Saggistica Aracne 266

Transcription

1 Saggistica Aracne 266

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3 Moreno Preto Welding Defects

4 Copyright MMXIII ARACNE editrice S.r.l. via Raffaele Garofalo, 133/A B Roma (06) ISBN I diritti di traduzione, di memorizzazione elettronica, di riproduzione e di adattamento anche parziale, con qualsiasi mezzo, sono riservati per tutti i Paesi. Non sono assolutamente consentite le fotocopie senza il permesso scritto dell Editore. I edizione: marzo 2013

5 Index Preface Introduction to Welding 1 Explanatory comments.3 Chamfer...4 Welding processes.6 Cracking 7 Hot cracks 8 Solidification cracks..8 Shrinkage cracks 8 Crater cracks 10 Liquation cracks.17 Cold cracks...20 Hydrogen cracks..20 Brittle crack..26 Fatigue cracks..32 Heat treatment cracks.38 Lamellar tearing..41 Cavity 45 Gas cavity.. 45 Pore 45 Uniformly distributed porosity..46 Cluster of porosity...46 Linear porosity.47 Elongated cavity..47 Worm-hole.48 Surface pore..48

6 Shrinkage cavity 55 Interdendritic shrinkage.55 Crater cavity.57 Final shrinkage cavity 60 Cavity visible only under the microscope...64 Solid inclusions 65 Slag inclusion...65 Flux inclusion...67 Oxide inclusion.67 Oxide film..67 Metallic inclusions..68 Lack of fusion and penetration 73 Lack of fusion 73 Lack of sidewall fusion...73 Lack of fusion between passes..73 Lack of root fusion...74 Lack of penetration..79 Lack of root penetration.79 Shape and dimensional defects 84 Continuous undercut...84 Intermittent undercut..89 Root undercut...89 Undercut between passes...92 Localized undercut..94 Excess weld metal 96 Incorrect weld toe..100 Excessive convexity Excessive penetration...104

7 Localized excessive penetration.104 Continuous excessive penetration..104 Melt through Overlap 108 Overlap of the last pass 108 Overlap of the root pass Misalignment..112 Misalignment between plates..112 Misalignment between pipes 112 Angular misalignment..115 Sagging 117 Sagging in horizontal position 117 Sagging in flat or overhead position.118 Sagging in a fillet weld.118 Sagging at the edge of the weld..119 Burn-through..122 Incompletely filled groove 124 Excessive asymmetry of a fillet weld.126 Irregular width Irregular surface 130 Root concavity 133 Poor restart.135 Excessive weld thickness..137 Excessive weld width 137 Insufficient throat height..138 Excessive throat height.138 Other imperfections 141 Stray arc..141 Spatter..144 Tungsten spatter.147 Torn surface 147 Grinding mark 151 Chipping mark 152

8 Excessive grinding.152 Footnote index 155

9 Preface The publication of this work has the objective to provide at all interested parties, a simple but effective tool, useful to the identification and assessment of weld defects. The work is therefore directed primarily to the technicians who carry out their tasks in the production offices and technical departments, the technical workshop and those in charge of inspecting the welded structures. The setting purely functional and simplified volume has the purpose of facilitating the reader in identifying and understanding of weld defects, without getting too in depth which would render the consultation little dynamic. In particular, after a general introduction on the subject of welding, will be analyzed and categorized many welding defects, followed by images, drawings and technical evaluations result of years of experience of specialists in the world of welded structures, metallic and non-metallic. Concluding, I must thank my colleagues inspectors who have contributed with great professionalism to the realization of this work. Good job! I. W. I. Moreno Preto

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11 Welding defects 1 Welding Welding is process that enables solid parts to be connected permanently to one another and produces continuity of the material. In its most common definition, welding requires the input of sufficient localized heat to cause fusion of the material. This fusion can be limited to only the material of which the parts to be joined are composed, or a different material, called filler material, can be used. When the filler material fuses together with the base material this is known as autogenous welding, while if the filler material does not fuse with the base material this is known as heterogeneous or brazing. Welding creates a permanent connection that differs from other permanent connections (such as riveting or bonding) which do not create continuity of the material. If performed correctly and according to certain principles, some autogenous welding processes also guarantee almost total continuity in the properties of the material of the welded parts. Welds represent the most important unknown factor in the assembly of metal structures, as the stresses that a welded joint will be capable of sustaining cannot be defined in advance, except empirically, i.e. by means of destructive testing. Therefore, from the design viewpoint, a welded joint is usually considered as a homogeneous joint, where high coefficients of safety are used, obtained empirically through destructive tests. Moreover, constructors undertake to produce the joints in the original conditions in which the empirical data was obtained. For this purpose, W.P.S.* are prepared, establishing the welding variables within narrow limits with which the operator must comply in order to make the weld as similar as possible to that of the test coupon and thus give meaning to the design calculations; therefore, failure to comply with even only one of the parameters of the W.P.S. can lead to a welding defect, in the broad sense of the term, as this affects theoretical performance. In any case, weld defects are normally intended as physical imperfections, that reduce the values of strength (*) W.P.S.: Welding Procedure Specification.

