Effect of structure on the properties of white chromium cast irons

Abstract

White chromium cast irons are used as wear-resistant materials operating under conditions of intense abrasive and impact-abrasive wear. The high wear resistance of white chromium cast irons is determined primarily by the presence of a large amount (30% or more) of solid chromium carbides of the M7C3 type. At the same time, an important effect on their operating characteristics is exerted by the state of the metallic base, which can have various structures depending on the alloying conditions and the heat treatment.It is known that in abrasive or hydroabrasive wear the best wear resistance is exhibited by white chromium cast irons with a martensite matrix [1] or a matrix of martensite with metastable austenite [2]. If the structure of the base conrains even small amounts of pearlite or troostite with similar characteristics of the carbide component, the wear resistance of the metal decreases considerably [3]. Under some wear conditions (high impact loads, high-hardness abrasive) an austenite matrix can be more advantageous than a martensite one, which can be explained [4] by the high capacity of austenite for cold-hardening. In the present work we investigated the effect of the structure of the base on the hardness and wear resistance of white chromium cast irons with the aim of increasing the service life of blades of shot-blast apparatuses.Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences; Ural Automobile Plant.The need to increase the hardness of the metallic base of cast irons has become clear in the investigation of the causes of disruption of shot-blast blades made of industrially produced cast iron ICh300Khl7G5 with a predominantly austenite structure, used in the Ural Automobile Plant. Fracture surfaces of such blades exhibited strong microplastic deformation, traces of plastic forcing back of the material (Fig. 1), and individual fatigue cracks. The blades were disrupted as a result of repeated plastic deformation and, possibly, of an adhesive interaction with the shot. This indicates that the material was disrupted due to insufficient strength and hardness of the metallic base. In order to increase the wear resistance of the blades, the hardness of the metallic base should be increased with retention of the most wear-resistant and hardest carbide phase, namely, carbides of the M7C 3 type. This can be provided by increasing the amount of martensite in the structure of the base by efficient alloying and choosing an optimum regime of heat treatment.We investigated experimental cast irons for shot-blast blades with the chemical composition presented in Table 1. In order to increase the martensite temperature M n and reduce the amount of retained austenite relative to the industrially produced metal 1, the concentration of magnesium was de-

Heat treatment forming a dissipative metal base in wear-resistant chromium-alloyed cast iron

“…It is characteristic in this case that the chemical composition of the metal base must have a carbon content of 0.6-0.7% and the corresponding combination of chromium and other alloying elements to ensure the condition 20 ºС < М s , М f > 20 ºС. Thus, martensite transformation develops mainly in the process of operation [2,3]. A high level of hardening is not achieved due to the formation of strain-induced martensite with carbon content below 0.6%.…”

“…The structure of wear-resistant chromium-alloyed white cast iron consists of solid special carbides (usually not less than 20-30% of eutectic and secondary carbides) and a metal substrate, which must be hard and ensure good fixation of carbide particles. Required chromium content ensures special type carbides, Cr 7 C 3 and Cr 23 C 6 . The most wear-resistant cast irons have an average carbon content of 2.3-2.7% and a chromium content of 12-18%, with carbides of the M 7 C 3 type.…”

“…The most wear-resistant cast irons have an average carbon content of 2.3-2.7% and a chromium content of 12-18%, with carbides of the M 7 C 3 type. In some cases, high-chromium cast iron with 23-28% of chromium is used, especially under conditions of abrasion-corrosion wear [1][2][3].…”

Niobium Additions to a 15%Cr–3%C White Iron and Its Effects on the Microstructure and on Abrasive Wear Behavior

“…Figure 5 shows evidence of the presence of martensite and M 2 C carbide in the 3.98% Nb iron; similar martensitic transformations were also observed for the other as-cast irons. It has been widely reported [1,3,9] that during cooling down to room temperature the austenite close to the eutectic carbide gets impoverished in carbon and it is prone to transform to martensite [1]. The presence of molybdenum rich carbide M 2 C has been reported in white irons containing Mo, since this element partitions partially to the matrix, partially to the M 7 C 3 carbide and also forms M 2 C [26][27][28][29].…”

“…High-chromium white irons are widely used in the mineral processing industry due to their excellent wear behavior under abrasive conditions. A microstructure consisting of a network of hard eutectic M 7 C 3 carbides in a mainly austenitic (as-cast) or martensitic (after a heat treatment) matrix makes these irons very suitable for severe wear applications [1][2][3][4]. To improve abrasive behavior in the as-cast iron, an increase in carbon and chromium would increase the carbide volume fraction.…”