Manufacturing and Service of an Engineering Component - Essay Example

?Non-Destructive Evaluation and Testing (NDE&T) Pressure Vessel Failure Case Study Introduction Pressure vessels are vessels or containers meant to contain high pressure fluids like water, chemicals, steam etc. These are indispensible for modern industrial activities. Pressure vessels find extensive usage in wide spectrum of industrial activities; some of these are listed below. a) Pressure Cooker in our kitchen for quick cooking of food b) Autoclaves in mineral beneficiation industry for high temperature leaching of minerals to increase leaching rate c) In chemical industry to carry out different reactions like ammonia synthesis etc at faster rate and for higher yield d) In thermal power industry to produce high pressure steam to run turbines e) In nuclear industry to keep coolant and / or moderator in liquid state even at higher temperature This is a brief list of activities where pressure vessels are used and from this list itself one can make out how important and indispensible are pressure vessels for our modern industrial civilization. Important milestones in the development of pressure vessel are reviews by Spence J. et al [1]. Therefore, it is important to know and understand the materials and manufacturing processes for pressure vessel and the role of non-destructive testing and evaluation in the entire manufacturing process and in-service monitoring of these vessels. This case study will cover these aspects in brief with emphasis on the failure of the vessel described in this case. 2. Materials for Pressure Vessel (PV) Fabrication Different materials can be used for fabrication of a pressure vessel depending on the service condition – temperature, pressure, chemical environment and the cost. For example one can afford to make a pressure cooker with aluminum alloy, but when it comes to a chemical reactor with severe corrosive environment, one needs to go for austenitic stainless steel. Mostly, the industrial pressure vessels are made of either low alloy steels or austenitic steels. Low alloy steels offer more strength and therefore one can afford to have a relatively thinner wall; that saves material and the vessel is less bulky. Besides, low alloy steels are much cheaper as compare to austenitic stainless steels and hence such pressure vessels are cost efficient. On the flip side these steels have lower ductility and much lower fracture toughness as compared to austenitic stainless steels. Besides, corrosion resistance is much weaker and therefore, in many applications a clad layer of austenitic stainless steel is placed on the inner wall of the pressure vessel by weld deposition process. On the other hand austenitic stainless steel as pressure vessel material offers much lesser strength and hence thick wall and bulky reactor and is much more costly. Hence except for highly corrosive applications, low alloy ferritic steels continue to be the material for pressure vessel in a vast majority of applications. 3. Manufacturing of Pressure Vessel (PV) Pressure vessel is essentially a shell structure and is made of thick plates. These plates are preferably hot forged plates to endure soundness of the plate and homogeneity of microstructure and also to ensure that dendritic solidification microstructure is completely broken and grain size becomes very fine. All this is done to improve fracture toughness of the plate. Subsequently these plates are bent and welded together to form a vessel. Wall thickness to diameter of a pressure vessel determines the pressure enduring capability of a vessel constructed with a material of certain strength. Besides, there are other structures like nozzles, manhole, dished end etc, which needs to be welded for different functional requirement. 4. Non-Destructive Evaluation & Testing (NDE&T) PV Manufacturing [2-4] Because plates are staring material for fabrication of a pressure vessel, therefore, it is very important to ensure quality of these plates. While one can perform destructive evaluation of these plates like metallography, chemical analysis, tensile property evaluation, fracture toughness measurement etc. but all these tests being destructive in nature can be performed on a representative samples only and not on the entire stock. Therefore, the only way to ensure quality of the entire stock in terms of soundness is ultrasonic examination. This examination in conjugation with other destructive tests on the representative samples will ensures quality of the entire stock and this practice is routinely used in industry to ensure