Deformation, densification, and corrosion studies of sintered powder metallurgy plain carbon steel preforms

Sintered plain carbon steels containing carbon up to 1% with density near theoretical have potential applications requiring high strength, high hardness, and wear resistance. In the forged conditions such steels are suitable materials for tools, cutters, bearing rollers, fasteners, and cams. In the present study, experimental investigations were under taken in order to understand the mechanism of densification and deformation during cold and hot upset forming operations on sintered cylindrical performs of Fe and Fe–1%C. Effect of addition of 1%C to Fe on the corrosion behavior of the steel has also been considered. Cylindrical preforms of aspect ratio of 0.5 were prepared out of mixed elemental powders of Fe and C, sintered at 1100 °C for 120 min and subsequently hot/cold forged in flat dies. Density as well as dimension measurements were carried out on the forged billets after each step of deformation during the forging operations. Further, aqueous corrosion tests were conducted on the forged alloys by immersing them in 1% HNO3 solution for 25 h. Weight loss measurements were made on the corroded samples. The densification rate is found to depend on the flow stress as well as the deformation rate of the preforms. Further, the densification rate increases monotonically up to an axial deformation level of about 0.5 (true strain) and thereafter slightly reduced in both Fe and Fe–1%C alloy. The applied stress during cold upsetting increases with densification. But the rate of increase is not uniform. The rate of densification is also observed to be on the higher side with the addition of 1%C with Fe during both cold and hot forging. The corrosion rate of pure iron samples has been observed to be invariably higher compared to that of Fe–1%C steels irrespective of the method of deformation. Corrosion rate is found to decrease drastically with increase in the percentage theoretical density in both materials. The highest density obtained in both alloys is seldom exceeding 98% of theoretical density irrespective of the mode of deformation. Presence of porosity leads to enhanced rate of uniform corrosion in both Fe and Fe–1%C carbon steels. Microstructure of the forged alloys revealed the presence of numerous fine rounded pores both within grains and along grain boundaries.