Evolution of Grain-Boundary Microstructure and Texture in Interstitial-Free Steel Processed by Equal-Channel Angular Extrusion

The equal-channel angular extrusion (ECAE) of Ti-bearing interstitial-free (IF) steel was performed following two different routes, up to four passes, at a temperature of 300 degrees C. The ECAE led to a grain refinement to submicron size. After the second pass, the grain size attained saturation thereafter. The microstructural analysis indicated the presence of coincident-site lattice (CSL) boundaries in significant fraction, in addition to a high volume fraction of high-angle random boundaries and some low-angle boundaries after the deformation. Among the special boundaries, Sigma 3 and Sigma 13 were the most prominent ones and their fraction depended on the processing route followed. A deviation in the misorientation angle distribution from the Mackenzie distribution was noticed. The crystallographic texture after the first pass resembled that of simple shear, with the {112}, {110}, and {123} aligned to the macroscopic shear plane.

Influence of roughness parameters and surface texture on friction during sliding of pure lead over 080 M40 steel

In the present investigation, tests were conducted on a tribological couple made of cylindrical lead pin with spherical tip against 080 M40 steel plates of different textures with varying roughness under both dry and lubricated conditions using an inclined pin-on-plate sliding tester. Surface roughness parameters of the steel plates were measured using optical profilometer. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the surface texture of hard surfaces. A newly formulated non-dimensional hybrid roughness parameter called 'xi' (a product of number of peaks and maximum profile peak height) of the tool surface plays an important role in determining the frictional behaviour of the surfaces studied. The effect of surfaces texture on coefficient of friction was attributed to the variation of plowing component of friction, which in turn depends on the roughness parameter 'xi'.

Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate

Surface texture of harder mating surfaces plays an important role during sliding against softer materials and hence the importance of characterizing the surfaces in terms of roughness parameters. In the present investigation, basic studies were conducted using inclined pin-on-plate sliding tester to understand the surface texture effect of hard surfaces on coefficient of friction and transfer layer formation. A tribological couple made of a super purity aluminium pin against steel plate was used in the tests. Two surface parameters of steel plates, namely roughness and texture, were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture and are independent of surface roughness (R-a). Among the various surface roughness parameters, the average or the mean slope of the profile was found to explain the variations best. Under lubricated conditions, stick-slip phenomena was observed, the amplitude of which depends on the plowing component of friction. The presence of stick-slip motion under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities. (C) 2009 Elsevier B. V. All rights reserved.

Phosphorus Partition between Liquid Steel and CaO-SiO2-P2O5-MgO Slag Containing Low FeO

CaO-SiO2-FeOx-P2O5-MgO bearing slags are typical in the basic oxygen steelmaking (BOS) process. The partition ratio of phosphorus between slag and steel is an index of the phosphorus holding capacity of the slag, which determines the phosphorus content achievable in the finished steel. The influences of FeO concentration and basicity on the equilibrium phosphorus partition ratios were experimentally determined at temperatures of 1873 and 1923 K, for conditions of MgO saturation. The partition ratio initially increased with basicity but attained a constant value beyond basicity of 2.5. An increase in FeO concentration up to approximately 13 to 14 mass pet was beneficial for phosphorus partition.

Influence of tilt angle of plate on friction and transfer layer—A study of aluminium pin sliding against steel plate

In the present investigation, unidirectional grinding marks were attained on the steel plates. Then aluminium (Al) pins were slid at 0.2°, 0.6°, 1.0°, 1.4°, 1.8°, 2.2° and 2.6° tilt angles of the plate with the grinding marks perpendicular and parallel to the sliding direction under both dry and lubricated conditions using a pin-on-plate inclined sliding tester to understand the influence of tilt angle and grinding marks direction of the plate on coefficient of friction and transfer layer formation. It was observed that the transfer layer formation and the coefficient of friction depend primarily on the grinding marks direction of the harder mating surface. Stick-slip phenomenon was observed only under lubricated conditions. For the case of pins slid perpendicular to the unidirectional grinding marks stick-slip phenomenon was observed for tilt angles exceeding 0.6°, the amplitude of which increases with increasing tilt angles. However, for the case of the pins slid parallel to the unidirectional grinding marks the stick-slip phenomena was observed for angles exceeding 2.2°, the amplitude of which also increases with increasing tilt angle. The presence of stick-slip phenomena under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities.

Pressure and thermally induced stages of wear in dry sliding of a steel ball against an aluminium-silicon alloy flat

Wear of etched near-eutectic aluminium silicon alloy slid against a steel ball under ambient is explored. The sliding velocity is kept low (0.01 m/s) and the nominal contact pressure is varied in a 15-40 MPa range. Four stages of wear are identified; ultra mild wear, mild wear, severe wear and post severe oxidative wear. The first transition is controlled by the protrusions of silicon particles, projecting out of the aluminium alloy matrix. Once these protrusions disappear under pressure and sliding, oxidation and bulk energy dissipation mechanisms take over to institute transitions to other stages of wear. The phenomenological characteristics of wear stages are explored using a variety of techniques including nanoindentation, focused ion beam milling, electron microscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS) and optical interferometry. (c) 2010 Elsevier B.V. All rights reserved.

