Formation of the Structure and Properties of Low-carbon Martensite During Quenching

Introduction. The development of low-carbon martensitic steels was preceded by the development of low-pearlitic, pearlite-free or bainitic steels. Both groups of steels did not require liquid cooling media for heat treatment, and the strength was at the level of 400-600 MPa. The bainite structure had a higher strength, but bainitic steels have significant drawbacks due to its manufacturability and relatively low viscosity, because it is difficult to avoid the appearance of upper bainite during heat treatment. Modern bainitic steels have strength of 1500 MPa, but it is still difficult to achieve the required reliability characteristics. With a Cr/C ratio greater than 35 wt. % (8 at. %), bainite transformation in low-carbon steels (0.04-0.1% C) is not observed, and such steels are referred to as low-carbon martensitic steels. In the work, steels marked with 07H3GNM, 15H2G2NMFB, 27H2G2NMFB are studied. Objective of the work is to determine the composition, morphology and mechanical properties of low-carbon martensitic steels with nonmetallic inclusions. To assess the effect of the martensite structure on the mechanical properties of low-carbon martensitic steels with strong carbide-forming elements after complete quenching and from intercritical temperature range is also the work objective. Methods of research. To study the structure, a microscope “Olympus GX-51”, a scanning electron microscope “Tescan MIRA3” with energy-dispersive analysis adapter were used. The fine structure and morphology of the phases were studied by transmission and scanning electron microscopy. Foils obtained by electropolishing were used for transmission electron microscopy. Electropolishing was carried out at temperatures close to 0 °C, in an electrolyte of 80% glacial acetic acid and 20% perchloric acid. A fine structure was examined on JEM 200CX and SM 30 microscopes at an accelerating voltage of up to 200 kV. The value of the parameter IC was determined in accordance with GOST 25. 506 - 85, according to the results of tests for static bending of the samples with a crack, type 4, with dimensions 5x10x60 mm. Mechanical properties during tensile tests were determined in accordance with GOST 1497-84, impact strength – according to GOST 9454-78. Critical points were established using differential scanning calorimetry and confirmed by dilatometric studies. Heat treatment of steels included quenching 950 °C, tempering 250 °C in the first case, and quenching from intercritical temperature range in the second. Results and discussion. The main inclusions in low-carbon martensitic steels were aluminum oxides, FeO, MnO, SiO2 oxides, and elongated sulfides (FeS, MnS), which form is close to globular. In steels with strong carbide-forming elements, carbides contained an increased amount of niobium and vanadium. Investigation of the destruction of samples with the structure of low-carbon martensite containing nonmetallic inclusions showed that the main reason for the decrease in viscosity with increasing carbon content is the increase in the fraction of the plate component. In the construction of a model for the destruction of steels with a rack and plate structure of martensite, it proceeded from the additive contribution to the strength of various morphological forms of martensite and the leading role in initiating the destruction of the impermeable interfaces for dislocations of the plate component.

low-carbon steel, nonmetallic inclusion, lath, plate, martensite, fracture, mechanical properties, boundary, stress concentration