Development of a mathematical equation describing the strain hardening behaviour of metastable AISI 301 austenitic stainless steel

The strain hardening behaviour of AISI 301 metastable austenite steel was analysed by evaluating tensile data against the empirical mathematical equations of Hollomon, Ludwik and Ludwigson. It was found that these equations were inadequate to model this TRIP steel with low stacking fault energy (SFE). It was found that the fraction of strain-induced martensite could be expressed as a sigmoidal function of the applied strain. The log-log plots of true stress and true plastic strain from 5% to εUTS performed with uniaxial isothermal tests at 30 oC were thereafter adequately fitted with a sigmoidal curve. The instantaneous strain hardening exponent was determined as the slope of the above-mentioned sigmoidal curve at a specific strain value. The strain hardening exponent and the rate of strain hardening (dσ/dε) increases with deformation due to formation of strain-induced martensite to a maximum and thereafter decreases as the volume fraction of strain-induced martensite approximates saturation. The variation of the instantaneous strain hardening exponent as a function of plastic strain and the strength coefficient, K, at 30 oC was deduced. A high value of K, 1526MPa, was determined. A correlation between the extent of martensitic transformation and the value of the instantaneous strain hardening exponent was observed. This work is part of the project that seeks to develop a constitutive model describing the flow stress during plastic deformation as a function of both plastic strain and the resulting martensitic transformation at different temperatures and strain rates and which accounts for the isotropic hardening process.Columbus Stainless Steel companyhttp://iopscience.iop.org/journal/1757-899Xam2020Materials Science and Metallurgical Engineerin

The direct observation of surface martensite formation upon cooling to temperatures close to ambient in a heat treated AISI 301 stainless steel

Martensitic transformation, either athermal or strain-induced, in metastable AISI 301 austenitic steel is of high technical interest as this facilitates manipulation of mechanical properties. This work is part of a project aimed at identifying the influence of variables that determine the athermal α′-martensitic transformation in this steel in order to ensure consistent martensite formation and optimized mechanical properties. The influences of initial austenitic grain size and surface concentrations of interstitial elements (C, N) on martensite start temperature were investigated. The surface concentrations of both carbon and nitrogen were decreased through decarburisation and the formation of martensite upon cooling from ambient temperatures was directly observed using a cryogenic Scanning Electron Microscope (cryo- SEM). It was found that martensite formed on the heat-treated surfaces at much higher temperatures than that expected for AISI 301 steel. The observations were confirmed using electron backscatter diffraction (EBSD). Other analytical techniques including glow-discharge optical emission spectroscopy (GDOES), and Neutron Diffraction were carried out to explain the observations.Columbus Stainless Steel companyhttp://iopscience.iop.org/journal/1757-899Xam2019Materials Science and Metallurgical Engineerin

Study of wear performance of wheel and rail steels under dry sliding conditions

The demand for efficient railway services has significantly increased in the past years due to an increased demand for the high-speed transportation of goods with high loads. The increase in loads and velocities has resulted in increased problems associated with rolling contact fatigue (RCF), rolling and sliding wear on the wheel and rail materials causing a reduction of service life of wheel/rail systems. Rail operating companies spend significant funds in maintenance and replacing damaged rails and wheels caused by wear. In addition, unscheduled maintenance due to wear and RCF often lead to poor availability of railway networks. For this study, dry sliding wear was investigated on wheel and rail steels using RTEC Multi-Function Tribometer. The results demonstrated that the rig was successful in simulating sliding wear, and that the fractions of the wear components could be varied, and it also provided instrumentation. Information on coefficient of friction against sliding distance and applied force were obtained which were used to compare sliding wear performance of both wheel and rail steels. The wheel was found to perform better than the rail under the same conditions due to its high initial hardness values and smaller interlamellar spacing.Mintekhttp://www.satnt.ac.za/index.php/satntam2022Materials Science and Metallurgical Engineerin

Re-austenitisation of thin ferrite films in C-Mn steels during thermal rebound at continuously cast slab corner surfaces

