Nucleation sites for ultrafine ferrite produced by deformation of austenite during single-pass strip rolling

An austenitic Ni-30 wt pct Fe alloy, with a stacking-fault energy and deformation characteristics similar to those of austenitic low-carbon steel at elevated temperatures, has been used to examine the defect substructure within austenite deformed by single-pass strip rolling and to identify those features most likely to provide sites for intragranular nucleation of ultrafine ferrite in steels. Samples of this alloy and a 0.095 wt pct C-1.58Mn-0.22Si-0.27Mo steel have been hot rolled and cooled under similar conditions, and the resulting microstructures were compared using transmission electron microscopy (TEM), electron diffraction, and X-ray diffraction. Following a single rolling pass of ∼40 pct reduction of a 2mm strip at 800 °C, three microstructural zones were identified throughout its thickness. The surface zone (of 0.1 to 0.4 mm in depth) within the steel comprised a uniform microstructure of ultrafine ferrite, while the equivalent zone of a Ni-30Fe alloy contained a network of dislocation cells, with an average diameter of 0.5 to 1.0 µm. The scale and distribution and, thus, nucleation density of the ferrite grains formed in the steel were consistent with the formation of individual ferrite nuclei on cell boundaries within the austenite. In the transition zone, 0.3 to 0.5 mm below the surface of the steel strip, discrete polygonal ferrite grains were observed to form in parallel, and closely spaced “rafts” traversing individual grains of austenite. Based on observations of the equivalent zone of the rolled Ni-30Fe alloy, the ferrite distribution could be correlated with planar defects in the form of intragranular microshear bands formed within the deformed austenite during rolling. Within the central zone of the steel strip, a bainitic microstructure, typical of that observed after conventional hot rolling of this steel, was observed following air cooling. In this region of the rolled Ni-30Fe alloy, a network of microbands was observed, typical of material deformed under plane-strain conditions.

A comparative study of microstructures and mechanical properties obtained by bar and plate rolling

Microstructures and mechanical properties of a low carbon steel were studied after plate rolling and bar rolling. Plate rolling is characterized as a monotonic compressive loading, while bar rolling is characterized as a cross-compressive loading. A four-pass plate rolling and bar rolling experiment was designed so that the material experiences the same amount of strain at each pass during rolling. The rolling experiment was performed at moderately high temperatures (450, 550 and 650 °C). The microstructures and mechanical properties of the low carbon steel acquired from the two types of rolling experiments were compared. The results revealed that differences of loading path attributed by monotonic loading (plate rolling) and cross loading (bar rolling) significantly influenced the microstructures and mechanical properties such as yield stress, ultimate tensile stress, strain hardening exponent and elongation of the low carbon steel.

Residual thermal stresses in a Fe3Al/Al2O3 gradient coating system

To combine the merits of both metals and ceramics into one material, many researchers have been studying the deposition of alumina coating using plasma spray on metal substrates. However, as the coatings are deposited at a high temperature, residual thermal stresses develop due to the mismatch of thermal expansion coefficients of the coating and substrate and these are responsible for the initiation and expansion of cracks, which induce the possible failure of the entire material. In this paper, the residual thermal-structural analysis of a Fe3Al/Al2O3 gradient coating on carbon steel substrate is performed using finite element modelling to simulate the plasma spray. The residual thermal stress fields are obtained and analyzed on the basis of temperature fields in gradient coatings during fabrication. The distribution of residual thermal stresses including radial, axial and shear stresses shows stress concentration at the interface between the coatings and substrate. The mismatch between steel substrate and composite coating is still the dominant factor for the residual stresses

Low strain processing of LC and ULC steels

Temper rolling and tension levelling are commonly used to manufacture flat rolled steel. Both processes lengthen the steel at strains up to 3% by applying a load and stretching the strip. By latering the balance between the load and the tension the formability of the low carbon and ultra low carbon steel may be optimised.

Effects of temperature in relation to sheet metal stamping

The demand to reduce the use of lubricants and increase tool life in sheet metal stamping has resulted in increased research on the sliding contact between the tool and the sheet materials. Unlubricated sliding wear tests for soft carbon steel sliding on D2 tool steel were performed using a pin-on-disk tribometer. The results revealed that temperature has an influencing role in the wear of tool steel and that material transfer between tool and sheet can be minimized at a certain temperature range in sheet metal stamping.

