The weld heat treatment of welded steel pipes is not mysterious

First, the purpose of induction heating weld heat treatment.

The performance of welded steel pipes is comprehensively evaluated by the performance of the two parts of the metal, the steel pipe body and the weld. The weakest point of any one side will determine the performance of the welded steel pipe. During the welding process, due to the high temperature of welding, the metal structure of the weld and the heat-affected zone changes and the grains coarsen, and there is also a large internal stress. As a result of these changes, the performance of the metal in the weld zone is very different from that of the parent material (steel pipe body). Moreover, the comprehensive performance of the weld metal is significantly lower than that of the steel pipe body. For this reason, the weld heat treatment must be used to eliminate the difference and make it a "seamless welded steel pipe" with consistent comprehensive performance.

(1) Eliminate the internal stress generated by welding. The heat-affected zone formed on both sides of the weld due to welding forms three completely different areas with different structures, grain sizes, and stress distributions from the weld and the steel pipe body. Due to the different heating temperatures and cooling rates of the weld, heat-affected zone, and steel pipe body, the internal stresses inside them are different. The internal stress in the weld is the largest, the heat-affected zone is the second largest, and the steel pipe body is the smallest. When the internal stress exceeds the yield limit, the metal will undergo plastic deformation, causing the welded steel pipe to bend; when the internal stress exceeds the strength limit, the metal will undergo grain boundary cracks, and after the cracks expand, the weld will crack. The metal grains in the weld area are coarse, and the yield strength and tensile strength are the lowest, making it the most prone to cracking. The heat-affected zone is also more prone to cracking than the steel pipe body. Therefore, in order to improve the comprehensive performance of the weld, it is necessary to eliminate the internal stress of the weld through heat treatment. Normalizing or high-temperature annealing is usually used to eliminate the internal stress of the weld.

(2) Improve the microstructure of the metal in the weld and heat-affected zone. The steel strips used for pipe making are mostly hot-rolled and air-cooled normalized, with a fine ferrite + pearlite structure; some micro-alloyed high-strength steel strips have a fine acicular ferrite structure. The high temperatures of welding cause two adverse effects on the weld metal. First, grain coarsening leads to a decrease in yield strength and embrittlement. Second, uneven post-weld cooling rates cause slow cooling in the weld zone, resulting in coarse ferrite and pearlite, while faster cooling in the heat-affected zone results in the formation of coarse martensite and retained austenite in addition to ferrite and pearlite. Combined with these factors, the grain coarsening and changes in the metal's microstructure cause embrittlement of the weld metal, resulting in significant differences in structure and properties from the steel pipe body. Therefore, weld heat treatment is necessary to refine the grain size and restore the metallographic structure to minimize the differences.

Second, Induction Heating Weld Heat Treatment Methods

Steel types requiring heat treatment to improve the mechanical properties of welds are primarily pipeline steels. The main heat treatment methods for improving the overall performance of welds include normalizing, normalizing + tempering, and quenching + tempering. Currently, weld normalizing is the most common method used in domestic welded steel pipe production, while other heat treatment methods are not widely used. The most advanced heat treatment for welded steel pipes is quenching + tempering. Normalizing treatment is mostly used in large-scale welded steel pipe production lines abroad, and tempering treatment is only used by a few companies in Japan, the United States and the European Union. Induction heating tempering treatment of welds is the future development direction.

(1) Weld induction heating normalizing treatment. Weld normalizing treatment includes annealing treatment, which is sometimes also called stress relief annealing. Weld induction heating normalizing treatment is to heat the weld to a temperature above Ac3, 900-950℃, air cool to below 400℃, and then water cool to room temperature. This eliminates weld internal stress, refines weld grains, improves microstructure, and improves weld plasticity and impact toughness. Weld induction heating normalizing treatment is suitable for ordinary low-alloy steel and some low-alloy high-strength steel, equivalent to welded steel pipes below X60 steel grade. Weld induction heating annealing treatment is to heat the weld to the dual-phase region of 700-750℃, and then air cool to room temperature. The purpose is to eliminate weld internal stress and improve plasticity. Annealing treatment is mainly used for welding carbon steel and some ordinary low alloy steel pipes.

(2) Weld induction heating normalizing + tempering treatment. When the hardness of the weld is still high but the plasticity is still low after normalizing treatment, high temperature tempering treatment can be used for remedial measures. Induction heating tempering treatment is to heat the weld to a temperature below Ac1, usually around 650℃ and then air cool it. After high temperature tempering treatment, the martensite structure in the original steel is transformed into tempered bainite and ferrite, the plasticity of the weld is improved, the hardness is reduced, and the strength does not change much.

