Metallurgical investigations were directed to probe into the genesis of "frizzle'' and fissure-type surface defects on 10-mm hot-rolled steel plates meant for application in flat-bed wagons for carrying heavy machinery. The thin hairline fissures on the steel plates were identified as skin laminations associated with long, shallow, and branched cracks, intruding from the plate surfaces to the interior with curved contours, replete with fragmented oxide scale entrapments and debris. The incidence of these superficial defects on the plates was linked to surface damage to the solidifying skin of continuously cast steel slabs, induced by the extensive pitting and cavitation of caster pinch rolls. It is presumed that during hot rolling of the steel slabs, surface blemishes like indentations and folds got rolled over by metal flow, entrapping copious amounts of primary as well as secondary scales underneath the laminated skin. Under the influence of shear forces during rolling, the defects are believed to have ingressed further into the plate interior leading to the formation of long, shallow, and branched cracks with curved contours and entrapped fragmented oxide scales.

A locked coil track rope (LCTR) is essentially composed of wires (round and rail-shaped) laid helically in different layers. These wire ropes are sometimes used in conveyors carrying empty and loaded buckets in mining areas. During service, such wire ropes may fail prematurely due to disintegration/failure of individual groups of wires. To understand the genesis of LCTR wire failures, a detailed metallurgical investigation of failed rope wires was made and included visual examination, optical microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Two types of failed wires were investigated; one is from a 40 mm diameter locked coil track rope and the other from a 53 mm locked coil track rope. Optical microscopy of failed round wires in the 53 mm diameter rope clearly revealed fully decarburized layers at the surface and a few grain-boundary cracks. From the location of the failure, it was clear that apart from static tensile loads, the wire ropes had been subjected to bending and unbending loads near the saddle, as fully loaded or empty buckets traveled access the conveyor. The SEM studies confirmed that the fracture had been caused by initiation of fatigue cracks in the decarburized zone under conditions of repeated bending and unbending stresses superimposed on the static tensile load.

Wire rods of high-carbon steel, in sizes ranging between 5.5 and 14 mm, are normally produced from continuously cast billets by hot rolling in a wire rod mill. These wire rods are usually supplied to wire drawing plants in either the hot rolled or the controlled-cooled condition. The microstructure of the hot rolled wire rods is a coarse lamellar pearlite and is unsuitable for large reductions by cold drawing. In contrast, the microstructure of controlled-cooled wire rods is a relatively fine pearlite, developed as a consequence of in-line water and forced-air cooling, and is suitable for large reductions by cold drawing. Although wire rod breakages in modern-day mills are comparatively rare, they nonetheless may take place due to a variety of factors. The failure of wire rods, hot rolled or controlled cooled, may occur as a result of improper rolling schedule, cobbles, sudden mill stoppages and/or accelerations, and processing inadequacies that lead to the formation of inappropriate microstructures. A comprehensive metallurgical investigation may therefore be necessary to discover the genesis of wire rod breakages during rolling and/or finish cooling operations.This paper focuses on the microstructural causes of breakage of controlled-cooled high-carbon steel wire rods during hot rolling and attributes most failures to the formation of hard martensite layers that facilitated crack generation.

A comprehensive metallurgical investigation was carried out on samples of prematurely failed cold-strip mill work-rolls used in an integrated steel plant to study the influence of microstructural characteristics on failure propensity and roll life. The samples pertained to 3 wt.% Cr-base forged steel work-rolls, which exhibited variations in roll life despite operation under similar mill environments. Optical and SEM revealed that while a uniform dispersion of fine globular carbides was conducive to higher roll life, carbides of angular and/or elongated morphologies acted as stress-raisers, induced microcracking of the tempered martensite matrix, and accentuated eventual spalling. From the standpoint of nonmetallic inclusions, higher life rolls were cleaner. Low/poor life rolls showed higher incidences of MnS and angular TiN inclusions, which often provided sites for the precipitation of undesirable elongated carbides. Although microprobe analysis indicated that carbides in these rolls were essentially M7C3, quantitative metallography revealed that, apart from morphology, roll performance was profoundly influenced by carbide content and count. Interestingly, while higher life rolls were characterized by carbide contents of > 4.5 vol.% and counts of > 200,000 number/mm(2), rolls exhibiting low and poor lives showed significantly lower values of these features. X-ray diffractometry of spalled roll specimens indicated that while higher life rolls contained minimal retained austenite, rolls exhibiting lower lives inevitably contained > 10% retained austenite. The deleterious effect of excessive retained austenite on the spalling susceptibility of cold-strip mill work-rolls was attributed to its possible transformation to martensite under imposed rolling stresses.

