Causes and Preventive Measures of Stainless Steel Welded Pipe Defects (2)
- By:Iris Liang
In last articles, we have discussed parts of causes and preventive measures of stainless steel welded pipe defects. Today, we keep overviewing the rest of them.
The sunken part at the end of the weld of the stainless steel welded pipe is called the arc crater. The arc crater not only seriously weakens the strength of the weld there, but also produces arc crater cracks due to the concentration of impurities.
Causes: The main reason is that the dwell time of arc extinguishing is too short; the current is too large when welding thin plates.
Preventive measures: When electrode arc welding is closed, the electrode should stay in the molten pool for a while or run in a circular motion, and then lead to one side to extinguish the arc after the molten pool is filled with metal; when tungsten argon arc welding, there must be enough The dwell time is attenuated and the arc is extinguished after the weld is filled.
When welding sanitary stainless steel welded pipes, the gas in the molten pool fails to escape when it solidifies and the cavities formed by remaining are called pores. Porosity is a common welding defect, which can be divided into internal porosity and external porosity in the weld. Stomata are round, oval, insect-shaped, needle-shaped and dense. The existence of pores will not only affect the compactness 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, moisture and other dirt on the surface and groove of the sanitary stainless steel welded pipe; the coating of the electrode is damp during arc welding and has not been dried before use; the arc is too long or partial blowing, the molten pool protection effect is not good Well, the air invades the molten pool; the welding current is too high, the electrode becomes red, the coating falls off early, and the protective effect is lost; the operation method is improper, such as the arc closing action is too fast, it is easy to produce shrinkage cavity, and the arc striking action of the joint is not correct, which is easy to produce Dense stomata, etc.
Preventive measures: before welding, remove oil, rust, and moisture within 20-30mm on both sides of the groove; bake in strict accordance with the temperature and time specified in the electrode manual; correctly select welding process parameters and operate correctly; use short arc as much as possible Welding, field construction must have windproof facilities; invalid electrodes are not allowed, such as welding core corrosion, coating cracking, peeling, excessive eccentricity, etc.
8. Inclusions and slag inclusions
Inclusions are non-metallic inclusions and oxides remaining in the weld metal produced by metallurgical reactions. Slag inclusions are molten slag that remains in the weld. Stainless steel welded pipe slag inclusions can be divided into two types: spot slag inclusions and strip slag inclusions. The slag inclusion weakens the effective section of the weld, thereby reducing the mechanical properties of the weld. Slag inclusions can also cause stress concentration, which can easily damage the welded structure when it is loaded. Causes: The interlayer slag is not clean during the welding process; the welding current is too small; the welding speed is too fast; the operation is improper during the welding process; the chemical composition of the welding material and the base metal is not properly matched;
Preventive measures: choose electrodes with good slag removal performance; carefully remove interlayer slag; reasonably select welding process parameters; adjust electrode angle and transport method.
When choosing a welded pipe production line, you can consider installing an intelligent PLC system. Hangao Tech (SEKO Machinery) PLC system can not only monitor the production data in real time, but also establish a database to store the production formulas of welded pipes of different specifications, so that the production process can access the database records at any time.
9. Burn through
During the welding process, the molten metal flows out from the back of the groove, and the perforation defect of the stainless steel welded pipe is called burn-through. Burn-through is one of the common defects in arc welding.
Causes: large welding current, slow welding speed, excessive heating of the welded pipe; large groove gap, too thin blunt edge; poor welder operation skills, etc.
Preventive measures: choose the appropriate welding process parameters and the appropriate groove size; improve the welder's operational skills, etc.
Cracks of sanitary stainless steel welded pipes can be divided into cold cracks, hot cracks and reheat cracks according to the temperature and time they occur; they can be divided into longitudinal cracks, transverse cracks, weld root cracks, arc crater cracks, Fusion line cracks and heat-affected zone cracks, etc. Cracks are the most dangerous defects in welded structures, which will not only make products scrapped, but may even cause serious accidents.
(1) Hot crack
During the welding process, the welding cracks produced by the welding seam and the metal in the heat-affected zone cooling to the high temperature range near the solidus line are called hot cracks. It is a dangerous welding defect that is not allowed to exist. According to the mechanism, temperature range and shape of welded pipe thermal cracks, thermal cracks can be divided into crystallization cracks, high-temperature liquefaction cracks and high-temperature low-plasticity cracks.
Cause: The main reason is that the low melting point eutectic and impurities in the molten pool metal form serious intragranular and intergranular segregation during the crystallization process, and at the same time under the action of welding stress. Along the grain boundaries are pulled apart, forming hot cracks. Hot cracks generally occur in austenitic stainless steel, nickel alloy and aluminum alloy. Low-carbon steel is generally not easy to produce hot cracks during welding, but as the carbon content of the steel increases, the tendency of hot cracking also increases. Preventive measures: Strictly control the content of harmful impurities such as sulfur and phosphorus in stainless steel welded pipes and welding materials, reduce the sensitivity of hot cracks; adjust the chemical composition of the weld metal, improve the weld structure, refine the grain, improve plasticity, Reduce or disperse the degree of segregation; use alkaline welding materials to reduce the content of impurities in the weld and improve the degree of segregation; select appropriate welding process parameters, appropriately increase the weld forming factor, and adopt multi-layer and multi-pass welding method; Use the same lead-out plate as the base metal, or gradually extinguish the arc, and fill the arc crater to avoid thermal cracks at the arc crater.
(2) Cold cracks
The cracks produced when the welded joint is cooled to a lower temperature (for steel below M. temperature) are called cold cracks. Cold cracks can appear immediately after welding, or it may take a period of time (hours, days or even longer) to appear. This kind of crack is also called delayed crack. great danger.
Causes: The hardened structure formed by martensite transformation, the welding residual stress formed by the large degree of restraint, and the hydrogen remaining in the weld are the three major factors that cause cold cracks.
Preventive measures: choose low-hydrogen welding materials, and bake them in strict accordance with the instructions before use; remove oil and moisture on the weldments before welding, and reduce the hydrogen content in the weld; choose reasonable welding process parameters and heat input to reduce The hardening tendency of the weld seam; the hydrogen elimination treatment is carried out immediately after welding to make the hydrogen escape from the welded joint; for the stainless steel welded pipe with a high hardening tendency, preheating before welding and heat treatment in time after welding can improve the structure and quality of the joint. Performance; adopt various technological measures to reduce welding stress.
(3) Reheat cracks
After welding, the stainless steel welded pipe is reheated within a certain temperature range (stress relief heat treatment or other heating process) and the cracks are called reheat cracks.
Causes: Reheat cracks generally occur in low-alloy high-strength steels, pearlitic heat-resistant steels and stainless steels containing vanadium, chromium, molybdenum, boron and other alloying elements. After a welding thermal cycle, they are heated to the sensitive area (550 ~650℃). Most of the cracks originate in the coarse-grained zone of the welding heat-affected zone. Most of the reheat cracks occur in stainless steel welded pipes and stress concentration places, and reheat cracks sometimes occur in multi-layer welding.
Preventive measures: On the premise of meeting the design requirements, select low-strength welding materials, so that the weld strength is lower than that of the base metal, and the stress relaxes in the weld to avoid cracks in the heat-affected zone; minimize welding residual stress and stress concentration; Control the welding heat input of the welded pipe, reasonably select the preheating and heat treatment temperature, and avoid the sensitive area as much as possible.
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