Q1. What are the advantages and disadvantages of solder connections?
Advantages of welded connection: simple structure, no weakening of component section, easy processing, many types of welding methods, automatic operation, saving steel, high efficiency, high rigidity, good integrity, and good sealing performance.
Disadvantages of welded connection: the metallographic structure of the steel in the heat-affected zone changes, and the local material becomes brittle; there is welding residual stress and residual deformation after welding, which reduces the bearing capacity of the compression member; the welded structure is very sensitive to cracks, and once local cracks occur , it is easy to expand to the whole, and the low temperature cold brittleness is more prominent.
Q2. Definition and influencing factors of weldability of steel?
Weldability of steel refers to the degree to which the material can be easily welded and meet structural properties under proper design and operating conditions. Weldability is often affected by factors such as the chemical composition of the steel, rolling method and plate thickness.
In order to evaluate the influence of chemical composition on weldability, it is generally expressed by carbon equivalent (Ceq). The larger it is, the worse the solderability.
The carbon equivalent Ceq (percent) value can be calculated by the following formula:
Q3. What are the causes of welding stress and welding deformation and how to reduce them?
The welding process of steel structure is a process of uneven heating and cooling. During welding, the temperature of the weld and its vicinity is very high, while most of the metal in the distance is not heated, and the expansion and contraction of the main metal are uneven.
After cooling, different degrees of shrinkage and internal stress (longitudinal and lateral) occur in the weld, resulting in various deformations of the welded structure.
Generally speaking, welding stress and welding deformation can be reduced from two aspects of design and processing technology.
Design measures: Arrange the position of the welds reasonably; select the size of the welds reasonably; the number of welds should be small, and the number of welds should not be too concentrated, and at the same time, the three-dimensional staggered welds should be avoided; the shrinkage stress in the thickness direction of the base metal should be avoided as much as possible.
Process measures: arrange the welding sequence reasonably; adopt reverse deformation; preheat before welding and temper after welding.
Process measures: arrange the welding sequence reasonably; adopt reverse deformation; preheat before welding and temper after welding.
Q4. Common welding methods for steel structures?
Commonly used welding methods for steel structures include manual arc welding, automatic (or semi-automatic) submerged arc welding, and gas shielded welding.
Manual arc welding: After the power is turned on, an arc is generated to melt the welding wire in the electrode and drip into the small groove molten pool formed by the arc on the weldment.
The slag and gas formed by the electrode coating cover the molten pool, preventing air from contacting the molten liquid metal and avoiding the formation of brittle, frangible compounds.
Submerged arc welding: An arc welding method in which the arc burns under the flux layer.
The welding wire is not coated with coating, but the welding end is covered by the granular flux flowing down automatically from the flux leakage head, the arc is completely buried in the flux, the heat of the arc is concentrated, and the penetration depth is large. High productivity with good welding quality and small deformation of weldments.
Gas shielded welding: An arc welding method that uses carbon dioxide gas or other inert gas as a protective medium. Relying on the shielding gas to form a local protective layer around the arc to prevent the intrusion of harmful gases and ensure the stability of the welding process.
The weld strength is higher than that of manual arc welding, and the plasticity and corrosion resistance are good. It is suitable for welding in all positions, and there are forward and backward methods.
Q5. The common welding position, joint form, groove form, weld type and tube structure node form code are as follows:
Q6. Common welding defects and their causes and treatment methods?
Weld defects are divided into six categories: cracks, cavities, solid inclusions, incomplete fusion, incomplete penetration, and shape defects.
Cracks: Usually there are hot cracks and cold cracks. The main reasons for the occurrence of hot cracks are poor crack resistance of the base metal, poor quality of welding materials, improper selection of welding process parameters, and excessive welding internal stress;
The main reasons for the occurrence of cold cracks are unreasonable welding structure design, improper welding seam layout, and unreasonable welding process measures, such as no preheating before welding and rapid cooling after welding.
The treatment method is to drill anti-cracking holes at both ends of the crack or remove the weld metal at the crack for repair welding.
Cavities: usually divided into two types: air holes and crater shrinkage holes. The main reasons for the generation of porosity are serious damage to the coating of the electrode, the electrode and flux are not baked, the base metal has oil stains or rust and oxides, the welding current is too small, the arc length is too long, the welding speed is too fast, etc. The treatment method is to shovel. Remove the weld metal at the porosity, and then repair the weld.
Cavities: usually divided into two types: air holes and crater shrinkage holes. The main reasons for the generation of porosity are serious damage to the coating of the electrode, the electrode and flux are not baked, the base metal has oil stains or rust and oxides, the welding current is too small, the arc length is too long, the welding speed is too fast, etc. The treatment method is to shovel. Remove the weld metal at the porosity, and then repair the weld.
The main reasons for arc crater shrinkage are that the welding current is too large, the welding speed is too fast, the arc is extinguished too fast, and the filler metal is not repeatedly added to the arc extinguishing place. The treatment method is repair welding at the arc crater.
