Reinforced construction is a critical process in engineering construction. It usually involves procedures such as reinforcement fabrication, bonding and installation, and welding.
Among these procedures, the quality of welding has a direct impact on the quality of reinforced construction.
1. Appearance defects
Appearance defects (surface defects) refer to defects that can be detected on the surface of a workpiece without relying on instruments.
Common appearance defects include undercut, weld tumor, depression, welding distortion, sometimes surface porosity and surface cracks, and incompletely penetrated root in one-sided welding.
A. Undercut
Refers to a groove or groove formed in the base metal part along the weld tip. It is caused by insufficient filling of the molten metal to the edge of the weld seam after the arc melts the edge of the weld seam.
The main causes of undercut:
The high heat of the arc, that is, too much current and too slow welding speed, results in inferior cuts. Incorrect angle between electrode and workpiece, excessive oscillation, arc that is too long, and improper welding sequence can cause undercuts.
Arc blowing from DC welding is also a reason that causes undercutting. Some welding positions (vertical, horizontal and overhead) will aggravate undercutting.
Undercutting reduces the effective cross-sectional area of the base metal, reduces the load-bearing capacity of the structure and also causes stress concentration, leading to sources of cracking.
Undercut Prevention:
Correcting the operating posture, selecting appropriate patterns and adopting a proper welding approach can eliminate undercuts.
Using AC welding instead of DC welding can effectively prevent undercutting when welding fillet welds.
B. Weld tumor
The liquid metal in the weld flows into the insufficiently heated base metal that is not melted or overflows from the weld root, forming a tumor of unmolten metal after cooling, which is called a weld tumor.
Strong welding specifications, too rapid electrode melting, poor electrode quality (such as decentralization), unstable welding power supply characteristics, and incorrect operating posture are prone to causing welding tumors.
Weld tumors are most likely to form in horizontal, vertical, and overhead positions.
Weld tumors are often accompanied by defects of incomplete fusion and slag inclusion, which can cause cracking.
At the same time, weld tumors change the actual size of the weld and cause stress concentration. The weld tumor inside the tube decreases its internal diameter and can cause blockages in the fluid flow.
Measures to prevent welding tumors:
Keep the weld flat during welding, correctly select the specification, choose an off-center electrode, and operate reasonably.
C. Pitting
Pitting refers to the part of the surface or back of the weld that is lower than the base metal.
Arc corrosion is mainly caused by the failure of the electrode (welding wire) to stop for a short period of time when the arc ends (the resulting corrosion is called arc corrosion). When welding in the upper, vertical and horizontal positions, internal corrosion often occurs at the weld root at the rear.
Pitting reduces the effective cross-sectional area of the weld, and arc pits often have arc cracks and arc contraction cavities.
Measures to prevent corrosion:
Use a welding machine with a current drop system, choose a flat welding position as much as possible, select suitable welding specifications, and let the electrode remain in the molten pool for a short time or swing circularly when the arc ends to fill the well arch.
D. Incomplete penetration
Incomplete penetration refers to continuous or intermittent grooves in the weld surface. Insufficient filler metal is the root cause of incomplete penetration.
Poor welding specifications, too thin an electrode, and improper operation can result in incomplete penetration.
Incomplete penetration also weakens the weld, making it prone to stress concentration. At the same time, poor welding specifications increase the cooling rate, which can cause porosity, cracking and other defects.
Measures to prevent incomplete penetration:
Increase the welding current and add cover pass welds.
E. Burn
Burn through refers to the defect that occurs during welding, in which the depth of fusion exceeds the thickness of the part and the molten metal flows through the back of the weld, forming a perforated defect.
Too high welding current, too slow speed and arc stay in the weld will cause burn defects. The gap between the workpieces is too large and the chamfer is too small, which may also burn.
Burning is not permitted in boiler pressure vessel products; completely destroys the weld, causing the joint to lose its connection and load-bearing capacity.
