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Brittle Failure and Low-Temperature Welding

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Brittle Failure and Low-Temperature Welding Brittle Failure and /:" . Low-Temperature Welding , c Downloaded from http://onepetro.org/jcpt/article-pdf/8/01/35/2166420/petsoc-69-01-06.pdf by guest on 30 September 2021 ,;,.'.:-. K. WINTERTON Head, Welding Section, Physical Metallu1'gy Divisicn, Mines B,'ancl!, Ottawa , 'J : .'- '., .! .,:, J ABSTRACT INTRODUCTION , :) The causes of brittle failure are explained, and the THERE ARE ALL KINDS OF TROUBLES that can develop aspects of crack propagation and crack initiation treated in equipment in northern climates. These may result separatel)·. The selection of steels for service at low temperatures is considered, and some instances are given from improper selection of materials, unsuitable over­ of special code requirements. General advice is presented all design, inappropriate design or choice of particular for the minimization of the problems of brittle failure. components, manufacturing defects with special sig­ 'Velding in cold weather affects personnel, materials and nificance for operation at low temperatures, etc. equipment. Metallurgieal factors are also involved. as revealed by experimentation and practical experience_ It is not surprising that suitable equipment and Code requirements are explained. Practical advice is materials could in most cases be provided, but that given for overcoming the problems that may be en­ countered. this would entail unwelcome increases in costs. More­ over, in Canad~l at least, the problems have been at­ tacked piecemeal, and there is no systematic ap­ proach available to those who are not immediate!)' deterred by the prospect of paying more for what the}" need. At the University of Alaska, the engineer­ ing faculty places emphasis on special training of, students in the unique aspects of work in the North. Dr. K. \VINTERTON e;raduated in In the U.S.S.R., a more unified approach to northern metallurgy from Binnfngham Uni­ versity in 1940. Subsequently, he problems has been developing, and they are consider­ joined a team working on the welding ing the establishment of a Research Institute for Tech­ of armour steel and was awarded his nical Problems of the North(l). Doctorate in 1943. He joined the Tin Research Institute as a l·esearch met­ The present paper is limited to the consideration allurgist, workinl{ on problems con­ of two aspects of the problem; firstly, brittle frac­ nected with the casting of brass and bronze_ In 1946, he joined the staff of ture, which is certainl}' the most important single ma­ the British Welding Research Asso­ terial problem, and secondl).', the effect of low tem­ ciation, later becoming chief metallur­ peratures on ,,,,'elding-. gist. He emigrated to Canada in 1955 to tal;:e the post of assistant director of the Department ..' of Engineering and Metallurgy of the Ontario Research Foundation. In 1958. he joined the Mines Branch of the BRITTLE FAILURE Department of Ener,gyJ Mines and Resources to take the post of head of the 'Welding Section, Physical Metallurgy Strength and Brittleness Division. On considering the metals used in oil-field opera­ THE PAPER WAE PRESENTED: at the 19th Annual tions, it is apparent that steels are by far the most Technical Meeting of The Petroleum Society of CIM; important. These may be plain-carbon steels or alloy held jointly with the Spring Meeting, Rocky ·Mountain steels, cheap steels or expensive steels, structural District, Division of Production, American Petroleum In­ stitute; Calgary. iVIay, 1968. steels. machinen' steels or special steels. A common characteristic is that they sometimes do not behave as the).' should in cold ·weather. Technology, January-March, 1969, Montreal 35 ~CoO.--- ~__...,..__.,.._..., In order to consider huw the strength of steel is aff~cted by the cold, it is neces8ary to agree on whnt SRITTlE STPENGr... __~ is meant by stl-ength_ Almost all designs are LJil.:'Ied on yield strength, 01" the load t.hat unit sections will ._-._/ sil~nificantly. Figlll'(~ 1 sustain without deforming -. ISO plots strength against temperature for pure inHl ill \ simple tension, and shows that yield strength actually \ increases as the temperature falls'~l. This may appear to be good, but it has some Lad consequences. At H ­," c.:ertain temperature, the yield stress and fracture \ ~tress ~ 'DO meet; below this temperature, the pure iron " ~SnELD will break sllddenly without deformation. lili:e glass. For pure iron. very low temperature!; are needed ,--.., to achieve this in a simple tensile test'~), as shown in Figure 1. However, there are other factors that can increase the yield stress relative to the fracture stress; '"'--__-::: -::::-_.l.-_-:!