Ni mai quench tips?

[Ben Thomas]

I have started a ni mai ko deba and am getting ready to harden. This is my first try with ni mai and I know that I'm going to have warping issues, so I picked a short, thick style to give myself plenty of material to work with.

From older posts, it looks like quench plates will save me a lot of headache, but are there other tips and tricks to make this as painless as possible?

Additional info: Starting materials were 0.11" 26C3 and 0.25" mild steel. It has been forged down to about 0.2". Blade is about 4" x 1.5". I've cleaned up the 26C3 side on the grinder but haven't done anything to the mild steel side.

Thanks all.

 

[DevinT]

It will warp less if you heat treat at the lowest end of the austenitizing range.

Hoss

 

[Stacy E. Apelt - Bladesmith]

Many of us have seen a san-mai blade tear itself down the middle in a quench if it was too hot. In yaki-ire the blade may break in half as the ha hardens, especially if it was too hot. Ni-mai is not as violent a phase change but should still be done at the lower limit of austenitization.

HT Regimen:
Normalize
Grind all edges clean and smooth with no "I'll grind that out later" places. Slightly round the edges and sand to 220 grit. You don't want any sharp angles, visible cracks, or bad welds.
HT at the lowest temp for the hardenable steel - Normally, 26C3 is done at 1475°F, so1450° should be fine. This puts the minimum needed amount of carbon in solution and leaves the other structures alone.
Quench in fast oil. Room temperature Parks #50 works well.
Clamp in quench plates immediately after 3 seconds in the oil. Do this quickly. Let cool one minute.
Immediately temper at 400°F. Two tempers recommended.

What will happen in the quench:
As the bar rapidly cools past the pearlite nose at 1000°F the 26C3 will stay as soft rubbery austenite. As the low carbon mild steel side converts back to a predominately ferrite structure (FCC to BCC), it will contract a slight amount. This will harmlessly curve the blade slightly away the 26C3 side. As the 23C3 steel reaches around 400° the high carbon steel will start converting to martensite and expand rapidly. This will suddenly curve the blade more to the mild steel side. Since the mild steel will be soft it should not be a problem, but the chance of a crack in the 26C3 or tear along the welds between the two steels propagating along the super brittle martensite is a concern. That is why the edges should all be rounded and smooth - no stress risers.

 

[Ben Thomas]

Following up on this post to say that following this advice worked great. It took a little longer for extra prep work Stacy suggested and also to fit a thermocouple to the forge, but worth it in the end.

There are so many helpful voices on this forum, but the two of you stand out post after post.