32 research outputs found
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 ΔUTS 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 (dÏ/dΔ)
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 αâČ-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 CâMn 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 CâMn 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 âswing backâ 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 â utilizing the unloading
compliance method of the ASTM E561 Standard â 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 (â 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 â i.e., it increased with the slip ratio,
with class B wheels performing better than the softer R260 rail.ân 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 beĂŻnvloed. 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 beĂŻnvloed. 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 sĂȘ, 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 wheelârail 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 ânext
generationâ 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