-::_---"_~ u ~ompositioll llotche~ <) SO 100 ISO 200 lI. e_g_. anu c.ertain irnpul"ities, and ~1i9 -280 -IBO IOO"F stress ~()nc.entl"ations, and the high speed of loa<l ap­ rFMPEJlATlJi=lF plication. Although apparently quite diffel'ent, thesC'Downloaded from http://onepetro.org/jcpt/article-pdf/8/01/35/2166420/petsoc-69-01-06.pdf by guest on 30 September 2021 Figure J.-Low-Telll]JcrQturc BrittleHel>·.<; in PlIl'e Troll. fac.:turs have the common effect of hindering the duc­ tile deformatiun of steel (slipl. and lead to a prefer­ ence fol' cleavage fractun~ and brittle behaviour. On an atomic scale, dislocation mo\'ement."l al'e inhibited. Taken together, theSe fadors opel'ating in ordinary steels can raise the tempel"atul'es at \...·hich brittleness is encountered to the levels of atmospheric \Iariation. rt might he argued that thi!:'i phenomenon is not of great importance, because. if the design is good, the yield point !:'ihould not be exeeNled. Unfortunately, it i~ possible to have lo\\"·stres~ britlle failure. 01" [ail· ure well below the nominal yield stress. C1'ack P,"opagafiml Once a brittle cratk is nlIIning, iL requires very little energy to keep it going. There is ample energy stored in a pipeline under high pressure to result in a Fi.rJJtre 2.-F'ailurc in a fLigh-p1·cssun gas pipeline. failure several miles long. as shown in Fi!Jlf,re 2_ Simi­ larly, the residual stresses and service stre8ses in a ship can re­ sult in complete frHcture. breaking it in half. A brittle crack can extend at high speed, in Lhe order of a mile per second. verr often faster than the speed at which the shess which sllstains it ("an relax. In a pipeline, (a) Fracture on ship plate 0.339 in. thick. the cracking speed is faster than the speed of sound in natural gas; that is, fa8tel' than the speed at which a prl~SSUl"e wave can travel in the gas, ~md t.herefore faster than the speed at which the pres­ sure can relax, The speed of sound in oil or in water is much faster than in ail', and if failure occurs when testing with liquid media, the cracks do not travel as far. 'When a brittle crack occurs, it lb) Fracture on ship plate 0.91 ih_ thick_ leaveg behind a trail on the frnc­ hIred 1;urfa,~e, called chevron mark­ ings. as shown in Fir/lire ,9. The apices of the V-shaped marldngs point towal'd the origin of the fail. ure_ This is very cOllvenient for analyzing the sequence of long or complex failures, and in particular f(Jl" locating the probable origin. (c) Fracture produced in laboratory on plate 0.532 in_ thick. Apart from catastrophic failures of the type mentioned, it must be 36 The Journal of Canadian Petroleum obvious that short failures of exactly the same type thought die hard, especially when the alternative is to may' occur which may seriously affect the working spend more money. Not unnaturally, the most strik~ operation of a crane, derrick or other steel fabrication. ing advances have been made in the military and spac·e fields. For rocket casings and nuclear submarine hulls, critical crack sizes are worked out for acceptable ma­ terials. The very searching non-destructive test meth­ C1'ack Initiatian ods are aimed at eliminating flaws excee'ding the size specified for the desired safetJ.~ level. In most practical It is known that once a crack gets started, and work situations, on the other hand, we continue to reaches a critical size. a low stress is sufficient to work with outmoded design methods, with inferior keep it going. There still remains the question of how steels) with indiscriminate good and bad steels, with 'such cracks are initiated. There are several different irrelevant acceptance criteria, and with palliatives ways. Cracks may be present already as the result of rather than cures. faulty welding or mis-handling. Fatigue cracks may Slowly, but inevitably, a change is coming. The rate occur at notches because of repeated or reversed load­ at which it can occur depends more upon the diffusion ing_ However, these extraneous causes are not strictly of widespread knowledge among all those concerned, necessar}~. A notch may be present which raises the and upon the ultimate economic realities, than upon 10caU)~ Downloaded from http://onepetro.org/jcpt/article-pdf/8/01/35/2166420/petsoc-69-01-06.pdf by guest on 30 September 2021 stress above the yield stress) so that a short any further technical advances. crack may be produced. In addition, the material in the vicinit}~ ma}~ have been previously degraded by There is still not enough quality control of steels straining or strain-ageing. so that its ductility is de­ intended for low-temperature service. A suitable and pleted or exhausted. common test for this purpose is the Charpy-V test. The best way of using the test is to make a number Some actual instances of brittle failure origins may of tests at different temperatures, and then plot a be helpful in illustrating the foregoing remarks.
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