Continuous Cooling Transformation of Under-Cooled Austenite of SXQ500/550DZ35 Hydropower Steel

The expansion curves of the continuous cooling transformation of undercooled austenite of SXQ500/550DZ35 hydropower steel at different heating temperatures and cooling rates were measured by use of a DIL805A dilatometer. Combined with metallography and Vickers hardness measurement, the continuous cooling transformation diagrams (CCT) of the studied steel under two different states were determined. The results show that in the first group of tests, after the hot-rolled specimens were austenitized at 920 °C, when the cooling rate was below 1 °C·s−1, the microstructure was composed of ferrite (F), pearlite (P) and bainite (B). With the cooling rates between 1 °C·s−1 and 5 °C·s−1, the microstructure was mainly bainite, and martensite (M) formed as the cooling rate reached 5 °C·s−1. When the cooling rate was up to 10 °C·s−1, the microstructure was completely martensite and the hardness value increased significantly. In the second group of tests, after the hot-rolled specimens were quenched at 920 °C and then heated at an intercritical temperature of 830 °C, in comparison with the first group of tests, and except for additional undissolved ferrites in each cooling rate range, the other microstructure types were basically the same. Due to the existence of undissolved ferrite, the microstructures of the specimens heated at intercritical temperatures were much finer, and the toughness values at low temperatures were better.

Microstructure and Properties Formed at Different Cooling Rates of Low Carbon Alloy Steel for Welding Wire Production

Physical simulation of steel Mn3Ni1CrMo continuous cooling with different speeds from austenitic state was performed using GLEEBLE 3500 complex. The phase transformations are analyzed and the effect of the cooling rate on the structure and hardness is investigated. A continuous cooling transformation diagram of the undercooled austenite decomposition is constructed. It was concluded that it is possible to reduce the hardness of the hot-rolled billet by reducing the cooling rate compared to the existing in the processing at the STELMOR line of PJSC “MMK”, and this will eliminate the heat treatment of welding wire on the hardware processing.

Application of Scanning Electron Microscopy and EDS Microanalysis for Microstructural Investigation of Hypoeutectoid Steel

The results of a microstructure, energy dispersive X-ray spectroscopy (EDS) analysis and hardness investigations of the hypoeutectoid steels with 1% Ni, imitating by its chemical composition toughening steels, are presented in the paper. According to PN-EN 10027 standard this steel should have a symbol 37NiMo4-3.The kinetic of phase transformation of undercooled austenite of investigated steel was presented on CCT diagram (continuous cooling transformation). The CCT diagram of tested steel is characterized in that the curves of beginning of the diffusion transitions form a letter "C" and are not separated from bainitic transformation by the stability range of the undercooled austenite.The EDS analysis performed for the selected cooling rates did not reveal any changes in the distribution of alloying elements of hypoeutectoid 37NiMo4-3 steel. However, these studies, from the point of view of the effect of alloying elements (acting jointly) are only the preliminary assessment analysis (beginning) of the impact of the chemical composition gradient in micro-scale in iron-based alloys.

The Phase Transformations in Hypoeutectoid Steels Mn-Cr-Ni

The results of a microstructure and hardness investigations of the hypoeutectoid steels Mn-Cr-Ni, imitating by its chemical composition toughening steels, are presented in the paper. The analysis of the kinetics of phase transformations of undercooled austenite of steels containing different amounts of alloying elements in their chemical composition, constitutes the aim of investigations. Metallographic examinations were carried out on a Axiovert 200 MAT light microscope. Sections were etched with a 3% HNO3 solution in C2H5OH. Dilatometric tests were performed using L78 R.I.T.A dilatometer. Using dilatometer the changes of elongation (Δl) of the samples with dimensions Ø 3×10 mm as a function of temperature (T) were registered. Obtained heating curves were used to precisely determine the critical temperatures (critical points) for the tested steels, while the differentiation of obtained cooling curves allowed to precisely define the temperatures of the beginning and the end of particular transition to draw CCT diagrams. Four CCT diagrams worked out for the tested hypoeutectoid steels (for quenching of steel) are - in the majority of steels - separated by the undercooled austenitic range and are of the letter „C” shape. However, for steels with Mn and Ni the separation of diffusive transformations from the bainitic transformation by the stable austenitic range is not observed. Hardenability of four investigated hypoeutectoid steels is similar, but still not high. To obtain martensite in the microstructure of these steels, it is necessary to apply the cooling rate higher than 25°C/s. The exception constitutes the Mn - Ni steel, in which only cooling with the rate higher than 50°C/s allows to achieve the martensitic microstructure and to avoid diffusive transformations (pearlitic and ferritic).