The solidification structure of the as-cast consists of the matrix structure that is predominantly austenite and precipitated chromium carbide along the grain boundary. Under these circumstances and where the level of...The solidification structure of the as-cast consists of the matrix structure that is predominantly austenite and precipitated chromium carbide along the grain boundary. Under these circumstances and where the level of impact is relatively modest, such alloys in as-cast condition will perform. However, at higher levels of impact energy, a point is reached where excessive stress are built up within the component and eventually the materials strength is exceeded and the outcome is complete failure in a characteristic stress fracture mode. If this is to be prevented, it is therefore imperative that the casting be subjected to appropriate heat treatment, to obtain a structure which consist of Cr7C3 carbide and martensite at a hardness range of 650-750HB. The microstructure of NF6357A cast chromium steel containing 2.59% C- 0.7%Si-0.91%Mn-18.54%Cr-0.019%P-0.01%S- balance–Fe after appropriate heat treatment such as quenching and tempering process have been characterised by means of optical microscope, micro hardness tester, optical emission spectrometer and charpy testing machine. The results show that oil quenched samples were found to retained microstructural consistency for casting thicker than 120mm section. For economic argument, air quenched castings of less than 120mm thickness is not only cheaper alternative, but it is also environment friendly. The fracture toughness was found to be fairly consistent between 2.4-2.6%C range. However, at higher carbon level, the fracture process is dominated by the presence of segregated carbide network which act as a weak link in the microstructure. This weak link encourages dislocation pile-up and impaired material toughness.展开更多
目的研究时效-超声冲击(A-UIT)复合处理方法对铝合金激光焊缝耐磨性的影响。方法对7075铝合金激光焊缝进行时效处理、超声冲击处理、A-UIT复合处理,对比时效处理、超声冲击处理、A-UIT复合处理后焊缝的表面硬度、表面粗糙度、3D形貌及...目的研究时效-超声冲击(A-UIT)复合处理方法对铝合金激光焊缝耐磨性的影响。方法对7075铝合金激光焊缝进行时效处理、超声冲击处理、A-UIT复合处理,对比时效处理、超声冲击处理、A-UIT复合处理后焊缝的表面硬度、表面粗糙度、3D形貌及扫描显微组织,并分析A-UIT复合处理、时效处理和超声冲击处理对焊缝耐磨性能的影响及机理。结果超声冲击处理的焊缝与焊态进行比较,粗糙度减小了8.65μm,表面硬度增大了47HV,摩擦系数降低了0.17,磨损率降低了70%;6、24、32 h时效处理后的粗糙度分别比焊态减小了10.18、4.19、5.88μm,表面硬度比焊态高出37、55、44HV,平均摩擦系数比焊态降低0.10、0.08、0.09,磨损率比焊态低51%、54%、61%;6、24、32 h A-UIT复合处理分别与6、24、32 h时效处理相比,粗糙度减小了9.88、10.58、8.7μm,表面硬度增大了43、44、31HV,平均摩擦系数降低了0.1、0.1、0.07,磨损率降低了35%、41%、27%。A-UIT复合处理焊缝主要以磨粒磨损为主,且存在局部剥层磨损。单独时效处理焊缝主要以浅色的剥层磨损为主,且存在少量的磨粒磨损,磨损面积较大,并伴有少量的氧化磨损。结论时效和超声冲击处理均可有效提高焊缝的耐磨性能,超声冲击处理的作用要大于时效处理,摩擦系数比24 h时效处理小15.3%,磨损率为时效处理的36.1%。A-UIT复合处理后的焊缝在纳米晶和时效强化相的共同作用下,比时效处理和超声冲击处理的焊缝耐磨性能提升更为明显。24 h A-UIT复合处理后,焊缝的摩擦系数比24 h时效处理提高了28.5%,比超声冲击状态提高了8.6%,磨损率比24 h时效处理提高了40.9%,比超声冲击状态提高了76.9%。展开更多
文摘The solidification structure of the as-cast consists of the matrix structure that is predominantly austenite and precipitated chromium carbide along the grain boundary. Under these circumstances and where the level of impact is relatively modest, such alloys in as-cast condition will perform. However, at higher levels of impact energy, a point is reached where excessive stress are built up within the component and eventually the materials strength is exceeded and the outcome is complete failure in a characteristic stress fracture mode. If this is to be prevented, it is therefore imperative that the casting be subjected to appropriate heat treatment, to obtain a structure which consist of Cr7C3 carbide and martensite at a hardness range of 650-750HB. The microstructure of NF6357A cast chromium steel containing 2.59% C- 0.7%Si-0.91%Mn-18.54%Cr-0.019%P-0.01%S- balance–Fe after appropriate heat treatment such as quenching and tempering process have been characterised by means of optical microscope, micro hardness tester, optical emission spectrometer and charpy testing machine. The results show that oil quenched samples were found to retained microstructural consistency for casting thicker than 120mm section. For economic argument, air quenched castings of less than 120mm thickness is not only cheaper alternative, but it is also environment friendly. The fracture toughness was found to be fairly consistent between 2.4-2.6%C range. However, at higher carbon level, the fracture process is dominated by the presence of segregated carbide network which act as a weak link in the microstructure. This weak link encourages dislocation pile-up and impaired material toughness.
文摘目的研究时效-超声冲击(A-UIT)复合处理方法对铝合金激光焊缝耐磨性的影响。方法对7075铝合金激光焊缝进行时效处理、超声冲击处理、A-UIT复合处理,对比时效处理、超声冲击处理、A-UIT复合处理后焊缝的表面硬度、表面粗糙度、3D形貌及扫描显微组织,并分析A-UIT复合处理、时效处理和超声冲击处理对焊缝耐磨性能的影响及机理。结果超声冲击处理的焊缝与焊态进行比较,粗糙度减小了8.65μm,表面硬度增大了47HV,摩擦系数降低了0.17,磨损率降低了70%;6、24、32 h时效处理后的粗糙度分别比焊态减小了10.18、4.19、5.88μm,表面硬度比焊态高出37、55、44HV,平均摩擦系数比焊态降低0.10、0.08、0.09,磨损率比焊态低51%、54%、61%;6、24、32 h A-UIT复合处理分别与6、24、32 h时效处理相比,粗糙度减小了9.88、10.58、8.7μm,表面硬度增大了43、44、31HV,平均摩擦系数降低了0.1、0.1、0.07,磨损率降低了35%、41%、27%。A-UIT复合处理焊缝主要以磨粒磨损为主,且存在局部剥层磨损。单独时效处理焊缝主要以浅色的剥层磨损为主,且存在少量的磨粒磨损,磨损面积较大,并伴有少量的氧化磨损。结论时效和超声冲击处理均可有效提高焊缝的耐磨性能,超声冲击处理的作用要大于时效处理,摩擦系数比24 h时效处理小15.3%,磨损率为时效处理的36.1%。A-UIT复合处理后的焊缝在纳米晶和时效强化相的共同作用下,比时效处理和超声冲击处理的焊缝耐磨性能提升更为明显。24 h A-UIT复合处理后,焊缝的摩擦系数比24 h时效处理提高了28.5%,比超声冲击状态提高了8.6%,磨损率比24 h时效处理提高了40.9%,比超声冲击状态提高了76.9%。
