回火熱處理對YOKE 8625MX鋼的機械性質與氫脆性質影響

Abstract

YOKE 8625MX 的設計在性質上主要有幾點要求：良好的硬化能、高強度與 高衝擊韌性，且希望能減少回火脆性。YOKE 8625MX在喬米尼實驗中可以得知，即使在冷卻速度最慢的一端，依舊保有40 HRC 以上的硬度，代表具有良好的硬化能。YOKE 8625MX 在水淬態下具有高於1800 MPa 的高強度，在-40 °C 下衝擊韌性達40 J 以上，且在回火後並無明顯回火脆性。600 °C 回火態具有1109 MPa的高強度，其原因應該為二次強化效應。 麻田散鐵鋼屬於較容易受到氫脆效應影響的材料，故評估各熱處理狀態下的抗氫脆能力是相當重要的。本實驗藉由陰極電化學充氫將氫充入材料內，綜合SEM、TEM、拉伸實驗與熱脫氫實驗結果，以推測YOKE 8625MX 顯微結構與氫脆現象的關聯。由實驗結果可得，水淬態與200 °C 回火態下，主要氫捕集位置為差排；在400 °C 回火態下，氫捕集位置為差排與雪明碳鐵；在600 °C 回火態下，氫捕集位置為差排、雪明碳鐵與M7C3，其中差排與雪明碳鐵為較弱的氫捕集位置，M7C3 為較強的氫捕集位置。 水淬態有著0.96 ppm 最高的吸氫量與最差的抗氫脆能力；200 °C 回火態與400 °C 回火態的吸氫量下降至0.6 ppm，拉伸曲線能達降伏強度，抗氫脆能力較強；600 °C 回火態雖然有著0.95 ppm 的高吸氫量，但較低的差排密度與較強的氫捕集位置M7C3，使得600 °C 回火態有著最佳的抗氫脆能力，充氫拉伸延伸率可達14 %。

There are some requirements for the design of YOKE 8625MX：good hardenability, high strength and high impact energy. Also, the reduction of tempered embrittlement is expected. By Jominy test, the part of sample with lowest cooling rate still possesses hardness over 40 HRC, which means that YOKE 8625MX actually has good hardenability. The UTS of YOKE 8625MX reaches 1800 MPa in quench state, and the impact energy reaches 40 J at -40 °C. Moreover, tempered embrittlement is slight. 600 °C tempered state still reaches 1099 MPa. Therefore, secondary hardening is expected. Martensitic steels are prone to hydrogen embrittlement, so the assessment of resistance of hydrogen embrittlement is important. In this study, hydrogen would be charged into steels by electrochemical method. By combining the results of SEM, TEM, tensile test and TDS, the relationship between microstructure and hydrogen embrittlement can be clarified. Results shows that the dominant trapping site of quench state and 200 °C tempered state is dislocation; dislocation and cementite in 400 °C tempered state; dislocation, cementite and M7C3 in 600 °C tempered state. Compared with dislocation and cementite, the activation energy of M7C3 is higher. Quench state shows the poorest resistance of hydrogen embrittlement with the highest hydrogen content, 0.96 ppm. The hydrogen content drops to around 0.6 ppm in 200 °C tempered state and 400 °C tempered state, with improvement on resistance of hydrogen embrittlement. Although 600 °C tempered state has high hydrogen content, 0.95 ppm. For the lower dislocation density and stronger trapping sites, M7C3, 600 °C tempered state shows the best resistance of hydrogen embrittlement. 14 % elongation can be got after charging.