Cold chisels and setts Dark purple 285 Cold chisels for cast iron Very dark

Cold chisels and setts dark purple 285 cold chisels

This preview shows page 40 - 43 out of 73 pages.

Cold chisels and setts Dark purple 285 Cold chisels for cast iron Very dark purple 290 Cold chisels for iron, needles Full blue 295 Circular and band saws for metals, screwdrivers Dark blue 300 Spiral springs, wood saws 450-700 To produce great toughness, but at the expense of hardness
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~ Page 41 of 73 ~ SURFACE HARDENING There are three methods of surface hardening which are discussed below. 1. FLAME HARDENING Localized surface hardening can also be achieved in medium- and high-carbon steels and some cast irons by rapid local heating and quenching. Fig 1(a) shows the principles of flame hardening. A carriage moves over the workpiece so that the surface is rapidly heated by an oxy-acetylene or an oxy- propane flame. The same carriage carries the water-quenching spray. Thus the surface of the workpiece is heated and quenched before its core can rise to the hardening temperature. This process is often used for hardening the slideways of machine tools, for example, lathe bed-ways. 2. INDUCTION HARDENING Fig 1(b) shows how the same surface-hardening effect can be produced by high-frequency electromagnetic induction. The induction coil surrounding the component is connected to a high-frequency alternating current generator. This induces high-frequency eddy currents in the component causing it to become hot.
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~ Page 42 of 73 ~ When the hardening temperature has been reached, the current is switched off and a water spray quenches the component. The induction coil can be made from copper tube which also carries the quenching water or oil. This technique is often used for hardening gear teeth. The induction coil can be tailored to suit the profile of the component. The depth of the case can be controlled by the frequency of the alternating current. The higher the frequency, the nearer to the surface of the component will be the eddy currents resulting in a shallower depth of heating and, therefore, a shallower depth of hardening. 3. NITRIDING This process is used to put a hard, wear-resistant coating on components made from special alloy steels, for example, drill bushes. This process is not suitable for the hardening of plain carbon steels, as iron nitrides could be formed to a considerable depth below the surface of the steel thereby embrittling the material. Steels for nitriding are low-alloy steels containing chromium and molybdenum, together with, in some cases, nickel and vanadium Some nitriding steels used for this process contain either 1.0% aluminium, or traces of molybdenum, chromium and vanadium. Nitrogen gas is absorbed into the components in ammonia gas at between 500 and 600 ° C for upwards of 40 hours in a gas tight chamber. At this temperature, the ammonia gas breaks down and the atomic nitrogen is readily absorbed into the surface of the steel.
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