Development of High Performance Sintered Alloy Steels by Metal Injection Molding

这项工作的目的是测试的可行性，使用金属注射成型（MIM）的方法，以达到高性能，通过一个新的制造路线。然而，仍然存在通过粉末类型的差异来控制最终产品中的碳含量和结构的问题。本文首先研究了4600钢的金属注射成形工艺，采用含0.71wt.% carbon.对于脱脂方法，溶剂脱脂，然后是最终的热脱脂产生均匀的结构和足够的刚性紧凑的处理。在最后一次热脱脂过程中，改变H_2和N_2混合气体中H_2的含量是控制残碳含量的主要因素。预合金细粉的烧结和热处理的MIM压坯的力学性能劣于混合元素粉末的相同处理的MIM压坯，特别是在延展性。这是由于预合金细粉（相对烧结密度为93-94%）致密度不够以及显微组织的差异造成的。对于接近无碳的预合金Cr-Mo粉末，在渗碳气氛（如CO气体）中脱脂时可发生渗碳，通过调节H_2和CO混合气体中的CO浓度可使碳恢复到所需水平。然而，与预合金粉末钢相比，该回火钢显示出优异的强度。这似乎是由于由回火马氏体网络包围的马氏体组成的结构。结果表明，采用金属注射成形技术可以获得高性能的铁基材料。

The purpose of the work is to test the feasibility of using the mental injection molding (MIM) approach to attain the high performance through a new fabrication route. However, there still remains the issue of controlling the carbon content and structure in the final product by the difference of powder type. In this study, MIM process for the 4600 steels has been studied at first by using prealloyed fine powder (particle size : 12mum) contained 0.71wt.% carbon. For the debinding approach, solvent debinding followed by final thermal debinding produced the homogeneous structure and stiff compact enough for handling. The residual carbon content was mainly controlled by changing the fraction of H_2 in gas mixture of H_2 and N_2 during the final thermal debinding. Mechanical properties of the sintered and the heat treated MIM compacts with prealloyed fine powders were inferior to those of the same treated MIM compacts with mixed elemental powders, especially in the ductility. This is due to the insufficient densification of prealloyed fine powder (relative sintered density : 93-94%) and the difference of microstructures.Subsequently, the same experiment has been performed for the Ni-Mo and Cr-Mo steels. For the nearly carbon-free prealloyed Cr-Mo powder, carburization could take place in the debinding by using a carburizing atmosphere such as CO gas and carbon could be restored to the desired level by adjusting the CO concentration in gas mixture of H_2 and CO.The mixed elemental powder was used for the Ni-Mo steels, resulted in the heterogeneous structure by segregation of Ni. However, this tempered steel showed a excellent strength compared to that of the prealloyed powder steel. This seems to be due to the structure consisted of martensite surrounded by a network of tempered martensite. The obtained results indicate the attainability of high performance ferrous materials by the MIM techniques.