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.     

Metallurgical investigation into the failure of a chopper blade used for cutting of hot-rolled steel coils

In the present work, failure investigation of a chopper blade received from an integrated steel plant has been presented. Chopper blades are used in chopping machines for cutting trimmed edges of hot-rolled coils into pieces to convert them into scrap. These blades are manufactured from hot forged or rolled billets or flats of high carbon high chromium cold work tool steel. The investigation consists of visual examination, chemical analysis, microstructural analysis through optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and hardness measurement. The chemical analysis confirmed the steel as equivalent to D2 grade in AISI notation. Carbide volume fraction of the broken blade was in the normal range of 10–15% as commonly observed in D2 tool steel. Microstructural examination under light and scanning electron microscopy showed non-uniform distribution of large eutectic primary carbides of irregular morphology forming strings or bands in tempered martensite matrix preferentially aligned in a specific direction. The uneven carbide arrangement in the matrix made the structure highly anisotropic and susceptible to localized stress concentration. The carbides were identified mainly as M₂₃C₆ type. Cracks were observed to initiate at the edges of the blade and propagate to the interior through clustered zones of carbides. SEM study suggests that the crack initiation was associated with decohesion of carbide particles in the cluster which culminated into final fracture by the mechanism of void coalescence and subsequent crack growth.     

Metallurgical investigation into the failure of an iron ore sintering car pallet

A failure analysis on premature cracking of a sintering car pallet in an integrated steel plant is presented. A detailed study on the microstructural and mechanical properties of the failed component has been carried out. Microstructurally, the amount of pearlite in the ferritic–pearlitic matrix was found to be significantly higher (~ 65.8%) than normally expected in materials used for elevated temperature application. A thin layer of flake and vermicular graphite (degenerate graphite) was found just beneath the casting skin although interior of the matrix contained normal spheroidal graphite. In few occasions, the presence of undesirable spiky and exploded graphites was also noticed. The material exhibited lower yield stress (312 MPa), tensile strength (457 MPa) and Charpy impact energy (3.4 J) indicating poor strength and toughness of the casting. The improper graphite morphologies in the sinter car pallet acted as stress raisers and produced cracks under dynamic thermal cycling and external loads experienced during the sintering process resulting in its premature failure.     

Metallurgical investigation of a failure in 316L stainless steel orthopaedic implant

An orthopaedic implant (nail for shinbone) made of 316L stainless steel (SS) that failed prematurely was examined to determine the root cause for the fracture. Detailed scanning electron microscopy was carried out to conclusively establish the evidence(s). Based on the results of extensive fracture surface analysis as well as the background information provided on the implant, it was determined that the implant (stainless steel nail) failed by the mechanism of predominantly ductile fracture facilitated by the presence of non-metallic inclusions.     

Metallurgical investigation on fatigue failure of stainless steel chain in a continuous casting machine

Stainless steels strips (chains) are used for the connection of dam blocks in belt casting machines. Thermal cycling and repetitive stressing under complex loading conditions due to tension and bending are the most frequent function modes during production. Samples from fractured stainless steel strips used for the connection of dam blocks in a copper rod continuous casting line, were sent for failure investigation. Optical and scanning electron microscopy for structural and fractographic evaluation along with mechanical testing are used as the principal analytical techniques in the context of the present investigation. Failure analysis findings suggest strongly that the failure was caused by bending fatigue which assisted also by thermal cycling, initiated from the strip surface and followed by ductile final overload fracture. Final fracture occurred via ductile failure, when the remaining strip cross sectional area reaches a critical size, becoming unable to sustain the operating load. Review of the service history (operating conditions, e.g. process design, applied loads, thermal cycles), in combination to the examination of a potential substitution of the material to a more heat (and fatigue) resistant one are suggested as further fatigue damage preventive actions.     

Metallurgical failure analysis of a horse trailer: A criminal investigation

Following the deaths of three people involved in a vehicular collision with an ejected horse, criminal charges were brought against the horse’s owner. Failure analysis was performed on the horse’s trailer to determine its roadworthiness. The failure mode of the hinge between the door and frame became the focus of the investigation. The analysis concentrated on proving whether or not the hinge was broken prior to the collision; that the trailer was not roadworthy; and whether the trailer was capable of safely transporting two horses on a public freeway. Analysis using optical microscopy and the scanning electron microscope indicated the hinge was significantly corroded and mechanically deformed. Additionally, there was evidence of paint on an alleged fresh fracture surface. The results were presented at the trial of the defendant.     

