Shallow hardening steel?

[Martin S.]

Forgive my ignorance but what does the term "shallow hardening steel" mean? I've seen 1095 and 1084 described that way but really don't know what it means.

 

[richard j]

google shallow hardening steel and take your pick

 

[Stacy E. Apelt - Bladesmith]

In the thicknesses of knife blades, it really doesn't have any relevance.
It refers to the trait of the steel to harden only a certain way into a block, leaving the inner portion a softer structure. As the "skin" hardens, it shields the inner portion. These steels have faster cooling rates to create martensite. In a six inch casting it matters a lot. In a 1/4" blade is will harden through the entire thickness if the quenchant is right. W-1 and 1095 are the main ones that have to be quenched carefully to get the best hardening. 1084 is a bit more forgiving. Steels that harden at a slower rate (alloy steels) are referred to as deep hardening.
Stacy

 

[mete]

Where it does matter in blades is in the making of a katana where the spine is clay coated .There you need low hardenability and would use a 1050, 1070 or 1084.Hardenability also tells us what quenchant to use .Lowest [shallow ] hardenability use water .then oils and finally for the highest hardenability it will harden in air.The test to measure hardenability is the Jominy End Quench Hardenability Test !

 

[Martin S.]

Thanks guys. That answers the questions I had .

 

[Kevin R. Cashen]

Perhaps a brief explanation of the Jominy end test would further clarify things. In the test a cylinder approx 25mm (1 inch) in diameter of the given steel is heated and then quenched by being subjected to a water spray aimed directly at the bottom of the piece. The specimen is then cross sectioned or ground back and hardness tests are done to determine the depth of hardening from the surface to the interior. This is what gives us our concept of "shallow hardening" versus "deep hardening", which is really a much better descriptor than "water" hardening Vs "oil" or air hardening because effective quenchants are entirely dependant upon cross section size. In thickness less than 3/16" the term "water quenching" becomes irrelevant, or even disastrous if rigidly adhered to.

L6 is a deep hardening alloy that could reach maximum hardness throughout a 1" diameter piece even with slower oils. 1095 is a shallow hardening steel that would struggle to harden to the center of a 1" bar even with gentler water quenches. The #50 oil that is all the buzz right now for its speed should be able to harden 1095 or 1084 blades up to a 3/16" thickness, above this there will most likely be varying degrees of pearlite in the mix. Slightly slower oils will reduce the hardenable thickness much more. This is why one will get natural striking hamons on 10XX series steels when quenching into oils even though no clay was applied; as the thickness approaches the maximum for the steels cooling curve, the pearlite start to appear.