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在许多实际场合,象AISI 4140,4340这样的超高强度低合金钢,由于在低温条件下低劣的延性和缺口韧性,使其应受到用限制。近年来已作了相当多的努力,采用不同的热处理技术或进行成分调整,以改善室温下的机械性能。遗憾的是,以上方法对于超高强度低合金钢的低温机械性能没有显效。根本问题在于保持高强度水平的回火马氏体组织本身对低温脆性敏感。我们已经有了一个可能的解决办法,就是使4140及4340型超高强度钢具有马氏体和下贝氏体的混合组织。混合组织改善机械性能的效果出于以下事实:下贝氏体以针状出现,分割了转变前的原奥氏体晶粒,有效地引起马氏体精细结构(即板条宽度和板条束
In many practical applications, ultra high strength low alloy steels such as AISI 4140 and 4340 should be limited due to their poor ductility and notch toughness at low temperatures. In recent years, considerable efforts have been made to employ different heat treatment techniques or compositional adjustments to improve the mechanical properties at room temperature. Unfortunately, the above method is not effective for the low temperature mechanical properties of ultra-high strength low alloy steel. The fundamental problem is that tempered martensite, which maintains high levels of strength, is itself susceptible to low temperature brittleness. One possible solution we have already had is to make 4140 and 4340 ultra-high strength steels with mixed martensite and lower bainite structures. The effect of the mixed microstructure on improving the mechanical properties is due to the fact that the lower bainite appears in the form of needles that segregate the pre-transformation austenite grains and effectively cause the fine structure of the martensite (ie the width of the slats and the thickness of the slat