Quality Management Of Steel Structure Welding Projects - Welding Defects

As a green and low-carbon building form, steel structure buildings have huge development potential in the future market. Actively promoting steel structure buildings is an important starting point to achieve the green and sustainable development goals of the construction industry. During the "14th five-year plan" period, our country will continue to implement the concept of green development to achieve the goal of "carbon peaking and carbon neutrality", and the proportion of steel structures will gradually increase.

Steel structures are widely used because of their light weight, high strength, and good seismic performance. At the same time, they have novel shapes, complex structures, numerous nodes, high technical content, complex construction, and difficult engineering construction. Quality management and quality assurance measures are particularly important.

Steel structure engineering mainly includes steel structure welding and fastener connection, steel parts and steel component processing, component assembly and pre-assembly, steel structure installation, envelope and curtain wall engineering and anti-corrosion and fire-proof coating engineering, among which welding engineering is very important for steel Structural engineering construction is both a key point and a difficult point.

There are various quality problems in steel structure welding projects, which are caused by many factors, including problems caused by improper processes, problems caused by violations of process operations, and problems caused by the limited technical level of construction personnel, etc.

Welding engineering issues

Welding defects: refers to defects produced during the welding process, such as material discontinuity, unevenness, etc. "Defects" that exceed the standards are called "defects."

1. The size of the weld does not meet the requirements

Phenomenon: The appearance of the weld is uneven, the width of the weld is uneven, the weld reinforcement is too large or too small, the weld width is too wide or too narrow, the transition between the weld and the base metal is not smooth, etc.

Cause Analysis

The weld groove angle, width and assembly gap are not uniform;

The welding method is improperly selected, the filling layer is too high, the groove contour is lost, and the cover surface is of varying width;

The quality of root cleaning and gouging on the back side is poor, and the width of the weld bead is different;

The welding current is too large or too small, the strip transport method and angle are improper, and the welding speed is uneven.

2. Undercut

Phenomenon: The arc or flame melts the base metal at the edge of the weld, without filling the welding metal, leaving a dent or groove.

Cause Analysis

The welding current is too large, the arc is too long, the rod transport angle is inappropriate and the rod transport operation is unskilled;

When transporting the strip, the residence time on both sides of the groove edge is too short, causing the deposited metal and the base material to not be fused;

The filler metal of the weld seam is too low, the welding meat of the cover surface is too thick, the arc residence time is too long, and the temperature of the welding area is too high, causing meat bite.

3. Welding flash

Phenomenon: The molten metal flows to the unmelted base metal outside the weld to form metal nodules, often accompanied by local lack of fusion. It is customary to call the excess lumpy part of the weld metal a weld nodule.

Cause Analysis

The dirt on the edge of the groove is not cleaned; the current is too large and the temperature of the molten pool is too high, causing the liquid metal to solidify slowly and fall under the action of its own weight;

Improper strip transport angle, unskilled operation, too slow welding speed and too large assembly gap, etc.

4. Arc crater

Phenomenon: The depth of the depression formed at the end of the weld bead during welding is greater than 0.5mm.

Cause Analysis

If the weld pool metal is not fully filled, arc extinguishing is too fast or the current is too large (for thin plates), arc craters may easily occur.

5. Splash

Phenomenon: During manual welding, general spatter and severe fusion spatter occur on the weld and the base metal on both sides.

Cause Analysis

If the polarity of the alkaline welding rod is incorrect, the arc will be unstable and cause spatter;

Improper grounding cable connection can cause serious magnetic deflection and cause serious splashing;

Improper storage of the welding rod may cause the surface coating to deteriorate and contain a large amount of gas inside.

6. Arc scratches on the surface of the base metal

Phenomenon: Mostly due to accidental contact between the welding rod or welding handle and the welding workpiece, or poor contact between the ground wire and the workpiece, which briefly causes an arc, leaving scars on the surface of the welding workpiece, forming many small round holes and pits.

Cause Analysis

Careless operation means that the exposed part of the welding rod or welding handle comes into contact with the non-welding part, causing an arc for a short time, scratching the surface of the base metal, and forming small round holes and pits.

7. Stomata

Phenomenon: The gas in the welding pool failed to escape during solidification and remained in the weld to form pores; it is divided into external pores and internal pores, single pores, continuous pores and dense pores, etc.

