Redistribution of Mn between α-Fe matrix and θ cementite during long-term thermal aging in a low alloy steel
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摘要: 利用原子探针层析技术研究了核反应堆压力容器（RPV）模拟钢调质处理后在370和400 ℃长期时效以及淬火后在400 ℃长期时效后Mn在α-Fe基体与渗碳体间重分布的特征。研究结果表明，在所有热处理条件下，Mn均会从α-Fe基体向渗碳体内扩散，引起渗碳体内Mn浓度升高。其中淬火后直接在400 ℃时效条件下试样中渗碳体内的Mn浓度最高。即使在400 ℃经过35000 h长时间时效，Mn在渗碳体内的浓度仍未达到平衡，需要进一步延长时效时间，这与Mn在400 ℃在α-Fe基体中扩散速率极其缓慢有关。此外，Mn在渗碳体内的分布也不均匀，在靠近α-Fe基体/渗碳体界面附近的渗碳体一侧存在Mn的原子偏聚区，偏聚区Mn浓度随时效温度升高而增加。长时间时效后，Mn在两相间重分布特征与Mn在渗碳体内扩散速率低于Mn在α-Fe基体中扩散速率有关。Abstract: The addition of certain amounts of Mn in steel has long been known to retard the growth and coarsening of cementite during tempering, which can increase the tempering resistance of carbon steels. It is now well-established that the retarding effect is inherently correlated with the partitioning of Mn between ferrite (α) matrix and cementite (θ). According to the equilibrium thermodynamics, Mn would diffuse from α-Fe matrix to θ cementite after the initial stage of tempering until equilibrium is reached. However, the manner in which Mn diffuses from α-Fe matrix to θ cementite is unclear, which is key in understanding the mechanism in which the partitioning of Mn can retard the growth and coarsening of cementite. Therefore, the measurement of Mn content across the α-Fe/θ interface is of importance to achieve this goal. In this study, the redistribution characteristics of Mn between α-Fe matrix and θ cementite after long-term aging at 370 or 400 °C with quenched–tempered or quenched samples of reactor pressure vessel model steel was investigated by atom probe tomography. Results show that Mn diffuses from the α-Fe matrix and enriches in the θ cementite under all heat treatment conditions. The concentration of Mn in cementite is the highest when the specimen is thermally aged directly after quenching. Moreover, Mn is not distributed uniformly within cementite after long-term aging at 400 °C for 35000 h. Instead, a Mn-segregated zone exists within cementite adjacent to the α-Fe/θ interface, with concentration increasing by aging temperature, which acts as a barrier to the coarsening of cementite by hindering the dissolution of small-sized cementite. The redistribution characteristics of Mn between the two phases is correlated with the difference of diffusivities in the α-Fe matrix and θ cementite during thermal aging, and the diffusivity of Mn in θ cementite is slower than that in α-Fe matrix.
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图 1 调质处理的试样在370 ℃时效28800 h后C和Mn原子的分布图（a），沿垂直于α/θ界面方向各合金元素成分的分布图：Fe、C、Mn（b），Mo、Si、Ni、Cu（c），P（d）
Figure 1. Atom maps of C and Mn in a quenched-tempered sample after thermal aging at 370 ℃ for 28800 h (a), composition profiles of Fe, C, and Mn (b), Mo, Si, Ni, and Cu (c), and P (d) across the α/θ interface
图 2 调质处理的试样在400 ℃时效35000 h后C和Mn原子的分布图（a）和Fe、C、Mn沿垂直于α/θ界面方向成分的分布图（b）
Figure 2. Atom maps of C and Mn in a quenched-tempered RPV steel sample after thermal aging at 400 ℃ for 35000 h (a) and composition profiles of Fe, C, and Mn across the α/θ interface (b)
图 3 淬火试样在400 ℃时效35000 h后C和Mn原子分布图（a）和Fe、C、Mn沿垂直于α/θ界面方向成分分布图（b）
Figure 3. Atom maps of C and Mn in a quenched sample after thermal aging at 400 ℃ for 35000 h (a) and composition profiles of Fe, C, and Mn across the α/θ interface (b)
表 1 A508-Ⅲ钢的化学成分
Table 1. Nominal chemical composition of A508-III steel with high Cu content
Content/% Cu Ni Mn Si P C S Mo Fe Atomic fraction 0.53 0.81 1.60 0.77 0.03 1.00 0.011 0.31 Bal. Mass fraction 0.60 0.85 1.58 0.39 0.016 0.22 0.006 0.54 Bal. 
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