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在(1)的基础上用复型及薄膜透射电镜技术观察与分析了形变热处理50CrVA 钢丝的显微组织结构,目的在于探索钢丝形变热处理的强韧化机理。在透射电镜下观察了马氏体形态及结构、发现仅在～1μm 范围内即存在着多种组织组成物:碳化物沿位错析出,弥散而均匀。马氏体板条间存在残余奥氏体膜,此膜在400℃回火时转变为渗碳体,引起材料韧性下降。组织中的未溶碳化物,周围环绕着一层高度畸变区。能谱分析证实它为一高铬合金渗碳体。用复型及薄膜透射电镜分析了组织中 M—A 岛、复型电镜观察到多种 M—A 形态、M—A 岛呈延伸变形状,表明它产生于分级之间,形变之前。透射电镜分析明确 M—A 岛内部亚结构为胞状位错网络及孪晶,位错网络上析出有微细碳化物。经400℃回火后渗碳体沿(112)孪晶面析出,其方向符合孪生方向。观察与分析确定 M—A 岛为一高碳区,其禽碳量约为1%,其断裂特征为解理断裂。当 M—A 岛的粒度由3μm 细化至～1μm 时,钢丝扭转断裂周次由3～5周提高至16～23周。结论指出形变淬火钢丝的主要强化因素为弥散强化及位错亚结构强化,而碳化物在位错处择优析出增强了这两种强化效果,M—A 岛作为“硬质”颗粒,对强化亦有所贡献,材料的韧化因素归结为:组织细化,亚结构细化,马氏体板条间存在残余奥氏体膜等。
On the basis of (1), the microstructure of 50CrVA steel wire was observed and analyzed by double-layer and thin-film transmission electron microscopy. The purpose of this study is to explore the mechanism of strengthening and toughening steel wire. The morphology and structure of the martensite were observed under a transmission electron microscope. It was found that there are many kinds of microstructures in the range of ~ 1μm: carbides dislocated along the dislocation, diffused and uniform. Residual martensite lath between the austenite film, the film at 400 ℃ tempering transformed into cementite, causing the material to decline in toughness. The organization of undissolved carbides, surrounded by a layer of highly distorted area. Energy spectrum analysis confirmed it as a high-chromium alloy cementite. The M-A island was observed by complex and thin-film transmission electron microscopy. Multiple M-A morphologies were observed under SEM, and the M-A island was in an extended shape, indicating that it originated between grading and before deformation. Transmission electron microscopy revealed that the intrastructural structure of M-A island was a dislocation network and twins, and fine carbides precipitated on the dislocation network. After 400 ℃ tempering cementite along the (112) twin crystal surface, the direction in line with twin direction. Observation and analysis confirmed that M-A island is a high-carbon zone with about 1% of its bird’s carbon, and its fault is characterized by cleavage and fracture. When the grain size of the M-A island was refined from 3 μm to -1 μm, the wire twisting and breaking cycle was increased from 3 to 5 weeks to 16 to 23 weeks. It is concluded that the main strengthening factors for the deformation quenched steel wire are dispersion strengthening and dislocation sub-structure strengthening, while the carbide preferential precipitation at dislocations enhances the two strengthening effects. The M-A island acts as a “hard” Contributions, material toughening factors attributed to: refinement, sub-structure refinement, there are residual martensite lath between the austenite film.