Influence of Tempering and Cryogenic Treatment on Retained Austenite and Residual Stresses in Carbonitrided 18CrNiMo7-6 Low Alloy Steel

This work investigated the influence of tempering conditions coupled with cryogenic treatment on thermal stabilization of retained austenite and residual stress distributions in carbonitrided 18CrNiMo76 low alloy steel samples. The carbonitriding conditions were set to enable attaining surface carbon and nitrogen content of 0.87 and 0.34 mass.-percent respectively. After carbonitriding, some of the samples were subjected to varying tempering conditions followed by cryogenic treatment at -120 °C using nitrogen gas. Analysis of both retained austenite and residual stresses was conducted using X-ray diffraction. In the as-quenched state, carbonitrided samples contained 52 mass.-percent. Samples that were directly subjected to the cryogenic treatment after quenching retained only about 20 mass.-percent of austenite. Samples subjected to variant tempering conditions coupled with cryogenic treatment retained at least 30 masses.-percent of austenite. A thermal stabilization of retained austenite which increases with increasing temperature was identified. On tempering at 240°C for 14 hours retained austenite becomes unstable and decomposes to bainite leading to the low initial amount of retained austenite before cryogenic treatment. It can be concluded that the tempering process coupled with cryogenic treatment leads to an increasing hardness, to higher compressive residual stresses as well as to a shift of the location of maximum compressive residual stress toward the surface

In-situ Observation of Retained Austenite and Residual Stress Evolutions during Tempering of carbonitrided DIN 1.6587 Alloy Steel

This paper investigates the evolution of retained austenite and residual stresses during and after tempering of carbonitrided 18CrNiMo7-6 low alloy steel carried out using in-situ X-ray diffraction technique. In this case, two carbonitriding treatments with different surface the retained austenite contents of 20 and 54 mass.-% are investigated. The tempering is carried out in a continuous heating mode to 650°C as well as in isothermal mode at holding temperature of 170, 240, and 300°C for 2 hours. During continuous heating at a heating rate of 10°C/min, the retained austenite started to decompose at 290°C. On isothermal holding at 170°C for 2 hours, the retained austenite remained relatively stable at 20 and 54 mass.-% while readily decomposed to less than 5 mass-% on holding at 300°C. On continuous heating, residual stress in martensite continuously relaxes and reaches full relaxation (0 MPa) at about 400°C. During isothermal holding, residual stresses in martensite are increasingly relaxed with increasing holding tempering. Further relaxation of residual stresses is observed during cooling whereas a cyclic variation of the residual stresses in the retained austenite could be determined

Investigation of the Effects of Low-Pressure Carburizing Process Parameters on Microstructural Evolution by Means of In Situ Synchrotron X-Ray Diffraction

In situ X-ray diffraction experiments during low-pressure carburizing processes are performed at the German Electron Synchrotron Facility, Beamline P07, in Hamburg, Germany, with a specially developed process chamber. Microstructural evolution is precisely analyzed based on diffraction data, and several process parameters are varied. The investigations focus on boost and diffusion steps in which carbon donor gas interacts with the hot steel surface and carbon atoms diffuse through the sample. An increased process temperature leads to higher carbon absorption during the boost step, especially at the early stages of the process. Regardless of process parameters, austenite saturation is reached in a few seconds. Therefore, longer boost step duration and/or a higher acetylene amount does not directly increase the carbon profile; instead, this would only increase the amount of carbides formed on the surface, which would contribute to the carbon profile by dissolution in the following steps. Therefore, shorter and a high number of boost steps are recommended for high efficiency. The cementite formation rate shows a similar trend with austenite saturation. It is very fast at the beginning and then stays almost constant. Therefore, introducing acetylene to the furnace after that point has no positive effect on the carburization

Effect of compressive deformations on the final microstructure of a low carbon high silicon bainitic steel thermomechanically processed

Due to a combination of advantages, high-performance steel components, especially for automotive manufacturing applications, are generally forged parts. In the forging industry, bainitic steels are being increasingly used, because they can reduce the processing chain and energy consumption. In this case, the bainitic microstructure can be obtained immediately after forging, with controlled cooling, and without any subsequent heat treatment. In the present work, the effect of thermomechanical routes performed in the austenitic and bainitic fields on the final microstructure and final hardness of 18MnCrSiMo6-4 bainitic steel has been discussed. Thermomechanical processing routes were tested and evaluated in a Gleeble® 3800 testing machine with one and two-step deformation. In both cases, the sample had its height reduced by 40% and the strain rate used was 0.1s-1. It could be shown that the plastic deformation promoted in the bainite field induces the bainite transformation. The results also show a strong dependence of bainite morphology concerning the deformation temperature of the steel. Moreover, the knowledge of the hot and warm stress-strain curves is an important result because it allows estimating the necessary stress and the energy consumption per volume unit to deform the material

Austenitic grain size prediction in hot forging of a 20MnCr5 steel by numerical simulation using the JMAK model for industrial applications

Yield strength and toughness in steels are directly associated with hot forging processes, especially by controlling austenitic grain size and cooling conditions. The phenomenological JMAK model in macroscale has been applied in different material classes to predict grain size after hot forming. However, on an industrial application, there is still a lack of understanding concerning the synergic effects of strain rate and temperature on recrystallization. This preliminary study aimed at investigating the applicability of coupled semi-empirical JMAK and visco-elastoplastic models in numerical simulation to predict austenitic grain size (PAGS). Hot forging of cylindrical samples of a ferritic-perlitic DIN 20MnCr5 steel was performed followed by water quenching. The main influences, such as temperature, strain and strain rate fields following the recrystallization model were investigated using the subroutine of FORGE NxT 2.1 software. The results were evaluated by comparing experimentally measured and simulated PAGS at process end. The forging process generates different strain and strain rate fields in the workpiece, which in turn lead to a variation in the PAGS and recrystallization fractions. The simulation was able to detect the PAGS variation showing a good agreement between the experimental forging results and the applied model

