Failure Analysis Case Studies II Episode 11 pot

338 Fig. 7. Oxide spike under the fractured surface of primary crack. which was located far from the oxide layer and on the tube metal in the vicinity of a primary crack whose composition may be affected by oxide scale formation. These two analysis results were compared, to predict the cause of oxidation. Also, a piece of oxide scale was detached from the tube’s interior surface and analyzed. Compositional analysis of the oxide layer at the crack tip was also conducted. EDAX analysis in the electron microscope was used for composition analysis and the results are summarized in Table 2. It is shown that the Cr content of the metal right beneath the primary crack decreased compared to the sound part of the tube. This is due to the formation of Cr203 at the crack surface resulting in a decrease of the Cr content in the neighboring base metal. Analysis results of three oxide scales (two pieces detached from different locations and one attached at the crack tip) showed that the Ni content did not vary while the Cr content increased and the Fe content decreased considerably. Thus, it can be predicted that Cr,03 is the main Table 2 EDAX analysis results of the radiant tube at several locations (in wt%) Composition Locations Ni Cr Fe Si Mo S V Sound tube metal far from the cracked region 18.67 30.16 49.74 1.43 - - - Tube metal near to the cracked region 19.56 27.34 51.57 1.53 - - - Crack tip oxide scale 17.19 33.52 44.86 2.44 1.21 0.47 0.30 Detached oxide scale 1 20.22 31.24 44.84 1.41 1.53 0.54 0.22 Detached oxide scale 2 18.05 41.24 35.48 3.06 1.33 0.61 0.22 339 Fig. 8. Ni distribution map of crack tip region (same location as shown in Fig. 6(b)) oxidation product in the oxide scale. Other minor elements appeared in the scales such as 0.4- 0.6% of S and 0.2-0.3% of V apparently produced by contact with combustion gas. A high content of S may generate a corrosion problem; vanadium usually forms V205, which, together with high velocity combustion gas, may cause erosion problems. In this tube failure, however, S and V seemed not to contribute to the failure. Figure 8 is a composition map obtained by EDAX that shows the Ni distribution at the crack tip region of Fig. 6(b). This picture shows that the oxidation layer is not correlated with Ni. Figure 9 shows the Cr distribution at the same location. Focusing on the Cr concentration of the oxide scale in Fig. 9, it is evident that the oxide layer at the crack tip is of Cr oxide product. 3. Discussion Abnormal oxidation problems of high temperature steels are generally caused by improper operating temperature exceeding the recommended temperature range. The manufacturing process of the radiant tube of this study is similar to a well-known centrifugally-cast HK steel, but the chemical composition of the tube is different from that of the HK steel since Si was added to the tube metal. Ifwe consider the composition only, which is Cr = 23-26%, Ni = 19-22%, C = 0.25%, Si = 1.5-3.0%, and Fe = balance, it is a typical composition of 314 stainless steel (UNS31400 steel). This material is known to have excellent high temperature oxidation resistance by forming Cr203 protective oxide film. However, if the service temperature exceeds 1 OOO'C, the stabilized Cr203 film becomes unstable and transforms into volatile Cr03 losing its protective effect. There- fore, the current radiant tube material should be used in a temperature range which does not exceed 1000°C, to prevent abnormal oxidation. Also, it was reported that for high Cr-Ni 340 Fig. 9. Cr distribution map of crack tip region (same location as shown in Fig. 6(b)) steel, excessive oxidation can occur in a short period if the operation temperature is higher than 1090°C [4]. In order to know whether the failed radiant tubes had been in service below or above 1000°C, the degradation level of the microstructure due to thermal aging was assessed. In most of the cases, the service temperature of the tube can be predicted by comparing the microstructure with that under known service conditions. Figure 10 shows a typical microstructure of the failed tube. IWF- Fig. 10. Microstructure of uncracked region of the tested radiant tube. 34 1 * Fig. 11. Microstructure of cracked region of the tested radiant tube showing void formation Referring to the known reference microstructures of HK tubes degraded at high temperature [3], the microstructure shown in Fig. 10 corresponds to the microstructure similar to one aged at 950- 1000°C for 60,000 h. Since the service period of the failed tube was only 15,000 h which is much less than the 60,000 h of the corresponding microstructure, it can be predicted that the service temperature of the tube was above 1000°C. Figure 11 shows a microstructure near the cracked area which shows internal void formation. Voids of this kind were reported to be formed when the service temperature reaches 1090-1230°C in the case of Ni-Cr steel [4]. Hence, it can be argued that the local metal temperature during the service must go up to this high temperature. This overheating can be induced by touching of the flame to the tubes near the supporting guide A in Fig. 1. Therefore, to prevent radiant tube failures methods should be sought to lower the tube metal temperature below lOOO"C, particularly in the vicinity of supporting guide A. Modification of burner tips or improving combustion systems can be considered. 