High-temperature deformation of delta-processed Inconel 718

© 2017 Elsevier B.V. The hot-flow behavior of Inconel 718 subjected to delta processing (DP) was analyzed. Hot compression tests were subsequently performed at 960 °C and 1020 °C at the four different strain rates of 0.001, 0.01, 0.1, and 1 s-1. The two deformation temperatures were located below and above the d-solvus of IN718 respectively. Microstructural characterization was performed by means of optical (OM) and scanning electron microscopy (SEM). The high temperature deformation results in the fragmentation of the existing d-phase by means of partial dissolution and/or deformation, leading to an improved grain size control. A classic dynamic recrystallization (DRX) behavior was observed in the flow curves, which is typical of low-medium stacking fault energy (SFE) alloys. This flow behavior was modeled according to various approaches. Peak stress modeling was performed using two different approaches, referred to as ‘apparent’ and ‘physically-based’. The hot-flow behavior before the peak stress was modeled according to the Estrin-Mecking-Bergstrom approach, with Avrami kinetics employed to describe the DRX behavior.Peer ReviewedPostprint (author's final draft

X-ray determination of compressive residual Stresses in spring steel generated by high-speed water Quenching

Automotive components manufacturers use the 5160 steel in leaf and coil springs. The industrial heat treatment process consists in austenitizing followed by the oil quenching and tempering process. Typically, compressive residual stresses are induced by shot peening on the surface of automotive springs to bestow compressive residual stresses that improve the fatigue resistance and increase the service life of the parts after heat treatment. In this work, a high-speed quenching was used to achieve compressive residual stresses on the surface of AISI/SAE 5160 steel samples by producing high thermal gradients and interrupting the cooling in order to generate a case-core microstructure. A special laboratory equipment was designed and built, which uses water as the quenching media in a high-speed water chamber. The severity of the cooling was characterized with embedded thermocouples to obtain the cooling curves at different depths from the surface. Samples were cooled for various times to produce different hardened case depths. The microstructure of specimens was observed with a scanning electron microscope (SEM). X-ray diffraction (XRD) was used to estimate the magnitude of residual stresses on the surface of the specimens. Compressive residual stresses at the surface and sub-surface of about -700 MPa were obtained.Peer ReviewedPostprint (published version

Texture evolution of experimental silicon steel grades. Part I: Hot rolling

The metallurgical understanding of the deformation processes during the fabrication of non-oriented electrical steels plays a key role in improving their final properties. Texture control and optimization is critical in these steels for the enhancement of their magnetic properties. The aim of the present work is to study the texture evolution of six non-oriented experimental silicon steel grades during hot rolling. These steels were low carbon steel with a silicon content from 0.5 to 3.0 wt%. The first rolling schedule was performed in the austenitic (¿-Fe) region for the steel with a 0.5 wt% of silicon content, while the 1.0 wt% silicon steel was rolled in the two-phase (a+¿) region. Steels with higher silicon content were rolled in the ferritic (a-Fe) region. The second rolling schedule was performed in the a-Fe region. Samples of each stage were analyzed by means of Electron Backscatter Diffraction (EBSD). Findings showed that the texture was random and heterogeneous in all samples after 60% of rolling reduction, which is due to the low deformation applied during rolling. After the second rolling program, localized deformation and substructured grains near to surface were observed in all samples. The Goss {110}texture-component was found in the 0.5 and 1.0 wt.-%silicon steels. This is due to the thermomechanical conditions and the corresponding hot band microstructure obtained after the first program. Moreover, the a//RD and the ¿ //ND fiber components of the texture presented a considerable increment as the silicon content increases. Future research to be published soon will be related to the texture evolution during the cold-work rolling process.Peer Reviewe

Analysis of strain-induced precipitates by delta-processing in Inconel 718 superalloy

