The influence of austenitisation temperature and time on the martensitic and isothermal bainite transformation of 51CrV4 spring steel was analysed. Based on the analysis of dilatometric curves, the martensite start temperatures (MS) were determined at different austenitisation temperatures (800–960 °C) and austenitisation times (5–30 min). At a temperature of 800 °C, a partial austenitic transformation occurred, and undissolved chromium carbides were present in the matrix. At higher temperatures, the austenitic transformation was complete, and the temperature MS increased with the austenitisation temperature. The temperature of the isothermal phase transformation has a stronger effect on the bainite transformation, which has different effects on the stability of the austenite and the diffusion processes. The microstructure of isothermal bainite transformation samples at 330, 430 and 520 °C was characterised by optical microscopy and dilatometric curves. Lower bainite was formed at a bainitic transformation temperature of 330 °C, and a combination of upper and lower bainite was characterised at a transformation temperature of 430 °C. In the samples transformed at 520 °C, a smaller proportion of lower bainite formed in addition to the upper bainite and martensite. Some allotriomorphic ferrite formed along the boundaries of the austenitic grains.
51CrV4 spring steel is a martensitic steel grade that is heat treated by quenching and tempering. Therefore, austenitisation is an important step that influences steel properties. The main goal of austenitisation is to obtain a single-phase austenite structure that will transform into martensite. We studied the influence of austenitisation parameters on grain growth and martensite transformation temperatures. The samples were quenched from different austenitisation temperatures (800–1040 °C) and were held for 5, 10 and 30 min. The martensite start transformation temperatures (MS) were determined from dilatometric curves, and the hardness was measured using the Vickers method. The microstructure of the samples and the size of the prior austenite grains were characterised using optical microscopy. The increase in the size of the prior austenite crystal grains increases the MS temperature. However, this trend is visible up to 960 °C, where the results start to deviate. High temperatures, 960 °C and above, cause both grain growth and increased carbide dissolution along with chemical homogenization of the steel. The added influence of strong solute diffusion caused a big deviation in the results. The stability of carbides during austenitisation were evaluated with scanning electron microscopy (SEM) and thermodynamic calculations of equilibrium phases using the Thermo-Calc program. MC-type vanadium carbides are stable up to 956 °C under equilibrium conditions, but the SEM results show that they were present in the microstructure even after annealing at 1040 °C. This means that crystal growth is slowed down, which is positive, and that the austenite contains less carbon, so the hardness is lower.
The influence of the nickel content and cooling rate on niobium nitride precipitation in as-cast stainless steels were analysed. Niobium microalloying is important for mechanical properties and the prevention of intergranular corrosion in stainless steels. However, coarse precipitates can negatively affect steel properties. The precipitation of NbN depends on thermodynamic conditions, which are dictated by the chemical composition and temperature. The thermodynamic computations were used to estimate niobium nitride precipitation. Additionally, segregation models were used to predict precipitation. Three steel batches with different nickel contents (9 wt.%, 4.7 wt.%, and 0.16 wt.%) were prepared in an induction furnace and cast into sand moulds. The polished and etched samples were examined with an optical microscope, followed by a more detailed examination using a scanning electron microscope. An automatic scanning electron microscope analysis of the niobium particles was performed to obtain particle number and size distribution. Primary niobium carbonitrides, eutectic phases, and heterogenous nucleations on MnS inclusions were observed. As the proportion of nickel in the solution decreased, the solubility of nitrogen in the melt increased, which is manifested by a lower formation of primary and eutectic niobium carbonitrides, while MnS non-metallic inclusions played an important role in the heterogeneous nucleation.
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