Macroscopic investigations of spalled 3% Cr-variety forged steel work-rolls used in the tandem mill of an integrated steel plant showed steel strip welding on roll surfaces. Microstructural observations of roll samples at regions away from strip-welded zones showed the desired uniform dispersion of fine globular carbides in tempered martensite. Quantitative image analysis of all investigated rolls also showed desirable carbide characteristics, with >4 vol.% carbides and >200,000 individual carbides/mm2. The carbide sizes ranged from 0.69 to 0.83 µm. In contrast, optical and scanning electron microscopy (SEM) observations of the strip-welded regions showed microstructural degeneration thought to have occurred from surface and/or subsurface damage caused by localized thermal shock and intense pressure. This possibly resulted in the formation of a rehardened and heavily retempered zone at the strip-welded region. Cracks originated in the heavily retempered zone due to residual tensile stresses and propagated under the applied rolling stresses to produce spalling.

Continuously processed hot dip galvanized steel sheets that exhibited bare spots, exfoliation/peel-off, and matte coating appearance were investigated to gain an insight into the genesis of such defects. Scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM/EDS) and electron probe microanalysis (EPMA) were employed in conjunction with analysis of the galvanizing bath. Analyses revealed a bath condition sensitive to surface cleanliness of the steel substrate and showed significant levels of carbonaceous residues and rolling debris on the annealed steel substrates. This improper steel surface condition has been attributed to excess oil carry-over on the cold rolled strip surface and poor burnoff in the annealing furnace.

The microstructural features of prematurely spalled tandem mill work-rolls were examined in an attempt to correlate microstructure with spalling behavior and roll performance. Spalled samples were collected from work-rolls that had shown variations in roll life under similar conditions of mill usage. Optical microscopy revealed that a fine dispersion of spheroidal carbides in a matrix of tempered martensite was conducive to superior performance in terms of roll life (i.e., tonnage rolled), and that coarse angular and irregular shape carbides were detrimental to roll life.

A metallurgical investigation of failed samples of hot-strip mill work-rolls used in an integrated steel plant was made to determine the influence of microstructural characteristics on failure susceptibility and roll life. The samples investigated pertained to prematurely failed indefinite chill double-poured (ICDP) iron work-rolls, which exhibited varying roll lives under similar mill operating environments. Although microstructures of all the investigated rolls showed similar graphite morphologies irrespective of their mill performance, discernible differences in carbide characteristics could be observed between high and low life rolls. Microstructural observation of nital-etched roll specimens revealed that lower life rolls were characterized by carbide microcracking. The propensity for cracking was particularly high in carbides exhibiting microhardness greater than 1020 VPN. Electron-probe microanalysis (EPMA) indicated that carbides in the spalled rolls were mostly of M 3 C type, where M was Fe and Cr. Quantitative image analysis of phases in the investigated rolls revealed that while graphite volume fraction in the range of 4.0 to 6.4% did not significantly affect roll life, carbide content higher than 28.5 vol% was found detrimental. In fact, a carbide content in the range of 24.0 to 28.50 vol% was found to be desirable for higher roll life. The study thus revealed that although carbides are indispensable for high hardness, resistance to wear, and thermal cracking, an excessive volume fraction (>30 vol%) of high hardness (microhardness > 1020 VPN) carbides accentuated microcracking, which ultimately induced premature spalling of hot-strip mill work-rolls.

Locked coil wire ropes, by virtue of their unique design and construction, have specialized applications in aerial ropeways, mine hoist installations, suspension bridge cables, and so forth. In such specialty ropes, the outer layer is constructed of Z-profile wires that provide not only effective interlocking but also a continuous working surface for withstanding in-service wear. The compact construction and fill-factor of locked coil wire ropes make them relatively impervious to the ingress of moisture and render them less vulnerable to corrosion. However, such ropes are comparatively more rigid than conventional wire ropes with fiber cores and therefore are more susceptible to the adverse effects of bending stresses. The reasons for premature in-service wire rope failures are rather complex but frequently may be attributed to inappropriate wire quality and/or abusive operating environment. In either case, a systematic investigation to diagnose precisely the genesis of failure is desirable. This article provides a microstructural insight into the causes of wire breakages on the outer layer of a 40 mm diam locked coil wire rope during service. The study reveals that the breakages of Z-profile wires on the outer rope layer were abrasion induced and accentuated by arrays of fine transverse cracks that developed on a surface martensite layer.

An extensive metallurgical investigation was carried out on samples of a failed roller bearing from the support and tilting system of a basic oxygen furnace (BOF) converter used in the steel melting shop of an integrated steel plant. The converter bearing was fabricated from low-carbon, carburizing grade steel and had failed in service within a year of fitting to a repaired shaft. Microscopic observations of both the broken roller and inner-race samples revealed subsurface cracking and preponderance of brittle oxide and other macroinclusions. Electron probe microanalysis (EPMA) studies confirmed that the brittle oxides that formed stringers were alumina, and the other macroinclusions were complex silicates. Both the alumina and silicate inclusions were deleterious to contact-fatigue properties. Microstructurally, the carburized regions of the broken roller and of inner-race samples contained high-carbon tempered martensite. Microhardness measurements revealed that although the core hardness of the roller and the inner-race samples were similar, the surface hardness of the roller was approximately 8.5 HRC units harder than that of the inner-race. Scanning electron microscope (SEM) observations of the roller fracture surface revealed striations indicative of fatigue, and energy-dispersive spectrometric (EDS) analyses corroborated a high incidence of silicate inclusions at crack sites. The study suggests that the failure of the bearing occurred because the hardness difference between the roller bearing and the inner-race surfaces resulted in wear of the inner-race. The wear led to shaft misalignment and play during service. The misalignment, coupled with the presence of inclusions, caused fatigue failure of the roller bearing.

