The Impact of Retained Austenite on the Mechanical Properties of Bainitic and Dual Phase Steels

Abstract

This paper presents the microstructural changes and mechanical properties of carbide-free bainitic steel subjected to various heat treatment processes and compares these results with similarly treated ferritic–pearlitic steel. A key feature of the investigated steel, which is common among others described in the literature, is that the Si content in the developed steel was >1 wt.% to avoid carbide precipitation in the retained austenite during the bainitic transformation. The phase identification before and after various heat treatment conditions was carried out based on microstructural observations and x-ray diffraction. Hardness measurements and tensile tests were conducted to determine the mechanical properties of the investigated materials. In addition, following the tensile tests, the fracture surfaces of both types of steels were analyzed. Changing the bainitic transformation temperature generated distinct volume fractions of retained austenite and different values of mechanical strength properties. The mechanical properties of the examined steels were strongly influenced by the volume fractions and morphological features of the microstructural constituents. It is worth noting that the bainitic steel was characterized by a high ultimate tensile strength (1250 MPa) combined with a total elongation of 18% after austenitizing and continuous cooling. The chemical composition of the bainitic steel was designed to obtain the optimal microstructure and mechanical properties after hot deformation followed by natural cooling in still air. Extensive tests using isothermal transformation to bainite were conducted to understand the relationships between transformation temperature and the resulting microstructures, mechanical properties, and fracture characteristics. The isothermal transformation tests indicated that the optimal relationship between the sample strength and total elongation was obtained after bainitic treatment at 400 °C. However, it should be noted that the mechanical properties and total elongation of the bainitic steel after continuous cooling differed little from the condition after isothermal transformation at 400 °C.

A Review of the Mechanical Properties of 17-4PH Stainless Steel Produced by Bound Powder Extrusion

“…Porosity analysis by less steel demonstrated a predominately martensitic microstructure, with some reverted inter-lath austenite formed in the heat-affected areas throughout printing [23,36]. The reverted austenite induces the transformation-induced-plasticity (TRIP) effect, where austenite transforms to martensite throughout plastic deformation, resulting in reduced yield strength and increased ductility commonly among steels [42]. The non-lath austenitic regions are most evident in Scanning Electron Microscopy (SEM) imaging as displayed in Figure 3, contrasting the cast martensite microstructure in Figure 2 Energy Dispersive Spectroscopy (EDS) analysis identifies the formation of Niobium Carbide (NbC) precipitates up to 10 μm in size, responsible for high-stress sites and subsequent cracks.…”

The Role of Microstructure Morphology on Fracture Mechanisms of Continuously Cooled Bainitic Steel Designed for Rails Application

“…[10,16] These also contain the so-called blocky retained austenite, and due to its low thermal stability, it may be fully or partially transformed into martensite during continuous cooling, forming M/A islands. [10] Also, Adamczyk-Cies´lak et al [17] compared the mechanical properties of bainitic steel rails after isothermal heat treatment and continuous cooling processes. It was found that low-carbon steel after a continuous cooling process is characterized by a more favorable combination of strength and elongation compared to the isothermally heat treatment process.…”

Decomposition mechanisms of continuously cooled bainitic rail in the critical heat-affected zone of a flash-butt welded joints

“…In just 2 years, significant research reports have been published in terms of low-carbon bainitic steels intended for applications in railway infrastructure. The latest investigations cover a wide range of issues including microstructure optimization considering alloying additives [1] and processing techniques [2]; manufacturing process [3]; heat treatments methods [4]; wear resistance [5]; rolling contact fatigue [6]; lowcycle fatigue performance [7]; fatigue crack growth rate [8]; and analysis of welding processes [9]. The mechanical properties of bainitic steels, compared with those of conventional ones, are promising and * E-mail: aleksandra.krolicka@pwr.edu.pl indicate the possibility of enhancing the durability of railway tracks, especially under heavy loads.…”