High-nitrogen-chromium alloyed powder metallurgy (PM) tool steels offer many attractive features including high strength and corrosion resistance. The PM route offers various advantages such as advanced alloy composition, high homogeneity, and well-defined size distribution of hard phase particles. This study presents microstructure and mechanical properties of a PM Cr-Mo-V-N alloy. The conventional manufacturing route for this alloy is hot isostatic pressing (HIP) followed by hot working. To investigate the possibility of near-net-shape manufacturing, a comprehensive comparison of the performance was made between steels produced by as-HIPed and HIPed followed by hot working. Both steel types were heat treated in the same way to obtain martensitic matrix with limited retained austenite. In the present investigation, microstructure and phase analyses were performed by X-ray diffraction and scanning electron microscopy. Mechanical tests were carried out by hardness measurements and tensile fatigue tests in the very high cycle fatigue regime using ultrasonic fatigue testing. 

This paper investigated the microstructure and fatigue behavior of PM-HIPed Inconel 625. The microstructure was composed of γ phase and (Mo, Nb) carbonitrides located mostly on prior particle boundaries. Despite the presence of these carbonitrides, the samples showed good tensile properties with high elongation. Two different surface conditions, pickled and machined, were used for high cycle fatigue testing under a four-point bending test. The results indicated that pickled samples had 6% lower fatigue strength (at 106 cycles) with three times higher standard deviation compared to the machined ones. Fatigue failure mechanisms were found to be dependent on surface conditions and showed different failure modes due to non-metallic oxide inclusions and surface defects in samples with machined and pickled surfaces, respectively. The effect of type, size, and location of defects, multiplicity of crack initiations, as well as surface roughness were analyzed and discussed.

Hot isostatic pressing (HIP) is a near-net shape powder metallurgy (PM) technique, which has emerged as an efficient technique, offering precise control over the microstructure and properties of materials, particularly in high-performance alloys. This technology finds applications across a wide range of industries, such as aerospace, automotive, marine, oil and gas, medical, and tooling. This paper provides an overview of powder metallurgy and hot isostatic pressing, covering their principles, process parameters, and applications. Additionally, it conducts an analysis of PM-HIPed alloys, focusing on their microstructure and fatigue behavior to illustrate their potential in diverse engineering applications. Specifically, this paper focuses on nickel-based superalloys and martensitic tool steels. The diverse microstructural characteristics of these alloys provide valuable insights into the PM-HIP-induced fatigue defects and properties.

This study investigated the impact of stress ratio on the fatigue performance and fatigue crack initiation characteristics of PM-HIPed Inconel 625 in the very high cycle regimes. Ultrasonic fatigue tests, operating at a frequency of 20 kHz, were conducted on PM-HIPed Inconel 625 samples under stress ratios of R = -1 and 0.1 up to the ultimate fatigue life of 109 cycles. Detailed fractographic and microstructural analyses were conducted to identify the mechanisms of crack initiation. The results revealed that stress ratio played a critical role in the crack initiation process. At R = 0.1, cracks predominantly initiated at carbonitrides and non-metallic inclusions, with neighboring crystallographic facets assisting in the formation of microcracks. Conversely, at R = -1, crack initiation was driven by large gains and triple junctions. Microstructural characteristics resulting from the HIP process significantly influenced fatigue crack initiation. Prior particle boundaries were found to affect fatigue crack initiation behavior through the presence of large grains within the boundaries, as well as carbonitrides and non-metallic inclusions networks along the rim. The discussion explored fracture mechanics, fracture surface analyses, and associated microstructural properties to elucidate the observed phenomenon.