Abstract
The effects of the post-weld treatment on the impact performance, microstructure, and carbide precipitation behavior of pressure vessel steel were evaluated under simulated post-weld conditions. The continuous cooling transformation and isothermal transformation curves of undercooled austenite for the steel were constructed based on the expansion curve, serving as a guide for the potential heat treatment of the steel plates. A more detailed study was conducted on the simulated post-weld process with an insulation temperature of 690 ℃ and an insulation time of 24 h, based on the delivery status of the steel plate. The microstructure was characterized using transmission electron microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, and electron probe microanalysis. The Charpy V-notch impact test was used to assess the impact performance of the steel plates. The results showed that refining the microstructure to 50% bainite and 50% ferrite, along with a high proportion of large-angle grain boundaries and large-angle misorientation grains at half the thickness of the steel plate, contributed to enhanced low-temperature impact toughness in its delivered state. Additionally, the steel predominantly consists of chromium-containing carbides. In the as-delivered state, the carbide size was measured at 110 nm. However, after post-weld heat treatment (PWHT), the carbide size significantly increased to 360 nm, reflecting a 227% growth. This coarsening is observed along the grain boundaries and through intragranular aggregation. Additionally, there was a change in carbide type from Cr7C3 in the as-delivered state to Cr23C6 following the heat treatment. This transformation was accompanied by a significant reduction in impact toughness, as evidenced by the impact energy dropping from 116 J to an unacceptable 43 J.
The core equipment used in petrochemical and coal chemical plants operates in harsh environments characterized by high temperatures, high pressures, and prolonged exposure to hydrogen. The steel utilized in manufacturing these components is particularly vulnerable to hydrogen damage and temper embrittlement, among other issues1,2. Currently, there is no unified and universally accepted theory explaining the hydrogen embrittlement mechanism. The primary theories include the hydrogen pressure theory and others3,4,5,6,7,8,9. The most widely accepted explanation for high-temperature hydrogen corrosion suggests that the level of thermal activation energy increases in hydrogen-rich environments. As hydrogen penetrates the steel, it reacts with carbon to produce methane, leading to surface or internal decarburization of the steel. Simultaneously, methane can accumulate at defects, such as inclusions or grain boundaries, resulting in the formation of methane bubbles. As the pressure within these bubbles rises, microcracks develop, which can be detected through acoustic emission technology10. This ultimately leads to a reduction in the strength and toughness of the steel11,12,13,14. To improve the stability and efficiency of core equipment, alloying elements such as chromium (Cr) and molybdenum (Mo) can be added to the steel. This not only enhances the mechanical properties and overall performance but also decreases the reactivity of carbon. Additionally, the carbides formed in the nanophase or at their interfaces serve as irreversible hydrogen traps, effectively reducing the diffusion rate of hydrogen15,16,17,18,19,20 and enhancing the steel’s resistance to high-temperature hydrogen corrosion21,22,23.
