The necessary of preheating high speed steel
The hardening process of high speed steel
High speed steels get their name from the fact that they may be operated as cutting tools at much higher speeds than is possible with plain carbon tool steels. Since the maximum hardness of high speed steels is obtained on tempering at high temparatures, they can be operated as cutting tools in the same temparature range without loss of hardness. That is the rise in temaprature due to friction will not reduce the temper of the tool pointas it cuts into the steel. Tool steels are often annealed for softening before machining or forming, and also for obtaining grain refinement.
Annealing
Soak at 900° C for about four hours. Then cool slowly at not more than 20°C per hour to 600°C. It may then be cooled to room temparature in still air.
Hardening
Pre -heat to 850°C slowly to prevent cracking. Then heat rapidly to approximately 1250°C depending upon the alloy used. This rapid heating reduces grain growth. Quench in air blast or oil depending on the size of the component. Because of the high temparature involved, oxidation of the steel can be severe. This can be minimised in a double chamber muffle by using the excess fuel to give a carburising atmosphere.
However, this reduces the combustion efficiency, and there may be some difficulty in reaching salts available, modern practice favours the use of the salt bath furnace.
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Secondary hardening
This is sometimes called tempering However, this term is not strictly true. Not only does secondary hardening increase the toughness of the steel, it also increase the hardness whereas tempering increases the toughness at the expense of hardness. The effect of secondary hardening helps these steels to work effectively at very much higher temparature than plain carbon steel.
Heat treatment of non ferrous metals
Annealing
Like steel non ferrous metals can be softened by heating and allowing to cool. However since they cannot be hardened by rapid cooling, the rate of cooling is comparatively unimportant. In fact copper components are often quenched in water. This not only saves time but also cleans the black oxide film from the surface of the components because of the rapid contraction.
Non ferrous metal annealing temparatures
Metal temaprature ° C
Cold working brasses 600-650
Copper 650-700
Aluminum 500-550
Duralumin 480-500
Hardening
Only a very few non ferrous alloys can be hardened by heating and quenching, like steel and most non ferrous metals are hardened by cold working. As anyone who has tried coppersmithing will konw the metal starts to work harden as it beaten to shape and it must be annealed from time to time to prevent it from cracking.
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