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Technology|Common defects in pressure vessel welding and prevention measures

Industry news
2018/09/05 10:22
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JiangsuXinfangyuanElectricCo.,Ltd.Withtheadvancementofindustrialmodernization,pressurevesselshavebeenwidelyusedinthenationaleconomysuchaspetrochemicalindustry.Theweldingqualityofthepressurevesseliscloselyrelatedtothestrength,compactnessandservicelifeofthepressurevessel.Atthesametime,thereareoftensomedefectsintheweldingofpressurevessels,whichhaveahugeimpactonthestabilityandworkoftheweldedcontainers.Therefore,itisofgreatpracticalsignificancetostudyanddiscussthecommondefectsandpreventionmeasuresofpressurevesselwelding.Thisarticlewillexplorethisindetail. Withtheadvancementofindustrialmodernization,pressurevesselshavebeenwidelyusedinthenationaleconomysuchaspetrochemicalindustry.Theweldingqualityofthepressurevesseliscloselyrelatedtothestrength,compactnessandservicelifeofthepressurevessel.Atthesametime,thereareoftensomedefectsintheweldingofpressurevessels,whichhaveahugeimpactonthestabilityandworkoftheweldedcontainers.Therefore,itisofgreatpracticalsignificancetostudyanddiscussthecommondefectsandpreventionmeasuresofpressurevesselwelding.Thisarticlewillexplorethisindetail.1.ThesignificanceofcontrollingtheweldingdefectsofpressurevesselsApressurevesselisacloseddevicethatholdsaliquidorgasandcarriesacertainpressure.Thepressurevesselincludesastoragecontainer,areactioncontainer,aheatexchangecontainer,aseparationcontainer,andthelike.Pressurevesselsaremainlyusedinthenationaleconomiclifeofthepetrochemicalindustry,energyindustry,materialstorageandtransportation,scientificresearch,andmedicalcare.Morethan30%ofthetotalworkloadofthemanufactureofthepressurevesselisrelatedtotheweldingwork.Atthesametime,studieshaveshownthat90%ofaccidentsinpressurevesselsarerelatedtoweldingdefects.Therefore,controllingtheweldingqualityistheguaranteeofthecompactnessandstrengthofthepressurevessel,whichhelpsthepressurevesseltoworknormally,safelyandimprovetheservicelife,andtoprotectpeople'slivesandproperty.2.CommonweldingdefectsandcausesofpressurevesselsDependingonthelocationofthedefectdistribution,thewelddefectsofthepressurevesselcanbedividedintoexternaldefectsandinternaldefects,whereinexternaldefectscanusuallybefoundbythenakedeye.Commonexternaldefectsinweldingincludecraters,welds,weldshapeandsizedefects,surfacesplashes,undercuts,andthelike.Internaldefectsincludecracks,slaginclusions,pores,incompletepenetration,unfused,etc.Amongthem,cracksarethemostdeadlydefectsaffectingpressurevesselsandarethekeyitemsthatmustbetested.2.1UndercutUndercutreferstothedepressionoftheedgeoftheweld.Undercutsareusuallycausedbyimproperselectionofweldingprocessparametersorimproperoperation.Themainreasonsfortheundercutare:improperoperationmethod,incorrectweldingspecification,suchasexcessivearclength,improperoperationmodeandangle,excessivecurrent,andinappropriateresidencetimeonbothsidesofthegroove.Throughexperimentalresearch,itisfoundthatthedistancebetweenthecenterofthewireoffsetwillcausetheresidencetimeofthemoltenpooltoincreaseandtheundercutwilloccur.Atthesametime,theweldingspeedwillbetoofast,andthecraterwillnotbefilledafterthearcisclosed.2.2stomataTheporesmainlymeanthatthebubblesinthemoltenpoolarenotreleasedintimebeforesolidificationoccurs,butremaininthegapoftheweld,andfinallytheformedvoids.Thegenerationofporesaffectsthedensificationoftheweldmetal.Therootcauseofthestomataiscausedbytheoutsideairorthegasgeneratedduringtheweldingprocessenteringthemoltenpoolandnotescapingintimebeforethemoltenpoolsolidifies.Thecausesoftheporesare:theedgeofthegrooveisnotclean,thereisrust,etc.;theelectrodeorfluxisnotbakedaccordingtotheoperationregulations;thebubblecannotpassthroughtheslag;theweldingspeedistoofast;theweldingvoltageistoohigh;theweldingenvironmentisrelativelyhumid.2.3slaginclusionTheformationofslagismainlyduetosomeslagremainingintheweld.Thepresenceofslaginclusionsisamajorhazardandgreatlyreducesthestrengthandcompactnessoftheweld.Themaincausesofslaginclusionare:aerobiccuttingattheedgeoftheweldorslagremaininginthecarbonarcbubble;thespeedistoofastduringwelding;thegrooveangleselectionduringweldingisunreasonable;thecurrentisinsufficientduringwelding.Inaddition,whenabasicelectrodeisselected,theremaybeadefectofslaginclusionduetoanabnormalpolarityoranexcessivearc.2.4Notwelded,notfusedUnfusedandunfusedarerelativelyseriousweldingdefects,whichwillreducethestrengthoftheweld,andevenleadtocracks,resultinginaccidents.Amongthem,inthewelding,whenthejointrootisnotcompletelypenetrated,therewillbenopenetration,andwhentheweldandtheweldmentorbetweentheweldandtheweldmetalarenotdissolved,somewillappear.Unfusedphenomenon.Thecausesoftheseproblemsareusuallycausedbyexcessivearc,toolargediameteroftheelectrode,toosmallcurrent,oxidizedandoilysurfaceofthegroove,insufficientweldfittingclearanceorgrooveangle,andincompletecleaningofthebottomseal.2.5WeldedtumorDuringtheweldingprocess,themetaloverflowsontotheinsufficientlyheatedbasemetalorthefrontweld.Whenthemetalsolidifies,thetinyflawsformedbythemetalunderitsowngravityaretheweldbead.Theweldcan'tbefusedwiththebasemetalorthepreviousweld.Iftheweldislocatedinside,itwillreducethes

Jiangsu Xinfangyuan Electric Co., Ltd.

With the advancement of industrial modernization, pressure vessels have been widely used in the national economy such as petrochemical industry. The welding quality of the pressure vessel is closely related to the strength, compactness and service life of the pressure vessel. At the same time, there are often some defects in the welding of pressure vessels, which have a huge impact on the stability and work of the welded containers. Therefore, it is of great practical significance to study and discuss the common defects and prevention measures of pressure vessel welding. This article will explore this in detail.
With the advancement of industrial modernization, pressure vessels have been widely used in the national economy such as petrochemical industry. The welding quality of the pressure vessel is closely related to the strength, compactness and service life of the pressure vessel. At the same time, there are often some defects in the welding of pressure vessels, which have a huge impact on the stability and work of the welded containers. Therefore, it is of great practical significance to study and discuss the common defects and prevention measures of pressure vessel welding. This article will explore this in detail.
1. The significance of controlling the welding defects of pressure vessels
A pressure vessel is a closed device that holds a liquid or gas and carries a certain pressure. The pressure vessel includes a storage container, a reaction container, a heat exchange container, a separation container, and the like. Pressure vessels are mainly used in the national economic life of the petrochemical industry, energy industry, material storage and transportation, scientific research, and medical care. More than 30% of the total workload of the manufacture of the pressure vessel is related to the welding work. At the same time, studies have shown that 90% of accidents in pressure vessels are related to welding defects. Therefore, controlling the welding quality is the guarantee of the compactness and strength of the pressure vessel, which helps the pressure vessel to work normally, safely and improve the service life, and to protect people's lives and property.
2. Common welding defects and causes of pressure vessels
Depending on the location of the defect distribution, the weld defects of the pressure vessel can be divided into external defects and internal defects, wherein external defects can usually be found by the naked eye. Common external defects in welding include craters, welds, weld shape and size defects, surface splashes, undercuts, and the like. Internal defects include cracks, slag inclusions, pores, incomplete penetration, unfused, etc. Among them, cracks are the most deadly defects affecting pressure vessels and are the key items that must be tested.
2.1 Undercut
Undercut refers to the depression of the edge of the weld. Undercuts are usually caused by improper selection of welding process parameters or improper operation. The main reasons for the undercut are: improper operation method, incorrect welding specification, such as excessive arc length, improper operation mode and angle, excessive current, and inappropriate residence time on both sides of the groove. Through experimental research, it is found that the distance between the center of the wire offset will cause the residence time of the molten pool to increase and the undercut will occur. At the same time, the welding speed will be too fast, and the crater will not be filled after the arc is closed.
