Analysis On Mechanism Of Cleavage Fracture In A Low Alloyed Steel At Different Low Temperature

Based on observations of fracture surfaces and measurements of local critical parameters, analysis and comparison of nature of cleavage triggering sites at various temperature for different microstructure steel, analysis of relationship between ductile crack propagation length and fracture toughness, sophisticated FEM calculations of distributions of stress, strain and triaxiality and simulations of short cracks initiated and extended at precrack tips, the cleavage fracture are studied for three point bending(3PB) precracked specimens with three various microstructure, the followed main conclusions are obtained. (1) Ductile to brittle transition temperature (DBTT) of fine grain fine carbide specimens is lower than that of fine grain coarse carbide specimens and coarse grain coarse carbide specimens, fracture toughness of fine grain fine carbine specimens is higher than that of fine grain coarse carbide specimens and coarse grain coarse carbide specimens. For precracked specimens, the sizes of carbides particles play a leading role on ductile to brittle transition temperature (DBTT) for fracture toughness, grain sizes have a minor role on ductile to brittle transition temperature (DBTT) for fracture toughness. The scatter of measured values of global fracture toughness are mainly caused by the variation of ductile crack length, especially, it is caused by random volatility of propagation process of ductile crack tip width and random distribution of the weakest microstuctures.(2)On the ductile to brittle transition region, firstly, ductile crack is produced for 3PB precracked specimens. Because the intrinsic propagation energy of ductile crack is not concerned with test temperature, toughness of various temperatures is attributed to stretch zone width and length of ductile. The energy is consumed on bluntness of crack tip and propagation process of ductile crack. On the lower temperature zone of ductile to brittle transition temperature (DBTT), much energy is consumed on the the blunting process, so the toughness is higher. On the lower temperature zone of ductile to britttle transition temperature (DBTT), fracture toughness is determined by propagation resistance of stretch zone width (SZW) ,stable crack length (SCL)and the time of cleavage fracture. On the lower temperature zone of ductile to brittle transition temperature (DBTT) and on the lower platform, direct ratio relationship is appear between fracture toughness (COD) and the SZW+SCL, so fracture toughness is evaluated by the stretch zone width and stable crack length.On the cleavage fracture process, crack nucleation has an important effect on cleavage fracture. Five nature of cleavage triggering sites are found, such as: grain boundary carbide cracking, inclusion cracking, the debonding interface of larger carbide, sphere carbide and ferrous matrix, grain boundary cracking. Based on observation of fracture surface, on the sharp upturn temperature zone of ductile to brittle transition temperature (DBTT), nature of cleavage triggering sites is changed for fine grain microstructure, inclusion cracking is domianted. On the lower temperature zone of ductile to britttle transition temperature (DBTT), grain boundary carbide cracking is the main initiation origins for coarse grain microstructure. It is means that sharp upturn of toughness is concerned with inclusion cracking for fine grain microstructure and fracture toughness is concerned with carbide for coarse grain microstructure.