Failure Analysis Case Studies II Episode 9 potx

268 B type hosepipe Fig. 1. Appearance of bursting hosepipes (arrowheads aim at openings). 3. Dissection of hosepipes The hosepipes were dissected from the outer to the inner layers in order to study the charac- teristics of bursting hosepipes. The results showed that: (1) The steel wire layer was intact. (2) In all A type hosepipes, there was only one opening in the inner rubber layer. Similar results were obtained for most B type hosepipes except two, where two openings were observed. (3) The opening in the inner rubber layer was situated in the outside of the bend near the metal elbow adapter. The distance between the opening and the metal core is about 2.0-12.5 mm (see Fig. 2). (4) The openings in the inner rubber layer are circumferential, as shown in Fig. 2. (5) No other defects were observed in the inner layers of the hosepipes. 269 A type hosepipe B type hosepipe Fig. 2. The openings in the inner rubber layer. (6) There were small openings in the outer surface of the inside middle layer in one B type hosepipe (see Fig. 3). 4. Fracture analysis of layers 4.1. Fracture analysis of inner layer Opening the fracture and inspecting by SEM, it was found that the fracture surfaces in both A and B type hosepipes have the same characteristics, showing flats and radiating ridges (see Figs 4 and 5). Fatigue markings in the later period of propagation could be clearly observed (Fig. 6). The 270 Fig. 3. The openings in the outer surface of the inside middle layer. Fig. 4. The fracture appearance of an A type hospipe. fatigue cracks were possibly initiated at the interface between the cotton thread and the inner layer. Finally, there were no obvious defects in the fatigue initiation zone (Fig. 5). Smaller cracks had similar fatigue characteristics (Fig. 7). 4.2. Fracture analysis of outer layer Opening the outer layer fractures and observing by SEM, it was concluded that cracks initiated from the outer surface and propagated towards the inner surface. Multiple initiation sites and some fatigue striations were observed (Fig. 8). 27 1 ~ _______ Fig. 5. The fracture appearance of a B type hosepipe. Fig. 6. Fatigue beach marks. 5. Analysis of hosepipe bursting failures Fracture analysis shows that hosepipe bursting is caused by fatigue failure, but the characteristics of failure are different in the inner and outer layers. In structure, the hosepipe is composed of seven layers (Fig. 9). Steel wire is the main load- bearing layer. The inside layer was projected from the steel wire by a middle layer and a cotton layer internal to the steel wire layer. The inner layer was in direct contact with kerosene and carried 272 Fig. 7. The fracture appearance of small cracks 'liP.V- mi- Fig. 8. The fracture appearance of the outer layer. alternating stress caused by pressure variations. The outer layer bears stress and bursts after the inner layer bursts. When kerosene has penetrated the inner and middle layers as well as the steel wire layer, kerosene gathered between the steel wire and the outer layer. Large fatigue cracks initiated in the most highly bent position of the outer layer and formed the multiple fatigue crack sources. There were three openings in the inner layer of one B type hosepipe; two of them were fully penetrating and the other was not. Fatigue cracks originated from the interface between the cotton 273 I inner layer 2-cotton 3 mnddlc layer 4-stccl wrc laycr 5-middle layer 6 conon 7-outer layer Fig. 9. Sectional diagram of the bursting hosepipe structure. layer and the inner layer. The two penetrating cracks propagated both inwards and outwards. The non-penetrating crack outwards. From the above-mentioned, the course of hosepipe bursting failure can be described as follows: fatigue cracks originated first from the interface between the cotton layer and the middle layer, and then propagated into the middle layer. Afterwards, further fatigue cracks initiated at the interface between the cotton layer and the inner layer and propagated into the inner layer. When the crack had penetrated the inner layer under the steel wire layer, the outer layer could not bear the pressure of the kerosene, resulting in a large stress fatigue failure leading to bursting. Improper design is the main reason for inner layer fatigue failure. The interface between the cotton and the inner layer is a weak position. Under working conditions, pressured pulses cause a radial bulge in the hosepipe. At the same time, there were bending deformations in the hosepipe. Insufficient fatigue resistance of the hosepipe is the most important reason for failure. However, only static pressure requirements were