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Description
This research investigated the effects of copper (Cu) and molybdenum (Mo) on the isothermal transformation behavior of high chromium cast irons. Two series of 15%Cr and 25%Cr cast irons with Cu and Mo contents of 1 and 2% each were produced by an induction furnace. Test pieces fixed to silica holder were austenitized at 1050 oC for 10 minutes and then quenched into a salt bath which was maintained at target temperatures between 200 and 800 oC and kept there until the transformation was completed. Transformations during isothermal holding were detected by a differential transformer and the isothermal transformation (IT) diagram was constructed for each specimen from the data employed.
The IT curves of 15%Cr cast irons consisted of pearlite (P) transformation curve at upper part and bainite (B) transformation curve at lower part in C shape. On the other hand, those of 25%Cr cast iron were composed of P in C shape and B in an incomplete C shape jutting toward the short time side. There existed a space between both transformations where any transformations were hard to occur even if the test piece was held isothermally for a very long time. The nose temperatures of P transformation (Tn-P) and that of B transformation (Tn-B) did not vary so much, i.e., around 650 °C for P and 300 °C for B transformation curves in both 15% and 25% Cr cast irons, respectively. However, the nose time (tn) that is the earliest start of each transformation changed depending on the kind and amount of alloying elements like Cu and Mo in the cast iron of this experiment.
From IT curves, the following effects of Cu and Mo additions were clarified. In the 15%Cr cast iron, Cu did not affect almost the nose time of P transformation (tn-P) but increased the nose time of B transformation (tn-B) outrageously or the transformation was postponed. Mo increased the tn-P a little over 1% but Mo seemed rather to make the tn-B decrease. In the 25%Cr cast iron, on the other hand, Cu increased the tn-P gradually but greatly enlarged the tn-B with an increase in Cu content. Mo increased the tn-P fairly or postponed the P transformation but it did not affect almost the tn-B.
Summary
High Cr cast iron, which is one of alloyed white cast irons, can be called a sort of composite material because it shows a composite structure consisting of crystallized chromium carbides of (FeCr)7C3 [M7C3] embedded in the matrix. This cast iron is excellent in abrasive wear resistance and has been mainly used for the hot working rolls, parts and components of a blast furnace in steelmaking industry, mineral digging machine, pulverizing mills in mining industry and those clinkers in cement factories.
During the solidification of high Cr cast iron, Cr is initially consumed to form M7C3 carbides. The remaining Cr dissolves into austenite (γ), contributing to solid-state transformations. Cr in γ not only strengthens the matrix by precipitating secondary carbides but also enhances hardenability by delaying pearlite transformation and lowering the Ms temperature. For applications requiring very high hardness, additional alloying elements such as Ni, Cu, Mo or V are recommended. Ni and Cu have been shown to improve hardenability. Moreover, Mo and V further contribute to achieving high hardness. Typically, high chromium cast irons are used in a heat-treated state.
In the heat treatment process, the crystallized M7C3 carbide remains unchanged, and Cr is involved in the transformation of the matrix. The tempered matrix microstructure is composed of precipitated secondary (M23C6), tempered martensite, and residual austenite (γR). Since the bainite is harder and tougher than pearlite, bainitic high Cr cast iron with more improvement in toughness can be expected like an acicular cast iron. However, it is difficult to obtain the bainitic matrix by normal cooling. To facilitate this, constructing the IT diagram for the high Cr cast iron becomes necessary. Up to now, many IT diagrams have been reported on steels but quite few on the high Cr cast irons.
In this study, 15% and 25%Cr cast irons with Cu and Mo contents of 1 and 2% each were prepared and the behaviors of IT transformation were investigated. The results indicated that the IT curves for both 15% and 25%Cr cast irons revealed the presence of P (pearlite) and B (bainite) transformations. The nose temperatures for P (Tn-P) and B (Tn-B) transformations were relatively consistent at approximately 650 °C for P and 300 °C for B in both 15% and 25% Cr cast irons. However, the nose time (tn) displayed variations depending on the amount of Cu and Mo contents. In the 15%Cr cast iron, Cu had a small effect on tn-P but significantly delayed tn-B. Mo, by contrast, slightly increased tn-P and decreased tn-B. In the 25%Cr cast iron, Cu gradually increased tn-P but greatly enlarged tn-B with increasing Cu content. Mo had a moderate effect on tn-P, either postponing or slightly delaying the P transformation, but it did not significantly affect tn-B.
| Speaker Country | Thailand |
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