Constitutive modelling of high temperature flow behaviour for a low carbon high silicon bainitic steel

It is well recognized the importance of the rheological characterization for the development of the steel in thermomechanical treatments, especially for the mechanical properties improvement of bainitic steels in subsequent hot forging optimization. Therefore, the plastic strain behaviour of a low carbon high silicon bainitic steel was studied through isothermal compression tests using a thermomechanical simulator at temperatures of 1123 K – 1423 K and strain rates of 0.1 – 5 s-1. Arrhenius equation was used to obtain the constitutive constants, which represents the material behaviour of flow stress in high temperature. Besides, work hardening, dynamic recovery, and the JMAK model in the dynamic recrystallization (DRX) of the steel parameters were determined. The second part of this research compared two proposed modified models from the literature, which showed the differences in modelled flow curves behaviour when they are applied for high strain levels. The flow curves were modelled in high strain levels for further implementation in numerical simulation, thus allowing an adjustment of parameters in hot forming processes for this bainitic steel. The proposed models presented an agreement with experimental values. However, only the Avrami equation to DRX showed the dynamic recovery mechanism in high strain levels, which has represented physical behaviour during the thermomechanical process

In situ investigation of the bainitic transformation from deformed austenite during continuous cooling in a low carbon Mn-Si-Cr-Mo steel

The effects of hot deformation on the bainitic transformation of a low carbon steel during continuous cooling were comprehensively studied through in situ high-energy synchrotron X-ray diffraction, dilatometry, and ex situ microstructural characterizations. The obtained results indicated that the prior deformation of austenite at 950 C accelerates the bainite formation at the early stages. During the ongoing of the transformation, both the overall kinetics of bainite and carbon enrichment of austenite are lower in deformed austenite. The bainitic microstructure developed from deformed austenite is more refined and presents the same retained austenite content at room temperature with slightly lower carbon content when compared with the undeformed sample. Besides, a significant higher dilatation strain was measured during the bainitic transformation in the deformed sample, which can be explained by the crystallographic texture in hot deformed austenite. The evolution of the peak broadening of the {220}c and {211}a reflections during bainitic transformation are discussed in detail

Investigation of the influence factors on distortion in induction-hardened steel shafts manufactured from cold-drawn rod

In this study, the distortion of steel shafts was investigated before and after induction hardening. Several essential influencing factors in the manufacturing process chain regarding cold drawing, cutting method, notches on the shafts, and induction hardening were analyzed by design of experiment (DoE). Further necessary examinations of microstructures, hardness profile, segregation of chemical composition, and residual stress state were conducted for understanding the distortion behavior. The results of the statistical analysis of the DoE showed that the drawing process is the most important factor influencing distortion. The surface hardening depth of induction hardening is the second main factor. The relationship between inhomogeneities in the work pieces and the distortion was finally discussed

Investigations of residual stress distributions in retained austenite and martensite after carbonitriding of a low alloy steel

Abstract. Specimens of low alloy steel were carbonitrided under different conditions to attain varying levels of carbon and nitrogen contents. The residual stress depth distribution was evaluated in martensite and retained austenite by X-ray diffraction. Beside standard evaluations, triaxial residual stress states with σ 33 ≠0 in both phases were also considered. High values of residual stresses in both phases were observed. The sign, magnitude and location of maximum compressive residual stresses were greatly influenced by the level of carbon and nitrogen contents. Introduction The carbonitriding process is the modified form of carburizing [1] during which ammonia gas is added into the carburizing atmosphere. The main advantage of this process in comparison to carburizing is the rapid diffusion of carbon (C) and nitrogen (N) as well as the formation of high compressive residual stresses (RS) in the case. High compressive RS and hardness enhance the fatigue properties of components like shaft, gear and bearing and improves resistance to wear, bending fatigue, and rolling fatigue In the present work, carbonitrided specimens made of steel grade 18CrNiMo7-6 were investigated in terms of RS distribution in both martensite and RA phases according to the standard si

In Situ X-Ray Diffraction Analysis of Microstructure Evolution during Deep Cryogenic Treatment and Tempering of Tool Steels

Deep cryogenic treatment of tool steels, incorporated in conventional hardening and tempering, has been a topic of intensive research in recent years. Yet, the governing microstructural mechanisms involved in the deep cryogenic treatment of high-alloyed tool steels are still controversial. Thus, an in situ X-ray diffraction study is conducted on three tool steels X38CrMoV5-3, X153CrMoV12, and ~X190CrVMo20-4 to shed light on microstructural evolution during cryogenic treatment and subsequent tempering. For these high-alloyed tool steels, the transformation of retained austenite into martensite is detected during the cooling phase of the cryogenic treatment. A change in tetragonality of martensite occurs mainly in the heating phase of the subsequent tempering process, which indicates the diffusion of carbon and carbide precipitation from the martensite. The microstructure evolution of the tool steels after hardening, cryogenic treatment, and tempering is further examined by scanning electron microscopy. © 2021 The Authors. Steel Research International published by Wiley-VCH Gmb