DATA AVAILABILITY STATEMENT : Data presented in this article is available at request from the corresponding author.The influence of primary cooling and rebound temperature at C–Mn slab corner surfaces during continuous casting on ferrite film transformation and AlN precipitation was investigated. Laboratory simulations included primary cooling to minimum temperature, Tmin, rebounding to various maximum temperatures, Tmax, followed by secondary cooling. The negative effect of a low Tmin on hot ductility could not be readily reversed, even at relatively high temperatures. Quantitative metallography was employed to study the evolution of the microstructure during rebounding and secondary cooling. Following primary cooling to temperatures just above the Ar3, thin films of allotriomorphic ferrite formed on the austenite grain boundaries. These films did not completely transform to austenite during the rebound at 3 C/s up to temperatures as high as 1130 C and persisted during slow secondary cooling up to the simulated straightening operation. Whilst dilatometry did not indicate the presence of ferrite after high rebound temperatures, metallography provided clear evidence of its existence, albeit in very small quantities. Coincident with the ferrite at these high temperatures was the predicted (TC-PRISMA) grain boundary precipitation of AlN in bcc iron during the rebound from a Tmin of 730 C. Importantly no thin ferrite films were observed, and AlN precipitation was not predicted to occur when Tmin was restricted to 830 C. Cooling below this temperature promotes austenite grain boundary ferrite films and AlN precipitation, which both increase the risk of corner cracking in C–Mn steels.https://www.mdpi.com/journal/metalsam2023Materials Science and Metallurgical Engineerin

Study of one-step and two-step quench and partition heat treatments on a medium carbon high silicon alloy using dilatometry

This study evaluated the microstructural evolution in a medium carbon high silicon steel during one-step, and two-step quench and partition (Q&P) processes using dilatometry experiments. The two-step Q&P process was carried out using different quench temperatures ranging from 180 to 260 oC. In the one-step process, Q&P heat treatment samples were held isothermally for ten minutes after quenching at specified temperatures ranging between 200 and 450oC. The two-step Q&P process yielded a higher fraction of retained austenite than a one-step Q&P process. During the isothermal hold step, the volume expansion due to carbon partitioning and austenite decomposition behavior was interpreted by experimentally determined strain values. For the one-step Q&P process, the austenite decomposition kinetics above and below the Ms temperature differed, as evidenced by the JMAK parameters. The TTT diagram generated for the one-step Q & P process showed a “swing back” at a temperature of around 355oC.Ferrous Metal Development Network (FMDN).http://www.satnt.ac.za/index.php/satntam2022Materials Science and Metallurgical Engineerin

The role of hydrogen in the corrosion-induced reduction of plane-stress fracture toughness and strain-induced intergranular cracking of AA2024

Paper presented at International Conference on Structural Integrity 2023 (ICSI 2023).The role of diffusible hydrogen in the embrittlement of AA2024 after short-term exposure in the standard exfoliation corrosion (EXCO) test solution (ASTM G34) was investigated. Slow strain rate KR-curves were established – utilizing the unloading compliance method of the ASTM E561 Standard – on 3.2 mm thick compact tension (C(T)) specimens for the following four (4) sets of samples: (i) as-received (unexposed), (ii) 2 h EXCO exposed, (iii) unexposed and heat-treated, and (iv) 2 h EXCO exposed and heat-treated samples of AA2024. A significant degradation (≈ 12.0 ± 1.8 %) was observed in the effective slow strain rate KC toughness after short-term exposure of the AA2024-T3 specimens to the EXCO-solution. Post-exposure heat-treatments appear to have restored the plane-stress fracture toughness to its original values. The formation of secondary and primary intergranular cracks in the plastic zone of the C(T) samples were studied using SEM. The presence of intergranular secondary surface cracks in the plastic zones of the C(T) samples was, however, not altered by the heat treatment, and did not appear to influence the fracture toughness results. Thermal desorption mass spectroscopy was exploited to evaluate the extent of hydrogen absorption due to the corrosive exposure, and the effect of the subsequent heat treatment in removing it.The Light Metals Development Network (LMDN) forming part of DSI.https://www.sciencedirect.com/journal/procedia-structural-integrityhj2024Materials Science and Metallurgical EngineeringSDG-09: Industry, innovation and infrastructur

