Part 1 Articles in knife magazines and discussions on the Internet concerning knife blade steels are getting pretty technical these days. The problem is that many terms are used with the assumption the reader is familiar with them. This article will address the mechanical properties of blade steels in general. Starting with that background we can apply the definitions outlined herein to illustrate how certain properties pertain to the selection of blade materials for 3 different knife types. The following 6 properties are important for knife blade steel: STRESS AND STRAIN, TOUGHNESS/DUCTILITY, STRENGTH, HARDNESS, RESISTANCE TO WEAR, CORROSION RESISTANCE STRESS AND STRAIN All materials have a stress and strain relationship. Stress is the force applied to a member and strain is the distance that the member moves (deformation) under the applied force. Imagine a rubber band stretched between your two hands. The force applied to move your hands apart shows up as a resistance force (stress) in the rubber band. The distance the band moves under the force is the strain. If the stress and strain for the band are plotted on a graph a line is drawn representing the behavior of the band from the initial load to the time it finally breaks. This is called a stress/strain curve. This curve is unique for all materials and gives a vast amount of information on how the material acts under applied force. Steel acts like a rubber band up to a certain point. That is, it has an elastic range. An automobile spring, a folder spring, a skyscraper swaying in the wind and a very flexible fillet knife all are working in the elastic range. If force is applied beyond the elastic range then it enters the plastic range. In this area the material acts more like a soft plastic, hence the name. The material will yield under the force applied but when the force is removed it won't spring back completely. It has yielded or in the case of a knife blade, "bent". Most knife blades are made from a grade of steel called cold work tool steel. This is the same material used for dies, bearings, and some types of machining cutters. We all know from experience that tool steels in general have a limited elastic range, and a narrow plastic range. They will "flex", take a small permanent bend and then fracture suddenly. This property is called bending fracture strength. We see an example of this every day in the knife shop working with drill bits, taps and milling cutters. TOUGHNESS/ DUCTILITY When a material can absorb forces from many different load types with out breaking then it is very tough or ductile. Tool steels tend to exhibit the opposite characteristics. That is they are elastic up to a point but have a relatively narrow ductility range. The high alloy types, especially those with carbon over about 1.0% and chrome above 5.0% at high hardness (over HRC 57) are moving out of the ductile range and are approaching brittle behavior. This is most apparent under dynamic or impact load conditions. Impact loads magnify the force that the steel feels and can cause large failures very quickly. These steels are also notch sensitive. They will fracture at a much lower load if an imperfection is present. This can be in the form of a deep scratch, corrosion spot, small crack, or sudden change in cross section. Steel company data sheets refer to toughness in terms of "Charpy C Notch" values. This is a measure of the ability of a notched test piece to resist breaking under an impact load. An example of an impact load is a lawnmower blade spinning at high RPM and hitting a rock, or worse yet, an ATS-34 blade (HRC 60) used to try to chop through a bone. The lawn mower blade (low alloy and soft) is going to get a pretty good dent but the knife blade is probably going to suffer a local break or "chip" out of the thin edge. 2 STRENGTH From the above discussion you might say the knife blade was not very strong because it ended up with damage. However the opposite is true. Tool steel heat-treated to the high hardness necessary for a knife blade is very strong. It can be about 10 times stronger than mild steel. The ability of a steel part to withstand force trying to pull it apart is called tensile strength. The ability of the part to withstand force trying to push it together or compress it is called compressive strength. ASTM A-36, structural steel, or mild steel used for general construction has a tensile strength of 36,000 pounds per square inch. Tool steel at HRC 60 has a tensile strength of about 350,000 pounds per square inch. Note that the ability to take load is related to the cross section area of the piece. The more material the greater the load capacity. For example a two square inch (1 inch x 2 inch) bar of mild steel will withstand 72,000 pounds in tension and a similar size piece of tool steel can withstand about 700,000 pounds in tension. The two steels have a very different stress strain curve (figure 1). Mild steel is very forgiving. It will plastically yield a large amount before breaking therefore avoiding a catastrophic structural failure. Tool steel has to be very stiff and strong because it must endure very high loads at the cutting interface between a tool and the part machined. When it breaks the fracture is usually very sudden. To put it another way mild steel would make a poor drill bit and tool steel would be a poor material to use for a highway bridge. Strength is a very important consideration for a knife blade because we like to make the cutting edge as thin as possible for cutting efficiency. A very high strength material like tool steel is necessary to prevent the very thin edge from yielding under load. Good toughness is necessary to prevent the thin edge from breaking off under bending or impact loads. HARDNESS The ability to work in the soft or annealed form and then to be hardened is one of the most important properties of tool steels. Steels are delivered from the mill in the annealed form. This allows the knife maker to cut, drill, file, bend and grind the steel. . In the annealed form it acts somewhat like the mild steel described above. To exhibit the positive properties of tool steel it must be heat treated so that the final crystal structure of the steel is changed. The tensile strength of steel is proportional to its hardness. The harder it is the more is resists deformation forces. In other words the edge will resist bending or breaking while cutting some very hard materials. Hardness is the most critical property for a knife blade because it is an indication all the other properties. It is the most convenient property to measure. A hardness tester forces a cone shaped diamond penetrator into the sample. The depth of penetration is measured very precisely and is read on a calibrated scale. Hardness for tool steels is usually given in reference to the Rockwell C Scale, abbreviated as HRC. The normal range for knife blades is HRC 50-63. Knife making steels have a "sweet spot" hardness range in which they will perform best. At this hardness all the properties are balanced to provide the best overall performance. Very good steel that is heattreated out side this range will not perform up to expectations. WEAR RESISTANCE Abrasive wear resistance (most important type for a knife blade) is a measure of the tool to resist being worn away by contact with other materials. Wear resistance correlates with hardness of the material in general. Even mild steel in contact with an abrasive surface will wear much longer than brass for example. In that case mild steel will make a better knife blade than brass. It gets more complicated with high alloy tool steels because of the carbides present in the material. Steels like O-1, A-2, 52100 will all have about the same wear resistance at equal hardness. The addition of chrome in steels like 440C, 154CM, ATS-34 and D-2 adds another dimension. During the initial melt the chrome combines with carbon to form chrome carbides. These carbides are harder than the iron carbides present in the lower alloy grades and will therefore add some abrasive wear resistance to the cutting edge (see figure 3). Imagine the carbides as the aggregate (rocks) and the 3 surrounding steel matrix as the cement in a concrete road surface. The cement wears away over time leaving the hard aggregate as the road wear surface. The harder the rocks the better the surface will resist wear over time. The same thing is true for a knife edge. The harder the carbides and the more densely they are packed the better the edge will resist wear. This is carried a step further with the high vanadium, high carbon CPM (Crucible Particle Metallurgy) steels and to an extreme with high-speed tool steels. High-speed tool steels like M2, M4, M42, and T15 will resist wear even when subjected to the red-hot temperatures at the cutting edge of a metal cutting tool.