1. IMPERFECTIONS OF THE WELDED CONNECTION

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1 1. IMPERFECTIONS OF THE WELDED CONNECTION A. Classification of imperfections in the welds acc. to EN (ISO 6520) Crack (100) - imperfection produced by a local rupture in the solid state which can arise from the effect of cooling or stresses. Cracks can arise as a hot crack in temperature range C. Reasons: large level of carbon C and other impurity (S and P); large welding speed; high internal stress level caused a large stiffnes of the structure. Cold cracks arise in temperature under 300 C, caused mainly by cold shortness i. e. hydrogen shortness. Lameral cracks arise mainly on as a result of losing its shape metal sheets in the thickness direction, at low her ductilities in this direction. Group No.1 Cracks 1001 microcrack 101 Ea 102 Eb 103 E 104 Ec 105 E 106 E longitudinal crack transverse crack radiating cracks crater crack group of disconnected cracks branching crack crack visible only under the microscope crack essentially parallel to the axis of the weld It can be situated: in the weld metal, in the weld junction 1013 in the heat affected zone 1014 in the parent metal crack essentially transverse to the axis of the weld It can be situated: in the weld metal, 1023 in the heat affected zone 1024 in the parent metal cracks radiating from a common point It can be situated: in the weld metal, 1033 in the heat affected zone 1034 in the parent metal crack in the crater at the end of a weld group of disconnected cracks in any direction group of connected cracks originating from a common crack -1-

2 Cavities (200) free space in weld material formed by entrapped gas. They are created in the result of gas making of metallurgical reactions. The influence on the creation of cavities have: moisture of the coating in alkaline electrodes; polluting welded edges (rust, paint, fat); too high rate of speed of cooling of the joint (small amperage, too high rate of speed of the welding). Group No. 2 Cavities 201 Aa gas cavity cavity formed by entrapped gas gas pore gas cavity of essentially spherical form 2015 Ab elongated cavity large, non-spherical cavity with its major dimension approximately parallel to the axis of the weld 2016 Ab 2012 worm-hole uniformly distributed porosity tubular cavity in weld metal caused by release of gas. The shape and position of worm-holes are determined by the mode of solidification and the sources of the gas. number of gas pores distributed in a substantially uniform manner throughout the weld metal; not to be confused with linear porosity (2014) and clustered porosity (2013) 2013 Ad clustered (localized) porosity group of gas pores having a random geometric distribution 2014 linear porosity row of gas pores situated parallel to the axis of the weld 2017 surface pore 2025 K end crater pipe gas pore that breaks the surface of the weld open crater with a hole reducing the cross-section of the weld -2-

3 Solid inclusion solid foreign substances entrapped in the weld metal: slag, flux or foreign metal. Influence on the creation of the slag inclusion have: infusible and with difficulty removable slag; inaccurate cleaning individual layers of the weld; too low linear energy at welding. Group No. 3 Solid inclusions 301 Ba 302 G 303 J slag inclusion flux inclusion oxide inclusion 3034 puckering 304 H metallic inclusion solid inclusion in the form of slag Slag inclusions can be: linear isolated 3013 clustered. solid inclusion in the form of flux Flux inclusions can be: linear isolated 3023 clustered. solid inclusion in the form of metallic oxide Oxide inclusions can be: linear isolated 3033 clustered. in certain cases, especially in aluminium alloys, gross oxide film enfoldment can occur due to a combination of unsatisfactory protection from atmospheric contamination and turbulence in the weld pool solid inclusion in the form of foreign metal Metallic inclusions can be: 3041 tungsten, 3042 copper, 3043 other metal. -3-

4 Lack of fusion and penetration - lack of union between the weld metal and the parent material or between the successive layers of weld metal. Influence on the lack of fusion have: too low linear energy at welding; wrong technique of welding (e.g. one-way lowering the electrode); presence of difficult-fusible oxides at joined surfaces. Group No. 4 Lack of fusion and penetration 401 Ba lack of fusion lack of union between the weld metal and the parent material or between the successive layers of weld metal It can be one of the following: 4011 lack of side-wall fusion; 4012 lack of inter-run fusion (also referred to as cold laps ), lack of root fusion, 4014 micro-lack of fusion. 402 G incomplete penetration (lack of penetration) difference between the actual and the nominal penetration 4021 incomplete root penetration; (one or both fusion faces of the root are not melted) 403 spiking extremely non-uniform penetration occurring in electron-beam and laser welding giving a sawtooth appearance. This can include cavities, cracks, shrinkages, etc. -4-

5 Imperfect shape and dimensions imperfect shape of the external surfaces of the weld or defective joint geometry. Causes of the uprising of such defects are different, out of most frequent it is possible to exchange: Inability to weld; Welding in compulsory positions; Wrong parameters of the welding (amperage, speed of the welding). Group No. 5 Imperfect shape and dimensions 501 F 502 undercut excess weld metal irregular groove at a toe of a run in the parent material or in previously deposited weld metal 5011 continuous undercut; intermittent undercut, shrinkage grooves, 5014 inter-run undercut, local intermittent undercut. reinforcement of the butt weld on the face is too large excessive convexity excessive penetration incorrect weld toe 506 overlap 507 linear misalignment 510 burn-through 512 excessive asymmetry of fillet weld reinforcement of the fillet is too large reinforcement of the butt weld on the root side is too large 5041 local excessive penetration; continuous excessive penetration, excessive melt-through. too small an angle (α) between parent material and weld surface 5051 incorrect weld toe angle; incorrect weld toe radius. excessive weld metal covering the parent material surface but not fused to it 5061 toe overlap; root overlap. misalignment between two welded pieces such that they are not in the same required parallel plane, even though their surface planes are parallel collapse of the weld pool resulting in a hole in the weld Excessive unequal leg length explanation not necessary -5-

