Grade 300 maraging steel or 18Ni-300 maraging steel is a class of low carbon iron-nickel based maraging steel with high strength, superior toughness, good weldability, and machinability. The lower carbon content in the Fe-Ni martensitic matrix along with the intermetallic precipitates formed during aging promotes a superior combination of strength and ductility in this class of steels. Considering the superior strength-ductility trade-off (which can be tuned with the amount of precipitation during aging), wrought maraging steels have been used extensively in the aerospace industries for the rocket motor casings, hydrospace industries in the pressure hulls of submarines, tooling industries, and even for structural applications. Several studies have been published on the feasibility of laser powder bed fusion (LPBF) of Grade 300 maraging steels, optimizing the process parameters, studying the effect of anisotropy with respect to the build direction on the mechanical properties, and microstructure/mechanical properties post aging of the LPBF fabricated parts. However, majority of these studies neglect the effect of stress induced phase transformations on the resulting mechanical properties. The purpose of this study is to characterize the origins of stress induced phase transformations in Ti-free variant of maraging steel parts manufactured using LPBF and understand their effect on the mechanical properties.
To characterize stress induced phase transformations, microstructure characterization was carried out before and after tensile testing using EBSD and μXRD. High resolution EBSD/EDS scans were acquired using a Zeiss Crossbeam 550 Focused Ion Beam (FIB)/Field Emission Scanning Electron Microscope (FESEM) equipped with Oxford Symmetry EBSD detector and Oxford Ultim Max EDX detector. EBSD measurements were conducted at an accelerating voltage of 25 kV, probe current of 5 nA, with a step size of 20 nm. EBSD data were post-processed using AZtecICE and Channel5 post processing software. EBSD scans were conducted on samples before tensile testing, and after tensile testing (close to the fracture surface) to characterize the phase fraction evolution. In order to corroborate the EBSD measurements, μXRD measurements were conducted close to the fracture surface using Rigaku Micromax source with Dectris Pilatus 200K detector, with a beam size of approx. 40 μm focused close to the fracture surface. Rietveld refinement of the XRD data before and after tensile testing was carried out using open source software Profex with BGMN. All the thermodynamic and kinetic calculations presented were performed using TCFE9 and MOBFE4 database of ThermoCalc and PRISMA.
Results and discussions
In the aged condition, the microstructure of the LPBF maraging steel part consisted of lath martensite along with reverted austenite and precipitates. During uniaxial tensile deformation, the fraction of reverted austenite decreases with a corresponding increase in the fraction of epsilon martensite and martensite. The increase in martensite fraction during tensile deformation resulted in an increase in strain hardening rate in the plastic regime resulting in an overall increase in strength and ductility of maraging steel. Such a stress induced transformation during room temperature tensile testing is not observed in grade 300 maraging steel manufactured using conventional manufacturing routes. Chemical composition analysis using EDX and associated thermodynamic calculations using CALPHAD approach suggest that the extent of segregation associated with faster cooling rates inherent to LPBF (which is not the case with conventional manufacturing techniques) resulted in increasing the stability of austenite at room temperature, which in-turn facilitated the transformation of austenite into martensite during tensile deformation resulting in increasing the strength and ductility.
In this study, stress induced phase transformation in Ti free grade 300 maraging steel manufactured using LPBF was characterized/identified using Electron Backscatter Diffraction (EBSD), micro-X-Ray diffraction (XRD), and CALPHAD approach. It was found that the rapid cooling rate associated with LPBF process resulted in the segregation of solutes during solidification, which in turn stabilized austenite at room temperature in the as-fabricated condition. The presence of retained austenite increased the kinetics of austenite reversion during aging, thereby stabilizing reverted austenite at room temperature after aging. The presence of large amounts of reverted austenite at conventional aging temperature, resulted in the transformation of reverted austenite into epsilon martensite during tensile deformation, thereby increasing both the strength and ductility of maraging steel parts manufactured using LPBF process. The results go to show that the rapid cooling rates, finer microstructure, segregation of solutes inherent to powder bed based additive manufacturing processes can be leveraged to engineer microstructure with superior strength and ductility which cannot be achieved using conventional manufacturing processes.