Metallurgical investigation of the aging process on tensile fracture welded joints in pipeline steel

The effects of aging in the tensile fracture behavior of welded joints of API5L-X52 pipe steel were studied by accelerating aging at 250 °C for different periods of time. The weld metal, heat affected zone and base metal, showed an increase in yield strength while the strain-hardening exponent decreased at early stages of aging. A maximum strength and minimum hardening exponent was found at 500 h due to peak-aging. Subsequently, both properties exhibited an opposite behavior due to over-aging. Tensile fractured specimens for the three different zones exhibited ductile failure, presenting microvoid morphology associated with the coalescence of microcavities. An increase in void density and a reduction in diameter during short periods in the fractured specimens were observed. The maximum density and minimum diameter of voids were obtained at 500 h and were linked to the improvement of strength and precipitation of nanoparticles. Afterward, the fractured surfaces exhibited a reduction in density and the diameters of voids were larger, having been induced by the deterioration of strength and coarsening of particles.     

火灾下平行钢丝束温度膨胀及高温蠕变试验研究

为研究开放空间拉索的火灾响应,该文考虑火源的空间辐射理论和拉索表面的环境换热边界,结合内部空腔辐射、接触传导、间隙导热三大基本传热理论,建立开放火灾下拉索腔体传热计算方法,并通过试验结果验证数值分析模型的准确性,分析预应力拉索在不同的内部传热方式、不同的包裹环境和不同的风环境下截面的温度场和应力场的时空分布特征,以及截面的轴力和弯矩的内力时变特征。结果表明：进行拉索的火灾响应分析时,考虑拉索的完整腔体传热模型可以比较准确地计算开放火灾下拉索截面的瞬态温度分布,随着曝火时间的增加,拉索截面的温度场和应力场分布呈现反对称状态且从二次分布特征向线性分布特征逼近,拉索截面会出现轴力损失和弯矩效应。相比于完整腔体传热模型,圆钢传热模型的截面应力分布均匀、轴力损失偏大且弯矩效应很小,空腔辐射模型的截面应力分布过于集中、弯矩效应偏大且轴力损失偏小。烟气包裹环境会加剧拉索截面的轴力损失和削弱拉索截面的弯矩效应。迎风环境会严重加剧拉索截面的轴力损失和弯矩效应。该文的研究成果可为索结构的抗火设计与防护提供理论依据。

     

Metallurgical failure analysis of cold cracking in a structural steel weldment: Revisiting a classic failure mechanism

Cold cracking of structural steel weldments is a well-characterized, well-documented, and well-understood failure mechanism. Extensive effort has been put forth to recognize the welding and materials selection parameters that are conducive to cold cracking; however, these engineering efforts have not fully eliminated the occurrence of such failures. This article describes cold cracking failure specifically related to the construction industry. This particular failure was successfully identified prior to final erection of the structural member, and the weld was successfully reworked. These actions potentially prevented a serious catastrophic event that could have occurred have occurred either later in the construction process or possibly during the use of the building. Individual welding parameters, such as electrode/wire selection, joint design, and pre/postheating, played a role in the failure, and a number of human factors relating to the actual fabrication practices also contributed to the failure process.     

Fatigue failure of a stainless steel wires used in a hydraulic pressure line

A Boeing B767-300 experienced a right engine fire during taking off. The inspection of the right engine’s fuel system revealed a failure in the turbine case cooling (TCC) air valve actuator hydraulic pressure line, a flexible component made of AISI 304 stainless steel wire braid. The failure occurred in an area of broken wires in the braid that was located at the edges of a kink in the line associated with a bending. The examination of the broken ends of the wires by means of the use of field emission scanning electron microscope (FESEM) revealed fractures features that were indicative of fatigue failure, with very fine fatigue striation spacing, thus indicating at least 1000 cycles to failure. The energy dispersive X-ray (EDX) analysis did not reveal any defects acting as initiation source. Rather, the failure resulted from the permanent deformation of the line, thus developing a stress concentration at the kink. Following these results, new safety recommendations were suggested and subsequently adopted.     

