Effect of thermal treatment of chromium iron melts on the structure and properties of castings

Abstract

The article describes the results of experimental studies for the effect of thermal treatment (TTM) of G-X300CrMo27-1 high-chromium cast iron samples in the solid-liquid and liquid state on the structure, phase composition and properties of ingots. For ingots with a carbon content of 2.8 to 4.5 wt.%, cooled at a rate of 3.3 * 10-2 deg/s, the dependencies of the structure, phase composition, composition of primary, eutectic carbides and matrix, hardness HV and microhardness of its individual phases and resistance to abrasive wear from the temperature of isothermal holding at TTM. A significant effect of the TTM temperature of melts on the structure and properties of high-chromium cast iron ingots were detected. The temperatures of inflection on the plotted curves for the characteristics of the structure and properties of the ingots were associated with a phase transition in iron at 1400 0С and with a point on the phase diagram liquidus of the Fe-Cr system. This as well as an increase in the concentration of chromium in the composition of primary carbides with an increase in the TTM temperature up to 1480 0С, made it possible to assume that (Cr, Fe)7C3 clusters stable in composition were formed in the melt of such cast irons below 1500 0С. The reasons for the decrease in the size of primary carbides during G-X300CrMo27-1 molten chromium iron overheating above 1500 0С were substantiated based on these data. During the TTM of fine-crystalline ingots made of hypereutectic cast iron in the temperature range between the liquidus and solidus lines, it was detected that the primary carbides (Cr, Fe)7C3 recrystallized resulting in a significant decrease in the chromium content and an increase in the iron content in them. At the same time, their sizes did not change significantly. Their share increased, and the share of eutectic carbides decreased. Such process also had a significant impact on the properties of the resulting ingots. The most preferable temperatures of hot metal cast irons in the liquid and solid-liquid states were identified based on the results of the studies.

Effect of Carbon and Cooling Rate on the Structure of Hypereutectic High Chromium Cast Iron in the Cast State and after Heat Treatment

“…It is possible to influence the melt with HHCCI via thermal and temporary treatments when it is in the liquid and solid-liquid states. Thus, [32] presents data indicating a decrease in the size of primary carbides caused by overheating the melt to above 1500 • C or by the heat treatment of a pre-melted alloy in the range of 1260-1320 • C. At the same time, the reductions in the cross-section of primary carbides in HHCCI castings reached 27-50% and 70-80%, respectively. However, these studies did not elucidate the direct influence of the sizes of primary carbides on the mechanical properties of castings.…”