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

Abstract

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-Fe 3 N and γ'-Fe 4 N 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 (ε s ) 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.

Modeling and Numerical Validation of Stress-Strain Curves of Maraging Steels, Grades 300 and 350 Under Hydrogen Embrittlement

“…On Figure 4, the fractographic specimens for both ST alloys by air shows the specimens suffered a fracture with ductility (dimples) characteristics with high strain and a general morphology showed a cup and cone type of failure. The equiaxed dimpled rupture surrounded by shear lips (characteristic of ductile fracture) was observed in a higher magnification image like the Reis et al studies 35 .…”

Analysis of the Surface Treatments Effect on the Creep Behavior of Ti-6Al-4V Alloy

“…It is observed that the plasma nitrided samples suffered the least elongation, which makes it the most suitable in applications that require a little dimensional variation. Reis et al 25 also associated the lower ductility of the nitrided samples to the hard surface layer. Figure 8 shows the Ti-6Al-4V alloy fracture surface after the creep test at 500 °C and 319 MPa.…”

Avaliação dos parâmetros de nitretação por laser de CO2 no aço maraging 18Ni (300) para melhoria da resistência à oxidação

“…Camadas nitretadas contendo a fase ε-Fe 3 N aumentam a dureza superficial e promovem um barreira para a difusão de oxigênio dentro da liga, reduzindo a taxa de oxidação e, consequentemente, melhorando o comportamento mecânico em elevadas temperaturas [10]. A fase FeN 0,056 é uma fase de não equilíbrio, resultado da segregação do nitrogênio durante a refusão por laser [11].…”

“…Estudos comprovaram ainda que o tratamento de nitretação por plasma neste material é suficiente para promover um ganho real em suas propriedades, principalmente em resistência a oxidação [5,6]. Quando aplicado em temperaturas até 480 ºC o aço maraging possui uma taxa de oxidação estável, porém quando aplicado em temperaturas mais altas o aço maraging perde esta propriedade e estas taxas sobem gradativamente com o aumento da temperatura, apresentando uma alta taxa de oxidação a partir de 600 ºC [7][8][9].…”