Additive manufacturing of an Fe–Cr–Ni–Al maraging stainless steel: Microstructure evolution, heat treatment, and strengthening mechanisms

Additive manufacturing of a low carbon Fe–Cr–Ni–Al maraging stainless steel (with the brand name CX) through the laser-powder bed fusion (LPBF) process is studied. Since the strength of this material is enhanced through precipitation hardening, the effect of different heat treatment cycles on the hardness and microstructure is assessed. The LPBF-CX is heat treated through a standard heat treatment procedure consisted of austenitization at 900 °C for 1 h followed by air cooling and aging at 530 °C for 3 h. Moreover, the effect of aging treatment (with no austenitization) on the as-built sample is studied. The microstructure of the as-built, austenitized-aged, and aged samples is studied using multiscale electron microscopy techniques. The as-built LPBF-CX consists of the typical lath martensitic structure and minor retained austenite. The martensite laths are featured by high dislocation density, with no evidence of precipitates. Austenitization-aging treatment shows a detrimental effect on the strength of LPBF-CX, due to martensite laths growth and retardation of precipitates evolution. Aging of the as-built LPBF-CX results in strength enhancement due to the evolution of nanometric and coherent β-NiAl precipitates, and martensite laths refinement. Moreover, the pre-existing dislocation networks play a key role in the strength of the aged material. The strength enhancement of the aged LPBF-CX is investigated through the fundamentals of alloy hardening

Microstructure and corrosion behavior of a novel additively manufactured maraging stainless steel

This study aims to achieve a clear understanding of relationships between the anisotropy in the microstructure and corrosion behavior in a newly developed maraging stainless steel (Corrax® or SS CX) fabricated by the laser-powder bed fusion (L-PBF) technique. The focus was to compare the properties of the planes parallel (side) and perpendicular (top) to the building direction of the fabricated sample. Various electrochemical testing and multiscale electron microscopy techniques were used to investigate the microstructure and corrosion behavior of both planes. The results indicated that different thermal histories experienced on the top versus the side planes during the L-PBF lead to microstructural variations between the planes. A slight increase in the laths size, and the lower fractions of dislocation density and low angle grain boundaries, and plausibly the lower level of residual stresses on the side plane were found to contribute to the improved corrosion response of the side plane as compared to the top plane

Corrosion resistance of 13wt.% Cr martensitic stainless steels: Additively manufactured CX versus wrought Ni-containing AISI 420

The corrosion resistance of heat-treated additively manufactured (AM) precipitation hardening martensitic stainless steel (SS) CX and wrought components of AISI 420-SS with lower Ni content, were compared. The microstructure of heat-treated AM-CX comprised of a martensitic matrix and a few nano-scale particles, while the wrought AISI 420-SS contained approximately 8% of austenite and Cr-rich carbides in a martensitic matrix. Superior corrosion resistance was detected for the AM-CX part as compared to the AISI 420-SS ascribed to nearly absence of Cr-rich carbides and martensite-retained austenite interfaces in the AM-CX SS, while their presence in the AISI 420-SS destabilized the passive film

Selective laser melted stainless steel CX : role of built orientation on microstructure and micro-mechanical properties

In this work, the effect of built direction on the small-scale mechanical properties and microstructure of a novel maraging stainless steel (SS-CX) manufactured through the selective laser melting (SLM) process was studied. Advanced electron microscopy and nanoindentation techniques were utilized to evaluate retained austenite fraction and micro-mechanical properties, respectively. Different thermal histories caused by the change of the built direction resulted in microstructures with different volume fractions of retained austenite and grain morphology. Furthermore, the slower cooling rates in the vertically built sample was found to result in the formation of large elongated grains and lower hardness values during the nanoindentation experiments

A hybrid additively manufactured martensitic-maraging stainless steel with superior strength and corrosion resistance for plastic injection molding dies

The customization of plastic injection molding dies is technologically and economically limited by conventional manufacturing processes. Recent advances in hybrid additive manufacturing (HAM) have provided more geometrical freedom for the manufacturing of parts with desired properties. In this paper, we report manufacturing of a hybrid 420/Corrax stainless steel with a reliable interface that can be applied in the manufacturing of next-generation geometrically complex plastic injection molding dies with enhanced strength and corrosion resistance. AISI 420 martensitic stainless steel is used as a cost-effective substrate, and a maraging stainless steel grade, known as Corrax, is printed on top of it using laser powder bed fusion (LPBF). A hybrid heat treatment cycle is applied to improve metallurgical properties and to enhance mechanical compatibility between the martensitic and the maraging stainless steels. Tensile tests coupled with scanning electron microscopy are carried out for analysis of failure, which show the development of shear bands in the microstructure of the 420 stainless steel substrate while a limited amount of deformation occurs in the interface region and Corrax microstructure. Void nucleation, growth, and coalescence are found at the 420/Corrax interface due to mechanical incompatibility and decohesion; however, microstructural instability mainly occurs along the shear bands on the 420 side and leads to fracture, which is quantified using high-resolution X-ray computed tomography. Nanoindentation tests show that the maximum level of hardness occurs at the interface due to the existence of sub-micron grains and the formation of AlN nanoparticles. Also, the formation of β-NiAl precipitates enhances the Corrax strength after heat treatment. In addition to a high strength, elevated corrosion resistance of the cooling channels is essential to extend the service life of plastic injection molding dies. Potentiodynamic corrosion testing at the interface shows that Corrax has remarkable corrosion resistance compared to 420. Therefore, additive manufacturing of the critical die areas such as the cooling channels using Corrax increases the service life of the mold

Microstructural Evolution in Additively Manufactured Fe-Cr-Ni Maraging Stainless Steel

In this work, the effect of heating rate on the phase transformation temperatures was investigated using dilatometry analysis. Continuous heating and isothermal holding above Ac3 temperature on microstructural evolutions in additively manufactured (AM) parts of Fe-Cr-Ni maraging stainless steel were studied. The microstructural features developed within the heating processes were characterized employing electron backscatter diffraction and transmission electron microscopy. Austenite reversion was found to take place in two steps for the AM parts by a diffusive mechanism as well as the precipitation reactions. Although grain refinement occurred during the austenite reversion of the continuously heated samples, the microstructure showed a coarser grain size after isothermal heating. The crystallographic orientations developed after the heating processes were different from those of the initial ones implying the absence of the austenite memory effect.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Team Jilt Sietsm