The petrochemical and coal chemical industries are using increasingly large and complex equipment. As a result, the state is placing greater emphasis on the safety of these industries and is upgrading energy equipment through the development of new methods and technologies24,25,26,27,28. This trend presents unprecedented challenges for chrome-molybdenum steel, which is used in core equipment. Certain design parameters, such as die welding temperature, time, and impact temperature, are pushing the limits of this material. As die welding temperatures increase and welding durations extend, the mechanical properties of the steel deteriorate significantly, leading to irreversible damage to its matrix29,30. Research dating back to the 1980s by Japanese scholar Goukou Shuichiro indicated that the reheat embrittlement of Cr Mo steel used in pressure vessels worsens when heat treatment temperatures are elevated or when holding times are excessively prolonged. This suggests that higher tempering parameters can have detrimental effects. Shuichiro’s research also indicated that improving the toughness of this steel typically involves adjusting the levels of elements such as carbon (C), silicon (Si), and manganese (Mn) to enhance hardenability and reduce oxygen content, rather than merely modifying tempering parameters. Recently, some Chinese scholars have developed a systematic process for producing clean steel with low carbon, ultra-low phosphorus, and ultra-low sulfur to meet the complex design requirements of large-scale pressure vessels. Notably, they introduced the Kr hot metal pretreatment technology, effectively controlling sulfur and phosphorus content in the hot metal. Additionally, they devised an external refining and deep desulfurization process that can lower sulfur content in finished products to below 0.002%. They also implemented innovative processes using single and double slag methods, as well as a top-and-bottom blown converter combination to ensure phosphorus levels remain below 0.005%. Moreover, they applied vacuum degassing technologies, including vacuum degassing (VD) and Richard’s Ham (RH) vacuum degassing, to significantly reduce impurities such as arsenic (As), tin (Sn), copper (Cu), oxygen (O), and hydrogen (H). To improve product toughness, they created a heat treatment process that combines quenching, sub-temperature quenching, and tempering (QLT), while also optimizing these processes. These technological advancements significantly enhance the physical properties of pressure vessel steel. To address new challenges, it is crucial to study the characteristics of precipitates formed during the heat treatment of chrome-molybdenum steel. This involves controlling the levels of harmful elements in steel plates, enhancing the purity of the steel, minimizing temper embrittlement, and adjusting the proportions of alloying elements. The aim is to further refine the alloy carbides, improve high-temperature corrosion resistance, enhance low-temperature impact toughness, and ensure performance stability, toughness matching, and homogeneity. These improvements will ensure the reliability of materials during processing and usage, ultimately guaranteeing the safe and stable operation of core equipment31. The rolling process has a direct effect on the structure and properties of chrome-molybdenum steel. By optimizing this rolling process, it is possible to significantly reduce the genetic effects of the rolled structure, eliminate performance variations in the thickness direction of steel plates, homogenize the structure, refine grain size, and reduce the size of carbide particles. This optimization lays a solid foundation for the subsequent heat treatment of the steel plates.
Materials and methods
Specimens preparation
The material used in this experiment is a chrome molybdenum steel plate sourced from a steel plant, with a thickness of 150 mm. The plate is manufactured through a controlled rolling and controlled cooling process, which is followed by normalizing and tempering. Specifically, controlled rolling and controlled cooling involves managing the rolling temperature during Phase II and preventing the steel plate from re-heating by using a water-cooling device (ACC) right after the rolling process is completed. The chemical compositions of the steel plates are outlined in Table 1. This table also details the range of chemical compositions for pressure vessel steel according to the GB/T 713 standard. As shown in Table 1, the chemical composition of the materials used in this experiment fully complies with the GB/T 713 standard. The sample size is 250 mm × 350 mm. A band saw is then used to split the sample in half along its width, resulting in two identical pieces measuring 125 mm × 350 mm each. One piece is processed into an impact sample blank at half the plate thickness, as per GB/T 2975 − 2018 “Sampling Position and Sample Preparation for Mechanical Property Tests of Steel and Steel Products.” This piece is then shaped into a standard V-notch impact sample measuring 10 mm × 10 mm × 55 mm, following the GB/T 229–2020 “Metallic Materials - Charpy Pendulum Impact Test Method. “The other piece undergoes maximum simulated post-weld heat treatment in a small resistance furnace. The post-weld heat treatment process involves holding the material at 690 °C for 24 hours, then cooling it to 350 °C before removing it from the furnace. The temperature is raised and lowered at a rate of 55 °C/h, followed by air cooling once it is unloaded. After cooling to room temperature, the impact sample is processed according to the aforementioned standards. Impact property tests are conducted on the pressure vessel steel under various heat treatment conditions. These tests are performed according to industry standards at a test temperature of -20 °C. Please refer to Fig. 1 for the processing method and Fig. 4 for the impact test results of the impact samples.
Table 1 Chemical composition of standard and experimental pressure vessel steel (mass fraction/%).
