steel for tools, reading the tables
Bill Tindall, E. TN
>I composed the following to help people interpret data they might find on tool steels. It is not meant to be a guide to steel selection as I don't know enough to write such a guide and the more I learn the more I know I don't know. But as more people poke into this matter we will eventually learn to optimize steel selection just as the knife makers have done. The pictures in the Brent Beach article referenced in a previous sharpening post should encourage people that there are significant improvements possible in the steels used for our tools. The convenient and cheap sources of steel and heat treating make any of these steels available to us for experimentation.
A variety of steels are used for woodworking tools and an even greater variety of steels are available for those that might want to make their own tools. Data are available on the properties of these steels. For example, Crucible Service Centers provides comprehensive data on many steels, www.crucibleservice.com. Manufacturers of steel usually provide data on hardness, toughness, wear resistance and red hardness. How can woodworkers use this data to choose the best steel for a woodworking tool?
A woodworking tool cutting edge can fail in use by crumbling, chipping, and/or wearing. All these factors dull the edge or in extreme cases might result in breakage. Therefore, steel properties that affect these factors are of interest. Red hardness is only relevant for steel used at very high temperature, for example cutting metal, so it is not a property of interest to woodworkers.
Hardness is a measure of the steel's resistance to deformation-bending or denting. This hardness is measured with a Rockwell Hardness scale, abbreviated Rc, which uses a diamond point to test how easily the metal dents. Diamond has a Rc hardness of 100, chisels and plane irons are about 60 and tools that can be filed such as saws have Rockwell hardness in the low 50's. The edge of a harder chisel will be less prone to bending or crumbling in use than one that is less hard. As will be discussed below, steels with similar hardness may differ significantly in wear resistance and brittleness.
Toughness is a measure of steel's brittleness. So, for example, the cutting edge of a tougher steel will be less prone to chipping or even breaking than one less tough. Hard steels can be brittle. Dulling can occur as a result of tiny chips at the cutting edge if the steel is not sufficiently tough. Ideally, a steel will be both hard and tough for best edge retention.
There are many kinds of wear and not all of these measures are relevant to woodworking. The wear reported most frequently, for example Crucible Service site Tables, is called adhesive wear. This kind of wear occurs when two metals rub together, for example in gears, and it has less relevance to woodworking tools. Abrasive wear is a measure of wear resulting from abrasive particles rubbing on a metal. Abrasive wear is a good measure of how difficult a metal is to sharpen or how fast it might wear while rubbing on wood. The Rockwell hardness is not a good measure of wear resistance. For highly alloyed tool steels the amount and hardness of the alloy carbides, or example vanadium or molybdenum carbide, best predict wear. Vanadium carbides formed during heat treating are greatly harder than the iron carbides in a simple carbon steel. Therefore, a simple carbon steel with hardness Rc 60 will be less wear resistant than a Rc 60 steel that is alloyed with vanadium. It follows that a more wear resistant steel can be difficult to sharpen unless a very hard abrasive is used, for example diamond. Another kind of wear is called chemical wear. A better description could be corrosive wear. In this case, chemicals in the wood, plus heat and air, change the composition of the cutting edge surface. The softer products formed are more prone to abrasive wear. Although abrasive, and possibly chemical, wear properties are most important to woodworkers, I do not know where to readily find these data for tool steels. Adhesive wear may provide some indication of these other wear properties but I think I have seen a case (CPM 3V) where the abrasive wear of a steel is greater than the adhesive wear value would indicate.
No steel delivers exceptional hardness, toughness and wear. Tradeoffs are made depending on which property is most important for an application and personal preferences. Wear resistance might be most important for a lathe tool, so a steel high in vanadium would be selected. But, this steel would be too brittle for a tool subject to impact such as a dovetail chisel. Or, an individual might decide the difficulty sharpening such a tool was too great and opt for something easier to sharpen. For a mortise chisel hardness and toughness is important to prevent the tip from breaking or crumbling. Wear resistance can be sacrificed some to get this toughness. For a plane iron all these factors must be balanced. Understanding where to get data on steel properties as well as an understanding of how to read the tables will help woodworkers make better tool and tool steel selections. But remember, the optimum properties of steel are only realized when it is precisely heat treated.