Development of High Performance Sintered Alloy Steels by Metal Injection Molding

本工作的目的是测试通过一种新的制造路线，使用金属注射成型（MIM）方法获得高性能的可行性。但是，通过粉末类型的不同来控制最终产品中的碳含量和结构仍然是一个问题。在这项研究中，首先研究了4600钢的MIM工艺，使用含有0.71wt的预合金细粉（粒度：12mum）。%碳。对于脱脂方法，溶剂脱脂后，最后的热脱脂产生均匀的结构和刚性紧凑，足以处理。在最终热脱附过程中，通过改变H_2与N_2混合气体中H_2的组分来控制残余碳含量。预合金细粉烧结和热处理后的MIM材料的力学性能比混合元素粉烧结后的MIM材料的力学性能差，特别是在延展性方面。这是由于预合金细粉致密性不足（相对烧结密度：93-94%）和显微组织差异所致。随后，对Ni-Mo和Cr-Mo钢进行了同样的实验。对于几乎无碳的预合金化Cr-Mo粉末，通过CO气体的渗碳气氛进行脱碳，通过调节H_2和CO混合气体中的CO浓度，可以使碳恢复到理想的水平。然而，与预先合金化的粉末钢相比，这种回火钢表现出优异的强度。这似乎是由于组织由马氏体包围的回火马氏体网络。研究结果表明，利用MIM技术制备高性能铁材料是可行的。

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.