S. K. Dhua

Effect of tempering temperatures on the mechanical properties and microstructures of HSLA-100 type copper-bearing steels

Abstract Two copper-bearing high-strength low-alloy (HSLA) steels with chemistry similar to HSLA-100, were made on a laboratory scale, one in an air induction (100 kg) furnace and the other in a vacuum induction (50 kg) furnace. The ingots cast were hot-rolled to 25 mm thick plates which were subsequently austenitized and tempered at different temperatures (400–700°C) for 1 h. Evaluation of mechanical properties and microstructure of as-quenched and tempered plates revealed that substantial improvement in strength (YS-1024 and 1025 MPa; UTS-1079 and 1111 MPa for steels 1 and 2) occurred at the expense of impact toughness on tempering at 500°C owing to profuse Cu precipitation in the matrix. With increase in tempering temperature however, the notch toughness improved considerably, reaching peak values of 53 and 123 Joules (J) at −85°C for steels 1 and 2 at 650 and 700°C tempering temperatures, respectively. The partially recovered matrix and the coarsened Cu precipitates in this temperature range presumably enhanced dislocation movement and notch toughness.

Influence of nonmetallic inclusion characteristics on the mechanical properties of rail steel

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/mm2 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 107 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.

Microstructural manifestations in color: Some applications for steels

Abstract 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.

Microstructural manifestations of fractured Z-profile steel wires on the outer layer of a failed locked coil wire rope

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 diameter 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.

Electron‐probe microanalysis: some applications in the assessment of steel product quality

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

Genesis of spalling in tandem mill work-rolls: Some observations in microstructural degeneration

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.

Microstructural features of prematurely failed hot-strip mill work rolls : Some studies in spalling propensity

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.

Metallurgical investigation into the causes of premature failure of high-carbon steel wire rods during hot rolling

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.

The ancient 11th century iron pillar at Dhar, India: a microstructural insight into material characteristics

Abstracts are not published in this journal

Metallurgical Investigation of a Cracked Splice Plate Used in a Power Transmission Line Tower

Steel splice plates are used in power transmission line towers. During service, some of these plates may fail due the loading stress as well as stress generated due to inclement weather condition. One such failed splice plate, cracked in the heel region of the plate has been studied in detail to find out genesis of such failure. Metallographic study of the failed plate material near the crack and away from the crack indicated presence of white hard martensite layer near the crack. Evaluation of mechanical properties of the plate material carried out at heel and edge regions had also supported the above findings. There was enhancement of hardness, yield strength, tensile strength of the plate near heel which had indicated localized hardening of the plate in this region due to presence of martensite. A substantial deterioration of the Charpy impact toughness of the material at the heel region could also be observed particularly at subzero testing temperatures due to the same reason. The material inadequacies and operational stresses must have caused the initiation and propagation of the cracks in the heel region of the plate leading to its premature failure.