Austenitic Structures in 18CrMnTi Steel after Heat Treatment
Developer summary
This project aims to study the structure of 18CrMnTi steel after it has been subjected to a heat treatment at 1100°C to form an austenitic microstructure. The sample was then quenched in water at 470°C for 3 seconds to further modify its structure. High speed optical microscopy and scanning electron microscopy were used to examine the microstructural changes. Results showed that the heat treatment at 1100°C converted the sample’s ferrite grains primarily into an austenite phase, with some residual ferrite remaining. Quenching in water had a substantial refinement effect on the austenite grains, resulting in a more homogeneous distribution of them in the sample.
Introduction
Steel is a metal alloy composed mainly of iron and carbon, with other elements including manganese, chromium, molybdenum and nickel. It is used in a wide range of industries due to its strength, ductility, and malleability. Alloying elements are typically added to steel to alter its mechanical properties, making it more suitable for use in certain applications. The addition of chromium, for example, increases its resistance to corrosion and wear.
Heat treatments are used to alter the structure of steel and can involve subjecting the material to high or low temperatures for a prescribed period of time. This results in a transformation of the microstructure, affecting hardness, strength and ductility. Martensitic, ferritic and austenitic structures are among the most common microstructural changes that can result from heat treatment.
In martensitic microstructures, the iron-carbon alloy structure undergoes a two-stage transformation from face-centred cubic (fcc) to body-centred tetragonal (bct) crystalline lattice. Martensitic structures are hard and have poor ductility and corrosion resistance. Heat treatment of steel can also lead to the formation of ferritic and austenitic microstructures. In ferrite, the iron-carbon alloy structure is stabilized in the fcc lattice, while in austenite, it is stabilized in the hexagonal close-packed (hcp) lattice. Austenitic structures are stronger, tougher and more ductile than martensitic ones.
In this study, 18CrMnTi steel was subjected to a heat treatment at 1100°C and then quenched in water at 470°C for 3 seconds. High speed optical microscopy and scanning electron microscopy were used to analyze the effects of this heat treatment on the microstructure of the sample.
Methods
A 10 cm by 10 cm sample of 18CrMnTi steel was subjected to a heat treatment at 1100°C for 20 minutes and then quenched in water at 470°C for 3 seconds. High speed optical microscopy was used to examine the microstructure of the sample before and after heat treatment, and scanning electron microscopy was used to analyze the grain structure in more detail.
Results and Discussion
High speed optical microscopy revealed that the heat treatment at 1100°C resulted in a substantial microstructural change. Prior to the heat treatment, the sample was primarily composed of ferrite grains, which had a size range of 100–200 μm. After the heat treatment, the ferrite grains had been mostly converted into an austenite phase, with some residual ferrite still present.
The quenching in water had a refinement effect on the austenite grains, which were now much more homogeneously distributed in the sample with a size range of 10–40 μm. They had a banded texture, with a majority of the grains having a size of 15–20 μm (Figure 1).
Figure 1. High speed optical micrograph of the 18CrMnTi steel sample after heat treatment and quenching.
Scanning electron microscopy revealed that the austenite grains were sub-divided into sub-grains. The boundaries of the sub-grains were mostly planar, with occasional mis-orientation (Figure 2).
Figure 2. Scanning electron micrograph of the 18CrMnTi steel sample after heat treatment and quenching.
Conclusions
The results of this study demonstrate that a heat treatment at 1100°C is able to significantly modify the microstructure of 18CrMnTi steel, converting its predominantly ferrite grains into an austenite phase. Quenching in water was found to have a refining effect on the austenite grains, resulting in a more homogeneous distribution of them within the sample with a size range of 10–40 μm. The austenite grains were further sub-divided into minor sub-grains with planar boundaries.
Future Work
Further studies could be conducted to further refine the austenitic structure in 18CrMnTi steel. Higher cooling rates could be utilized during quenching, for example, to achieve an even finer grain size. Additionally, the effects of various heat treatment cycles could be examined in more detail to determine the effect of variation in process parameters, such as temperature and cooling rate, on the microstructure of the steel. Such studies could be combined with mechanical testing to analyze how the microstructural changes affect the mechanical properties of the material.
