Metastable Austenite in 17–4 Precipitation‐Hardening Stainless Steel Produced by Selective Laser Melting

Abstract

Selective laser melting (SLM) is a rapid prototyping technique based on melting and solidification of powder layers to build up a 3-D solid body. [1][2][3] It was developed as a prototyping technology, but is attracting ever-increasing interest as a rapid manufacturing technology for the production, for instance, of orthopedic prostheses made of titanium and cobalt alloys [4,5] and die inserts made of low alloys and maraging steels for the plastics industry. [6,7] Because of the large solidification undercooling, SLM results in the formation of either very fine or metastable microstructures, which sometimes do not find a correspondence in the alternative technologies and in the international standards, as in the case of titanium alloys. [8] An in-depth investigation on the effect of the microstructure on mechanical properties is necessary to evaluate whether or not such a peculiar microstructure is acceptable for practical applications.SLM is used for the production of surgical tools, as well. In this case, a stainless steel combining mechanical strength, wear resistance and corrosion resistance is the best choice. This requirement is satisfied by the precipitation-hardening (PH) stainless steels. Among them, the 17-4 PH grade is a high-chromium steel with a martensitic microstructure (resulting from the transformation of austenite on cooling), which has to be heat treated (solution annealing and aging) to promote the precipitation of Cu phases. [9] In this material, the large solidification undercooling may prevent the formation of martensite in the as-built material, leading to a metastable austenitic microstructure.Metastable austenite may transform into martensite on straining, resulting in the well-known transformation-induced plasticity (TRIP) phenomenon. TRIP is exploited in some steel grades, namely the low alloyed Si-Mn steels as well as the high alloyed Mn-Si-Al steels, to improve their plastic ductility in cold forming. TRIP is also displayed by austenitic stainless steels. Its occurrence depends on the stacking fault energy (SFE) of austenite: [10] on decreasing SFE, deformation occurs by perfect and partial dislocation gliding, by twinning and then by martensitic transformation. With a SFE below 15-20 mJ m À2 , austenite transforms to e and a' martensite, resulting in an increase in strain hardenability and, in turn, ductility.An increase in toughness due to the presence of retained austenite in a 17-4 PH stainless steel was reported by Nakagawa and Miyazaki; [11] up to 30% austenite was stabilized in the microstructure by a special heat treatment. However, a mostly austenitic 17-4 PH stainless steels has never been investigated.In this paper, a study on the microstructure of a 17-4 PH stainless steels, produced by SLM, and on its effect on the tensile properties is presented. The as-built microstructure contains a large amount of metastable austenite. Tensile tests show a large plastic ductility coupled to significant strain hardenability, resulting in a high tensile strength around 1 GPa. The trans...

Reduction of Surface Roughness by Means of Laser Processing over Additive Manufacturing Metal Parts

“…Indeed, as-built samples are approximately composed of 70% mass fraction austenite and 30% martensite on average [21]; a reference microhardness of 265 HV is found, in agreement with similar results on the same alloy [22] and the material data sheet of the supplier.…”

Mechanical Properties of Austenitic Stainless Steel Made by Additive Manufacturing

“…Wrought 17-4PH stainless steel is supplied by the mill as a fully martensitic material, which is then heat treated to form copper precipitates, which increase the strength. In contrast, Facchini et al’s [13] analysis showed that the as-fabricated UNS S17400 stainless steel was 70 percent mass fraction metastable austenite, which transformed to the martensite phase during tensile deformation. The material of our study is similar to that of Facchini et al [13], in that the as-fabricated material is not magnetic, and therefore not martensitic, but after deformation the specimens are strongly ferromagnetic.…”

“…In contrast, Facchini et al’s [13] analysis showed that the as-fabricated UNS S17400 stainless steel was 70 percent mass fraction metastable austenite, which transformed to the martensite phase during tensile deformation. The material of our study is similar to that of Facchini et al [13], in that the as-fabricated material is not magnetic, and therefore not martensitic, but after deformation the specimens are strongly ferromagnetic. The deformation-induced austenite-to-martensite transformation is common in austenitic stainless steels and has been studied extensively [14,15].…”

“…Figure 12 compares the stress-strain behavior for the UNS S17400 stainless steel in the horizontal orientation for the as-fabricated and heat-treated condition to the corresponding curves reported by the the MMPDS [12], the manufacturer [11], and one literature report from Facchini et al [13] that used the manufacturer’s material and fabrication process. The tensile properties of the UNS S17400 stainless steel are completely different from the accepted values for 17-4PH stainless steel from the MMPDS for all heat treatments.…”

Prediction of porosity in metal-based additive manufacturing using spatial Gaussian process models

“…For example, Facchini et al [49] analyze the microstructure and Kumar and Kruth [50] study the wear behavior of SLMfabricated 17-4 steel parts. Murr et al [51] provide a good review on efforts in processing 17-4 PH steel and other metallic alloys using laser-and electron beam-based AM, with a focus on reporting the microstructure and phase structures.…”