Knowledge of welding defects and heat treatment of pressure vessels (2)

In order to prevent cracks, you can limit the S and P content of steel and welding materials: adjust the chemical composition of the steel; refine the weld grains; improve the basicity of the welding material; improve segregation; control the welding specifications; Welding, using a small line of energy; the casting is broken, reducing arc pits.

You can also choose low-hydrogen alkaline electrodes, which are strictly dried and taken as needed; use reasonable welding specifications; eliminate hydrogen immediately after welding; improve the quality of steel and reduce layered inclusions in steel; financially reduce various welding process measures . Reduce residual stress and stress concentration; slow cooling by preheater, heat treatment after welding. These methods, as long as they are used properly, can improve the quality of welding and prevent defects.

As for unwelded, unfused, slag inclusions, blowholes, and weld surface defects such as meat bites, weld size, etc., can pass non-destructive inspection to determine the location of the defect, take a reasonable and effective repair process, and operate carefully To achieve the purpose of eliminating weld defects and ensuring the inherent quality of the product.

The heat treatment can eliminate the residual stress and prevent deformation, that is, it can relax the welding residual stress and stabilize the size and shape. Post-weld heat treatment can also improve the performance of the base metal and welded structure: Specifically: it can soften the heat affected zone, increase the ductility of the weld metal, improve fracture toughness, discharge harmful substances such as hydrogen, improve corrosion resistance and improve creep And increase fatigue strength.

However, improper selection of post-weld heat treatment process will degrade the performance of welded joints. Therefore, post-weld heat treatment has become an important part of container manufacturing. The most widely used post-weld heat treatment of welded joints is high temperature tempering, normalizing and solid-melting. Tempering at high temperature can solve the adverse effects of pressure vessel quality caused by welding and deformation.

1. Post-weld heat treatment can relax welding residual stress

2. Softening of HAZ in hardened zone of welded joint

3. Reduced hydrogen in welded joints

4. Influence on weld metal tensile strength

5. Effect on weld metal impact toughness

6. Effect on width of decarburized layer

In order to achieve the desired post-weld heat treatment effect, we must carefully study, it is very important to choose the appropriate post-weld heat treatment process.

a. the temperature of the furnace when the weldment enters the furnace,
b. Upper and lower temperature limits during heating,
c. Upper and lower heating speed limits,
d. Upper and lower holding time,

e. Upper and lower limits of cooling rate,

f. Out-of-bath temperature,
g. The temperature difference of each part of the welded part being heated during the heating process,
h. The temperature difference of each part of the welded part to be heated during heat preservation,
i. Atmosphere in the furnace. It can be seen that there are many factors that affect heat treatment, and the choice should be cautious and cannot be selected at will.

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