What are the key inspection points for T91/T92 in power plant boilers?

How can one determine the differences between domestically produced materials and imported materials?

【Abstract】 It introduces the development process of T9l/P91 steel, analyzes the main welding problems associated with this steel, and discusses the key points of its welding processes as well as its applications. The results show that the new T9l/P91 steel grade has been widely used in the high-temperature pipes of high-parameter thermal power units, thanks to its range of excellent performance characteristics. The main problems regarding the weldability of this steel are its high sensitivity to cold cracking and a certain tendency to hot cracking; furthermore, the deterioration in joint properties (degradation of toughness in the weld zone and softening in the heat-affected zone) cannot be ignored ； A proper welding process is an important technical means for controlling and improving the weldability of this steel. After the welding method and welding materials are determined, the key process measures for obtaining high-quality joints are: preheating before welding, controlling the layer temperature, and carrying out \"timely and effective\" post-weld heat treatment. For welding T91/P91 steel pipes with different joint combination types (same steel or different steels) and various specifications, the corresponding welding processes each have their own characteristics ； The new process of using specialized flux-cored wires for TIG backing welding has pushed the welding technology for this steel grade to a new level of development. I. Overview: Thanks to its excellent comprehensive properties such as high-temperature creep resistance, thermal stability, and high-temperature endurance strength, T91/P91 steel is being increasingly used in the superheaters, reheaters, and main steam pipes of power plant boilers. Although T91/P91 steel has been in use and studied in China for over a decade, some organizations have accumulated experience in mastering its welding techniques. Moreover, the Power Generation Construction Department of the **Electricity Company issued a guiding document titled \"Guidelines for Welding Techniques of T91/P91 Steel\"; yet welding quality issues related to this steel still occur from time to time at construction sites. This indicates that, on the one hand, there is an insufficient understanding of the weldability of this steel ； On the other hand, the control of key technologies in the supporting welding process is still insufficient. In other words, the work of digesting, absorbing, and localizing the introduced steel grades and their welding processes must continue. The number of research papers on the welding of T91/P91 steel is increasing year by year, and every successful application in power plant units is closely related to the welding processes used. Due to the different combination types of joints and the specifications of pipes (diameter and wall thickness), various welding processes are required. Therefore, it is essential to continue researching the weldability of T91/P91 steels and the associated welding processes, in order to understand the mechanisms by which process parameters affect joint performance and to develop innovative core technologies for these processes. To this end, this paper specifically links typical welding processes with the welding problems of this steel, and provides a comprehensive review of the characteristics of its welding processes as well as their applications. This work holds positive significance and reference value for further improving the welding process of T91/P91 steel and extending the service life of boilers. II. Introduction to T91/P91 steels: With the rapid development of the power industry, high-parameter, large-capacity units are emerging in increasing numbers, which places higher demands on the high-temperature creep resistance and stress corrosion resistance of steel tube materials. To this end, the world’s major industrialized countries have conducted extensive research, developing a series of new types of heat-resistant steels in the chain-link shape, which have been successfully used in large-capacity thermal power generation units. Among these, the high-CT 9Cr1MoVNbN heat-resistant steel is equivalent to the T91/P91 steel. In the 1970s, the United States improved the existing 9Cr1Mo steel in laboratories; at the beginning of the 1980s, the improved version of this steel was designated as T91/P91 steel. Subsequently, in 1983, T91/P91 steel was approved by the American ASME. At the end of the 1980s, Germany shifted from F12 steel to T91/P91 steel, and in the early 1990s, Japan actively promoted the use of T91/P91 steel. At present, all the major steel mills in the world that produce boiler tubes and large-diameter thick-walled tanks have completed research on the industrial production of T91/P91 steel. Steel mills in countries such as Japan, Germany, and France have already begun supplying T91/P91 steel tubes to the whole world. Our country introduced this steel grade in 1987 and applied it in power plants. The localization of this steel has been mandated for implementation by the Ministry of Metallurgy. T91/P91 steel is widely used in high-parameter thermal power generators because of the following advantages of its performance: ① Compared with stainless steels, this steel has a low coefficient of thermal expansion and good thermal conductivity. ②This shear has a high tensile strength at room temperature, with δb reaching up to 770 MPa, and it also exhibits good plasticity. ③The impact toughness and material brittle transition temperature of this steel are significantly superior to those of similar X20 and EMl2 steels. ④This steel possesses higher high-temperature creep strength and allowable stress; its high-temperature creep strength after 105 hours of operation at 550°C is twice that of T22 steel, and its allowable stress in the range of 540–610°C is significantly higher than that of T22, TP304H, and X20 steels. ⑤This steel exhibits good