The formation of ultrafine ferrite and low temperature bainite through thermomechanical processing

In the current study, a novel approach was employed to produce a unique combination of ultrafine ferrite grains and low temperature bainite in a low carbon steel with a high hardenability. The thermomechanical route included warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to bainitic transformation regime (i.e. 400-250°C). The resultant microstructure was ultrafine ferrite grains (i.e. <4μm) and very fine bainite consisting of bainitic ferrite laths with high dislocation density and retained austenite films. This microstructure offers a unique combination of ultimate tensile strength and elongation due to the presence of ductile fine ferrite grains and hard low temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy techniques.

Sliding wear behavior of plasma sprayed Fe3Al–Al2O3 graded coatings

Fe3Al–Al2O3 double-layer coatings (DC), Fe3Al-Fe3Al/50%Al2O3–Al2O3 triple-layer coatings (TC) and Fe3Al-Al2O3 graded coatings (GC) were produced from a series of Fe3Al/Al2O3 composite powders with different compositions on low carbon steel substrate using PLAXAIR plasma spraying equipment. Friction behaviors and wear resistance of the three kinds of coatings have been investigated under different loads. Tests were carried out using an MRH-3 standard machine, in lineal contact sliding under dry condition against hardmetal, at a sliding velocity of about 1.57 ms−1. Wear rates under different loads were measured and the friction coefficients were recorded. SEM analysis was carried out to identify the wear mechanisms. The results show that the GC has higher wear-resistance than DC and TC. The tribological characteristics of graded coating were different along the depth of the coatings, and the surface of coatings with pure Al2O3 does not show the best wear resistance. The wear rate and friction coefficients were also different under different loads. The failure types of plasma-sprayed Fe3Al-Al2O3 graded coatings in lineal contact were: loosening of ceramic particles, crack nucleation and propagation, brittle fracture, plastic deformation, and adhesive wear.

Characterization of microstructures and tensile properties of TRIP-aided steels with different matrix microstructure

Three different heat treatment processes have been proposed as a fundamental method to produce three kinds of TRIP-aided steels with polygonal ferritic matrix (F-TRIP), bainitic matrix (B-TRIP) and martensitic matrix (M-TRIP) in a newly designed low alloy carbon steel. By means of dilatometry study and detailed characterization, the relationships among transformation, microstructure and the resulting mechanical behavior were compared and analyzed for the three cases. The work hardening of the samples was evaluated by calculating the instantaneous n value as a function of strain. The M-TRIP sample exhibits the highest strength with the highest work hardening rate at low strains and subsequent rapid descending at high strains. In contrast, the B-TRIP sample has relatively high continuously constant work hardening behavior over strain levels greater than 0.067. The difference in work hardening behavior corresponds directly to the rate of the retained austenite-martensitic transformation during straining, which can be attributed to the carbon content, the morphology of the retained austenite and the matrix microstructure in the respective TRIP-aided samples. © 2014 Elsevier B.V.

Recrystallization kinetic study of copper-bearing strip cast steels

The effects of copper on as-cast structure, recrystallization and precipitation kinetics of strip cast low carbon steel were investigated. As-cast microstructure mainly consists of polygonal ferrite and Widmanstatten ferrite. Recrystallization responses were strongly dependent on initial microstructure and Cu content. Precipitation strengthening was observed in high copper content alloys.