2.2 stomata
The pores mainly mean that the bubbles in the molten pool are not released in time before solidification occurs, but remain in the gap of the weld, and finally the formed voids. The generation of pores affects the densification of the weld metal. The root cause of the stomata is caused by the outside air or the gas generated during the welding process entering the molten pool and not escaping in time before the molten pool solidifies. The causes of the pores are: the edge of the groove is not clean, there is rust, etc.; the electrode or flux is not baked according to the operation regulations; the bubble cannot pass through the slag; the welding speed is too fast; the welding voltage is too high; the welding environment is relatively humid.
2.3 slag inclusion
The formation of slag is mainly due to some slag remaining in the weld. The presence of slag inclusions is a major hazard and greatly reduces the strength and compactness of the weld. The main causes of slag inclusion are: aerobic cutting at the edge of the weld or slag remaining in the carbon arc bubble; the speed is too fast during welding; the groove angle selection during welding is unreasonable; the current is insufficient during welding. In addition, when a basic electrode is selected, there may be a defect of slag inclusion due to an abnormal polarity or an excessive arc.
2.4 Not welded, not fused
Unfused and unfused are relatively serious welding defects, which will reduce the strength of the weld, and even lead to cracks, resulting in accidents. Among them, in the welding, when the joint root is not completely penetrated, there will be no penetration, and when the weld and the weldment or between the weld and the weld metal are not dissolved, some will appear. Unfused phenomenon. The causes of these problems are usually caused by excessive arc, too large diameter of the electrode, too small current, oxidized and oily surface of the groove, insufficient weld fitting clearance or groove angle, and incomplete cleaning of the bottom seal.
2.5 Welded tumor
During the welding process, the metal overflows onto the insufficiently heated base metal or the front weld. When the metal solidifies, the tiny flaws formed by the metal under its own gravity are the weld bead. The weld can't be fused with the base metal or the previous weld. If the weld is located inside, it will reduce the strength, reduce the effective cross-sectional area, and affect the appearance. The main cause of the weld is that the temperature of the molten pool is too high due to uneven transport, and the liquid metal slowly falls and then forms a weld on the surface of the weld. When standing and over-welding, the welding current may be too large if the welding current is too large and the arc is too long.
2.6 crack
Crack refers to the joint action of welding stress and other brittle factors, so that the atomic structure of the metal material is destroyed to form a new interface, thereby creating a gap. Welding cracks in pressure vessels are a highly hazardous defect. Most of the damage to the pressure vessel structure begins at the crack. Cracks are classified into hot cracks and cold cracks.
Cracks generated during the crystallization of weld metal from liquid to solid are called hot cracks. Hot cracks often appear in the weld. The characteristics of hot cracks are visible immediately after welding and are mostly located at the center of the weld. The cause of the hot crack is the low melting point impurities such as FeS in the weld pool. In general, such impurities are mostly long solidification time, low melting point, and low strength after solidification. Therefore, when the restraining stress applied to the surface by the outside is sufficiently large, and the contraction force generated when the weld metal solidifies, it is liable to be pulled apart or pulled apart to form a crack shortly after solidification.
The formation of the cold crack is a crack generated outside the weld line of the weld joint and the base material during the cooling of the weld metal, or after the cooling, or may be a crack generated at the weld line of the base metal. Most of the locations where they occur are concentrated in locations where stress is concentrated, such as undercuts and roots. The temperature at which cold cracking occurs is generally in the range of martensite transformation temperature. Cold cracks are extremely harmful and generally occur a few hours or more after welding.
There are three main reasons for the occurrence of cold cracks:
(1) The joint is subjected to a large restraint stress;
(2) due to the heat cycle of the weld, a hardened structure is formed in the heat affected zone;
(3) Some excess diffused hydrogen accumulates at the weld and a constant concentration occurs.
The crack generated at the weld line where the base metal or base metal meets the weld seam during the cooling process or after cooling is called cold crack. The temperature at which cold cracks are formed is usually in the range. Cold cracks occur mostly at locations where stresses such as roots and undercuts concentrate. Cold cracks can occur either immediately after welding or a few hours or more after welding. Cold cracks are more harmful to pressure vessels than hot cracks.