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Fig. 1
figure 1
Schematic diagram of impact specimen processing.
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Measurement methods
The aforementioned interrupted impact samples underwent grinding and polishing, followed by metallographic corrosion using 4% (by volume) nitric acid alcohol. Subsequently, the microstructure was examined and analyzed via a 200MAT metallographic microscope and a Quanta 400 scanning electron microscope. Simultaneously, samples for transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) were prepared using a double-jet electrolytic thinning instrument. The preparation process entails several steps: initially, the sheet sample is thinned to approximately 50 μm, then punched into a circular piece with a diameter of 3 mm using a sample punch. Subsequently, the film sample is further thinned using the double-jet thinning instrument. The double-jet solution consists of anhydrous ethanol mixed with 10% (by volume) perchloric acid, and the temperature is maintained between − 25 and − 20℃. The morphology and distribution of precipitated phases in the sample were observed using a FEI Tecnai F30 TEM, while the chemical composition of carbides was analyzed via the TEM’s built-in energy dispersive spectroscopy (EDS). The fine microstructure of chrome-molybdenum steel was analyzed using a Zeiss Ultra55 SEM equipped with EBSD, focusing on the microstructural and carbide transformation characteristics under both heat treatment and post-weld heat treatment conditions. The CCT and TTT curves of the steel were determined using a DIL805A/D phase change instrument coupled with JMatPro image analysis software. The critical point testing standards adhered to were YB/T 5127-93 and YB/T 5128-93. The test parameters were as follows: austenitizing temperature (℃): 930, holding time (s): 600, room temperature (℃): 20, temperature rise rate (℃/h): 200, and temperature drop rate (℃/h): 200.
Results
Determination of the CCT curve and TTT curve
Steel is a metal characterized by multi-type phase transitions, with its high-temperature structure (austenite) and transformation products (ferrite, pearlite, bainite, and martensite) exhibiting varying bulk densities. Consequently, when a steel sample undergoes heating or cooling, the volume effects resulting from its phase transitions are superimposed on the expansion curve, disrupting the linear relationship between expansion and temperature. This allows the determination of the phase transition temperature based on the change point on the expansion curve, which corresponds to the critical phase transition point of steel in the solid state32.
Upon measurement, the CCT and TTT curves for the pressure vessel steel studied in this paper are presented in Figs. 2 and 3. Respectively, with the corresponding critical point data outlined in Table 2. Specifically, the phase transition points Ac1 are at 751.7℃, and Ac3 at 903.4℃. The heat treatment process for the steel plate in this study, encompassing normalizing and tempering, is based on these measured values.
Table 2 Critical points of experimental steel.
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Fig. 2
figure 2
CCT diagram of experimental steel.
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Fig. 3
figure 3
TTT diagram of experimental steel after isothermal conditions for 2 h.
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The influence of high-temperature and long-term mold welding on the low-temperature impact toughness of steel plates
Figure 4 illustrates the processing standards for the samples and the impact performance in the normalized tempered state (representing the delivery state, the same applies hereinafter) as well as in the maximum die welding state. The average impact energy of the steel plate in the delivery state is 160 J (with a peak value of 252 J), indicating stable and satisfactory impact energy (refer to the blue dotted trend line). However, upon undergoing high-temperature, long-duration die welding, the stability of the impact energy significantly deteriorates (as indicated by the orange dotted trend line), accompanied by fluctuations, with the impact energy ranging from 43 to 216 J. Based on the standard requirements (average impact value ≥ 47 J, individual value ≥ 33 J), there is essentially no surplus impact energy. Consequently, it is evident that post-weld heat treatment significantly affects the low-temperature impact toughness of the steel.
Fig. 4
figure 4
Distribution diagram of impact energy under various conditions.
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Discussions
Organizational structure analysis
Fig. 5
figure 5
Metallographic structure of delivery status (a) and PWHT state (b).
