Welding instructions for large diameter carbon steel elbows

Large diameter carbon steel elbows occupy an important position in modern industrial pipe systems, especially in the transport of high temperature and high pressure media, high-quality elbows can ensure the tightness and durability of the entire pipe system.
In order to achieve high quality products, the manufacture of large diameter carbon steel elbows needs to go through multiple processes such as stamping, preheating, welding, tempering and annealing. This paper will analyze each process link in detail, and discuss the key points, technical requirements and precautions of process control.

Stamping process

In the production of large diameter carbon steel elbows, stamping is usually divided into two processes: cold stamping and hot stamping. The choice of which process is mainly based on the following factors:

Material properties: For carbon steel with good cold forming performance and good plasticity, the cold stamping process can achieve higher forming accuracy and dimensional stability; For some steels with high carbon content or poor toughness, the hot stamping process can reduce the difficulty of processing and reduce the risk of stress concentration and cold cracking because it is processed at high temperatures.
Equipment capacity: The technical level and capacity of the plant equipment determine the choice of stamping process. Some high-end equipment can achieve high precision cold stamping, while some equipment is limited to heating systems, suitable for hot stamping processes.
Product requirements: For elbow products with beautiful shape, uniform wall thickness and small size deviation, especially stainless steel thin-wall elbow, it is mostly manufactured by internal and external die cold extrusion process; The most suitable process for carbon steel elbow is selected according to the specific design parameters.

Stamping process
For carbon steel elbows, the stamping process generally uses a tube blank equal to the outer diameter of the elbow as the raw material. The process is as follows:
Tube blank preparation: The tube blank equal to the outer diameter of the elbow is selected as the raw material.
Die assembly: The tube blank is first placed on the lower die, and then loaded into the inner core and end die.
Stamping process: the upper die moves down, through the constraints of the outer die and the support of the inner die, the tube blank moves along the reserved gap under the push rod, and the final shape is a bend.
This process requires high precision of the internal and external die, and strict requirements for the wall thickness deviation of the tube blank, so as to ensure that the shaped elbow is beautiful, the wall thickness is uniform and the size deviation is small.

Preheating and welding process

Preheating process

Objective: To reduce the maximum hardness of heat affected zone during welding of medium carbon steel and prevent cold crack; Improve the plasticity of welded joint and reduce the residual stress after welding.
Local preheating: For large weldparts, when the overall preheating is difficult, local preheating can be used, usually within the range of 150~200mm on both sides of the welding joint.

Welding process precautions

When welding, the basic electrode with permitted conditions should be preferred to reduce the hydrogen content in the base metal and prevent hydrogen embrittlement.
When welding the first layer weld, due to the high melting ratio of the base metal, small current and slow welding speed should be used to reduce the penetration depth of the base metal and ensure the stable metallographic properties of the weld.

Post-welding treatment

If stress relief heat treatment cannot be performed after welding, post-heat treatment should be performed immediately to ensure that the welded joint and the base material are in a stress release state, thereby reducing the risk of cracks in the welding area.

Heat treatment

Tempering process: Tempering is to reheat the quenched carbon steel elbow to a certain temperature, and then use a specific cooling method to eliminate the internal stress generated by quenching, while reducing hardness and brittleness, in order to obtain the expected mechanical properties. Tempering can be divided into high temperature tempering, medium temperature tempering and low temperature tempering according to temperature.
Annealing process: Annealing is the carbon steel elbow heated to the temperature of phase change or partial phase change (such as Ac₁, Ac₃ interval), heat treatment process of slow cooling after heat preservation.

Bevelling and welding preparation

Bevel finish

Purpose:
Before welding, the welding part is processed into a U shape or V shape groove to reduce the proportion of the base material into the weld metal, thereby reducing the carbon content in the weld and preventing cracks caused by excessive carbon content.
Processing requirements:
The shape of the groove should be smooth, and the Angle of the groove should be processed according to the design requirements to ensure the welding quality and joint strength.

Special fixture and positioning

For large diameter elbows, the tube billet is positioned and calibrated using a special elbow forming machine and fixture to ensure a good mold fit.
If necessary, the chuck is modified to ensure a firm grip and avoid uneven stress during the welding process.

The welding of large diameter elbow is a process with high technical content, and the manufacturer must strictly control the whole process from pre-welding preparation, process selection, welding parameter control to post-welding treatment and testing. Only on the premise of ensuring that all links meet the requirements of the code, can we ensure the long-term safe and reliable operation of large caliber elbows under complex conditions such as high pressure and high temperature.

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