6 Miscellaneous imperfections all imperfections which cannot be included in groups 1 to 5. Group No. 6 Miscellaneous imperfections 601 arc strike stray arc local damage to the surface of the parent material adjacent to the weld, resulting from arcing or striking the arc outside the joint preparation 602 spatter globules of weld metal or filler metal expelled during welding and adhering to the surface of parent material or solidified weld metal tungsten spatter. 603 torn surface surface damage due to the removal by fracture of temporary welded attachments 604 grinding mark local damage due to grinding 605 chipping mark local damage due to use of a chisel or other tools 606 underflushing reduction in the thickness of the tack weld imperfection misalignment of opposite runs temper colours (visible oxide film) 613 scaled surface 614 flux residue 615 slag residue 617 incorrect root gap for fillet welds 618 swelling workpiece due to excessive grinding imperfection resulting from defective tack welding, e.g broken run or no penetration, defective tack has been overwelded. difference between the centrelines of two runs made from opposite sides of the joint lightly oxidized surface in the weld zone, e.g. in stainless steels 6101 discolouration. visibly tinted surface layers in the weld metal and heataffected zone caused by the weld heat and/or by lack of protection, heavily oxidized surface in the weld zone flux residue that is not sufficiently removed from the surface adherent slag that is not sufficiently removed from the surface of the weld excessive or insufficient gap between the parts to be joined imperfection due to a burning on welded joints in light alloys resulting from a prolonged holding time in the solidification stage -6-

7 Metal structures Laboratory 2. NON-DESTRUCTIVE TESTING METHODS OF WELD JOINTS A. Visual inspection Visual inspection is probably the most underrated, and often misused, method of welding inspection. Because of its simplicity, and the absence of sophisticated equipment, the potential of this method of inspection is quite often underestimated. The control consists in the visual observation and measurements of joints. Visual inspection can be carried out as pre-weld inspection, inspection during welding or post-weld inspection. They enclose: Inspection, or all welds were carried out and correctly situated; Visual inspection weld surface and shape; Measurement of the thickness and lengths of the welds; Detecting surface defects (of e.g. undercuts, spatters or chips). With visual inspection of the weld surface is possible to detect: Surface and shape defects (e.g wrong dimensions of the welds, undercuts, toe defects, craters or swellings); Lack of fusion and excessive penetrations one side butt welds; Cracks in the weld material or in the heat affected zone detecting of such defects is possible to confirm by inspection of the effective segment through the magnifying glass or with penetrative surveys. For checking dimensions of fillet welds is applying weld gauge (special calipers). Weld gauge simplified measurement of thickness of the fillet welds or heights of the spure of butt welds. To visual inspection internal surfaces of the pipes or small tanks (eg. Penetration quality), the endoscope can be used. -7

8 B. Radiographic Testing (RT) Method of inspecting materials for hidden flaws by using the ability of short wavelength electromagnetic radiation (high energy photons) to penetrate various materials. Either an X-ray machine or a radioactive source, like Ir-192, Co-60, or in rarer cases Cs-137 are used in a X-ray computed tomography machine as a source of photons. Neutron radiographic testing (NR) is a variant of radiographic testing which uses neutrons instead of photons to penetrate materials. This can see very different things from X-rays, because neutrons can pass with ease through lead and steel but are stopped by plastics, water and oils. Since the amount of radiation emerging from the opposite side of the material can be detected and measured, variations in this amount (or intensity) of radiation are used to determine thickness or composition of material. Penetrating radiations are those restricted to that part of the electromagnetic spectrum of wavelength less than about 10 nanometres. Advantages of the radiographic method: To conduct the evaluation of the defectiveness of welds it is possible with the following methods: Through comparing the radiogram of the weld with model radiograms included in special catalogues; Through the direct evaluation of the size of defects (in mm) and of increasing them (number of defects or their summary length on the chosen stretch); With establishing the so-called general defectiveness expressed in % of loss of a cross section of the weld as a result of defects; Based on the description stipulated in the contract of defects; The method allows for the detection of internal defects; The film can be recorded and store to confirmation of weld quality. The defects detected with radiographic method. description cross-section radiogram description cross-section radiogram worm hole lack of inter run fusion linear slag inclusion longitudinal crack gas pore porosity (linear) lack of root fusion traverse crack radiating crack -8-

9 C. Ultrasonic testing (UT) Ultrasonic testing (UT) is a testing techniques based on the propagation of ultrasonic waves in the object or material tested. In most common UT applications, very short ultrasonic pulse-waves with center frequencies ranging from MHz, and occasionally up to 50 MHz, are transmitted into materials to detect internal flaws or to characterize materials. In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing. However, when ultrasonic testing is conducted with an Electromagnetic Acoustic Transducer (EMAT) the use of couplant is not required. There are two methods of receiving the ultrasound waveform: reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the "sound" is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. The diagnostic machine displays these results in the form of a signal with an amplitude representing the intensity of the reflection and the distance, representing the arrival time of the reflection. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence. Using the couplant increases the efficiency of the process by reducing the losses in the ultrasonic wave energy due to separation between the surfaces. Advantages ultrasonic method: High penetrating power, which allows the detection of flaws deep in the part; High sensitivity, permitting the detection of extremely small flaws; Some capability of estimating the size, orientation, shape and nature of defects; Non hazardous to operations or to nearby personnel and has no effect on equipment and materials in the vicinity. Disadvantages ultrasonic method: Manual operation requires careful attention by experienced technicians. The transducers alert to both normal structure of some materials, tolerable anomalies of other specimens (both termed noise ) -9-