冶金专业英语Unit18.ppt

* * * Text Heat treatment is a term applied to a variety of procedures for changing the characteristics of metal by heating and cooling. By proper heat treatment, it is possible to obtain certain characteristics in metal such as hardness, tensile strength (ability to resist stretching), and ductility. The procedures of heat treatment of steel include hardening, tempering, annealing, and case hardening. Text 1.Hardening Hardening is a process of heating and cooling steel to increase its hardness and tensile strength, to reduce its ductility, and to obtain a fine grain structure. The simple experiments described below may be enlightening. Two old-fashioned steel knitting needles, a gas ring, a bowl of water, a piece of sandpaper and a pair of pliers are needed. Experiment 1: Bend one needle slightly to feel how tough and springy it is. Now hold the needle in the flame, using the pliers, the bowl of water being close at hand. When it is bright red, dip the end of the needle as quickly as possible into the water. The needle should still be red hot as it is being quenched. Then try to bend the quenched end; it is hard and brittle and will snap off. Text 2.Tempering Tempering is done by reheating the metal to low or moderate temperature, followed by quenching or by cooling in air. It is a process that follows the hardening procedure and makes the metal as hard and tough as possible and removes the brittleness from a hardened piece . Two important points are to be noted in connection with the quenching and tempering of steel: (1)To get a fully hardened structure, the steel should be heated to the appropriate temperature. And then rapidly cooled. The appropriate temperature before quenching depends on the carbon content of the steel concerned. (2) Tempering does not restore the pearlitic structure. Before that can be done ,the steel has to be heated to the appropriate temperature and then slowly cooled. Te