What little I know about blade making metallurgy

[Paracelsus]

I like forged knives because I think there is something mystical about a blade made by melting a lump of steel and pounding it out with a hammer on an anvil to near the final shape and profile. I think such a blade carries with it something of the Heart, Soul and Intent of the maker. But it is also a science. Experiments are conducted and the results guide future questions and work. What follows my simplistic way of describing blade metallurgy.

After forging to shape, the blade is ground to some degree to create the profile and edge. Next comes the heat treatment in which the blade is heated in the forge to about 1800 degrees F and suddenly quenched (cooled) in water, or oil, or sometimes even liquid nitrogen, depending on the particular steel alloy being worked. This forces the steel to crystallize into a material known as Martensite. This is the really hard stuff. It will shatter like glass if struck on a hard surface and is completely unsuitable for using knives.

Different alloys of steel containing varying amounts of iron, carbon, silicon, vanadium, magnesium, molybdenum and chromium have varying potential to form different sized particles of Martensite. The size of these particles influences the abrasion resistance of an edge. The matrix in which these crystals are imbedded determines the relative strength and brittleness of the steel. The rate of cooling is the predominate factor in forming the crystallization pattern of the steel. Slower is softer and faster is harder.

In the Tempering Process the blade is heated up only a few hundred degrees and melts down some of the Martensite, leaving a much more stable composite material containing some really hard granules surrounded by more pliable stuff. Ideally, the edge is left Really Hard and the back of the blade and the tang are tempered (low heat) to a shock resistant and flexible state. This produces an "optimal" balance of edge hardness (resistance to wear) and brittleness (causing the blade to crack or even shatter like glass when struck).

Sometimes the tempering process is performed in reverse as done by the ancient Japanese and others, using different thickness' of clay coatings on the blade to cause differential heat retention during the initial quench, producing different rates of cooling and therefore predictable crystallization patterns in the metal in different parts of the blade. This often results in visible "temper lines".

Do any of you makers out there have any comments? What have I got wrong (I'm sure a lot)? I would greatly appreciate makers comments to another thread I'm posting on the custom forum: Knife Making Metallurgy: Science or Mysticism? I didn't really know which forum was most appropriate for these two topics. I think I'll post this in the custom forum also and post a link in the shop talk area. Thanks to all who take the time to read and reply.

Paracelsus

 

[Ed Caffrey]

I'll give you my input, just remember, opinions and methods will vary greatly from person to person. This is just what I do.
Experimentation is the key. I have read all the heat treat books, have tried their foumulas, and now have very little faith in any of them. The blades produced with their formulas where good, but not even close to what I have achieved by taking their information and going to the next logical step. Most who are "hard core" bladesmiths spend as much time experimenting to increase the characteristics in a blade that they desire, as they do with filling orders. (at least I do)
The steps that I take are numbered below:
1. Forge
2. Normalize (stress relieve)
3. Anneal (refine grain structure/soften)
4. Grind (to finish shape)
5. normailize (relieve any grinding stress)
6. Harden (to take to it's hardest state)
7. Temper (to reduce hardness to a "working level")

Possibly one of the most important things to understand (the science part) is what each of the elements present in a given steel will do for the finished product. They must be understood in how they will react alone, and in combination with other elements.
With a standard oil quench steel will form a combination of austinite (an end to end plate type structure) and Martinsite (an overlapping plate type structure) There are a variety of other structures that can, and do form, but with standard methods, these are the most common. When hardening a given steel the idea is to get all elements into "solution" (evenly distributed throughout the steel) and then hardening "freezes" them in place (for lack of a better term)
Grain sturcture plays a more important role with blades than hardness alone. A very fine, tight grain structure is essential to strenght and durability. This is achieved through thermal cycles, and varies greatly from maker to maker.
Tempering is nothing more than reducing the hardness to a workable level. This is why you hear of so many different Rc levels recommended throughout the knifemaking community. Again, personal preference is the key here. Although most would agree that a blade edge should be hard enough to hold up under extreme duty, yet soft enough to allow it to be supple and easy to sharpen.
The clay method of heat treating is nothing more than using clay to form a "heat sink" that insulates a portion of the blade from reaching critical temp (hardening temp), in many ways it is very similar to "edge quenching" a blade, it's just another method to achieve a given end. The "temper line" seen on these types of blades are simply a boundary line where steel transitions from harder to softer zones. The visibility of these lines relies more on the variety of steel utilized, than on the method of hardening. As an example, 1095 is difficult to achive bold temper lines on, however, using the same method with a 1050 steel will provide a bold/brilliant temper line.
I truely believe that there are as many ways to achieve a given end, as there are knifemakers, and that's what makes us such a unique group. Over time you will find that those who endure as knifemakers will have there own individual ways and methods, as well as a distinct and appealing style, not only with their product, but with the ways that they openly give and recieve information. Above all, they will also be those who seek constant improvement in what and how they produce their knives.



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Ed Caffrey
"The Montana Bladesmith"

 

[raker]

That was well said Ed. The experimentation is also another good reason to work with a few steels and if possible, steels from the same batch. If you don't know for sure what your are working with, it is hard to get the most out of it the first time. Ray Kirk

 

[magnum .44]

about the tempering, authensite( hope i spelled that correct) is the brittle stuff. see you bring the heat to the authensiting temp, then quench, turning the authensite into martensite. after the blade is a room temp, there is still remaining authensite, you get rid of this by stress releiving the blade. then you temper, and bring the blade down to a specified hardness. i have my blades heat treated by rob simonich, and he does a fantastic job.
i use the stock removal method, becuase i don't have my forge hooked up to gas yet. there is also some mysticism in that, even though it's hard to believe. while grinding your blades they carry a part of your heart and soul. i beleive this to be the better way, or at least the way for me to do this, becuase you can use stainless, and tool steels, even high speed, and super alloys, and talonite too. i have attempted to forge blades, with other peoples forges, and i don't like the forging route. although, when i do get my forge running i will attemp making damascus, and mokume.

[This message has been edited by magnum .44 (edited 27 November 1999).]

 

[Paracelsus]

Having seen a couple of old threads brought Back To The Top recently, I thought I would see if this thread can gather a few more good responses. I'm hoping some of the new Maker members will contribute their knowledge to this thread. I would be especially grateful if Paul Bos could take the time to enlighten us ignorant folks here at BFC.

I think the ideas I'd like to be discussed should increase folks awareness that there Is No Ideal Steel. There is Always a balance of factors at play in the making of blade steel. What happens to the steel while forging, annealing, normalizing, hardening, and tempering seems to be MUCH more important in determining the functional characteristics of a blade than exactly the alloy composition of the steel.

I really appreciate Ed Caffrey's, Ray's and Magnum's posts. Any others?