in Figure 3. The rapid decrease in solubility with decreasing temperature for these two phases produces the precipitation hardening effect.
equilibrium amount of ε-Cu and Cr23 C6
CB7Cu-1 steel with the addition of niobium, vanadium, and nitrogen was studied in an attempt to increase the strength and toughness of the steel. Hence, strong carbide formers, Nb and V, were added to remove soluble carbon and thereby suppress the formation of Cr23
with vanadium and nitrogen was used to study additional hardening by vanadium carbo-nitride precipitation. A V: N of 4:1 is the stoichiometric ratio for vanadium nitride formation.3
C6. In an earlier study4 C6 . , V: N ratio of 4:1 strength-
ened wrought 17-4 PH steel and prevented the formation of Cr23
The aim of the present work is to study the structure trans- formations in CB7Cu-1 steel and to investigate the pos- sibility of attaining improved mechanical properties by micro-alloying with Nb, V, and N. The project’s goal for the mechanical propertiee have been set by the American Metalcasting Consortium (AMC) and are discussed later in this paper.
Experimental Procedures
Commercially produced electric induction melted and Ar- gon Oxygen Decarburization (AOD) refined continuous cast bars of CB7Cu-1 were used as charge material. Melting of these bars was carried out in a 45 kg (100 lb) induction furnace under Ar flow to provide a protective atmosphere. Fe75Si was added to the furnace as a silicon raiser for all the heats. Fe65Nb was added to the furnace during melting for heat 1 while FeV and FeNb were added to the furnace dur- ing melting for heat 2. Heat 3 was produced by the addition of FeNb during melting and nitrided FeV (78 percent V, 16 percent N) just before tapping. Steel was tapped into a ladle at 1650°C (3000°F) and was cast into no-bake, phenolic bonded silica sand molds (for Charpy impact bars) at room temperature. Tensile bars were cast from preheated ceramic shell molds, which were heated to 980°C (1800°F) (see Fig- ure 4). The chemical composition of the precast ingot bar and the four heats cast at Missouri S&T are given in Table 1 and Table 2.
Heat treatment was done in an electric resistance heated fur- nace. Heat treatment procedure included 3 steps:
1. Homogenization treatment at 1200°C (2190°F) or 1250°C (2280°F) to reduce alloy segregation.
2. Austenite conditioning treatment at 1050°C (1922°F). The primary goal of austenite condition- ing was to dissolve Cu into the austenite followed by air, water, or liquid nitrogen quenching.
3. Age-hardening at 482° to 538°C (900° to 1000°F). 60 Temperature (°C)
Figure 3. Thermo-chemical predictions of formation of austenite, ferrite, ε-copper and chromium carbide under equilibrium condition.
Temperature (°C) (b)
Figure 2. Scheil model prediction of (a) remaining liquid composition during steel solidification and (b) Nb carbides formation in interdendritic regions.
Alloying
Temperature (°C) (a)
in the solid is shown
International Journal of Metalcasting/Spring 10
Austenite, Ferrite (Weight %)
Weight, g
Weight, %
ε-Cu, Cr23
C6
(Weight %)
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