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MICROSTRUCTURE AND PROPERTIES OF NB, V AND N MODIFIED CB7CU-1 (17-4 PH) STEEL


A. Murthy, S. Lekakh, V. Richards and D. Van Aken Missouri University of Science and Technology, Rolla, MO, USA


Copyright © 2010 American Foundry Society Abstract


Microstructure and mechanical properties of CB7Cu-1 (17-4 PH) steel modified by addition of niobium, vanadium, and nitrogen were explored. Four heats of CB7Cu-1 steel: base, niobium modified to remove carbon from solution, vanadium modified, and vanadium plus nitrogen modified were melted in a 45 kg (100 lb) induction furnace under Ar atmosphere and cast into no-bake phenolic bonded sand molds and preheated ceramic shell molds. Computational thermodynamics, Scanning Electron Microscopy (SEM), X-ray diffraction, and optical microscopy techniques were used to characterize microstructures produced during homogenization, austenite conditioning, and quenching treatments. Age hardening kinetics were studied at 460°C


Introduction and Thermodynamic Calculations


CB7Cu-1 stainless steel is a martensitic, age-hardenable stain- less steel similar to the wrought product 17-4 PH that yields a lath martensite microstructure upon quenching after austenite conditioning. This steel is age hardened by the precipitation of ε-Cu and Cr23


CB7Cu-1 is often considered as a substitute for forged steel products to assist rapid changes in prototype designs, avoid directionality of mechanical properties, and to produce near net shape products.2 to 17-4 PH steels3-4


ity.1 C6. However, Cr23 C6 is reported to reduce ductil- Previous alloy modification studies have focused on the mechanical properties


of wrought product and there has not been much examination of modifying cast CB7Cu-1 steels.


Thermo-chemical modeling software was used to predict the solidification behavior of CB7Cu-1 steel under equilibrium conditions. In addition, a non-equilibrium Scheil model was used to estimate the possible phase formations and transforma- tions resulting from alloy segregation during solidification. The predicted proportion of austenite formed during steel solidifi- cation and cooling is presented in Figure 1. Nickel, being an austenite stabilizer and chromium a ferrite stabilizer, the com- bination of lower Cr (16%) and higher Ni (4%) helps to fully austenitize the steel and to minimize the formation of ferrite.


Temperature (°C)


The Scheil (non-equilibrium) model was used to predict the composition of the last liquid to solidify. In this model,


International Journal of Metalcasting/Spring 10


Figure 1. Equilibrium solidification in CB7Cu-1 (17-4 PH) steel: percent austenite versus temperature.


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(860°F) and 482°C (900°F). Tensile an Charpy impact properties were measured in peak-aged and over-aged conditions. Fracture surfaces were observed using SEM and complex niobium-vanadium carbonitrides were identified as fracture initiation sites. For a fixed elongation to failure, a higher tensile strength was obtained by over-aging the CB7Cu-1 steel modified with niobium, vanadium, and nitrogen; however, the Charpy impact energy was lower as compared to the peak aged base CB7Cu-1 alloy.


Keywords: 17-4 PH, stainless steel, precipitation hardening, niobium, vanadium, nitrogen, charpy impact, tensile strength, heat treatment, X-ray diffraction


it was assumed that no diffusion occurs in the solid phase and that the remaining liquid is chemically homogeneous. This computation was done by eliminating the solid phases from the free energy calculation after temperature steps of 20°C (68°F). The real solidification process should take place between the limits estimated by equilibrium and those predicted by the non-equilibrium Scheil model. Figure 2(a) and Figure 2(b) illustrate that Cr and Nb enrich the inter- dendritic liquid and that the liquid becomes supersaturated with respect to niobium carbide at 1370°C (2500°F). The


Austenite (%)


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