HIGH-TEMPERATURE STRENGTH OF CHROMIUM WHI TE CAST-IRON CONTAINING CRYSTALLIZED GRAPHITE

Chromium white-cast iron containing crystallized graphite (abbreviated as the A-alloy) may be considered as a composite-casting-material due to its structure consisting of three phases with different properties ; M(7)C(3) type eutectic carbide, austenite matrix, and graphite. Chro mium white-cast iron containing crystallized graphite possesses both g ood wear resistance and strength similar to high chromium white-cast i ron. Furthermore, it also possesses good lubricity due to the graphite ; therefore, it is used as a material for finish rolls of stainless st eel. The purpose of this study is to clarify the high-temperature comp ression-strength mechanism of the A-alloy compared with the B-alloy, a n alloy without graphite but with a similar composition to that of the A-alloy. The form of the stress-strain curve shows the usual form wit h work hardening occurring at temperatures below 673 K; however, on th e stress-strain curves at temperatures above 773 K, the maximum compre ssion stress is reached at an early stage of deformation followed by a gradual decrease in the stress. The maximum compression strength of t he A- and B-alloys at room temperature are approximately 2200 MPa and 2400 MPa respectively and decrease to about 1300 MPa at a temperature of about Tm/2 (700 K) on both alloys. The maximum compression strength of the A- alloy is slightly less than the B-alloy throughout all test temperatures. However, the strain rate dependence of maximum compress ion strength appears at temperature above 773 K on both the A-alloy an d the B-alloy, that is, the strain rate sensitivity (m-value) can be o btained from the slope of linear relationship between ln sigma(B) and ln epsilon over dot at each temperature; namely, m=0.03 at 773 K, m=0. 08 at 873 K, and m=0.14 at 1023 K. Compressive failure is initiated by the deformation of graphite on the A-ahoy, whereas on the B-alloy com pressive failure is initiated by the crack of eutectic carbide caused by deformation of the large size matrix near the carbide. Therefore, i t is presumed that in both alloys the compressive strength is maintain ed by the eutectic carbide, and their high-temperature deformation beh avior is governed by the deformation of matrix.