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，这一点至关重要。这些性能是由它们的显微组织决定的，这些组织通常由一种称为“奥氏体化、淬火和回火”的工艺路线控制。奥氏体化步骤是一种高温保温，它使整个材料的显微组织均质化为单一相-奥氏体相。然后，钢在淬火步骤中冷却，形成贝氏体和马氏体等强烈的微观成分。然后需要回火步骤，通过消除内应力和在马氏体/贝氏体中析出碳化物来增加韧性。最近的工作表明，低合金钢组织中存在不利于韧性的大碳化物，这可能是富碳残余奥氏体岛在回火过程中分解的结果(残余奥氏体=淬火后残留的奥氏体)。我们已经测量到，在淬火步骤后的大型锻件中可能存在大量的残余奥氏体量(&gt；10%)，但回火对这些岛的影响仍然不太清楚。例如，这些岛在回火热处理的哪个阶段分解，以及当使用不同的回火温度时，它们是否形成不同的微观组织，目前还不清楚。本项目旨在详细描述低合金钢(SA540、SA508 3级和SA508 4N级)中残余奥氏体的分解过程，并了解粗大碳化物形成的条件。它将使用扫描和透射电子显微镜、光学显微镜、膨胀测量法和同步辐射X射线衍射等技术来表征奥氏体分解和由此产生的微观结构。利用显微硬度测试和夏比冲击试验来评估不同回火后组织对力学性能的影响。如果时间允许，将对化学异质材料(即标准锻造材料)和均化材料中的残余奥氏体行为进行比较，以确保整个过程中化学成分的一致性。

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.