Taking the new and old P92 pipes as the object,the rod samples of fine grain heat affected zone(FGHAZ)was prepared by using rapid heat treatment process.The microstructure and properties of the two FGHAZ samples were compared through observation by SEM,TEM,microhardness and creep tests,and the effect of the pipe original state on the microstructure and mechanical properties of FGHAZ of P92 steel was revealed.The results show that compared with that of the FGHAZ of new P92 pipe,the dislocation density of the FGHAZ of old P92 pipe is lower,the size of the precipitates is bigger,which decreases the hardness and creep resistance.During creep test,the FGHAZ of the old P92 pipe has higher precipitating tendency of Laves phase,which promotes the formation of creep void,accelerates the creep fracture and shortens the creep rupture life.

Super 304H has been a crucial material for ultra-supercritical boilers. However, the relationship between microstructure evolution, strengthening mechanism, and embrittling behavior during long-term aging was lacking investigation. This investigation aimed to reveal the strengthening and embrittling mechanism from precipitates in Super 304H. The results showed that the hardness increment came from the grain boundary’s M23C6 (GB’s M23C6) and intragranular nano Cu-rich particles. After being aged for 5000 h, the GB’s M23C6 and nano Cu-rich particles provided a hardness increment of approximately 10 HV and 30 HV, respectively. The impact toughness gradually decreased from 213 J/cm2 to 161 J/cm2 with the extending aging time. For the aged Super 304H, the GB’s M23C6 provided a higher cracking source. In addition, the nano Cu-rich particle restricted the twin-induced plastic deformation of austenitic grain and depressed the absorbed energy from austenitic grain deformation.

Upon careful examination, numerous wind turbine collapses can be attributed to the failure of the tower bolts. Nowadays, the Schmidt–Neuper algorithm is extensively accepted in wind turbine tower bolt design. It is not advisable to utilize the finite-element method, notwithstanding the effect of the flange gap. To quantitatively investigate the influence of flange gaps on bolt fatigue, a nonlinear finite-element model of a flange segment incorporating bolt pretension and contact elements is herein proposed. Three distinct types of flange gaps are defined intentionally. It is possible to determine the nonlinear relationship between the wall load and bolt internal force. The fatigue damage of bolts was thus computed using the obtained nonlinear curve. Comparing with the results with those of Schmidt–Neuper method revealed the bolt fatigue damage is susceptible to a specified flange gap.

Microstructure and mechanical properties of P92 steel after 70,000 h service at about 601 °C were investigated, and the normalizing and tempering state P92 steel sample was produced for comparative research. Microstructure changes and carbide precipitation were observed by OM, SEM and TEM + EDS. Tensile properties and impact toughness changes were tested, and the influence of microstructure on the mechanical properties was analyzed. The results showed that microstructure changes were martensite recovery, dislocation density decline and Laves phase precipitation and coarsening during long-term service. Creep damage formed in the service P92 pipe was not very homogeneous along the thickness direction. The creep cavities distributed in the central area of the pipe were more and larger than those in other areas. After 70,000 h service at 601 °C, tensile strength and yield strength at room temperature decreased slightly, but the yield strength at high temperature decreased sharply. The impact toughness was reduced to approximately the min. value of the standard requirement. The re-heat treatment (normalizing + tempering) can recover the hardness, room temperature tensile properties and impact toughness, but cannot completely recover the high temperature tensile properties.

Creep performance evaluation of fine-grained heat affected zone (FGHAZ) of new and service exposed P92 pipes is crucial for repairing P92 pipe of ultra-super critical (USC) power plants. In this work, bulk FGHAZ of new and service exposed P92 pipes was prepared using a rapid heat treatment process (RHTP) to investigate the creep performance degradation mechanism of fresh FGHAZ of service exposed P92 steel. The creep performance and microstructural evolution of the FGHAZ of new and service exposed P92 pipes were investigated. The results showed that W concentration in the body-centered cubic (BCC) matrix was the key to creep performance. The W concentration of BCC matrix of FGHAZ of new pipe was higher than of service exposed pipe as the Laves could not be completely dissolved in the FGHAZ of service exposed pipe after RHTP. Higher W concentration in the BCC matrix of FGHAZ of new pipe contributed to a higher dislocation density and finer M 23 C 6 particle. Higher W concentration helped keep the high dislocation density during creep. Therefore, the FGHAZ of new pipe always had a stronger BCC matrix. The stronger BCC matrix contributed to the creep deformation relied on GBS FGHAZ of new pipe. With the pinning effect from finer M 23 C 6 , the FGHAZ of new pipe exhibited a smaller creep strain rate. On the contrary, the weaker BCC matrix of FGHAZ of service exposed pipe contributed to the creep deformation of IDC which had a larger creep strain rate. Finally, it resulted in a shorter creep rupture time of FGHAZ of service exposed pipe. In addition, the higher dislocation density of BCC matrix of FGHAZ of new pipe also contributed to a higher hardness at room temperature.

