The transformation behaviour and hot strength of 3CR12 during the continuous casting process

Extensive research has been done over the years and has contributed quite a lot to the development of 3CR12 stainless steel. Nevertheless, there is still much to be understood about the behaviour of this steel during its production. One of the problems that are occasionally encountered is the side bulging effect i.e. the unconstrained narrow faces of the strand plastically bulge due to ferrostatic pressure from the liquid core of the strand at high temperatures. In general, this problem is prevalent in ferritic stainless steels as they exhibit a weaker hot strength than austenitic stainless steels. Coupled with side bulging, there is also strand width variation at high temperatures i.e. when the steel is in the ferrite-austenite dual phase region. Both of these dimensional changes to the slab profile create some processing problems in subsequent hot rolling operations when unacceptable width variations are encountered. This work was, therefore, motivated by the requirement to quantify the role that the metallurgical behaviour of this steel plays with regard to the above width variation problem. The research work involved studies of the d-ferrite to austenite phase change during continuous cooling (simulating cooling during continuous casting) and the establishment of CCT diagrams, the influence of chemical composition on the austenite start temperature and the hot ductility and hot strength visà-vis the side bulging effect. The casting conditions in the mould are crucial to ensure that the solidification shell is thick enough to withstand the ferrostatic pressure exerted on the unconstrained narrow sides of the strand as it exits from the mould. Therefore, part of the solution lies in the study of the optimisation of the cooling rate, mould flux properties, casting speed, mould taper angle and the chemical composition of this steel, among other factors. Additions of austenite formers, within the specification range of 3CR12, should be favourable for a stronger solidification shell since austenite exhibits superior hot strength to d-ferrite. The good hot ductility observed in 3CR12 may also provide a leeway for increasing the secondary cooling rate in order to form a thicker solidification shell soon after emerging from the mould. This may reduce the side bulging effect. This can be achieved without a risk of transverse cracking if the cooling is adjusted carefully. As long as the d-ferrite to austenite phase ratio keeps fluctuating due to variations in (i) the chemical composition within the specification range and (ii) the cooling rate in the dual phase region from cast to cast, the strand width variation effect will persist. This is simply because of the effect these have on the ferrite to austenite phase ratios through the differences between the lattice structures of these two phases. Ferrite being less dense than austenite, occupies more volume than austenite, thereby affecting the slab width. The remedy to this problem is to control within stricter chemical composition limits in order to reduce or completely avert this width variation effect.Dissertation (MSc (Metallurgy))--University of Pretoria, 2006.Materials Science and Metallurgical Engineeringunrestricte

The effect of soaking and tempering temperatures on high vanadium alloys for grinding media

Vanadium produces the hardest carbides and can influence wear properties positively. However, due to the minerals’ price fluctuations as a result of the current economic climate, vanadium is currently expensive and seldom used in wear resistant materials despite its valuable properties. Nevertheless, it is still important to study the effects of vanadium carbides on wear properties and later compare the cost/durability ratio to the other currently used industrial alloys. Vanadium has been used as a secondary alloying element in high chromium cast iron and has produced positive results through the influence of orientation and morphology of the eutectic carbide M7C3. Vanadium carbides are known to be discontinuous, which is helpful with regards to impact properties. Vanadium carbides alone cannot maintain high macro-hardness values and for improved results carbon is needed to increase the strength of the matrix. In this paper, the influence of the soaking and tempering temperatures on hardness and wear properties in a high vanadium alloy, 1.91C-0.82Mn-0.96Si-0.90Ni-1.35Cr- 0.25Mo-6.12V, were investigated. The soaking temperatures were 1150 and 1250°C while the tempering temperatures were varied at 100°C interval from 300 °C to 600 °C. It was found that soaking at 1250°C increased the amount of retained austenite, but at the same time also improved the wear resistance in as quenched condition as well as after tempering up to 500 °C.The Advanced Materials Division at Mintekhttp://iopscience.iop.org/journal/1757-899Xam2020Materials Science and Metallurgical Engineerin

Constitutive constants for hot working of steels : the critical strain for dynamic recrystallisation in C-Mn steels

Please read abstract in the article.The University of Pretoriahttp://link.springer.com/journal/116652016-01-30hj201

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

Using the physically based constitutive model and processing maps to understand the hot deformation behavior of 2304 lean duplex stainless steel

RESEARCH DATA : The research data for this article are available on the University of Pretoria Bahr Dilatometer machine and the SEM post-processing machine, available on request.Please read abstract in the article.Mintek.https://link.springer.com/journal/11661hj2023Materials Science and Metallurgical Engineerin

An evaluation of the thermal fatigue performance of three alloys for casting mould applications

A petrochemical company experiences premature thermal fatigue failure of the casting moulds used in catalyst production. The aim of the project was to find an alternative alloy that would outperform the current low-alloy cast steel used for the moulds. Based on their thermo-fatigue properties, 3CR12 ferritic stainless steel and H11 tool steel were chosen for testing and comparison with the currently used BS3100 B7 cast steel. Samples of each material were subjected to temperature cycling in a Gleeble 1500TM thermo-mechanical processing simulator, followed by surface analyses. The main parameters derived from the test work were the total true strain, the hot strength of the materials, and the number of cycles to failure. Additionally, the coefficient of thermal expansion for each material was measured using a Bähr dilatometer. H11 tool steel yielded the best performance by way of having the fewest surface cracks, the lowest total true strain per cycle, the most cycles to failure, the highest hot strength, and the lowest coefficient of thermal expansion.Paper written on project work carried out in partial fulfilment of B. Eng. (Metallurgical Engineering)Coaltech and NRF (National Research Foundation)http://www.saimm.co.za/journal-papersam2016Materials Science and Metallurgical Engineerin

