Microstructure and fracture mechanics properties of 13% Cr-4% Ni martensitic stainless steels

Abstract

13% Cr-4% Ni martensitic stainless steels are used to manufacture the hydroelectric runners due to their high strength and toughness, as well as their good resistance against corrosion and cavitation erosion. As hydraulic turbine runners are subjected to cyclic loadings, fatigue cracking is the main source of damage during their useful lifetime, and regular inspections are performed to verify the severity of the damage, and carrying out the required reparations. The main purpose of this study is characterizing the fracture behaviors of the hydroelectric runner component materials i.e. the cast (CA6NM), the wrought (415), the matching weld metals (410NiMo). The possibility of establishing relationships among fracture behavior, inclusion characteristics and austenite contents has motivated the study. The stress triaxiality ST ratios were measured on the necked region from the tension broken samples, and fracture micromechanics were characterized using the modified Rice and Tracey model describing ductile rupture in terms of void growth-ST ratio relationship. Linear-Elastic Fracture Mechanics (LEFM) is not applicable for small laboratory specimens of ductile materials, such as 13% Cr-4% Ni martensitic stainless steels, as large-scale yielding occurs during KIc testing. Elastic-Plastic Fracture Mechanics (EPFM) approach is therefore necessary to measure the strain energy release rate (J) for this category of stainless steels. The stable crack growth behavior of the substrate steel (CA6NM), the weld metals, and the Heat-Affected Zone (HAZ) was characterized during JIc-testing. The substrate steel was tested in heat-treated condition, whereas the weld metal and the HAZ were examined in both as-welded and heat-treated conditions. Fracture surfaces were examined using a Scanning Electron Microscope (SEM) coupled with an Energy Dispersive X-ray (EDX) spectrometer to reveal the nature of the second phase particles. It was shown that fracture strain after necking can be estimated using size and spacing of inclusions when considerable void growth occurs before final rupture; however, in the cases in which negligible void growth occurs due to a very small inclusion spacing, the proposed model is not appropriate. The austenite content and the mechanical properties of the matrix should also be considered. The heat-treatment significantly improved the fracture toughness JIc of the weld metal and the HAZ by factors of 2 and 2.7, respectively. However, unstable propagations were occurred into the weld metal in the case of all the JIc tests performed on the HAZ. This is due to the detrimental effect of the very small inclusion spacing in the microstructure of the weld metal, resulting in an immediate coalescence of the nucleated micro-voids.

Date	12 Jul 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Philippe Bocher (Supervisor)