By Paul Coppinger
Hello ET Turners,
Steels and Turning Tools – Both have been around for a long time and woodturning would be very difficult, if not impossible, without steel. Hopefully, this will be informative and helpful as you acquire and use today’s steel woodturning tools.
History - The first recorded woodturning was from Egypt and has been dated from about 1300 BC. It consisted of a 2-man lathe, one turning the wood with a rope and the other using a sharp tool to cut the shapes in the wood. Presumably, this tool was made from some type of steel. All steels begin with making an alloy of iron with a carbon content of between 0.2% and 2.1%. Several processes have developed to produce steel.
The first known steel implement came from a region of Turkey and is dated from about 4000 BC. Other ancient steel items came from East Africa in about 1400 BC and from China around 300 BC.
Wootz or Damascus steel (characterized by visible bands on the surface) came from about 300 BC in India and was made in crucibles by combining iron ore, charcoal and glass or sand. When heated, the impurities mixed with the molten glass to produce a slag on the surface, while the charcoal produced carbon which combined with the molten metal to form a carbon steel button in the bottom of the crucible. Skimming off the slag left a button which could then be forged into tools or forged with other buttons to form ingots. Tools made with this Wootz or Damascus steel were renowned for their sharpness and toughness.
Henry Bessemer, in 1855 patented a fast technique of steel making where air was blown through molten iron ore to remove most impurities and carbon. Then carbon was added back to produce the desired carbon levels.
Modern steel is made using an open-hearth process. Slower than the Bessemer process, it allows time to continually check the carbon content as it is being removed. Thus, a very controlled carbon content is obtained. Also, desired elements can be added to form different alloys. The open-hearth process makes it very easy to control the composition and uniformity of steel. An example is High Speed Steel (HSS) used in machine milling and turning to shape metals. Percentages of the elements molybdenum and vanadium are added to carbon steel during the open-hearth process to produce HSS.
Working Steel - Carbon in steel is important because steel’s strength and hardness increase with carbon while ductility decreases. In other words, as steel gets stronger and harder, it gets more difficult to work. In addition, carbon steel responds well to heat treatment. Heat treatment allows steel to have different characteristics, i.e., hardness and toughness. To understand heat treatment, it is important to discuss several important items:
? Annealing-heating steel to soften and allow shaping
? Curie Point-the temperature steel looses its magnetism
? Quenching-quickly cooling hot steel
? Tempering-reheating to a desired temperature.
As steel is heated, the crystals that make-up steel grains become plastic and start to stretch when pulled or compressed (hammered). This is how steel can be shaped and is called annealing. As steel is heated, it reaches a point called the Curie Point where it looses its magnetic properties and no longer will be attracted to a magnet.
This Curie Point is the temperature that steel changes its crystal form allowing a carbon atom to attach to the inside of the crystal. This new crystal form is harder and under stress. If allowed to cool slowly, the carbon atom returns to the outside of the crystal and the crystal is back to its original state. If cooled quickly, quenching, the carbon atom is “frozen” in the middle of the crystal resulting in a very hard, brittle, highly stressed form of steel. Reheating the metal to a temperature below the Curie Point will relieve some of this stress producing a controlled hardness and durable form of steel. So in summary, putting the steel in the stressed state is hardening; removing some of the stress is tempering and removing all the stress is annealing. This is called heat treating steel. As an example of heat treating a shop-made turning tool of carbon steel, first heat with a torch until a magnet is no longer attracted to the tool tip. Second, immediately quench in water. Third, place the torch tip below the cutting edge and heat until the tool tip color progresses from pale yellow to yellow to straw color (about 475 degrees F) – then quench immediately again in water. The edge temper will be at a hardness/durability equivalent to a good knife. To make and temper a scraper, stop after the initial quenching. This produces a very hard scraper edge that is difficult to sharpen but very durable.
Grain is important to understanding turning tools because as the grain grows larger, the edge sharpness becomes less, but the durability increases. Think about folding 80 grit versus 600 grit sandpaper to form an edge. Because the 600 grit grain of sand is much smaller than the 80 grit, the folded edge is much sharper. And as with many things, there is a tradeoff between sharpness and durability. The 600 grit edge is sharper but not near as tough as the larger grained 80 grit. The same is true of steel grains. As you add elements to carbon steel to make better alloys, the grain size increases, producing a very durable edge, but one that will not cut as well as plain carbon steel.
All the above brain dump leads up to what I want to discuss this month…how to buy turning tools. Prior to the ‘80s all turning tools were made from high carbon steel with a Curie Point of about 1400 degrees F. If ground on most high speed grinders, the edge will often reach temperatures that ruin the heat treatment resulting in a softening of the edge. This is visible by a bluing of the steel at the edge. Alloys of steel made by adding other elements during the batch making process have much higher Curie Points meaning that when sharpening by grinding, it is much more difficult to get the edge temperature hot enough to destroy its heat treating properties. Therefore, when acquiring turning tools, stay away from the older high carbon steels and as a minimum, buy High Speed Steel tools such as M2. All of the name brand tool makers today have gone to M2 or better grades of steel to avoid this grinding problem.
Also today, another class of turning tool steel is available – powdered metal. When steel is being formed in the processes above, certain elements can be added to produce unique alloys. The percentages of these elements are limited by the chemistry of the alloy. Adding too much of an element results in floating of the excess element on top of the molten alloy like grease on top of a hot soup. To get these higher percentages into the alloy, metallurgist have discovered that by grinding both the steel and the elements into powder, mixing them to the desired percentages, then heating under very high pressure in a shaped crucible, a new alloy is produced with additional unique properties. The result for woodturners is tools that hold an edge very long.
Although not a steel, carbide is beginning to be used for turning tools. Carbide is actually tungsten carbide meaning it has equal amounts of tungsten and carbon. It is a powder that can be formed under heat and pressure to produce a material 3 times stiffer than steel and much denser than steel. It produces a very long lasting edge but is even less sharp than powdered steel. And it is expensive. Because of its hardness, it can not be sharpened by high speed grinding with a typical grinding wheel. It requires a diamond coated wheel to power sharpen or a diamond honing stick to hand sharpen. To get around this sharpening problem, most carbide turning tools today have tips that can be rotated to a fresh edge or easily replaced.
In summary, high carbon steel can be honed (not ground) to a very sharp edge but will dull easily. HSS is a tougher steel but can not be sharpened to as fine an edge as carbon steel. Powdered metal tools have the greatest toughness meaning they stay sharp longer but they produce the least sharp edge of the steel cutting tools. And finally, carbide can be used very effectively for roughing cuts but doesn’t cut as fine as steels.
When buying tools, I suggest buying all you can afford. The best starting point for new turners is to purchase name brand M2 HSS tools. As you improve in your turning, you will be able to appreciate the more expensive powdered metal tools. One final thought – most name brand manufacturers, and some of the Chinese brands, offer turning tool sets with 6-8 tools. I recommend NOT buying these sets because usually you will only use 2 out of the set. The rest will stay in a drawer and may never be used. In a future article, I will discuss the specific tools you need to start turning, then what can be added to enhance your turning as your technique advances.