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The final mechanical properties of steel are influenced by several factors, including the type of microstructure, the proportions of different phases within the steel, the level of structural refinement, and the precipitation behavior of second-phase particles. Generally, a mixed microstructure comprising multiple phases is considered beneficial for achieving desirable overall mechanical properties33. In Fig. 5, sections (a) and (b) illustrate microstructural images at 500× magnification of the as-delivered and maximum die-welded specimens, respectively. According to Fig. 5, the structure at the midpoint of the thickness of the as-delivered sample consists of a homogeneous mixture of 50% bainite and 50% ferrite, characterized by a uniform and refined grain size exceeding grade 6.5. Non-uniform nucleation of defects in ferrite can cause pre-eutectoid ferrite to preferentially nucleate at grain boundaries, inclusions, and other locations34. Most of the remaining austenite transforms into a bainite structure, which then develops into a bainite tempered structure along with a small amount of polygonal ferrite during the tempering process. It is important to control the formation of ferrite, as higher ferrite content at low temperatures can lead to decreased toughness. This occurs because a significant presence of the soft ferrite phase causes cracks to propagate around the harder phases (such as pearlite or bainite) within the ferrite. When cracks enter a large-sized ferrite phase, their capability to relieve stress concentration at the crack tip diminishes35,36. In contrast, the structure at the midpoint of the thickness of the maximum die-welded specimen shows no significant change. However, due to the specimen undergoing a high-temperature die welding process at 690 °C for 24 h, stress was further relieved, leading to the precipitation of carbides within the ferrite. The residual austenite in the M-A islands has largely decomposed into carbides (as indicated by the arrow in Fig. 5b), and the number, size, and distribution of carbides within the bainite structure have significantly increased, becoming more dispersed and clustered. The original grain boundaries have become indistinct or even vanished37. Further research has identified that the mechanical properties of the steel are primarily influenced by two mechanisms: solid solution strengthening, which results from the presence of alloy elements like carbon (C), chromium (Cr), and molybdenum (Mo) dissolved in the steel, and precipitation strengthening due to the formation of chromium-containing molybdenum carbides. These two mechanisms counterbalance each other, with solid solution strengthening having a more substantial impact. In the specimen subjected to post-weld heat treatment (PWHT), as shown in Fig. 5b, excessive carbide precipitation enhances the precipitation strengthening of the matrix. However, under the predominant influence of solid solution strengthening, a significant increase in the precipitation of major alloy elements such as Cr, Mo, and nickel (Ni), combined with carbon diffusion in the matrix, results in the formation of larger and more numerous carbides, which significantly diminishes the solid solution strengthening effect.
To analyze the effective grain size accurately, Electron Backscatter Diffraction (EBSD) was used to observe the orientation difference Inverse Pole Figure (IPF) distribution map of the as-delivered microstructure, as shown in Fig. 6. Grain boundaries with orientation differences exceeding 15° are marked with blue lines. Statistically, the proportion of large-angle grain boundaries (greater than 15°) in the sample structure is 37.2%. A higher density of large-angle grain boundaries enhances resistance to crack propagation, allowing the sample to absorb more energy during fracture. The increase in the proportion of large-angle grain boundaries, along with the refinement of grains with significant orientation differences, contributes to improved low-temperature impact toughness in steel. Therefore, one of the primary strategies to enhance the impact toughness of the test steel is to carefully control the multiphase heat treatment process, promoting the refinement of the bainite/martensite plate bundle and block dimensions.
Fig. 6
figure 6
(a) IPF pic. and (b) Large and small angle grain boundaries of chromium molybdenum steel EBSD samples.
