Effects of austenizing temperature, cooling rate and isothermal temperature on overall phase transformation characteristics in high carbon steel

Abstract

Phase transformations in high-carbon steel has been investigated by dilatometry during continuous cooling and isothermal transformations, following austenizing at temperatures of 900 °C–1200 °C (1173–1473 K). Optical microscopy and high-resolution scanning electron microscopy techniques revealed the presence of martensite, complex bainite, pearlite and metastable retained austenite under a variety of heat treatments. Magnetic measurements proved to be a useful technique to determine the fraction of retained austenite. Quantitative image processing with ImageJ software and magnetic techniques were used to calculate phase fractions. The rate of phase transformations increased by an increase in austenizing temperature for transformations occurring during continuous cooling and isothermal transformations. A mere increase in cooling rate from 3 °C (276 K) per second to 5 °C (278 K) per second changed the relative phase fractions and hardness significantly. The hardness of isothermally cooling specimens was a strong function of temperature and microstructure. Although isothermal holding at 500–525 °C (773–798 K) resulted in a mixture of bainite and pearlite, the hardness decreased significantly by decreasing the isothermal transformation temperature from 525 °C (798 K) to 500 °C (773 K). These outcomes provide valuable guidelines for the development of novel microstructure in high carbon steel in industrial practice.

Keywords

• High carbon steel
• Phase transformation
• Multiphase microstructure
• Phase fraction
• Hardness