Implementation and control of field welding technology for steel structure column joints

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This paper briefly introduces the basic properties, welding technology and basic essentials of field welding operation of Q345C steel.

Keywords: Q345C field welding process

The steel structure of the main workshop of 2×600MW unit in the first phase of Hebei Dingzhou Power Plant was designed by Hebei Electric Power Design Institute and manufactured by Sanmenxia Hydraulic Machinery Factory. The steel structure of the main workshop is divided into: deoxygenated coal bunker, heater platform and A0-A column of steam turbine room. The main column, beam and vertical support of the steel frame are all made of "H" section steel, and the base material is Q345C (low alloy structural steel). The main columns of the steel frame are connected into a whole by subsection butt joint, in which the webs of H-shaped steel are connected by high-strength bolts and the flanges are connected by butt joint welding.

I. Introduction of materials

The chemical composition of 1 Q345 is as follows (%):

Element C≤ Mn Si≤ P≤ S≤ Al≥ V Nb Ti

The content is 0.21.0-1.60.55 0.035 0.0150.02-0.150.015-0.06 0.02-0.2.

The mechanical properties of Q345C are as follows (%):

Mechanical property index elongation (%) Test temperature 0℃ Tensile strength MPa yield point MPa≥

Value Δ 5 ≥ 22j ≥ 34σ b (470-650) σ s (324-259)

When the wall thickness is between 16-35mm, σ s ≥ 325mpa; When the wall thickness is between 35 and 50 mm, σ s is ≥ 295 MPa.

2. Welding characteristics of 2.Q345 steel

2. 1 carbon equivalent (Ceq) calculation

CEQ = C+Mn/6+Ni/ 15+Cu/ 15+Cr/5+Mo/5+V/5

Ceq=0.49%, exceeding 0.45%. It can be seen that the weldability of Q345 steel is not very good, and strict technological measures need to be formulated during welding.

2.2 Problems in Welding of Q345 Steel

2.2. 1 hardening tendency of heat affected zone

During the welding and cooling process of Q345 steel, the quenched martensite is easy to form in the heat affected zone, which increases the hardness and reduces the plasticity in the near-seam zone. As a result, cracks appeared after welding.

Cold crack sensitivity

The welding cracks of Q345 steel are mainly cold cracks.

Second, the welding construction process

Groove processing → spot welding → preheating → root cleaning (carbon arc gouging) → external welding → internal welding → self-inspection/special inspection → post-weld heat treatment → nondestructive testing (weld quality reaches Grade I).

Third, the selection of welding process parameters

By analyzing the weldability of Q345 steel, the following measures are formulated:

1. Selection of welding materials

Because Q345 steel is prone to cold cracking, low-hydrogen welding materials should be selected. Considering the principle that the strength of welded joints should be equivalent to that of base metal, E50 15 (J507) covered electrode should be selected.

See the following table for chemical composition (%):

Element C, Mn, Si, P, Cr, Mo, V and Ti

The content is 0.071.1.1.530.009 0.0160.020.01.0/.

See the following table for mechanical properties:

Mechanical property index σ b (MPa) σ s (MPa) δ 5 (%) ψ (%) AKVJ-30℃

Value 440 540 317916411476.

2. Groove form: (provided according to drawings and equipment)

3. Welding method: Manual arc welding (D) is adopted.

4. Welding current: In order to avoid coarse weld structure and reduce impact toughness, small-scale welding must be adopted. The specific measures are as follows: select small-diameter covered electrode, narrow bead, thin welding layer and multi-layer and multi-pass welding process (see Figure 1 for welding sequence). The weld bead width is not more than 3 times the width of covered electrode, and the thickness of welding layer is not more than 5 mm The first to third layers adopt Ф 3.2 covered electrode, and the welding current is100-130a; Covered electrode Ф 4.0 is used for the fourth to sixth floors, and the welding current is 120- 180A.

5. Preheating temperature: Since the CEQ of Q345 steel is more than 0.45%, it should be preheated before welding, with the preheating temperature T0= 100- 150℃ and the interlayer temperature Ti≤400℃.

6. Post-weld heat treatment parameters: In order to reduce welding residual stress, reduce the hydrogen content in the weld and improve the metal structure and properties of the weld, the weld should be heat treated after welding. The heat treatment temperature is 600-640℃, the constant temperature time is 2 hours (when the plate thickness is 40mm), and the heating and cooling rate is125℃/h.

Fourth, the welding sequence:

1. preheat before welding

Before welding the flange, preheat the flange and keep it at a constant temperature for 30 minutes before welding. The preheating, interlayer temperature and heat treatment of welding are automatically controlled by the heat treatment temperature control cabinet. Using far infrared crawler heating plate, microcomputer automatically sets and records curves, and thermocouple measures temperature. During preheating, the measuring point of thermocouple is 15mm-20mm away from the edge of groove.

weld

2. 1 To prevent welding deformation, each column joint is symmetrically welded by two people, and the welding direction is from the middle to both sides. When welding the inner mouth (the inner mouth is a groove near the web), the first to third layers must be operated in small specifications, because its welding is the main reason that affects the welding deformation. After welding one or three layers, clean the back. After the completion of carbon arc gouging, the weld must be mechanically polished, and the surface of the weld must be cleaned to carburize and expose the metallic luster, so as to prevent the surface from cracking due to serious carbonization. The welding of the outer port shall be completed at one time, and then the remaining inner ports shall be welded.

2.2 When welding the second layer, the welding direction should be opposite to that of the first layer, and so on. The welded joints of each layer shall be staggered by 15-20mm.

2.3 When two welders are welding, the welding current, welding speed and welding layers should be consistent.

2.4 When welding, it should start from the arc striking plate and end on the arc closing plate. Cutting and polishing after welding.

3. Post-weld heat treatment: The heat treatment shall be carried out within 12 hours after the joint is welded. If heat treatment can't be carried out in time, heat preservation and slow cooling measures should be taken. During heat treatment, two thermocouples should be used to measure the temperature, and the thermocouples should be spot-welded on the inside and outside of the weld.

The welding temperature curve of Q345 steel is shown below.

4. Welding inspection

According to the requirements of Code for Construction and Acceptance of Steel Structure Engineering, the weld is inspected by ultrasonic flaw detection, and the flaw detection ratio is 100%.

Verb (abbreviation of verb) on-site technical management

1. Prepare detailed welding construction work instructions.

2. Controlling the whole welding process is the core to ensure the quality.

When welding each column joint, there should be a special person to monitor the welding process. If the welder does not follow the work instruction, the welding shall be terminated immediately. During the welding process, the heat treatment personnel should monitor the interlayer temperature throughout the whole process, and notify the welder to stop immediately if it exceeds the standard.

3. Improving the quality awareness of construction personnel is the key to the implementation of welding technology.

Before construction, all personnel shall make disclosure and receive the construction process card. In the disclosure, the characteristics of welding process and the necessity and control points of strictly controlling on-site welding process are explained in detail.

Conclusion of intransitive verbs

According to this welding process, 102 welded joints were welded on site, and the first pass rate reached 100% after nondestructive testing. The actual construction shows that the welding process can not only guide the welding of Q345 steel on site, but also ensure the welding quality.