There are many kinds of covered electrode involved in the oilfield productivity construction project. The characteristics, composition, mechanical properties, working conditions and welding materials of covered electrode coating are summarized and analyzed, and the selection requirements of covered electrode are listed, which provides a comprehensive explanation for the correct selection and use of covered electrode.
Keywords: covered electrode characteristics commonly used selection points
There are many kinds of covered electrode, and each covered electrode has certain characteristics and uses. Because of different coating types, even the same covered electrode has different application characteristics. In addition, the physical and chemical properties, working conditions (structural shape and stiffness) and construction conditions of the welded parts are different, and factors such as production efficiency and economy should be considered, which will inevitably bring certain difficulties to the selection of covered electrode.
Therefore, it is necessary to determine some basic principles to guide covered electrode's choice.
There are many kinds of covered electrode involved in oilfield productivity construction projects. In addition to carefully understanding the composition, properties and uses of various covered electrode materials, it is also necessary to comprehensively consider the situation of welding parts, construction conditions and welding technology in order to correctly select and use covered electrode.
Covered electrode characteristics commonly used in 1
Characteristics of 1. 1 cellulose electrode
Cellulose covered electrode coating contains about 30% organic matter, such as wood flour, starch, cellulose, etc. It also contains proper amount of titanium dioxide and asbestos. It is reduced by ferromanganese.
In the welding process, the decomposition of organic matter produces a large amount of gas to protect the arc and molten pool metal, so it can be called gas protection covered electrode.
Because relying on gas protection, slagging agent can be reduced, coating thickness becomes thinner, arc blowing force is large during welding, penetration depth is large, slagging is easy, but splashing is more.
The covered electrode is suitable for all-position welding, and is mainly used for downward vertical welding. At present, it is widely used as the primer coating covered electrode in pipeline engineering with oil field diameter 159 mm or more.
It is best to use DC power supply to reverse the polarity when welding: the welding current should be as small as possible, otherwise the coating will be overheated, the organic matter will be decomposed in advance, and the weld pool protection will be insufficient, which will easily lead to welding defects.
1.2 low hydrogen covered electrode characteristics
The low-hydrogen coating covered electrode is not named after the main raw materials in the coating, but is characterized by the low content of diffused hydrogen in the deposited metal.
The coating mainly contains fluoride such as carbonate and fluorite, and the slag has high alkalinity, which is also called alkaline coating covered electrode.
The slagging components are CaO, CaF2, MgO, SiO2 _ 2, TIO2 _ 2 and ZrO2 _ 2.
When the coated covered electrode is made, the drying temperature is very high.
So as to further remove the moisture in the coating and reduce the source of hydrogen.
Another important advantage of low-hydrogen coated covered electrode is that the diffusion hydrogen content of deposited metal is very low, which is only 1/5 to110 of other types of covered electrode.
The low diffusion of hydrogen plays an important role in improving the cold cracking ability.
Therefore, thick or rigid structures should be welded with low hydrogen covered electrode, otherwise cold cracks will occur in the weld or heat affected zone. With the increase of steel grade or weld strength, the allowable diffusion of hydrogen will decrease.
Therefore, in order to avoid welding cold cracks, ultra-low hydrogen covered electrode should be used when welding high strength steel.
After the moisture absorption of coated covered electrode coating, the amount of hydrogen diffused in deposited metal increases obviously, which is also one of the important reasons for welding cold cracks.
Low-hydrogen covered electrode is widely used in transporting toxic and flammable media and important structural engineering in oilfield productivity construction projects; At the same time, it is also the first choice for restoration of coated covered electrode.
1.3 acid covered electrode characteristics
The coating composition of acid covered electrode is highly oxidizing and insensitive to pores caused by water and rust. The coated covered electrode is baked at 65438 050℃ for 65438 0 hours before use, and the cellulose coated covered electrode does not need to be dried if it is not wet.
Stable arc, AC welding, large welding current and long arc operation.
The transition effect of alloying elements is poor, the weld seam is well formed, the melting depth is shallow except for iron oxide mold, and the slag structure is glassy, which is convenient for slag removal.
The impact properties of weld at room temperature and low temperature are high, but the crack resistance is poor except for iron oxide type.
The high hydrogen content in the weld is easy to produce white spots, which affects plasticity and has less smoke when welding.
1.4 alkaline covered electrode characteristics
The composition of alkaline covered electrode coating is highly reducible and sensitive to pores caused by water and rust. Covered electrode was dried at 300~400℃ and 65438 0 ~ 2 hours before use.
The fluoride in the coating makes the arc stability worse, so DC welding is needed, and the welding current is relatively small, which is about 10% less than the acid covered electrode of the same specification. Short arc operation is required, otherwise it is easy to cause porosity and the transition effect of alloy elements is good.
The weld seam is well formed, easy to accumulate, with deep penetration, crystalline slag structure, difficult slag discharge in the first layer of groove and easy slag discharge in subsequent layers.
The weld has high impact performance at room temperature and low temperature, good crack resistance, low hydrogen content in the weld and a lot of smoke during welding.
2 covered electrode's basic conditions
2. 1 Consider the mechanical properties and chemical composition requirements of the weldment.
① From the point of view of equal strength, choose covered electrode that meets the requirements of mechanical properties.
Or, considering the weldability of the base metal, covered electrode with different strength and good toughness should be adopted, but the weld structure should be changed to meet the requirements of equal strength and stiffness.
(2) Make the alloy composition of the deposited metal conform to or close to the base metal.
