Heat-resistant steel refers to steel that has both thermal stability and thermal strength under high temperature conditions. Thermal stability refers to the ability of steel to maintain chemical stability (corrosion resistance, non-oxidation) under high temperature conditions. Thermal strength refers to the sufficient strength of steel under high temperature conditions. The heat resistance is mainly ensured by alloy elements such as chromium, molybdenum, vanadium, titanium, and niobium. Therefore, the selection of welding materials should be determined based on the alloy element content of the base metal. Heat-resistant steel is widely used in the construction of petroleum and petrochemical industry equipment. Most of the pearlitic heat-resistant steel we often come into contact with has lower alloy content, such as 15CrMo, 1Cr5Mo, etc.
1 Weldability of chromium-molybdenum heat-resistant steel
Chromium and molybdenum are the main alloy elements of pearlitic heat-resistant steel, which significantly improve the high-temperature strength and high-temperature oxidation resistance of the metal. However, they worsen the welding performance of the metal and have a quenching tendency in the weld and heat-affected zone. After cooling in the air, it is easy to produce a hard and brittle martensite structure, which not only affects the mechanical properties of the welded joint, but also generates large internal stress, resulting in a tendency of cold cracking.
Therefore, the main problem when welding heat-resistant steel is cracks, and the three factors that cause cracks are: structure, stress and hydrogen content in the weld. Therefore, it is particularly important to develop a reasonable welding process.
2 Pearlitic heat-resistant steel welding process
2.1 Bevel
The bevel is usually processed by flame or plasma cutting process. If necessary, the cutting should be preheated. After polishing, PT inspection should be performed to remove cracks on the bevel. Usually a V-shaped groove is used, with a groove angle of 60°. From the perspective of preventing cracks, a larger groove angle is advantageous, 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 (removing hydrogen and preventing pores).
2.2 Pairing
It is required that the assembly cannot be forced to prevent internal stress. Since chromium-molybdenum heat-resistant steel has a greater tendency to crack, the restraint of the weld should not be too large during welding to avoid excessive stiffness, especially when welding thick plates. The use of tie bars, clamps and clamps that allow the weld to shrink freely should be avoided as much as possible.
2.3 Selection of welding methods
At present, the commonly used welding methods for pipeline welding in our petroleum and petrochemical installation units are tungsten arc welding for the base layer and electrode arc welding for the filling cover. Other welding methods include molten inert gas shielded welding (MIG welding), CO2 gas shielded welding, Electroslag welding and submerged arc automatic welding, etc.
2.4 Selection of welding materials
The principle of selecting welding materials is that the alloy composition and strength properties of the weld metal should basically be consistent with the corresponding indicators of the base metal or should meet the minimum performance indicators proposed by the product technical conditions. In order to reduce the hydrogen content, a low-hydrogen alkaline welding rod should be used first. The welding rod or flux should be dried according to the prescribed process and taken out as needed. It should be installed in a welding rod insulation bucket and taken away as needed. There should be no more than 4 in the welding rod insulation bucket. hours, otherwise it should be dried again, and the number of drying times should not exceed three times. There are detailed regulations in the specific construction process. When hand 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 situations where the weldment cannot be heat treated after welding.
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 welding quality, whether it is spot welding or during the 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 chromium-molybdenum heat-resistant steel. This must be done even in hot summer. Generally, asbestos cloth is used to cover the weld and the near seam area immediately after welding. Small weldments can be placed 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 occurrence of delayed cracks, eliminate stress and improve the structure.
Xinfa welding equipment has the characteristics of high quality and low price. For details, please visit: Welding & Cutting Manufacturers – China Welding & Cutting Factory & Suppliers (xinfatools.com)
3 Precautions for welding
(1) When welding this type of steel, measures such as preheating and slow cooling after welding must be taken. However, the higher the preheating temperature, the better. The welding process requirements must be strictly followed.
(2) Multi-layer welding should be used for thick plates, and the inter-layer temperature should not be lower than the preheating temperature. The welding should be completed in one go, and it is best not to interrupt. If there is a need to pause between layers, thermal insulation and slow cooling measures should be taken, and the same preheating measures should be taken before welding again.
(3) During the welding process, attention should be paid to filling the arc craters, polishing the joints, and removing crater cracks (hot cracks). Moreover, the greater the current, the deeper the arc crater. Therefore, the welding process instructions should be strictly followed to select welding parameters and appropriate welding line energy.
(4) Construction organization is also an important factor affecting the quality of welding, and the cooperation of various types of work is particularly important to avoid wasting the quality of the entire weld due to failure to connect to the next process.
(5) Attention should also be paid to the influence of weather environment. When the ambient temperature is low, the preheating temperature can be appropriately increased to prevent the temperature from dropping too fast, and emergency measures such as wind and rain protection can be taken.
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. If any link is omitted, the consequences will be serious. Welders must strictly implement welding procedures and strengthen the guidance of welders’ sense of responsibility. We should not take chances and guide welders to implement the process with seriousness and necessity. As long as we strictly implement the welding process during the construction process, cooperate well with various types of work, and arrange the process reasonably, we can ensure the welding quality and technical requirements.
Post time: Nov-01-2023
