Undoing the work of the FUF with 1095

[jkf96a]

I'm bringing along a group of knifemakers who are relatively inexperienced. One of them brought me a batch of knives to rockwell test last night. One of them tested around 59, but the others were all still soft. The steel was 1095.

The reported heat treat was to heat it to "nonmagnetic" in a charcoal forge, then quench in warm/hot motor oil. All of the blades had some of the bubbly look that I typically think of as signs of over-heating. Just for kicks, I also ground one down a bit to ensure I wasn't just testing surface de-carb.

My theory is that the HT failed because he overheated the steel, then used too slow a quench. If I understand right, that leaves grain growth and mixed pearlite, both of which contribute to a soft blade.

If I'm right, and the steel is currently mixed structures with large grain, what needs to be done to get things back to good? I assume the steel is enough of an unknown mess so that another heat/quench alone won't do much good.

I pointed him toward less heat and a faster quench, but what else? Normalize in descending heats? Sub-critical anneal? He doesn't have the temp control to do these steps well, but I could let him use my oven.

How do I best help this guy? I offered him a couple feet of 1084 already.

 

[HHH Knives]

Jason, I would start with a full anneal cycle then do 2 or 3 normalizing cycles and then follow with a proper heat/quench

Sounds to me like your on the right track. in guiding him to a fast quench oil. With better heat control.

Good luck and have fun!
Randy

 

[Kevin R. Cashen]

Hello Jason, You got good input from Randy that you can take to the bank. In examining your description the largest “red flag” I see is the hot motor oil. Believe it or not with most steels when doing heat treating troubleshoots for folks the quench is seldom the culprit, despite how often it is suspected, however with 1095 the odds change a bit in that area. Motor oil at any temperature is going to have a hard time getting 1095 to where you want it, but heated much beyond 150F and it will have even more variables and quirks.

Also, believe it or not, the suspicion of large grains is actually contrary to the hardness readings. Large grained steel that has been over soaked in regard to carbide will actually harden more deeply than fine grained steel with residual carbide. This is due to a suppression of pearlite formation due to eliminating points for the process to get started. So a large grained steel can actually top out in Rockwell hardness but will be rather weak/brittle as well.

This is not to say, however, that your hardness does not suffer from another issue. With steels that have more than .8% carbon you may avoid making pearlite but you will then have an incomplete conversion to martensite because the overheat put too much carbon into solution, and austenite is even softer than pearlite.

The bubbly appearance is probably effects of heavy oxidation which can be the result of overheating or can also be the result of too much time on an oxidizing fire. He should back off the air draft and cover his fire with a fire brick tunnel to contain a reducing atmosphere.

As Randy said, I would renormalize to get everything back to a clean slate- heat to nonmagnetic and then air cool. And re-heat treat using the brick tunnel idea and a much better quenchant.

 

[jkf96a]

So the overheat problem is too much carbon in solution, not grain growth. Thanks for the clarification. It's been a while since I've seen his forge, but if I remember right, he does have it closed in a little. Not sure what he's using for air source. As far as oil goes, he was looking at getting some canola. Not Parks, but beats motor oil any day.

After 3 or 4 years of oven HT only, my son wants to forge a little. I may mess with HT a little and see if I can re-gain the temp control/eyeball/magnet judgment I used to have. I think Don would rather see how it's done at his house with his equipment than to have me throw it all in the oven and "fix" it.

 

[Kevin R. Cashen]

So the overheat problem is too much carbon in solution, not grain growth. Thanks for the clarification...

Don’t get me wrong, one of the very real signs and results of overheating is indeed grain growth, in fact overheating is THE cause of grain growth, however lower Rockwell is not a sign of grain growth since large grained steel will actually harden up more easily than fine grained steel. This is one of the pitfalls of taking Rockwell hardness as a definitive test of everything, Rockwell only tells you the level of penetrative hardness in steel, but it really cant’ tell you anything about the internal condition responsible for that hardness. Is a 42 hardness the result of retained austenite, pearlite or ferrite? The Rockwell tester has no idea, all it knows is that it is 42HRC, but the more dangerous of the two blind spots is a Rockwell that is 66 with grains as big as your fist, in this case the hardness tester says it is good to go but the utter edge failure will tell otherwise.