10 Critical Steel Structure Welding Problems & Complete Troubleshooting

1. Advantages & Disadvantages of Welded Steel Joints

Welding connection is the most widely used joint form for modern steel structures, but it has inherent pros and cons that must be fully considered in design:

Core Advantages

1. Simple structural layout, no bolt hole cutting, complete cross-section retention, higher steel utilization rate

2. High overall rigidity and good airtight performance, suitable for container, pipeline and sealed structural parts

3. Diverse welding processes including SAW, MIG/MAG and MMA, easy to realize automatic robotic welding for mass production

4. Flexible connection shape, capable of complex irregular structural splicing

Key Disadvantages & Hidden Risks

1. Uneven heating forms heat-affected zones (HAZ), altering metal grain structure and causing local embrittlement

2. Large residual tensile stress and permanent deformation after cooling, lowering bearing capacity of compression members

3. High crack sensitivity; tiny internal cracks easily expand under load, leading to overall structural failure

4. Obvious low-temperature cold brittleness risk, unsuitable for ultra-low temperature outdoor structures without process optimization

2. Steel Weldability & Carbon Equivalent Evaluation Standard

Steel weldability determines whether high-quality defect-free welds can be obtained under normal construction conditions, mainly affected by chemical composition, rolling technology and plate thickness.

Carbon equivalent (Ceq) is the core evaluation index:

• Low Ceq value: Low hardening tendency, excellent weldability, no preheating required for thin plates

• High Ceq value: Severe hardening risk, prone to cold cracks; mandatory preheating & slow cooling needed for thick plates

Designers must calculate carbon equivalent before formulating welding procedures to match corresponding preheating temperature, welding wire and flux materials.

3. Welding Residual Stress & Deformation: Generation Mechanism & Control Measures

Welding is an uneven rapid heating-cooling cycle: the weld zone expands sharply under high temperature while base metal remains cold, restrained by surrounding steel during cooling, forming longitudinal & transverse residual shrinkage stress and structural bending/twisting deformation.

Design Optimization Measures

1. Disperse weld layout, avoid multi-weld three-dimensional intersection to prevent stress concentration

2. Reduce redundant weld size, control weld load reasonably

3. Set relief holes at stiffener crossing positions to disconnect secondary welds and avoid superimposed shrinkage stress

On-Site Process Control Methods

1. Optimize welding sequence: skip welding, segmented back welding, layered multi-pass welding to balance shrinkage force

2. Pre-set reverse deformation offset to counteract natural weld shrinkage

3. Implement preheating before welding and post-heat tempering to release internal residual stress

4. Three Main Welding Processes For Steel Structure Fabrication

1) Manual Metal Arc Welding (MMA)

Low equipment cost, flexible full-position operation for complex small components. Welding rod coating generates protective slag & gas to isolate air. Disadvantages: low efficiency, unstable manual forming, mainly used for on-site assembly and repair welding.

2) Submerged Arc Welding (SAW)

Arc burns under granular flux, concentrated heat, deep penetration, minimal deformation, ultra-high efficiency for thick straight long welds. Widely applied for H-beam, large steel plate splicing in factories, only suitable flat/horizontal welding with auxiliary fixtures.

3) Gas Metal Arc Welding (GMAW/MIG/MAG)

Shield gas isolates molten pool, stable arc, smooth weld surface, supports all-position welding. Two operation modes:

• Push welding (forward): Shallow penetration, wide flat bead, beautiful molding

• Drag welding (backward): Deep penetration, narrow high reinforcement, for thick plate root welding

5. Standard Welding Symbols & Joint/Pocket Code Rules

Unified international welding symbols unify communication between designers, technicians and inspectors, covering welding position, joint form and groove type:

1. Welding Position Code: F(Flat), H(Horizontal), V(Vertical), O(Overhead)

2. Main Joint Code: B(Butt), T(T-joint), C(Corner), L(Lap), X(Cross)

3. Groove Code: I, V, X, K, U, J (U/J groove for plates over 50mm thickness)

6. Six Major Categories Of Steel Welding Defects, Causes & Repair Solutions

All weld defects directly reduce structural safety; below are the most frequent faults in steel workshops:

1) Cracks (Hot Crack & Cold Crack)

• Hot crack: Excessive welding heat, high impurity content, concentrated shrinkage stress

• Cold crack: High carbon equivalent steel, insufficient preheating, fast cooling speed

• Repair: Drill crack stop holes at both ends, completely remove cracked metal then re-weld

2) Voids (Porosity & Crater Shrinkage Cavity)

Caused by rust/oil on workpiece surface, damp welding consumables, excessive travel speed. Remove porous layer and perform patch welding.

3) Solid Inclusions (Slag Inclusion & Tungsten Inclusion)

Multi-layer welding without slag cleaning, too small welding current. Dig out inclusion area before re-welding.

4) Lack Of Fusion / Lack Of Penetration

Too fast travel speed, narrow groove gap, low welding heat input. Back gouging and double-sided welding for critical joints.

5) Shape Defects (Undercut, Overlap, Misalignment)

Improper gun angle, mismatched current/voltage. Grind defective area and cosmetic repair welding.

6) Burn Through, Weld Bead Overflow

Excessive welding current, thin base plate, too slow travel speed. Fill and polish after removing excess metal.

7. Prevention Measures For Lamellar Tearing Of Thick Steel Plates

Lamellar tearing occurs on T/cross joints with plate thickness ≥20mm, generated by thickness-direction tensile shrinkage stress. Standard optimization schemes:

1. Adopt narrow V/K groove to reduce weld filling volume and shrinkage force

2. Use symmetrical double-sided groove instead of single-sided asymmetric groove

3. Extend the end of stressed steel out of weld zone to transfer tensile stress

4. Replace cross joints with cast steel transition segments to avoid multi-direction stress superposition

8. Two Main Weld Quality Inspection Technologies

1) Visual Inspection (VT)

First-step inspection: Check weld size, surface undercut, cracks, porosity, misalignment with naked eye or magnifying glass. All projects must pass before internal flaw detection.

2) Non-Destructive Testing (NDT)

1. Ultrasonic Testing (UT): Mainstream internal defect inspection, fast detection, low cost, judges defect position and depth via waveform, widely used for steel structure on-site inspection

2. Radiographic Testing (RT): X/γ ray penetrates weld to form defect images on film, accurate defect type identification. X-ray for plates under 30mm; γ-ray for super-thick components.

9. Acceptance Standard For Random Weld Sampling Inspection

Steel structure engineering follows unified sampling judgment rules:

1. Unqualified rate ＜2%: Entire batch passes acceptance

2. Unqualified rate ＞5%: Entire batch rejected, full inspection required

3. Unqualified rate 2%~5%: Double sampling test; total unqualified ≤3% passes, otherwise reject

4. Any crack detected: Double re-inspection; secondary crack found → full inspection of all welds

10. Conclusion

Steel structure welding involves systematic control from design drawing, material selection, welding process to post-inspection. Targeted optimization of weld layout, preheating parameters, welding sequence and inspection standards can eliminate over 95% of common defects including deformation, cracks and lamellar tearing. Standardized construction procedures are the core guarantee of long-term structural safety.

If you need customized welding process schemes, defect troubleshooting or robotic welding transformation for steel fabrication, contact professional engineers at heavth.com for full-process technical support.