Effect of directionality of unidirectional grinding marks on friction and transfer layer formation of Mg on steel using inclined scratch test

Surface topography has been known to play an important role in the friction and transfer layer formation during sliding. In the present investigation, EN8 steel flats were ground to attain different surface roughness with unidirectional grinding marks. Pure Mg pins were scratched on these surfaces using an Inclined Scratch Tester to study the influence of directionality of surface grinding marks on coefficient of friction and transfer layer formation. Grinding angle (i.e., the angle between direction of scratch and grinding marks) was varied between 0 degrees and 90 degrees during the tests. Experiments were conducted under both dry and lubricated conditions. Scanning electron micrographs of the contact surfaces of pins and flats were used to reveal the surface features that included the morphology of the transfer layer. It was observed that the average coefficient of friction and transfer layer formation depend primarily on the directionality of the grinding marks but were independent of surface roughness on the harder mating surface. In addition, a stick-slip phenomenon was observed, the amplitude of which depended both on the directionality of grinding marks and the surface roughness of the harder mating surface. The grinding angle effect on the coefficient of friction, which consists of adhesion and plowing components, was attributed to the variation of plowing component of friction. (c) 2006 Elsevier B.V. All rights reserved.

Effect of copper addition on the fracture and fatigue crack growth behavior of solution heat-treated SUS 304H austenitic steel

Small additions of Cu to the SUS 304H, a high temperature austenitic stainless steel, enhance its high temperature strength and creep resistance. As Cu is known to cause embrittlement, the effect of Cu on room temperature mechanical properties that include fracture toughness and fatigue crack threshold of as-solutionized SUS 304H steel were investigated in this work. Experimental results show a linear reduction in yield and ultimate strengths with Cu addition of up to 5 wt.% while ductility drops markedly for 5 wt.% Cu alloy. However, the fracture toughness and the threshold stress intensity factor range for fatigue crack initiation were found to be nearly invariant with Cu addition. This is because the fracture in this alloy is controlled by the debonding from the matrix of chromium carbide precipitates, as evident from fractography. Cu, on the other hand, remains either in solution or as nano-precipitates and hence does not influence the fracture characteristics. It is concluded that small additions of Cu to 304H will not have adverse effects on its fracture and fatigue behavior. (C) 2010 Elsevier B.V. All rights reserved.

Effect of prestrain on fatigue crack propagation in mild steel containing non-metallic inclusions

The effect of tensile prestrain on fatigue crack propagation behaviour of commercial mild steel with significant amount of stringer inclusions has been studied. In prestrained materials the usual stable stage II crack growth region is preceded by a phase wherein a retardation in crack growth rate occurs. No such behaviour is observed in annealed material. The amount of retardation is found to increase with increase in prestrain. A mechanism for the observed retardation in crack growth rate is also presented.

Effect of Prestrain on Fracture Toughness of Rolled Mild Steel with Significant Inclusion Content

The basic method of JIc calculation using a single specimen is discussed. Dokouipil's approach for evaluating the JIc value is extended further and the effect of prestrain on rolled mild steel with significant inclusions is studied using this modified approach. Although this method does not give an accurate value of JIc, it is quite effective for a comparative study. While the fracture toughness of annealed and 7% prestrained materials are about the same, the fracture toughness of 3% prestrained material is significantly lower.

Micromechanics Of Fatigue Crack-Growth At Low Stress Intensities In A High-Strength Steel

An attempt to systematically investigate the effects of microstructural parameters in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels has been made for a high strength low alloy steel to observe in general, widely different trends in the dependence of both the total threshold stress intensity range, DELTA-K(th) and the intrinsic or effective threshold stress intensity range, DELTA-K(eff-th) on the prior austenitic grain size (PAGS). While a low strain hardening microstructure obtained by tempering at high temperatures exhibited strong dependence of DELTA-K(th) on the PAGS by virtue of strong interactions of crack tip slip with the grain boundary, a high strength, high strain hardening microstructure as a result of tempering at low temperature exhibited a weak dependence. The lack of a systematic variation of the near-threshold parameters with respect to grain size in temper embrittled structures appears to be related to the wide variations in the amount of intergranular fracture near threshold. Crack closure, to some extent provides a basis on which the increases in DELTA-K(th) at larger grain sizes can be rationalised. This study, in addition, provides a wide perspective on the relative roles of slip behaviour embrittlement and environment that result in the different trends observed in the grain size dependence of near-threshold fatigue parameters, based on which the inconsistency in the results reported in the literature can be clearly understood. Assessment of fracture modes through extensive fractography revealed that prior austenitic grain boundaries are effective barriers to cyclic crack growth compared to martensitic packet boundaries, especially at low stress intensities. Fracture morphologies comprising of low energy flat transgranular fracture can occur close to threshold depending on the combinations of strain hardening behaviour, yield strength and embrittlement effects. A detailed consideration is given to the discussion of cyclic stress strain behaviour, embrittlement and environmental effects and the implications of these phenomena on the crack growth behaviour near threshold.

Criteria for prediction of flow instabilities and microstructural manifestations during warm working of AISI 304L stainless steel

The deformation characteristics of 304L stainless steel in compression in the temperature range 20–700°C and strain rate range 0·001–100 s−1 have been studied with the aim of characterising the .flow instabilities occurring in the microstructure. At higher temperatures and strain rates the stainless steel exhibits flow localisation, whereas at temperatures below 500°C and strain rates lower than 0·1 s−1 the flow instabilities are due to dynamic strain aging. Strain induced martensite formation is responsible for the flow instabilities at room temperature and low strain rates (0·01 s−1). In view of the occurrence of these instabilities, cold working is preferable to warm working to achieve dimensional tolerance and reproducible properties in the product. Among the different criteria tested to explain the occurrence of instabilities, the continuum criterion, developed on the basis of the principles of maximum rate of entropy production and separability of the dissipation function, predicts accurately all the above instability features.