Cutting tools most commonly used are made of carbon steel and high-speed steel. They are heat treated to develop certain desired qualities in the cutting tools, particularly to have high hardness and high wear -resistance, which are needed to prevent the cutting edges of a tool from becoming blunt by wearing away in service.
The first operation is forging, which should be carried out at 850*C to 950*C for carbon steel depending upon the carbon content of steel and at 1100*C to 1300*C for high speed steel. Forging should never be continued when the tool has cooled to a temperature of about 723*C for carbon steels and about 920*C for high speed steels.
The next operation is normalizing or annealing. Plain carbon steels are usually normalized, while high-speed steels are annealed. This is essential, and must be carried out in advance of hardening in order to have uniform grain structure and to relieve the strains set up by forging. Tools of carbon steels are heated for normalizing to 760*C to 840*C. Heating should be slow about 723*C, after which cooling is done freely in air. The first golden rule in heat- treating any high speed tool is to warm the steel before it is put into either the fire or furnace, and then to heat it up slowly, thoroughly, and evenly. If this simple hint is observed, there is virtually no risk of spoiling. The reason is that high-speed steel is denser, i.e., about 10 per cent heavier than ordinary steel and its thermal conductivity is lower. High - speed steels are heated for annealing to about 900*C, allowed to cool in still air, reheated to about 800*C, and again cooled in still air. To put the steel into the softest condition it is heated to about 850*C and cooled in the furnace. Powered charcoal, charred bone, charred leather, slaked lime, sand , fire clay, etc. can be used to pack the closed box in which the steel is annealed , to prevent carburization. The steel is held at the annealing temperature for not less than 3 to 4 minutes for each is held at the each 1mm of section thickness of the longest pieces being treated.
The next step is hardening. General recommendations for hardening temperatures for any given tool steel based on carbon content are as follows : For steels of 0.70 to 0.80 percent carbon content, 780 to 850*C ; for steels of 0.80 to 0.95 per cent carbon, 765*C to 790*C.; for steels of 0.95 to 1.10 percent carbon, 750* to 775*C ; and for steels of 1.10 percent and over carbon, 750* to 770*C. They are next quenched by immersing them in a bath of fresh water, but water is not an ideal quenching medium. In order to secure more even cooling and reduce danger of cracking, brine solution with 10 % brine or caustic soda solution with 3 to 5 percent caustic soda may be used. The hardening of high speed steel requires several steps or separate operations. The first consists in pre-heating the tool to about 800*C in a pre-heating furnace. The steel is held at this temperature for 15 to 20 seconds for each 1mm of the section thickness of the article to be treated. The second step is to transfer the steel to a furnace where it can be heated rapidly to the hardening range of about 1100*C to 1300*C. In treating tools care should be taken to prevent carburization. The tool is held at this temperature for one or two minutes depending upon the bulk , and then removed and cooled in a blast of cold dry air, or quenched in oil.
Hardened steel is then tempered. Tempering is designed , as previously indicated, to remove the excessive brittleness characteristic of a newly hardened tool, without reducing to any appreciable degree of hardness at the cutting edge. Tempering Temperatures for carbon steels usually vary from 150 to 300*C and or high speed steel from 550 to 650*C, depending upon the type of tool and its use. In heating steels for tempering, either a furnace or some kind of bath such as oil, various salt mixtures or lead may be used.
The heat - treating processes applied to various tools of carbon and high-speed steel are essentially similar to one another. Some differences are caused by the fact that certain tools , e.g., screw taps, screw dies , lathe, planer and shaping tools , are best not hardened throughout, where as others, e.g., drills, milling cutters, should be hardened throughout.
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