The rapid development in the advanced manufacturing industry asks for better service performance of hot work die steels, especially their high-temperature strength. In this work, a modified SF01 steel based on H13 steel was spray-formed with the carbon content increased to 0.5 wt %. The yield strength (YS) and elongation of SF01 steel at 25°C are about 1824 MPa and 8.6%, respectively, with YS improving by 14.6% and elongation decreasing by 9% compared to conventional H13 steel (about 1591 MPa, 9.6%). It was also found that the YS of SF01 steel was at least 10% larger than H13 steel over the temperature range from 25 to 650°C. The average grain size of tempered SF01 steel is smaller, the dislocation density is lower, and the volume fraction of precipitations is larger than conventional H13 steel (CH13). Furthermore, M3C and VC were observed in tempered SF01 steel, which were not found in CH13. The finer grain size is ascribed to the combined effect of spray forming and the pinning of grain boundaries by VC during 1040°C austenitization process in quenching. A mathematical model was utilized to correlate microstructural characteristics with yield strength. Calculations revealed that the superior YS of SF01 is ascribed to a higher contribution of grain boundary strengthening and precipitation strengthening. The unsatisfied toughness of SF01 steel results from the presence of VC with an average size of 440 nm, which could promote cracks nucleation in the brittle temperature range when subjected to tension loading, and help micropores initiation during plastic deformation. The brittle-to-ductile transition temperature of SF01 is about 200°C, and the deformation mechanism from room temperature to 650°C is briefly discussed. 

Microstructure and high-temperature tensile behavior of spray-formed modified 2000MPa H13 hot work die steel with 0.5wt % carbon

M300 maraging steel was fabricated by selective laser melting (SLM), and the effect of heat treatments on the corrosion and stress corrosion resistance of the alloy was evaluated in 3.5% NaCl solution. The microstructure of the alloy in the solution-treated and aged condition is typical of lath martensitic structure along with metallurgical defects typical of SLM-processed material. The results indicated that M300 steel processed by SLM is highly susceptible to stress corrosion cracking (SCC). The fracture morphology of the samples exhibited brittle intergranular cracking along with ductile cracking features in NaCl. HIP treatment after SLM and before solution treatment and aging improved the SCC resistance of the alloy in terms of marginal increase in elongation in NaCl solution. However, this treatment could not completely eliminate the SCC susceptibility of the maraging steel in NaCl solution. The SCC susceptibility of M300 maraging steel processed by SLM can be attributed to the presence of defects such as pores which assisted the early initiation of cracks and the resultant intergranular (IGC) failure. The defects were considerably healed by subjecting the SLM-processed alloy to HIP treatment, thus improving the SCC resistance. In addition to the reduction in pore density, the increased austenite content present in the alloy after heat treatment played significant role toward improving the corrosion resistance of the as-printed M300 steel.

Stress Corrosion Cracking Behavior of Selective Laser-Melted M300 Maraging Steel in 3.5 wt.% NaCl Solution

Design freedom, complex geometry fabrication capability, minimal parts assembly and excellent material utilization attributes of additive manufacturing processes have provided a leverage g...

Defects and post-manufac- turing processes of additively manufactured steels: A Review (Part 2)

Effect of hot isostatic pressing on microstructural and micromechanical properties of additively manufactured 17–4PH steel

Additive manufacturing of magnesium matrix composites: Comprehensive review of recent progress and research perspectives

Additive manufacturing is a good alternative to conventional methods for the production of near net shape geometries with high geometric complexity shorter lead times, being a good option for the manufacturing of dies for die casting process. In this research, a novel hot-work tool steel for aluminum die casting applications manufactured by laser powder bed fusion was investigated. As-built and stress-relieved (AS-B + SR) state was established and used as the reference condition, and subsequent post-treatments were added and compared to the reference condition. Test parts were evaluated using tensile, impact, hardness and thermal fatigue testing. Compared to the reference condition, heat treatment (HT), significantly increased the hardness, yield and ultimate tensile strengths of the material, due to the obtained tempered martensite microstructure. Hot isostatic pressing (HIP) prior to HT significantly increased the impact toughness and ductility, and slightly increased the yield and ultimate tensile strength values compared to the HT condition. The addition of nitriding treatment after HT, without intermedium HIP step, resulted in the highest surface hardness and lowest impact toughness. Thermal fatigue was mostly affected by the hardness and the softening of the material during the thermal fatigue testing. Results showed that a high surface hardness resulted in a higher thermal fatigue crack nucleation, meanwhile conditions with a high softening during thermal fatigue performance resulted in a higher crack propagation.

Evaluation of post-treatments of novel hot-work tool steel manufactured by laser powder bed fusion for aluminum die casting applications

Microstructure and properties of as-built laser powder bed fusion (LPBF) steels differ from the conventional ones, and they may contain some porosity and lack of fusion. Therefore, post-treatments, including hot isostatic pressing (HIP), are used to densify the material, and tailor the properties of the final product. Usually, HIP is performed as an operation separate from heat treatment. In the present investigation a new approach was used, in which the whole cycle of the heat treatment was carried out in HIP under pressure, and the influence of HIP on microstructure of an advanced stainless maraging tool steel manufactured by LPBF was investigated. For a comparison, a conventional steel grade of the same chemical composition, after a heat treatment at the same temperature-time conditions, was also characterized. The microstructure of the steel was investigated by means of advanced microscopy and atom probe tomography. The influence of the manufacturing route, heat treatment and HIP on microstructure, austenitic phase fraction and size distribution of precipitates was investigated, and the role of high pressure in stabilization of austenite in the microstructure was discussed. It was concluded that since HIP influences phase transformations, a fundamental understanding of the influence of HIP on microstructure is necessary, and development of new post processing regimes guaranteeing the best performance of the material is required.

Influence of heat treatment under hot isostatic pressing (HIP) on microstructure of intermetallic-reinforced tool steel manufactured by laser powder bed fusion

C. Oikonomou

Papers

Evaluation of post-treatments of novel hot-work tool steel manufactured by laser powder bed fusion for aluminum die casting applications

Influence of heat treatment under hot isostatic pressing (HIP) on microstructure of intermetallic-reinforced tool steel manufactured by laser powder bed fusion