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 performance is mainly ensured by alloy elements such as chromium, molybdenum, vanadium, titanium, niobium, etc. Therefore, the selection of welding materials should be determined based on the alloy element content of the base material. Heat-resistant steel is widely used in the construction of petroleum and petrochemical equipment. We often come into contact with pearlite heat-resistant steel with low alloy content, such as 15CrMo, 1Cr5Mo, etc.

1. Weldability of chromium molybdenum heat-resistant steel

Chromium and molybdenum are the main alloying elements of pearlite heat-resistant steel, significantly improving the high-temperature strength and high-temperature oxidation resistance of the metal. However, they deteriorate the welding performance of the metal and have a tendency to harden in the weld seam and heat affected zone. After welding, cooling in the air can easily produce hard and brittle martensitic texture, which not only affects the mechanical properties of the welded joint, but also generates significant internal stress, resulting in a tendency to cold crack.

Therefore, the main problem when welding heat-resistant steel is cracks, and the three elements that form cracks are: microstructure, stress, and hydrogen content in the weld seam. Therefore, it is particularly important to develop a reasonable welding process.

2. Welding process of pearlite heat-resistant steel

2.1 Beveling

The processing of the groove usually uses flame or plasma cutting technology, and if necessary, the cutting should also be preheated. After polishing, PT inspection should be conducted to remove cracks on the groove. Usually, V-shaped grooves are selected with a groove angle of 60 °. From the perspective of preventing cracks, a larger groove angle is beneficial, but it increases the welding amount. At the same time, the groove and both sides of the inner part are polished clean to remove oil, rust, and moisture (hydrogen removal, prevention of pores).

2.2 Pairing

It is required that assembly cannot be forced to prevent internal stress. Due to the large tendency of cracks in chromium molybdenum heat-resistant steel, the restraint of the weld seam during welding should not be too large to avoid excessive stiffness. Especially in thick plate welding, tie bars, fixtures, and fixtures that hinder the free shrinkage of the weld seam 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 inert gas arc welding for backing, electrode arc welding for filling the cover, and other welding methods include melting inert gas shielded welding (MIG welding), CO2 gas shielded welding, electroslag welding, and submerged arc automatic welding.

2.4 Selection of Welding Materials

The principle of selecting welding materials is that the alloy composition and strength performance of the weld metal should be basically consistent with the corresponding indicators of the base metal or should meet the minimum performance indicators proposed by the product technical conditions. Moreover, in order to reduce the hydrogen content, low hydrogen alkaline welding rods should be used first. The welding rods or flux should be dried according to the specified process and taken as needed. They should be placed in the insulation barrel of the welding rod and taken as needed. The welding rod should not be kept in the insulation barrel for more than 4 hours, otherwise it should be re dried, and the drying frequency should not exceed three times. This is detailed in the specific construction process. When manual arc welding chromium molybdenum heat-resistant steel, austenitic stainless steel welding rods, such as A307 welding rods, can also be used, but preheating is still required before welding. This method is suitable for situations where the welded parts cannot be heat treated after welding.

2.5 Preheating

Preheating is an important process measure for welding cold cracks and relieving stress in pearlite heat-resistant steel. In order to ensure welding quality, preheating and maintaining a certain temperature range should be carried out during both spot welding and welding processes.

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. Even in hot summer, this must be achieved. Generally, immediately after welding, cover the weld seam and the near seam area with asbestos cloth, and small welding parts can be placed in asbestos cloth for slow cooling.

2.7 Post weld heat treatment

Immediately after welding, heat treatment should be carried out to prevent the occurrence of delayed cracks, eliminate stress, and improve microstructure. The preheating temperature before welding and the temperature table for post welding heat treatment of heat-resistant steel are shown in the table below.

3. Welding precautions

(1) When welding this type of steel, measures such as preheating and slow cooling after welding must be taken, but the higher the preheating temperature, the better. The welding process requirements must be strictly followed.

(2) Thick plates should be welded in multiple layers, with the interlayer temperature not lower than the preheating temperature. Welding should be completed in one go, 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, polishing the joint, and removing crater cracks (hot cracks). Moreover, the larger the current, the deeper the arc crater. Therefore, the welding process guide book should be strictly followed to select welding parameters and appropriate welding line energy.

(4) Construction organization is also an important aspect that affects the quality of welding, and the coordination of various types of work is particularly important to avoid the quality of the entire weld joint being compromised due to inadequate connection of the next process

(5) Attention should also be paid to the impact of weather conditions. 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 prevention should be taken.

4. Summary

Preheating, insulation, and post weld heat treatment are necessary process measures for welding chromium molybdenum heat-resistant steel, and the three are equally important and cannot be ignored. Any negligence in any aspect can have serious consequences. Welders should strictly follow the welding process and strengthen the guidance of their sense of responsibility. We should not take chances and guide welders to carry out the seriousness and necessity of the welding process. As long as we strictly follow the welding process during the construction process, cooperate well between different types of work, and arrange the processes reasonably, we can ensure the welding quality and technical requirements.