straight-tube bending properties. ⑥The high-temperature fatigue performance of this steel is superior to that of T22 and TP304H steels, and its high-temperature oxidation resistance is also much higher than that of T22 steel. III. Weldability of T91/P91 steel: T91/P91 steel is a new generation of medium-alloy heat-resistant steel developed on the basis of 9Cr1Mo steel, by employing purification and fine-graining metallurgical techniques, as well as micro-alloying and controlled rolling/controlled cooling processes. In terms of chemical composition (see Table 1), the levels of C, S, and P in T91/P91 steel have been reduced, with V, Nb, and N elements being used for microalloying ； In terms of mechanical properties (see Table 2), the strength and toughness of T91/P91 steel have improved. This is due to the fundamental difference in the strengthening mechanism of this steel compared to older steel grades; that is, in addition to solid solution strengthening and precipitation strengthening, it also achieves a high density of dislocations and highly refined grains through microalloying, controlled rolling, deformation heat treatment, and controlled cooling. The supply condition of this steel is normalizing + tempering (730–760°C), and its microstructure is tempered martensite. Compared with T9 steel, T9l/P9I steel has a significantly reduced sensitivity to welding cracks due to the lower content of carbon and impurity elements, which in turn lowers the preheating temperature required to prevent crack formation. It can be said that the weldability of this steel has improved. But this does not mean that satisfactory joint performance can be achieved in all cases. Studies show that the main problems regarding the weldability of this steel are as follows: (1) sensitivity to welding cracks. Steel contains a variety of alloying elements, with their total content reaching around 10%; it possesses quite high air-quenching properties. Due to the strong tendency for hardening in the HAZ, its sensitivity to cold cracking remains high ； At the same time, the steel contains elements such as C and Nb that promote thermal cracking; therefore, it also has a certain tendency to undergo thermal cracking, with sensitivity lying between SA213—T9 steel and SA213—TP304H steel. (2) The plasticity of HAZ decreases. Due to the influence of the welding heat cycle, the HAZ exhibits a strong tendency for grain growth, which reduces the plasticity of this region. (3) Softening of HAZ. Due to the influence of the welding heat cycle, a softened layer forms in the HAZ, deteriorating the mechanical properties of this region. (4) Deterioration of weld metal toughness. Since the weld metal has not undergone controlled rolling or deformation heat treatment, the grains cannot be refined as a result ； At the same time, during the cooling and solidification process, it is difficult for Nb and V elements in the weld to form fine C and N compounds, resulting in the weld having much lower toughness compared to the base material. (5) Studies have shown that, under certain conditions, this steel also exhibits a certain sensitivity to stress corrosion cracking. Overall, the main problems regarding the weldability of this steel are its high sensitivity to cold cracking and a certain tendency toward hot cracking; moreover, the deterioration in joint properties (degradation of toughness in the weld zone and softening in the heat-affected zone) cannot be ignored. A proper welding process is an important technical means for controlling and improving the weldability of this steel. IV. Key points of welding technology for T91/P91 steel: The common specifications for T91 steel tubes used in the superheaters and reheaters of power station boilers include ψ54mm and ψ57mm, etc., with wall thicknesses of 4mm, 8mm, and 9mm, among others. Large-diameter main steam P91 steel pipes are available in various specifications such as ψ457mm×45mm and ψ347mm×46mm. In on-site construction of small-diameter pipes, horizontal butt joint fixation is commonly used; single-sided welding with double-sided formation is required, and it is also necessary to prevent the welds from oxidizing. To ensure that the joints exhibit satisfactory performance, early welding processes primarily relied on two methods: one was full TIG welding (TIG for root pass + TIG for fill pass) ； Second is TIG welding for the root pass + shielded metal arc welding for the fill pass. The key points of its welding process are as follows: (1) Use the correct groove shape and dimensions. (2) Select appropriate welding materials (TIG welding wire and filler welding rod). (3) Select the correct welding parameters (including wire and electrode grades and diameter, tungsten electrode diameter, welding current, argon flow rate, power supply polarity, number of weld layers and passes, etc.). (4) Select the correct preheating temperature for the workpiece and the interpass temperature for the weld. (5) Select appropriate post-weld heat treatment specifications (including heating and cooling rates, tempering temperature, and holding time, etc.). (6) Adhere to correct joint assembly and tack welding, as well as skilled and proficient manual operation techniques. According to literature reports, the application of the aforementioned methods and processes has achieved fairly satisfactory results. However, the drawback or difficulty of this process is the complexity of protecting the back side of the weld during welding. If proper protection is not provided, the back side of the weld will oxidize, resulting in an unguaranteed weld quality and posing safety hazards. To this end, self-protecting flux-cored wire welding methods and their processes have been introduced in recent years. The greatest advantage of this process is that the gases and slag generated by the metallurgical reactions during welding not only provide effective protection for the molten pool on the front side, but also effectively protect the weld on the back side as well ； As the so-called gas-slag combined protection welding method and process, it