高温服役对HP40Nb炉管焊接接头组织及性能的影响

以一段服役7年左右的HP40Nb制氢转化炉管为对象,研究了高温服役过程对其焊接接头微观组织及力学性能的影响。观察分析了不同区域显微组织的特征和析出相的形态及分布情况,并测定了各区域的硬度、强度和冲击韧性等性能参数。结果表明:HP40Nb炉管焊接接头的熔敷金属中只有少量碳化物析出,而在母材中有大量的碳化物析出并聚集在奥氏体晶界上形成网链状;接头的抗拉强度只有少量下降,但其延伸率和冲击韧性均发生严重下降。     

Damage characterization in two reformer heater tubes after nearly 10 years of service at different operative and maintenance conditions

Reformer furnaces are used in petrochemical industry to produce hydrogen. Their most critical components are radiant tubes, where extreme temperature and pressure conditions required the deployment of high alloyed austenitic HP grade steels, owing to their superior strength to creep rupture and good corrosion resistance. Nevertheless, these high strength alloys undergo damage when process conditions allow coke deposition and maintenance procedures are not carried properly.Two radiant tubes, coming from two different plants and made of HP40Nb and HP40Nb microalloy steels respectively, are investigated here to highlight the endured damage after 85,000–96,000 h of service at maximum temperature of ～950 °C. Light optical microscopy (LOM) and scanning electron microscopy (SEM) analyses both confirmed significant aging in both tubes, as revealed by a substantial microstructural evolution consisting of phases transformation and precipitation, as well as cavitation and incipient microcracking in the bulk. Mechanical testing demonstrated the considerable worsening of mechanical properties following such a microstructural deterioration.However, noticeable differences between the two tubes were discovered as far as type and distribution of damage, and extent of the aged zone. The small differences in alloys composition seem not sufficient alone to explain such dissimilarity in behaviour because, in addition to creep damage, one tube showed also clear evidence of carburization. Instead it is argued that differences in process conditions (i.e. temperature, pressure chemical composition of process fluids and steam–hydrocarbon ratio) and cleaning maintenance for decoking (i.e. type and frequency) may have synergistically triggered different damage mechanisms responsible for the different form of degradation on each reformer tube.     

Microstructural changes caused by yttrium addition to NbTi-modified centrifugally cast HP-type stainless steels

Centrifugally cast heat-resistant HP stainless steels are particularly suitable for applications where service conditions comprise high temperatures and aggressive environments; thus, they are extensively used in reformer furnaces, in which hydrogen production takes place. The demand for better performance has motivated developments in these steels. The addition of Nb and Ti as microstructural modifiers has proved successful in providing a more stable microstructure. In this work yttrium was added to centrifugally cast NbTi-modified HP steels. It was observed that its presence increased the level of fragmentation of the chromium carbides, a positive aspect for creep resistance. The main cause of the fragmentation is the formation of yttrium carbides, which serve as heterogeneous nucleation sites for the other carbides. One of the tubes, with a lower titanium content, showed the best creep performance among those tubes studied owing to the presence of a smaller volume fraction of the deleterious G phase.     

Effect of overheating temperature on the microstructure and creep behavior of HP40Nb alloy

HP40Nb alloy has been widely used as a high temperature material in petrochemical plants. However, overheating or local temperature excursion occurs occasionally in service and leads to serious damage on the material. Effect of temperature on the microstructure and creep performance of the HP40Nb alloy is investigated in the present work. Several specimens are cut from serviced components of the alloy and heat-treated at different temperatures from 900 °C to 1250 °C for its possible working conditions, in which the temperature of 1200 or 1250 °C is used to simulate the overheating condition of HP40Nb tubes. The microstructure of specimens is examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). The creep behavior is evaluated through using impression creep tests with a flat-ended cylindrical indenter. The content of inter- and intra-dendritic carbides in the specimens is represented by the surface fraction of each phase, which has been estimated by image processing method. The results show that the total of the surface fraction of inter- and intra-dendritic carbides in the HP40Nb alloy does not significantly change at the temperature lower than 1100 °C. However, the surface fraction of inter-dendritic carbides reaches the maximum at 1100 °C. A maximal steady state impression rate is also observed at 1100 °C. The results suggest that the content of inter-dendritic carbides is the main influencing factor on the creep performance of HP40Nb alloys comparing with that of the intra-dendritic carbides.     

