17-4 precipitation hardened (PH) martensitic grade stainless steel has been shown to have varying as-deposited chemical compositions depending on starting feedstock compositions. These composition changes can lead to variations in austenite volume fraction and therefore microstructure. Austenite levels can then cause significant changes in mechanical behavior. Material made from nitrogen and argon atomized powders were used to evaluate the effect of composition and aging and mechanical behavior. Material made using argon atomized powder had little to no austenite, leading to a mechanical response with minimal work hardening and a continuous yield behavior, similar to wrought. Samples made with nitrogen atomized powder had increases in strength and ductility as well as exhibiting strain induced austenite-to-martensite phase transformations. Very large ductility from strain induced transformations was defined by discontinuous yielding behavior and increased work hardening while the added nitrogen also improved the ultimate tensile strength. Aging behavior of the material changed with increasing nitrogen content. In the nitrogen atomized samples peak-aging temperatures increased, strain induced phase transformation occurred in all aging cases, and discontinuous yielding in the as-deposited condition changed to continuous yielding in the peak-aged and over-aged conditions. Retained austenite was shown in these ways to be a primary contributor to the mechanical behavior of this material.
To evaluate the mechanical responses to previously found structural differences, uniaxial tensile tests were performed. Quasi-static conditions and a strain rate on the order of 8 x 10-4 s-1 were utilized. The force load was measured by a 10 kN MTS load cell while the strain was measured via digital image correlation (DIC). DIC was taken on a Point Grey GRAS-50S5M-C at 1 Hz. Software then correlated the deformation of the samples using a cubic B-spline interpolation algorithm with a subset size of 21 pixels and step size of 5 pixels. Using a virtual line extensometer on the software that is 16mm in length, the engineering strain was measured. Tensile bars had a gauge length of 16.5 mm, a width of 3.75mm and a thickness of 1.45mm. The grip region is 6.3 mm wide and 1.45 mm in thickness with a radius of 3.75 mm joining the regions. Microhardness of some tensile samples was also measured to evaluate the Transformation Induced Plasticity (TRIP) effect along with X-Ray Diffraction (XRD). A Leco M-400-G1 Hardness Tester and a load of 300 gf was used to obtain Vickers microhardness. XRD was done using PANalytical X’Pert Pro MPD with an Empyrean Co radiation source (λ=1.78899 Å) at 40kV and 40mA. A 300 micron monocapillary source optic was used to obtain data at discrete locations along the fractured tensile bars. A Fe β filter, 0.04 mm Soller slit, and an X’Celerator detector on an 85 mm radius reduction arm with a programmable anti scatter slit was used. 2ϴ ranged between 48° and 126° with a fixed slit size. MDI Jade was used to get the integrated intensities for all peaks in order to measure the amount of retained austenite along fractured tensile bars.
The nitrogen atomized as-built and under-aged samples appear to have the constant stress elongation region from strain induced phase transformation. Peak-aged and over-aged cases do not show the constant stress elongation region even though the retained austenite volume fraction appears to be fairly similar to the under-aged case. Collapse of this region points towards a change in strain reactions upon aging. It is already known that the increased austenite from the added nitrogen content causes more copper to remain dissolved in the matrix during aging. Overall properties of the 17-4 PH grade stainless steel change significantly with nitrogen and austenite content. Increase in nitrogen and retained austenite content decreases yield, increases ultimate tensile strength, and increases ductility. Added nitrogen content likely acts as a solid solution strengthening mechanisms in that it increases ultimate tensile strength, but since it also acts as an austenite stabilizer, the ductility is increased, and the yield is dropped. In comparison to the argon atomized case, the nitrogen atomized material has a higher ultimate tensile strength likely due to the solution hardening effect of adding nitrogen to the matrix. The austenite that falls out because of the increased nitrogen content causes a decrease in yield strength and strain induced phase transformation that leads to increased ductility. These effects contrast each other, but here, the ultimate tensile strength is driven by the material behavior after transformation while the ductility, and yield was driven by the increased austenite phase volume fraction.
Tests of the mechanical behavior help point to the way the material reacts to strain after certain processing conditions. Other techniques help understand how the structure changes with respect to the mechanical deformation. These tests of behavior and structure have led to several conclusions including:
•Specimens with high nitrogen content exhibited high levels of retained austenite in the as-deposited condition (81 vol%) that then decreases with increasing aging temperatures.
•While material made with nitrogen atomized powder portrayed high amounts of retained austenite, the ultimate tensile strength and elongation were found to be higher than or comparable to the material made with argon atomized powder and the ASTM standard specifications.
•As-deposited and under-aged material made with nitrogen atomized powder portrayed discontinuous yielding as a result of strain induced austenite to martensite transformations. The extent of elongation during discontinuous yielding can be estimated by Bain distortion and hard sphere models that correspond to the noted orientation relationships (N-W) between the parent austenite and child martensite.
•After peak-aging, high nitrogen material did not have discontinuous yielding characteristics. The strain induced austenite to martensite phase transformation still occurs which contributes to increased ductility and hardening. At nearly the same austenite phase fraction (only a 2 vol% difference) as the under-aged sample, the discontinuous yielding behavior is eliminated. Since the austenite appears to transform similarly as in the as-deposited and under-aged material, the change in yielding behavior is likely linked to the interaction between the nanoscale copper precipitates developed during aging and the movement of the transformation fronts in the austenite.