eliminates the need for complex procedures and equipment for argon filling on the back side of the weld, overcoming the drawback of the TIG method in terms of protecting the weld on its back side. However, this method is not yet mature, as it is difficult to ensure the quality of welds produced with such specialized self-protecting flux-cored wires, and moreover, the weldability of these wires is also unsatisfactory. In recent years, there has also been a TIG welding method that uses flux-cored wire for filling and backing. The feature of this method is that, during the root welding, the slag from the cored wire is used to protect the back side of the weld, eliminating the need for protection on that side. The filler layer of the weld can be created either by TIG welding or by shielded metal arc welding. This type of flux-cored wire designed for backing has easy adjustment of its composition and properties; the performance of the joints meets the required standards, and the weldability is also satisfactory, making it a promising welding process. The key aspects of the TIG welding method using flux-cored wire for root pass welding are, aside from the selection of welding materials and welding parameters as well as the operating techniques which differ from those of conventional argon arc welding, largely similar to those of other methods. For the welding of large-diameter pipes (P91), in addition to the welding method of TIG welding for the root pass followed by shielded metal arc welding for the fill passes (multi-layer, multi-pass), some organizations have recently adopted the submerged arc welding process (TIG welding for the root pass + shielded metal arc welding for a 5–8 mm layer + arc welding for further filling), which significantly improves welding productivity while also enhancing the quality of the welds. From the welding process characteristics of T9l/P91 steel, it can be seen that whether it is the TIG welding method using solid wire for root pass welding or the TIG method using flux-cored wire for root pass welding, the key aspects of such welding processes are aimed at achieving two objectives: one is to prevent the occurrence of welding defects ； Second is to ensure the performance of the joint. To prevent welding defects, it is necessary to effectively control process parameters such as the workpiece preheating temperature, interpass temperature, welding heat input, and welding techniques ； To ensure the performance of the joint, it is necessary to effectively control process parameters such as the composition of the filler wire (solid or flux-cored), the welding heat input, and the post-weld heat treatment of the workpiece. It should be noted that once the welding method and welding materials have been determined, the key process measures for obtaining high-quality joints are: preheating before welding, controlling the layer temperature, and \"timely and effective\" post-weld heat treatment. The essence of preheating before welding is to control the behavior of hydrogen in the welding area, reduce the hardness and stress peaks in that area, and improve toughness. The temperature control at the control layer serves, on one hand, to prevent the weld from overheating ； On the other hand, it is to maintain slow cooling. Timely and effective post-weld heat treatment implies two things: first, this process can indeed improve the structure of the weld and its HAZ, enhance the toughness and high-temperature endurance of the joint, and eliminate welding internal stresses ； Secondly, when implementing this process, it is necessary to strictly control the minimum temperature during the post-weld cooling of the welded parts, as well as the time interval before heat treatment is applied after welding. The latter is crucial for achieving excellent joint performance. V. Application of welding processes for T91/P91 steels 1. Application of butt welding processes for T91+T91 (P91+P91) steel pipes. The exhaust ducts of the final superheater in Unit 1 of the first phase of a certain power plant were manufactured using ψ57mm×8mm T91 steel pipes; the 576 welds were welded using the umbrella TIG method or TIG backing + SMAW filling method. While implementing the key welding process points shown in Example 1 of Table 3, special emphasis is placed on pre-welding preparations, back-gassing procedures, and strict operating protocols. The welded joints exhibit excellent performance, ensuring the reliability and stability of the unit operation. At present, the unit is operating normally, and this process has been successfully completed. The chemical composition of the welding material and the mechanical properties of the deposited weld: In order to fundamentally resolve the problem of tube failures in the superheaters and reheaters of 6 ultra-high pressure power plant boilers, a certain power plant replaced G102 steel with ψ42mm×5mm T91 steel, and employed the welding process described in Example 2 in Table 3. The joints were inspected using 100% X-ray testing, and all met standards of grade 2 or higher. The replaced T91 steel pipes and their joints have maintained good quality during over a year of operation. The above is a successful case in which satisfactory results were achieved without preheating the workpiece before welding. The results of the process application in this case are consistent with the conclusion reached by Tao Yongshun and others, which states that for T91 steel pipes with a diameter of less than 60 mm and a wall thickness of less than 6 mm, cold cracks do not occur when TIG welding is used without preheating. In the installation project of 2×660MW units at a power plant, SA335—P9l large-diameter thick-walled steel was used for the main steam pipes, the hot sections of the reheat steam pipes, and the outlet headers of the boiler’s final superheater. There are 6 different pipe specifications, ranging from ψ281mm×49.5mm to ψ762mm×31.5mm, with wall thicknesses ranging from 31.5mm to 78.0mm. In total, there are 156 weld joints. The welding process specified in Example ③ of Table 3 was employed for construction; the joints were inspected using 100% X-ray testing, achieving a first-pass qualification rate as high as 99.71%, indicating excellent welding quality. This is a successful case of the P91 steel welding process applied to large-diameter, thick-walled pipes. This application shows that under highly restrictive conditions, processes such as pre-welding heating, welding with low heat input, and post-weld heat treatment are crucial for achieving satisfactory joint properties. 