Work rolls made of indefinite chill double-poured (ICDP) iron are commonly used in the finishing trains of hot-strip mills (HSMs). In actual service, spalling, apart from other surface degeneration modes, constitutes a major mechanism of premature roll failures. Although spalling can be a culmination of roll material quality and/or mill abuse, the microstructure of a broken roll can often unveil intrinsic inadequacies in roll material quality that possibly accentuate failure. This is particularly relevant in circumstances when rolls, despite operation under similar mill environment, exhibit variations in roll life. The paper provides an insight into the microstructural characteristics of spalled ICDP HSM work rolls, which underwent failure under similar mill operating environment in an integrated steel plant under the Steel Authority of India Limited. Microstructural features influencing ICDP roll quality, viz. characteristics of graphite, carbides, martensite, etc., have been extensively studied through optical microscopy, quantitative image analysis (QIA), and electron-probe microanalysis (EPMA). These are discussed in the context of spalling propensity and roll life.

An extensive investigation has been carried out on six commercial heats of pearlitic rail steel to study the influence of nonmetallic inclusion characteristics on the tensile, fatigue, and fracture toughness properties. The steels investigated were made through the basic oxygen furnace (BOF)-continuous casting route and rolled in the rail and structural mill into 90 kg/mm 2 ultimate tensile strength (UTS) grade rails. While tensile properties (yield strength [YS], UTS, and elongation) of the rail steels investigated were found to be insensitive to inclusion type and volume fraction at their present level (0.23 to 0.45%), the fracture toughness and high-cycle fatigue properties were found to be inclusion sensitive. The fracture toughness values of the steels were found to range between 42.33 and 49.88 MPa √m; higher values, in general, were obtained in heats exhibiting lower volume fractions (0.15 to 0.19%) of sulfide inclusions. The high-cycle fatigue limit, i.e. , stress corresponding to 10 7 cycles, was found to be higher in cleaner steels, particularly in those with lower volume fractions of oxide inclusions. This phenomenon was corroborated by scanning electron microscopy (SEM) observations of fracture surfaces, where oxide inclusions in particular were found to be instrumental in crack initiation. Although fatigue life did not show any direct correlation with the volume fraction of sulfides, elongated MnS inclusions were sometimes observed at crack initiation sites of fatigue-tested specimens.

Electron-probe microanalysis was employed concurrently with optical metallography to understand the genesis of surface and internal defects which are critical to steel product quality. Investigations of surface defects such as ‘slivers’ on hot-rolled sheets, ‘edge-cracking’ of hot-rolled coils and longitudinal surface cracking of rolled round bars revealed that such defects in finished products are often inherited from upstream processing stages and eventually accentuated in the final stages of processing. Inadequately deoxidized steel with a high gas content, improper casting practices, usage of inferior quality mould powder and bad reheating practice in respect of temperature and furnace atmosphere were found to impair synergistically the quality of finish products. Microprobe analysis of internal defects such as cracks in billets and axial discontinuities/cracks in thick steel plates produced from continuously cast slabs elucidated the role of specific macrosegregation-causing elements during continuous casting and their ultimate effect in inducing transformation to undesirable microstructures which enhance cracking propensity. Some of these metallurgical applications where microprobe analysis, both qualitative and quantitative, has been used for the evaluation of steel product quality are described. Copyright © 1999 John Wiley & Sons, Ltd.

Abstracts are not published in this journal

The interpretational advantages of colored microstructures over conventional black-and-white images is recognized in view of the natural response of the human eye to color variations. This applies well for steels, where ambiguities in phase and feature discrimination can often arise from intrinsic lack of contrast or otherwise subtle grey-level differences of the observed microstructural constituents. Although staining techniques are advantageous from this standpoint, their use, even today, is rather limited in routine metallography. Textbook instructions and standard reagent formulations alone cannot guarantee optimum results unless sample preparation is meticulous and the etching technique is perfected. Moreover, the etch response of different steel chemistries being unique to each grade demands experimentation as a prerequisite for obtaining optimum results. This article provides a pictorial insight into the fascinating world of microstructures obtained in a gamut of plaincarbon, dual-phase, low-alloy, stainless, and high-alloy tool steels investigated in our laboratory. Issues concerning the revelation of anodic matrix and second phases and crystallographic orientation aspects of microstructural features, such as grains, twins, and colonies, are also elucidated and discussed.