Microstructure evolution during laser direct energy deposition of a novel Fe-Cr-Ni-W-B hardfacing coating

Abstract

The paper presents a detailed study on the microstructure evolution and cracking tendency of a novel Fe-Cr-Ni-Mo-W-B hard facing alloy deposited on a carbon steel substrate using blown powder directed energy deposition technique. CALPHAD approach was used to understand the phase equilibrium in the system and to optimize the volume fraction of the hard facing precipitates. The preliminary deposits showed extensive cracking in the hard facing deposit, which was attributed to solidification cracking. To evaluate the cracking tendency of the alloy solidification sequence of the current alloy was evaluated using a Scheil Gulliver simulation and contrasted with the solidification sequence of the commercially available alloys. The simulations showed that the alloy used in this current study showed a primary FCC mode of solidification as opposed to a primary BCC mode of solidification in the commercial alloys. Crack free deposits were then fabricated by preheating the substrate to 400 °C thereby reducing the thermal strains. The segregation of the elements and phase fractions calculated using the THERMOCALC's® TCFE8 data base were then compared with the experimentally measured phase fractions and compositions. Both the simulations and the experimental work showed that the as solidified microstructure consists of a (Cr,Fe)2B phase in a FCC matrix. However, while CALPHAD is able to capture the phase fractions of the various phases during build fabrication, the partitioning behavior of W is not accurately captured. In this work we have developed a novel alloy and demonstrate that crack free deposits can be obtained with this new alloy with a hardness close to 600 VHN (55 HRC).