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Description
The effect of carbon balance (Cbal), which is defined as the amount of carbon (C) dissolving in the matrix in an equilibrium state, on the microstructure and the abrasive wear behavior of heat-treated multi-component white cast irons for steel hot work rolls was investigated. The test specimens with a wide range of the Cbal value from -0.68% to +0.53% were prepared by altering Cr content in the constant basic alloy compositions of 2%C, 2%Mo, 1%W and 5% V. After annealing the cast specimen at 1223K for 18 ks, it was hardened by fan air cooling from 1323K and 1373K for 3.6 ks austenitizing. The hardened specimens were tempered at three levels of temperatures from 673 to 893K; the temperature giving the maximum tempered hardness (HTmax), the lower and higher temperatures than HTmax (L-HTmax and H-HTmax). The abrasive wear resistance was assessed using Suga (two-body-type) and Rubber wheel (three-body-type) wear testers.
Results indicated that the solidification structure of all specimens consisted of primary austenite dendrite and eutectic structures. The (γ+MC) eutectic existed in all the specimens but the (γ+M2C) eutectic appeared in the specimens with Cbal more than -0.2%, while the (γ+M7C3) eutectic came into existence in those with Cbal less than 0%. The matrix of as-hardened specimens was composed of secondary carbide, martensite and retained austenite except for the specimen with -0.68% Cbal. The macro-hardness and micro-hardness increased up to 0%Cbal and then decreased with an increase in the Cbal value. The volume fraction of retained austenite (Vγ) went up with a rise of Cbal value and austenitizing temperature.
Abrasion tests revealed a linear relation between wear loss (Wl) and wear distance (Wd) in all the specimens. The lowest wear rate (Rw, mg/m) or highest wear resistance obtained in the as-hardened (As-H) or HTmax specimen. The highest Rw or lowest wear resistance was obtained in L-HTmax or H-HTmax specimen, irrespective of the Cbal value and austenitizing temperature. In both abrasion tests, the Rw decreased continuously until 0%Cbal, and then, it gradually increased as the Cbal values rose. It was found that the Rw lowered with rising of hardness. In the region of Vγ less than 10%, the Rw values varied widely but over 10%Vγ, the Rw of two-body-type wear was reduced until 20%Vγ and then, rose gradually. In the three-body-type wear, however, the Rw did not change significantly even if the Vγ value increased up to about 60%. The grooving, scratching and pitting wears appeared on the worn surface and the pitting was observed predominantly in the eutectic region and the grooving and the scratching in the matrix.
Keywords: Carbon balance, abrasive wear resistance, hot work roll, multi-component white cast iron, heat treatment, hardness, volume fraction of retained austenite
Summary
In the steel-making, mining and cement industries, various kinds of machines are used for steel rolling, crushing and pulverizing of minerals and their products. Severe abrasive wear takes place frequently in the parts or components of such machines. The Ni-hard and high Cr cast irons have been available as such materials because of both the higher hardness and wear resistance compared with steels. According to the demand for upgrading the productivity and quality of the product, a new alloy with higher performance is required. A multi-alloyed white cast iron containing several kinds of strong carbide forming elements such as chromium (Cr), molybdenum (Mo), vanadium (V) and tungsten (W) was developed for the purpose. Compared with conventional rolls made by high Cr and Ni-hard cast irons, the roll made of multi-alloyed white cast iron shows better wear resistance and longer service life in spite of less volume fraction of eutectic carbides.
The basic chemical composition of multi-alloyed white cast iron is 5 wt% of Cr, Mo, W, V each and 2 wt%C (hereafter wt% is expressed by %). The C content of the cast iron is higher than that of high speed tool steel in order to obtain suitable amounts of hard eutectic carbides for superior wear resistance. The mechanical and wear properties of multi-alloyed white cast iron depend on the kind and amount of carbides as well as the matrix structure. Therefore, a heat treatment must be provided to improve the matrix structure for desirable properties. Generally, the heat treatment process for multi-alloyed white cast iron consists of annealing, hardening and tempering in the same manner as steels and other alloyed cast irons.
The role of C is classified into two parts, formation of eutectic carbides during solidification and dissolution of the remainder into the matrix. The latter affects the transformation of the matrix. Therefore, a parameter of carbon balance (Cbal), which is defined as the amount of C dissolving in the matrix in equilibrium state, was introduced. It was reported by our previous work that the Cbal showed great effect on the phase transformation of multi-alloyed white cast iron.
The abrasive wear behavior in the industrial application is very complex and varies according to wear environment, type of abrasives, contacting angle, magnitude of load applied and the microstructure of materials. In spite of the fact that the wear tests in many laboratories have been carried out, the test data are often invalid for practical use. Therefore, it is considered that more research to evaluate the abrasive wear have to be tried using different types of wear testers. Regarding the multi-component white cast iron with basic alloy composition, the solidification, heat treatment behavior and abrasive wear resistance have already been reported by our research groups. To enhance further the work roll performance made of this material, the effects of varying single or double alloying elements in the basic alloy composition, specifically focusing on phase transformation, heat treatment, wear resistance and fracture toughness must be explored. Up to the present, the effect of Cbal on wear behavior of heat-treated multi-component white cast irons has not been clarified.
In this work, the multi-component white cast irons containing Cbal of -0.68 to +0.53% were prepared by controlling the Cr content from 3 to 9% under the fixed contents of 2%Mo, 1%W and 5%V. After annealing, the specimens were hardened from 1323 and 1373 K, and then tempered at 673 to 873 K, the temperature at which the maximum tempered hardness (HTmax) was obtained, the lower and higher temperatures than that at HTmax (L-HTmaxand H-HTmax, respectively). The abrasive wear resistance was evaluated using Suga (two-body-type) and rubber wheel (three-body-type) abrasive wear testers. Then, the relationship between Cbal, microstructure and wear resistance associated with heat treatment conditions were clarified.
Results showed that the solidification structure of all specimens consisted of primary austenite dendrites and eutectic structures. The (γ+MC) eutectic crystallized in all specimens, while the (γ+M2C) eutectic appeared in the specimens with Cbal values exceeding -0.2%, and the (γ+M7C3) eutectic formed in specimens with Cbal values less than 0%. The matrix of all as-hardened specimens composed of secondary carbide, martensite, and retained austenite, with the exception of the specimen with -0.68% Cbal. Both macro-hardness and micro-hardness increased up to 0%Cbal, followed by a subsequent decrease with higher Cbal values. The volume fraction of retained austenite (Vγ) increased with increasing Cbal value and austenitizing temperature. Abrasion tests showed a linear relationship between wear loss (Wl) and wear distance (Wd) across all specimens. The lowest wear rate (Rw, mg/m) was observed in the as-hardened (As-H) or HTmax specimen. Conversely, the highest Rw was found in L-HTmax or H-HTmax specimens, irrespective of Cbal value and austenitizing temperature. Rw decreased continuously until 0%Cbal, after that it gradually increased with rising Cbal values. Notably, Rw showed an inverse correlation with hardness. In the Vγ region below 10%, Rw values displayed significant variability. Over 10%Vγ, the Rw of two-body-type wear test decreased until 20%Vγ, followed by a gradual increase. In a three-body-type wear test, Rw did not show significant changes even with an increase in Vγ up to about 60%. The worn surface consisted of grooving, scratching, and pitting wear types.
| Speaker Country | Thailand |
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