A study of the effect of heat treatments on EN9 steel - grain structure and mechanical properties

Medium carbon steel 070M55, also known as EN9 or AISI/SAE 1055, was heat treated with five different heat treatment procedures to promote varying microstructures resulting in enhanced properties of the metal. The aim of this study is to investigate the effect of different heat treatment procedures on a medium carbon steel, regarding the grain microstructure and mechanical properties. Different heat treatment procedures were researched, and five different methods were chosen. The five procedures were normalising, quenching, and tempering at 250°C, 450°C and 650°C. Additionally, all the tempered specimens were quenched before being raised to their designated temperatures. An Instron 5582 was set to record the tensile modulus (E), tensile stress (), and tensile strain () every 0.1 seconds, with the crosshead speed set at 5mm/min. Sudden impact energy absorption, or toughness, was tested on an Avery 6703 Charpy impact machine. The Vicker’s hardness was tested on a Buehler MicroMet 5104 using a 10N force. The test measured the hardness every mm, starting at 0.5mm from the centre left of the specimen to centre right. The EBSD specimens were then mounted in a Buehler SimpliMet 2000 hot mould press and polished to 1000 FEPA grit. The Covid-19 pandemic prevented further development on the microscopy; however, the specimens would have been processed as follows: further polishing with diamond solutions would have been completed before Colloidal Silicia was applied and then finished with a Vibromet. The EBSD microscopy would have then been completed using a JEOL 6610 VP-SEM with Aztec analysis software. The microscopy would have been used to measure the grain size and identify the phases and orientation, before relating the mechanical data to the microscopy data. It was found that a higher tempering temperature correlated to a greater ductility and toughness of the medium carbon steel. There was an additional average of 47.1J of energy absorbed between a 250°C temper and a 450°C one, and then again to a 650°C temper. It was also found that heat treating medium carbon steel does not significantly affect the tensile modulus. The Hall-Petch relation applies to EN9 steel which indicates a positive increase of yield stress to the finer the grains. However, this is less evident when relating the grain size to tensile strength