Short-term Creep Properties and Fracture Surface of 18 Ni (300) Maraging Steel Plasma Nitrided

Plasma nitriding of a solution annealed and aged 300 grade maraging steel was studied aiming to increase the creep resistance. The surface microhardness reached 1,140 HV, producing 50 µm layer composed of ε-Fe3N and γ'-Fe4N nitrides at the uppermost sample layer. The inner core remained unaltered presenting typical plate-like martensite microstructure of maraging steels with average microhardness of 604 HV. Surface RMS roughness in the nanometric scale increase from 52 nm to 71 nm. The continuous layer of iron nitrides seems to behave as a barrier for oxidation and for inward oxygen diffusion improving the creep resistance by reducing the steady-state creep rate (εs) in 52-65% when compared with the literature results. Dominant creep mechanism is controlled by dislocations climb. Fracture surfaces of specimens presented ductile failure consisting of equiaxed and bi-modal dimples in the fibrous zone surrounded by 45º shear lip. Nitrided sample presented a reduced ductility, associated to the hard surface layer.

本研究针对经固溶退火及时效处理的300级马氏体时效钢（maraging steel）开展等离子氮化（plasma nitriding）实验，旨在提升其抗蠕变性能。试样表层显微硬度可达1140 HV，最表层形成厚度为50 μm的复合层，该层由ε-Fe3N与γ'-Fe4N两种氮化物构成。试样内部基体保持原始组织不变，呈现马氏体时效钢典型的板条马氏体显微结构，平均显微硬度为604 HV。表面纳米级均方根粗糙度从52 nm升至71 nm。连续氮化铁层可作为氧化与向内氧扩散的阻隔层，相较于现有文献研究结果，其通过将稳态蠕变速率(εs)降低52%~65%，进而提升了抗蠕变性能。主导蠕变机制由位错攀移（dislocations climb）控制。试样断口呈现韧性断裂特征，纤维区存在等轴韧窝与双峰韧窝，周围环绕45°剪切唇。经氮化处理的试样塑性有所降低，这与其表层硬化效应密切相关。