demanded for the hosepipes. Serious bending deformation in mounting is another important reason for hosepipe bursting. Limited by the space, bending deformation could not have been avoided in mounting the hosepipes. It was reported that the minimum bending radius in a high pressure hosepipe is about 6-7 times the external diameter [I]. The pressure capacity of the hosepipe will drop rapidly if the radius is too small. When the hosepipe works in normal conditions the working life will drop. For example, the service pressure of A style hosepipes is 700 MPa. When the bending radius is 70% of the minimum required in mounting only 58% of the rated working pressure of the hosepipe can be applied in service, i.e., 406 MPa (Fig. IO). If the working pressure is kept at 700 MPa, the working life is shortened to 12% (Fig. 10). 6. Conclusions (1) The bursting of the hosepipes was caused by fatigue failure. The fatigue cracks originated from the interfaces between the cotton and the inner and middle layers. (2) Fatigue cracks first propagated into the middle layer. Afterwards, cracks propagated into the inner layer. After the inner layer had been penetrated, a layer fatigue stress was applied to the outer layer. 274 working pressure life(%) the highest pressure Fig. 10. Relation between bent radius, pressure capacity and working life of hosepipe. (3) The main reasons for hosepipe failure are as follows: (a) inadequate structure; (b) low fatigue resistance; (c) severe bending deformation in mounting. Reference [I] Beijing Institute of Aeronautical Materials, Aeronautical Material Science, Shanghai Science and Technology Press, 1985, p. 6. Environmental attack 277 FAILURE OF AUSTENITIC STAINLESS STEEL COMPONENTS USED IN NITROGEN OXIDE PLANT V. M. J. SHARMA, A. K. JHA, P. RAMESH NARAYANAN, S. ARUMUGHAM and T. S. LAKSHMANAN* Material Characterisation Division, Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Thiruvananthapurarn, 695 022, India (Received 2 1 March 1997) Abstract-Austenitic stainless steel components of a nitrogen oxide plant have been found to leak in service. The failed components, namely pipe-to-pipe joints and pipe-to-flange joints, have been studied through standard metallographic techniques to analyse the cause of the failure. In case study I, involving the failure of pipe-to-flange joints, cracking was observed in the pipe wall next to the pipe-to-flange weld. In case study 11, involving the failure of a sight port flange, cracking was observed in the flange adjacent to the pipe-to-flange weld. In both cases, cracking was by an intergranular mechanism, and carbon contents were much higher than permitted for "L" grades of austenitic stainless steel. Q 1997 Elsevier Science Ltd Keywords: Metallography, residual stress, fractography 1. INTRODUCTION Liquid propellants, with their higher specific impulse than solid propellants, have emerged as efficient fuels for satellite hunch vehicles. For combustion during flight, various types of oxidizers have been used. One such oxidizer is dinitrogen tetroxide (N,O,). In view of the nonusability of dinitrogen tetroxide with some of the storage materials, mixed oxide of nitrogen (MON) has emerged as a better choice. Its lower freezing point (- 14°C) makes this one of the best oxidizers. MON mainly consists of a mixture of N204, NO and NO,. Further classification of MON has been done as MON- 3 (3% NO) and MON-10 (10% NO), depending on the content of NO. The plant, which produces mixed oxides of nitrogen (MON-3), consists of reactors, absorption columns, collection tanks, storage tanks, interconnecting piping, flanges and valves. One such plant, commissioned 7 years back, is in operation. However, for the last 2 years, leaks were observed in some of the components, mainly in pipe-to-pipe joints and pipe-to-flange joints. While attempting to rectify such a leak by tightening the flange, the flange broke into pieces. Various components, which have cracked and leaked at different locations of the process plant, have been grouped into two categories. In the first, the failure is in the pipe portion, and, in the second, the failure is in flange portion. As such, two failed components, (i) a pipe-to-flange joint and (ii) a sight port flange, were selected for detailed studies. 2. CASE STUDY I: FAILURE OF PIPE-TO-FLANGE JOINT The joint consists of a tube (25mm ID and 32mm OD) made of AIS1 304 L grade stainless steel welded to a flange made of the same steel (Fig. 1) The joint is used to transfer HNO, from a storage tank to an NO reactor. A leak was noticed in the pipe portion of the joint. *Author to whom correspondence should be addressed. Reprinted from Engineering Fuihre Analysis 4 (3), 17 1-1 78 (1 997) [...]