12 2 Welding defects and tenacity of the welded joint, as they are possible crack initiation points and amply forces that in the more or less long term cause the weld to fracture. The fundamental aspects that affect the final properties of a welded joint are: Operating imperfections: these are due to the welder s practical experience in relation to the type of joint to be welded and to its position. Metallurgical imperfections: these are due to the metallurgy of the weld and therefore depend on the welding parameters. Mechanical properties: the weld joint must always ensure specific mechanical properties.

13 Welding defects 3 Explanatory comments To make the cataloguing of defects more coherent with the world of standards, we have taken the standard UNI EN ISO Classification of geometric imperfections in metallic materials as a reference.

14 4 Welding defects Chamfer Edge Opening Edge Root face Root gap Structure of a welded joint Fusion limit zone Fusion zone Base material HAZ FZ = Fusion Zone HAZ = Heat Affected Zone BM= Base Material

15 Welding defects 5 Joint types: Butt joint Ripples Bead Subsequent passes First pass Fillet joint z a a: Throat height - z: Bead side

16 6 Welding defects Welding processes As stated above, welding processes are divided mainly into two macro groups, namely autogenous welding processes and heterogeneous welding processes. Autogenous welds are those in which the base metal fuses and takes part in forming the joint. Heterogeneous welds are those in which the base metal does not fuse and take part in forming the joint. Autogenous welding procedures include electric arc, flame, laser, electron beam, etc., while for heterogeneous welding procedures include soldering, braze welding, explosion welding, compression welding, etc. In this paper we shall be dealing mainly with defects encountered in welds performed with the following procedures: The most common abbreviations for European standard. - TIG Tungsten Inert Gas - MIG Metal Inert Gas - MAG Metal Active Gas - MMA Metal Manual Arc (Shielded electrodes) - SAW Submerged Arc Welding The most common abbreviations for American standard: - GTAW Gas tungsten arc welding - GMAW Gas metal arc welding - SMAW Shielded metal arc welding - SAW Submerged arc welding

17 Welding defects 7 CRACKING Description: Cracks are the most dangerous defects that can be encountered in a welded joint, as in the long term they can lead to failure of the joint. Cracks can be defined as imperfections caused by local fracture in solid state as a result of stresses; these are indicated as twodimensional defects with irregular trend. Intergranular or transgranular cracks can develop, depending on whether the fracture is along the grains or passes through the grains. Depending on their position with respect to the weld, cracks can be categorized as fusion zone (FZ) cracks, heat affected zone (HAZ) cracks and base metal (BM) cracks. Micro-cracks These are cracks only visible under the microscope (50X). Welding cracks These form during or after welding. During the cooling phases the joint passes through temperature ranges in which situations of brittleness can occur. In these ranges, hot cracks can develop from the time of solidification of the metal up to around 950 C, while cold cracks can develop from around 150 C up to ambient temperature (this brittle phase can continue even for several hours after complete cooling of the welded joint) (fig. 1).

18 8 Welding defects C Solidification temperature Hot cracks 950 C Cold cracks 150 C Fig. 1 Graph showing the brittleness ranges Time Hot cracks Solidification cracks Shrinkage cracks These form as a result of excessive amounts of carbon, sulphur and phosphorous in the molten pool. These elements form compounds (segregation*) which delay solidification and are usually concentrated in the central part of the weld, which is the part that solidifies last, and are intergranular in nature. The residual stresses that start to occur already in the initial stages of solidification can create openings in which there are parts that have still not solidified, which are accentuated as cooling continues. This type of defect is often found in aluminium alloys due to some alloying elements such as copper, zinc and manganese, which have a higher solidification time. (*) Segregations: Inhomogeneity of composition in the metal alloy during solidification.

19 Welding defects 9 Other factors that influence this type of cracks are: The shape of the bead: very narrow and deep beads facilitate the concentration of impurities in the centre of the weld during fusion; moreover, due to their geometry, very concave beads facilitate the concentration of stresses on the surface, which can give rise to cracks. Heat input A high heat input causes a decrease in the cooling speed of the fusion zone, thereby increasing the probability of cracks forming. Preparing the faces Preparation of the joint with too great a distance between faces promotes the formation of hot cracks caused by an increase in the welding surface and an increase in solidification times. This situation leads to an increase in transverse shrinkage stresses. Fillet joints are particularly sensitive to this type of problem, as excessive distance between faces facilitates an overall reduction in the strength of the joint. Degree of restraint of the joint The greater the restraints to which a joint is subjected during welding are, the greater the stresses that are created on it will be; consequently the possibility of hot cracks forming is also greater. As stated above, hot cracks mainly develop between the grains of the metal and can run longitudinally or at times transverse (fig. 2) to the weld bead but only on the inside thereof.

20 10 Welding defects Crater cracks These are a kind of hot cracks and form on the end part of the bead due to the progressive concentration of impurities in the final part of the weld pool and to very high shrinkage stresses. According to their position in relation to the weld bead, crater cracks can be divided into: Crater cracks Cracks that can appear in the crater after a weld. (fig. 3). Longitudinal to the bead Transverse to the bead Star shaped