Prediction of solidification phases in Cr-Ni stainless steel alloys manufactured by laser based powder bed fusion process

Abstract

The microstructure and strength of chromium-nickel (Cr-Ni) stainless steel alloy parts is highly dependent on the chemical composition of the material and the thermal cycling it experiences during each processing step. This is particularly true when utilizing a laser powder bed fusion (LPBF) process to create the part. LPBF is an additive manufacturing (AM) process that quickly scans a laser over thin layers of powder to melt and solidify the powder to create the part. This results in rapid heating and cooling of the material that is dependent on the processing conditions and part geometry. Consequently, it is necessary to understand how these heating and cooling rates affect the resulting material phases of the final part. Only then the manufacturer can have confidence that the part will perform as intended. The objective of the report is to provide LPBF users with the necessary background to develop processing conditions that will achieve their microstructural design objectives.This report outlines the established methods, using chemical-composition-based phase diagrams (Schäffler and DeLong diagrams), to predict the material microstructure of stainless steel weld metals and applies those methods to LPBF manufactured Cr-Ni stainless steel (S17-4 1 ). Predictions of the solidification phases in Cr-Ni stainless steel alloys, based on the ratio of the Cr and Ni equivalent, are shown. Incorporating these ratios into the phase solidification diagram helps to predict whether the solidification of a Cr-Ni stainless steel occurs in primary ferritic or austenitic phase. This approach also helps users to understand how the increased nitrogen content in additively manufactured S17-4 results in the greater retention of austenite compared to the same material produced by traditional methods. These diagrams can also inform the users about the stability of the retained austenite and its likelihood to decompose into other phases, such as martensite and cementite. In addition to outlining how phase solidification diagrams can help AM users better understand the material they produce, this report also compares results from literature describing microstructure of LPBF fabricated S17-4 with the predicted microstructure before and after different heat treatments. The report also shows that the fine columnar austenitic-martensitic-ferritic microstructure of as-manufactured S17-4 has changed into a predominant martensitic microstructure by a cryogenic treatment, resulting in an increase of hardness.Furthermore, results of mechanical property measurements on additively manufactured S17-4 from other research work are compared and discussed in the result section to explain possibilities of the material phase transformations during different heat treatments, which lead to changes in the mechanical material properties.

Effect of shot peening on corrosion resistance of additive manufactured 17-4PH steel

“…SP-induced changes in the surface layer, as well as changes in the microstructure, can potentially affect the corrosion resistance of steel. During cold processing, a transformation of metastable austenite into martensite and intermediate carbides like Fe 3 C called "Martensitic stress-induced transformation" (MIST) can occur [36], which can affect corrosion resistance. One of the dangers during SP is the potential penetration of burnishing medium particles into surfaces, thus reducing or increasing corrosion resistance.…”

Comparative Study on the Elongation of Low-Carbon Steel and Stainless Steel at Different Creep Temperatures

“…Meanwhile, the (alpha + gama) phase appear at temperature 740 o C, which is the ratio of the alpha phase dominates the gamma phase because it is close to the alpha zone, and, at this temperature the size of the particles is larger, therefore it is characterized by greater ductility and lower hardness. As for the martensitic stainless steel, the martensite phase (cementite + ferrite) which contains cementite needles, which are by nature hard and needle-shaped, and have a lower ductility due to the phase shift from (cementite + ferrite + perlite), due to the (austenite + ferrite) phase, which led to decrease in the hardness value with increase in the ductility [17]. From the obtained results, it was concluded that the temperature and load have a direct relationship in the two samples for short time creep test.…”

Strategies and Outlook on Metal Matrix Composites Produced Using Laser Powder Bed Fusion: A Review

“…Meanwhile, N 2 gas is more typically selected for the less-reactive alloys, such as stainless steels and Ni alloys [165,170]. The N 2 gas employed in the LPBF may produce nitrogen-based compounds in as-built microstructures, which can improve corrosion resistance and alter mechanical properties [171][172][173]. Therefore, shielding gases must be carefully selected based on their application.…”