Tham khảo tài liệu 'the science and technology of materials in automotive engines part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 288 Science and technology of materials in automotive engines method of causing martensitic transformation. The temperature at which martensitic transformation takes place decreases with increasing carbon content and falls below room temperature in steels with a composition above C. Therefore a high amount of retained austenite is likely to appear in high-carbon steels and carburized steels. Although retained austenite is unstable and causes distortion when martensitic transformation takes place during operation it is reported that a small amount of retained austenite increases toughness considerably. Normalizing austenite steel transforms into a mixture of ferrite and pearlite during slow cooling. If the cooling rate is slightly faster than aircooling the transformed pearlite becomes fine. It raises the strength but the value is lower than that produced by quench-hardening. This treatment is called normalizing. Figure c shows a normalized microstructure. Annealing when steels are gradually cooled from the austenite state by turning off the power to the furnace the resultant pearlite becomes coarse and soft. This heat treatment is called annealing. Figure d shows an annealed microstructure. Isothermal annealing austenitizing steels first and then cooling and holding at just below the A1 temperature generates a ferrite and cementite microstructure. The original austenitizing produces a soft annealed state within a shorter period of time than annealing at a temperature below A1. Austempering holding a part at a constant temperature during cooling below A1 gives rise to bainite a tough microstructure with an intermediate hardness between pearlite and martensite. This treatment is referred to as austempering and is often used in spring steels for cushion springs etc. Spheroidizing annealing this treatment spheroidizes cementite to increase ductility and malleability. It is used in a cold forging billet and is appropriate for severe working conditions. .
