| dc.description.abstract | Abstract
The purpose of this research is to study the mechanical properties of S50C steel under various austempering treatments. Through systematic experiments, the variation of mechanical properties (tensile strength, hardness, and elongation) with the temperature (900 and 745oC) and time (5, 15, 30, and 60 min) in austenitizing or intercritical annealing and temperature (300, 325, 350, 375, and 400oC) and time (15, 30, 60, 120, and 180 min) in isothermal tempering was characterized. In this way, it is hoped that a simple low-cost heat-treatment technique suitable for mass production could be developed to meet the various needs of mechanical properties of S50C steel in industry.
Results showed that the S50C could have a microstructure of bainite or mixed martensite and lower bainite after going through the given austempering treatments and that the tensile strength was reduced when the tempering temperature was increased. On the other hand, S50C can have a microstructure of mixed proeutectoid ferrite, bainite and globoular pearlite after going through the given intercritical annealing plus isothermal tempering treatments and that the tensile strength and hardness were increased with tempering temperature. This trend is opposite to that in conventional austempering treatment and the main reason is that the S50C mechanical properties are controlled by the extent of recrystallization of the residual pearlite after going through the intercritical annealing and isothermal tempering. The content of residual pearlite was reduced when the time of intercritical annealing is increased. Accordingly, the degree of increase in tensile strength with tempering temperature would be reduced with an increase in intercritical annealing time. The effects of tempering time on the microstructure became saturated after 30 min in both austempering treatment and intercritical annealing plus isothermal tempering treatment. This means that for a given tempering temperature the transformation of microstructure was almost completed within 30 min. In this regard, increasing tempering time longer than 30 min will not change the microstructure and mechanical properties. Under consideration of cost, it is therefore suggested to let the isothermal tempering process last just for 30 min in practical use.
By the combination of different austempering conditions, S50C steel can have a variety of mechanical properties with the tensile strength ranging from 596 to 1930 MPa, the hardness ranging from HR30N 36 to 72, and the elongation ranging from 4.6 to 28.7%. This wide range of mechanical properties could make S50C steel suitable for use in various mechanical components. | en_US |