6533b861fe1ef96bd12c5566

RESEARCH PRODUCT

High chromium-carbon’s steel microstructural and properties evolutions after Hot Isostatic Pressing (HIP) process : From powder to final product.

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Fe-Cr-C alloys are frequently used as hard coating coatings for nuclear valves in pressurized water reactors (PWRs), due to their good mechanical resistance and corrosion. The Norem02 alloy (Fe - 25% Cr - 4% Ni - 4% Mn - 4% Si - 2% Mo) is one of the most promising candidates. Hardfacing coatings are typically deposited by a plasma transferred arc welding (PTAW) process, which results in a coarse and heterogeneous microstructure, exhibiting unsatisfactory wear resistance properties, mainly at high temperature. In the present study, Norem02 is developed by Hot Isostatic Compression (CIC), a pressure assisted sintering technique where a powder is placed in a plastically deformable container under sintering conditions. This results in the development of dense materials with a fine-grained microstructure and improved durability properties. CIC can be used as an alternative hard coating technique if the substrate to be coated forms part of the container.The Norem02 powder was sintered for 3 hours at 1100 ° C and 1200 bar in a 304L container. The microstructure consists mainly of austenite in which chromium carbides of the M23C6 type (M = Cr, Mn, Mo) are dispersed as well as a small amount of ferrite. Despite a measured density of 100% characteristic of complete sintering, the microstructure of the initial powder is still visible after the CIC process. However, a more defective microstructure and globular carbides formed between the old powder particles.A transition zone was revealed inside the Norem02, near the interface with the 304L. In this zone, the microstructure is formed of a duplex matrix (austenitic / ferritic) and mainly of globular carbides of the M23C6 type.In order to study the phase transformations and the densification mechanisms of the Norem02 powder, interrupted HIP cycles at different temperatures (from 800 to 1100 ° C) were carried out. The microstructural evolutions of the volume and of the interface zone were studied by optical observations, SEM and EBSD analyzes coupled with characterizations by X-ray diffraction. A vacuum sintering test without any applied pressure was also carried out, in order to understand the influence of the application of pressure on phase transformations.All the results made it possible to begin understanding the phenomena of densification as well as the influence of the various sintering parameters on the final microstructure of the material. Preliminary results were obtained concerning the tribological and mechanical performances of the material. These indicate an improvement in performance over the initial development process.