Prevention and control measures:
Use a lower current and appropriate welding speed, reduce mounting clearance, add a bracket or buffer to the back of the weld. The use of pulse welding can effectively prevent burning.
F. Other surface defects
(1) Poor training
The appearance and geometric dimensions of the weld do not meet the requirements. There are welds that are too tall, have an uneven surface, and the weld is too wide or transitions poorly into the base material.
(2) Misalignment
Two workpieces are displaced from each other in the thickness direction, which can be viewed as both a surface weld defect and an assembly formation defect.
(3) Collapse
In one-sided welding, due to excessive input heat and excess molten metal, the liquid metal collapses to the back of the weld, and the back of the weld protrudes after forming, while the front collapses.
(4) Surface porosity and shrinkage cavities
(5) Various welding deformations such as angular deformation, twisting, wave deformation, etc. are also welding defects. Angular deformation is also an assembly formation defect.
2. Porosity and slag inclusions
A. Porosity
Porosity refers to the cavities formed in the weld due to weld pool gas that did not escape before the metal solidified.
The gas can be absorbed by the molten pool from the external environment or generated during the metallurgical welding process.
1. Porosity classification
According to its shape, porosity can be classified into spherical porosity and worm-shaped porosity.
According to the number of pores, it can be divided into single pores and grouped pores. Clustered pores include uniformly distributed pores, densely distributed pores and linearly distributed pores.
According to the composition of the gas inside the pore, there are hydrogen pores, nitrogen pores, carbon dioxide pores, carbon monoxide pores, oxygen pores, etc. The pores generated during fusion welding are mainly hydrogen pores and carbon monoxide pores.
2. Mechanisms of porosity formation
The solubility of gas in the metal in the solid state at room temperature is only one-tenth to one-hundredth of that in the metal in the liquid state at high temperature.
When molten metal solidifies, a large amount of gas must escape from the metal. When the rate of solidification is greater than the rate of gas escape, porosity forms.
3. Main causes of porosity
Rust and oil stains on the surface of the base metal or filler metal, and the welding rod or non-dry flux can increase the amount of pores in the weld because the moisture in the rust, oil stains and coating of the welding rod and the flux decomposes into gas at high temperatures, increasing the gas content in the molten metal.
When the welding energy is too low, the cooling rate of the weld pool is too high, which is not conducive to gas escape. Insufficient deoxidation of the weld metal can also increase oxygen pores.
4. Porosity risks
Porosity reduces the effective cross-sectional area of the weld, making the weld loose, thus reducing the strength and plasticity of the joint. It can also cause leaks.
Porosity is also a factor that causes stress concentration. Hydrogen pores can promote cold cracking.
5. Measures to prevent porosity
- Clean oil, rust, moisture and debris from the surface of the welding wire, working groove and its surroundings.
- Use alkaline soldering rods and flux and dry them completely.
- Weld using direct current with reverse polarity and short arc.
- Preheat before welding to slow the cooling rate.
- Use a slightly stronger specification for welding.
B. Slag inclusions
Slag inclusion refers to the phenomenon of residual slag remaining in the weld seam after welding.
1. Classification of Slag Inclusions
- Metal Slag Inclusion: Refers to the residual metal particles such as tungsten or copper in the weld seam, commonly known as tungsten inclusion or copper inclusion.
- Non-Metallic Slag Inclusion: Refers to the residue of unmolten flux coating or flux, sulfides, oxides, nitrides in the weld seam. If the metallurgical reaction is incomplete, slag removal will be difficult.
2. Distribution and shape of slag inclusions
There are single-point slag inclusions, linear slag inclusions, chain-shaped slag inclusions and dense slag inclusions.
3. Causes of Slag Inclusions
- Inadequate groove size;
- Impurities in the groove;
- Incomplete slag removal between layers in multilayer welding;
- Low energy in the welding line;
- Rapid cooling of the weld seam, resulting in very rapid solidification of the metal;
- The flux coating or flux composition of the welding rod is unreasonable, with high melting point;
- During tungsten inert gas welding, improper polarity of power supply, high current and current density, and tungsten electrode melting and falling into the molten pool;
- Weak welding rod oscillation during manual welding, unfavorable to slag floating.
