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Microstructure evolution during Heat Treating on Steel

If a γ solid solution is quenched in water to prevent diffusion of carbon atoms, the carbon remains fixed in (though not part of) a lattice structure, setting up intense local lattice strains that block movement of dislocations. As a result, the structure is hard and extremely strong, but also very brittle. Under an optical microscope it looks like an array of needles, completely different from pearlite (Fig. 1). The needle-like structure formed when carbon is trapped by the iron crystal lattice is called martensite. It represents the maximum hardness obtainable with any given carbon content.

The degree of hardness of a quenched austenitic structure is proportional to the lattice strain. The lower the carbon content, the lower the strain. The maximum hardness appears to be reached in the region of 0.6 to 0.8% carbon. Industrially, steels in which the carbon is too low to give a useful hardening effect on quenching (below 0.25%) are known as mild steels, and those that do show a useful hardening effect are termed medium carbon or structural steels. Those containing more than 0.8% carbon not only offer greater strength and hardness on heat-treatment, but also contain excess cementite. Cementite particles confer excellent resistance to wear and these so-called high-carbon steels are often used for cutting and forming tools.

Fig. 1: Microstructure of martensite formed by quenching steel from the austenite region: very hard and brittle: magnification, 300x (in the book Alexander, et al [1])

Unfortunately, the high hardness and strength of a wholly martensitic structure is difficult to exploit in practice because it is so brittle. A steel in this condition requires considerable support from a tougher material to be of use in service. An added disadvantage of quenched steels is dimensional change during transformation, due to the different lattice spacing of iron and carbon atoms in a- and y-iron. Martensite expands by a few percent on formation, the expansion increasing with the carbon content. This expansion causes distortion and internal stresses that can lead to cracking. A water- or oil-quenched steel with more than about 0.4% carbon will almost certainly develop quench cracks, and the main reason why the welding of such steels is difficult is that cracking can occur even on cooling in air.

There are two ways of overcoming the brittleness of a quenched martensite steel to give desired combinations of hardness and toughness throughout the section. The first is to produce martensite and then temper it. Tempering is a controlled heat-treatment allowing some of the trapped carbon to escape from the interstitial spaces between the iron atoms and eventually form particles of cementite. The second is to cool the Y-iron from the high-temperature austenitic state in such a way as to cause it to change into another type of structure intermediate between the equilibrium pearlite and the metastable martensite. This intermediate structure has properties resembling those of a tempered martensite and is called bainite. In bainite some of the carbon atoms remain in the lattice, trapped between the iron atoms, and some are precipitated as a compound with iron. The particles of this compound are so fine that they become visible only with the help of an electron microscope. Under an optical microscope the structure, when suitably etched, appears as a dark mass of needles or acicular blocks (Fig. 2).

Fig. 2: Microstructure of bainite transformation product of austenite: fairly hard and reasonably tough: magnification, 300x (in the book Alexander, et al [1])

Fig. 3: Property changes with tempering of a martensitic (quenched) carbon steel. Notice how the most drastic changes take place between 300 and 400°C [1]

The temperature at which tempering is carried out is critical. Between 200°C and 300°C diffusion rates are slow and only a small amount of carbon is released. As a result the structure retains much of its hardness but loses some of its brittleness. Between 500°C and 600°C diffusion is much faster, but allows most of the carbon atoms to diffuse from between the iron atoms to form cementite. The dramatic change in mechanical properties brought about by tempering a martensitic 0.4% carbon steel is shown in Fig. 3.



1. W.O. Alexander, G.J. Davies, K.A. Reynolds and E.J. Bradbury: Essential metallurgy for engineers. 1985. Van Nostrand Reinhold (UK) Co. Ltd. ISBN: 0-442-30624-5


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