Effect of Porosity on Deformation, Damage, and Fracture of Cast Steel
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STEEL castings are under-utilized because of uncertainties in their performance and lack of expertise in casting design. Discontinuities in castings, like porosity, play an important role in casting underutilization. Porosity creates uncertainty in a design’s robustness, since there are no methodologies for including its presence in the design. As a result, designers employ overly large safety factors to ensure reliability leading to heavier components than necessary. Contributing to the issue, the processes of designing and producing castings are usually uncoupled except for the specification of nondestructive evaluation (NDE) requirements. Unless design engineers have test data or experience for a part, they call for NDE requirements without knowing how this relates to part performance. By predicting porosity accurately from casting simulation and realistically modeling its effects on the part performance, engineers can develop robust designs that are tolerant of the porosity and reliable. In the current study, engineering approaches have been applied to simulate the effect of porosity on deformation, damage, and fracture for a cast steel during tensile testing, and the simulation results are compared with measurements.
R.A. HARDIN, Research Engineer, and C. BECKERMANN, Professor, are with the Department of Mechanical and Industrial Engineering, University of Iowa, Iowa City, IA 52242. Contact e-mail: [email protected] Manuscript submitted August 31, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
The material used in this study is ASTM A216 Grade WCB steel. It is a cast carbon steel having a combination of good ductility and strength. It has the following chemical composition (maximum wt pct): C 0.3; Mn 1.0; P 0.035; S 0.035; Si 0.6; Cu 0.3; Ni 0.5; Cr 0.5; Mo 0.2; and V 0.03; and the total of Cu, Ni, Cr, Mo, and V cannot exceed 1.0 wt pct. At room temperature, Grade WCB steel has a yield strength and a tensile strength of 248 and 485 MPa, respectively, and 22 pct elongation as minimum tensile requirements in ASTM A216. Failure of such ductile metals occurs on the microscopic scale by mechanisms of void nucleation, growth, and coalescence.[1] Voids can pre-exist as microporosity and can also nucleate from imperfections like second-phase particles. After nucleation, voids grow with increasing hydrostatic stress and local plastic straining. As voids nucleate and grow, the void (or porosity) volume fraction increases. The voids begin to interact, and the porosity fraction at which interactions between voids begins is the critical porosity volume fraction fc. As plastic strain continues to increase, local necking and coalescence occur in the material between voids until a connected chain of voids forms and failure occurs. The porosity fraction at which fracture occurs is the failure porosity volume fraction fF. The effects of porosity on the structural performance of carbon and low alloy steel castings on the macroscopic scale are not as clearly defined as they are on the microscopic scale. In previou
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