Retained Austenite Decomposition, and its Effect on Microstructure and Properties in Low-Alloy Steels.

能源应用中使用的低合金钢必须具有优异的强度和韧性。这些性能由它们的微观结构决定，微观结构通常由称为“淬火、淬火和回火”的加工路线控制。结晶步骤是高温保持，其使整个材料的微观结构均匀化为单相-奥氏体。然后在淬火步骤中冷却钢，以形成强的微观成分，如贝氏体和马氏体。然后需要回火步骤以通过消除内应力和在马氏体/贝氏体中沉淀碳化物来增加韧性。最近的研究表明，在低合金钢显微组织中存在对韧性有害的大碳化物，可能是回火过程中富碳残余奥氏体岛分解的结果（残余奥氏体=淬火后的残余奥氏体）。我们已经测量到在淬火步骤之后，在大型锻件中可能存在显著水平的残余奥氏体（>10%），但是回火对这些岛的影响仍然不太清楚。例如，尚不清楚这些岛在回火热处理期间的哪个阶段分解，以及当使用不同的回火温度时它们是否形成不同的微观结构。本项目旨在详细研究低合金钢（SA 540、SA 508 Grade 3和SA 508 Grade 4 N）中残余奥氏体的分解过程，并了解粗碳化物形成的条件。它将使用诸如扫描和透射电子显微镜、光学显微镜、硬度计和同步加速器X射线衍射等技术来分析奥氏体分解和由此产生的微观结构。它将使用显微硬度测试和夏比冲击测试来评估不同回火后显微组织所带来的机械性能变化。如果时间允许，将在化学不均匀的材料（即，标准锻造材料）和已均质化的材料，以确保整个过程中化学性质一致。

It is critically important that the low-alloy steels used in energy applications have excellent strengths and toughnesses. These properties are determined by their microstructures, which are typically controlled by a processing route referred to as 'austenitise, quench and temper'. The austenitise step is a high-temperature hold, which homogenises the microstructure across the material as a single phase - austenite. The steel is then cooled in the quench step, to form strong microconstituents such as bainite and martensite. A tempering step is then required to increase toughness, by relieving internal stresses and precipitating carbides in the martensite/bainite. Recent work has indicated that the presence of large carbides in low-alloy steel microstructures, which are detrimental for toughness, may result of the decomposition of islands of carbon-enriched retained austenite during tempering (retained austenite = austenite retained after quenching). We have already measured that significant levels of retained austenite (>10%) are likely to be present in large forgings following the quenching step, but the effect of tempering on these islands remaining less well understood. For instance, it is not clear at what stage during the tempering heat treatment these islands decompose, and whether they form different microstructures when different tempering temperatures are used. This project aims to characterise the process of retained austenite decomposition in low-alloy steels (SA540, SA508 Grade 3 and SA508 Grade 4N) in detail, and understand the conditions under which coarse carbides form. It will use techniques such as scanning and transmission electron microscopy, optical microscopy, dilatometry and synchrotron X-ray diffraction to characterise the austenite decomposition and the resulting microstructures. It will use microhardness testing and Charpy impact testing to assess the change in mechanical properties brought about by different post-temper microstructures. If time permits, a comparison will be made between the retained austenite behaviours in material that is chemical heterogeneous (i.e., standard wrought material) and material that has been homogenised to ensure a consistent chemistry throughout.