Hydrogen induced premature failure of massive cast medium carbon steel anchor fluke

A 17 tons cast medium carbon steel anchor fluke of an ocean going container ship suffered catastrophic premature failure during demooring from anchorage made after maiden voyage of one month duration. It was evident upon recovery that the fluke simply fractured into halves. Microstructural analysis showed ferritic–pearlitic microstructure and decarburized ferritic surface with massive twinning around the crack initiation zone. Fractography showed brittle (intergranular mode dominating at the crack initiation zone at surface followed by cleavage mode towards the core) crack propagation suggesting embrittlement plausibly induced by hydrogen and grain boundary impurities. Casting process apparently incorporated the hydrogen and could not eliminate completely during the post-casting annealing considering the massive size. Embrittlement of decarburized surface was caused by trapping of diffusing out hydrogen in annealing process.     

Evolution of microstructure and tensile properties of cold-drawn hyper-eutectoid steel wires during post-deformation annealing

Manufacturing temperatures of severely cold-drawn hyper-eutectoid steel wires are sufficiently high to influence the mobility of dislocations and alloy elements, thereby affecting the materials' mechanical properties. Herein, we describe the evolution of microstructure and tensile strength of the as-drawn 3.45 GPa steel wire during post-deformation annealing for 30 min at 150-450℃. Annealing at 150℃ raised the strength to 3.77 GPa by age-hardening through activation of dislocations pinning by carbon, while further temperature rising up to 450℃ caused a severe loss of strength. It was proved that annealing at 300 and 450℃ destabilizes the lamellar microstructure, promoting the formation of carbon-deficient(Fe,Mn,Cr)_3 C-type cementite particles with preferentially rounded and partially faceted hetero-interfaces. Annealing at 450℃ yielded the accumulation of Mn and Cr at the ferrite/particle interfaces, and their concentrations at the interfaces were dependent on the interface structure; i.e., lower concentrations at rounded interfaces(formed through capillarity–driven coarsening of the spheroidized cementite), and higher concentrations at faceted interfaces(that are initially existing in the as-drawn state). Our proof-of-principle observations, supported by thermodynamic calculations and kinetic assessments, provide a pathway for understanding the changes in microstructural and tensile properties during manufacturing of the hyper-eutectoid steel wires.     

Experimental investigation into the axial plastic collapse of steel thin-walled grooved tubes

Experimental investigations were conducted into the quasi-static collapse and energy absorption characteristics of thin-walled steel tubes containing a number of geometrical discontinuities in the form of grooves of constant depth and axial length. The failure modes and load-deflection characteristics are discussed and compared with the corresponding theoretical values. An inextensional collapse mechanism is used to describe a non-symmetrical diamond mode shell folding which takes into account the concept of stationary circumferential and inclinded travelling hinges.     

Reliability-based fatigue failure analysis for causes assessment of a collapsed steel truss bridge

This paper is intended to demonstrate conventional and reliability-based approaches to the collapse cause assessment in order to identify the effects of mis-installed bracket and H-beam members on the collapse of a steel truss bridge over the Han river in Korea only 15 years after opening to traffic. Based on extensive numerical investigations with parametric studies on various possible failure causes in terms of failure probability and expected fatigue life, it has been found that the mis-installation of bracket and H-beam members accelerated the fatigue failure of the vertical pin-connected hanger. Moreover, it may be observed that both the conventional and reliability-based S-N and linear elastic fracture mechanisms (LEFM) approaches in terms of the expected fatigue life and the fatigue failure probability provide about similar and compatible results. This indicates that any of the reliability approaches could be used as effective and rational techniques for the quantitative investigation of the complex collapse causes, together with the aids of conventional S-N/LEFM fatigue analysis.     

Metallurgical investigation of a prematurely failed roller bearing used in the support and tilting system of a steel making converter used in an integrated steel plant

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.     

Efficient and economical manufacture of heavy steel castings

Heavy steel castings, a symbol of the level of heavy industries, are high-tech products assembling materials, metallurgy, casting, etc. The feasible schemes for economizing manufacturing heavy steel castings were analyzed. The effects of riser design, sand mold, oxidation and deformation of casting during heat treatment, machining allowance, etc., on the economizing manufacture heavy steel castings were reviewed. Realization of efficient and economical manufacture of heavy steel castings will improve the output ratio of metal, shorten manufacturing period, save energy and resource, reduce pollution, and improve the competitiveness of enterprises.