Corresponding measures must be taken to prevent slag inclusion based on the reasons stated above.
4. Damage from slag inclusions
The damage from point slag inclusions is similar to that from pores. Slag inclusions with a sharp tip will generate stress concentration, and the sharp tip will also develop into a crack source, which is more harmful.
3. Cracks
The breaking of the atomic bond in the weld, resulting in a new interface and a gap, is called a crack.
A. Crack Classification
According to the size of the crack, it can be divided into three types:
- Macroscopic cracks: cracks visible to the naked eye.
- Microcracks: can only be detected under a microscope.
- Ultramicro cracks: can only be detected under high-power microscope, generally referring to intergranular cracks and intracrystalline cracks.
From the point of view of production temperature, cracks can be divided into two categories:
- Hot cracks: cracks produced close to line Ac3. They usually appear immediately after welding and are also called solidification cracks. This type of crack occurs mainly at grain boundaries, and there is an oxidized color on the surface of the crack, which loses its metallic luster.
- Cold cracks: refers to cracks produced when cooled below the transformation temperature of M3 martensite after welding, which generally appear after a period of time after welding (several hours, several days or even longer). Therefore, they are also called delayed cracks.
According to the reasons for the generation of cracks, cracks can be divided into:
- Reheat Cracks: Cracks produced when the joint is reheated to 500 ~ 700 ℃ after cooling. Reheat cracks occur in the coarse-grained region of the heat-affected zone of welding precipitation-strengthened materials (such as metals containing Cr, Mo, V, Ti, Nb) and generally develop from the fusion line to the melting region. coarse grained. of the thermally affected zone, presenting characteristics of intergranular cracking.
- Laminar rupture is mainly due to the inclusion of impurities such as sulfides (MnS) and silicates in the steel during the rolling process, forming anisotropy. Under welding stress or external restraint stress, the metal cracks along the direction of rolling impurities.
- Stress corrosion cracking: cracks produced under the combined action of tension and corrosive media. In addition to residual stress or confining stress factors, stress corrosion cracking is mainly related to the structure and morphology of the weld.
B. Risks of cracking
Especially for cold cracks, the damage is catastrophic. Most pressure vessel accidents in the world are caused by brittle fractures caused by cracks, except in some cases caused by irrational design or inappropriate material selection.
Hot cracks (solidification cracks)
- Solidification crack formation mechanism
- Hot cracking occurs during the final solidification stage of the weld metal, and the sensitive temperature range is generally in the high temperature zone close to the solid phase line.
- The most common hot crack is the solidification crack, which forms when impurities that generate low melting point eutectics are enriched at the grain boundary due to the segregation of crystallization during the solidification process of the weld metal, forming the so-called “ liquid film”. .”
In a specific sensitive temperature range (also known as brittle temperature range), its strength is very small and it will crack due to the tensile stress caused by the solidification contraction of the weld, eventually forming a crack. Solidification cracks most commonly occur longitudinally along the center length of the weld and are called longitudinal cracks.
They also sometimes occur between two columnar crystals within the weld, called transverse cracks. Arch cracks are another form of solidification cracks and are common hot cracks.
Hot cracks generally occur along grain boundaries and typically occur in gas-welded joints of materials with many impurities, such as carbon steel, low-alloy steel, and austenitic stainless steel.
(2) Factors affecting solidification cracks
- The influence of alloying elements and impurities: The increase of carbon elements and impurity elements such as sulfur and phosphorus will widen the sensitive temperature range and increase the opportunity for solidification cracks.
- The influence of cooling rate: Increasing the cooling rate will increase the degree of crystallization segregation and widen the crystallization temperature range, which will increase the chance of solidification cracks.
