Tham khảo tài liệu 'the science and technology of materials in automotive engines part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The connecting rod 213 10 pm Microstructure of a needle roller under scanning electron microscopy. Spherical carbide around 2 pm disperses in tempered martensite matrix. The steel containing fine round carbide is ductile although the carbide itself is hard and brittle. Net carbide Lamellar pearlite before spheroidizing a Spheroidized carbide after spheroidizing b a Net-shaped carbide at grain boundaries and lamellar pearlite in grains of a hyper-eutectoid steel. The microstructure resembles the super-carburized microstructure shown in Fig. of Chapter 8. b Spheroidized carbide cementite . The next stage is the spheroidizing process Fig. b at the mixed region of austenite and cementite point b in Fig. above the A1 line . After this slow cooling to below the A1 line spheroidizes the carbide. In this procedure round carbide is generated spontaneously because the spherical shape has less surface energy. A sufficient number of nuclei are required in order for fine carbide to be dispersed. If carbide nuclei are not present above A1 the supersaturated carbon in the austenite generates lamellar pearlite during cooling to below A1 and spheroidization fails. 214 Science and technology of materials in automotive engines Austenite area in the iron-carbon phase diagram. The temperatures corresponding to a and b in Fig. are indicated. 880-920 C Gradual heating Forced air cooling 30 min 25 mm Air cooling from 600 C a 780-810 C 10 C h 120 min 25 mm 720 C 4-6 h b Gradual cooling 120 min 25 mm Air coolin from 600 C g Spheroidizing diagram. a The process removes net carbide and refining lamellar pearlite. The representation 30 min 25 mm means that the treatment requires 30 min. for a 25 ộmm rod. b Spheroidizing treatment. Additional quenching and tempering are necessary for a roller bearing. The fine carbide in a homogeneous sorbite microstructure see Appendix F dissolves into the austenite above the A1 point. However carbide nuclei are eliminated
