The carbide and graphite formation and redistribution of alloy elements during solidification were investigated on high-speed steel (HS) and Ni-hard type cast irons with Nb and V. The crystallization of hypereutectic HSS proceeds in the order of primary MC,
In the roll material for hot rolling, the alloys which disperse a large amount of carbide in the matrix are widely used because they are superior in abrasion resistance. High-alloy white cast irons, in which a large amount of carbides disperse in the hardenable matrix, are widely used for abrasion resistant parts. Steel strip mills are also one of their important application fields, though the durability of high-alloy cast iron rolls is superior to conventional low-alloy ones. The scoring could sometimes shorten the life service and impair the surface quality of products [
The chemical compositions of specimens tested are shown in Table
Chemical composition for tested samples.
Chemical composition (mass %) | Carbide | ||||||||||
C | Cr | V | Nb | Ni | Si | Mo | W | Co | |||
High-speed steel type (HS) | 1 | 2.64 | 4.58 | — | 2.94 | 7.01 | 3.27 | 2.21 | — | — | MC + M6C + M7C3 |
2 | 2.66 | 0.99 | 2.30 | 2.05 | 4.98 | 3.46 | — | — | — | MC | |
3 | 2.53 | 2.88 | 1.82 | 1.92 | 6.06 | 3.01 | — | — | — | MC + M7C3 | |
4 | 2.63 | 2.97 | 2.08 | 1.86 | 5.92 | 3.35 | 5.05 | — | — | MC + M6C + M7C3 | |
5 | 2.73 | 3.02 | 3.22 | 0.97 | 6.09 | 3.25 | 5.15 | 4.93 | — | MC + M6C + M7C3 | |
6 | 2.79 | 3.05 | 2.95 | 0.99 | 6.10 | 3.23 | 5.16 | — | 5.11 | MC + M6C + M7C3 | |
7 | 2.81 | 2.97 | 2.89 | 0.94 | 6.45 | 3.45 | 5.14 | — | — | MC + M6C + M7C3 | |
8 | 2.74 | 3.09 | 2.10 | 0.87 | 5.99 | 4.46 | 5.09 | — | — | MC + M6C + M7C3 | |
Ni-hard type | 1 | 3.37 | 1.87 | — | — | 4.26 | 0.90 | 0.55 | — | — | M3C |
2 | 3.25 | 1.85 | 0.04 | — | 4.20 | 0.94 | 0.49 | — | — | MC + M3C | |
3 | 3.16 | 1.81 | 1.86 | 0.53 | 4.24 | 0.93 | 0.46 | — | — | MC + M3C | |
4 | 3.20 | 1.67 | 1.93 | 0.75 | 4.16 | 1.19 | 0.51 | — | — | MC + M3C | |
5 | 3.25 | 1.59 | 0.96 | 0.90 | 4.12 | 1.24 | 0.52 | — | — | MC + M3C | |
6 | 3.31 | 1.04 | 0.76 | 0.74 | 4.08 | 0.84 | 0.50 | — | — | MC + M3C | |
7 | 3.35 | 1.24 | 1.94 | 0.35 | 4.32 | 0.88 | 0.54 | — | — | MC + M3C | |
8 | 3.30 | 1.20 | 1.94 | 0.62 | 4.40 | 0.88 | 0.54 | — | — | MC + M3C | |
9 | 3.33 | 0.71 | 0.97 | 1.80 | 4.41 | 0.90 | 0.53 | — | — | MC + M3C | |
10 | 3.35 | 0.96 | 0.97 | 1.82 | 4.40 | 0.96 | 0.54 | — | — | MC + M3C |
Furthermore, almost all Nb crystallizes in MC carbide; therefore, the effect of Nb addition on graphitization is very small. On the other hand, it is commonly recognized that NbC-
Ni-hard type cast iron series were based on alloy no. 1, and variable amounts of vanadium (V) and niobium (Nb) which are MC carbide formers were changed systematically. The amounts of Cr and Si were also controlled in some specimens. Thermal analysis was carried out for each specimen, and every specimen was melted over 1773 K in a siliconit furnace under argon atmosphere and then cooled at 10 K/min until 1173 K and quenched in oil. Distribution of carbides and graphite was analyzed in relation with the solidification sequence of the alloy. Moreover, EPMA analysis was carried out for the specimen quenched during solidification, and the relation between the behavior of alloy elements during solidification and crystallization of graphite was investigated.
Inoculation tests were also carried out for High-speed steel type cast iron and Ni-hard type cast iron. The specimens were remelted in a carbon resistance furnace, inoculated with Fe-75% Si, and then poured into the sand mold preheated at 1173 K. The amount of graphite in all specimens was examined metallographically.
The typical microstructures are shown in Figure
Microstructures of HS type specimens.
The amount of graphite in all specimens measured is from 0.5 to 2% showing a tendency to decrease as the amount of Nb and V increases as is shown in Figure
Amount of graphite-tested specimens.
Thermal analysis curves of hyper- (no. 4) and hypoeutectic (No. 6) alloys are shown in Figure
Thermal analysis curves of HSS samples.
The influence of each element on graphite formation is commonly evaluated based on the solubility of C in molten iron [
The higher values of
Relation between graphite fractions and
Conventional Ni-hard specimen consists of austenite (
Microstructures of Ni-hard type cast irons.
Fine graphite particles crystallize in the specimen with Nb and V content. Primary MC crystallizes in the specimen contained over 0.5% Nb (alloy no. 9). As shown in Figure
Effect of Nb and V content on the amount of graphite.
Thermal analysis curves of Ni-hard samples.
The specimen no. 1 crystallizes in the order of primary austenite (
Since the partition coefficients of Nb and V to primary
Moreover, the growth of
The relation between the amount of graphite and
Relation between graphite’s fraction and
The amount of graphite of Ni-hard type cast iron increases with the
The graphite distribution on alloy no. 7 specimens inoculated with Fe-75% Si alloy (0.5 and 1.0% Si added) is shown in Figure
Inoculation effect on high-speed steel type cast iron.
The specimen’s composition no. 8 was used to study the effect of inoculation by using Fe-75% Si, and the results are shown in both Figures
Effect of inoculation on Ni-hard type cast iron.
Relation between the amount of graphite and the amount of inoculant.
The effects of alloy elements and inoculation on graphite formation were investigated for high-speed steel type cast iron and Ni-hard type cast iron. The following conclusions were obtained.
(1) In case of high-speed steel type cast iron, the crystallization of hypereutectic alloy proceeds in the order of primary MC,
(2) In case of Ni-hard type cast iron, by the addition of Nb and V,
(3) The amount of graphite increases almost linearly with decreasing of solubility parameter
(4) In both high-speed steel type cast iron and NI-hard type cast iron, the inoculation with Fe-75% Si alloy effectively increases the amount of graphite, and higher amount of inoculant results in more uniform distribution of larger flakes and particles graphite.