Delta-Processing (DP) is a suitable heat treatment process aiming at controlling grain growth during hot deformation of the well-known Ni-base superalloy Inconel 718 (IN718). DP is often applied by heating up to 900 °C for 24 h in order to promote d-phase saturation prior to deformation. In addition to the d-phase, Mo and Cr-rich carbides in the form of M23C6 may also precipitate during the subsequent hot deformation of this alloy in the range of 950–1000 °C These carbides are induced by deformation and can affect the static and dynamic recrystallization of the matrix. The objective of the current work is to understand the precipitation of M23C6 carbides when the material has been saturated with d-phase precipitation, as well as their interaction with recrystallization phenomena. For this purpose, a series of double-strain hot compression test (double-hit) were carried out. Several holding times were applied between the first and the second hit in order to derive the softening/hardening fraction progress with time. Microstructural characterization of the samples was performed through Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) in order to identify the precipitating/dissolving phases under each testing condition.Delta-Processing (DP) is a suitable heat treatment process aiming at controlling grain growth during hot deformation of the well-known Ni-base superalloy Inconel 718 (IN718). DP is often applied by heating up to 900 °C for 24 h in order to promote d-phase saturation prior to deformation. In addition to the d-phase, Mo and Cr-rich carbides in the form of M23C6 may also precipitate during the subsequent hot deformation of this alloy in the range of 950–1000 °C These carbides are induced by deformation and can affect the static and dynamic recrystallization of the matrix. The objective of the current work is to understand the precipitation of M23C6 carbides when the material has been saturated with d-phase precipitation, as well as their interaction with recrystallization phenomena. For this purpose, a series of double-strain hot compression test (double-hit) were carried out. Several holding times were applied between the first and the second hit in order to derive the softening/hardening fraction progress with time. Microstructural characterization of the samples was performed through Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) in order to identify the precipitating/dissolving phases under each testing condition.Peer ReviewedPostprint (author's final draft

High-temperature deformation of delta-processed Inconel 718

© 2017 Elsevier B.V. The hot-flow behavior of Inconel 718 subjected to delta processing (DP) was analyzed. Hot compression tests were subsequently performed at 960 °C and 1020 °C at the four different strain rates of 0.001, 0.01, 0.1, and 1 s-1. The two deformation temperatures were located below and above the d-solvus of IN718 respectively. Microstructural characterization was performed by means of optical (OM) and scanning electron microscopy (SEM). The high temperature deformation results in the fragmentation of the existing d-phase by means of partial dissolution and/or deformation, leading to an improved grain size control. A classic dynamic recrystallization (DRX) behavior was observed in the flow curves, which is typical of low-medium stacking fault energy (SFE) alloys. This flow behavior was modeled according to various approaches. Peak stress modeling was performed using two different approaches, referred to as ‘apparent’ and ‘physically-based’. The hot-flow behavior before the peak stress was modeled according to the Estrin-Mecking-Bergstrom approach, with Avrami kinetics employed to describe the DRX behavior.Peer Reviewe

X-ray determination of compressive residual Stresses in spring steel generated by high-speed water Quenching

Automotive components manufacturers use the 5160 steel in leaf and coil springs. The industrial heat treatment process consists in austenitizing followed by the oil quenching and tempering process. Typically, compressive residual stresses are induced by shot peening on the surface of automotive springs to bestow compressive residual stresses that improve the fatigue resistance and increase the service life of the parts after heat treatment. In this work, a high-speed quenching was used to achieve compressive residual stresses on the surface of AISI/SAE 5160 steel samples by producing high thermal gradients and interrupting the cooling in order to generate a case-core microstructure. A special laboratory equipment was designed and built, which uses water as the quenching media in a high-speed water chamber. The severity of the cooling was characterized with embedded thermocouples to obtain the cooling curves at different depths from the surface. Samples were cooled for various times to produce different hardened case depths. The microstructure of specimens was observed with a scanning electron microscope (SEM). X-ray diffraction (XRD) was used to estimate the magnitude of residual stresses on the surface of the specimens. Compressive residual stresses at the surface and sub-surface of about -700 MPa were obtained.Peer Reviewe