Study of the effect of carbon on the structure, mechanical properties, and nature of failure for the metal of steel Kh25N20S2 castings

Abstract

Results are provided for a study of the metal of shaped castings and centrifugal pipes manufactured correspondingly by static and centrifugal casting methods. The effect of carbon on the structure, properties and nature of damage (failure) for the metal of castings made from heat-resistant austenitic steel Kh25N20S2 is demonstrated. Results are presented for metallographic study of specimens of this steel and cast metal.In order to manufacture highly thermally loaded reaction coil pipes for pyrolysis units and heating units of furnaces for light annealing of rolled metal product operating at 800-1000°C, there is extensive use of lightly-alloyed heat-resistant austenitic steel Kh25N20S2 with a different carbon content. The most critical elements of these structures, i.e., pipes and fittings, are normally manufactured correspondingly by centrifugal and static casting methods from steel Kh25N20S2 with a low (0.1-0.2%) or high (0.35-0.45%) carbon content.Carbon within the chemical composition of steels and alloys is one of the most important elements for efficiency of influencing the phase composition, structure and mechanical properties of steel. Therefore, performance of comprehensive studies with the aim of determining the effect of carbon content on mechanical properties, structure and phase composition of the metal of cast components, manufactured by centrifugal and static casting methods is of scientific and practical interest.Comprehensive studies were carried out for the metal of shaped castings (taps, angle 180°, wall thickness 10-15 mm) and centrifugal pipes (∅125 × 10 mm, length up to 3500 mm) manufactured correspondingly by static and centrifugal casting methods that are basic in the production of assembled massive (up to 100 kg) components made of heat-resistant high-temperature steels and alloys.The chemical composition of the metal of cast components is given in Table 1, from which it follows that the test metal containing chromium, nickel, silicon and manganese within these limits almost corresponds to metal of the basic composition of Kh25N20S2, although it is characterized by different carbon content C C . In order to manufacture rupture specimens, templates and metallographic sections, pipe elements 100 and 300 mm long were used, cut correspondingly from the cylindrical part of fittings and long centrifugally-cast pipes.Macroscopic analysis of the metal structure in the cross section of pipe elements showed that zones of columnar crystals in the metal of centrifugally-cast pipes and fittings occupies about 80 and 100% respectively through the wall thickness.Here columnar crystals in the metal of centrifugally-cast pipes grow in a direction from the outer to the inner surface of the wall, and in the metal of fittings crystals grow to meet each other forming at the junction a boundary of a so-called