Damage analysis in Fe–Cr–Ni centrifugally cast alloy tubes for reforming furnaces

Reformer furnaces tubes work under high temperature and pressure for a long time, which are very critical conditions for creep deformation and life of most common materials. Cast austenitic Fe–Cr–Ni alloys in the widely know HP grades are used for reformer tubes to allow a good service at temperatures that can be close to 1000 °C. This paper reports a study devoted to the damage analysis of reformer furnace tubes after more than 100,000 h of service. Tubes, made of a HP grade modified with Nb and Ti additions, were inspected in situ by a laser optic system to measure their internal diameter and evaluate creep deformation. With the aim of developing a criterion for deciding the substitution of components, samples of as cast material and samples, cut from the most deformed tubes put out of service, were considered to check changes of mechanical properties and metallurgical characteristics. Tensile and creep tests were carried out; moreover the metallographic observations included optical and scanning electron microscopy and energy dispersive X-ray microanalysis in order to measure locally the chemical composition.     

Microstructure and properties of alloy HP50–Nb: comparison of as cast and service exposed materials

Abstract

Spun cast alloy HP50–Nb tubing retired from service in a steam reformer after approximately 8 years of service had stress rupture properties suggesting very short remaining life. Microstructural examination, however, indicated little or no creep damage (cavitation) but did reveal extensive microstructural modification. The microstructure, mechanical properties, and stress rupture properties of the ex service material were determined and compared with virgin as cast material. The microstructural changes that occurred during service included the formation of phases rich in alloying elements such as chromium and niobium that would otherwise be expected to contribute to high creep strength. The creep life of the service exposed material was correspondingly short. The creep ductility of the service exposed material, however, was significantly higher than that of the as cast material. The implications of these findings for remaining life assessments of reformer furnaces using these materials are discussed.

MST/3207     

Nb对HP合金组织与蠕变断裂强度的影响

随 Nb 含量的增加,HP 合金蠕变断裂强度的变化表现出极大值特征。在含碳0.4(wt%)的HP 合金中添加0.5(wt%)以下的 Nb,主要起固溶强化作用;当 Nb 量约1.5(wt%)时,可以获得最佳的蠕变断裂强度。其原因是在共晶组织中形成了形态复杂的共晶 NbC,在晶内析出了高度弥散的二次 NbC,以及由于 Nb 的加入提高了组织稳定性;当 Nb 量进一步提高时,由于共晶 NbC 形成了片层状共晶团以及晶内 M_(23)C_6大量减少使蠕变断裂强度下降。     

Magnetic Hysteresis Loop Technique as a NDE Tool for the Evaluation of Microstructure and Mechanical Properties of 2.25Cr–1Mo Steel

This study intends to understand the effect of short-term overheating on microstructure modifications, and variation in mechanical and magnetic properties of boiler tubes. It is based on the hypothesis that short-term overheating on boiler tubes leads to microstructural changes degrading their mechanical properties, thus resulting in their failure. As part of this study, fresh 2.25Cr–1Mo boiler tube samples were heat treated in the range of 700–950 \(^{\circ }\hbox {C}\). Magnetic hysteresis loop (MHL) measurements were carried for the non-destructive evaluation (NDE) of the mechanical properties that get altered due to microstructural modifications. For comparison, MHL was also carried out on a service-exposed boiler tube, which had failed due to short-term overheating. The magnetic parameters were correlated with the change in microstructure and micro-hardness of the samples. A decrease in coercivity, remanence, maximum induction and micro hardness were found at the lower soaking temperatures, due to: easy magnetic domain wall and dislocation motions for the stress relaxation; annihilation and recovery of dislocations; increase in inter carbide distance; and the decrease in the number density of carbides for the coarsening of carbides. A subsequent increase in coercivity, remanence and maximum inductions along with hardness are due to the nucleation and growth of fresh bainitic phase, obstructing the magnetic domain wall and dislocation motion. A drastic decrease in coercivity for the service-exposed tube is due to the transformation of MX type carbides to \(\hbox {M}_{23}\hbox {C}_{6}\) type and accumulation of such carbides at the grain boundaries along with the decrease in number density of the carbides. The presence of scale has less effect on the coercivity, but its demagnetizing effect largely decreases the remanence and maximum induction. The results of the study show that the MHL can be a suitable NDE technique for the evaluation of change in microstructure and degradation of mechanical properties in power plant steels.     

Effect of austenitising temperature on microstructure and mechanical properties of 30Si2CrNi4MoNb ultrahigh strength steel

Abstract

The microstructure and mechanical properties of 30Si2CrNi4MoNb ultrahigh strength steel were investigated after austenitising over a range of temperature between 1133 and 1483 K. The experimental results show that the isotropy of impact toughness and mechanical properties were greatly improved due to the disappearance of undissolved aligned second phase when the austenitising temperature was over 1233 K. When the austenitising temperature was over 1383 K, martensite lath and packet abnormally grew up due to dissolution of spheroidal Nb rich carbonitrides; both the platelet size and morphology of martensite were changed, which has an effect on the mechanical properties of the samples. It was noticed that the finer self-tempered carbides, which strengthened martensitic matrix, appeared after austenitising temperature over 1283 K. The strength profiles show a marked plateau for the samples austenitised from 1283 to 1433 K; however, the strength was deteriorated due to coarsening of these self-tempered carbides at 1483 K. It was confirmed that calcium treatment can help improve the isotropy of mechanical properties by modifying sulphide inclusion morphology.     

