engleski

Influence of Hardening and Tempering vs. Austempering on Mechanical and Fracture Properties of 65Si7 Steel

Abstract. 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 properties, such as strength, hardness, ductility, contraction, toughness etc., as well as the influence of these procedures on the magnitude of some fracture mechanics parameter for example stress intensity factor K. The experimental investigations were performed on the specimens made of 65Si7 steel (chemical composition: 0.69% C, 1.56% Si and 0.90% Mn). A basic precondition for a successful performing of austempering of steel is the choice of the adequate steel quality. It should be pointed out that this procedure of heat treatment is applicable and feasible only with elements of relatively thin cross-section (thickness of about 25 mm). It is related to a possibility of undercooling of austenite, or to the real curves of cooling for both the surface and the core of the product to the temperature of isothermal transformation. The transformation of austenite should not start before the temperature of isothermal transformation is reached. Consequently, the choice of the adequate steel for this procedure should be made on the basis of the right isothermal TTT diagram. It can be said that in general some non-alloyed steels are suitable for austempering and low-alloyed steels for hardening and tempering. The procedure of austempering is performed in the way that a product is cooled from the temperature of austenisation in a soline bath and is kept in it as long as all of the undercooled austenite is turned into bainite and is in turn cooled in the air, Fig. 1b. Since the researches aimed to investigate the influence of different heat treatment procedures or 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, Fig. 2. 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 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.

Heat treatment procedures; Hardening and tempering; Austempering; Mechanical properties: Strength; Hardness; Ductility; Toughness; Fracture mechanics parameter; Stress intensity factor.

Rad je objavljen u časopisu "Key Engineering Materials - Materials Structure & Micromechanics of Fracture - MSMF6" u broju za veljaču 2011. godine. Časopis je indeksiran u bazi "Scopus".