- The influence of crystallization stress and restraint stress: In the brittle temperature range, the strength of the metal is extremely low, and the welding stress makes some metal parts subject to tensile stress. When the tensile stress reaches a certain level, solidification cracks will occur.
(3) Measures to prevent solidification cracks
- Reduce the content of harmful elements such as sulfur and phosphorus and use materials with a lower carbon content for welding.
- Add a certain amount of alloying elements to reduce columnar crystals and segregation. Elements such as aluminum, zirconium, iron and molybdenum can refine grain size.
- Use a weld with a shallow melting depth to improve heat dissipation conditions, causing low-melting point substances to float on the surface of the weld and not exist within the weld.
- Reasonably select welding specifications and adopt preheating and postheating to reduce the cooling rate.
- Adopt a reasonable assembly sequence to reduce welding stress.
Reheat Cracks
(1) Characteristics of reheat cracks
- Reheat cracks occur in the superheated coarse-grained areas of the weld heat-affected zone. They occur during the reheating process, such as post-weld heat treatment.
- The temperature range of reheating crack production: Carbon steel and alloy steel 550 ~ 650 ℃; austenitic stainless steel ~300℃.
- Reheating cracks are intragranular cracks (along the grain boundary).
- They are more likely to occur in precipitation hardening steels.
- Associated with welding residual stresses.
Crack reheating mechanisms
There are several explanations for the mechanism of reheat cracking, and the model fracture theory explanation is as follows: In the area near the weld, under the action of high-temperature thermal cycling, phase-reinforced carbides (such as iron carbide, carbide, chromium carbide and misplaced carbide) are deposited in the dislocation area within the crystal, making the internal reinforcement resistance much higher than the intergranular reinforcement resistance.
Especially when the reinforced phase is evenly distributed in the grain, it makes the local adjustment of the interior of the grain difficult and also makes the general deformation of the grain difficult.
Therefore, the plastic deformation caused by stress relaxation is mainly supported by the metal at the grain boundary, so the stress at the grain boundary is concentrated and cracking occurs, which is called model fracture.
Preventing reheat cracking
- Pay attention to the strengthening effect of metallurgical elements and their effects on reheating cracks.
- Reasonable preheating or adopting postheating to control the cooling rate.
- Reduce residual stress to avoid stress concentration.
- During tempering treatment, try to avoid the sensitive temperature range of reheating cracks or reduce the residence time within this temperature range.
Cold Cracks
Characteristics of cold cracks
- Cold cracks occur at lower temperatures and after a period of time after welding, which is why they are also called delayed cracks.
- They occur mainly in the heat-affected zone and can also occur in the welding zone.
- Cold cracks can be intergranular cracks, transgranular cracks, or a mixture of the two.
- Component failure caused by cold cracking is a typical brittle fracture.
Cold cracking mechanisms
- The hardened structure (martensite) reduces the metal's plastic reserves.
- Residual stress in the joint causes the weld to pull.
- There is a certain amount of hydrogen in the joint.
Hydrogen content and tensile stress are two important factors in the formation of cold cracks (here referring to hydrogen-induced cracks).
Generally, the arrangement of atoms within metals is not completely ordered, but contains many microscopic defects. Under the action of tensile stress, hydrogen diffuses and accumulates in the high stress area (defect area). When the hydrogen concentration reaches a certain level, it will break the bond between the metal atoms, resulting in some microscopic cracks.
Under the continuous action of stress, hydrogen continuously accumulates, microscopic cracks continuously expand, until they turn into macroscopic cracks and eventually break. The critical hydrogen concentration and the critical stress value determine the occurrence of cold cracks.
When the hydrogen concentration within the joint is less than the critical hydrogen concentration, or the applied stress is less than the critical stress, cold cracking will not occur (i.e., the delay time is infinitely long). Among all cracks, cold cracks are the most harmful.
Measures to prevent cold cracks
- Use alkaline electrodes with low hydrogen content, dry them thoroughly and store them at 100-150°C, and use them as soon as possible after removing them.
