AP steel: https://www.materials.sandvik/en/materials-center/material-datasheets/wire/exera-20ap-free-cutting-medical-wire/

Hope someone can give me advice.

Best regards
Rolf

[bnewberry]

I am not an expert, I just make knives. That is a relatively simple steel. That is good. I would quench in hot oil instead of water. Water temper can cause cracking if it is too fast, Also you don’t need a long soak at temperature with a simple steel. 5 minutes is plenty.

I would temper the broach up to 300 but heat it up slowly. With a small item you may be causing stress in the item by putting it in a hot oven. Put it in a cool oven and let it warm up to 200 and keep it there for 10 minutes, then slowly increase to 300 and keep it there for 30 minutes.

[bnewberry]

It occurs to me to ask, how hard is the material you are broaching? Also, looking at the broken broaches, do the ends look grainy? This might mean uour heat treat is causing grain growth which can cause brittleness.

[Rolf]

 Thank you for your advice, bnewberry.
 I'm broaching hammers made from mild steel.

The steel on the broken broaches is very fine grained. But I'll try reducing  the heat soak to 10miutes

I've tried quenching in oil. Does not work.  I think the layer of anti-scaling compound slows the cooling. When I tempered the broach, I heated the oven to 300 Celsius before putting it the oven. I'll try your tempering suggestions on the next broach .

Best regards
Rolf

[bnewberry]

Good luck and let us know how you make it work!

[bob in the woods]

I'm an amateur at this, but like to surround myself with folks who are smarter and more experienced .  None of my teachers used cold water for quenching. Room temp is what seems to work for me.

[Bsharp]

"I've started on a third broach and need advise on how to temper it. Should I stay with 300 celsius and lengthen the time to 1 hour or more?
Or should I increase the heat to 400 celsius?"

That is what I would try, 400 C.....or even 450 C.

I have shattered chucking reamers when they were dropped on a cement floor.

Use plastic instead of brass to tap them?

[P.W.Berkuta]

For your quench try 50/50 water and automatic transmission fluid. Float the water on the trans oil and use enough of both for the quench. You will have to try different tempering temps as I can't help you there - good luck and keep us posted on you results

[rich pierce]

I would make test pieces 3” long, simply flat stock, and smack them with a light hammer with half the length sticking over the anvil edge. If it snaps it’s not tempered enough. This sort of testing saves a lot of heartache when making tools or knives for those of us who work unknown steel and have no accurate equipment (heating to quench by eye and tempering by colors).

[shortbarrel]

I will go with the above post. Made many springs and tooling out of a Model T car main spring.