Abstract For the first time, cyclic re-austenitization is carried out for additively manufactured high-strength low-alloy (HSLA) steels in order to refine the microstructure by reducing the prior austenite grain (PAG) size. In this work, HSLA-100 steels processed using laser powder bed fusion (LPBF) technique are subjected to several cycles of re-austenitization using quenching dilatometry. Microstructure characterization for every cycle revealed the presence of bainite, martensite and martensite/austenite (M/A) islands. From the analysis of the dilatometry curves and extensive microstructure characterization, it was found that till the 2nd cycle of re-austenitization, both PAG size and martensite start (Ms) temperature reduces, while the amount of bainite transformed decreased and the retained austenite content increased. Concomitantly, the highest microhardness along with peak nanohardness of the constituent phases was achieved at the 2nd cycle. Conversely, from the 3rd cycle, the microhardness, as well as the nanohardness of the constituent phases, are found to decrease due to an increase in the PAG size. This behavior is in contrast to the general tendency where a saturation limit is reached after the peak refinement is achieved. It is found that retained austenite can act as a pinning particle to obstruct the PAG boundary movement and its fraction is found to decrease from the 3rd cycle. Hence, the increase in PAG size after the 3rd cycle can be attributed to the destabilization of effective pinning particles to hinder the PAG boundary movement during the re-austenitization.

中文翻译：

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激光粉末床熔融法制备含铜高强度低合金钢的循环再奥氏体化

摘要 为了通过减小​​原始奥氏体晶粒（PAG）尺寸来细化显微组织，首次对增材制造的高强度低合金（HSLA）钢进行循环再奥氏体化。在这项工作中，使用激光粉末床融合 (LPBF) 技术加工的 HSLA-100 钢使用淬火膨胀法进行了几次再奥氏体化循环。每个循环的显微组织表征显示存在贝氏体、马氏体和马氏体/奥氏体 (M/A) 岛。从膨胀曲线分析和广泛的显微组织表征发现，直到第二次再奥氏体化循环，PAG尺寸和马氏体起始（Ms）温度均降低，而贝氏体相变量减少，残余奥氏体含量增加. 同时，在第二个循环中达到最高的显微硬度以及组成相的峰值纳米硬度。相反，从第 3 次循环开始，发现组成相的显微硬度和纳米硬度由于 PAG 尺寸的增加而降低。这种行为与在实现峰值细化后达到饱和极限的一般趋势相反。发现残余奥氏体可以充当钉扎粒子来阻碍 PAG 边界运动，并且发现其分数从第 3 次循环开始减少。因此，第三次循环后 PAG 尺寸的增加可归因于有效钉扎粒子的不稳定，从而阻碍了再奥氏体化过程中 PAG 边界的运动。相反，从第 3 个循环开始，发现组成相的显微硬度和纳米硬度由于 PAG 尺寸的增加而降低。这种行为与在实现峰值细化后达到饱和极限的一般趋势相反。发现残余奥氏体可以充当钉扎粒子来阻碍 PAG 边界运动，并且发现其分数从第 3 次循环开始减少。因此，第三次循环后 PAG 尺寸的增加可归因于有效钉扎粒子的不稳定，从而阻碍了再奥氏体化过程中 PAG 边界的运动。相反，从第 3 个循环开始，发现组成相的显微硬度和纳米硬度由于 PAG 尺寸的增加而降低。这种行为与在实现峰值细化后达到饱和极限的一般趋势相反。发现残余奥氏体可以充当钉扎粒子来阻碍 PAG 边界运动，并且发现其分数从第 3 次循环开始减少。因此，第三次循环后 PAG 尺寸的增加可归因于有效钉扎粒子的不稳定，从而阻碍了再奥氏体化过程中 PAG 边界的运动。这种行为与在实现峰值细化后达到饱和极限的一般趋势相反。发现残余奥氏体可以充当钉扎粒子来阻碍 PAG 边界运动，并且发现其分数从第 3 次循环开始减少。因此，第三次循环后 PAG 尺寸的增加可归因于有效钉扎粒子的不稳定，从而阻碍了再奥氏体化过程中 PAG 边界的运动。这种行为与在实现峰值细化后达到饱和极限的一般趋势相反。发现残余奥氏体可以充当钉扎粒子来阻碍 PAG 边界运动，并且发现其分数从第 3 次循环开始减少。因此，第三次循环后 PAG 尺寸的增加可归因于有效钉扎粒子的不稳定，从而阻碍了再奥氏体化过程中 PAG 边界的运动。