2. Application of the butt welding process for T91+12Cr1MoV steel pipes: The sixth phase expansion project of a power plant involved two 125MW units; T91 and 12Cr1MoV steel pipes were used for the high-temperature reheaters. The pipe specifications were 42mm×4mm, with 672 butt joints per unit. The welding process specified in Example ④ of Table 3 was used for construction; the non-destructive testing passed, and the unit has been operating well to date. This is a successful example of the T91 steel welding process applied in the welding of dissimilar metals with pearlitic heat-resistant steel. This application shows that the welding process and associated measures used are well-suited for welding this type of dissimilar steels. 3. Application of the TIG welding process using cored wire for T91 steel. In 1995, a power plant carried out maintenance work on the high-temperature superheater and the outer ring of the reheater in Boiler No. 8; the tubes were made of T91 steel with dimensions of ψ42mm×5.5mm. For this work, the specialized cored wire YR91W designed for T91 steel TIG welding, along with the welding process outlined in Example ⑤ in Table 3, was used. All post-welding inspections passed, and the boilers have been operating safely to this day. The above is a typical successful example of advancing welding technology by using a new type of welding filler material to enable TIG welding of T91 steel without argon shielding on the back side. 4. Application of the butt welding process for T91 steel + 1Cr18Ni9Ti steel pipes: During the installation of the superheaters for the 2×300MW units in the second phase of a power plant, butt joints made of different steel types, namely T91 + 1Cr18Ni9Ti, with dimensions of ψ51mm×6mm, were encountered. The welding process involved using nickel-based wire WELTIG82TIG for the root pass, along with nickel-based electrode INCONEL182 for the fill and cover passes, following the welding procedure outlined in example ⑥ in Table 3. In total, more than 160 welds were completed; after welding, 100% non-destructive testing was carried out, and the first-pass qualification rate reached 97.2%. The application examples of this welding process have the following characteristics: (1) The composition, microstructure, and properties of the pipes on both sides of the joint differ greatly. (2) The welding process used is quite different from that in the aforementioned case: ① Nickel-based wires and electrodes were employed. ②The selected preheating temperature and interlayer temperature have been significantly reduced compared to before. ③The biggest process feature is that no heat treatment is carried out after welding. All the aforementioned process variations are determined by the properties of the dissimilar steel materials and the microstructural characteristics of their joints. As can be seen from these typical applications, welding T91/P91 steel pipes with different joint combination types (same steel or different steels) and various specifications requires welding processes that are tailored to each case. In particular, a new process that uses specialized cored wires for TIG backing has been developed, taking the welding techniques for this type of steel to a new level. The progress and development of welding technology have improved the performance of joints, and facilitated the use of new steel grades and their corresponding welding processes ； The increased demand for welding processes for new steel grades has driven faster development of these process technologies. At present, compared with the major industrialized countries in the world, there is still a certain gap in the research and development of T91/P91 steels as well as in the corresponding welding processes in my country; much work remains to be done regarding the process suitability of these steels and their evaluation. It is hoped that, in a relatively short period of time, more valuable innovative results can be achieved in the development of new specialized welding materials and corresponding processes suitable for this purpose, as well as in research on aspects such as the high-temperature creep properties of joints, microstructural changes, failure mechanisms, and the weldability of dissimilar steels. VI. Conclusion (1) T91/P91 steel is a modified version of 9Cr–1Mo steel; thanks to its range of excellent mechanical properties, this new steel grade has been widely used in the high-temperature pipes of high-parameter thermal power generation units. (2) The main problems regarding the weldability of T91/P91 steel are its high sensitivity to cold cracking and a certain tendency to hot cracking; meanwhile, the weakening of joint properties—such as the deterioration of toughness in the weld zone and the softening of the heat-affected zone—cannot be ignored. A proper welding process is an important technical aspect for controlling and improving the weldability of this steel. (3) After the welding method and welding materials are determined, the key process measures to obtain high-quality joints include preheating before welding, controlling the layer temperature, and carrying out \"timely and effective\" post-weld heat treatment. (4) Different joint combination types (same steel or different steels), as well as the welding of T91/P91 steel pipes of various specifications, each require welding processes that are tailored to their specific characteristics. The new process of using specialized flux-cored wires for TIG backing has pushed the welding technology for this steel grade to a new stage of development.