Improper Heat Treatment Research Articles

Fracture Analysis for Pitman Shaft of Automotive Steering Gear

The fractographic appearance, microstructure, chemical elements and hardness of the pitman shaft of automotive steering gear were analyzed. The fracture is induced by improper heat treatment and non-taking anti-carburization measures on thread part. Some feasible measures, including copperizing, coating anti-carburization paint and keeping localized manufacture allowance, were proposed for preventing the occurrence of similar fractures.

Failure analysis of Grade-80 alloy steel towing chain links

An experimental investigation of the causes of failure of chain links that occurred during towing operation of heavy-weight army vehicles is reported in this study. All failures took place at the weld area of the links after a short service life. Tensile tests on both base metal and the weld samples indicated high tensile strength of 800 MPa for both materials. However, the weld exhibited brittle fracture at relatively small strains of ε ≈ 0.05, while the base metal failed at strains ε ≈ 0.20, indicating moderate ductility. Optical metallography and scanning electron microscopy (SEM) analysis revealed that fatigue was initiated due to inherited cracks at the outer circumference of the weld. Fatigue crack propagation was evident by progressive marks and intervening striations. Their distribution was not rotationally symmetric, indicating a possibility of combined cyclic loading on the links. The large area of final rupture indicated a ductile rupture in the weld center and brittle fracture in the outer region of the weld due to overloading. The results suggest that the major causes of chain failure are as follows: high cyclic loading, weld defects, improper post-weld heat treatment, and decrease in material hardness and corrosion resistance due to insufficiency of some alloying elements.

Evaluation of Alloy 146, 279, 900, and 926 sensitization to intergranular corrosion by means of electrochemical methods and image analysis

Properties of stainless steels are affected by improper heat treatment. When that happens, the material can become sensitive to intergranular corrosion. This loss of properties is due to a microstructural changes. In this paper, electrochemical potentiokinetic reactivation tests ( single and double loop) were used to evaluate steel sensitization to intergranular corrosion. Four alloys, Alloy 146 (UNS 1.4331) and 279 (UNS 1.4429) (austenitic stainless steels), Alloy 900 (UNS 1.4462) (duplex stainless steel), and Alloy 926 (UNS N08926) (high alloyed stainless steel), were employed as working electrodes. The specimens were sensitized at different temperatures (600–825 °C) for periods of 1 or 2 h in argon atmosphere. Electrochemical tests were conducted using the electro-optical devices P-200002525 and P-200002526. These devices permit to obtain images of the electrode surface and electrochemical data simultaneously. In addition, these results were compared to standardized etching with oxalic acid (ASTM A-262), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Two alloys showed sensitization to intergranular corrosion, Alloy 900 and Alloy 926 at 725 and 825 °C. Additionally, the single and double loop methods showed different passivation results. In the double loop test, the formation of a layer over the entire electrode surface was visible during the process of activation. The film remained until the end of experiment. However, in the single loop method no significant variations were observed on the electrode surface during passivation process, but if material is sensitized a film was formed during reactivation branch.

Failure Analysis of a High-Pressure Component

Martensitic stainless steel nuts were received for failure analysis. A few of these nuts cracked at the immediate stress loading while the others fractured after 2 years of service. These nuts were subjected to different metallurgical tests such as visual examination, optical microscopy, chemical analysis, hardness testing, and fractography. It was concluded that improper heat treatment weakened the structure and subsequently fractured on the application of load.

Fracture Analysis on a Cylindrical Shell Section of the Low-Pressure Absorption Tower

Cracking of a cylindrical shell section from an absorption tower occurred during the hydraulic pressure testing. In order to find out the cause of failure, the cracked cylindrical shell section was inspected and destructively analyzed. Optical microscopy was performed to evaluate the basic microstructure of the material used to fabricate the cylinder and the effect of welding on the microstructure. The fracture surface was examined in a scanning electron microscope, and the effects of the heat treatment temperature on the structure and properties of 410S/16MnR explosively clad plate were investigated. Detailed metallographic studies indicated that bainite existed in the base layer of the explosively welded material. The weld-induced residual stress in the crack origin was investigated by a three-dimensional (3D) finite element model. The numerical result was consistent with the fracture analysis supporting the conclusion that cracking was caused by the poor mechanical properties of the explosively clad plate and that the poor properties were induced by improper heat treatment after explosive welding. The welding residual stresses also contributed to the failure process.

