Causes and preventive measures of stainless steel welded pipe defects

- Apr 01, 2019-

Stainless steel welded pipe in the welding process will appear some defects, stainless steel welded pipe defects will lead to stress concentration, reduce the bearing capacity, shorten the service life, or even cause brittle fracture.General technical regulations stipulate that cracks, incomplete welding, incomplete fusion and surface slag inclusion are not allowed;Defects such as undercut, internal slag inclusion and porosity cannot exceed a certain allowable value, and defects exceeding the standard must be thoroughly removed and welded.The causes, hazards and preventive measures of welding defects in common stainless steel welded pipes are summarized as follows.

Stainless steel welded pipe

 I. the welding seam size does not meet the requirements

The size of the welding seam does not meet the requirements mainly refers to the weld residual height and height difference, weld width and width difference, the amount of the wrong side, after welding deformation does not meet the standard requirements of the size, the weld uneven, uneven width, large deformation.Different widths of welds not only cause unaesthetic appearance of welds, but also affect the bonding strength of welds and base metal.Excessive weld residual leads to stress concentration, while lower than the base metal, insufficient joint strength can be obtained.When the wrong side and deformation are too large, the stress transfer distortion and stress concentration will occur, resulting in a decrease in strength.

Causes: improper groove Angle of stainless steel welded pipe or uneven edge and assembly gap;Unreasonable selection of welding process parameters;Welder's operation skill level is lower.

Preventive measures: select the appropriate groove Angle and assembly clearance;Improve assembly quality;Select suitable welding process parameters;Improve welder's operation skill level.

Second, the biting edge

Due to the incorrect selection of welding process parameters or incorrect operation process, the grooves or depressions formed by burning along the base material of the welding toe are called undercut.Undercut not only weakens the strength of welded joints, but also easily causes cracks due to stress concentration.

The reasons are: the current is too large, arc is too long, the Angle of the welding rod is not correct, the method of moving the rod is improper.

Preventive measures: welding electrode arc welding to choose the appropriate welding current and welding speed, arc can not be stretched too long, welding rod Angle to be appropriate, the correct method of moving.

Iii. Incomplete welding

Incomplete penetration refers to the phenomenon of incomplete penetration at the root of welded joint of stainless steel welded pipe during welding.Lack of penetration will cause stress concentration and crack.Important welded joints are not allowed to be incomplete.

Causes: too small groove Angle or clearance, too large blunt edge, poor assembly;Improper selection of welding process parameters, too small welding current, welding speed is too fast;Bad welder operation skill etc.

Preventive measures: correct selection and processing groove size, reasonable assembly, ensure the gap, choose the right welding current and welding speed, improve the welder's operating technology level.

Iv. Incomplete fusion

Incomplete fusion refers to the part that is not completely fused between weld bead and base metal or between weld bead and weld bead during fusion welding.The failure of fusion directly reduces the mechanical properties of the joint.

Causes: the welding speed of stainless steel welding pipe is fast and the welding current is small, the welding heat input is too low;Electrode eccentricity, improper Angle between electrode and welding parts, arc pointing deviation;There are rust and dirt on the side wall of groove and incomplete slag cleaning between layers.

    Stainless steel welded pipe      Preventive measures: correctly select welding process parameters, carefully operate, strengthen the cleaning between layers, improve the level of welder operation technology.

Five, the flash

Weld nodules refer to the metal nodules formed in the welding process when molten metal flows to the unmelted base metal beyond the weld.Welding nodules not only affect the weld formation of stainless steel welded pipe, but also slag inclusion and incomplete welding often exist in the part of welding nodules.

Causes: blunt edge is too small and root clearance is too large;High welding current and fast welding speed;Welder operation skill level is low.

Preventive measures: according to the different welding position to choose the appropriate welding process parameters, strictly control the size of the melting hole, improve the level of welder operation technology.

Six, arc pit

The sunken part at the end of welding seam of sanitary stainless steel welded pipe is called arc pit.The arc crater not only seriously weakens the strength of the weld, but also produces arc crater cracks due to the concentration of impurities.

The main reason is that the dwell time of arc extinguishing is too short;Too much current when sheet metal is welded.

Preventive measures: when arc welding electrode arc, the electrode should be in the melting pool for a little stay or as a circular conveyor, until the molten pool metal filled to the side of the extinguishing arc;In tig welding, sufficient dwell time should be allowed, and the arc extinguishing should be attenuated after filling the weld.

Seven, stomatal

When welding sanitary stainless steel welded pipe, the gas in the molten pool failed to escape during solidification and the holes formed by residual gas are called pores.Stoma is a common welding defect, which can be divided into inner stoma and outer stoma.Stomata are round, oval, wormlike, needle-like and dense.The porosity will not only affect the density of the weld, but also reduce the effective area of the weld and reduce the mechanical properties of the weld.

Causes: there are oil, rust, water and other dirt on the surface and groove of sanitary stainless steel welded pipe;When electrode arc welding, electrode coating is affected by damp and there is no drying before use;If the arc is too long or blown off, the molten pool will not be well protected and the air will invade the molten pool.If the welding current is too large, the electrode will become red and the coating will fall off in advance, losing its protective effect;Improper operation methods, such as too fast arc closing action, easy to produce shrinkage hole, joint arc starting action is not correct, easy to produce dense porosity.

