Hi A druid!
Swamp Rat Knifeworks and occasionally Northwinds and Bark River use 52100, too. It's a very nice steel.
Marquenching is the process of rapidly cooling heated steel from its austenitic crystal shape (the microscopic crystals change shape, size, and number depending on times they're held or moved to certain temperatures) to the very starting point of the temperature range wherein it would change to martensite. Austenite is a soft and fluffy crystal shape and martensite is harder, more needly, and full of built-in stresses (many of those stresses are relieved by tempering the steel so the closely-packed shape of the martensite can relax and spread out).
By dropping the steel's temperature to the hottest starting point at which martensite occurs and holding it there for a pre-determined amount of time, the steel (in our case, a Spyderco Muleteam Knife! :thumbup: ) can transform into hard and tough martensite with less internal stressors threatening to make it brittle. You can also use marquenching just a hair above the martensite starting point and form a similar crystal shape called bainite. There are two types of bainite and knife-users are interested in the tougher type called "lower bainite." Marquenching can allow for either an almost entirely martensitic blade, entirely bainitic blade, or mixture of both crystals (often called a duplex structure blade - though these days, condex seems more relevant...).
Changing steel crystals from austenite into bainite can make the steel more impact-resistant and more flexible at a given thickness, but at the expense of hardness and wear-resistance. To combat those negatives, folks such as Randal Graham, Bob Kramer, and BFC's own Galloglas use marquenching to produce a mixture of martensite and bainite in their blades. Howard Clark, and maybe some others, go as far as making the spine portions of their knives bainitic while making the edges out of pure, delicious martensite. For more info on bainite, look for free, online stuff written by Dr. Harry Bhadesia of Cambridge University. Also look for critiques of said steel crystal by Jerry Bennett and Kevin Cashen.