Quality control measures for large diameter steel pipes

Quality control measures for large diameter steel pipes

Large-diameter steel pipes (generally straight seam submerged arc welded pipes or spiral submerged arc welded pipes) have large wall thickness, good material, and stable processing technology. In the welded joints of large-diameter steel pipes, the weld seams and heat-affected zones are the places where various defects are most likely to occur. Welding undercuts, pores, slag inclusions, lack of fusion, lack of penetration, welding flashes, burn-through, and welding cracks are the main forms of welding defects and are often the source of quality accidents in large-diameter steel pipes.

Large-size seamless pipe forming technology

The forming technologies of large-size seamless pipe fittings mainly include: hot spinning, hot extrusion, hot piercing and rolling, and hot pushing and expanding. details as follows:

1.hot spinning method

It is to fix the flat blank or pre-formed blank to the rotating mandrel, apply pressure to the blank with the rotating wheel, and the rotating wheel is fed axially at the same time. After one or more processing, various thin-walled hollow rotating systems are obtained. The process method of the product. Its advantages are high product precision, good mechanical performance and wide size range. The tool mold is simple, easy to replace, and the production cost is lower than other methods. The main disadvantages are the long production hours and low output. The processed pipe fittings are limited by the length of the equipment and the size is short.

2.hot extrusion method

The billet needs to be pre-processed by machining before extrusion. When extruding pipes with a diameter of less than 100mm, the equipment investment is small, the material waste is small, and the technology is relatively mature. However, once the diameter of the pipe is enlarged, the hot extrusion method requires equipment with large tonnage and high power, and the corresponding control system must be upgraded.

3.hot piercing and rolling method

Hot piercing rolling is mainly based on longitudinal rolling and cross rolling. Longitudinal rolling and extension rolling are mainly limited-moving mandrel continuous tube rolling, small-stand limited mandrel continuous rolling tube rolling, three-roll restricted mandrel continuous rolling tube rolling and floating mandrel continuous tube rolling. These methods have high production efficiency, low metal consumption, good product quality, advanced control systems, and increasingly widespread applications. The skew extension rolling method has also been widely used because of its low investment, short process, suitable for small batches, multiple varieties, multiple specifications and high-precision production of medium-thick wall seamless pipes.

4.thermal expansion method

The diameter expansion equipment is simple, low cost, convenient maintenance, economical and durable, and flexible product specification change. If you need to prepare large diameter elbows and other similar products, you only need to add some accessories. It is suitable for the production of medium and thin-walled and large-diameter seamless steel pipes, as well as thick-walled pipes that do not exceed the equipment capacity.

How to control the quality of large diameter steel pipes?

1. Quality control before welding
1) First check the quality of raw materials. Only after passing the inspection can you officially enter the construction site, and resolutely put an end to the use of unqualified steel.
2) Next is the management of welding materials. Check whether the welding materials are qualified products, whether the storage and baking system is implemented, whether the surface of the distributed welding materials is clean and rust-free, whether the electrode coating is intact and whether there is mildew.
3) The third is the clean management of the welding area. Check the cleanliness of the welding area. There should be no water, oil, rust, oxide film and other dirt. This plays an important role in preventing the appearance of external defects in the weld.
4) When choosing the appropriate welding method, the principle of test welding first and then welding should be followed.

2. Welding quality control
1) Check whether the welding wire and flux specifications are correct according to the welding process regulations to prevent welding quality accidents caused by incorrect use of welding wire and flux.
2) Supervise the welding environment. When the welding environment is not good (temperature below 0°C, relative humidity above 90%), corresponding measures should be taken before welding.
3) Before pre-welding, check the groove dimensions, including gaps, blunt edges, angles and gaps, etc., to see if they meet the process requirements.
4) Whether the welding current, welding voltage, welding speed and other process parameters selected in the submerged arc automatic internal and external welding process are correct.
5) Supervise the welding personnel to make full use of the length of the arc striking plate at the end of the steel pipe when performing automatic internal and external submerged arc welding.
6) Supervise the welding personnel to see whether the welding slag has been cleaned first, whether the joints have been thoroughly treated, and whether there is oil, rust, slag, water, paint and other dirt on the groove.

Summarize

In the production process of large-diameter steel pipes, since the initial processing is performed in a hot state, the heating operation is a very important process that determines product quality. Heating furnaces are divided into heating furnaces and reheating furnaces according to their functions; the former is used to heat the blank from room temperature to the processing temperature; the latter is used to reheat the blank to the temperature required for processing. Improper heating will cause cracks, folds, pain and other waste products on the inner or outer surface of the tube blank.

At the same time, the forming process of large-diameter steel pipes should be reasonably designed. Springback control design is usually adopted, which can effectively control the rebound state of the tube blank, thereby ensuring that the rebound state of the tube blank meets the design requirements. The main factor affecting the elastic recovery state of the tube blank is the yield strength of the material.

For example, when designing the structure of large-diameter steel pipes, we can use materials under different spring recovery conditions to process them into qualified pipe blanks. In fact, process design can be expressed with a complete mathematical model, so that each link is presented more clearly.

There are many types of hot air furnaces, but the most important one is the annular furnace. The bottom of this heating furnace is ring-shaped and can slowly deform. The billets are loaded from the entrance along the diameter of the furnace bottom, and can be heated and evenly heated to the specified temperature as soon as the exit is used up. A type of stove. The key to this heating operation is to heat the blank evenly to a temperature suitable for processing. Since perforation has a great impact on quality, that is, the temperature during the bending process is an important condition affecting quality, so it is generally necessary to control the temperature of the blank during the perforation process.