Formability Prediction of Laser-Welded Stainless Steel AISI 304 and AISI 430

The effect of laser welding on the mechanical properties and the prediction of formability for austenitic stainless steel AISI 304 and ferritic steel AISI 430 when welded by a YLS-5000 fiber laser, were studied in the paper. The microstructure of the welded joint was analyzed using light microscopy. The mechanical properties were determined by static tensile testing. The forming limit diagrams were produced from notched samples at R5, R17, and R25 mm. The hardness values of the welded joint and the base material were determined using the Vickers method. Samples made of AISI 430 showed that the formability suffered due to laser welding. Longitudinal coarse ferrite grains were observed in the microstructure of the AISI 430 weld metal. The coarse-grained structure of the welded joint and the continuous interface along the centerline caused the failure of the AISI 430 laser-welded samples at significantly lower actual stress and strain values than were required to break the base material. No significant changes in the formability were observed in the AISI 304 samples after laser welding. The growth of dendrites was observed in the microstructure of the AISI 304 welded joint in a direction towards the centerline of the welded joint. A comparison of the experimentally determined FLD0 values and the values calculated from predictive equations showed that a better agreement was achieved for uniform elongation than for the strain hardening exponent. The manufacturability and economic efficiency of selected parts of an exhaust system by hydromechanical drawing were evaluated on the basis of the process capability index Cpk.

The effect of cold rolling and high-temperature gas nitriding on austenite phase formation in AISI 430 SS

Austenitic stainless steel is the most commonly used material in the production of orthopedic prostheses. In this study, AISI 430 SS (0.12 wt. % C; 1 wt. % Si; 1 wt. % Mn; 18 wt. % Cr; 0.04 wt. % P and 0.03 wt. % S) will be modified by creating austenite and removing its ferromagnetic properties via the high-temperature gas nitriding process. Cold rolling with various percentage reduction (30, 50, and 70 %) was followed by gas nitriding at a temperature of 1200 °C with holding times of 5, 7, and 9 hours, then quenching in water was carried out on as-annealed AISI 430 SS. The formation of the austenite phase was examined by XRD (x-ray diffraction). The microstructure and element dispersion were observed using SEM-EDS (scanning electron microscope-energy dispersive spectrometry), whereas the mechanical properties after gas nitriding and water quenching were determined by Vickers microhardness testing. At all stages of the gas nitriding process, the FCC iron indicated the austenite phase was visible on the alloy's surface, although the ferrite phase is still present. The intensity of austenite formation is produced by cold rolling 70 % reduction with a 5-hour gas nitriding time. Furthermore, the nitrogen layer was formed with a maximum thickness layer of approximately 3.14 µm after a 50 % reduction in cold rolling and 9 hours of gas nitriding process followed by water quenching. The hardness reached 600 HVN in this condition. This is due to the distribution of carbon that is concentrated on the surface. As the percent reduction in the cold rolling process increases, the strength of AISI 430 SS after gas nitriding can increase, causing an increase in the number of dislocations. The highest tensile strength and hardness of AISI 430 SS of 669 MPa and 271.83 HVN were obtained with a reduction of 70 %.

Analysis of the Visual Appearance of AISI 430 Ferritic Stainless Steel Flat Sheets Manufactured by Cool Rolling and Bright Annealing

This article reports on the relation between the surface topography and the optical reflectance, both total and diffuse, of different samples of AISI 430 ferritic stainless steel. Gaussian filters with different cutoff wavelengths were applied to the height maps of the surface topography of the samples, to separate the different scales of surface roughness involved in optical scattering in the visible range of the spectrum. Significant anisotropy, related to the rolling process, was found in the topography. An effective roughness slope parameter was defined from the dependence of the ratio between the root mean square height and the autocorrelation length on the cutoff wavelength. This roughness slope demonstrated an exceptionally good linear relationship with CIE 1931 luminance, which was calculated from the diffuse reflection spectra. The color uniformity of the samples was analyzed based on their CIE L*a*b* coordinates under daylight and LED illumination. The results confirmed the strong influence of manufacturing process on the surface characteristics of AISI 430 ferritic stainless steel sheet products with a bright finish.