Improvement of corrosion resistance of maraging steel manufactured by selective laser melting through Intercritical heat-treatment

Existing studies suggest that martensite-to-austenite reversion can increase the overall mechanical strength of maraging steel. Their effect on corrosion properties, however, is unclear. Selective laser melted (SLM) specimens were tempered near austenite finish temperatures to investigate the electrochemical effect of reversed austenite. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS) were used to characterize their microstructure. To define and test pitting performance, potentiodynamic polarization and open-circuit potential were performed in a 3.5 wt. % NaCl solution. The reversed austenite precipitated mainly along the martensite lath boundaries during the Intercritical heat treatment at 720°C. The nucleation of reversed austenite is allowed by the local Ni enrichment caused by the dissolution of intermetallic particles. As a result, the tempered 720°C specimens reported a higher pitting potential, lowest corrosion current density, and lowest corrosion rate than the as-printed, aged, and homogenized specimens. No investigations have been performed to date that demonstrate the impact of austenite reversion on the corrosion susceptibility of SLM maraging steel. Other than being nobler, austenite is lighter than martensite due to reduced precipitant density, accounting for fewer galvanic cells and a lower corrosion rate.

Effect of Reversed Austenite on Mechanical Properties of ZG06Cr13Ni4Mo Repair Welded Joint

In this work, gas tungsten arc welding (GTAW) was used to repair ZG06Cr13Ni4Mo martensitic stainless steel. Repair welding occurred either once or twice. The changes in the microstructure and properties of the repair welded joints were characterized by optical microscope (OM), scanning electron microscope (SEM), electron backscattering diffraction (EBSD), tensile and impact tests. The effects of reversed austenite in repair welded joints on microstructure and mechanical properties were studied. The results show that the microstructure of the welded joint after repair welding consists of a large amount of martensite (M) and a small amount of reversed austenite (A), and the reversed austenite is distributed at the boundary of martensite lath in fine strips. With the increase in the number of welding repairs, the content of reversed austenite in the welded joint increases. The microstructure in the repair welded joints is gradually refined, the microstructure in the once and twice repaired joints is 45.2% and 65.1% finer than that in the casting base metal, respectively. The reversed austenite presented in the repair welded joints decreases the tensile strength by 4.8% and 6.7%, increases the yield strength by 21.3% and 26.4%, and increases the elongation by 25% and 56%, respectively, compared with the casting base metal. In addition, the reversed austenite mainly nucleates and grows at the boundary of lath martensite. The refinement of the martensite structure was due to the generation of reversed austenite and the refinement of original austenite grain by the welding thermal cycle. After repair welding, the reverse austenite appeared in the repair welded joints and the tensile strength decreased slightly, but the plastic toughness was significantly improved, which was conducive to the subsequent service process.

Modelling of Reversed Austenite Formation and its Effect on Performance of Stainless Steel Components

The kinetics of reversed austenite formation in 301 stainless steel and its effect on the deformation of an automobile front bumper beam are studied by using modelling approaches at different length scales. The diffusion-controlled reversed austenite formation is studied by using the JMAK model, based on the experimental data. The model can be used to predict the volume fraction of reversed austenite in a temperature range of 650 - 750 C. A 3D elastoplastic phase-field model is used to study the diffusionless shear-type reversed austenite formation in 301 steel at 760 C. The phase-field simulations show that reversion initiates at martensite lath boundaries and proceeds inwards of laths due to the high driving force at such high temperature. The effect of reversed austenite (RA) on the deformation of a bumper beam subjected to front and side impacts is studied by using finite element (FE) analysis. The FE simulations show that the presence of reversed austenite increased the critical speed at which the beam yielded and failed. RA fraction also affects the performance of the bumper beam.

Effect of Post-weld Heat Treatment on Microstructures and Properties of Laser-Welded FV520B Steel

For the laser-welded FV520B steel welds, the experiments of post-weld aging treatment have been conducted at different temperatures. The microstructural transformations under various heat-treatment conditions are investigated by optical microscopy (OM), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Vickers hardness and impact toughness are also measured after the aging treatments. The results of microstructural observations show that before heat treatment the fusion zone (FZ) mainly includes low carbon martensite and δ-ferrite. With the increase of aging temperature and time, in FZ the δ-ferrite phase spheroidizes and disappears. When the aging temperature is lower than 400 ºC, the carbides grow with increasing aging time. After aging treatment at 550 ºC for 12 h, a small amount of reversed austenite appears in FZ. The volume fraction of reversed austenite increases with the further increase of aging temperature and time. However, in FZ all the reversed austenite disappears after aging at 650 ºC for 12 h. This indicates that at room temperature the amount of reversed austenite is related not only to the volume fraction of the reversed austenite obtained from aging, but also to the stability of reversed austenite during cooling. The Vickers hardness and impact toughness of the welds mainly depend on the balance between the dispersion strengthening effect of the Cu-rich precipitates and the weakening effect of tempered martensite. The presence of reversed austenite can also improve the toughness of welds. In addition, as the aging temperature and aging time increase, the hardness difference between the three zones of FZ, heat affected zone (HAZ) and base metal (BM) decreases.