Tool Steel & Heat Treatment

Steve Costain replies: Some small tools made for O-1 (oil hardening tool steel) can be done in the shop, but for a plane blade A-2 steel holds an edge best. A-2 steel requires a heat treat furnace. The steel needs to be brought up to temperature, held at temperature for a certain time depending on thickness and then tempered at a lower temperature. The minimum charge for heat treatment of A-2 steel is about $85 plus the cost of steel and fabrication. Considering that you can buy a very nice A-2 plane blade from Lee Valley for less than $60, I wouldn’t recommend making your own.

Bruce Wedlock replies: Tool steels are alloys of iron, with varying small amounts of carbon, chromium, manganese, nickel and possibly molybdenum, tungsten and vanadium. These alloy compositions result in characteristics after heat treating which enhance their particular application. For woodworking tools, two hardening methods are used—A-2 in which the steel is air hardened and O-1 where the steel is oil quenched.

A-2 steels have a balance of toughness and wear resistance. They are typically used for mortise and bench chisels where the bevel angle is 30° or higher.

O-1 steels have increased harden ability and are less likely to crack. Since they can hold a sharper edge, they are used when the bevel angel is 25° or less. So O-1 steel is preferred for paring chisels and low-angle plane blades as well as knives.

In cryogenic tempering the steel is gradually lowered in temperature to 300°F below zero and held for 24 hours. The steel is then slowly raised to +300°F and annealed for several hours. Cryogenically treated steels have improved wear resistance and toughness. Typically the edge will last 50% longer than an oil-quenched tool.

Garrett Hack replies: I am assuming you want to make some cutting tools. The easiest steel to work with would be an oil or water hardening steel that you can either scrounge up (other tools or the like) or buy in many shapes and sizes. Once you have your new tool shaped—either by grinding or by forging—you need to harden the cutting edge. Heat it glowing red hot and then quench it rapidly into a can containing cooking oil. Plunge vertically and agitate the tool as you plunge. Oil works for both oil and water hardening steel. To make sure you really got the edge hard, file across it and the file should skate and not cut. To temper the steel, reducing its brittleness, polish up an area of the edge and heat back from the edge slowly and carefully with a torch. As soon as you see the polished spot turn a light straw color, quench immediately. If the tool proves too prone to chipping, you can always temper further, to a slightly darker straw.