Table of Contents:
- Why you need to know
- What is Steel
- What is carbon steel
- Affects of Carbon
- Carbon Steel Review
- What is Stainless Steel
- Stainless additives (alloying elements) and their affects
- Stainless Review
1. Intro- In blade metals #1 we discussed the comparative “real world” properties of carbon steel to stainless steel. For this second installment we will be looking into the metals on a more atomic level, and lightly covering what causes these metals to be different from each other, and how those differences affect their usage as knife steels. Keep in mind we are only scratching the surface of metallurgy. Furthermore, we will do our best to keep the text strictly pertaining to knife metals.
2. Why do we need to know about this stuff? At some point in our lives we all want to learn more about the things that interest us, and we discover rather quickly that we in fact knew a lot less than we thought we did. “the more you learn the less you come to know” type of thing. If knives interest us we owe it to ourselves and the knife industry to have a basic understanding of what makes them tick. You will thank yourself in the end having purchased tools that best fit your needs. The following is a simplified introduction to carbon and stainless knife steel.
3.1 Adding carbon to iron allows us to harden the resulting steel, so that we may manipulate the steel into useable tools such as knives. Other common alloying elements include manganese, nickel, chromium, molybdenum, vanadium, silicon, and boron. Each element adds it’s own characteristics like toughness, hardenability, wear resistance, corrosion resistance and so forth.
Adding each of these elements to iron will result in a different recipe. There is always a trade off though. Too much of any one ingredient affects the entire recipe. Too much carbon (usually over 2%) makes the steel less ductile (more brittle).
Knife steels, to be good performing for your needs, need to fall into a certain “recipe” range. It would be a poor choice to choose a 1050 medium carbon steel for a small everyday chore knife because you would be sharpening it all the time. This does not make 1050 a poor steel, it simply means it would be better suited to heavy duty chopping work as an axe.
3.2 Affects of Carbon- This is only small sampling of available carbon steels.
1095- The 10xx series of steels are considered simple carbon steels. Simple because they only consist of a few alloying elements; manganese, phosphorous, and sulfur. The last two digits, “95” in this case, denotes the carbon content. For example, here is the AISI (American iron and steel institute) property chart for 1075. (courtesy http://www.azom.com)
You will see iron makes up 98% with carbon contributing only .75 percent or (75 points). This represents of very small precentage, but is critical to the hardenability of the steel. The higher the carbon content the more brittle the steel becomes when hardened. Less carbon content and you lose edge holding capabilities. 1095 is usually used for knives whereas the lower numbers like 1050 are used in larger blades like swords.
3.3 Review: Like most things, everything comes at a cost. Generally, as hardness increases, toughness decreases. Often the lower the carbon, the higher the toughness. Toughness on the other hand is the maximum amount of energy a material can absorb before fracturing, it can also be thought of as the opposite of brittleness.
4. Stainless steel- Chromium content dictates whether a steel is classified as stainless or not. Knife steels generally have around 13-15% cr. Any amount under 13% usually triggers debates of the steels validity in the stainless family. ASM (American Society for Metals) says “STAINLESS STEELS are iron-base alloys that contain a minimum of about 12% Cr, the amount needed to prevent the formation of rust
in unpolluted atmospheres (hence the designation stainless).” (Courtesy of http://www.asminternational.org/documents/10192/1849770/06940G_Chapter_1.pdf).
Generally, as chromium increases it will contain more chromium carbides, which tend to make sharpening these blades more difficult. Some stainless steels may have many elements added to boost performance or alter its characteristics. (see below)
4.1 Alloying elements of stainless steel-
(courtesy of http://www.smt.sandvik.com/en/products/strip-steel/strip-products/knife-steel/knife-steel-knowledge/important-factors/chemical-composition/)
4.2 Review: As with carbon steel- the Carbon element also plays a very important role in stainless steel, but as seen above, a multitude of other alloys are used to alter the final characteristics, to best fit the needs of each maker and user.
Once we move past these basic concepts we get into the more involved specifics like; solutions of steel, phases of steel (ferrite, austenite, martensite, pearlite…), heat treatments, time temp transformation curves, quench mediums and so forth. I will not be covering any of that until I feel confident enough in my knowledge to explain it.
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