Inspection standard and welding control problem of thick-walled steel pipe

Through observation, it is not difficult to find that whenever thick-walled steel pipes, thermally expanded pipes, etc. are produced, the pipes obtained by thick-wall welding on high-frequency welding equipment with strip steel as the production material are called thick-walled steel pipes. Among them, according to different uses, and different production processes, (can be roughly divided into scaffolding pipes, fluid pipes, wire casings, bracket pipes, guardrail pipes, etc.). Thick-walled welded pipe specification GB/T3091-2008 and low-pressure fluid welded pipe is a kind of thick-walled welded pipe. Usually, water and gas are transported. After welding, one more water pressure test is added than usually welded pipe. Therefore, the low-pressure fluid pipe is thicker than usual. Welded pipes are usually quoted a little higher.

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

1. Thick-walled steel pipes should be submitted for inspection in batches, and the batching rules should conform to the rules of the corresponding commodity specifications.

2. The inspection items, sampling quantities, sampling locations, and experimental methods of thick-walled steel pipes shall be following the rules of the corresponding commodity specifications. With the approval of the purchaser, the hot-rolled seamless thick-walled steel pipe can be sampled in batches according to the rolling root group.

3. As for the experimental results of thick-walled steel pipes, when a certain item does not meet the requirements of the product specification, the unqualified ones should be picked out, and from the same batch of thick-walled steel pipes, double the number of samples should be taken to carry out the unqualified items. 's retest. If the re-inspection results are unqualified, the batch of thick-walled steel pipes shall not be delivered.

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

5. If there are no special rules in the product specification, the chemical composition of the thick-walled steel pipe shall be checked according to the smelting composition.

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

7. The supplier has rules to ensure that the delivered thick-walled steel pipes conform to the corresponding commodity specifications. The buyer has the right to inspect and inspect according to the corresponding commodity specifications.

In addition, there are some things we need to know about the welding control 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 the formula, the high-frequency eddy current thermal power is affected by the current frequency. The eddy current thermal power is proportional to the square of the current encouragement frequency, and the current encouragement frequency is also affected by the encouragement voltage and current. and capacitance and inductance. For low carbon steel, the welding temperature is controlled at 1250~1460°C, which can meet the penetration requirements of the pipe wall thickness of 3~5mm. In addition, the welding temperature can also be adjusted by adjusting the welding speed.

When the input heat is lacking, 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 lacking, the edge of the heated weld exceeds the welding temperature and occurs. Overburning or dripping makes the weld form a hole.

2. Control of the weld gap of thick-walled steel pipes

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 circular tube blank with an open gap. Make the ends of the welds flush. If the gap is too large, the nearby effect will be reduced, the eddy current heat will be lacking, and the intergranular bonding of the weld will be poor, resulting in incomplete fusion or cracking. If the gap is too small, the proximity effect will increase, and the welding heat will be too large, resulting in weld burnout; perhaps the weld will form deep pits after being kneaded and rolled, which will affect the weld surface.