1. Advantages and disadvantages of welding connection?
Advantages of welding connection:
Simple structure, no weakening of component section, simple processing, multiple welding methods, automatic operation, steel saving, high efficiency, great rigidity, good integrity and good sealing performance.
Disadvantages of welding connection:
The area of steel affected by heat undergoes changes in its metallographic structure, causing local materials to become brittle.
Welding results in residual stress and deformation, which decreases the bearing capacity of the compressed members.
Welded structures are highly susceptible to cracking. If local cracks occur, they are prone to extensive propagation and low-temperature cold brittleness is more pronounced.
2. Definition of steel weldability and factors that influence it?
Weldability of steel refers to the ease with which materials can be welded while meeting required structural performance under suitable design and working conditions. The weldability of steel is often influenced by its chemical composition, rolling method and plate thickness.
To evaluate the impact of chemical composition on weldability, it is generally expressed as carbon equivalent (Ceq). The weldability of steel is better when the Ceq is lower, as it indicates a lower tendency for the material to harden. On the other hand, when the Ceq is higher, the weldability of the steel is worse due to its greater tendency to harden.
The carbon equivalent value of Ceq (percentage) can be calculated using the following formula:
3. What are the causes of welding stress and deformation and how to reduce them?
The process of welding a steel structure involves uneven heating and cooling. During welding, the temperature of the weld and the surrounding area is very high, while the metal in the distance remains unheated. As a result, the expansion and contraction of the main metal are uneven.
After cooling, the weld seam will experience varying degrees of contraction and internal stress (both longitudinal and transverse), leading to different deformations in the welding structure.
To reduce welding stress and deformation, two aspects can be addressed: design and processing technology.
Project measurements:
The weld position should be reasonably organized.
Weld size selection should be made based on reasonableness.
The number of welds must be minimal and must not be excessively concentrated. Furthermore, it is essential to avoid creating a three-dimensional intersection of welds.
It is important to avoid shrinkage stress in the direction of the base metal thickness as much as possible.
Process measurements:
Arrange the welding sequence reasonably;
Adopt reverse deformation;
Preheating before welding and tempering after welding.
4. 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.
Related Reading: Manual Arc Welding vs CO2 Gas Protected Welding
Manual arc welding:
After being electrified, an electric arc is generated to melt the welding wire on the electrode and release it into the small weld pool melted by the electric arc during welding.
The slag and gas formed by the electrode coating cover the molten pool to prevent air from coming into contact with the molten liquid metal and forming brittle, crack-prone compounds.
- a) Circuit:
- b) Welding process;
- 1 – Electric welding machine:
- 2 – Driver;
- 3 – Welding;
- 4 – Arc:
- 5 – Coated leather;
- 6 – Gas for protection;
- 7 – Slag:
- 8 – Weld metal;
- 9 – Base metal;
- 10 – Welding wire;
- 11 – Melted pool
Submerged Arc Welding:
This welding method uses an arc that burns under a layer of flux. Unlike traditional methods, the welding wire is not coated with any additional material. Instead, the welding end is covered by granular flux, which automatically flows from the flux casting head.
As a result, the arc is completely buried in the flow, which causes the heat to concentrate and penetrate deeply. This makes it an ideal method for welding thick sheets with high productivity while maintaining good welding quality and minimal welding deformation.
- Welding Wire Turntable
- Wire Feed Motor
- Flow Funnel
- Power supply
- Molten flow
- weld metal
- Welding
- Flow
- Direction of movement
Gas shielded welding:
This is an arc fusion welding method that employs carbon dioxide or other inert gases as a shielding medium. The shielding gas forms a local protective layer around the arc, which helps prevent the invasion of harmful gases and ensures the stability of the welding process.
Compared to manual arc welding, this method produces welds with greater strength, excellent plasticity and corrosion resistance. It is suitable for welding in all positions, including forward and reverse methods.
5. Common Welding Codes?
Common welding positions, joint shapes, groove shapes, weld types and node shape codes of pipe structure are shown below:
| Code | Welding position |
| F | downward welding |
| H | Welding in a horizontal position |
| V | Vertical welding |
| O | Aerial Position Welding |
Related Reading: What do 1G, 2G, 3G, 4G, 5G and 6G mean in welding?
Joint type and groove shape code
| Joint type | Groove shape | ||||
| Code | Name | Code | Name | ||
| I | I-groove | ||||
| Plate gasket | B | Butt joint | V | V-groove | |
| T | T-Joint | X | X-Slot | ||
| X | tube cross | I | Single Side V-Groove | ||
| W | fillet joint | K | K-slot | ||
| F | Lap joint | U1 | U-groove | ||
| Pipe Joint | T | T-Joint | J1 | Single Side U-Groove | |
| K | K joint | Observation: 1. When the thickness of the steel plate is ≥ 50mm, U-shaped or J-shaped groove can be used |
|||
| s | Y joint | ||||
6. Common welding defects, causes and treatment methods?
Weld defects are divided into six categories: cracks, cavities, solid inclusions, incomplete fusion, incomplete penetration and shape defects.
To snap:
Hot cracks and cold cracks are two common types of welding defects.
Hot cracks are mainly caused by poor crack resistance of the base metal, poor-quality welding materials, improper selection of welding process parameters, and excessive internal welding stress.
On the other hand, cold cracks are often the result of irrational design of the welding structure, improper arrangement of welding seams, and inadequate welding process measures such as lack of preheating before welding and rapid cooling after welding. .
To treat these types of cracks, one method is to drill holes to prevent cracking at both ends of the crack or remove the weld metal from the crack for repair welding.
Cavity:
Welding defects are generally divided into two types: air holes and crater shrinkage.