... Engineering, 3rd edn McGraw-Hill,New York, 198 6 7 Cotton, J B and Jacob, W R., British Corrosion Journal, 197 1,6,42 8 Stott, J F D., Metals and Materials, 198 8,4,224 9 Rozenfeld, I L., Corrosion Inhibitors McGraw-Hill, New York, 198 1 10 Scully, J C., The Fundamentals ojCorrosion, 2nd edn Pergamon Press, Oxford, 197 5  299 11 Polmear, I J., Light Alloys Arnold, London, 198 1 12 Hollingsworth, E H and Hunsicker,... l 2 l 4 l 6 l 8 l 10 12 l 14 Ii 16 PH I 1 mol I-' of H+ t Neutral solution Fig A5 I mol I-' of OH- Failure Analysis Case Studies II D.R.H Jones (Editor) 0 2001 Elsevier Science Ltd .All rights reserved 30 1 CREVICE CORROSION OF 316L CAUSED BY CHLORIDE PARTITION IN WATER-BUTANONE MIXTURES J H CLELAND Cambcor Ltd, 30 Windsor Road, Cambridge CB4 3JW, U.K (Received 9 May 199 7) Abstract-Chloride-induced... paper 284 31 29 21 25 23 21 19 17 BRAGG ANGLE 'ze' Fig 9 X R D pattern of corrosion product (Fe,O,) on sight port flange REFERENCES 1 Metals Handook, Vol 13: Corrosion, 9th edn ASM, Metals Park, OH, 198 7, p 348 2 Metals Handbook, Vol 7: Atlas of Microstructures, 8th edn ASM, Metals Park, OH, 197 2, p 133 3 Fontana, M G and Greene, N D , Corrosion Engineering.McGraw-Hill, New York, 196 7, p 59 4 Liberto,... water-butanone system is shown in Fig Table 1 Chemical analysis of a section from the corroded pump: composition in wt% C Mn Si P S Cr Ni Mo 0.0 19 0.03 maximum 1.73 2.00 maximum 0. 59 0.028 0.045 0.004 16.5 16.&18.0 10.2 10.0-14.0 2.W3.00 maximum maximum -~ ~ Pump body UNS S31603 1.oo maximum 0.030 Reprinted f o Engineering Failure Analysis 4 (4), 287- 291 (1 99 7) rm 2.14 302 160 Y @ 3 E n i I - I I I 1 140... applied to the analysis of three case studies of corrosion failure in heating systems: rusting through of mild steel radiators after only 2 years in service; premature pitting corrosion of aluminium heat-exchanger tubes; and external corrosion of mild steel water pipes Q 199 7 Elsevier Science Ltd Keywords: Bacterial corrosion, corrosion, corrosion protection, pitting corrosion, heat-exchanger failures 1... circumference of the pipe, and the crack l I Fig 2 Stereomicrograph of the crack on a pipe-to-flange joint x 16 Table 1 Chemical analysis (wtX) of the failed pipe-to-flange joint Component ~~ C S Cr Ni Mn Mo Fe 0 034 0 010 19 3 0011 19 7 98 94 13 0 093 12 03 02 Balance Balance ~~~ Flange Pipe 2 79 (d) Fig 3 Optical micrographs of the failed pipe-to-flange joint (a) Microstructure near the crack, showing equiaxed... Press, Oxford, 196 6 2 Shreir, L L., ed., Corrosion, Vol 1: MelallEnvironment Reactions, 2nd edn Newnes-Buttenvorths, Oxford, 197 6 3 Boffardi, B P., in Metals Handbook Vol 13,9th edn: Corrosion American Society for Metals, Metals Park, OH, 198 7, p 487 4 yon Fraunhofer, J A., British Corrosion Journal, 197 1,6,23 5 Butler, G., Ison, H C K and Mercer, A D., British Corrosion Journal, 197 1,6,32 6 Fontana,... Pourbaix diagram for iron at 25 "C Mains water usually ~ ~ 1 rmq= 7 1 Radiator """ Return Fig 1 Schematic diagram of a closed recirculating heating system Reprinted from Engineering Failure Analysis 4 (3), 1 79- 194 (I 99 7) 286 Waterway Spot weld Fig 2 Construction of a typical mild-steel radiator has a pH of 6.5-8 [2], so as long as the potential of the iron is kept below - 0 6 V (standard hydrogen-electrode... in Merals Handbook, Vol 13, 9th edn: Corrosion American Society for Metals, Metals Park, OH, 198 7, p 583 13 Evans, U R., An Introduction to Metallic Corrosion, 2nd edn Arnold, London, 197 5 14 Durrant, P J., General and Inorganic Chemistry, 2nd edn Longmans, London, 195 2 15 LaQue, F L and Copson, H R.,Corrosion Resistance o Metals and Alloys, 2nd edn Reinhold, New York, 196 3 f 16 Uhlig, H., The Corrosion... would give a corrosion rate of x0. 09- 0.27 mm/year If it is assumed that rain-water had been leaking into the building for x 7 years, the depth o attack would be x 0.6f 1 .9 mm If the steel develops pitting corrosion, the rate of attack in the pit can be up to 10 times the rate of general corrosion [ 161 In this case, the pits would easily be capable of perforating the wall 298 of the pipe after 7 years Localized . Chemical analysis (wtX) of the failed pipe-to-flange joint Component C S Cr Ni Mn Mo Fe ~~ ~~~ Flange 0 034 0 010 19 3 98 13 03 Balance Pipe 0 093 0011 19 7 94 12 02. of a closed recirculating heating system. Reprinted from Engineering Failure Analysis 4 (3), 1 79- 1 94 (I 99 7) 286 Waterway Spot weld Fig. 2. Construction of a typical mild-steel. radiators to perforate after only 2 years in service. Fig. 3. 2.0- -1.0 - potential IIII IIII -2 0 2 4 6, 8Ll;H 12 14 16 + Mains water The Pourbaix diagram for iron at 25