Hello Jesse. There are 3 distinct phases to a standard quench. There is the initial vapor jacket formation when the blade is first produced, then the nucleate boiling collapse of the vapor jacket, and the final phase is the direct conductive cooling phase. The first two phases correspond with the range where the steel needs to be cooled the fastest to avoid the diffusion based processes that will result in unwanted products from 1,100F to around 700F; this is the range where the fast cooling is critical. The final cooling via direct conduction is the most efficient and thus the quickest, but it also falls in the range that requires the least speed for the steel. In fact it is actually undesirable speed because the steel is now undergoing a transformation that is not time dependent at all, and the faster cooling can stress things a bit too much.

Products designed specifically for quenching have these cooling curves carefully engineered into them. Canola oil, various mineral oils, automotive products etc… were not made for quenching and so do not have the same chemistry adjustments to match the desired curves exactly. I always tread very lightly here because people can get very defensive about this topic and can really resent hearing that a favorite fluid may not be the best tool for the job, but these are just the facts. A good quench oil is designed to suppress the vapor jacket formation and the stability of the same for much faster cooling but then has the ability to slow the cooling curve at the lower end when the steel needs a gentler quench. Water, for example, is about the worst by doing the exact opposite, it has a massive and very stable vapor jacket and then cools incredibly fast in the final phase; it is in this phase that people hear that dreaded “ping of death” sound when quenching into water. Canola oil has a much lower vapor jacket formation and stability, on par with some quenching oils in fact, but does not slow down on the final cooling, it also degrades very quickly in use.

I still remember well the first time I did an interrupted quench with a well-made quench oil. I counted off the number of seconds I thought would work, noticed that the vibrating boil was completely done and whipped the blade out to grab with my gloved hands. I didn’t hold it very long! With smoking gloves I began to ponder the new situation. This oil was done boiling at a much higher temperature and would also take a few more seconds just to reach 400F. It was then that the reality, that this tools properties had been made distinctly different to do this job, kicked me right between the eyes.

The interrupted quench is not a true marquenching operation, but is the next best thing. It approximates the marquench by using a normal quenching oil and extending the direct conductive cooling phase much longer by replacing the oil with air. I may have used #50 with an alloy steel in an extreme pinch at a demo, but I don’t remember it and if I did I was desperate. This is because I would not want to leave the impression in one of my talks or demos that one can match deep hardening steels with fast oils. Not necessarily that it is impossible to do, just it is not optimum. My personal philosophy after twenty years of teaching bladesmithing is that the learning curve is steep enough even with optimal tools and supplies, without having me lead you down bumpy side tracks.

Now all that being said, you can, in a pinch, alleviate some of the negative effects of a fast quench oil on an alloy steel by interrupting, but the timing will need to be worked out and well done and it would be best on thinner and simple blade cross sections. I guess if I had to heat treat a 52100 blade and all I had was #50 I would go for the interrupt, the signs of given temperature from the oil are not steel dependent and so they would be the same. The point at which you want to interrupt could be different depending on your soak procedures, since the hardening heat will determine the amount of carbon in solution and that carbon in solution will determine the temperature at which the blade will begin to harden via martensite formation. But I think the 400F degree mark would still be your safest bet.

Thanks for answering this Kevin, i was hoping you would. And i didn't mean to suggest that you condone a fast oil quench for these alloy, the time in question was indeed a demo and#50 was all that was there and you made it very clear that it is s bad idea, just not impossible.

As for the process here. I got a mystery oil from a friend of mine about 20 gallons that he said was part of some closed down machine shop inventory sell off or something. Ii ran some 1/16 slabs of 01 5160 and 10xx thru it for full quench from just non magnetic. The first two come out screaming hard but it seemed to take a long while for the quench to finish and the 10xx was just as soft as butter. Lead me to believe it was a slow oil. So if i run a few tests and find where and when the 400 degree interrupt comes in at from a1500 quench, i should be in good shape. Thanks again. I'll post my findings.

Jesse Bartram

The 1/16 1095 hot hardening tells a lot about what you are asking. For me the ideal way to configure these test coupons is to make a blade shaped wedge cross section the tapers from 1/16" to 5/16" and then cross section them for Rc testing on the ends. You could do the same without the trouble of the wet saw etc... by cutting a sharp notch around the outside before quenching and then breaking the piece. Although, if the quench was a complete failure things are not going to break, but then I guess you would also have an answer as well.

Alright, Thank you for the control configuration. I will try this hopefully this weekend. I will probably have to send the pieces down to Jims to Rockwell them, but i should be able to garner so info from grains and types of fractures i get i imagine. Thanks Kevin.

Jesse