低碳低合金鋼時效過程中Mn在α-Fe與滲碳體間重分布特征

摘要: 利用原子探針層析技術研究了核反應堆壓力容器（RPV）模擬鋼調質處理后在370和400 ℃長期時效以及淬火后在400 ℃長期時效后Mn在α-Fe基體與滲碳體間重分布的特征。研究結果表明，在所有熱處理條件下，Mn均會從α-Fe基體向滲碳體內擴散，引起滲碳體內Mn濃度升高。其中淬火后直接在400 ℃時效條件下試樣中滲碳體內的Mn濃度最高。即使在400 ℃經過35000 h長時間時效，Mn在滲碳體內的濃度仍未達到平衡，需要進一步延長時效時間，這與Mn在400 ℃在α-Fe基體中擴散速率極其緩慢有關。此外，Mn在滲碳體內的分布也不均勻，在靠近α-Fe基體/滲碳體界面附近的滲碳體一側存在Mn的原子偏聚區，偏聚區Mn濃度隨時效溫度升高而增加。長時間時效后，Mn在兩相間重分布特征與Mn在滲碳體內擴散速率低于Mn在α-Fe基體中擴散速率有關。
- 核反應堆壓力容器鋼 /
- 原子探針層析技術 /
- Mn原子偏聚 /
- 合金元素重分布 /
- 準平衡析出
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.
- nuclear reactor pressure vessel steel /
- atom probe tomography /
- Mn segregation /
- redistribution /
- para-equilibrium precipitation

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圖 1 調質處理的試樣在370 ℃時效28800 h后C和Mn原子的分布圖（a），沿垂直于α/θ界面方向各合金元素成分的分布圖：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

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圖 2 調質處理的試樣在400 ℃時效35000 h后C和Mn原子的分布圖（a）和Fe、C、Mn沿垂直于α/θ界面方向成分的分布圖（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)

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圖 3 淬火試樣在400 ℃時效35000 h后C和Mn原子分布圖（a）和Fe、C、Mn沿垂直于α/θ界面方向成分分布圖（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)

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表 1 A508-Ⅲ鋼的化學成分

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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