quality of the entire stock. Coming to manufacturing process, it is mostly welding and that too multipass deposition of the weld metal or filler wire in the U or V groove made for preparing the joint. Welding is known to introduce many defects in the fusion zone and degradation of mechanical properties in the fusion zone and in the vicinity of the fusion zone also known as heat affected zone or HAZ. Some of the common defects associated with welding are the following: a) Incomplete Root Fusion i.e. root of the joint does not get completely fused and this is one potential crack initiation site. b) Slag Inclusion: This results from insufficient cleaning of the slag from the previous weld pass. c) Lack of Fusion: If there was a slag or non-conduction coating from previous pass, then arc deviates away from such a location, leading to lack of fusion. This becomes a potential crack initiation region. d) Hot Cracking: The weld metal solidifies over a temperature range and if there are impurities like S and P then these impurities form low melting eutectic and solidify towards the end on the surfaces of the already solidified grain which start to shrink causing tensile loading on the partially solidified grain boundaries and thus continuity gets broken and solidification crack emerges. e) Liquation Cracking: This occurs in partially melting zone where only the low melting phases on the grain boundaries melt and the grains remain solid. When these regions cool, then microcracks emerge. Alloys with impurities and wide melting range are susceptible for this type of cracking. f) Cold Cracking or Hydrogen Cracking: Ferritic / Martensitic steels are more susceptible against this kind of cracking. Hydrogen can come into the weld from various sources including moisture in the filler rod, shielding gases or moist air in the surrounding etc. to name a few. Once the weld solidifies all the hydrogen gets dissolved in austenitic phase, which is high temperature phase of steel. As the weld cools, this hydrogen dissolved in the austenitic phase field starts to come out and migrates towards high stress regions like micro-cracks, martensite grain boundaries etc. and starts to build up there. Once sufficient hydrogen has got accumulated it leads to increase pressure locally and cracking takes place. Still more hydrogen migrates towards the crack tip and the crack grows, this vicious cycle continues. Hydrogen cracking causes cracks with very sharp tip [5]. g) Martensitic Phase Transformation: This is relevant for ferrtic / martensitic steel (as in the present case). As the weld solidifies it cools very rapidly leading to martensitic transformation. In case of carbon steels, it happens for steels with carbon equivalent more than 0.3. The martensite is very hard and brittle phase and its fracture toughness is very low. Therefore, it is recommended that such steels must be subjected to post-weld tempering heat treatment. This is a heat treatment that causes softening of the martensitic phase and introduces significant amount of toughness. h) Residual Stress: Weld and its surrounding regions undergoes a complex heat cycle and with the heat cycle the materials in the vicinity of the weld joints undergo thermal expansion / contraction. The extent of this expansion and contraction is more near the joint and less farther away from it. This differential thermal expansion / contraction and the requirement of continuity introduces considerable stress in and around the weld joint. This is normally tensile in the fusion zone and the HAZ and thus putting HAZ and fusion zone at much higher risk of failure. This list of defects introduced by welding process can go on. The main point is that welding is inevitable for pressure vessel manufacturing and welding introduces a wide variety of defects, so what is the way ahead? The solution lies in 1) Laying down a qualified welding procedure (QWP) 2) Strict adherence to the QWP 3) Laying down an effective quality assurance plan by employing suitable NDE&T procedures Let us concentrate on the third point i.e. effective quality assurance plan by employing suitable NDE&T procedures Whenever welding is performed for fabrication of an actual component, one cannot afford to do destructive testing as that would destroy the component. But there must be a method to ensure that the component does not fail in actual test. For this purpose, NDE of weld joint is employed. There are many methods depending upon the joint under evaluation a suitable combination of these techniques can be employed. The first important method is visual inspection. This, itself