Pressure Vessel Welding Defects and Heat Treatment Knowledge

Pressure vessels play a very important role in petrochemical production. Pressure vessels can act as petrochemical equipment such as reaction, energy exchange, separation, tower, storage, transportation, etc. They have the danger of bombing, and their safe operation is directly related to enterprise production and personal safety.

Welding has become a key process in the production of pressure vessels, and the quality of welding is a very important link to ensure the quality of pressure vessels. The quality of single welding is affected by many factors: welder skills, chemical composition, mechanical properties, welding materials, welding process and equipment, environment, etc. can all affect the welding quality.

Welding defects
1. Cracks in welded joints
As we all know, the welded joint is an inhomogeneous body and an inhomogeneous body of mechanical properties. During the welding process, near the fusion line of the welded joint, the temperature is between the solid phase and the liquid phase. After cooling, the microstructure belongs to the overheated structure, the grains are coarse, the chemical composition and structure are extremely uneven, the strength increases, and the plasticity decreases. The outer side of the fusion line is the "overheating zone", where the grains are coarse, and Widmandarin and sorbite often appear, so the toughness is significantly reduced.
The outer side of the superheated zone is the "normalizing zone". Due to the recrystallization process of heating and cooling, fine and uniform ferrite and pearlite are obtained. The outer side is "unsafe recrystallization", and the heating temperature is between AC1-AC3. When this area is heated, the pearlite and part of the ferrite in the steel are transformed into austenite with relatively fine grains, and part of the iron is still retained. Element body, austenite transforms into fine ferrite and pearlite during cooling, while the ferrite that is not melted into austenite does not transform, the grain size is relatively coarse, forming a uniform crystal grain size, and still retains the original Ribbon properties in tissue.
Since the crystallization and heat transfer directions of the molten pool in the heat-affected zone are just opposite, that is, the crystallization direction from the heat-affected zone to the fusion line to the weld is the crystallization direction. The impurities in the molten pool move from the fusion line to the center, so that slag inclusion defects are prone to occur in the center of the molten pool, and cracks are prone to occur at the fusion line due to the fast cooling rate.
Welded corrosion cracked joints can cause cracks due to the hardenability of steel, cold cracks due to hydrogen diffusion, reheat cracks, intergranular grains, and welding defects due to welding specifications and worker skills. Practice has proved that cracks are the most harmful to the quality of pressure vessel products.

1) Hot crack
It is due to the segregation phenomenon in the crystallization process of the welding molten pool. The segregated substances are mostly co-products and impurities with low melting point. The crystallization process exists as a liquid interlayer. Low. When the welding tensile stress is large enough, the liquid interlayer is pulled apart or is pulled off soon after solidification to form a crack.

2) Cold crack
It refers to the cracks generated during welding at the lower temperature of A3 or after cooling to heat preservation. The crack formation temperature is low, in the martensitic transformation range, that is, below 200-300 ° C, so it is called cold crack. Sometimes cracks appear hours or days after welding, or even for a long time, so they are also called delayed cracks. Its more dangerous.
Cold cracks are often caused by the intrusion of air or the decomposition of the drug skin material during arc combustion. Hydrogen enters the molten pool and melts in the molten iron. Because a large amount of hydrogen is dissolved in the molten iron at high temperature, the solubility is greatly reduced at low temperature, and the hydrogen dissolved in molten iron is precipitated from the molten iron. Hydrogen diffuses and accumulates at the defects in the steel, resulting in an increase in local pressure, which promotes cracks in the steel, so cold cracks are also called hydrogen-induced cracks.
There are serious layered non-metallic inclusions in the steel during rolling, which makes the tensile plasticity in the thickness direction very poor, and there is a high tensile pressure in the thickness direction of the plate, resulting in stepped layered spalling.

3) Reheat cracks
Some steels containing Cr, Mo, V, B and other alloy elements do not produce cracks after welding. During stress relief treatment, or after long-term use at a certain temperature, cracks occur along the grain boundaries of the heat-affected zone, which are called reheat cracks, or SR cracks for short.
Reheat cracks are caused by supersaturated and solid solution carbides (mainly carbides of Cr, Mo, and V) in the first post-heating process, which precipitate again during reheating, resulting in intragranular strengthening and slipping strain. The original austenite grain boundaries are concentrated, and reheat cracks occur when the plasticity of the grain boundaries is insufficient to withstand the strain generated by the relaxation stress process.
This type of steel has a sensitive area around 600°C. Sensitivity decreases when it exceeds 650°C.