(3) Weld induction heating quenching + tempering treatment. This heat treatment method is also called tempering treatment. Online weld induction heating tempering treatment is the most advanced heat treatment technology at present. After tempering treatment, the comprehensive mechanical properties of the weld fully reach the level of the steel pipe body, and the performance of the weld and the steel pipe body are homogenized. The core of realizing this heat treatment process technology is to master the transverse magnetic field heating technology to ensure the uniformity and accuracy of the heating temperature. For welds made of low-alloy high-strength steel and slightly alloyed high-strength steel, the quenching temperature is 900-950°C, and the tempering temperature is 600-650°C. Quenching utilizes spray cooling, while tempering utilizes a combination of air and water cooling. When using longitudinal magnetic field heating for quenching and tempering, the temperature can be controlled with an accuracy of ±10°C, a level of control essential for maintaining stable performance of high-strength welded steel pipe. Using transverse magnetic field heating for welds also requires high temperature control accuracy.

Third, Induction Heating Normalizing of Welds

Normalizing is currently the most commonly used heat treatment method for welded steel pipes in China, sometimes also referred to as weld annealing. The difference between the two is that normalizing requires a heating temperature above Ac3, while annealing requires a lower heating temperature around Ac1. For pipeline steel welded pipe, weld normalizing is a heat treatment method specified in the API standard.

Induction heating normalizing of welds can be categorized as either in-line or off-line, depending on the production configuration. Offline normalizing can be used for products with a wide range of specifications and small batch sizes. Online normalizing should be used for products with a small number of specifications and large batch sizes.

Induction normalizing of welds is primarily used for low-carbon, low-alloy steel pipes reinforced with vanadium, niobium, and titanium. The purpose of normalizing welds is to eliminate internal stresses generated by welding, refine the weld metal grain size, improve the metal's plasticity and toughness, and minimize the performance difference between the weld and the pipe.

Fourth, Selecting the Temperature for Induction Normalizing of Welds

Traditional normalizing temperatures for low-alloy steels are 30-50°C above Ac3. However, with an induction heating rate of 20-50°C/s⁻¹, Ac3 increases with increasing heating rates. Based on empirical experience with induction quenching temperatures, Ac3 increases by 20-50°C. Therefore, the normalizing temperature for induction normalizing low-alloy steel welds should be 50-100°C above Ac3. The upper limit is used when the steel contains alloying elements such as Cr, V, and Ti; the lower limit is used when the steel does not contain these alloying elements.

Normalizing temperatures should not be too high. Under normal heating conditions, the initial austenite grains of low-alloy hypoeutectoid steel begin to grow at around 900°C, with rapid growth above 950°C. Therefore, excessively high normalizing temperatures hinder the refinement of the weld's grain structure and reduce the effectiveness of normalizing. Furthermore, excessively high normalizing temperatures can exacerbate metal oxidation in the weld zone, affecting the surface quality of the welded steel pipe and reducing steel yield. Similarly, excessively low normalizing temperatures fail to improve the weld metal's plasticity and toughness, defeating the purpose of normalizing.

Fifth, Holding Time and Cooling Methods for Induction Heating Normalizing Welds

Selecting the Holding Time for Induction Heating Normalizing Welds. Quenching, solutionizing, and normalizing are all heat treatments that involve heating steel to the austenitizing temperature and then cooling it at varying rates. While the goal of austenitizing is the same, all three methods share the same goal. Experiments with eliminating the austenitizing hold time during induction quenching of low-alloy steel and solution treatment of austenitic heat-strengthening steel have shown that by increasing the induction quenching and solution treatment temperatures by 50-100°C above conventional treatment temperatures, equivalent heat treatment results can be achieved without the need for a hold time. The concentration of strengthening elements in solution is consistent, and the room-temperature and high-temperature mechanical properties after tempering and aging surpass those of conventional treatments. This demonstrates that eliminating the hold time during induction normalizing does not affect subsequent heat treatment results.

In summary, for low-alloy hypoeutectoid steel, as long as the induction normalizing temperature is appropriately selected and the weld heating temperature is uniform, the normalizing hold time can be completely eliminated without compromising the heat treatment results. This is because, considering the rate of metallographic phase transformation, temperature plays a decisive role, and its effect far outweighs that of time. Trading temperature for time is a hallmark of induction rapid heat treatment.