Effects of slip ratio on wear performance of class B wheel steels against softer R260 rail steels using the twin disc setup

A train experiences different slip ratios at the wheel/rail contact point as it moves along the rail track, which influences the rolling contact fatigue (RCF) and wear properties of wheel and rail materials. This variation in slip ratios is caused by a change in contact area between the wheel and rail head at curves, as the slip ratio increases compared with when a train is moving on a straight track. When the train is moving on a straight track, the wheel is found to be in contact with the rail head; but that changes when moving around curves, as the wheel flange will now be in contact with the gauge corner of the rail, affecting the severity of wear. Therefore, more research needs to be done to understand the role that slip ratio plays in the wear performance of wheel and rail materials in order to be able to develop models or systems that could be used to predict preventive maintenance. The aim of this work was to investigate the effect of the slip ratio on the wear performance of class B wheels against softer R260 rail steels under rolling and sliding conditions, using a twin-disc setup developed at the University of Pretoria. The results showed that the severity of wear was heavily dependent on the slip ratio – i.e., it increased with the slip ratio, with class B wheels performing better than the softer R260 rail.’n Trein ervaar verskillende glipverhoudings by die wiel/spoorkontakpunt soos dit langs die spoorlyn beweeg, wat die rolkontakmoegheid (RCF) en slytasie-eienskappe van wiel- en spoormateriaal beïnvloed. Hierdie variasie in glipverhoudings word veroorsaak deur 'n verandering in kontakarea tussen die wiel en spoorkop by kurwes, aangesien die glipverhouding toeneem in vergelyking met wanneer 'n trein op 'n reguit spoor beweeg. Wanneer die trein op 'n reguit spoor beweeg, word gevind dat die wiel in kontak is met die spoorkop; maar dit verander wanneer om die trein om kurwes beweeg, aangesien die wielflens nou in kontak sal wees met die spoorhoek van die spoorstaaf, wat die hoeveelheid slytasie beïnvloed. Daarom moet meer navorsing gedoen word om die rol wat glipverhouding speel in die slytasieprestasie van wiel- en spoormateriaal te verstaan, ten einde modelle of stelsels te kan ontwikkel wat gebruik kan word vir die voorspelling van voorkomende instandhouding. Die doel van hierdie werk was om die effek van die glyverhouding op die slytasieprestasie van klas B-wiele teen sagter R260 spoorstaal onder rol- en glytoestande te ondersoek, met behulp van 'n tweeskyf-opstelling wat by die Universiteit van Pretoria ontwikkel is. Die resultate het getoon dat die erns van slytasie baie afhanklik was van die glipverhouding - dit wil sê, dit het toegeneem met die glipverhouding, met klas B-wiele wat beter presteer as die sagter R260-reling.Presented at the 23rd Annual International Conference of the Rapid Product Development Association of South Africa (RAPDASA) Institute for Industrial Engineering, held from 9 to 11 November 2022 in Somerset West, South AfricaMintek.http://sajie.journals.ac.zaam2023Materials Science and Metallurgical Engineerin

Effects of lubrication on wear and rolling contact fatigue behavior of class B wheel steels against R350HT rail steels using a twin-disc wear simulator