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Morphology analysis of carbides
A detailed observation of carbides in the microstructure was conducted using a Quanta 400 scanning electron microscope, as shown in Fig. 7. The SEM images illustrate the presence of nanoscale dispersed precipitates. For this type of steel, normalizing is akin to a re-austenitizing process. The effects of quenching stress and phase change volume differences lead to many dislocations, resulting in a high dislocation density and significant residual stress in the steel plate after subsequent quenching and water cooling. After tempering, there is recovery of dislocations, and changes occur in the width of the bainite laths. Meanwhile, the carbon-rich hard phases, such as M/A islands, in the quenched microstructure begin to decompose during tempering. The carbon atoms released during this process combine with alloy elements like chromium (Cr) and molybdenum (Mo) to form a dispersed distribution of chromium-molybdenum carbides. These fine precipitates effectively impede dislocation movement, delay the recovery of the matrix, and reduce the tendency towards softening, thereby ensuring high strength in the tempered steel plate— a phenomenon known as precipitation strengthening. Observations also reveal significant variations in the distribution and morphology of the carbonized precipitates. In the as-delivered state, the precipitated carbides are relatively small, measuring approximately 110 nm, and predominantly exhibit a granular or short-rod shape, distributed both within the crystals and at the grain boundaries. In contrast, after prolonged high-temperature die-welding processes, the originally distinct grain boundaries weaken or may even disappear. This results in uneven carbide precipitation and signs of aggregation, with some granular carbides undergoing a morphological transformation into spheroidized shapes. The size of these carbides increases to approximately 360 nm, indicating a pronounced growth effect. This prolonged high-temperature die welding essentially acts as a detrimental tempering process.
Fig. 7
figure 7
SEM structure of delivery status (a) and PWHT state (b).
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Based on the energy spectrum analysis presented in Fig. 8, further investigation into the various forms of car-bides shows that short-rod carbides contain the highest proportions of chromium and molybdenum. Larger ag-glomerated granular carbides follow, containing the second-highest proportion of these elements, while smaller agglomerated granular carbides have the lowest proportion. These findings suggest that larger-sized carbides are enriched in Cr and Mo, resulting in a stronger precipitation-strengthening effect. However, this also signifies a substantial weakening of the solid solution strengthening effect. Additionally, the strengthening effect of these larger carbides is significantly inferior to that of spheroidized carbides. Their dense distribution at the grain boundaries makes them prone to becoming sources of crack propagation, ultimately leading to a considerable decline in the performance of pressure vessel steel.
Fig. 8
figure 8
Energy spectrum analysis of carbides in pressure vessel steel.
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Analysis of the elemental composition of carbides
The distribution of alloy elements in the structure of chrome-molybdenum steel is characterized in detail using the Energy Dispersive Spectroscopy (EDS) integrated into the Scanning Electron Microscope (SEM), as illustrated in Fig. 9. The upper figures represent the distribution of each element in the sample’s as-delivered state. The lower figures then highlight the distribution of key elements after the maximum die-welding process. The findings reveal that displacement alloy elements, including Cr and Mo, exhibit a distinct distribution within both ferrite and bainite, with a particularly abundant presence in bainite, predominantly manifesting as chromium carbide precipitation. Upon closer inspection, it becomes evident after PWHT, key alloy elements such as C, Cr, and Mo become more concentrated at the grain boundaries (indicated by the arrows).
Fig. 9
figure 9
Distribution diagram of the main alloying elements in the microstructure of pressure vessel steel.
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Analysis of carbide morphology
Fig. 10
figure 10
TEM bright field image and carbide SAED pattern of pressure vessel steel.
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The TEM bright field image and carbide SAED pattern of pressure vessel steel in different states are presented in Fig. 10. It is evident that the sample retains numerous bainite and ferrite lath structures. Specifically, in the delivery state, the matrix still contains a large number of dislocations, and the carbide precipitation is relatively uniform and dispersed, without aggregation or growth. Through SAED pattern analysis combined with EDS results, it was determined that the precipitated granular second phase is primarily Cr7C3 carbides. These carbides pin the relative dislocations, preventing the recovery and recrystallization of the ferrite matrix in the tempered state. The dual strengthening mechanisms of solid solution strengthening and precipitation strengthening contribute to the steel’s excellent low-temperature toughness. In contrast, in the PWHT state, the bainite and ferrite lath structures significantly widen, and the carbide type transitions from Cr7C3 to Cr23C6. As the high-temperature, long-duration die welding process persists, the matrix gradually softens. Simultaneously, the type of Cr23C6 carbide significantly reduces the fracture strength of the grain boundaries38, turning it into crack sources and propagation paths, thereby decreasing impact toughness. In other words, the smaller size of Cr7C3 carbides helps improve the low-temperature impact toughness of the test steel, while the larger size of Cr23C6 carbides is just the opposite.
