Micromechanism of the transition of fibrous cracking to cleavage of C-Mn base and weld steel
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I.
INTRODUCTION
THE authors have shown11j that critical events for cleavage fracture could be different for notched and precracked specimens made of the same weld steel. For notched specimens, the critical event is the extension of a ferrite grain-sized crack; however, for a precracked specimen, it is the extension of a second-phase particlesized crack. It is interesting to identify the critical event for cleavage fracture after a fibrous crack has been initiated and propagated in either a notched or a precracked specimen, because the critical event determines the criteflon of the fibrous-cleavage transition. There are opposing viewpoints of the factors affecting the transition of a fibrous crack to cleavage. Some researchers have claimed that the transition is caused by the increase of the local tensile stress 12,41which is caused in turn by the increase of the triaxiality of the stress and the strain hardening at the tip of a fibrous crack with its extension. But other researchers ~5,61 concluded by calculation that the triaxiality of tensile stress does not increase with the extension of a fibrous crack, the above transition being caused by the increased probability of cleavage with increasing area of the high stress zone in the plastically strained region ahead of the tip of a fibrous crack during its extension. Given the controversy, there is still room for investigation. A salient feature of toughness values measured in the transition temperature region is great variability. Some researchers tT,sl have attributed this variability to the randomness of the unstable extension of a main crack formed by the initiation and coalescence of microcracks. Other researchers tg't~ have related the variability to the J.H. CHEN, Professor and President, and G.Z. WANG, Lecturer, are with Gansu University of Technology, Lanzhou, Gansu, People's Republic of China 730050. Manuscript submitted July 17, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
heterogeneity of the microstructure of materials. It is necessary to investigate the mechanism of the scattering of toughness values by detailed observation on a microscopic scale. In this investigation, based on results of macroscopic tests and by detailed microscopic observations and measurements, the critical event, the mechanism of transition from fiber cracking to cleavage, and the factors causing the considerable scattering of toughness values were investigated. II.
EXPERIMENTS
A. Materials and Specimens Materials for these experiments are C-Mn base and weld steel. Their compositions are shown in Table I, and the microstructures and morphology of inclusions are shown in Figure 1. The C-Mn base steel was supplied in the form of hot-rolled plate. The specimens of the crack opening displacement (COD), four-point bending (4PB), three-point bending (3NB), and Charpy V tests were sampled, as shown schematically in Figure 2. The welding parameters used are shown in Table II. The dimensions of the specimens are shown in Figure 3.
B. Mechanical Testing The COD, 4PB, and 3NB tests
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