I. Chemical Composition and Mechanical Properties of Q345 Steel
1. The chemical composition of Q345 is shown in the following table (%)
| Element | Happy |
| W | ≤0.2 |
| Mn | 1.0-1.6 |
| Yes | ≤0.55 |
| P | ≤ 0.035 |
| s | ≤0.035 |
| Al | ≥0.015 |
| V | 0.02-0.15 |
| No. | 0.015-0.06 |
| You | 0.02-0.2 |
2. The mechanical properties of Q345C are as follows (%)
| Mechanical performance indicators | Value |
| Elongation rate (%) | ≥ 22 |
| Test temperature 0 ℃ | ≥ 34 |
| Tensile strength MPa | 470-650 |
| Yield point MPa | 324-259 |
When the wall thickness is between 16-35mm, σs≥325Mpa; when the wall thickness is between 35-50mm, σs≥295Mpa.
II. Welding characteristics of Q345 steel
2.1 Calculation of Carbon Equivalent (Ceq)
Ceq=C+Mn/6+Ni/15+Cu/15+Cr/5+Mo/5+V/5
The calculated Ceq is 0.49%, higher than 0.45%. This indicates that the welding performance of Q345 steel is not very good and strict process measures need to be formulated during welding.
2.2 Common problems with Q345 steel during welding
2.2.1 Hardening Trend of the Heat-Affected Zone
During the Q345 steel welding cooling process, the heat-affected zone tends to form a tempered structure – martensite, which increases the hardness of the area close to the seam and reduces its plasticity. As a result, cracks may occur after welding.
2.2.2 Sensitivity to Cold Cracking
The main type of welding crack in Q345 steel is cold cracking.
III. Welding construction process
Bevel preparation → Spot welding → Preheating → Internal welding → Post gouging (with carbon arc gouging) → External welding → Internal welding → Self-inspection/special inspection → Post-welding heat treatment → Non-destructive inspection (bead quality welding grade one pass)
4. Selection of Welding Process Parameters
Through the weldability analysis of Q345 steel, the following measures were developed:
1. Selection of welding materials
Due to the high cracking tendency of Q345 steel, low hydrogen welding materials must be used. Considering the principle that the welded joint should be as strong as the base material, the E5015 (J507) type welding rod is chosen.
The chemical composition and mechanical properties are as follows (in%):
| Element | Happy |
| W | 0.071 |
| Mn | 1.11 |
| Yes | 0.53 |
| s | 0.009 |
| P | 0.016 |
| Cr | 0.02 |
| Mo | 0.01 |
| V | 0.01 |
| You | 0.01 |
The mechanical properties are shown in the following table:
| σb/Mpa | σs/Mpa | δ5（%） | Ψ（%） | Akv/J-30℃ |
| 440 | 540 | 31 | 79 | 164/114/76 |
2. Beveled Shape:
According to drawings and equipment supply.
3. Welding method:
Manual electric arc welding will be used (D).
4. Welding current:
To avoid a coarse weld seam structure, which may result in a decrease in impact resistance, small specification welding should be adopted. Specific measures include small diameter welding rods, narrow welding paths, thin welding layers, and multilayer and multipath welding processes.
The width of the welding path should not exceed three times the diameter of the welding rod, and the thickness of the welding layer should not exceed 5 mm.
For the first to third layers, a Ф3.2 welding rod with a welding current of 100-130A should be used; for the fourth to sixth layers, a Ф4.0 welding rod should be used with a welding current of 120-180A.
5. Preheating temperature:
As the Ceq of Q345 steel is >0.45%, preheating should be done before welding, with the preheating temperature T0 = 100-150 ℃ and the interlayer temperature Ti≤400 ℃.
6. Post-weld heat treatment parameters:
To reduce the welding residual stress, decrease the hydrogen content in the weld seam, and improve the metal structure and properties of the weld seam, post-welding heat treatment should be carried out on the weld seam.
The heat treatment temperature is 600-640°C, with a holding time of 2 hours (for a board thickness of 40mm), and the heating/cooling rate is 125°C/h.
V. On-site welding sequence
1. Pre-welding preheating
Before welding the flange plate, preheat it first and start welding after a constant temperature of 30 minutes.
Preheating, interlayer temperature and heat treatment are automatically controlled by the heat treatment temperature control cabinet using far infrared range type heating plates, with a microcomputer automatically adjusting and recording the curve and a thermocouple measuring the temperature .
During preheating, the measuring point of the thermocouple is 15mm-20mm from the chamfer edge.
2. Welding
2.1 To avoid welding deformation, each column joint is welded symmetrically by two people, with the welding direction from the middle to the sides.
When welding the inner mouth (the chamfer facing the belly plate), the first to third layers should use a small specification operation, as their welding is the main cause of welding deformation.
After welding the first to third layers, subsequent gouging is carried out. After carbon arc gouging is completed, the weld seam must be mechanically polished, the carbon penetration on the surface of the weld seam cleaned, and the metallic luster exposed, to prevent severe surface carbonization from causing cracking.
The outer mouth must be welded in one go, and finally the remaining part of the inner mouth must be welded.
2.2 When welding the second layer, the welding direction should be opposite to that of the first layer and so on. Each layer of the welding joint must be staggered by 15-20 mm.
2.3 The two welders must maintain consistency in welding current, welding speed and number of layers welded.
2.4 When welding, start at the impact plate and end at the back plate. After welding is complete, cut and polish.
3. Post-welding heat treatment:
After welding is completed, heat treatment must be carried out within 12 hours. If heat treatment cannot be carried out in time, insulation and slow cooling measures must be taken. During heat treatment, the temperature must be measured with two thermocouples, spot-welded on the inside and outside of the weld.
4. Welding inspection
According to the requirements of the “Steel Structure Engineering Construction and Acceptance Specifications”, welds are inspected using ultrasonic flaw detection, with an inspection rate of 100%.
SAW. Onsite Technical Management
1. Prepare detailed welding construction work instruction manuals.
2. Full control of the welding process is the core to ensure quality.
During welding of each column joint, a dedicated person must monitor the welding process. If the welder does not follow the work instructions manual, welding must be stopped immediately.
During the welding process, heat treatment personnel must monitor the interlayer temperature throughout the process. If it exceeds the standard, the welder must be immediately notified to stop.
3. Increasing the quality awareness of construction personnel is critical to implementing the welding process.
Before construction, a full informational meeting should be held with the team and a construction process card issued. The briefing must explain in detail the characteristics of the welding process and the need and control points to strictly control the welding process on site.
VII. Conclusion
Following this welding process measure, a total of 102 welds were made on site, with a unique 100% pass rate in non-destructive testing.
Through practical construction verification, this measurement of the welding process can not only guide the welding of Q345 steel on site, but also ensure the welding quality.
