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INFLUENCE OF AUSTEMPERING ON FRACTURE MECHANICS PARAMETERS OF 65 SI 7 STEEL Dragan Pustaić, Franjo Cajner and Martina Lovrenić Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Lučića 5, 10000 Zagreb, Croatia dragan.pustaic@fsb.hr, franjo.cajner@fsb.hr and martina.lovrenic@fsb.hr Summary 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. An advantage of austempering over hardening and tempering is in obtaining the bainite microstructure which has greater toughness, strain, 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]. All investigations were performed on 65Si7 steel (chemical composition: 0.69% C, 1.56% Si and 0.90% Mn). The specimens were first submitted to heat treatment by the parameters previously determined by our own investigations 2, 3]. 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 in the starting conditions had two different microstructures: • by cooling down in oil the obtained microstructure was a hardened microstructure with visible plates of martensite common to the steel with this share of carbon, 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. These investigations comprise two groups of experiments, namely the static tensile tests (2 specimens) and tests to investigate the fracture toughness of the material (3 specimens). A laboratory testing of fracture toughness of steel was carried out and diagrams of force relationships - CMOD, CTOD- a and J-integral- a were recorded. A standard three points bending test specimen (SENB) was investigated. All investigations of the fracture mechanics parameters were performed according to the British standard BS 7448: 1991 – Fracture mechanics toughness tests: Part 1. Method for determination of KIc , critical CTOD and critical J values of metallic materials. Crack mouth opening displacement (CMOD) in dependence of the load on the specimen (force F) is measured. The measured values are recorded in the form of a diagram F-CMOD, Fig.1. Also, the diagrams CTOD- a and J-integral- a were recorded. After the critical value of notch opening displacement Vc has been determined, it is recalculated into the critical value of the CTOD parameter in the crack tip, i.e. in c. Elastic-plastic analysis of FE models is performed using the commercial FE package ABAQUS. For non-linear analysis, the Ramberg-Osgood type stress-strain relationship is used , (1) where and n are material constant and strain hardening exponent, respectively. FIGURE 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 Austempering without tempering gives significantly higher ductility in comparison to the most favourable case of hardened and tempered state. Austempering with a subsequent tempering at the temperature of t = 480oC contributes to a more significant decrease in tensile strength m, for about 20%. The analysis of the critical value of the stress intensity factor KIc shows that with higher temperature of tempering t, higher values of the parameter KIc in hardened and tempered specimens are obtained. For example, parameter KIc is approximately twice as high at the temperature of tempering t = 480oC than the one at the temperature of tempering t = 300oC. The best critical values of the stress intensity factor KIc are obtained by austempering without tempering and they are approximately 50% higher than the ones obtained by tempering at the temperature t = 480oC, or even three times higher than the values obtained by hardening and tempering at the temperature t = 300oC. By hardening and tempering at the temperature t = 480oC, the obtained critical values of crack tip opening displacement c are approximately 3.5 to even eight times higher than those obtained by hardening and tempering at the temperature t = 300oC. By austempering without tempering, the obtained critical value of crack tip opening displacement c is about seven times higher than the value obtained by hardening and tempering at the temperature t = 300oC. References 1. Pustaić, D. and Cajner, F., Inžynieria materialowa, vol. 5 (124), 733-736, 2001. 2. Cajner, F., Strojarstvo, J. Theo. Apl. Mech. Engng., vol. 33 (5/6), 289-296, 1991. 3. Cajner, F., Kovine, zlitine, tehnologije, vol. 28 (6), 533-538, 1994. |
