Investigation of the process of surface decarburization of steel 20 after cementation and heat treatment

Introduction. In industry, the method of carburizing with a solid carburizer is used to saturate the surface layer with carbon. In practice, it is necessary to prevent or reduce surface decarburization of steel as much as possible, either by using a protective atmosphere or by heating under conditions in which the oxidation process of the metal surface layer occurs faster than the decarburization process. During decarburization, a ferrite structure is formed in the surface layer, which, under contact loads, reduces the resistance to crack initiation and increases the probability of fatigue failure of the product as a whole. The purpose of this work is to evaluate the effect of heating temperature during carburizing and subsequent hardening, as well as equalizing period, on the depth of the decarburized layer during chemical-thermal treatment of low-carbon steel. Research methods. The chemical composition of the steel as delivered was determined. The analyses were performed using an optical emission spectrometer, model LAVFA18B Spectrolab. For the study, unalloyed hypoeutectoid Steel 20 was selected, with an initial ferrite-pearlite microstructure. The samples had a rectangular shape with average dimensions of 50 mm × 10 mm × 10 mm. Carbon saturation was carried out on one side (from the side of the poured carburizer, while the reverse surface of the samples was protected by a layer of clay). The samples were placed in a metal container, filled with carburizer in a 25–30 mm layer, closed with a lid, and sealed. Carbon saturation was carried out at 900 °C for 4–8 hours. After that, the box with samples was taken out of the furnace and cooled in air. Quenching was carried out in a furnace in air (humidity was not measured) at furnace heating temperatures of T = 780 °C, 850 °C, and 950 °C with a equalizing period of 4.6 h in a laboratory electric resistance furnace with a chamber volume of V = 22 dm³. Metallographic examination and microhardness measurements were performed. Results and discussion. During the experiments, it was noted that the heating temperature for carburizing and quenching plays an important role in decarburization. At a temperature of 700 °C, the decarburization phenomenon was not observed, indicating that the decarburization reaction did not occur below this temperature. When the temperature exceeds 750 °C, the samples exhibit obvious decarburization, and the ferrite structure is columnar, oriented perpendicular to the decarburized surface. A partial decarburized layer appears in the samples at 850 °C, and the thickness of the full decarburized layer decreases. Above 900 °C, the sample mainly shows a partial decarburized layer because, at this temperature, the steel structure is fully austenitic. Above 1,000 °C, the layer thickness increases rapidly, showing exponential growth. The experiments also demonstrated the effect of heating and equalizing periods on the depth of the decarburized layer. The presented results will be useful in chemical-thermal treatment of products requiring high surface hardness.