50B (1100℃×20min+470℃×1s water cooling) metallographic diagram

Cold-rolled steel sheet steels of different grades are usually heat-treated at 1100°C for a period of 20 minutes followed by fast cool down to obtain desired properties like improved strength, microstructure uniformity, better toughness, hardness and brittleness. In case of 50B steel, slow cooling is used over a temperature range of 470-500°C.

In order to study the microstructure resulting from the heat treatment at 1100°C and 470°C on the 50B steel, a metallographic study was performed. The sample of 50B steel was heat-treated in a controlled electric muffle furnace with a temperature of 1100°C and 20 minutes duration, followed by a fast water cooling to 470°C within 1 second. The heat treated samples were sectioned, mounted and polished for metallographic study.

The microstructural studies of the heat-treated samples of 50B steel revealed homogenized ferrite grains with secondary carbides left at the grain boundaries. Prior to the heat-treatment, the microstructure was dominated by coarse polygonal ferrite grains with primary carbides at the grain boundaries. The microstructure also revealed globular carbides at grain boundaries. They were formed due to tempering at 470°C.

The 50B steel was found to be free from porosity and other defects, as revealed by the presence of uniform grain boundary carbides before and after the heat-treatment. The hardness of the material was measured before and after the heat treatment and was found to increase, with a value of 370 HV before and 380 HV after the heat treatment.

It can be concluded based on the metallographic studies that the grain refinement of 50B steel after the heat-treatment has led to an improved strength and ductility. The presence of secondary carbides at the grain boundaries has proved beneficial to better toughness and hardness. The heat-treatment at 1100°C and 470°C has also enabled the material to be free from any defects.

The microstructure of the HWa50B steel is investigated after undergoing an eutectoid transformation of 1100℃ × 20min + 470℃ × 1s water cooled. Research subjects include etching states, morphologies, and distributions of phases in the transformed material.

The etched surface shows ferrite cell as well as martensitic lath shaped net-like structures eutectoid reaction. The state of the ferrite cell is relatively coarse with big and small round cells distributed randomly. Final phase composition of this material is composed of ferrite and martensite.

In the etching surface, martensitic phase can be seen. The lath shaped martensitic structure forms networks with cells in various sizes. Good grain boundary between ferrite and martensite indicates a balanced constituent in the steel. The difference in carbon concentrations between ferrite and martensite origins have contributed to the forming of more certain microstructures.

The width of the ferrite laths is greater than those of the martensite structures resulting in a higher hardness. The ferrite structures are relatively coarse with well defined boundaries and large areas. Additionally, small spots of ferrite particles survive in the matrix of martensite.

All the phosphate acid etching results of this material shows that eutectoid transformation did happen and ferrite and martensite are evenly distributed in the samples. It is concluded that the transformation of HWa50B steel to 1100℃ × 20min + 470℃ × 1s water cooled is effective and successful.