Microstructural evolutions and phase transformations of a fe-cr-c alloy elaborated by hot isostatic pressing
Fe-Cr-C alloys are frequently used as hardfacing coatings for nuclear valves in pressurised water reactors (PWR), due to their good mechanical and corrosion resistance. Norem02 alloy (Fe – 25 % Cr – 4 % Ni – 4 % Mn – 4 % Si – 2 % Mo) is one of the most promising candidate. Hardfacing coatings are typically deposited by Plasma Transferred Arc Welding (PTAW) process, which results in a rough and heterogeneous microstructure, presenting unsatisfactory wear resistance properties, mainly at high temperature [1,2]. In the present study, Norem02 is elaborated by Hot Isostatic Pressing (HIP), an innovative pressure-assisted sintering technique resulting in the elaboration of dense materials with a fine-grained microstructure and improved durability properties. HIP can be used as an alternative hardfacing coating technique if the substrate to be coated constitutes a part of the container.
Norem02 powder was HIPed for 3 hours at 1100°C and 1200 bar in a 304L container The bulk microstructure composes primarily of austenite with a minor amount of ferrite and M23C6-type carbides M = Cr, Mn, Mo) . Despite a measured density of 100 %, the lath-like fine microstructure of the initial powder is still visible in the bulk of the sintered material after HIP process. On the other hand, a coarse microstructure formed at the prior particle boundaries. A transition zone was revealed inside Norem02, close to the interface with the container. In this area, the microstructure is formed by a duplex (austenitic/ferritic) matrix and mainly globular M23C6-type carbides.
In order to study the phase transformations and densification mechanisms of Norem02 powder, interrupted HIP cycles at different temperatures (from 800 to 1100°C) were carried out. Microstructural evolutions of the bulk and the interface zone were carefully investigated by optical observations, SEM and EBSD analyses coupled with X-ray diffraction characterisations. Moreover, to evaluate the influence of the pressure on the densification phenomena and phase transformations, Norem02 powder was HIPed at 1100°C but at different pressures (900 and 1500 bar). A sintering test under vacuum without any applied pressure was also performed.
The whole set of results allowed understanding the densification phenomena, in order to elaborate a material with a controlled microstructure and to establish a relationship between elaboration process, final microstructure and properties.