Improving the ductility and toughness of Fe-Cr-B cast irons

This thesis characterises the microstructures, phase transformations and their variation with composition and heat treatment in Fe-Cr-B cast irons and identifies the crucial microstructural factors that govern the ductility and toughness of these materials. Conventional metallographic techniques, such as optical microscopy, SEM, TEM, electron probe and x-ray diffraction, combined with the mechanical property tests - tensile, impact, hardness and fracture toughness - as well as specifically designed tests for the investigation of crack initiation and extension, were utilised in the course of this study. The major findings are summarised below: (1) The matrix in as-cast and solution treated Fe-Cr-B cast irons is supersaturated in boron and carbon. Microprobe analysis suggests that the boron solubility is probably between 0.185 to 0.515wt% in the as-cast condition and 0.015 to 0.0589wt% in the solution treated condition. These values are much higher than the commonly accepted solubility of 50ppm in pure iron. (2) In essence boron in Fe-Cr-B alloys behaves exactly as if it were in interstitial solution and its mobility is comparable to interstitial solute like carbon. (3) Boron in solution tends to migrate to the grain boundaries during heat treatment, via either equilibrium or non-equilibrium segregation. The type of segregation depends on the crystal structure of the matrix. Equilibrium segregation of boron dominates in martensite/bainite matrices and in solid solutions with a bcc crystal structure during tempering. The non-equilibrium segregation of boron is dominant in austenite and in solid solutions with a fee crystal structure during heating and cooling process. (4) The segregated boron atoms tend to displace carbon atoms from the grain/phase boundaries and the matrix regions near the grain/phase boundaries and this forces the carbon atoms to concentrate in the central regions of the matrix grains. (5) Various phase transformations in Fe-Cr-B cast irons are closely associated with the extent of boron segregation to the grain/phase boundaries. (6) Cracks in Fe-Cr-B cast irons preferentially initiate from and propagate along the grain/phase boundaries where the secondary precipitates M6(C,B) and M23(C,B)6 are formed. Sometime, cracks may initiate from the sub-grain boundaries in the matrix where there is a high concentration of the precipitated secondary phases. Therefore, the failure of Fe-Cr-B cast irons is a type of secondary precipitation induced embrittlement. (7) Overall boron content has relatively a smaller effect on the resistance to crack initiation and extension and the ductility and toughness, compared with boron in solution. Increases of carbon and molybdenum contents result in an increase of the amounts of secondary precipitation at grain/phase boundaries. (8) If boron in solution was used to promote the interaction between boron and carbon at the grain/phase boundaries and lead to an reduction of secondary precipitates at there, the boron in solution resulted in an increase in resistance to crack initiation and extension. Otherwise, boron in solution is detrimental because it increases the secondary precipitates at the grain/phase boundaries. On the basis of these findings, modification of Fe-Cr-B cast irons to improve ductility and toughness by eliminating or at least reducing the crucial, harmful microconstituents in microstructures via changes in composition and heat treatment has been attempted. Test results show that the approaches used are partly successful. However, the compositional and heat treatment modification can only reduce, but not entirely eliminate the secondary precipitates at grain/phase boundaries. Therefore, the embrittlemenet induced by these secondary precipitates at the grain boundaries still controls the ductility and toughness of Fe-Cr-B alloys. This occurs, even though the continuous, three-dimensional networks of eutectic M2B borides were spheroidised by heat treatment into bigger, isolated, equiaxed particles that were uniformly distributed in the matrix grains.