edge quenched 5160, but it fully hardened, why?

[John Frankl]

I have been getting soft edges with my 5160 triple edge quenched and tempered at 350. I thought part of it might be the shallowness of my quench, about 3/8". Today I went to about 5/8" and a longer soak time, yes I am heating in a forge, not with a torch. On the final quench, most of the blade came up to temp, but I onle quenched the edge. I waited till all red left the blade, then submerged in oil to cool to room temp. When I submerged, the oil bubbled alot, but did not flash. I had a less distinct hardening line than usual, so I tested the whole blade with a file. The back is about as hard as the edge, a file won't touch it. I know some steels have a fairly long window from critical to Ms, but I waited a good 15-20 seconds before submerging the blade. Any ideas what happened?

Thanks,

John

 

[peter nap]

That's one of the great mysteries of steel John. It happens to me sometimes as if the back somehow air quenched. That may be what happens. Do it over again and see what happens. You may also want to change your temper temps. I find with a triple quench that my edges are better from 300 to 325.

 

[Greg Covington]

John,

What are the appromixate dimensions of the blade that you are quenching?

I have great luck edge quenching medium to large blades, but smaller knives of thinner stock have a tendency to harden all the way even if it is just the edge going into the quench.

In the triple quench on 5160, I recommend the oil to be at 160F and the temper to be around 350F. Edge testing with a file after each quench you will feel the blade to become progressively softer feeling, but this is due to the grain refinement.

I would say that you should play a little more with the quenchant heat and if that doesnt improve your edge at 350F temper, then drop down to 325 or so.

 

[B . Buxton]

John,

I've never had that problem with 5160 or 52100 and thats all I do is edge quench, I usually quench 1/3 to 1/2 depth of the blade depending on size and intended use, with quench heated to 150 - 160 range. I always hold the blade in the quenchent until it quits bubbling at the top of the quench line, that will mean the spine has cooled below hardening temps, then just slip the blade in the quench and let it cool to the touch, about 30 - 45 minutes, then I check with a file. If you are quenching in a bright area and just go by color I'm afraid your not letting the spline cool enough, I know with 52100 at quenching heats it shows very little color in my quenching area, and its all enclosed, so it would be hard to judge the spine temp, pay close attention to your magnet and don't over heat, then be sure and hold the spine out until there is absolutely no bubbling at the quench line.

Good luck
Bill

 

[John Frankl]

Thanks for the replies (keep'em coming). At this point would you recommend a soft back draw, or a complete re-normalizing, re=quenching?

John

 

[B . Buxton]

I would normalize 1 time then start my quenching process again, bringing it up to my first quench heat fairly slow. Unless you think you over heated the first quenching process you shouldn't have gotten any grain growth, so 1 normalizing should relieve any quenching stress and soften the spine back.

Good Luck
Bill

 

[Ed Fowler]

Hello John: If you are planning to do a multiple quench, no normalizing cycle is necesairy. simply heat her up again and quench. The spine will be tempered in the following heats. When using the multiple quench, all you have to do is get barely above critical, always use the magnet. Quench on a rising heat, if you wait too long, the blade will remain nonmagnetic below the temperature where it will harden.

Although the spine may harden to some extent, it will be much more tough than if it had been fully quenched in the oil.

I would suggest that just for the heck of it, polish the blade and see what the etch tells you. Then after studying the tale of the etch, put the blade in a vice and see what happens over several 180 degree flexes. You will have gained some knowledge that at this time only remains a question. Naturally unless you have a large supply of this steel (same lot etc) your knowledge may be farily restricted, but the lesson learned will correlate polsitive on future excursions in to the unknown.