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Through-thickness ductility is used as a measure of the resistance of steel plate to lamellar tearing during and after welding. Lamellar tearing is initiated by delamination at inclusion/matrix interfaces in response to weld thermal-contraction strain, and this behavior is simulated in the through-thickness tension test. Although initiated at inclusions, lamellar tearing proceeds by microvoid growth and coalescense into terraces and, finally, by shear between terraces. These latter stages of lamellar tearing are thought to be sensitive to matrix strength and toughness. Accordingly, an attempt was made to investigate the influence of changes in strength and toughness on through-thickness reduction of area (TTRA).
Six high-strength low-alloy steel plates with a range of inclusion fractions were selected and heat-treated to develop ranges of strength and toughness. Measurements of inclusion fraction, longitudinal tensile properties, longitudinal Charpy impact properties, and TTRA were made. Analyses of these results indicated the following associations: 1. Doubling the inclusion concentration (for example, from 0.1 to 0.2 percent) resulted in a decrease in TTRA of about 12 percentage points. 2. Increasing the transition temperature (decreasing the toughness) by 56°C (100°F) resulted in a decrease in TTRA of about 4 to 12 percentage points and an average of about 7 percentage points. 3. Increasing the yield strength by 68.9 MPa (10 ksi) resulted in a decrease in TTRA of 1 to 8 percentage points and an average of about 4 percentage points.
These findings indicate that inclusions are the principal factors in restricting through-thickness ductility, but that high strength or low toughness may also be appreciably associated with reduced TTRA.
lamellar tearing, through-thickness tension test, matrix toughness, matrix strength, inclusion prevalence, transition temperature
Associate Research Consultant, Research Laboratory, U.S. Steel Corporation, Monroeville, Pa.