EN1.4418超麻田散鐵不鏽鋼顯微結構與機械性能之研究

Abstract

本研究探討了不同淬火及回火溫度搭配不同回火時間對QT900等級EN1.4418超麻田散鐵不鏽鋼顯微組織演進及機械性能的影響。研究挑戰了標準熱處理溫度，採用降低固溶溫度以達成晶粒細化，或降低回火溫度以減少由差排回復引起的強度損失，目標是在保持良好的延伸率的情況不下犧牲過多強度。研究結果顯示，當固溶熱處理溫度從950°C (Q950)降低至750°C (Q750)時，可有效地將原沃斯田鐵晶粒尺寸從20µm縮小至10µm，晶界密度的增加為後續回火過程中沃斯田鐵逆相變的成核成長提供了更多位點。相同回火參數下，Q750的殘留沃斯田鐵體積分率高於Q950，並且對後續殘留沃斯田鐵的分布及形貌也產生了影響。然而，晶粒細化未能貢獻強化效應，原因是Q750下會析出Cr23C6，導致強度下降；而在Q950條件下，Cr完全固溶，從而提供了碳原子的固溶強化作用。 研究中還發現，未回火樣品的拉伸結果已能符合QT900規範中的16%延伸率，這是由於極低碳含量的軟質麻田散鐵組織具有優異的抵抗孔洞膨脹特性，能夠彌補整體延伸性能的不足。回火後，殘留沃斯田鐵體積分率隨回火溫度從500°C至650°C呈現先升後降的趨勢，這與沃斯田鐵的穩定性有關。沃斯田鐵逆相變的程度對拉伸性質的表現具有顯著影響。殘留沃斯田鐵有助於提高均勻延伸率，延遲頸縮的發生。同時，隨著回火溫度的升高，差排回復與沃斯田鐵逆相變所造成的強度損失也隨之增大。在500°C低溫回火時，觀察到顯著的二次硬化現象，尤其在晶粒細化的Q750樣品中更為明顯，這使得降伏強度提升300MPa，而Q950則提升了70MPa。這一現象可能與碳化物在不同回火溫度下的變化有關，推論低溫回火可能使M3C溶解，或是析出細小M2X碳化物，從而貢獻固溶強化或析出強化作用。

This study investigates the relationship between the microstructural evolution and mechanical properties of QT900 grade EN1.4418 supermartensitic stainless steel, subjected to various quenching and tempering temperatures combined with different tempering times. The research challenges conventional heat treatment standards by employing lower austenitization temperature to achieve grain refinement and lower tempering temperature to reduce strength loss caused by dislocation recovery. The aim is to maintain a good elongation without sacrificing excessive strength. The experimental results indicate that reducing the solution treatment temperature from 950°C (Q950) to 750°C (Q750) effectively reduces the prior austenite grain size from 20 µm to 10 µm. The increased grain boundary density provides more nucleation sites for austenite reversion during subsequent tempering. When comparing the same tempering parameters, the volume fraction of retained austenite in Q750 is higher than in Q950, which also affects the distribution and morphology of the retained austenite. However, the grain refinement did not contribute to strengthening, as carbide precipitation (Cr23C6) occurs under the Q750 condition, resulting in a reduction in strength. In contrast, the chromium in Q950 remains fully dissolved, providing solid solution strengthening through carbon atoms. Additionally, tensile tests of the as-quenched samples reveal that the elongation already meets the QT900 specification of 16%, which can be attributed to the excellent resistance to void expansion of the soft martensitic structure, due to its very low carbon content. This allows the post elongation to compensate for the overall elongation performance. After tempering, the volume fraction of retained austenite increases and then decreases as the tempering temperature rises from 500°C to 650°C, which is related to the stability of the austenite phase. The degree of austenite reversion significantly influences the tensile properties. Retained austenite contributes to the better uniform elongation and delays necking. It was observed that as the tempering temperature increases, the dislocation recovery and austenite reversion cause greater strength loss. At a low tempering temperature of 500°C, a secondary hardening phenomenon was observed, particularly in the Q750 sample with refined grain, leading to an increase in yield strength by 300 MPa, while Q950 saw an increase of 70 MPa. This phenomenon may be related to the changes in carbides at different tempering temperatures. It is speculated that low-temperature tempering could lead to the dissolution of M3C or the precipitation of fine M2X carbides, thereby contributing to solid solution strengthening or precipitation strengthening.