Cause Analysis

Environmental atmosphere, gases dissolved in the base metal, welding wire and steel core of the welding rod, gases generated by decomposition when the welding rod coating melts, moisture in the protective gas, gases generated by thermal decomposition of oil, moisture, rust spots, etc. on the welding wire and base material, and The gases generated by the welding metallurgical reaction are the causes of pores, and the welding materials and welding processes are the main aspects.

8. Slag inclusion

Phenomenon: Some of the slag remaining in the weld after welding is trapped inside the weld, and some is trapped in the groove.

Cause Analysis

Slag inclusions brought in from the outside: slag inclusions in the base metal are mixed into the weld; high melting point components in the welding rod coating, oxides and slag shells at the edge of the groove are not cleaned and remain in the molten metal during welding. Forming slag;

Metallurgical products during welding: The products of metallurgical reactions during welding (oxides, sulfides, nitrides, etc.) do not have time to float to the surface of the molten metal when the molten pool metal solidifies, and remain in the weld, forming slag inclusions.

9. Incomplete welding

Phenomenon: The root of the joint is not completely melted during welding, leaving a gap.

Cause Analysis

The welding current is too small or the welding speed is too fast, causing the penetration to be too shallow;

The bevel angle is too small, the blunt edge is too large, and the root assembly gap is too narrow;

If the angle of operation of the welding rod is improper, the welding rod will blow sideways, causing the arc heat source to be lost or biased to one side, which can easily cause incomplete penetration on the other side;

During double-sided welding, the backside is not completely cleaned, or oxides, slag, etc. hinder the full fusion between metals.

10. Not fused

Phenomenon: The weld metal and base metal and the weld metal are not completely melted and combined, which is divided into side wall unfusion, interlayer unfusion and weld root unfusion.

Cause Analysis

The welding line energy or flame energy rate is too small, that is, the welding current is too small or the welding speed is too fast, the pause time of the rod movement on both sides of the weld is too short or the welding rod is biased to one side, and the base metal or interlayer metal is not fully melted.

When there are oxides or uncleaned slag and other dirt on the groove of the base metal or the surface of the previous weld bead, and when the welding temperature is not high enough to combine the melt, unfusion will also occur.

The starting temperature of welding is low and the welding speed is too fast, which may cause the beginning of the weld to remain unmelted.

11. Thermal cracks

Phenomenon: Cracks produced at high temperatures are also called crystal cracks. Usually occurs within the weld, sometimes also in the heat affected zone.

Cause Analysis

The generation of hot cracks is the result of the combined action of metallurgical factors and mechanical factors.

When the welding stress is large enough, the liquid interlayer or the solid metal that has just solidified will be pulled apart to form cracks.

12. Cold cracks

Phenomenon: Cracks are generated during the cooling process of the weld. The temperature at which the cracks form is usually below 300°C to 200°C, within the martensitic transformation temperature range.

Cause Analysis

The welded joint forms a hardened structure;

the presence and concentration of diffusible hydrogen;

There is a large welding tensile stress.

Under different circumstances, any of the three factors may lead to the occurrence of cold cracks, among which diffuse hydrogen is the most active factor inducing cold cracks.

13. Lamellar tearing

Phenomenon

Lamellar tearing caused by cold cracking in the welding toe or root in the welding heat affected zone.

Welding heat-affected zone cracking along inclusions is the most common lamellar tear in engineering.

Inclusion cracking along the middle edge of the base metal away from the heat affected zone generally occurs in thick plate structures with more Mn and S flaky inclusions.

Cause Analysis

The type, quantity and distribution form of non-metallic inclusions are the essential cause of lamellar tearing, which is the fundamental reason for the anisotropy and mechanical properties of steel.

Z-direction restraining stress. Thick-walled welded structures bear different Z-direction restraint stresses, post-weld residual stresses and loads during the welding process, which are the mechanical conditions that cause lamellar tearing.

Effect of Hydrogen. It is generally believed that hydrogen is an important influencing factor in lamellar tearing induced by cold cracking near the heat-affected zone.

14. Deformation

Phenomenon: During the welding process, there is non-equilibrium heating and cooling, and structural parts inevitably produce welding residual deformation that cannot be ignored.

Cause Analysis

Too many welding layers;

The base material cools too quickly;

Improper welding sequence;

Improper weld design, such as too many welds;

Improper restraint method.

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