4. Conclusions By conducting a failure analysis on the cracked radiant heater tubes used in a high temperature furnace, the following conclusions are derived. The radiant heater tube which was centrifugally-cast with the same chemical composition as a typical HK steel except additional Si could be used-without problems if it is operated at a temperature less than 1000°C so a protective Cr,03 oxidation film forms. However, as the operating temperature exceeded 1000-1 lOO"C, the stabilized Cr203 transformed into volatile Cr03 and abnormal oxidation or rapid oxidation occurred. The failed tube of the current study must have been used at or above the recommended temperature range and as a result, locally thinned areas were formed by excessive oxidation. Some of the oxidation pits were filled with oxide scales formed 342 by rapid oxidation. This thick oxide scale was usually cracked because the heat expansion coefficient of the oxide was different from that of the tube metal on which the scale is attached. Through the opening of the oxide crack, fresh tube metal which was located beneath the oxide crack tip suffered repeated oxidation resulting in small cracks initiating in the tube metal. Tube failure finally occurred as a result of propagation of these small cracks to the outer surface of the tube. The failure could be prevented by maintaining the temperature of the tube at the flame side of the burner, that is, in the vicinity of supporting guide A in Fig. 1, below 1000°C by improving the existing combustion system or by modifying the burner tips. Acknowledgements The authors are grateful for the support provided by a grant from the KOSEF (Korea Science and Engineering Foundation) through Safety and Structural Integrity Research Center in Sung Kyun Kwan University. The authors also would like to thank POSCO (Pohang Iron and Steel Co.) for providing samples. References [I] Williamson J, Shipley M. Life assessment and monitoring of furnace heaters, improving reliability in petroleum [2] Walter M, Schutze M, Rahmel A. Oxidation of Metals, 1993;40:37. [3] Life prediction of tubes for steam reformer and cracker. Document for Information Document No. 85, KHK, 1983. [4] Lai GY. High temperature corrosion of engineering alloy, ASM International, 1990. refineries and chemical and natural gas plants. Houston, TX, USA, Novembcr 9-12, 1992. Environmentally assisted cracking Failure Analysis Case Studies II D.R.H. Jones (Editor) 0 2001 Elsevier Science Ltd. All rights reserved 345 SUSTAINED LOAD CRACK GROWTH LEADING TO FAILURE IN ALUMINIUM GAS CYLINDERS IN TRAFFIC J. W. H. PRICE*, R. N. IBRAHIM and D. ISCHENKO Mechanical Engineering Department, Monash University, 900 Dandenong Road, East Caulfield, Victoria 3145, Australia (Received 26 June 1997) Abstract-Some common portable aluminium gas cylinders have shown a liability to develop cracking. This cracking has in some cases led to leaks and on occasions to violent and sometimes fatal failures. There are a number of features of this cracking which have not been properly explained. Previous modelling of the growth of these cracks under sustained load has been developed from specimen testing. As is shown in this paper these data produce results which appear to produce values of crack growth which are too slow by a factor of the order of lo* to explain the observed phenomenon. It also appears that crack growth can be rapid even in cylinders with low levels of lead. This paper presents a numerical simulation of the growth of these defects based on the local stresses in the vicinity of the crack edge. This information is related to cracks actually found in cylinders which have leaked or failed in service. From this procedure an equation for the crack growth rate is developed, This also leads to an explanation as to why “leak before break” is not always observed in these cylinders. 0 1997 Elsevier Science Ltd. Kejwords: Emhrittlement, pressure-vessel failures, residual stress, slow crack growth, sports equipment, failures. 1. INTRODUCTION Portable aluminium cylinders are in common use in the world for purposes such as self-contained underwater breathing apparatus (SCUBA), respirators for fire and medical use and other uses. In Australia about 1,700,000 of these cylinders are in circulation, and large numbers exist in all developed countries. There has been a history of cracking developing in some of these cylinders in the position shown in Fig. 1. 