Tao, P, et al. proposed a sequence for corresponding carbide transformation during tempering with initial precipitation of M3C and the subsequent precipitation of M7C3 and M23C6, simultaneously researched the decrease in hardness of the tempered specimens agreed well with the prediction of the weakening of precipitation strengthening owing to the coarsening of carbides. The calculation carried out by Janovec et al. showed that the second phases in Cr-Mo-V low alloy steels like M3C, M23C6, and M7C3 carbides had different Cr/Fe ratios and their chemical compositions changed during tempering. However, the above studies did not discuss the adverse effects of carbide coarsening on the properties of steel, especially low-temperature impact toughness. From the research results of this paper, it can be seen that appropriately reducing the PWHT time and controlling the excessive transformation of carbides from Cr7C3 to Cr23C6 have significant guiding effects on balancing post-weld processes with excellent low-temperature impact properties of pressure vessel steel, and further seeking the optimal balance for equipment safety.
Conclusions
In this paper, the mechanism of carbide precipitation behavior of the pressure vessel steel before and after PWHT was investigated. The effects of carbide transformation on the Charpy V-notch impact properties were also evaluated. Based on the above investigation, the following conclusions could be drawn:
(1)
The refinement of the steel plate comprising 50% bainite and 50% ferrite multiphase contributes to improved low-temperature impact toughness in its delivered state. This enhancement is due to the optimal balance between strength and toughness provided by this specific multiphase structure. Furthermore, the high proportion of large-angle grain boundaries and misorientation among grains at half the thickness of the steel plate increases resistance to crack propagation, allowing the material to absorb more energy during fracture.
(2)
The mechanism of carbide precipitation mainly includes the decomposition of the carbon-rich hard phase during the tempering process. During this process, carbon, chromium, and molybdenum together form dispersed carbides, hindering dislocation movement and ensuring the high strength and toughness of the steel plate.
(3)
The most significant change observed during the transformation from small-sized Cr7C3 carbides in the original state to larger-sized Cr23C6 after post-weld heat treatment (PWHT) is the dramatic reduction in the steel’s properties, particularly evident in the sharp decline in impact toughness. Therefore, design institutes and manufacturing plants should aim to avoid conducting high-temperature post-weld heat treatments that exceed 24 h for this type of steel.
Data availability
All data generated or analyzed during this study are included in this paper.
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Acknowledgements
The author is very grateful to HBIS Funded Project: HBIS GROUP’s Key Science and Technology Project in 2023, “Research on the Phase Transformation Mechanism and R&D Promotion of Heavy-Thickness Steel under Single Quenching Processes at Negative Temperatures (2023106)”.
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Wuyang Iron and Steel Co., Ltd., Wugang, 462500, Henan, China
Yang-bing Li
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Y.-L. conceived the experiment, collected and analyzed the data, wrote the initial draft, obtained the research fund.
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Li, Yb. Effect of PWHT process on carbide precipitation behavior and impact toughness of pressure vessel steel. Sci Rep 15, 9735 (2025). https://doi.org/10.1038/s41598-025-94900-7
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Received:07 November 2024
Accepted:18 March 2025
Published:21 March 2025
DOI:https://doi.org/10.1038/s41598-025-94900-7
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Keywords
Pressure vessel steel
PWHT
Carbides
Microstructure
Impact toughness