A comparison of fracture behavior of low alloy steel with different sizes of carbide particles
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I.
INTRODUCTION
A S revealed by the present authors in References 1 and 2, the critical event for cleavage fracture varies in various types of specimens made of the same steel. In precracked specimens, the critical event is the propagation of a secondphase particle-sized microcrack into the matrix grain, but in notched specimens, it is the propagation of a ferrite grain-sized microcrack into the adjacent grain. In References 3 and 4, a definite relationship between grain size and toughness-associated parameters measured in notched specimens (4PB and Charpy V) was verified, which supported the conjecture that grain size had a dominant effect on notch toughness. However, the effects of second-phase particle sizes on the toughness of precracked specimens have not been identified by direct observation. This article compares the fracture behavior of steels with similar grain sizes but different sizes of carbide particles and emphasizes the effect of the grain and carbide particle sizes on the respective toughnesses.
II.
EXPERIMENTAL
A. Materials A low alloy steel, of composition shown in Table I, was used. The steel was heat treated to obtain the ferritic microstructure with predominate grain-boundary carbides. Specimens were austenitized at 900 ~ and then air cooled to obtain the uniform grain size. Some of the specimens were spheroidized at 700 *C for 24 hours and the remaining for 120 hours to obtain fine (SC) and coarse (LC) carbide particles, respectively. The metallographic samples were etched by 2 pct nital and the SC and LC microstructures are shown in Figures l(a) and (b). A longer diameter dt and a shorter diameter d~ for each
G.Z. WANG, Associate Professor, and J.H. CHEN, Professor, are with the Welding Institute, Gansu University of Technology, Lanzhou, Gansu 730050, People's Republic of China. Manuscript submitted February 23, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the 400 ferrite grains and 500 carbide particles of both SC and LC were measured by scanning electron microscope (SEM) photographs, and the diameter d of ferrite grains and carbide particles was calculated by the following formulas: d = ~/d,. ds The ferrite grain and carbide size distribution of both SC and LC are shown in Figures 2 and 3. The ferrite grain size distributions of SC and LC are similar to each other and have a 9.3-pm average size. The SC shows a significant difference from LC in carbide size distribution, and the size of the largest particle of SC is evidently smaller than that of LC. B. Specimens and Experiments Eight-millimeter round bars with 50-ram gage length were made for the tensile tests, which were carried out over a temperature range of - 196 ~ to 20 ~ using a SHIMADZU AG -10TA universal testing machine with a crosshead speed of 2 mm/min. The sampling direction and dimensions of specimens for four-point bending (4PB), COD, and Charpy V tests are shown in Figure 4. The critical values of COD (6c) were measured over - 1 9 6 ~ to - 9 0 ~ according to British Standard DD19. Part of the specimens were unloa
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