engleski

Influence of Microstructure of 65 Si 7 Steel on its Mechanical Properties

Introduction. The aim of the performed investigations was to determine the influence of two different heat treatment procedures, i. e. hardening and tempering vs. austempering, on some mechanical and fracture properties of a particular type of steel (steel 65Si7 ; the chemical composition: 0.69% C, 1.56% Si and 0.90% Mn). An advantage of austempering over hardening and tempering is in obtaining the bainite microstructure which has greater toughness, ductility, contraction, fatigue strength and a better fracture toughness than a tempered martensite of the same type of steel [2, 3]. All these advantages of bainite refer to untempered state. By tempering of bainite microstructure steel its toughness decreases and in tempered bainite it is lower than the toughness of temepered martensite [3]. Since the researches aimed to investigate the influence of various microstructural states of steel on its properties, the specimens were first submitted to heat treatment. After heat treatment, photographs of microstructures of the starting conditions (prior to tempering) were taken. From the photos it is clearly seen that the specimens had two different microstructures: • by cooling down in oil the obtained microstructure was a hardened microstructure with visible plates of martensite, and its hardness was 850 HV1, • by austempering the obtained microstructure was a microstructure of lower bainite metal base, with probably lower share of residual austenite and with hardness of 440 HV1. Crack mouth opening displacement (CMOD) in dependence of the load on the specimen is measured. The measured values are recorded in the form of a diagram F-CMOD, Fig.1., BS 7448. Fig. 1. Dependence of crack mouth opening displacement CMOD on the force F, a) for hardened and tempered specimens, b) for austempered specimens, untempered and tempered References [1] Pustaić, D. and Cajner, F., Inžynieria materialowa, vol. 5 (124), (2001), 733-736. [2] Cajner, F., Strojarstvo, J. Theo. Apl. Mech. Engng., vol. 33 (5/6), (1991), 289-296. [3] Cajner, F., Kovine, zlitine, tehnologije, vol. 28 (6), (1994), 533-538.

Heat treatment procedures; Hardening and tempering; Austempering; Mechanical properties: Toughness; Strength; Hardness; Fatigue strength; Fracture mechanics parameters; Stress intensity factor

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