Reply To: Hardness of steel rod…

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@Bob Tascione wrote:

Now that’s a really good question!!

There will be a lot of space between the points I’ll mention here that can be filled with technical and complex explanation but I’ll leave that to someone that knows what they’re talking about.
There are commercial quenching oils available which work great but many trades people use different types of ‘home’ oils depending on the type of metal, complexity of the part and desired hardness. These oils range anywhere from say standard motor oils, different types of vegetable, linseed, mineral, fish, paraffin and fuel oils like kerosene etc. etc. Some of these oils will harden more than others. It depends on the specific heat of the oil. The higher the specific heat the harder the results. Oil quenching will produce a better balance between toughness, hardness, and warp-age than the brine used in the videos or plain water. Brine will produce the hardest surface with water following close behind. Both brine and water will leave a fairly clean surface. Oil will not harden as much as it doesn’t permit the metal to cool as quickly as the saline or pure water. The faster the cooling the harder the results. With rapid cooling comes the danger of cracking or shattering which is especially true with parts having sharp corners, shoulders and edges. Complex parts tend to warp much more with brine or pure water quenches than with oil. Also some metals are more suited to water quench and some to oil or air. David and I used to do our own heat treating in the tool rooms heat treating depts. These were highly controlled environments. We would always know the upper and lower critical points that a particular metals temperature needed to fall between. Under the lower critical point and no hardening took place after quench, a little above and results were unpredictable. We would normally take it right to the upper limit to allow for slight cooling during the transfer from oven to quench. The ovens pretty much did all the work. After setting them up you just waited ‘trance like’ until the parts reached the required temperature. There are times when it’s much more complicated than this as David mentioned in an earlier post. Some metals require vacuums and gases etc. but for normal heat treating of metals like 01, 52100 etc. it was pretty straight forward. I can’t cook a turkey without blowing it up so that should tell you something about how easy standard heat treating for manufacturing really is. Then there’s the way real people do it in ‘real’ life. That’s knife and sword makers, blacksmiths and home machinists and backyard inventors. That’s us. That’s more of an art form. Very enjoyable. We determine temps by testing and heating the metal and then watch the ‘heat rainbow’ change from one shade to another until we reach the color that will suit our needs then quickly quench and agitate the part in a witches brew of whatever we determine will produce the results we’re looking for. Maybe we dip it into a powder or paste of boric acid to keep scale from forming then wrap it up in iron binding wire to prevent warp-age. That’s the magic of it all. More of a dance than a trance.
So, what type of oil should you use? What do you have in the cupboard that you can sneak past the cook? :)

NOTE: CAUTION…any oil can ignite when a hot part hits it so keep an eye out. Also bigger parts make bigger flames!


I have a former student and now buddy (yea I know that’s rare) who makes swords, knives, and armor doing the blacksmith thingy, so I think I’ll pick his brain about how he heats he metals. I do know that he has a home built furnace that is fired by propane. Anyway – every time I talk to him I get the feeling that what he does is more alchemy than science. He just made a batch of mead (he has bees as well) so I think I’ll drink some of that. It’ll all make sense then. :D

This is really helpful!

tmac1956Reply To: Hardness of steel rod…