| dc.description.abstract | This study used BASF-Catamold MIM 4140 (42CrMo4) metal powder as the injection material. Tensile samples were injected and sintered at five different temperatures. Then, the density of the samples was measured to select the suitable sintering temperature and homogenize the samples to improve the hardness, abrasion resistance, and uniformity of the internal distribution of the material. Next, the test bars were divided into two categories, without shot peened and shot peened, for shot peening tests and Hv hardness tests. Based on the results of the Hv hardness tests, Taguchi′s method was used to optimize the experimental design and select the best experimental group. Finally, the test bars were subjected to nitriding heat treatment after shot peening, nitriding heat treatment only, quenching and tempering followed by shot peening before nitriding heat treatment, and quenching and tempering followed by nitriding heat treatment, subsequently, Hv hardness testing was conducted. Finally, tensile testing was used to get the ultimate strength, and then the microstructure was observed at crack.
The experimental results showed that a suitable sintering temperature of 1340°C was selected from the density test to minimize the need for net zero carbon emissions. The hardness test results (before heat treatment) found that the Hv hardness value at a depth of 0.1 mm increases with increasing grain size, pressure, and time in the shot peening condition. Taguchi′s method of optimizing the Hv hardness values from the hardness test (before heat treatment) resulted in the optimum combination of group 18 in the L18 (21×32) orthogonal array. The surface hardness of the nitrided test bars and the quenched-and-tempered test bars both increased significantly. Additionally, nitriding heat treatment after shot peening further increased surface hardness. According to the hardness results (after nitriding), the hardness of the shot peening test bars increased by approximately 113% compared to nitriding alone, and by about 59% compared to the quenched-and-tempered test bars. When comparing quenching and tempering followed by nitriding with the combination of shot peening and nitriding after quenching and tempering, the latter improved hardness by approximately 59%. Since both treatments increase the hardness of the test bars by more than 50%, shortening the current nitriding heat treatment time by 50% (to 3 hours) could still achieve comparable hardness to the original process. Additionally, different shot peening conditions can be selected based on hardness requirements, reducing energy consumption and carbon emissions, and helping to achieve the goal of energy conservation and carbon reduction.
Finally, the tensile test results showed that the highest and lowest tensile strength and elongation in the two nitriding treatments were found in the without-shot-peened group and the L18 group 18, respectively. In particular, the yield strength, tensile strength, and elongation in the without-shot-peened group of the nitriding treatment after quenching and tempering were the most balanced, with respective values of 1015.7 MPa, 1197.8 MPa and 7.6 %. This makes it the most suitable for applications requiring hardening, meeting the targets for applications that demand excellent mechanical properties without needing to further increase surface hardness. | en_US |