Air holes are mainly caused by various factors, including serious damage to the electrode coating, failure of electrode and flux baking, oil stains or rust and oxide on the base metal, insufficient welding current, excessive arc length long and fast welding speed.
Treatment for air holes consists of removing the defective weld metal at the location of the air hole and then performing a repair welding procedure.
Crater shrinkage, on the other hand, is mainly caused by excessive welding current, high welding speed, rapid arc extinction, and insufficient addition of filler metal to repeatedly extinguish the arc.
Treatment for crater retraction consists of carrying out a repair welding operation at the crater site.
Solid inclusion:
There are two types of defects that can occur during welding: slag inclusion and tungsten inclusion.
The main causes of slag inclusion are poor quality of welding materials, welding at too low a current, welding at too fast a speed, high slag density that blocks the buoyancy of the slag, and failure to clean the slag during multilayer welding. . .
To remedy slag inclusion, the weld metal around the affected area must be removed and then the welding process can continue.
Tungsten inclusion is typically caused when the tungsten electrode comes into contact with the molten pool metal during argon arc welding.
To repair this defect, the defective metal in the tungsten inclusion must be excavated and the welding process can then be resumed.
Incomplete fusion and penetration:
There are several main reasons for incomplete welding, including welding current that is too small, welding speed that is too fast, groove angle clearance that is too small, and inappropriate operating technology.
The treatment method for non-fusion is to remove the weld metal in the non-fusion position and then repair the weld.
To treat incomplete penetration, the method is to repair the incomplete penetration on one side of the structure with a good gap directly at the back of the weld.
For major welds that cannot be directly repaired by welding, the incomplete weld metal must be removed and the weld redone.
Shape defect:
Including undercut, overlap, sagging, root shrinkage, misalignment, angle deviation, weld superelevation, surface irregularity, etc.
7. Common measures to prevent lamellar rupture of plaques?
For T-shaped, cross-shaped and corner joints when the thickness of the flange plate is not less than 20 mm, in order to avoid or reduce large welding shrinkage stresses in the thickness direction of the base metal plate , the following design joint structure should be adopted:
- Smaller angles and gaps in the welding grooves (a) should be used, provided that the penetration depth and weld tightness requirements are met.
- For corner joints, a groove that is symmetrical or inclined in relation to the side plate (b) must be used.
- Bilateral groove welding should be symmetrical rather than asymmetrical one-sided groove welding (c).
- For T-shaped or corner joints, the end of the plate that bears the welding tensile stress in the plate thickness direction must extend outside the weld zone of the joint (d).
- Cast steel or forged steel transition sections should be used for T-shaped and cross joints, and butt joints should be used to replace T-shaped and cross joints (e, f).
Change the tension direction of the thick plate joint to reduce the tension in the thickness direction;
For nodes subject to static load, under the condition of meeting the joint strength calculation requirements, the fully penetrated groove weld shall be replaced by the partially penetrated butt and fillet weld.
8. Welding quality inspection method?
After welding and weld inspection are completed, the first inspection step will be appearance inspection. Visual inspection or magnifying glass should be used to observe any defects such as undercuts, burns, incomplete penetration, cracks, stepped edges, permanence and to verify that the overall dimensions of the weld meet the requirements.
Defects within the weld are usually detected by ultrasonic waves. This method is based on the principle that ultrasonic waves can spread within the metal and reflect and refract when encountering the interface of two media, which helps in inspecting flaws inside the weld. The waveform can be used to determine the presence and location of the fault.
Since there is a reflective surface between the probe and the test piece, during ultrasonic inspection, the coupling agent must be coated on the welding surface. However, the ultrasonic method cannot determine the type and size of defects.
Radiographic inspection is sometimes used in non-destructive testing to detect defects in welds. There are two types of radiographic inspection: X-ray inspection and γ-ray inspection. The principle is that when the ray passes through the inspected weld, any defects will result in less attenuation of the ray passing through that area.
As a result, the negative film on the back of the weld is highly sensitive to light, and black spots or streaks will appear at the defect location after washing off the film.
X-ray inspection has short exposure time, high speed and strong penetration ability, but the equipment is complex, expensive and suitable for testing weldments with a thickness of less than 30 mm. In contrast, gamma ray inspection equipment is portable, easy to operate, and has strong penetration ability.
9. What is the basis for judging sampling inspection results?
Lot acceptance criteria for welds:
- When the unqualified rate of the number of welds in sampling inspection is less than 2%, the batch is accepted.
- When the unqualified rate of the number of welds in sampling inspection is more than 5%, the batch is unacceptable.
- When the unqualified rate of the number of welds in the sampling inspection is between 2% and 5%, the sampling inspection should be repeated and a weld extension line on both sides of the original unqualified part should be added, except for the situation in the fifth paragraph of this article.
- When the unqualified rate of all welds in the sampling inspection is not more than 3%, the batch is qualified.
- When the unqualified rate is greater than 3%, the lot is unqualified.
Additional inspection criteria:
- If batch acceptance is not qualified, all remaining welds in the batch must be inspected.
- If a crack defect is found on inspection, repeat the spot check.
- If no crack defects are found in the double spot check weld, the batch will be accepted.
- If multiple cracks are found during inspection or cracks are found after double checking, the batch will be unacceptable and all welds remaining in the batch must be inspected.
10. What conditions require qualification in welding procedures?
In addition to the exemptions listed in the national steel structure welding code, any combination of steel, welding materials, welding methods, joint shapes, welding positions, post-weld heat treatment systems, welding process parameters, measurements of preheating and postheating and other parameters used by the construction unit for the first time, will be subject to evaluation of the welding process prior to fabrication and installation of steel structure members.
