Investigation of cleavage fracture initiation in notched specimens of a C–Mn steel with carbides and inclusions

Abstract Cleavage initiation in notched specimens of a C–Mn steel with carbides and inclusions is investigated at −196 and −130 °C. The mechanical tests, microscopic observations and measurements, and FEM calculations are carried out. Two types of cleavage initiation sites are found. One is related to approximately spherical inclusions ahead of notch root (IC initiation), the other is related to spherical inclusions situated ahead of the elongated larger string inclusions (SIC initiation). In both IC and SIC initiation mechanism, the crack nucleation is induced by inclusions and the final fracture is controlled by propagation of a ferrite grain-sized crack into matrix grain. In the case of SIC initiation, a plate-like defect caused by the early debonding of the elongated larger string inclusions promotes the cleavage fracture, and the cleavage initiates in front of the plate-like defect. Temperature has pronounced effect on the type of the cleavage initiation site. At −196 °C the fractures are mainly the IC initiation mechanism, and at −130 °C all the fractures are the SIC initiation mechanism. The reason for this has been analyzed. The notch toughness of the materials tested is mainly determined by the cleavage initiation occurring in the weak constituents composed of inclusions and matrix grains, and is independent on the sizes and numbers of carbide particles.

[1]  K. Sieradzki,et al.  The effect of a partially dissociated nitrogen environment on the fracture toughness of 4340 steel , 1978 .

[2]  D. Dimiduk,et al.  Notch fracture inγ-titanium aluminides , 1996 .

[3]  D. Munz,et al.  THE SIZE EFFECT OF MICROSTRUCTURAL IMPLICATIONS OF THE WEAKEST LINK MODEL , 1990 .

[4]  W. Clark,et al.  The mechanism of brittle fracture in a microalloyed steel: Part I. Inclusion-induced cleavage , 2000 .

[5]  Anthony G. Evans,et al.  A statistical model of brittle fracture by transgranular cleavage , 1986 .

[6]  J. H. Chen,et al.  A comparison of toughness of , 1992, Metallurgical and Materials Transactions A.

[7]  T. J. Baker,et al.  The Effect of Non-Metallic Inclusions on Cleavage Fracture , 1986 .

[8]  A. N. Stroh.  The formation of cracks as a result of plastic flow , 1954, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[9]  J. H. Chen,et al.  Fracture behavior of C-Mn steel and weld metal in notched and precracked specimens: Part II. micromechanism of fracture , 1990 .

[10]  C. Prioul,et al.  Fractographic observations of cleavage fracture initiation in a bainitic A508 steel , 1999 .

[11]  David R. Owen,et al.  An elastic-plastic stress analysis for a notched bar in plane strain bending , 1971 .

[12]  J. H. Chen,et al.  Fracture behaviour of C–Mn steel multipass MMA weld metals at −60°C in Charpy V testing , 1988 .

[13]  M. Yao,et al.  A metallographic and fractographic study of the origin of cleavage fracture in mild steel , 1996 .

[14]  K. Ono,et al.  Anisotropic mechanical and acoustic emission behavior of A533B steels , 1981 .

[15]  N. Petch,et al.  The influence of grain boundary carbide and grain size on the cleavage strength and impact transition temperature of steel , 1986 .

[16]  J. H. Tweed,et al.  Micromechanisms of failure in CMn weld metals , 1987 .

[17]  J. F. Knott,et al.  Effect of reheating on microstructure and toughness of C–Mn weld metal , 1983 .

[18]  D. Shetty,et al.  Fractographic observations of cleavage initiation in the ductile-brittle transition region of a reactor-pressure-vessel steel , 1983 .

[19]  John R. Rice,et al.  ON THE RELATIONSHIP BETWEEN CRITICAL TENSILE STRESS AND FRACTURE TOUGHNESS IN MILD STEEL , 1973 .

[20]  W. Clark,et al.  The mechanism of brittle fracture in a microalloyed steel: Part II. Mechanistic modeling , 2000 .

[21]  Qudong Wang,et al.  Change of critical events of cleavage fracture with variation of microscopic features of low-alloy steels , 2002 .

[22]  J. Knott,et al.  Cleavage fracture of A 533 B pressure vessel steel in martensitic condition , 1984 .

[23]  J. G. Wang,et al.  Effects of geometry of notched specimens on the local cleavage fracture stress σf of C–Mn steel , 2003 .

[24]  J. H. Chen,et al.  A statistical model for cleavage fracture in notched specimens of C-Mn steel , 2001 .

[25]  C. J. McMahon,et al.  Initiation of cleavage in polycrystalline iron , 1965 .

[26]  W. J. Sisak,et al.  Yielding and work hardening effects in notched bend bars , 1986 .

[27]  J. H. Chen,et al.  Cleavag Fracture Criterion of Low Alloy Steal and Weld Metal in Notched Specimens , 1998 .

[28]  H. Wadley,et al.  Indentation loading studies of acoustic emission from temper and hydrogen embrittled A533B steel , 1982 .

[29]  J. F. Knott,et al.  Effects of microstructure on cleavage fracture stress in steel , 1978 .

[30]  J. H. Chen,et al.  Effects of notch geometry on the local cleavage fracture stress σf , 1999 .

[31]  I. Bernstein,et al.  Cleavage fracture in pearlitic eutectoid steel , 1989 .

[32]  A. Echeverría,et al.  The role of grain size in brittle particle induced fracture of steels , 2003 .

[33]  J. H. Chen,et al.  On the scattering of the local fracture stress σf , 1990 .

[34]  J. Knott,et al.  Micromechanical modelling of fracture toughness , 1987 .

[35]  L. Zhang,et al.  Dependency of fracture toughness on the inhomogeneity of coarse TiN particle distribution in a low alloy steel , 2001 .

[36]  G. T. Hahn,et al.  The Influence of Microstructure on Brittle Fracture Toughness , 1984 .