Preventive measures: before welding, remove oil, rust and water in the range of 20~30mm on both sides of the groove;Bake strictly according to the temperature and time specified in the electrode specification;Correct selection of welding process parameters, correct operation;Short arc welding should be adopted as far as possible.It is not allowed to use the invalid electrode, such as the core corrosion, coating cracking, peeling, eccentricity is too large.

Inclusion and slag inclusion

Inclusions are nonmetallic inclusions and oxides left in weld metals by metallurgical reactions.Slag inclusion is residual slag in the weld.Stainless steel welding pipe slag can be divided into spot slag and strip slag.Slag inclusion weakens the effective section of weld, and thus reduces the mechanical properties of weld.Slag inclusion can also cause stress concentration, which is easy to damage the welded structure under load.Causes: slag cleaning between layers in the welding process is not clean;Welding current is too small;Welding speed is too fast;Improper operation during welding;Improper chemical composition match between welding material and base material;Improper groove design, machining, etc.

Preventive measures: select the electrode with good slag removal performance;Carefully remove slag between layers;Reasonable selection of welding parameters;Adjust electrode Angle and rod movement method.

Nine, burn through

In the process of welding, molten metal from the back of the groove outflow, stainless steel welded pipe perforation defects known as burn-through.Burn-through is one of the common defects in arc welding.

Causes: large welding current, slow welding speed, so that the welding pipe excessive heating;Groove clearance large, blunt edge too thin;The welder operation skill is poor and so on.

Preventive measures: select appropriate welding process parameters and appropriate groove size;Improve welder's operation skill etc.

Ten, crack

According to the temperature and time of the crack, it can be divided into cold crack, hot crack and reheat crack.It can be divided into longitudinal crack, transverse crack, welding root crack, arc pit crack, fusion line crack and heat affected zone crack.Crack is one of the most dangerous defects in welding structure, which will not only make the product scrapped, but also may cause serious accidents.

(1) thermal crack

In the process of welding, the welding cracks caused by the cooling of the metal in the weld and heat affected zone to the high temperature zone near the solid line are called thermal cracks.It is a dangerous welding defect that is not allowed.According to the mechanism, temperature range and morphology of the hot crack, the hot crack can be divided into crystallization crack, high temperature liquefaction crack and high temperature low plastic crack.

Causes: eutectic substances and impurities with low melting point in molten pool metal form severe intracrystalline and intercrystalline segregation during crystallization, and under the action of welding stress.Hot cracks are formed along the grain boundaries.Hot cracks generally occur in austenitic stainless steel, nickel alloy and aluminum alloy.It is not easy to produce hot crack when low carbon steel is welded.Preventive measures: strictly control the content of sulfur, phosphorus and other harmful impurities in stainless steel welded pipes and welding materials to reduce the sensitivity of hot cracks;Adjust the chemical composition of weld metal, improve weld structure, refine grain, improve plasticity, reduce or disperse segregation degree;Alkaline welding material is adopted to reduce the content of impurities in the weld and improve the segregation degree.Choose the proper welding process parameters, increase the weld forming coefficient properly, and adopt the multi-layer and multi-pass welding method;When the arc is broken, use the same drawing board as the base material, or gradually extinguish the arc, and fill the arc pit, to avoid the heat crack in the arc pit.

(2) cold crack

The welded joints are cooled to a lower temperature (at M for steel).A crack that occurs below temperature is called a cold crack.Cold cracks can appear immediately after welding, or it may take a period of time (a few hours, a few days or even longer time), this kind of crack is also known as delayed crack, it is a common form of cold crack, with greater risk.

Causes: hardened microstructure formed by martensite transformation, welding residual stress formed by high restraint and hydrogen remaining in the weld are the three main factors for cold crack.

Preventive measures: choose low hydrogen welding materials, baking in strict accordance with the instructions before use;Remove oil and water on the welding parts before welding, reduce the content of hydrogen in the welding seam;Select reasonable welding process parameters and heat input to reduce the hardening tendency of the weld;Dehydrogenation is carried out immediately after welding to make hydrogen escape from the welded joint.For stainless steel welded pipes with high tendency of quenching, preheating before welding and timely heat treatment after welding can improve the structure and performance of joints.Adopt various technological measures to reduce welding stress.

(3) reheat crack

After welding, the stainless steel welded pipe is heated again within a certain temperature range (stress relief heat treatment or other heating process) and the cracks produced are called reheat cracks.

Causes: reheat cracks usually occur in vanadium, chromium, molybdenum and boron alloy elements such as high strength low alloy steel, pearlite heat-resistant steel and stainless steel, through a welding thermal cycle, and then heated to sensitive areas (550 ~ 650 ℃).Most of the cracks originated from the coarse crystal zone of the welding heat affected zone.Most of the reheat cracks occurred in the welded stainless steel pipe and the stress concentration.

Preventive measures: on the premise of meeting the design requirements, choose low-strength welding materials, so that the weld strength is lower than the base material, stress relaxation in the weld, to avoid the heat affected zone crack;Minimize welding residual stress and stress concentration;Control the welding heat input, select the preheating and heat treatment temperature reasonably, and avoid the sensitive area as far as possible.

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