1.1. The nature of the cracking The cracking tends to grow from notches created during the forming process for the top end of the cylinders and is driven by stress not only from the pressure contained in the cylinder, but also residual stresses from their manufacture. Understanding the cracking and estimating the rate of cracking growth is an objective which has interested a number of researchers in order to achieve a basis for assessing acceptable defect sizes [I, 21. The crack growth has in many quarters been stated to involve a phenomenon called solid metal induced embrittlement (SMIE) where crack growth is aided by surface diffusion of certain elements, the most important of which is lead. The cracks grow under constant load, so it is also described as “sustained load cracking”. Since diffusion of elements is involved, there are some similarities to creep crack growth and this terminology has also been used. The fundamentals of this process are described elsewhere [2, 31. ~~~ ~ *Author to whom correspondence should be addressed. Reprinted from Engineering Failure Analysis 4 (4), 259-270 (1997) 346 I '0 ring seating surface Notches and cracks tend to be found in this area - 14 mm 185 mm Fig. 1. The top of an aluminium gas cylinder with the area of crack initiation and growth indicated. 1.2. Faiiure experience In 1983, two fatal ruptures of hoop-wrapped aluminium cylinders occurred in the United States. A significant report made of those incidents at the time was prepared by Failure Analysis Associates [4]. This report considered and compared three categories of cylinders which were provided by the manufacturers. This and other work Icd to the identification of risk factors associated with the aluminium cylinders in traffic in the U.S. These were: 0 cracking originating at the neck shoulder region as shown on Fig. 1; 0 folds in the neck region; and 0 lead levels of 100 ppm or higher in the aluminium alloy. There have been recent incidents in the US., namely the death of a fire fighter filling a cylinder in 1993 [5] and an injury in a Miami dive shop in June 1994 [6]. In Australia, most cylinders in circulation are made locally. These cylinders do not have high lead contents and are not made with recycled scrap, which were two of the factors featured in the early US. studies. Nevertheless, there has been some failure experience with Australian cylinders, though no serious injuries. Poole [7] describes a cylinder made to Australian standards which failed cata- strophically in 1994 in New Guinea during hydrotest in a shop. One key area of protection of the public is a requirement that all cylinders be inspected regularly. For scuba tanks the frequency in Australia is annual; for other aluminium cylinders, the required frequency is four-yearly. This inspection is visual and carried out with lights and dental mirrors. A number of testing stations in Australia have reported that a significant number of locally made cylinders experience cracking from the neck. The authors have themselves investigated a number of cracked cylinders. For some years of cylinder manufacture in the mid-l980s, the frequency of detection of cracking at the necks in 1995 was as high as 10% at some test stations. Because of the method of inspection, it is not clear how serious these detections are, and the cracking can often be shallow. Since 1995, all cylinders with visible cracking must be condemned. The alloy in use in Australia and other countries was changed from 6351 in T6 temper to 6061 T6 during the last few years. The 6061 alloy is believed by the industry to have less susceptibility to cracking, though the authors have one cylinder in this alloy which has a crack in the neck area. Stark and Ibrahim [8] present data comparing the two grades that do not indicate a significant difference in either K,, or crack growth rates. 347 2. EXAMINATION OF TWO SEVERELY CRACKED CYLINDERS Among several defected cylinders and samples the authors have investigated two Australian made SCUBA cylinders which have large defects in them. Cylinder A leaked during filling and cylinder B failed catastrophically during hydrotest. These cylinders do not conform to the model of failure as described by Failure Analysis Associates [4]. Although the origin of the cracking was in the same location on the neck as the U.S. experience, neither cylinder has significant neck folds and lead levels are below the limit of measurability using the standard spectral analysis test, that is, below 10 ppm. 2.1. Cylinder A Cylinder A was made in 1983 and leaked during filling in 1994. In 1994 the Health and Safety Organisation (HSO) in Victoria withdrew an aluminium cylinder from traffic and provided it to the authors for investigation. This cylinder exhibited cracking so large as to cause a leak, making it impossible to fill. There is a second defect of almost the same size almost opposite the leaking crack (see Fig. 2). The cylinder is of 8.65 kg water capacity with a test pressure of 32.4 MPa, manufactured in August 1983. The leaking defect penetrated through to the upper surface of the cylinder outside the O-ring contact surface and thus caused the leak. The fracture surfaces have been examined under scanning electron microscope as reported in Price et al. [9]. Most of the defect surface exhibits the features observed before (for example by Lewandowski et al.) and thus probably has the SMIE growth mechanism. However there is evidence of a defect about 3 mm deep by 15 mm long at the neck shoulder which has a different appearance and is probably a pre-existing defect from which the whole defect grew. 2.2. Cylinder B The other specimen is part of the cylinder which failed in New Guinea. Discussed by Poole [7], this was made in August 1987 but otherwise has similar specifications to cylinder A. This cylinder failed catastrophically during hydrotest in New Guinea on 13 February 1994 and was inside a water- filled concrete tank which also burst. The failure occurred in four places around the neck and the Fig. 2. Inside the top of cylinder A. Two large cracks almost opposite each other are marked. This specimen has been broken open and crack growth was found to proceed under the surface further than detectable using dye penetrant on the surface. The defect appearance conforms to the intergranular growth mechanism. [...]