In Inconel 783 alloy, there are mainly two kinds of strengthening phases of beta and gamma'. In addition, it is found that there are other precipitated phases in the alloy's beta phase. By means of multiple testing methods, the types and functions of the precipitated phase in the beta phase were determined. The results showed that the precipitated phases from beta phase were nanoscale Nb and Ti carbides and a large number of white needle-like Laves phases. The former, due to its small size, which can improve the grain boundary slip resistance of the alloy, thus improves the shaping of the material. While the latter, due to its sharp shape and due to its large amount, destroys the original structure of the beta phase and greatly reduces the room temperature plasticity of the alloy.

The creep rupture strength data of Grade 91 steel specimens with various hardness values were used to study the correlation of hardness with creep rupture strengths and maximum allowable stresses of the steel. The results show that creep rupture tests at a certain temperature and stress applied to the specimens with a series of hardness values receive the results in both overestimation and underestimation of rupture properties, leading to some unrealistic effect on 10(5) h creep rupture strengths determined in this way. An approach was thus proposed to determine the lower limits of hardness satisfying the maximum allowable stresses at given temperatures. It was found with this method that a hardness level of >= 201 (205) HBW of any of the Type 1- and Type 2- Grade 91 components running at a temperature of <= 575 (600) degrees C can satisfy the requirement of the maximum allowable stresses at the corresponding temperature specified by ASME BPVC 2019, and a hardness value of >= 204HBW is effective for the grade 91 components with a wall-thickness of mm running at a temperature of <= 575 degrees C to satisfy the requirement of the maximum allowable stresses at the corresponding temperature specified by ASME BPVC 2017. Therefore, the most recently modified specification brings, to some extent, about difficulty in continuously practicing the application of the lower limit of hardness values (190-250HBW) specified by ASME BPVC 2017-2019 because it is not satisfied with the requirement on the maximum allowable stress at some given temperatures. Thus, there is a need to raise the lower limit of hardness values to settle this issue in the future. In addition, the optimization of the function fitting the creep-rupture data currently used in the estimation calculation of service/remaining lives was studied, showing that the tendency of overestimation of rupture properties can be reduced by replacing the current power function with the logarithm one. Quite good fitness of the practical data with the logarithm function curves is contributed to separating the whole data group with a series of hardness values into the higher and lower hardness level groups in calculation. On this basis, the relationship of thickness, hardness and service life of the components with variable dimensions and hardness values can be obtained by integrating the technical parameters of the safety assessment, which is able to reflect the applicability, reliability and intuitiveness of this combination. The above results can be used as reference for both the revisions of the relevant technical standards and the practical applications of industry.

The evolution of the microstructure and mechanical properties of P91 steel during heat treatments at different cooling speeds during the normalization stage was investigated. Results showed that normalized martensite with high hardness and strength was obtained over a wide range of cooling rates (higher than 200 °C/h) during the normalization stage of P91 steel. Within this cooling rate range, twin martensite gradually generated as the cooling rate decreased, which enhanced the brittleness of the normalized P91 steel. When the cooling rate was decreased to 25 °C/h, the P91 steel exhibited completely annealed microstructures consisting mainly of bulk α-Fe grains, with the formation of continuous M23C6 carbides at the grain boundaries. As a result, the annealed P91 steel had poor hardness and strength, as well as demonstrating increased brittleness. Notably, after tempering, the hardness and strength of normalized martensite were greatly weakened, while the toughness of tempered martensite greatly increased. After the tempering process, the continuous M23C6 GBs in the completely annealed P91 steel partly dissolved, which also remarkably enhanced the toughness.

