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Manufacturing Engineer’s Reference Book P2

Furthermore, martensite, which is characterised by an acicular appearance, forms progressively over a temperature range as the temperature falls; if the temperature is held constant after the start no further action takes place. Martensite formation produces an expansion related to the carbon content. The mechanical properties of martensite depend on the carbon content; low carbon martensites (less than O.OS%C) have reasonable ductility and toughness, high carbon martensites have no ductility or toughness and extreme hardness and, because of the state of internal stress, are very liable to spontaneous cracking. Thus low carbon martensite can be used for industrial purposes, e.g | 1 24 Materials properties and selection Figure 1.18 Hardness of martensite related to carbon content Furthermore martensite which is characterised by an acicular appearance forms progressively over a temperature range as the temperature falls if the temperature is held constant after the start no further action takes place. Martensite formation produces an expansion related to the carbon content. The mechanical properties of martensite depend on the carbon content low carbon martensites less than 0.08 C have reasonable ductility and toughness high carbon martensites have no ductility or toughness and extreme hardness and because of the state of internal stress are very liable to spontaneous cracking. Thus low carbon martensite can be used for industrial purposes e.g. welded 9 Ni steels for low-temperature applications have low carbon martensitic heat-affected zones. High carbon martensite must be tempered before it is allowed to cool to room temperature e.g. carbon tool steels are water quenched to exceed the critical cooling rate but the tool is withdrawn from the bath while still hot and immediately tempered. 1.3.5.5 Isothermal decomposition of austenite Reference was made in the previous section to the fact that if the y to a transformation is suppressed by fast cooling the austenite is in an unstable condition. If before reaching the temperature at which martensite begins to form the cooling is arrested and the steel is held at a constant temperature the unstable austenite will transform over a period of time to a product which differs markedly from pearlite and has some visual resemblance to martensite in being acicular. This structure is called bainite it is formed over a range of temperatures about 550-250 C and its properties depend to some degree on the transformation temperature. Bainite formed at a lower temperature is harder than bainite formed at a higher temperature. It is tougher than pearlite and not as hard as martensite. It differs fundamentally .