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

低合金钢中残余奥氏体分解及其对显微组织和性能的影响。

• 批准号: 2879345
• 金额: --
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2023
• 资助国家: 英国
• 起止时间: 2023 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2879345
• 关键词: Retained; Austenite; Decomposition; its; Effect

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.

在能源应用中使用的低合金钢具有优异的强度和韧性是至关重要的。这些性能是由它们的微观结构决定的，而微观结构通常由被称为“奥氏体化、淬火和回火”的加工路线控制。奥氏体化步骤是一个高温保持，它使整个材料的微观结构均匀化，成为单相奥氏体。然后在淬火步骤中冷却钢，形成强微组分，如贝氏体和马氏体。然后需要回火步骤，通过消除马氏体/贝氏体中的内应力和析出碳化物来提高韧性。最近的研究表明，低合金钢显微组织中存在不利于韧性的大型碳化物，这可能是由于回火过程中富碳残余奥氏体岛的分解造成的（残余奥氏体=淬火后残余奥氏体）。我们已经测量到，在淬火步骤之后，大型锻件中可能存在显著水平的残余奥氏体（bbb10 %），但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的哪个阶段分解，以及在不同的回火温度下它们是否形成不同的显微组织。本项目旨在详细描述低合金钢（SA540、SA508 Grade 3和SA508 Grade 4N）中残余奥氏体分解的过程，并了解粗碳化物形成的条件。它将使用扫描和透射电子显微镜、光学显微镜、膨胀测量和同步加速器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。将采用显微硬度测试和夏比冲击测试来评估不同回火后微观组织所带来的力学性能变化。如果时间允许，将对化学非均质材料（即标准锻造材料）和均质材料中保留的奥氏体行为进行比较，以确保自始至终具有一致的化学性质。