is good enough in many cases to say whether the joint needs further evaluation or it can be tested right away. Subsequently one can perform Dye Penetrant Test (DPT): In this test a dye is employed on the test surface which gets sucked inside a crack and then dye is removed and a developer is applied to suck dye out of the crack. Thus crack length is revealed. This is very good to reveal surface cracks. One can also employ ultrasonic testing and X-ray Radiography to find out defects in the weld joint and then one can go ahead to see if these joint meet the acceptance criteria or should be rejected. In case the joint is not accepted, one may go for a repair joint. That will save the entire component from getting rejected or failing during hydro test and thus causing huge loss of man, material and other resources besides causing lot of inconvenience and or fatal accidents. 5. The Present Case Pressure vessel failure during hydro testing and also in – service has occurred in many cases and the same has been investigated by many teams [6-7]. Critical examination of the present case throws many regions for the failure of the vessel during the hydro test. 5.1 Wrong Material: The material of construction was a coarse grained material with very low fracture toughness. This was the first mistake. Coarse grained materials have poor fracture toughness; that too in case of ferritic steels, which are known for poor fracture toughness. Poor fracture toughness material for a welded construction is a bad idea as welding leads to further deterioration in the fracture toughness. This was evident from the mode of fracture which is brittle mode of fracture. 5.2 Hydrogen Ingress: There appears to a case of delayed cracking as the vessel has failed 7.5 hrs after it was loaded. This, points towards hydrogen cracking. This means, there was hydrogen ingress during welding. 5.3 No Post Weld Tempering: As this steel has ~0.3wt% C, this undergoes martensitic transformation in the fusion zone and HAZ. This transformations, hardens the fusion zone and HAZ and drastically reduces the fracture toughness of the same. Weld joints in such steel must be tempered after welding to introduce some toughness in the fusion zone and HAZ. But surprisingly, it was not done. It will amounts to foolishness on the part of the fabricators. Therefore, there is no surprise that it failed in catastrophic manner during hydro-test. 5.4 NDE&T of the Joint: It appears that the joint was not subjected to NDE&T after welding was done and prior to hydro test. Had it been done, one could have found the crack length and could have calculated the stress intensity factor at the crack tip. This could have helped in deciding whether to go for the hydro test or for repair of the weld joint. One would have gone for repair of the weld joint and this would have averted the accident during the hydro test of the vessel. 5.5 In-Service Inspection of the Vessel: NDE&T plays a very important role not only during manufacturing of a component but also during service. Once in services, the flaws in an engineering structure grow in size and its size can be monitored using NDE techniques. This helps in calculating residual life of a component and taking suitable remedial action before it fails catastrophically. 6. Conclusions: Based on the critical analysis of the case it can be said that NDE&T plays a crucial role during manufacturing and service of an engineering component. Besides, NDE&T is very important in avoiding catastrophic failures thus saving resources and precious human life. References: [1] Spence J., Nash D.H. “Milestones in pressure vessel technology”, International Journal of Pressure Vessels and Piping 81 (2004) 89–118 [2] Rao B. S. C., Madeswaran R. & Chandramohan R. “In-fabrication and pre-service care on stainless steel pressure vessels”, Int. J. Pres. Vessel & Piping 73 (1997) 53-57 [3] SOLOMONI K. A., OKRENT D. and William E. KASTENBERG “PRESSURE VESSEL INTEGRITY AND WELD INSPECTION PROCEDURE”, NUCLEAR ENGINEERING AND DESIGN 35 (1975) 87-153 [4] WELLEIN R. “INFLUENCE OF PRE/IN-SERVICE INSPECTIONS AND TESTS ON THE RELIABILITY OF REACTOR PRESSURE VESSELS”, Nuclear Engineering and Design 71 (1982) 391-392 [5] Gooch T. G. “Environment Sensitive Fracture: Materials & Welding Considerations”, Metal Tech., 9 (1982) 210 – 215. [6] John W.H. “The failure of the Dartmouth turbine casing”, International Journal of Pressure Vessels and Piping 75 (1998) 559–566 [7] Wang Yang, Lu Ye-Bo, Pan Hong-Liang “Failure analysis of a hydro-processing reactor”, Engineering Failure Analysis 16 (2009) 11–18 Read More