edge holding and toughness on...

[the possum]

On many of the smaller knives I used around the farm, just plain old strength (hardness) was usually the limiting factor to "edge holding" rather than wear resistance.

 

[dustfurn]

Obviously those elements heavily affecting impact toughness would be important to know. The problem comes in with methods of heat treatment. If you end up with grain boundaries laden with carbides the impact strength is going to be in the basement for even a fairly tough steels, until you fix that issue, also the morphology of plate instead of lathe martensite is going to substantially lower impact values.

Very many of the operations that bladesmiths are fond of will result in this in 52100 or 1095:

This will make these steels have the lowest impact values regardless of the hardness.

While it is beyond the scope of most guys not using very tight temperature controls, it is also possible to dissolve only enough of the carbon in steels above .8%C to achieve hardness and leave the rest in the form if very fine carbides. If this is done in solution amounts from .6% to .8% you can effect Ms enough to eliminate more plate type martensite in favor of lathe type which will greatly enhance impact strength. Thus if done correctly one could get much of the abrasion resistance from 1095 and still have it handle impact like it was 1084, all depending on where you put the carbon with the heat treatments.

Plate martensite forms at lower temperatures under greater strain and at irrational habit plane angles so that the plates impinge upon each other like chunks of ice in a jammed up river which can cause microfracturing, like this:

Lathe martensite forms at higher temps under less strain and in more orderly packets like this:

Thus it is much "tougher". Ms is affected by chemistry and carbon in solution is a major chemical factor. So one can adjust it up or down by mastering the ability to determine carbon in solution.

I like to bring this up to help illustrate that I am not just over thinking things when I stress what exacting controls in heat treating can accomplish. Guys who feel I may be disrespecting the idea that heating to non magnetic and dunking in oil is "good enough", need to understand that what I am really saying is that there is a whole world of limitless possibilities they may never even be aware of if they simply settle with basics of what appears to work. This stuff is so cool and fascinating that the exploration and experimentation and be endless even after you remove many of the unknown factors.

Thanks for the informative post.

How do you avoid the situation visualized so well in the first photo (carbide-laden grain boundaries) in hypereutectoid steels (something like 1095 or w1)? If you suspect that you have induced such a situation in your steel, how would you go about rectifying this? If this has been described in a previous post, i apologize, but does anyone have a link?

Thanks again for the great info. This kind of discussion is why i love reading bladeforums!

Dustin

 

[69_knives]

http://www.navaching.com/forge/steels.html

I stumbled upon that page while looking up some information for forging 01, looks like O1 has a good mix between edge holding and toughness between the low alloy steels.