4) Methods to prevent cracks
In order to prevent cracks, the S and P content of steel and welding consumables can be limited: adjust the chemical composition of steel; refine the grain size of the weld; improve the basicity of the welding material; improve segregation; control the welding specification; Welding, using small line energy; casting arc breaking, reducing arc crater.
Low-hydrogen alkaline electrodes can also be used, the electrodes are strictly dried, and can be taken as needed; select reasonable welding specifications; eliminate hydrogen immediately after welding; improve the quality of steel and reduce layered inclusions in the steel; . Reduce residual stress and stress concentration; slow cooling by preheating machine and post-weld heat treatment. These methods, as long as they are used properly, can improve the welding quality and prevent defects.
As for lack of penetration, lack of fusion, slag inclusions, pores, weld surface defects such as meat bite, weld size, etc., all can be inspected by non-destructive testing to determine the location of the defect, adopt a reasonable and effective repair process, and operate carefully. Achieve the purpose of eliminating weld defects and ensuring the inherent quality of products.
post weld heat treatment
Post-weld heat treatment can eliminate residual stress and prevent deformation, that is to say, it can relax welding residual stress and stabilize the size and shape. Post-weld heat treatment can also improve the performance of the base metal and structural parts in the welding zone: specifically: it can soften the heat-affected zone, increase the ductility of the weld metal, improve the fracture toughness, discharge harmful substances such as hydrogen, improve corrosion resistance, and improve creep performance. and improve fatigue strength.
However, improper selection of post-weld heat treatment process will reduce the performance of welded joints. Therefore, post-weld heat treatment has become an important part of container manufacturing in our eyes. The most widely used post-weld heat treatment of welded joints is high temperature tempering, normalizing and solid melting treatment. High temperature tempering can solve the adverse effects of welding and deformation on the quality of pressure vessels.

1. Post-weld heat treatment can relax welding residual stress
With the increase of the heat treatment temperature and the prolongation of the holding time, the residual stress in the welding zone decreases accordingly. When the temperature rises to over 550 °C, the residual stress can be considered to be completely eliminated. However, the effect of holding time is not as obvious as that of temperature increase.

2. Softening of the hardened zone in the heat affected zone of the welded joint
Since the residual stress is greatly reduced, the tempering improves the metallographic structure, improves the plasticity and toughness, so the hardenability is reduced, and the hardened area of the welded joint is softened.

3. Hydrogen reduction in welded joints
During heat treatment, the temperature of the welded joint increases, the diffusion rate of hydrogen continues to increase, and it escapes to the outside. Generally speaking, when heating below 300 °C and holding it for 2-4 hours, the purpose of hydrogen zone can be achieved, not to mention when it is heated to 550-650 °C, the The hydrogen purpose is fully achieved.

4. Influence on the tensile strength of weld metal
Post-weld heat treatment, the effect on the tensile strength of the weld metal is related to the heat treatment temperature and holding time. The higher the heat treatment temperature and the longer the holding time, the lower the normal temperature tensile strength of the weld metal, and the higher the alloy content, the carbon equivalent The larger the ratio, the larger the ratio of strength reduction.

5. Influence on the impact toughness of weld metal
Excessive heat treatment will cause a drop in impact value for any steel grade. For Cr-Mo, Cr-Mo-V and most pearlite and martensitic heat-resistant steels, proper post-weld heat treatment can improve the impact toughness. For some high-strength steels, the impact value decreases after heat treatment. For carbon steel and Mn-Nb-Ni steel, the impact value is basically unchanged after post-weld heat treatment.

6. Influence on the width of decarburization layer
The higher the heat treatment temperature and the longer the holding time, the greater the degree of decarburization layer. This is because the element content is not equal when the carbide is formed, which causes carbon diffusion. especially serious.
Tempering is when the weldment is heated to 500-650 °C, the carbides are further aggregated, and a mixture of ferrite and fine-grained cementite is obtained. Elasticity, plasticity and toughness.
Normalizing is to heat the weldment to 30-50 ℃ above Ac or Acm, take it out of the furnace after heat preservation, and cool it in the air. The purpose is to improve the structure and refine the grains. A single normalizing cannot eliminate residual stress after welding.
Solid solution heat treatment, heating the steel to 920-1150 ℃ and cooling it rapidly, so that the carbide or brittle phase precipitates at the grain boundary of the austenitic welded joint at 450-850 ℃ and re-melts into the austenite, and it is quickly fixed. , in order to obtain a uniform solid solution. Thereby eliminating its intergranular corrosion. It also improves the corrosion resistance and mechanical properties of welded joints and eliminates work hardening. The solid solution heat treatment should be uniformly heated as a whole, and local heating should not be adopted.

In order to achieve the expected effect of post-weld heat treatment, it must be carefully studied, and it is very important to choose a suitable post-weld heat treatment process.