Wear and rolling contact fatigue (RCF) are major causes of delays and unavailability of rail systems. The presence of lubricants at the rail and wheel interface influences wear and RCF. Lubricants include naturally occurring types, such as water from rain and leaves from trees next to rail lines, and materials applied on purpose to help improve adhesion and friction, such as friction modifiers, greases, and traction gels. The aim of this work was to study the wear behavior of AAR class B wheel versus R350HT rail materials in the presence of water and oil in comparison with the dry condition. There is currently a lack of knowledge regarding the combination of these materials in a twin-disc simulator, and this work provides information on their impact on RCF and wear performance for use by the rail industries. It was found that wear was much lower when water or oil was introduced at the wheel–rail interface compared to dry conditions, for all slip ratios. When water was used, the main cause of RCF was found to be fluid crack pressurization. The RCF cracks were also observed under dry contact.Mintek South Africa.http://www.tandfonline.com/loi/utrb202024-06-08hj2024Materials Science and Metallurgical EngineeringSDG-09: Industry, innovation and infrastructur

Carbide-free bainitic steels for rail wheel applications

In South Africa, forged wheels for rail cars are imported but cast wheels are manufactured locally, although recent developments indicate that forged wheels will in future be manufactured locally. The forged wheels are generally manufactured from AAR Class C steel, which is a high carbon, pearlitic steel used for rail application. Railway wheels are a costly component of the railway wagon as they experience wear and damage during application. Improvements in the mechanical properties are thus desired. Against this background, a project is reported on the development of durable rail steel alloys for railway wheel applications. Carbide-free bainite is a novel microstructure comprising bainitic ferrite and retained austenite/martensite but without coarse carbides in the interlath positions. The absence of carbides is achieved through the addition of a high silicon (~2wt%) content to the steel. This carbide-free bainite can achieve high tensile strength (>1000MPa) and toughness (40J, 20°C) as well as good wear resistance. These alloys have found application in areas where high strength, toughness and wear resistance are required, such as in rail steels, and have been deemed the “next generation” of rail steels. Carbide-free experimental alloys were produced in the laboratory and tested for mechanical properties such as hardness, tensile strength and impact toughness. The properties of the laboratory alloys were compared to those of standard Class C alloys used in South Africa to determine their potential use as railway wheel alloys.We would like to thank the following people and organisations, without which this study would not have been possible: the University of Pretoria for providing technical support and academic supervision, Mintek, for technical and financial support, The Department of Science and Technology (DST), Advanced Materials Initiative (AMI)-Ferrous Metals Development Network (FMDN) for their financial support, the Nelson Mandela University, Department of Physics and Centre for HRTEM for transmission kikuchi diffraction (TKD) analysis and the Nuclear Energy Council for South Africa (NECSA) for their neutron diffraction analysis.Mintek, the Department of Science and Technology (DST) and Advanced Materials Initiative (AMI)-Ferrous Metals Development Network (FMDN).http://iopscience.iop.org/journal/1757-899Xam2020Materials Science and Metallurgical Engineerin

Effect of intercritical annealing of normalised Nb-Ti-V microalloyed plate steel on microstructural evolution

A homogeneous microstructure is required for consistent mechanical properties in normalised Nb-Ti-V microalloyed plate steels. Frequently, as-hot rolled microalloyed plate steels have a banded microstructure that is persistent even after normalising heat treatment (NHT), and this leads to inconsistencies and some scatter in mechanical properties. Therefore, this work focused on the influence of single-cycle normalising heat treatment (SNHT), doublecycle normalising heat treatment (DNHT) followed by intercritical annealing normalising heat treatment (INHT) on the homogenisation and mitigation of a banded microstructure. The study was conducted on a 0.13C-Nb-Ti-V plate steel grade. The as-hot rolled microstructure was banded and had a 1.13 Anisotropy Index (AI) value. Results from the three thermal cycles revealed that the DNHT and INHT mitigated the pearlite microstructural banding and gave a more homogenized pearlite phase distribution throughout the microstructure, unlike the SNHT that retained the banding. The DNHT also exhibited the finest ferrite grain size, while the INHT exhibited the coarsest. From Vickers hardness measurements (153±5.8 HV, 157±3.6 HV and 166±4.5 HV), the UTS was approximately deduced as, 480, 490 and 530 MPa for the SNHT, DNHT and INHT respectively.http://iopscience.iop.org/journal/1757-899Xam2020Materials Science and Metallurgical Engineerin