回火時間對Fe-Cr-Ni-Mo高強鋼碳化物演變及力學性能的影響

回火時間對Fe-Cr-Ni-Mo高強鋼碳化物演變及力學性能的影響EFFECT OF TEMPERING TIME ON CARBIDES EVOLUTION AND ITS EFFECT ON MECHANICAL PROPERTIES IN A Fe-Cr-Ni-Mo HIGH-STRENGTH STEEL

利用透射電子顯微鏡（TEM）與掃描電子顯微鏡（SEM）探究了回火時間（10 min、20 min、40 min、120 min）對不同V含量（0%、0.08%、0.14%，質量分數(shù)）Fe-Cr-Ni-Mo高強鋼碳化物轉變和力學性能的影響。結果表明：淬火態(tài)0V鋼在馬氏體板條間析出了少量的M7C3型碳化物，而含V鋼中無碳化物析出，故淬火態(tài)0V鋼的強度最高（2060 MPa）?；鼗鹛幚磉^程中，短時間回火（20 min）時，0V鋼僅在板條間析出了M3C型碳化物，隨著回火時間的延長（120 min），該碳化物逐漸轉變?yōu)镸23C6，這兩種碳化物尺寸均較粗大（150~300 nm），對合金鋼強度的貢獻相對較弱，導致0V鋼的強度逐漸下降，由回火20 min時的1197 MPa下降到回火120 min后的1088 MPa。加入V后，合金鋼經(jīng)短時間回火（20 min）后不僅在晶界析出M3C，還在晶內析出了數(shù)量較多的M2C，且尺寸細小（不大于80 nm），隨著時間的延長，M3C逐漸分解并形成了數(shù)量較多的M6C和更穩(wěn)定的MC，對合金鋼的沉淀強化效果較強，且對塑韌性的影響相對較小。因此隨著回火時間的延長，含V鋼的強度基本保持不變，而塑韌性呈現(xiàn)增加的趨勢，獲得了良好的強韌性配合。

Fe-Cr-Ni-Mo steel is widely used in various industrial fields, such as water turbine in hydroelectric power station, pressure vessel, shipbuilding section etc. for their good combination of strength and impact toughness. In order to meet the needs of high-strength and good toughness, the quenching and then tempering are often used for this kind Fe-Cr-Ni-Mo steel. In particular, the carbides precipitated in the tempering process are the key to determine the strength and toughness. In this work the transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to investigate the carbides and their effect on mechanical properties of Fe-Cr-Ni-Mo steel with different V contents (0%V、008%V、014t%V，mass fraction) after tempered at 610℃ for different times (10min, 20min, 40min, 120min). The results show that some M7C3 type carbides were found between martensite lath interfaces in quenched 0V steel, but no carbides were found in the quenched 008V and 014V steel. As a result, the strength of 0V steel (2060MPa) is higher than 008V and 014V steel (1906MPa and 1857MPa, respectively). After tempering for 20min, a small amount of M3C type carbides with large size (150~300nm) were found in 0V steel. Besides M3C type carbides there also appears M2C type carbides with a small size of 50~80nm and 20~40nm in 008V and 014V steel respectively. With increasing tempering time, M3C in 0V alloy will transform into M23C6 carbides gradually, which also have coarse size of 195 nm. The difference in alloy with V addition is that the M3C will disappear and precipitates much more M2C, M6C or MC type carbides. Compared with coarsen M3C type carbides, the finer M2C, M6C and MC types carbides have better precipitation strengthening effect and less deterioration of ductility and toughness. As a result, the 008V and 014V steel have better tempering stability. For example, as increasing tempering time from 20min to 120min the tensile strength of 0V steel decreases from 1197MPa to 1088MPa, while the strength of 014V alloy keeps almost constant from 1286MPa to 1277MPa.

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