Steels are ubiquitous due to their affordability and the landscape of useful properties that can be generated for engineering applications. But to further expand the performance envelope, one must be able to understand and control microstructure development by alloying and processing. Here we use multiscale, advanced characterization to better understand the structural and chemical evolution of AISI 4340 steel after quenching and tempering (Q&T), including the role of quench rate and short-time, isothermal tempering below 573 K (300 °C), with an emphasis on carbide formation. We compare the microstructure and/or property changes produced by conventional tempering to those produced by higher temperature, short-time “rapid” tempering. We underscore that no single characterization technique can fully capture the subtle microstructure changes like carbon redistribution, transition carbide and/or cementite formation, and retained austenite decomposition that occur during Q&T. Only the use of multiple techniques begins to unravel these complexities. After controlled fast or slow quenching, η transition carbides clearly exist in the microstructure, likely associated with autotempering of this high martensite start temperature (M_s) steel. Isothermal tempering below 598 K (325 °C) results in the relief of carbon supersaturation in the martensite, primarily by the formation of η transition carbides that exhibit a range of carbon levels, seemingly without substitutional element partitioning between the carbide and matrix phases. Hägg transition carbide is present between 300 °C and 325 °C. After conventional tempering at or above 598 K (325 °C) for 2 h, cementite is predominant, but small amounts of cementite are also present in other conditions, even after quenching. Previous work has indicated that silicon (Si) and substitutional elements partition between the cementite, which initially forms under paraequilibrium conditions, and the matrix. Phosphorous (P) may also be preferentially located at cementite/matrix interfaces after high temperature tempering. Slower quench rates result in greater amounts of retained austenite compared to those after fast quenching, which we attribute to increased austenite stability resulting from “autopartitioning”. Rapid, high temperature tempering is also found to diminish tempered martensite embrittlement (TME) believed to be associated with the extent of austenite decomposition, resulting in mechanical properties not attainable by conventional tempering, which may have important implications with respect to industrial heat treatment processes like induction tempering. Controlling the amount and stability of retained austenite is not only relevant to the properties of Q&T steels, but also next-generation advanced high strength steels (AHSS) with austenite/martensite mixtures.

钢材因其价格适中以及工程应用可产生的有用特性而无处不在。但是，要进一步扩大性能范围，必须能够通过合金化和加工来理解和控制微观结构的发展。在这里，我们使用多尺度的高级表征来更好地了解淬火和回火（Q＆T）之后的AISI 4340钢的结构和化学演变，包括淬火速率和短时，低于573 K（300°C）等温回火的作用。强调碳化物的形成。我们将常规回火与高温，短时间“快速”回火所产生的微观结构和/或性能变化进行比较。我们强调指出，没有一种单一的表征技术可以完全捕获Q＆T过程中发生的细微微观结构变化，例如碳的重新分布，过渡碳化物和/或渗碳体的形成，以及残留的奥氏体分解。只有使用多种技术才能开始解决这些复杂性。经过控制的快速或缓慢淬火后，η过渡碳化物显然存在于显微组织中，可能与这种高马氏体起始温度（M _s）钢的自回火有关。低于598 K（325°C）的等温回火主要通过形成η来缓解马氏体中的碳过饱和。表现出一定碳含量范围的过渡碳化物，似乎在碳化物相与基体相之间没有替代元素分配。Hägg过渡碳化物存在于300°C至325°C之间。在598 K（325°C）或以上的温度下进行常规回火2 h后，渗碳体占主导地位，但即使在淬火后，在其他条件下也存在少量渗碳体。先前的工作表明，硅（Si）和替代元素在最初在超平衡条件下形成的渗碳体与基质之间分配。高温回火后，磷（P）也可能优先位于渗碳体/基体界面。与快速淬火后相比，较低的淬火速度会导致残留的奥氏体数量更多，我们将其归因于“自动分区”带来的奥氏体稳定性的提高。还发现快速高温回火可以减少回火马氏体脆化（TME），这被认为与奥氏体分解的程度有关，从而导致常规回火无法达到的机械性能，这对于诸如工业热处理的工艺可能具有重要意义。感应回火。控制残留奥氏体的数量和稳定性不仅与Q＆T钢的性能有关，而且与奥氏体/马氏体混合物组成的下一代高级高强度钢（AHSS）也有关。还发现高温回火可减少被认为与奥氏体分解程度有关的回火马氏体脆化（TME），从而导致常规回火无法获得的机械性能，这可能对工业热处理工艺（如感应回火）产生重要影响。控制残留奥氏体的数量和稳定性不仅与Q＆T钢的性能有关，而且与奥氏体/马氏体混合物组成的下一代高级高强度钢（AHSS）也有关。还发现高温回火可减少被认为与奥氏体分解程度有关的回火马氏体脆化（TME），从而导致常规回火无法获得的机械性能，这可能对工业热处理工艺（如感应回火）产生重要影响。控制残留奥氏体的数量和稳定性不仅与Q＆T钢的性能有关，而且与奥氏体/马氏体混合物组成的下一代高级高强度钢（AHSS）也有关。