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

Heat treatment response and characterization of Ti6Al4V + xMo produced by laser metal deposition

During the laser metal deposition additive manufacturing processing of Ti6Al4V ELI alloy, the parts produced were exposed to high levels of thermal gradients, which resulted from rapid heating and cooling rates in the material. This had an adverse effect on the material properties, as tensile residual stresses were created in the parts and increased the strength while significantly reducing ductility. Additionally, the presence of columnar grains compromised the material properties because it resulted in inhomogeneous microstructures that exhibit anisotropy in parts. This study investigated the influence of β annealing temperatures on the microstructure of Ti6Al4V ELI alloy produced during laser metal deposition, and the Ti6Al4V ELI in-situ alloyed with varying molybdenum content. The observations made included a temperature driven phase transformation, which resulted in a change from columnar to equiaxed grains due to heat treatment of the Ti6Al4V ELI alloy, while the solidification structure of the alloy changed from planar to cellular due to the addition of Mo. The Ti6Al4V ELI alloy heat treated at 1000 °C reported a hardness profile of 204 ± 5 HV0.3, which was comparable to the reported hardness (206 ± 34 HV0.3) of the Ti6Al4V ELI in-situ alloyed with 10 mass percent Mo (10% Mo). This implies that the effects of the in-situ alloying of Ti6Al4V ELI with 10% Mo are comparable to the heat treatment of Ti6Al4V ELI alloy at a β annealing temperature of 1000 °C, in terms of stabilization of the β-phase.The Department of Science and Innovation (DSI) and the Council for Scientific and Industrial Research-Young Researcher Empowerment Fund (CSIR-YREF).http://www.elsevier.com/locate/matprhj2022Materials Science and Metallurgical Engineerin

The influence of niobium content on austenite grain growth in microalloyed steels

The relationship between niobium content and austenite grain growth has been investigated through hot rolling simulation on a Bähr dilatometer. The effect of delay time between passes during rough rolling in Nbmicroalloyed steels with nitrogen contents typical for electric arc furnace (EAF) melting was studied. The results indicate that the grain growth constants n, Q, and A increase with an increase in Nb content. The activation energy for austenite grain growth Q was found to be in the range of 239 to 572 kJ/mol, the exponential constant n ranged from 2.8 to 6.2, and the material and processing condition constant A from 4.24 × 1012 to 4.96 × 1028, for steels with niobium contents ranging from 0.002% Nb to 0.1% Nb. A general constitutive equation for the prediction of austenite grain growth in these Nb-microalloyed steels under rough rolling conditions has been developed. Good agreement between the experimental and the predicted values was achieved with this constitutive equation.http://www.saimm.co.za/journal-papersam2016Materials Science and Metallurgical Engineerin

The influence of thermomechanical processing on the surface quality of an AISI 436 ferritic stainless steel

The need to reduce weight while maintaining good mechanical properties in materials used in the automotive industry has over the years seen an increased exploitation of various steels to meet this new demand. In line with this development, the ferritic stainless steel family has seen a wide application in this industry, with the AISI 436 type increasingly being used for automotive trims and mufflers for exhaust systems, as well as a significant part of this steel’s application being for the manufacture of wheel nuts and wheel nut caps in trucks, mainly through the deep drawing process. However, there have been reports of some poor surface roughening of this material during deep drawing, with tearing and/or cracking also reported in some instances. This has been suspected to possibly be associated with some local differences in localized mechanical properties between grains and grain clusters of the rolled and annealed material. In order to investigate the poor surface roughness exhibited by AISI 436 ferritic stainless steel (FSS) during deep-drawing, Lankford values (Rmean and Δr), grain size, and microtextures of various sheet samples from this steel were studied. The chemical composition range for the samples was 0.013–0.017% C, 17–17.4% Cr, 0.9–1% Mo, and 0.4–0.5% Nb. The steels were subjected to various hot and cold rolling processing routes i.e. involving industrial direct rolling (DR) or intermediate annealing rolling (IR), and the drawability and final surface qualities of the steels were compared. It was found that the DR route gave an average R-mean and Δr value of 1.9 and -1.4 respectively, while the IR route yielded an average Rmean and Δr value of 1.6 and 0.52 respectively. The high Δr value for the DR processing route had a substantial adverse effect on the drawability. IR samples exhibited a smoother surface finish on visual inspection, while clear flow lines were visible on the DR samples, despite the fact that DR is the preferred industrial processing route due to the reduced production costs it offers. This observation was also confirmed through SEM examinations. The difference in the surface quality was attributed to microtexture. However, the mechanism responsible for this difference still needs to be identified.The authors gratefully acknowledge the financial contribution provided by the Advanced Metal Initiative (AMI) of the Department of Science and Technology (DST) through the Ferrous Metals Development Network (FMDN), and Columbus Stainless (Middelburg, South Africa).http://www.saimm.co.za/journal-papersam2016Materials Science and Metallurgical Engineerin