Owing to its high creep rupture strength, good weldability, and low costs, T23 steel is an ideal material for manufacturing the heating components of water walls, superheaters and reheaters in ultra-super critical plants. However, its coarse grain heat affected zone (CGHAZ) is prone to stress-relief cracking (SRC) during post-weld heat treatment or high-temperature service. The mechanism of SRC is controversial and an effective method for forecasting and preventing SRC in T23 components is currently lacking. Clarifying the mechanism of SRC in the CGHAZ of T23 steel, and developing a practical engineering technique for predicting and preventing SRC generation, are therefore essential. In this work, CGHAZ specimens of T23 steel were simulated in a thermo-mechanical simulator, and aged at 650 degrees C for 0-48 h. After simulating the microstructure evolution of the as-welded CGHAZ during service, the SRC susceptibility of the CGHAZ was evaluated. The microstructural changes and carbide precipitation were observed by OM, SEM, TEM, and EDS. The as-welded CGHAZ of T23 steel was composed of mixed martensite and bainite with high hardness. After ageing at 650 degrees C, the structure recovered and recrystallized with a lower dislocation density and larger sub-grains than the as-welded CGHAZ. Carbides such as M23C6, M7C3, and MX gradually precipitated inside the grains and grain boundaries, decreasing the hardness. The SRC susceptibility was high in the as-welded CGHAZ, but decreased with increasing ageing time. When the ageing time exceeded 24 h, the sample was SRC-resistant. The main cause of SRC in the CGHAZ was precipitation and growth of M23C6 on the grain boundaries, which induced the formation of softened zones in the matrix near the grain boundary, and promoted the formation of micro-voids. During ageing, the unstable microstructure in the as-welded CGHAZ transformed as carbides precipitated and the matrix recrystallized, thereby reducing the intragranular strength. Meanwhile, the depletion of alloy elements near the grain boundary was eliminated. The microstructural evolution decreased the difference between the intragranular and intergranular strengths in the CGHAZ. Finally, the CGHAZ showed significantly improved ductility and low SRC susceptibility. The hardness of the aged CGHAZ was positively related to the SRC susceptibility. At hardnesses above 250 HB, the CGHAZ was SRC-susceptible, but at hardnesses below 250 HB, the CGHAZ was SRC-resistant.

Simulated coarse-grained heat-affected zone (CGHAZ), second reheated CGHAZ (UA-CGHAZ), super-critical reheated CGHAZ (SCR-CGHAZ) and inter-critical CGHAZ (ICR-CGHAZ) of T23 steel were produced via thermal simulation of welding. Their corresponding stress-relief cracking (SRC) susceptibility were assessed using isothermal slow strain rate tensile test. The fracture features and microstructures were characterized to reveal the cracking mechanisms. The simulated CGHAZ of T23 steel was highly susceptible to SRC at temperatures of 550–750 °C and the fracture mode exhibited micro-void coalescence. M23C6 carbides precipitated at grain boundary may promote the micro-void nucleation and weaken the grain boundary, which resulted in grain boundary cracking preferentially. The strain was concentrated on the weakened grain boundary under tensile load, and the micro-voids gradually grew and coalesced into micro-crack, which ultimately propagated along grain boundary until complete fracture occurred. The SRC susceptibility of SCR-CGHAZ was reduced partially and the SRC susceptibility of ICR-CGHAZ was eliminated. After a second thermal cycle with a peak temperature slightly higher than Ac3 (SCR-CGHAZ), the grain size of CGHAZ was significantly refined and the crack resistance was elevated. When the peak temperature of the second thermal cycle was limited between Ac1 and Ac3, partial grains near grain boundaries were austenitized and transformed to fine ferrite grains along the prior austenite grain boundaries. The fine ferrite grains inhibited M23C6 precipitation and improve the resistance of cracking propagation. Thus, the ductility was improved and the SRC susceptibility was reduced.

Simulated coarse-grained heat-affected zone (CGHAZ) of T23 steel was produced via thermal simulation of welding, and its stress-relief cracking (SRC) susceptibility was evaluated using isothermal slow strain rate tensile test. The evolution of inter-granular and intra-granular precipitates in CGHAZ during tempering, and the depletion of alloy elements adjacent to grain boundaries were characterized to clarify the cracking mechanism. The simulated CGHAZ of T23 steel was slightly susceptible to SRC at temperatures of 675-750 degrees C and the fracture mode exhibited micro-void coalescence. Massive coarse inter-granular M23C6 carbides precipitated during tempering, promoting the nucleation of micro-voids on grain boundaries as well as the depletion of Cr, W, and Mo near grain boundaries, thus weakening the grain boundaries. Many relatively fine intra-granular M23C6 and M7C3 carbides precipitated, enhancing the strength of the grain interiors. Therefore, the strain was concentrated on the weakened grain boundary under tensile load, and the micro-voids gradually grew and coalesced into micro-crack, which ultimately propagated along grain boundary until complete fracture occurred. The intragranular MX carbide was not a major factor for the SRC generation in T23 steel because very few MX carbides precipitated during short-time tempering.

The hardness test and microstructure characterization were adopted to analyze the cracking failure of dissimilar steel weld cracking on high and medium pressure steam lead pipe. The materials of the cylinder, the guide pipe and the welding material were ZG15Cr2Mo1, 1Cr9Mo1VNbN and AWS 5. 11 ENiCrFe-1, respectively. The study showed that the hardness of the fusion line was higher than that of the adjacent base metal (ZG15Cr2Mo1) and nickel base welding, which was the weak area. The pressure pipe cracking is high temperature fatigue crack. The grains in the weld joint region were coarse, indicating that the welding specification is too broad and the time for welding remaining in the sensitized temperature zone is too long. The improper welding process was the main reason for the welding cracking of the high and medium pressure guide pipe.