Investigation of alternative heat treatment applications on parts manufactured by forging method

In the production of automotive steering and suspension system parts, different heat treatment methods are applied in order to relieve the internal stresses in the material structure after hot forging, to increase its resistance to heat and corrosion, to increase its machinability, to bring it to a desired hardness value and to gain some mechanical properties. In this study, it is aimed to examine the effect of hardening heat treatment on microstructure and mechanical properties of C45 (SAE 1045) and 42CrMo4 (SAE 4140) quality steel parts used in steering and suspension system parts, which are shaped by hot forging, by applying an alternative hardening heat treatment (when the temperature of the parts left to cool after forging drops to the austenitizing temperature, they are immersed in the quenching medium and then tempered at the temperature and time determined in the conventional method) to the currently applied conventional method (parts cooled to room temperature after forging and applied a hardening heat treatment and then tempered). A total of 12 sample pieces has been prepared as; 3 from the C45 steel sample to which the conventional heat treatment method was applied, 3 from the C45 steel sample to which the alternative heat treatment method would be applied, 3 from the 42CrMo4 steel sample to which the conventional heat treatment method would be applied, and 3 from the 42CrMo4 steel sample to which the alternative heat treatment method would be applied. 3 Pieces of C45 Samples Conventional Method Heat Treatment Application: Ø32 steel bar material was supplied from steel companies, cut into ball joint forging draft lengths and prepared for hot forging. The draft material was preheated by induction before forging, and forging was carried out in an air hammer press at a temperature range of 1100 – 1250 °C. Immediately afterwards, a hot forged draft piece was removed with a 150-ton eccentric press shear. After hot forging, the samples, which were cooled in air and brought to room temperature, were annealed in the annealing furnace between 840 – 870 °C, brought to the austenite phase temperature and cooled in oil. With the annealing process, the hardness of the steel is reduced, its ductility is increased and its internal stresses are eliminated. The annealed and oil-cooled samples were tempered in the temperature range of 525 – 555 °C and then cooled in air. 3 Pieces of C45 Samples Alternative Method Heat Treatment Application: Ø32 steel bar material was supplied from steel companies, cut into ball joint forging draft lengths and prepared for hot forging. The draft material was preheated by induction before forging, and forging was carried out in an air hammer press at a temperature range of 1100 – 1250 °C. Immediately afterwards, a hot forged draft piece was removed with a 150-ton eccentric press shear. After hot forging, when the temperature of the C45 ball joint body sample, which was controlled in air, fell to the range of 850 - 880 °C, rapid cooling was done by dropping it into a pre-prepared 5% concentrated water - polymer liquid. Afterwards, the samples were tempered in the temperature range of 525 – 555 °C and cooled in air. 3 Piece 42CrMo4 Samples Conventional Method Heat Treatment Application: 42CrMo4 (SAE 4140) quality Ø60 diameter steel bar was supplied from steel companies as material, the pitman arm was cut into forging draft lengths and prepared for hot forging. The draft material was preheated by induction before forging, and forging was carried out in a 40 kJ Beche brand hydraulic hammer press at a temperature range of 1100 – 1200 °C. Immediately afterwards, the hot forged draft piece was removed with 250 tons of eccentric press shears. After hot forging, the samples, which were cooled in air and brought to room temperature, were annealed in the annealing furnace between 840 – 870 °C, brought to the austenite phase temperature and cooled in oil. The annealed and oil-cooled samples were tempered in the temperature range of 605 - 640 °C to reduce the internal stresses in the structure of the steel, and then cooled in air. 3 Pieces of 42CrMo4 Samples Alternative Method Heat Treatment Application: 42CrMo4 (SAE 4140) quality steel was supplied from steel companies as solid bar material in Ø60 diameter, pitman arm was cut in forging draft lengths and prepared for hot forging process. The draft material was preheated by induction before forging, and forging was carried out in a 40 kJ Beche brand hydraulic hammer press at a temperature range of 1100 – 1200 °C. Immediately afterwards, the hot forged draft piece was removed with 250 tons of eccentric press shears. After hot forging, when the temperature of the 42CrMo4 material pitman arm body sample, which is controlled in air, drops to the range of 850 - 880 °C, rapid cooling was done by dropping it into a pre-prepared 5% concentrated water - polymer liquid. Afterwards, the samples were tempered in the temperature range of 605 - 640 °C and cooled in air. The microstructure images of the C45 and 42CrMo4 steel samples used in the experiments, to which conventional and alternative heat treatment methods were applied, were examined with optical and scanning electron microscopy. Vickers hardness measurements of both material samples were made at the end of two different heat treatment methods. 6 different hardness measurements were taken from the edge and middle regions of each sample and the average of these 6 measurement results was recorded in the test results. 4 different groups of tensile test samples from the traditional and alternative heat treatment methods of both materials were prepared and tensile test was performed. In the tensile test results, the yield strength, tensile strength and elongation values were added to the test results for different heat treatment methods of both materials. In addition, the Charpy impact test was applied to the samples of both materials applied to the traditional and alternative heat treatment methods and added to the test results. The microstructures of the C45 and 42CrMo4 steel samples used in the experiments, to which conventional and alternative heat treatment methods were applied, were examined with an optical microscope, and the microhardness values were determined in the Vickers hardness device. The hardness values are given as the average of 6 different measurements, including the edge and center regions of the whole section. Tensile test samples were prepared and perfomed to tensile test for 4 different groups from the traditional and alternative heat treatment methods of both materials. The yield strength, tensile strength and % elongation values of the hardened and tempered steel parts were determined by the tensile test. In addition, the Charpy impact test was applied to the samples of both materials applied to the traditional and alternative heat treatment methods and added to the test results. According to the Vickers hardness measurement results obtained, it was observed that similar hardness values were obtained in both steel types (C45 and 42CrMo4) as a result of conventional and alternative heat treatment. While the average hardness values of C45 steel after the conventional heat treatment method were 280 HV, a similar hardness value of 280 HV was obtained after the alternative heat treatment. While the average hardness values of 42CrMo4 steel after conventional heat treatment method were 305 HV, 310 HV hardness values were obtained after alternative heat treatment. The measured hardness values show that the martensite structure can be formed after both hardening methods. Hardness values after hardening with the alternative method are slightly higher than conventional hardening. Although it has a coarser grain structure, the possible reason for the higher hardness value is the different quenching medium. Alternative heat treatment samples have higher tensile strength than conventional heat treatment samples in both steel types (C45 and 42CrMo4). The percent elongation amounts in the tensile test results of both steel types samples; It was observed that the samples applied alternative heat treatment were lower than the samples applied traditional heat treatment method. These results are also compatible with hardness values. The Charpy impact test results on samples of both steel types applied to the conventional and alternative heat treatment method are lower in the parts heat treated with the alternative method, confirming the tensile test. It has been observed that the steels subjected to heat treatment with the alternative method have a coarser grained microstructure, so the impact energy values are lower.