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Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic

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Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic Materials Transactions, Vol. 47, No. 1 (2006) pp. 72 to 81 #2006 The Japan Institute of Metals Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Chromium Cast Iron Sudsakorn Inthidech1;*, Prasonk Sricharoenchai1 and Yasuhiro Matsubara2 1Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok10330, Thailand 2Department of Metallurgical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok10330, Thailand The relationship between hardness and volume fraction of retained austenite (V ) was investigated in heat-treated 16 mass% and 26 mass%Cr hypoeutectic cast irons with and without addition of a third alloying element of Ni, Cu, Mo and V. In as-hardened state, hardness changed remarkably depending on the V . Overall, Ni and Cu decreased hardness but Mo increased it. Hardness increased in 16 mass%Cr cast iron but decreased in 26 mass%Cr cast iron by V addition. The V increased with Ni, Cu and Mo addition but diminished with V addition in 16 mass%Cr cast iron. In 26 mass%Cr cast iron, Ni and Mo increased the V but Cu and V reduced it. Higher austenitization caused more V . Curves of tempered hardness showed an evident secondary hardening due to precipitation of special carbides and transformation of destabilized austenite into martensite. High tempered hardness was obtained in the specimens with high V in as-hardened state. Maximum tempered hardness (HTmax) was obtained when V was less than 20% and it increased with an increase in Mo content. The HTmax slightly increased with V content in 16 mass%Cr cast iron and decreased in 26 mass%Cr cast iron. Ni and Cu did not show significant effects on HTmax. The highest value of HTmax was obtained in both series of cast irons containing Mo. (Received September 5, 2005; Accepted November 9, 2005; Published January 15, 2006) Keywords: high chromium cast iron, alloying element, heat treatment, hardness, volume fraction of retained austenite 1. Introduction cooling rate.4) Once the high chromium cast iron has solidified, the High chromium cast irons, which contain 12 to carbide morphology can be little modified by any means 30 mass%Cr and 1.8 to 3.6 mass%C are well known as except by plastic deformation. However, the matrix structure abrasion wear resistant materials. It is said that their can be widely changed by heat treatment. In the practical use microstructures consisting of hard eutectic carbides of of high chromium cast irons, heat treatment has been (Cr,Fe)7C3 or M7C3 and strong supportive matrix provide conducted in order to achieve the optimal combination of excellent wear resistance. Consequently, high chromium cast hardness and quantity of retained austenite for abrasion wear irons have been widely used for parts or components in the resistance and other mechanical properties. In particular, the various industries. Their main and typical applications are for secondary carbides precipitated during destabilization heat rolling mill rolls in the steel-making industry, and for rollers, treatment and the carbides formed during subsequent temper- tables and liners of pulverizing mills in cement, mining and ing plays an important role for the wear resistance and thermal power plant industries. mechanical properties. It is said that the maximum macro- The high wear performance of high chromium cast iron hardness obtained after tempering is at most 800 HV30 in depends on both the amount and the type of eutectic carbide high chromium cast iron without any alloy addition5) and that and on the matrix structure. Many papers report that the wear the hardness in eutectic high chromium cast iron can be resistance increases with an increase in volume fraction of increased by addition of Mo.6) In the case of hypoeutectic eutectic carbide under abrasive wear conditions.1,2) However, cast iron, some alloying elements which improve the hard- high volume fraction of eutectic carbide deteriorates the enability and simultaneously promote the formation of toughness which is a critical factor, particularly, when the secondary carbides with higher hardness than chromium high chromium cast iron is used for construction parts. In carbides should be added. In spite of many researches on high order to get higher toughness, the volume fraction of eutectic chromium cast irons containing alloying elements, system- carbides must be limited and also coarse primary carbides be atic and detailed investigations of the effects of alloying avoided. Hence, high chromium cast iron with hypoeutectic elements on the behavior of hardness and retained austenite composition is widely used. during heat treatment are very limited.7,8) Solidification of high chromium cast iron has been studied In this study, therefore, hypoeutectic high chromium cast by many researchers1–4) and it is found that austenite ( ) irons, to which Ni or Cu is separately added to improve dendrites solidify first in hypoeutectic cast iron, followed by mainly hardenability as well as Mo or V to promote a eutectic reaction of L !