The latter diffuses into the steel forming nitrides of iron aluminum chromium

The latter diffuses into the steel forming nitrides

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The latter diffuses into the steel forming nitrides of iron, aluminum, chromium and vanadium. Ion nitriding (plasma nitriding) is a surface hardening heat treatment, in which nitrogen is delivered to the workpiece surface in form of ionized gas (plasma). The case formed as a result of nitriding has a hardness of about 1100 HV (Vickers Hardness) which is higher than the hardness after carburizing. Nitrided part possess also better wear resistance , increased fatigue strength , enhanced toughness and good resistance to corrosion . No additional heat treatment is required after nitriding.
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~ Page 49 of 73 ~ LOCALISED CASE HARDENING It is often not desirable to harden a component all over. For example, it is undesirable to case harden screw threads. Not only would they be extremely brittle, but any distortion occurring during carburising and hardening could only be corrected by expensive thread-grinding operations. Various means are available for avoiding the local infusion of carbon during the carburising process. o Areas to be left soft could be heavily copper plated. The layer of copper prevents intimate contact between the component and the carbon, thus preventing carburization. Note that copper plating cannot be used for salt-bath treatment, as cyanide dissolves the copper from the component. o Areas to be left soft could be encased in fire clay. This method is mostly used when pack-carburising. o Surplus metal could be left on the component. This is machined off, together with the infused carbon, between carburising and hardening. Although more expensive because of the extra handling involved, this is the surest way of leaving local soft areas. Fig. 1 below shows this method.
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~ Page 50 of 73 ~ EXERCISE 1. Sketch a cooling curve for a 0.5% carbon steel. Identify and label the change points on the curve and explain what happens at each point. 2. Explain why calculations for the linear expansion of a steel with increase in temperature are only valid up to the Ac 1 point. 3. Show by means of a diagram the effect of carbon content on the hardness, tensile strength and ductility of annealed plain carbon steels. 4.With reference to the iron-carbon phase equilibrium diagram: (a) Explain what is meant by the following change points:(i)Ar1(b) Explain the difference between Ar points and Ac points.(c) Explain what is meant by the ‘Curie point’. (ii) Ar 3 (iii) Ar cm 5. Explain briefly what is meant by ‘limiting ruling section’ and how it is related to the hardenability of steels. 6. Explain why the ‘Ar’ points for steels are more likely to be of use than the ‘Ac’ points when heat treating steels. 7. Explain why plain carbon steels can benefit from heat treatment. 8. Describe the basic requirements for a heat treatment process. 9. State which heat treatment process you would choose (and give reasons for your choice) in order to: (a) Prepare a cold-rolled steel sheet for severe cold working (b) Stress relieve a forging prior to machining (c) Toughen a quench-hardened steel.
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