Comparisons of the two-body abrasive wear behaviour of four different ferrous microstructures with similar hardness levels

The abrasive wear resistance of four distinct metallurgical steel microstructures - bainite, pearlite, martensite and tempered martensite, with similar hardness levels was investigated. A pin-on-disc tribometer was used to simulate the two-body abrasive condition (i.e. the metallic surface abrading against the silicon carbide abrasive particles) and evaluate the specific wear rate of the microstructures. Each microstructure had a unique response towards the abrasion behaviour and this was largely evident in the friction curve. However, the multi-phase microstructures (i.e. bainite and pearlite) demonstrated better abrasion resistance than the single-phase microstructures (i.e. martensite and tempered martensite). Abrasion induced microstructural changes at the deformed surfaces were studied using sub-surface and topographical techniques. The properties of these layers (i.e. surface profile measurements) determined the amount of material loss for each microstructure. These were directly linked to the single-wear track analysis that highlighted a marked difference in their mode of material removal. Ploughing and wedge formation modes were dominant in the case of bainite and pearlite microstructures, whereas the cutting mode could be attributed to the higher material loss in the single-phase microstructures. The combination of brittle and ductile phases in the multi-phase microstructure matrix could be one of the driving factors for their superior abrasion resistance.

The distribution of grain boundary and interface plane orientations in transformed microstructures

The properties of interfaces depend not only on the lattice misorientation, but also on the interface plane orientation. Extensive studies of grain boundaries led to the conclusion that in systems evolving by grain growth, the relative areas of different grain boundary planes are inversely correlated to their relative energies. In other words, the low energy grain boundary planes make up a larger part of the population than the higher energy grain boundary planes. The hypothesis of this work is that the interface plane orientation distribution in transformed microstructures depends more on the mechanism of formation than on the relative energy. After a discussion of methods for measuring interface plane orientations, results will be presented for lath martensite in a low carbon steel and for martensite in a Ti-6Al-4V alloy processed in two different ways to promote a displacive transformation in one case and a diffusional transformation in the other.

The behaviour of praseodymium 4-hydroxycinnamate as an inhibitor for carbon dioxide corrosion and oxygen corrosion of steel in NaCl solutions

Praseodymium 4-hydroxycinnamate (Pr(4OHCin)3) was investigated as a novel corrosion inhibitor for steel in NaCl solutions, and found to be effective at inhibiting corrosion in both CO2-containing and naturally-aerated systems. Surface analysis results suggest that the corrosion inhibition ability of Pr(4OHCin)3 in the naturally-aerated corrosion system could be attributed to the formation of a continuous protective film. For the CO2-containing system, the corrosion inhibition efficiency of Pr(4OHCin)3 was predominantly because of formation of protective inhibiting deposits at the active electrochemical corrosion sites, in addition to a thinner surface film deposit. © 2013.

Dry sliding wear of active screen plasma carburised austenitic stainless steel

Low temperature diffusion treatments with nitrogen and carbon have been widely used to increase the tribological performance of austenitic stainless steels. These processes produce a layer of supersaturated austenite, usually called expanded austenite or S-phase, which exhibits good corrosion and wear resistance. The novel active screen technology is said to provide benefits over the conventional DC plasma technology. The improvements result from the reduction in the electric potential applied to the treated components, and the elimination of such defects and processing instabilities as edge effects, hollow cathode effects and arcing. In this study, AISI 316 coupon samples were plasma carburised in DC and active screen plasma furnaces. The respective layers of carbon expanded austenite were characterised and their tribological performance was studied and compared. Detailed post-test examinations included SEM observations of the wear tracks and of the wear debris, EDX mapping of the wear track, EBSD crystal orientation mapping of the cross sections of the wear tracks, and cross-sectional TEM. Based on the results of wear tests and post-test examinations, the wear mechanisms involved are discussed.

Frictional behaviour and wear performance of a nitrocarburised coating sliding against AISI 1019 steel

This work employed a commercial nitrocaburising process to diffuse a coating onto M2 grade high speed tool steel. Properties of the nitrocaburised coating (CN) such as thickness, roughness and hardness were characterised using a variety of techniques including Glow-Discharge Optical Emission Spectrometry (GD-OES) and Scanning Electron Microscopy (SEM). A tribological test has been developed in which two nominally identical crossed cylinders slide over each other under selected test conditions. The test has been employed to investigate the wear performance of both CN coated and uncoated M2 specimens and frictional behaviour of the sliding interface between the tool and a AISI 1019 steel workpiece under unlubricated (dry) and lubricated conditions. Fourier Transform Infrared Spectroscopy (FTIR) was used to monitor the formation of chemical species from the oxidation of lubricant during tribological testing.