Hardfacing of Iron and Steel Utilizing Cast Iron Scraps

在Fe-C-Cr三元系中出现的M_7C_3可形成纯碳化物，具有熔化温度低、硬度高、与铁基结合强度好等特点。本文研究了熔炼原料获得的块体碳化铬的组织和性能，发现低铬碳化物的组织由M_7C_3和莱氏体相组成，而50%和60%铬的碳化物则为纯的M_7C_3碳化物。在此基础上，以铸铁废料为原料，研究了M_7C_3碳化铬在钢铁堆焊中的应用。激光熔覆硬质合金涂层的硬度随铬含量的增加而增加，结果表明，经激光表面熔化处理的含60%铬的堆焊层由几乎纯的M_7C_3碳化物组成，具有良好的耐磨性。由于这些碳化物的热膨胀系数接近钢铁的热膨胀系数，因此堆焊层与铁基体结合紧密。以废铸铁为原料，一次激光熔化可获得硬度高、耐磨性好的显微组织，而用灰铁和球墨铸铁则无明显差别。激光堆焊材料的断面上出现了一些裂纹，垂直于碳化物与基材的界面。结果表明，采用耦合扩散工艺将铁基亚表面组织转变为功能梯度组织，可以有效地抑制界面附近的残余应力集中，从而抑制碳化物层的开裂。

M_7 C_3 appearing in the Fe-C-Cr ternary system can be formed as a pure carbide and is expected to have a low melting temperature, a high hardness and excellent bonding strength with iron substrate. In the present work, microstructure and properties of bulk chromium carbide specimen obtained by melting raw materials have been investigated and it is found that the microstructure of carbide specimen with low chromium content consists of M_7 C_3 and ledeburite, while those of 50%Cr and 60%Cr specimens are obtained as a pure M_7 C_3 carbide. On the basis of these findings, an application of M_7 C_3 chromium carbide as a hardfacing of iron and steel was examined utilizing cast iron scraps as starting materials. Hardness of laser-clad carbide layr was increased with chromium content and it was revealed that the hardfacing layr containing 60%Cr subjected to laser surface melting consists of almost pure M_7 C_3 carbide and shows excellent wear property. Since these carbide was found to posess a coefficient of thermal expansion near those of iron and steel, the hardfacing layr showed a tight bonding with iron sustrate. In the case of utilizing cast iron scraps as a raw material, the same microstructure having a high hardness and an excellent wear resistance was obtained through one-pass laser melting and no significant deference was observed when using gray iron scraps and S.G.iron scraps. Some cracking was seen on the section of laser-hardfacing materials, running perpendicular to the interface between carbide and substrate. It was indicated that the coupled-diffusion processing for the modification of the microstructure of subsurfacc of the iron substrate to functionally graded microstructure can be effective to supress the residual stress concentration near the interface and hence cracking in the carbide layr.