- Increase the preheating temperature, adopt post-heating measures, ensure that the interlayer temperature is not lower than the preheating temperature, choose reasonable welding specifications, and avoid the formation of hardened structures in the weld.
- Choose a reasonable welding sequence to reduce welding deformation and stress.
- Carry out dehydrogenation heat treatment in a timely manner after welding.
4. Incomplete penetration
Incomplete penetration refers to the phenomenon that the base metal does not melt and the weld metal does not enter the joint root.
1. Reasons for incomplete penetration
- Low welding current and shallow penetration depth.
- Inadequate groove and gap size, blunt edge too large.
- The influence of magnetic strike.
- Excessive electrode eccentricity.
- Poor cleaning of the intermediate layer and weld root.
2. Dangers of incomplete penetration
One of the dangers of incomplete penetration is that it reduces the effective cross-sectional area of the weld and decreases the strength of the joint.
Furthermore, the damage caused by stress concentration due to incomplete penetration is much greater than the damage caused by strength reduction. Incomplete penetration seriously reduces the fatigue strength of the weld.
Incomplete penetration can become a source of cracking, which is an important cause of welding failures.
The damage caused by stress concentration due to incomplete penetration is much greater than the damage caused by reduced strength. Incomplete penetration seriously reduces the fatigue strength of the weld.
3. Incomplete penetration prevention
Using higher welding current is a basic method to avoid incomplete penetration. In addition, when welding angle joints, using AC instead of DC to avoid magnetic strike, reasonably designing grooves and strengthening cleaning, and using short arc welding measures can also effectively prevent incomplete penetration.
5. Lack of fusion
Lack of fusion refers to the defect that the weld metal and base metal, or the weld metal and weld metal, are not fused.
According to its location, lack of fusion can be divided into three types: lack of fusion in the groove, lack of fusion between layers and lack of fusion at the root.
1. Reasons for lack of fusion defects
- The welding current is very low.
- The welding speed is very fast.
- The electrode angle is not correct.
- The arc blowing phenomenon occurs.
- The welding is in a downward position and the base metal that has not been melted has been covered by molten iron.
- The surface of the base metal is affected by pollutants or oxides, which affect the fusion bond between the deposited metal and the base metal.
2. Dangers of lack of fusion
Lack of fusion is an area-type defect. Lack of fusion in the groove and lack of fusion in the root significantly reduce the supporting cross-sectional area and cause severe stress concentration. Its harmfulness is second only to cracks.
3. Prevention of lack of fusion
Using a higher welding current, carrying out the welding operation correctly and paying attention to cleaning the groove are effective measures to avoid lack of fusion.
6. Other defects
(1) The chemical composition or microstructure of the welded joint does not meet the requirements:
Improper matching of welding material and base metal, or burning of elements during the welding process, can easily cause changes in the chemical composition of the weld metal or result in a microstructure that does not meet the requirements.
This can lead to a decrease in the mechanical properties of the welded joint and also affect the corrosion resistance performance of the joint.
(2) Overheating and burning:
If the welding specifications are used incorrectly, the heat-affected area will remain at a high temperature for a long time, which can cause the grain to become coarse, resulting in overheated microstructures.
If the temperature increases further and the duration is prolonged, it may cause local oxidation or melting of grain boundaries, resulting in burnt microstructures.
Overheating can be eliminated by heat treatment, while burning is an irreversible defect.
(3) Cracks in the bottom:
Cracks that form in the base metal adjacent to the weld metal or in the heat-affected zone; caused by welding stresses and strains combined with restricted dissimilar expansion and contraction rates.
Steel welding technology includes various types, and in order to strengthen construction quality control, the reasonable application of steel welding technology should be based on the specific conditions of the engineering project, to ensure the stability and safety of the entire building structure.
Therefore, it is important for everyone to pay attention to the welding defects of the above steel during construction.
