固体渗氮C422钢的显微结构及其缺口敏感性

Microstructure and Notch Sensitivity of C422 Steel via Solid Nitriding

目的针对固体渗氮C422(22Cr12NiMoWV)钢,开展渗氮层显微组织结构及其缺口敏感性的研究,为C422汽轮机阀杆服役可靠性评价提供技术支持。方法采用固体渗氮剂对C422钢表面进行渗氮,通过渗氮层的成分与物相检测、显微组织结构观察、力学性能测试及断口形貌分析,揭示不同温度及时间条件下C422钢渗氮层的显微组织特征,以及渗氮对该钢有无缺口试样拉伸力学性能,尤其是缺口敏感性的影响。结果C422钢表面渗氮层主要由Fe_(3)N、Fe_(4)N、Cr_(23)C_(6)、CrN、VN、WN等化合物构成。550℃、6 h渗氮后,渗氮层厚度为80μm,组织致密,硬度高。随着渗氮温度的升高,保温时间的延长,渗氮层的厚度变化不大,但渗氮层组织粗化,并产生表面裂纹。有无渗氮C422钢的缺口敏感参数均小于1,对缺口轻微敏感,渗氮增加了缺口敏感性,渗氮导致C422钢试样的屈强比升高,塑性下降,渗氮层呈解理断裂,脆性断裂特征明显。结论渗氮温度为550℃、保温时间为6 h是C422钢最佳的固体渗氮工艺参数,渗氮层结构致密,硬度高,渗氮对C422钢的缺口敏感性没有明显影响,但脆性有所增加。

Solid nitriding of the C422(22 Cr12NiMoWV) steel was carried out to investigate microstructure of the nitriding layer and notch sensitivity of the nitride steel,which provided a technical support for evaluating service reliability of the C422 steam turbine valve stem.Composition,phase and mechanical property tests,as well as cross-sectional and fracture surface observations were employed to clarify microstructure of the nitriding layers under different nitriding temperatures and times,and also effect of nitriding on mechanical properties of the C422 steel samples with/without notch.Accordingly,notch sensitivity of the nitrided steel was estimated.The results show that surface nitriding layer of the C422 steel is mainly composed of Fe_(3)N,Fe_(4)N,Cr_(23)C_(6),CrN,VN and WN.After nitriding at 550 ℃ for 6 h,the nitriding layer of 80 μm in thickness is dense,and has high hardness.As increasing the nitriding temperature and time,the thickness of the nitriding layer is not apparently changed,but the coarse microstructure and surficial microcracks are detectable.The C422 steel samples with/without nitriding both have a notch sensitivity ratio less than 1,indicating that the steel is sensitive to notch,and nitriding increases notch sensitivity.After nitriding,the yielding-to-tensile ratio/the plasticity of the C422 steel sample increases/decreases,meanwhile cleavage of the nitriding layer suggests an apparent brittle fracture characteristic.The optimal solid nitriding parameters of the C422 steel are the nitriding temperature of 550 ℃ and the holding time of 6 h.The nitriding layer is dense and high in hardness.Nitriding has no apparent effect on the notch sensitivity of the C422 steel,but results in embrittlement of the steel.