... AIME, Chicago, 1977, pp 379406 8 Troiano, A R., Trans A S M , 1960, 52, 54 1960 9 Beachem, C D., Metall Trans., 1972,3,437 - - - Faiilrre Analysis Case Studies 11 D.R.H Jones (Editor) 0 200 1 Elsevier Science Ltd All rights reserved 365 PIk S1350-6307(98)000094 FAILURE ANALYSIS OF CARRIER CHAIN PINS G A SLABBERT,* J J McEWAN and R PATON Physical Metallurgy Division, Mintek, Private Bag X3015, Randburg... Montreal, July 1996 Schr6der, R., Materials Science and Technology, 1985, l(Oct), 154 Failure Analysis Case Studies II D.R.H Jones (Editor) 0 2001 Elsevier Science Ltd All rights reserved 357 HYDROGEN-ASSISTED STRESS-CORROSION OF PRESTRESSING WIRES IN A MOTORWAY VIADUCT L VEHOVAR* Institute of Metals and Technology, Lepi pot 1I, IO00 Ljubljana, Slovenia V KUHAR National Building and Civil Enginering Institute,... mechanical/environmental failure process that results from the absorption of hydrogen into the metal, usually in combination with stress (residual or applied) [SI HE is one of four types of environmental cracking [6] and is mainly a problem in high strength steels The susceptibility of a material increases with : (i) The strength (e.g hardness) (ii) Increasing amounts of cold work (iii) Increasing residual... extreme situations Given the above analysis it is not clear why some cylinders, such as cylinder B fail catastrophically without leaking, while some cylinders such as cylinder A leak prior to failure The principal explanation of this probably lies in the fact that the growth of the defect as observed does not occur in the regular fashion assumed in classical crack growth analysis If crack growth can occur... This new position of the crack was then plotted on a drawing of the cylinder section For the next step of analysis a new mesh was constructed with the crack front moved to a new position The remeshing of the model is a key feature of the process used in this paper 4.2 Finite element analysis Stress analysis was carried out using the finite element package MSC/NASTRAN at each step of crack growth The problem... disruption during the sugar cane harvesting season, and equipment must be maintained in a high standard of repair When failure of equipment does take place, it is important to identify the cause to minimize the likelihood of any future problems This paper details the analysis of a failure of conveyor chain pins that had operated for only six weeks The pins had been heat treated so that they had become... an absolute minimum during this period Unforeseen failure of critical components does occur, however, and repair and maintenance is done rapidly in order to get the plant back into full production In many instances, it is imperative that the cause of failure be ascertained to avoid future problems and minimize costly breakdowns This paper details the failure of carrier chain pins from a conveyor used... the majority of fractures were in the middle portion of the pin *Author to whom correspondenceshould be addressed Reprinted fromEngineering Failure Analysis 5 (2), 121-128 (1998) 366 Table 1 Specifications for AISI 431 and ‘En 57’ stainless steel, and the chemical analysis of the chain pins Yc o Specification for AISI 43 1 Specification for ‘En 57’ Pin A (fractured) Pin B (fractured) Pin C (fractured)... and resistance to quench cracking [ 11 2.3 Metallographic analysis The metallurgical structure of all the samples, except for pin C, was tempered martensite and pin C contained ferrite stringers in a tempered martensite matrix, as shown in Figs 2 and 3 Since all the pins failed in a similar manner, the presence of ferrite stringers in pin C did not influence its failure All of the fractures, of which... pins 0.2% yield strength (MP4 Elongation Impact energy at room temperature W) (J) 1 I82 Pin Ultimate tensile strength (MPa) 1439 1458 1579 23.2 21.6 24.8 - - N2 N3 N4 N5 N6 N7 N8 N9 1 I82 116 2 - - - - 22 57 23 24 45 117 5 11. 5 635 Average Standard deviation 'En 51" AISI 431' AISI 4313 AISI 4314 - 1492 76 850-1 000 1210-1515 1370 1360 23.2 1.6 34 16 - - 1030 1080 ~ - - - 16 23 ' British standard specification . assisted cracking Failure Analysis Case Studies II D.R.H. Jones (Editor) 0 2001 Elsevier Science Ltd. All rights reserved 345 SUSTAINED LOAD CRACK GROWTH LEADING TO FAILURE IN ALUMINIUM. 1996. 12. Schr6der, R., Materials Science and Technology, 1985, l(Oct), 154. Failure Analysis Case Studies II D.R.H. Jones (Editor) 0 2001 Elsevier Science Ltd. All rights reserved. catastrophically during hydrotest. These cylinders do not conform to the model of failure as described by Failure Analysis Associates [4]. Although the origin of the cracking was in the same location