Dynamic Fracture Toughness of High Strength Cast Steels L.N. Bartlett, A. Dash, D.C. Van Aken, V.L. Richards, K.D. Peaslee Missouri University of Science and Engineering, Rolla, Missouri Copyright 2012 American Foundry Society ABSTRACT The dynamic fracture toughness of Cr and Mo steels with nickel contents of 0, 1.56, and 5.5 wt.% was evaluated and compared to a lightweight steel of composition Fe-30.40%Mn-8.83%Al-1.07%Si-0.90%C-0.53%Mo. Each steel was heat treated to a Rockwell C-scale hardness range of 36 to 38. The 4130, 4325, and HY130 steels were quench-hardened and tempered. The lightweight steel was solution treated, water quenched and age hardened. Of the alloys tested, the lightweight steel, the 4325 steel and the Al-killed and Ca-treated HY130 steel had similar dynamic fracture toughness values of 153, 153 and 165 kJ/m 2 , respectively. The 4130 steel had a much lower toughness of 94 kJ/m. 2 The lightweight Fe-Mn-Al-C alloy performed better at Rockwell C32, producing the highest measured dynamic fracture toughness of 376 kJ/m 2 . Toughness of the Cr and Mo steels was strongly dependent on deoxidation practice. Alloys treated with ferro-titanium showed a reduction in toughness, which was attributed to TiN particles and in one case eutectic Type II sulfides. Addition of misch metal to an aluminum and ferro-titanium treated HY130 steel eliminated the Type II sulfides and increased the dynamic fracture toughness from 58 to 88 kJ/m 2 . HY130 obtained the highest toughness (165 kJ/m 2 ) when aluminum deoxidation was followed by calcium treatment. INTRODUCTION The fracture toughness of high strength steel can be characterized by either a stress intensity or a J-integral approach to fracture mechanics. For materials with low toughness where cleavage fracture dominates, there is little plastic deformation around the crack tip and linear elastic fracture mechanics (LEFM) dominates. 1 Toughness is then evaluated as a critical stress intensity factor, K Ic . For sufficiently ductile materials, such as austenitic Fe-Mn-Al-C steels, failure is governed by the flow properties around the crack tip and the LEFM approach is no longer valid. 1 Therefore, for materials exhibiting crack tip blunting, an elastic-plastic fracture mechanics (EPFM) approach is required and the fracture behavior is described by the path independent J integral, which is equivalent to the energy release rate in elastic- plastic materials. 1 The fracture toughness of ductile materials, J Ic , is defined as the critical value of J near the onset of stable crack growth. 2 In both of the above determinations of fracture toughness, the material is assumed to be under quasistatic loading conditions of less than 2 MPa√m/s. 3 A material’s resistance to fracture is often dependent on the loading rate; therefore, the static J Ic may not be representative of material behavior at high loading rates. Dynamic fracture toughness (DFT) under high loading rates is often difficult to obtain because crack extension during impact loading is difficult to measure. Instrumented Charpy impact tests provide a reproducible way of measuring the time dependency of force and crack displacement at elevated loading rates and therefore, provide a means of measuring DFT. Brittle fracture or Type I failure for linear elastic materials is characterized by crack initiation at the maximum load, which is followed by unstable crack propagation to failure. Failure of elastic-plastic materials is characterized by Type II, Type III, or Type IV behavior. Type II failure occurs when there is enough plasticity around the crack tip to allow for a small amount of stable crack extension before fracture at the peak load. For structural applications subject to shock loading, Type III or Type IV failure of the steel is desired. In Type III failure, there is significant stable crack extension after the peak load followed by unstable fracture. Type IV fracture is characterized by stable crack growth and the material fails by ductile tearing only, which is the desirable behavior for military armor. Schindler 4 has proposed a method of determining toughness from instrumented Charpy impact tests that is based on the crack tip opening displacement (CTOD) and crack tip opening angle (CTOA) models of crack nucleation and growth. This results in an algebraic expression for the dynamic J-R curve from which J 1d can be evaluated in an analogous way to the determination of J Ic . 2 This method is a single specimen approach with experimental inputs of the peak load (P max ), the energy up to peak load (E max ) and the total facture energy (E tot ), which are easily determined from the instrumented Charpy results. A complete description of the test procedure and methods for the analysis of the data may be obtained from a reading of Schindler 4 , ASTM E 1820 3 and ASTM E 813. 2 FRACTURE OF HIGH STRENGTH STEELS The ability of a cast high strength steel to resist fracture is a function of many different metallurgical factors, including the inherent matrix toughness, the segregation of impurities that have limited solubility and the composition, morphology, and distribution of second phase particles. For steels of similar strength and microstructure, fracture toughness at elevated loading Paper 12-054.pdf, Page 1 of 17 AFS Proceedings 2012 © American Foundry Society, Schaumburg, IL USA