Inspection standards and welding control problems of thick-walled steel pipes

Through observation, it is not difficult to find that in the production of thick-walled steel pipes, thermal expansion pipes, etc., the pipes obtained by thick-walled welding on high-frequency welding equipment are called thick-walled steel pipes. Among them, according to the different uses and different post-production processes, (it can be roughly divided into scaffolding tubes, fluid tubes, wire casings, bracket tubes, guardrail tubes, etc.). Thick-walled welded pipe specification GB/T3091-2008 and low-pressure fluid welded pipe is a kind of thick-walled welded pipe. It is usually transported by water and gas. Welded pipes are usually quoted a little higher.

The inspection standards for thick-walled steel pipes mainly include the following points:

1. Thick-walled steel pipes should be submitted for inspection in batches, and the batching rules should meet the requirements of the corresponding product specifications.

2. The inspection items, sampling quantity, sampling location, and test methods of thick-walled steel pipes shall be by the provisions of the corresponding product specifications. With the consent of the purchaser, hot-rolled seamless thick-walled steel pipes can be sampled in batches according to the number of rolled roots.

3. In the test results of thick-walled steel pipes, when a certain item does not meet the specifications of the product, the unqualified ones should be picked out, and double the number of samples should be randomly taken from the same batch of thick-walled steel pipes to carry out unqualified items. retest. If the re-inspection results are unqualified, the batch of thick-walled steel pipes shall not be delivered. For the following inspection items, if the initial inspection fails, re-inspection is not allowed.

4. For thick-walled steel pipes that fail the re-inspection results, the supplier can submit them for inspection one by one; or conduct heat treatment again and submit a new batch for inspection.

5. If there is no special regulation in the product specification, the chemical composition of thick-walled steel pipes shall be inspected according to the melting composition.

6. The inspection and inspection of thick-walled steel pipes should be carried out by the technical supervision department of the supplier.

7. The supplier has rules to ensure that the delivery of thick-walled steel pipes conforms to the corresponding product specifications. The buyer has the right to inspect and inspect according to the corresponding commodity specifications.

Welding control problems of thick-walled steel pipes:

1. Welding temperature control of thick-walled steel pipes

The welding temperature is affected by the high-frequency eddy current thermal power. According to formula (2), the high-frequency eddy current thermal power is affected by the current frequency, and the eddy current thermal power is proportional to the square of the current encouragement frequency; The influence of voltage, current, capacitance, and inductance. The formula of encouraging frequency is f=1/[2π(CL)1/2]...(1) In the formula: f-encouraging frequency (Hz); C-capacitance (F) in the encouraging circuit, capacitance=electricity/voltage; L-inductance in the excitation loop, inductance = magnetic flux / current The above formula shows that the excitation frequency is inversely proportional to the square root of the capacitance and inductance in the excitation loop, or proportional to the square root of the voltage and current, and only needs to change the capacitance and inductance in the loop Or voltage, current can change the size of the excitation frequency, to achieve the purpose of controlling the welding temperature. For low-carbon steel, the welding temperature is controlled at 1250~1460℃, which can meet the penetration requirements of the pipe wall thickness of 3~5mm. In addition, the welding temperature can also be achieved by adjusting the welding speed. When the input heat is insufficient, the edge of the heated weld cannot reach the welding temperature, and the metal structure remains solid, resulting in incomplete fusion or incomplete penetration; when the input heat is insufficient, the edge of the heated weld exceeds the welding temperature, resulting in Overburning or dripping makes the weld form a molten hole.

2. Control of weld gap of thick-walled steel pipe

The strip steel is sent to the welded pipe unit, rolled by multiple rollers, and the strip steel is gradually rolled up to form a round tube blank with an opening gap. Adjust the reduction of the kneading roller to control the weld seam gap at 1~3mm. And make the two ends of the welding port flush. If the gap is too large, the proximity effect will be reduced, the heat of the eddy current will be insufficient, and the intergranular bonding of the weld will be poor, resulting in unfused or cracking. If the gap is too small, the proximity effect will increase, the welding heat will be too large, and the weld seam will be burned; or the weld seam will form deep pits after kneading and rolling, which will affect the weld seam surface.