Solid-Solid Phase Transformations in a Metastable Stainless Steel: Microstructural Control and Mechanical Properties

Abstract

In this thesis work the processing-microstructure-mechanical properties relationship has been studied in a cold-rolled semi-austenitic metastable stainless steel of composition: 12Cr-9Ni-4Mo-2Cu-1Ti-0.5Mn-0.4Al (wt. %). Due to its good corrosion resistance, good ductility in the annealed state, high strength in martensitic state and its ability to precipitation harden, this material is especially suitable for complicated designs that still have high requirements on the strength of the final product. However, the complex thermo-mechanical behavior of this steel is difficult to understand and limits its applications. Therefore, and in view of its good properties, it is worth investing time in studying the phase transformations that the material may undergo during the thermo-mechanical processing as well as the interlink microstructure-properties. In this way, it is possible to gain insight about the parameters controlling the thermal and mechanical stability and to propose new processing routes that lead to the adequate final properties depending on the application. The pronounced chemical banding present in the cold-rolled as-received state has turned up to be a thorny and difficult-to-solve problem that influences the stability and the microstructure of the material. The combination of techniques such as transmission electron microscopy (TEM), electron probe microanalysis (EPMA), magnetization measurements, micro-hardness Vickers and thermoelectric power (TEP) measurements have allow to perform a detailed characterization of the α’→γ transformation upon isochronal heating (0.1, 1 and 10 ºC/s). It was found to occur in a wide range of temperatures, in two steps due to the chemical banding, and through an interface-controlled mechanism for all heating rates. The isochronal heating allows the precise control of the microstructure and submicrometer size (0.35-0.41 μm) dual (α’/γ) and austenitic microstructures can be obtained. The mechanical behavior of these microstructures was studied by tensile testing, magnetization measurements and transmission electron microscopy (TEM) and it was found that the ultimate tensile strength (UTS) and elongation can be varied from about 1.20 GPa and 25 % to 2.20 GPa and 3 %, respectively. When the austenite grain size (AGS) is increased to the coarse-grained (CG) scale (~6.50 μm), the yield strength (YS) and UTS can drop to 0.40 and 0.80 GPa, respectively, and the total elongation increase up to 44 %. The main factors affecting the mechanical behavior in this steel are the mechanical stability of the austenite, the balance of austenite/martensite volume fractions, the presence of strengthening second-phase nano-particles and the chemical banding. Due to its metastability, the austenite is susceptible of transforming into strain-induced martensite (SIM) under applied uniaxial tensions (so-called TRIP effect), which results in outstanding work-hardening rates and enhanced mechanical properties. The refinement of the microstructures causes an increase in the critical stress required to initiate the formation of SIM and results in a faster transformation kinetics compared to the CG counterparts. Finally, the ability of precipitation hardening of this steel was thoroughly investigated in the cold-rolled state for aging temperatures of 300-550 ºC and times up to 72 h. The hardening rate during aging has been characterized using hardness and TEP measurements; and nano-precipitates formed have been analyzed by TEM and HRTEM. A semi-empirical model has been used to estimate the activation energy of the process. The mechanical behavior of selected microstructures aged at 400, 450 and 500 ºC has been also characterized and discussed based on the nanometer size of the precipitates, their coarsening and the formation of austenite after long aging treatments.