There is a thing about oil hardening steels that is deceptive and can lead us to think we got a good hardening when things may not quite be right.

L6 is kind of similar to O-1 in its tendency to harden in air, even though it is an oil quenching steel.  L6 is capable of reaching 62HRC by just air cooling.  When my lawnmower blades wear out, I like to weld in L6 inserts to make them many times more durable.  When I first started doing this I thought that letting that L6 air harden would be a whole lot easier than a full heat treatment on the blades.  The oil quenched blades allowed me to run over just about anything and continue mowing, they were great, but the air quenched L6, despite 62HRC before temper would simply chip, crack and come apart.  Why is this?  Upper bainite.

The problem with oil hardening steels is that they have enough alloying to suppress the formation of pearlite allowing them to be easily hardened in oil.  But there are phases that form after pearlite that are also undesirable and in oil they are defeated, leaving maximum austenite to convert to martensite (the fully hardened phase), but with slower cooling in air these phases have time to form and give you a mixed microstructure.  Actual air hardening steels have even more alloying to suppress the other phases so that you get a true hardening from them.

If optimum results could be had from simply air hardening oil hardening alloys industry would have loved to get rid of oil in the process a long time ago; indeed O-1, and other oil hardening alloys are being phased out, more and more, in favor of richer air hardening steels so that shops do not have to deal with the mess and health issues of oil quenching.

In short, it may Rockwell good, it may seem hard and cut as good as some other knives, but when you cross section and have a look at the inside of a steel that was quenched short of what it was designed for, you will find less than the homogenous condition that would have optimized its properties.       

Thank you Kevin, that's good to know. You mentioned Bainite, upper Bainite. I would love to achieve Bainite, but what is "upper" Bainite ?  Can I get O-1 to bainite w/o using salts ?

Secondly, Kevin, this may explain something weird that happened during another H-T session. After normalizing an O-1 blade, I noticed the tip was slightly bent. I thought it would be soft and was still quite warm, so I chucked it in the vise and gave a small tug to straighten. It snapped very easily and had a course grain. Is this what you are talking about ???

Yes, I normally avoid using the “B” word because it tends to derail discussions about desired knife properties. There is a lot of hype heaped upon bainite among knifemakers. What we have to remember is that there is no such thing as adamantium or unobtanium, the universe likes to make it so that there are always trade-offs in properties, otherwise mankind could have settled on one super material long ago.  Along with the great expectations comes the lack of other fine, but important, details about the reality of the phases above the Ms point. 

First is that there are two types of bainite, there is upper bainite, which forms from around 850°F to 600°F, and then there is lower bainite which forms from 600°F down to Ms (martensite start).  Upper bainite is miserable stuff, it lacks all the properties you want from a blade.  Lower bainite does have greater toughness but tends to hold the same HRC values related to tempering at the temperatures it was transformed at.  This means that for properties giving us excellent long-term cutting performance, martensite is still at the top.  Bainite may make a pretty good machete, or fencing foil, but I would have no use for it in a hunting knife or chef’s knife.   

Generally, knifemakers seem preoccupied with inordinate levels of toughness in their blades, but too often at the expense of qualities that lend to the very definition of what knife is.  High strength stable edges that allow the tool to cut is essential, performing like a spring or jackhammer bit should be secondary, if even worth consideration in most instances.

But say we are making a machete like camp knife that will require more toughness, there is actually a much easier, and more effective route to that goal than exotic heat treatment, and that is steel selection.  O-1, W2 or 1095 have chemistries that make them naturally form high hardness abrasion resistant blades, if we are looking for toughness, we are starting in the wrong place, and will have to resort to creative heat treating to force them to behave in ways they were not designed.

But if we start out with L6, 5160, 1075, or even 80CrV2, wea rea already on the tougher side of knife blades, before we even begin heat treatment, and then straight forward standard heat treatments for those alloys will give us exactly what we want.

Taking O-1 to a bainitic structure would be counterproductive as you would be defeating the properties it was designed for and thus not get the most for the properties you were aiming for.  If you wanted to really maximize bainite effects, starting with something like 5160 would be much better.  But to accomplish austempering effectively timed, multi stage quenches are best to avoid undesirable phases.            

Well, the grain size would have contributed to this significantly, but if the blade tip was still hot when it snapped it is a very good illustration of how mixed phases are not always good, especially when one is not stable.  During martensitic transformation there are load of crazy things occurring inside the steel, very stressful things.  Whole planes of iron atoms are tilting and shearing past each other at the austenite to martensite transformation interface, and it is happening very fast. The resulting distortion of the lattice creates incredible amounts of strain effects.  During this process the steel can easily come apart if acted upon by outside loads.   I have pulled my share of steel apart with my bare hands during the martensite transformation, and have also found that it will not tolerate sudden loading very well at all.  Just above the transformation it is like putty an you cannot break it, and after the transformation is complete it is brittle but surprising strong, often requiring impact to break.  But during the transformation (from around 400°F to 200°F) it is best handled gently.  You can carefully push it straight but don’t really crank on it and you really don’t want any sudden loading.             

Both of your answers have taken a big load off my mind. Thank you for such thorough information. This has kept me from going down a wrong road.