Heat-resistant steel refers to steel that has both thermal stability and thermal strength under high temperature conditions. Thermal stability means that the steel can maintain chemical stability (corrosion resistance, non-oxidation) under high temperature conditions. Thermal strength means that the steel has sufficient strength under high temperature conditions. Among them, the heat resistance is mainly guaranteed by alloying elements such as chromium, molybdenum, vanadium, titanium, and niobium. Therefore, the selection of welding materials should be determined according to the alloying element content of the base metal. Heat-resistant steel is widely used in the construction of petroleum and petrochemical industrial installations. Most of the pearlitic heat-resistant steels that we can often come into contact with are pearlite heat-resistant steels with low alloy content, such as 15CrMo, 1Cr5Mo and so on.
1 Weldability of chrome molybdenum heat-resistant steel
Chromium and molybdenum are the main alloying elements of pearlitic heat-resistant steel, which can significantly improve the high-temperature strength and high-temperature oxidation resistance of the metal, but they make the weldability of the metal worse, and have a tendency to quench stress in the weld and heat-affected zone. Cooling in air is easy to produce hard and brittle martensite structure, which not only affects the mechanical properties of welded joints, but also produces large internal stress, resulting in cold cracking tendency.
Therefore, the main problem when welding heat-resistant steel is cracks, and the three elements that form cracks are: structure, stress and hydrogen content in the weld, so it is particularly important to formulate a reasonable welding process.
2. Welding process of pearlitic heat-resistant steel
2.1 Bevel
The groove is usually processed by flame or plasma cutting process. If necessary, the cutting should also be preheated. After grinding, PT inspection is performed to remove the cracks on the groove. Usually V-shaped groove is selected, and the groove angle is 60°. From the perspective of preventing cracks, it is beneficial to have a larger groove angle, but it increases the amount of welding. At the same time, the groove and both sides of the inner part are polished to remove oil and rust. and moisture and other contaminants (dehydrogenation, prevention of stomata).
2.2 Group pairs
It is required that the alignment cannot be forced to prevent the generation of internal stress. Since the chromium molybdenum heat-resistant steel has a large crack tendency, the restraint of the weld during welding should not be too large, so as not to cause excessive stiffness, especially when welding thick plates, hindering the The use of tie bars, fixtures and fixtures with free shrinkage of the weld should be avoided as much as possible.
2.3 Selection of welding method
At present, the commonly used welding methods for pipeline welding in our petroleum and petrochemical installation units are argon tungsten arc welding for bottoming, electrode arc welding for filling cover, and other welding methods include MIG welding, CO2 gas shielded welding, Electroslag welding and automatic submerged arc welding, etc.
2.4 Selection of welding materials
The principle of matching welding materials, the alloy composition and strength properties of the weld metal should basically be consistent with the corresponding indicators of the base metal or should reach the minimum performance indicators proposed in the product technical conditions. In addition, in order to reduce the hydrogen content, low-hydrogen alkaline electrodes should be used first. The electrodes or fluxes should be dried according to the prescribed process, and they should be taken as needed. They should be placed in the electrode insulation bucket and taken as needed. The number of electrodes in the electrode insulation bucket should not exceed 4. hours, otherwise it should be re-dried, and the number of drying times should not exceed three times, which are specified in the specific construction process. In manual arc welding of chromium-molybdenum heat-resistant steel, austenitic stainless steel electrodes, such as A307 electrodes, can also be used, but preheating is still required before welding. This method is suitable for the case where the weldment cannot be heat treated after welding.
The selection table of heat-resistant steel welding consumables is as follows:
2.5 Preheating
Preheating is an important process measure for welding cold cracks and stress relief of pearlitic heat-resistant steel. In order to ensure the welding quality, whether it is spot welding or welding process, it should be preheated and maintained within a certain temperature range.
2.6 Slow cooling after welding
Slow cooling after welding is a principle that must be strictly followed when welding chrome-molybdenum heat-resistant steel. This must be done even in hot summer. Generally, asbestos cloth is used to cover the welding seam and the area near the seam immediately after welding. Small weldments can be placed in Cool slowly in asbestos cloth.
2.7 Post-weld heat treatment
Heat treatment should be carried out immediately after welding, the purpose of which is to prevent the generation of delayed cracks, relieve stress and improve the structure. The temperature table of heat-resistant steel preheating temperature before welding and post-welding heat treatment is shown in the following table.
3 Welding precautions
(1) Measures such as preheating and slow cooling after welding must be performed when welding this type of steel, but the higher the preheating temperature, the better, and the welding process requirements must be strictly implemented.
(2) Multi-layer welding should be adopted for thick plates, and the interlayer temperature should not be lower than the preheating temperature. The welding should be completed at one time, preferably without interruption. If interlayer suspension is required, thermal insulation and slow cooling measures should be taken, and the same preheating measures should be taken before re-welding.
(3) During the welding process, attention should be paid to filling the crater, grinding the joint, and removing the crater crack (hot crack). Moreover, the larger the current, the deeper the arc crater, so the welding parameters and appropriate welding line energy should be strictly implemented according to the welding process instructions.
(4) The construction organization is also an important part of the welding quality. The cooperation of various types of work is particularly important to avoid the failure of the next process to connect the quality of the entire weld.
(5) The influence of weather and environment should also be paid attention to. When the ambient temperature is low, the preheating temperature can be appropriately increased to prevent the temperature from dropping too quickly, and emergency measures such as wind and rain protection can be taken at the same time.
4. Summary
Preheating, heat preservation, post-weld heat treatment and other processes are necessary process measures for welding chromium-molybdenum heat-resistant steel. The three are equally important and cannot be ignored. The omission of any link has serious consequences. Welders should strictly implement the welding process and strengthen the guidance of the welder's sense of responsibility. Don't take chances and guide the welders to implement the seriousness and necessity of the process. As long as we strictly implement the welding process during the construction process, cooperate well between various types of work, and arrange the processes reasonably, the welding quality and technical requirements can be guaranteed.
