The structure of modern pressure vessels is basically a welded structure. Welding is one of the main process methods in pressure vessel manufacturing. And welding rods are commonly used welding materials. According to different uses and performance, welding rods can be divided into 9 categories.
① Structural steel welding rods (including ordinary low alloy steel): The deposited metal of this type of welding rod has certain mechanical properties in natural environments at room temperature.
② Molybdenum and chromium molybdenum heat-resistant steel welding rods: The deposited metal of these welding rods has varying degrees of high-temperature working ability.
③ Stainless steel welding rod: The deposited metal of this type of welding rod has varying degrees of corrosion resistance to atmospheric corrosive media and certain mechanical properties at room temperature, high temperature or low temperature.
④ Surfacing welding rod: This type of welding rod is specifically used for surfacing on metal surfaces, and its deposited metal has a certain degree of resistance to different types of wear or corrosion at room temperature and high temperature.
⑤ Low temperature welding rod: The deposited metal of this type of welding rod has a certain low-temperature working ability under different low-temperature medium conditions.
⑥ Cast iron welding rod: This type of welding rod is specifically used for welding repair or welding cast iron.
⑦ Nickel and nickel alloy welding rods: These types of welding rods are used for welding, repairing, or overlaying nickel and aluminum alloys. Some welding rods can also be used for repairing cast iron and welding dissimilar metals.
⑧ Copper and copper alloy welding rods: This type of welding rod is used for welding, repairing, or overlaying copper and copper alloys. Some welding rods can be used for repairing cast iron and welding dissimilar metals.
⑨ Aluminum and aluminum alloy welding rods: This type of welding rod is used for welding, welding repair, or overlay welding of aluminum and aluminum alloys.
The acidity and alkalinity of welding rods are essentially determined by the alkalinity of the slag.
The acidic electrode coating contains a large amount of acidic oxides such as SiO2 and TiO2, as well as a certain amount of carbonates, and the alkalinity of the slag is less than 1. Titanium type rods, titanium calcium type welding rods, ilmenite type welding rods, and iron oxide type welding rods are all acidic welding rods.
Alkaline welding rod coating contains a large amount of alkaline slagging substances such as marble and jadeite, as well as a certain amount of deoxidizers and alloying agents. Low hydrogen welding rods are all alkaline welding rods.
The principle of selecting welding rods:
1, The factors to consider are the physical and chemical properties of the welded parts, as well as the principles for selecting chemical composition:
1. Based on the viewpoint of equal strength, choose welding rods that meet the mechanical properties of the base material, or welding rods that combine the mechanical properties of the base material, or welding rods that strengthen the weldability of the base material, and switch to welding rods that are not strong but have good weldability, but consider the structural type of the weld seam to meet the requirements of equal strength and equal stiffness
2. Ensure that its alloy composition matches or is close to the base material
When the base material contains high levels of harmful impurities such as carbon, sulfur, and phosphorus, welding rods with good crack resistance and porosity resistance should be selected. It is recommended to use titanium calcium oxide and titanium iron ore welding rods. If this problem cannot be solved, low hydrogen welding rods can be used
2, When considering the working conditions and performance of the welded parts, the selection principle is:
When subjected to dynamic and impact loads, in addition to ensuring strength, there are high requirements for impact toughness and uniform elongation. Low hydrogen type, titanium calcium type, and iron oxide type welding rods should be selected in sequence
2. For those who come into contact with corrosive media, appropriate stainless steel welding rods must be selected based on the type, concentration, working temperature, and distinction between general corrosion and intergranular corrosion
When working under wear and tear conditions, it is important to distinguish whether it is general or impact wear, and whether it is wear at room temperature or high temperature
When working under extremely high temperature conditions, corresponding welding rods should be selected to ensure the mechanical properties at low or high temperatures
3, Factors to consider: the complexity and stiffness of the geometric beginning of the weld, the preparation of the welding groove, and the principles for selecting the welding position:
1. Welding parts with complex shapes or large thicknesses, where the weld metal undergoes significant shrinkage stress during cooling and is prone to cracking, must use welding rods with strong crack resistance, such as low hydrogen type welding rods with high toughness or iron oxide type welding rods
2. Weldments that cannot be flipped due to limitations must use welding rods that can be welded in all positions
3. For welding parts that are difficult to clean at the welding site, acidic welding rods with strong oxidizing properties and insensitivity to oxide scale and oil stains should be selected to avoid defects such as porosity
4, The consideration factor is the selection principle for welding site equipment:
In areas where there is no DC welding machine, it is not advisable to use welding rods with limited DC power supply. Instead, welding rods with AC/DC power supply should be used. Some steels (such as pearlite heat-resistant steel) require post weld heat treatment, but when heat treatment cannot be carried out due to equipment conditions (or structural limitations), non parent metal welding rods (such as austenitic stainless steel welding rods) should be used instead, and post weld heat treatment is not necessary
5, When considering factors such as improving welding processes and protecting the health of workers, the selection principles are:
In areas where both acidic and alkaline welding rods can meet the requirements, acidic welding rods should be used as much as possible
6, When considering factors such as labor productivity and economic rationality, the selection principle is:
When using the same performance, it is advisable to choose acidic welding rods with lower prices instead of alkaline welding rods