ð þ M7C3Þ. The eutectic is found precipitation hardening, were employed. The investigation to grow with a cellular interface and to solidify as a colony was focused on the variation of hardness and the volume structure.4) The carbide morphology in the eutectic structure fraction of retained austenite including their correlation with depends on the type and the amount of carbide forming the heat treatment conditions. elements, namely Cr, Mo, V and carbon content. The eutectic morphology, i.e., the sizes of eutectic colony and individual 2. Experimental Procedure carbide particle can be controlled by solidification rate or 2.1 Preparation of test specimens *Graduate Student, Chulalongkorn University 16 mass%Cr and 26 mass%Cr hypoeutectic cast irons Effect of Alloying Elements on Heat Treatment Behavior of Hypoeutectic High Chromium Cast Iron 73 Table 1 Chemical composition of test specimens. Element (mass%) Number CCrSiMnNiCuMoV No. 1 3.01 16.48 0.62 0.78 — — — — No. 2 2.90 16.23 0.51 0.53 1.21 — — No. 3 2.95 16.00 0.55 0.56 2.14 — — — No. 4 2.94 15.84 0.54 0.56 — 1.00 — — No. 5 2.96 15.90 0.54 0.53 — 2.02 — — No. 6 2.97 16.12 0.52 0.50 — — 0.96 — No. 7 2.97 15.93 0.51 0.51 — — 2.03 — No. 8 3.05 15.82 0.55 0.51 — — 3.05 — No. 9 2.96 16.13 0.50 0.51 — — — 1.00 No. 10 3.13 16.08 0.53 0.53 — — — 2.10 No. 11 3.06 16.06 0.47 0.50 — — — 3.05 No. 12 2.65 25.56 0.37 0.51 — — — — No. 13 2.65 26.16 0.48 0.48 1.18 — — — No. 14 2.64 25.91 0.47 0.51 2.06 — — — No. 15 2.62 26.31 0.50 0.50 — 1.03 — — No. 16 2.68 26.04 0.51 0.51 — 2.10 — — No. 17 2.66 25.84 0.48 0.50 — — 0.98 — No. 18 2.61 25.90 0.50 0.50 — — 1.95 — No. 19 2.64 26.40 0.49 0.47 — — 2.96 — No. 20 2.61 25.97 0.51 0.50 — — — 1.05 No. 21 2.64 25.91 0.51 0.51 — — — 2.02 No. 22 2.71 26.46 0.52 0.51 — — — 3.02 without alloy addition and with a separate addition of Ni or tion were (200) and (220) planes for ferrite ( ) or martensite Cu up to 2 mass% and Mo or V up to 3 mass% were produced and (220) and (311) planes for austenite ( ). using a 30 kg-capacity high frequency induction furnace with alumina lining. Raw materials such as mild steel, pig iron, 3. Experimental Results and Discussions ferro-alloys and pure metals were used as charge materials. The charge materials were melted down and superheated up 3.1 Microstructure of test specimen to 1853 K. After holding at the temperature, each melt was Typical as-cast microstructures of 16 mass%Cr cast irons poured from 1773–1793 K into preheated CO2 mold with a with and without alloying elements are shown in Fig. 1. The cavity size of 25 mm in diameter and 65 mm in length, and microstructures consist of primary austenite dendrites and the surface of the top riser was immediately covered with dry eutectics structures. The microstructure of the alloy-free cast exothermic powder to prevent the riser from cooling. The iron consists of pearlitic matrix with rod-like and massive chemical compositions of the test bars are shown in Table 1. eutectic M7C3 carbides. The ( þ M7C3) eutectic shows a The bars were sectioned by a wire-cut machine to obtain colony morphology that contains fine carbides in the central disk-shaped test pieces, 7 mm in thickness. region and coarse carbides at the boundary region. In the cast irons with alloying element, the carbide morphology changes 2.2 Heat treatment little whereas the matrix remarkably changes depending on First of all, the as-cast specimens were annealed for the kind and the amount of alloying element. The matrix in homogenizing by holding at 1173 K for 10.8 ks and then the cast irons alloyed with Ni or Mo is austenitic and that in cooled in a furnace. The annealed specimens were then the cast iron with V is pearlitic. A major part of matrix in austenitized at 1273 and 1323 K for 5.4 ks and hardened by the cast iron with 1 mass%Cu is pearlitic and that with forced air cooling with approximate cooling rate of 12 K/s. 2 mass%Cu is mostly austenitic. The hardened specimens were tempered at 50 K intervals As for the microstructures of 26 mass%Cr cast irons, all the between 573 and 873 K for 7.2 ks.
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