Gradient structure and fracture characteristics of steel castings

Abstract

The effect of melt overheating over the equilibrium liquidus to temperatures of 1570 ºС, 1620 ºС and 1670 ºС and heat removal conditions (normal and rapid cooling) on the formation of a gradient structure and fracture characteristics over the cross section of castings of 25L and 30HGSL steels was investigated. The differentiated phase-structural state of rectangular castings (60x60x100 mm) was provided by the predominant one-sided heat removal in foundry molds with different heat removal capacity and average cooling rate 2 – 5 ºС/s and 300 – 350 ºС/s. Regular changes in the length of the main macrostructural zones in castings are established depending on the crystallization conditions, in particular, an increase in the zone of near-surface small crystals and the transcrystallization zone during rapid cooling of the melt. It is shown that depending on the temperature conditions of melt preparation and cooling during crystallization, the grain size in the cross section of castings varies by 4-5 numbers (steel 25L) and by 6-8 numbers (steel 30HGSL). The rapid cooling of the melt causes a significant increase in the impact strength of the cross section of the castings in the range of 50-35 J/cm2 (steel 25L) and 40-20 J/cm2 (steel 30HGSL), even under conditions of significant overheating of the melt. Under normal cooling conditions, there is a significant decrease in the values of impact strength in the cross section of the castings from 30 to 15 J/cm2. According to the results of determining the toughness in castings with differentiated structure depending on the crystallization conditions and test temperature (+20 ºС… -60 ºС) the regression equation for changing its values in the range of viscous-brittle transition temperatures and brittleness temperature (Tbrtl50) of steels in the volume of castings. The obtained results create preconditions for the development of technological principles of engineering of cast steel products to ensure the specified properties in accordance with the requirements of their operation. Keywords: steel, crystallization, gradient structure, toughness.