Failure analysis of railroad wheels

Failure of three railroad wheels, each belonged to a different train but to the same heat, was analyzed. It was found that the wheels were failed due to improper heat treatment.

Failure of refurbished turbine blades in a power station by improper heat treatment

Turbine blades in a power station exposed to a temperature of about 800 °C were refurbished after 40,000 h of operation. However, after only 900 h of service at the same temperature, a number of refurbished blades were fractured. Various electron microscopy techniques as well as computational modeling were used to develop a one-to-one correspondence between microstructure and temperature distribution. Fracture was found to occur by a ductile intergranular mode. Although there was no evidence for overheating during service, the results showed that the blades initially contained γ′ precipitate-free zones alongside grain boundaries, which could accelerate intergranular failure consistent with the observed fracture mode. It was concluded that the blades were damaged by improper heat treatment during refurbishing.

Effect of heat treatment on magnetic properties and microstructure of bulk Fe3B/Nd2Fe14B nanocomposite permanent magnets

Melt spun ribbons of Nd4·5Fe77B18·5 alloy were consolidated into bulk permanent magnets by spark plasma sintering (SPS) method. Effects of heat treatment on the density, microstructure and magnetic properties of the magnets are studied. Further observation shows that the sample consolidated by SPS possesses ultrafine nanocrystalline structure and small fraction of amorphous phase. After proper heat treatment, magnets with fine crystal structure and good magnetic properties were obtained simultaneously. On the other hand, excessive grains growth, which will lead to undesirable magnetic properties, can occur due to improper heat treatment.

Failure investigation of a tie rod end of an automobile steering system

A failure analysis of a tie rod end of a sports utility vehicle (SUV) steering mechanism has been carried out in this study. The tie rod end is composed of two parts fitted together: a threaded part and an embracing part. Failure took place in the threaded part which is made of AISI 8620 steel. The vehicle had been in service for approximately two years and accumulated less than 30,000 km. An evaluation of the failed part was undertaken to determine the cause of failure and assess its integrity. Visual examination, photo documentation, chemical analysis, hardness measurement, and metallographic examination were all conducted. The failure surface was examined with the help of a scanning electron microscope (SEM) equipped with EDAX facility that determines chemical composition at desired locations within the part. Results indicated that the tie rod end had failed by fatigue with a crack initiation at the throat (minimum) area of the threaded part due to material deficiency and improper heat treatment.

Evaluation of properties and microstructure as a function of tempering time at intercritical temperatures in HY-80 steel castings

Prior work on a failed HY-80 Bridge Access Trunk (BAT) casting indicated the cause of failure to be improper processing techniques. As-received HY-80 casting material showed a non-homogenous microstructure with two distinct microconstituents present: undertempered martensite and a layered martensite–ferrite structure. Heat treatment temperatures from in the intercritical range (721–799 °C) produced microstructures that differ from the desired uniform microstructure of tempered martensite and were similar to those found in the failed casting. In order to further examine the relationship between processing and microstructure, it was decided to vary the time for which the steel was held in the intercritical temperature range. This additional work was warranted by the medium intercritical heat treatment results in the previous study [Holthaus JE, Koul MG, Moran AL. Property and microstructure evaluation as a function of processing parameters: large hy-80 steel casting for a US navy submarine. Eng Fail Anal 2006;13(1):1397–409] and its similarity to the failed casting microstructures. An important finding of this study is that, contrary to normal behavior during tempering, HY-80 steel tempered in the intercritical range demonstrates a severe loss of toughness; which can be exaggerated for longer hold times and higher temperatures. To confirm the hypothesis that the presence of brittle martensite formed by improper heat treatment was the cause of failure, SEM section fractography was employed to directly examine the microstructure underlying the fracture surface and to identify a correlation between microstructure and fracture mode.