Solid inclusions: There are two defects of slag inclusion and tungsten inclusion. The main reasons for slag inclusion are poor quality of welding materials, too small welding current, too fast welding speed, too high slag density, hindering slag from floating, and the slag is not cleaned up during multi-layer welding. The treatment method is to remove it. The weld metal at the slag inclusions is then repaired.
The main reason for the occurrence of tungsten inclusion is that the tungsten electrode is in contact with the molten pool metal during argon arc welding. The treatment method is to dig out the defective metal at the tungsten inclusion and re-weld.
Lack of fusion and penetration: The main reasons are that the welding current is too small, the welding speed is too fast, the groove angle gap is too small, and the operation technology is not good.
The treatment method for lack of fusion is to remove the weld metal at the lack of fusion and then repair it by welding.
The treatment method for incomplete penetration is that the single side of the structure with good openness is not penetrated, and the welding can be directly repaired on the back of the weld.
For important weldments that cannot be directly repaired by welding, the incomplete weld metal should be removed and re-welded.
Shape defects: including undercut, weld bead, slump, root shrinkage, misalignment, angle deviation, super-high weld, surface irregularity, etc.
Q7. Common measures to prevent laminar tearing of sheets?
In T-shaped, cross-shaped and fillet joints, when the thickness of the flange plate is not less than 20mm, in order to avoid or reduce the large welding shrinkage stress in the thickness direction of the base metal plate, the following joint structure design should be adopted:
Under the condition that the penetration depth requirements and the weld compactness are met, a smaller welding groove angle and gap (a) shall be adopted;
In fillet joints, a symmetrical groove or a groove (b) biased towards the side plate is used;
Use double-sided bevel symmetrical welding instead of single-sided asymmetrical welding (c);
In a T-shaped or fillet joint, the end of the plate subjected to the welding tensile stress in the plate thickness direction protrudes from the joint weld area (d);
In T-shaped and cross-shaped joints, cast steel or forged steel transition sections are used to replace T-shaped and cross-shaped joints (e, f) with butt joints;
Change the force direction of the thick plate joint to reduce the stress in the thickness direction;
For joints under static load, under the condition that the joint strength calculation requirements are met, the full penetration groove welds are replaced by the butt and fillet joint welds with partial penetration.
Q8. Weld quality inspection method?
When the weld is inspected after welding, the appearance inspection should be carried out first, and the naked eye or magnifying glass should be used to observe whether there are any defects, such as undercut, burn-through, incomplete penetration, crack, wrong edge, bed, etc., and check whether the weld size conforms to Require.
Defects inside the weld are commonly detected by ultrasonic. The principle is to use the ultrasonic energy to propagate inside the metal, and the principle of reflection and refraction will occur when encountering the interface of the two media to inspect the internal defects of the weld, and it can be judged according to the waveform. There are defects and defect locations.
Since there is a reflective surface between the probe and the test piece, couplant should be applied to the surface of the weldment during ultrasonic inspection, and ultrasonic waves cannot determine the type and size of the defect.
Non-destructive testing is also sometimes used for radiographic inspection. There are two types of X-ray inspection and γ-ray inspection. The principle is that when the ray passes through the inspected weld, if there is a defect, the attenuation of the ray passing through the defect is small, so the photosensitive film on the back of the weld is stronger, and after the film is washed, it will be displayed on the defect. Black spots or streaks appear.
The X-ray irradiation time is short, the speed is fast, the equipment is complex, the cost is high, and the penetrating ability is small, and the thickness of the weldment to be detected is less than 30mm. The γ-ray inspection equipment is light, easy to operate, and has strong penetrating ability.
Q9. What is the basis for judging results during sampling inspection?
When the unqualified rate of the number of welds in the sampling inspection is less than 2%, the batch of acceptance is qualified;
When the unqualified rate of the number of welds in the sampling inspection is greater than 5%, the batch of acceptance is unqualified;
In addition to the situation in the fifth paragraph of this article, when the unqualified rate of the number of welds in the sampling inspection is 2% to 5%, the sampling inspection shall be doubled, and an extension line of the welding seam on both sides of the original unqualified part must be added. When the unqualified rate in the seam is not more than 3%, the batch acceptance is qualified; when it is greater than 3%, the batch acceptance is unqualified;
When batch acceptance fails, all remaining welds of the batch shall be inspected;
If one crack defect is found in the inspection, double spot inspection shall be performed. If no crack defect is found in the double spot inspection weld, the batch acceptance shall be accepted; If qualified, all remaining welds in the batch shall be inspected.
Q10 What situations need to go through welding procedure qualification?
In addition to the conditions exempted from assessment in the national steel structure welding code, the steel, welding materials, welding methods, joint forms, welding positions, post-weld heat treatment systems, welding process parameters, preheating and post-heating measures adopted by the construction unit for the first time According to the combination conditions of these parameters, the welding procedure evaluation shall be carried out before the fabrication and installation of the steel structural components.
