Effect of Cr and N on the Stress Corrosion Cracking Behavior of Fe-18Mn Steel

High-Mn steels developed for offshore industries require good resistance to stress corrosion cracking (SCC) in seawater. Elements like Cr and N are often added to improve the resistance to SCC. In this study, the SCC behavior of Fe18Mn3Cr0.1N and Fe19Mn19Cr0.6N steels in artificial seawater was examined. Slow strain rate tests were conducted at a nominal strain rate of 10(-6)/sec in air and artificial seawater under anodic and cathodic applied potentials. The tensile ductility drop in artificial seawater was compared to air and evaluated as the resistance to SCC. It was found that both specimens showed intergranular cracking in artificial seawater under both anodic and cathodic applied potentials. The intergranular SCC was more severe under anodic applied potential than cathodic applied potential. However the sensitivity to SCC in artificial seawater was substantially reduced in Fe19Mn19Cr0.6N specimen with higher Cr and N content, as compared to the Fe18Mn3Cr0.1N specimen under both applied potentials. Potentiodynamic tests in artificial seawater showed an increase in pitting corrosion potential, rather than corrosion potential, with increasing Cr and N content in high-Mn steel. The SCC behavior of high-Mn steels with different Cr and N contents was discussed based on micrographic and fractographic observations.11Ysciescopuskc

Growth behavior of fatigue cracks in ultrafine grained Cu smooth specimens with a small hole

In order to study the growth mechanism of fatigue cracks in ultrafine grained copper, stress controlled fatigue tests of round-bar specimens with a small blind hole as a crack starter were conducted. The hole was drilled on the surface where an intersection between the shear plane of the final ECAP processing and the specimen surface makes an angle of 45° or 90° with respect to the loading axis. At a low stress ( a = 90 MPa), the direction of crack paths was nearly perpendicular to the loading direction regardless of the location of the hole. Profile of crack face was examined, showing the aspect ratio (b/a) of b/a = 0.82. At a high stress ( ? a = 240 MPa), although the growth directions inclined 45° and 90° to the loading-axis were observed depending on the location of the drilling hole, crack faces in these cracks were extended along one set of maximum shear stress planes,  corresponding to the final ECAP shear plane. The value of aspect ratios was b/a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions, respectively. The role of deformation mode at the crack tip areas on crack growth behavior were discussed in terms of the mixed-mode stress intensity factor. The crack path formation at high stress amplitudes was affected by the in-plane shear-mode deformation at the crack tip

Effect of stabilization annealing on fatigue crack propagation behavior of inconel 706 alloy at 25 and 650 °C

Stabilization annealing (SA) treatment is often performed on precipitation-hardened IN706 (Ni-37.7Fe-15.6Cr–3Nb-1.7Ti-0.23Al) before aging to improve the resistance to creep, while sacrificing the tensile properties. In this study, the effect of SA treatment on fatigue crack propagation (FCP) behavior of IN706 at 25 and 650 °C was examined based on detailed fractographic and micrographic analyses. IN706 specimen with SA treatment at 845 °C for 3 h showed a considerable amount of η precipitates, that were located along grain boundaries and within grains. It was found that η precipitates, particularly those linearly aligned within grains, provided easy crack path, reducing the resistance to FCP of IN706 at both 25 and 650 °C

Re-heat treatment effect on the microstructure and mechanical properties of the Inconel 706 alloy for repair

Although the understanding of microstructure and mechanical property changes in re-heat-treated Inconel alloys is important to evaluate the performance of repaired components, recent research has mainly focused on the heat treatment effect of as-fabricated products. Thus, in this work, the performance change of the used Inconel 706 rotor-disks under varying re-heat treatment conditions was investigated. Because of the heat exposure of the used rotor-disk component, a high fraction of plate-like η phase (6.36%) and compact γ'/γ'' co-precipitate structure were initiated in the as-received sample. After re-heat treatment, η phase dissolution breakage and compact to non-compact γ'/γ'' co-precipitate structure transition occur, resulting in mechanical properties changes of the Inconel 706 alloy. As a result, η phase fraction affects the tensile strength and ductility of Inconel 706 alloy, while the non-compact γ'/γ'' co-precipitate structure in the matrix degrades the creep lifetime. These results indicate that re-heat treatment during the repair of operated components induces microstructural and mechanical properties changes. However, to investigate the detailed history of used Inconel 706 components, additional research on the microstructural degradation during operation of Inconel components is required

Effects of Nitrogen and Tensile Direction on Stress Corrosion Cracking Susceptibility of Ni-Free FeCrMnC-Based Duplex Stainless Steels

Stress corrosion cracking (SCC) behavior of Ni-free duplex stainless steels containing N and C (Febalance-19Cr-8Mn-0.25C-(0.03, 0.21)N, in wt %) was investigated by using a slow strain rate test (SSRT) in air and aqueous NaCl solution with different tensile directions, including parallel (longitudinal) and perpendicular (transverse) to the rolling direction. It was found that alloying N was effective in increasing the resistance to SCC, while it was higher along the longitudinal direction than the transverse direction. The SCC susceptibility of the two alloys was assessed based on the electrochemical resistance to pitting corrosion, the corrosion morphology, and the fractographic analysis

Microstructural effects on the tensile and fracture behavior of selective laser melted H13 tool steel under varying conditions

The microstructural-mechanical correlative study has been conducted for characterization of selective laser melted H13 tool steel. Transformation behavior from austenite to martensite has been observed with partitioning of C in matrix with correlative atomic diffusivity during selective laser melting process. During solidification, columnar grain structures are formed due to epitaxial growth following the build direction of H13 tool steels. Columnar microstructures are mostly composed of martensite with small amount of retained austenite. Supercooling of H13 with high laser scan speed increased the nucleation sites, which reduced the diameter of columnar grain. During tensile test, deformation appeared in grain boundary while there was no significant martensitic phase transformation confirmed by X-ray diffraction (XRD) method and electron backscattered diffraction (EBSD) analysis. S2 (scan speed of 200 mm/s specimen had the better tensile property with tensile strength of 1700 MPa and elongation of 1.6% than the rest(<1300 MPa, 0.3%). Fractographic observation confirmed that porosity, pore size, and pore types are key metallic parameters affecting to tensile properties. Various carbides were identified by transmission electron microscopy (TEM) and 3D atom probe tomography (APT) analysis to confirm these microstructural characteristics and fracture behaviors of selective laser melted H13 tool steels. ?? 2019 Elsevier Inc

Effect of heat treatment conditions on the plastic deformation behavior of the Inconel 706 alloy

Although the Inconel 706 alloy has been developed to reduce the manufacturing cost for mass production, the effect of post-heat treatment on the deformation mechanisms of this alloy system has not been well investigated yet. In this study, the effect of heat treatment conditions on the plastic deformation behavior of the Inconel 706 alloy was analyzed. The size of the coherent γ'/γ'' co-precipitates in the sample subjected to 3-step aging (3STEP) is larger than that in the sample subjected to 2-step aging (2STEP). However, the yield strength of 3STEP is lower than that of 2STEP due to the precipitate-free zone near its η phase which reduces the volume fraction of the γ'/γ'' co-precipitates. The coarsened η phase in 3STEP also acts as a crack initiation site, inducing intergranular fracture. Meanwhile, serrated flows were observed in the tensile stress-strain curve of 2STEP tested at a high temperature condition, which resulted in ductility degradation. The present result indicates that the different heat treatment conditions for the Inconel 706 alloy affect the morphology of the precipitate in the matrix, which can be attributed to their deformation behaviors