The prediction of mechanical behavior for steel wires and cord materials using neural networks

The tensile strength of the steel wire material is required to be sufficiently high for better performance. Steel with a high cleanness will prevent problems during drawing and the heat treatment. The studies show that among many defects the most important ones are the non-metallic inclusions and undesirable phases encountered during improper heat treatment. Especially different non-metallic inclusions will play an important role during crack propagation due to their weak matrix bond. In this study typical wire and cord failures due to non-metallic inclusions are examined. A generalized regression neural network was developed to predict the tensile strength as a function of experimental conditions. The predicted values of the tensile strength estimated by neural network are found to be in good agreement with the actual values from the experiments.

Effect of low copper content and heat treatment on intergranular corrosion of model AlMgSi alloys

Certain 6000-series extrusions may develop susceptibility to intergranular corrosion (IGC) by improper heat treatment, especially if copper is present as an alloying element. Although occurrence of IGC in such cases is documented, the underlying mechanisms are not adequately explained. We present corrosion data for two model alloys, having different Cu content and Mg:Si ratio, showing that the susceptibility to IGC depended primarily on the Cu content and secondly on thermal processing. Low Cu samples (0.0005 wt.% Cu) were essentially resistant to IGC. High Cu samples (0.12 wt.% Cu), which were air cooled after extrusion, exhibited significant IGC. However, IGC susceptibility was reduced significantly as a result of artificial aging to peak strength. Water quenched high Cu samples were essentially resistant to IGC. However, slight IGC susceptibility was introduced after aging. Electron optical characterisation revealed Al 4Mg 8Si 7Cu 2 (Q-phase) grain boundary precipitates on all the variants susceptible to IGC. The susceptibility was attributed to microgalvanic coupling between Q-phase grain boundary precipitates (noble) and the adjacent depleted zone (active).

Failure analysis of a pinion

A pinion, which was a part of an air-motor driving a diesel generator (DG) of a power plant’s emergency power-supply system, was found to have failed into three parts during a routine check-up of the DG. This pinion, attached to the air-motor shaft is active only for a short duration during the start up, when the motion of the air-motor shaft is transferred to the ring gear attached to the DG which in turn starts the DG. Once the DG has started, the pinion disengages from the ring gear and the air-motor stops. The pinion that failed had been in operation for a long time and experienced several start-up operations before failure. A systematic failure analysis was carried out to find out the reasons for this unexpected fracture of the pinion during operation. The results indicate that the fracture was caused by fatigue with a fatigue crack initiating from the fillet of one of the pinion teeth. Due to misalignment that was present, the pinion did not mesh properly with the ring gear during the start-up operation and this led to a high stress concentration at the root of the pinion. The misalignment also led to severe wear and excessive heat generation at the mating surface. Misalignment, improper heat treatment, non-uniform distribution of sulphide inclusions and sharp corners would have all contributed to the failure of the pinion.

Effects of Improper Heat Treatment on Steel Crane Wheels

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Waste package performance evaluations for the proposed high-level nuclear waste repository at Yucca Mountain.

