Design of low-carbon, low-temperature bainite

Abstract

The aim of the work presented here was to investigate the possibility of a low-carbon version of superbainite. One of the limiting parameters in the design is the martensite-start temperature. The dependence of it on the austenite grain size has been investigated. The Fisher model for the geometrical partitioning of austenite grains by plates of martensite was used to develop a theory to explain the observations, based on the ability to detect transformation as a function of the austenite grain size. The relationship derived has been tested on a wide range of published data.The work described in above paragraph leaded to the investigation on effect of grain size on athermal martensite kinetics. The study revealed that the martensite transformation rate starts from zero, rapidly increases until a fraction ~0.15, and then decreases smoothly. Small grain-sized samples transform rapidly at the early stages because of larger number of initial nucleation sites. However, the grain size dependence diminishes at the later stages because of autocatalysis and decreasinguntransformed austenite.Compiled knowledge on martensite has been applied to assess the ε martensite-start temperatures. Methods have been created for calculating the temperature at which ε-martensite forms when austenite is cooled. It is demonstrated that the thermodynamic method used in similar calculations for α` can be applied for some alloy systems whereas it cannot be implemented for highly solute concentrated alloys because of inaccurate thermodynamic data, a conclusionwas confirmed using first-principles calculations. A neural network model has been developed which is capable of predicting the transformation temperature for a wide range of compositions and complex phenomena.Three alloys have been designed, manufactured and investigated to evaluate the validity of low-carbon version of low-temperature bainite. Several problems such as low transformation temperature, merging of BS into MS, presence of coalesced bainite and slow kinetics could be solved. However, small fraction of retained austenite coming from low-carbon concentration of initial alloy could not be avoided. That resulted in ordinary mechanical properties.