(3) When the content of harmful impurities such as carbon, sulfur and phosphorus in the chemical composition of the base material is high, the coated covered electrode with strong crack resistance and porosity resistance, such as low-hydrogen coated covered electrode, should be selected.
It must be pointed out that the requirements for mechanical properties and chemical composition of welded components are unbalanced. Some weldments may emphasize the requirements for strength and toughness, but the requirements for chemical composition are not necessarily the same as those of the base metal. For example, when selecting structural steel covered electrode, the equal strength between weld metal and base metal or the high toughness of weld metal should be considered first; Some weldments may emphasize the requirements of chemical composition, such as the consistency of chemical composition between weld metal and base metal and the selection of heat-resistant steel and stainless steel covered electrode; Sometimes there may be strict requirements for both. Therefore, when choosing covered electrode, we should prioritize and consider comprehensively.
2.2 Consider the working conditions and performance of weldments.
(1) Under the conditions of dynamic load and impact load, the workpiece requires not only tensile strength and yield strength, but also high impact toughness and plasticity.
At this time, low hydrogen type, titanium calcium type or iron oxide type covered electrode should be selected.
② When the workpiece works in corrosive medium, it is necessary to distinguish the medium type, concentration, working temperature and corrosion type (general corrosion, intergranular corrosion, stress corrosion, etc.). ), in order to choose the appropriate stainless steel covered electrode.
(3) When the workpiece works under wear conditions, it is necessary to distinguish whether it is general wear or impact wear, and whether it is intermetallic wear or abrasive wear.
It should also be considered whether to work in corrosive medium to select a suitable surfacing covered electrode.
(4) Workpieces working at low temperature or high temperature shall be made of covered electrode which can ensure the mechanical properties at low temperature or high temperature.
2.3 Consider the structural characteristics, stress state, rigidity, welding groove preparation and welding position of the workpiece.
(1) The workpiece with complex shape or large thickness is prone to crack due to the large internal stress generated by the weld metal during cooling shrinkage.
Therefore, it is necessary to use covered electrode with good crack resistance, such as low hydrogen covered electrode, high toughness covered electrode or iron oxide covered electrode.
(2) When the welding position can't be turned over, the covered electrode that can perform all-position welding must be selected.
(3) When some welding parts are difficult to clean due to conditions, acidic covered electrode with strong oxidation and insensitivity to rust, scale and oil stain should be considered to avoid defects such as blowholes.
3 covered electrode selection point when welding the same steel.
3. 1 mechanical properties and chemical composition requirements
For ordinary structural steel, the strength of weld metal and base metal is usually required, and covered electrode with tensile strength of deposited metal equal to or slightly higher than that of base metal should be selected; For alloy structural steel, it is mainly required that the mechanical properties of weld metal match the base metal, and sometimes the alloy composition is the same or close to the base metal; When the welding structure is rigid, the joint stress is high, and the weld is prone to cracks and defects, covered electrode, which is one level lower than the base metal strength, can be considered. When the content of carbon, sulfur, phosphorus and other elements in the base metal is high, the weld is prone to crack, so low hydrogen covered electrode with good crack resistance should be selected.
3.2 Requirements for service performance and working conditions of weldments
For weldments subjected to dynamic load and impact load, in addition to meeting the strength requirements, the weld metal should also be ensured to have high impact toughness and formability, and low hydrogen covered electrode with high formability and toughness index can be selected; Stainless steel covered electrode or other corrosion-resistant covered electrode should be selected according to the nature and corrosion characteristics of corrosive medium. The weldments working at high temperature or low temperature shall use corresponding heat-resistant steel or low-temperature steel covered electrode.
3.3 Structural characteristics and stress state of weldments
For weldments with complex structural shape, high stiffness and large thickness, cracks are easy to occur during welding due to large stress, so low hydrogen covered electrode with good crack resistance should be selected; For the weldments whose welding parts are difficult to clean, acidic covered electrode with strong oxidation and insensitivity to rust, scale and oil stain should be selected. For weldments that cannot be turned over due to conditions, and some welds are in non-flat welding positions, covered electrode with all-position welding shall be selected.
4 Construction conditions and equipment
When there is no DC power supply, and the welding structure requires the use of low hydrogen covered electrode, AC -DC dual-purpose low oxygen covered electrode should be selected; In narrow or poorly ventilated places, choose acid covered electrode or low dust and low hydrogen covered electrode.
4. 1 Operation Process Performance
Under the condition of meeting the product performance requirements, try to choose acid covered electrode with good process performance.
4.2 Economic benefits
Under the condition of satisfying the use performance and operation process performance, covered electrode with low cost and high efficiency shall be selected as far as possible.
5 concluding remarks
To sum up, in the process of selecting covered electrode, we need to consider many aspects, choose a suitable range, and verify the technological performance of covered electrode. Through practical operation, we can see whether covered electrode can easily carry out welding operation, investigate the fluidity of slag in the state of arc occurrence, the transition of droplets during welding, the removal degree of slag and the appearance of weld after welding, and determine which covered electrode is the best choice through the performance and experimental evaluation of the whole welding operation.
Because the welding process performance is also affected by many factors, such as welder's technical level, welder's performance, welding tools, parent metal's performance (material, plate thickness, joint type), welding parameters and so on. It is difficult to objectively and accurately evaluate the technological performance of covered electrode. Therefore, when evaluating the technological performance of covered electrode, we should also determine the welding machine, welding machine, other welding tools, test materials and welding parameters.