The evaluation studies of the proposed repository for long-term storage of spent nuclear fuel and high-level nuclear waste at Yucca Mountain, Nevada, are underway. Fulfillment of the requirements for limiting dose to the public, which includes containment of the radioactive waste emplaced in the proposed repository and subsequent slow release of radionuclides from the Engineered Barrier System (EBS) into the geosphere, will rely on a robust waste container design, among other EBS components. Part of the evaluation process involves sensitivity studies aimed at elucidating which model parameters contribute most to the waste package and overlying drip shield degradation characteristics. The model parameters identified for this study include (1) general corrosion rate parameters and (2) stress corrosion cracking (SCC) parameters. Temperature dependence and parameter uncertainty are evaluated for the general corrosion rate model parameters while for the SCC model parameters, uncertainty treatment of stress intensity factor, crack initiation threshold, and manufacturing flaw orientations are evaluated. Based on these evaluations new uncertainty distributions are generated and recommended for future analyses. Also, early waste package failures due to improper heat treatment were added to the waste package degradation model. The results of these investigations indicate that the waste package failure profiles are governed by the manufacturing flaw orientation model parameters.

Failure Analysis Approach to Fracture Studies in Powder Metallurgy Parts

This paper presents a failure analysis of tool steel and brass powder metallurgy (P/M) parts that failed during service. A detailed failure investigation of fractured tool steel and brass parts was carried out to assess the causes for their premature failures. The fractured surfaces of the broken pieces and the component surfaces were subjected to detailed examination. Investigations were carried out by visual methods, microhardness measurements, and using optical and scanning electron microscopes. In the case of the brass sample, visual examination of the surface indicated flat surface features. Detailed optical and electron microscopic studies corroborated by microhardness indentations have conclusively established that the failure was mainly due to the presence of very small impurities in the brass component material. In the case of the tool steel sample, the fractured surfaces of the component were subjected to destructive and nondestructive tests. Representative fractured pieces were examined visually and tested for their yield strength using simple tensile tests. Optical and scanning electron microscopies at appropriate magnifications were also performed to characterize its microstructure and fracture morphology. Detailed investigations of the tool steel part established that the failure was mainly due to inferior yield strength of the component resulting from improper heat treatment.

Failure analysis of coal pulverizer mill shaft

Failure of a ball and race type coal pulverizer mill shaft has been analysed. It was found that the shaft, made of EN 25 steel, failed by fatigue. The fatigue cracks originated from the keyway area, unusually from the top edge. The presence of elongated manganese sulphide inclusions due to improper heat treatment had reduced the ductility and CVN toughness of the material, and thus made the material more prone to failure.

Failure of locked coil wire rope of coal handling system

The outer Z-shaped wires of locked coil wire ropes used in coal transportation were found to fail frequently. A systematic metallographic investigation was carried out to understand the cause of failure. The wire was found to fail due to decarburised layers that form due to improper heat treatment procedure followed during production. The steps to avoid decarburisation and a comprehensive mechanical test were suggested to ascertain the life of the wire ropes.

Failure analysis of an accessory bevel gear installed on a J69 turbojet engine

The fracture of a drive bevel gear (base material: AISI 8617) used for an accessory gear box of a turbojet engine was investigated. From the visual examination of the fracture surface of the failed bevel gear it was shown that the cracking mode was fatigue. Careful observation of the fatigue crack origin using SEM (scanning electron microscopy) indicated that there were no surface defects. The fatigue crack initiated at the upper root area of the gear teeth and propagated intergranularly. The cracking route was identical with that of typical tooth bending fatigue (except) for deviations attributed to improper surface heat treatment. Microstructural analysis revealed that the fractured gear was not properly case hardened and this lowered the surface hardness dramatically. The remedial action is to pinpoint the exact process parameters that resulted in the wrong case hardening.

Failure of a valve rod caused by improper heat treatment and metallurgy

Fractography analysis was used to investigate a structural component which failed during its normal service life. It has been found that imperfections in the metallurgy and heat treatment were the major causes of failure. Some detrimental impurities were found in the material. A number of cracks were initiated at the interface between the impurities and the base material. These cracks were the main reason for the failure. Also several kinds of metallography defects contributing to crack initiation have been found. The metallographic structure of the material is bainite+ferrite; the amount of ferrite increases gradually from the outside to the inside of the part. This was the wrong metallographic structure for this kind of material; it should be sorbite. Cracks initiated and propagated under a relatively low stress level by stress corrosion cracking.