HP40Nb合金钢高温劣化的非线性超声评价

耐高温合金材料在服役过程中会发生高温劣化,主要表现为微观组织结构的变化,从而导致在其中传播的超声波能产生显著的高次谐波.针对HP40Nb合金钢材料高温劣化的检测及评价,提出采用归一化超声纵波非线性参量来表征材料的高温损伤状态.对HP40Nb损伤试样进行了非线性超声信号的测量结果显示,归一化超声纵波的非线性参量随着材料高温加载时间呈现出显著的上升-平稳-下降趋势.对材料微观组织演化过程进行了扫描电镜(SEM)的观察和分析,结果表明,超声非线性参量在早期阶段的上升与HP40Nb材料高温加载过程中η相等第二相的析出和聚集密切相关;其在后期阶段的下降归因于析出相的减少和脱落以及微孔洞的产生.由此可见,非线性超声对HP40Nb材料高温劣化后微观组织的变化非常敏感.     

Thermal Degradation Evaluation of HP40Nb Alloy Steel After Long Term Service Using a Nonlinear Ultrasonic Technique

Nonlinear ultrasonic analysis has been proposed to characterize the thermal degradation of HP40Nb steel associated with microstructure evolution. Measurements of ultrasonic nonlinear signals were performed in the thermally degraded damaged HP40Nb specimens and the results showed a clear change of increase-plateau-decrease tendency of the normalized nonlinear parameter with respect to the different thermal loading time. Based on metallographic studies, the variation in the measured acoustic nonlinearity reveals that the normalized acoustic nonlinearity increases due to the increases of the second phase precipitates and dislocation density in the early stage and it decreases as a combined result of the reduction of dislocation density, coarsening of precipitates and initiation of micro-voids after long-term high temperature exposure. Moreover, an analytical model calculation of precipitate–dislocation interaction has been performed to interpret the correlation between microstructural evolutions and the measured ultrasonic nonlinearity. Consequently, ultrasonic nonlinearity is found to be strongly sensitive to the microstructure evolutions during thermal degradation of HP40Nb steels.     

Carburisation layer evolution of Fe–Cr–Ni alloy in furnace after long term service: experimental study and numerical prediction

Abstract

Owing to high temperature comprehensive properties, Fe–Cr–Ni alloys are designed to operate in corrosive gaseous environments of ethylene pyrolysis furnace. However, most premature failed tubes were caused by carburisation. In the present study, based on the Fick’s second law and equilibrium constant method, study on the carburised layer evolution of HP40Nb and KHR45A alloys by pack carburising experimental investigation and numerical simulation by MATLAB software were carried out. The results show that the experimental and simulated data agree with each other acceptably. The carburising layer rate of KHR45A alloy is much smaller than that of HP40Nb alloy due to higher contents of Cr and Ni element in the former. With increasing operating temperature ranging from 1000 to 1100°C, the maximum service lives of the two alloys sharply decrease.     

Precipitation and hot deformation behavior of austenitic heat-resistant steels: A review

The austenitic heat resistant-steels have been considered as important candidate materials for advanced supercritical boilers, nuclear reactors, super heaters and chemical reactors, due to their favorable combination of high strength, corrosion resistance, perfect mechanical properties, workability and low cost.Since the precipitation behavior of the steels during long-term service at elevated temperature would lead to the deterioration of mechanical properties, it is essential to clarify the evolution of secondary phases in the microstructure of the steels. Here, a summary of recent progress in the precipitation behavior and the coarsening mechanism of various precipitates during aging in austenitic steels is made. Various secondary phases are formed under service conditions, like MX carbonitrides, M_(23)C_6 carbides, Z phase, sigma phase and Laves phase. It is found that the coarsening rate of M_(23)C_6 carbides is much higher than that of MX carbonitrides. In order to understand the thermal deformation mechanism, a constitutive equation can be established, and thus obtained processing maps are beneficial to optimizing thermal processing parameters, leading to improved thermal processing properties of steels.     

Failure Analysis and Remaining Life Assessment of Methanol Reformer Tubes

High-quality failure analysis and good engineering judgment can turn plant shutdowns resulting from methanol reformer tube failures into an opportunity to improve the future performance of the reformer furnace. The plant down time can be used to evaluate remaining tube life and provide some insight into the effect of tube operating history, especially tube metal temperature on tube performance. The results can be used to minimize potential future failures and economic losses because of reformer shutdowns. In this article, the failure mechanism of a ruptured reformer tube is determined and an assessment of the remaining life of non-ruptured tubes in the reformer is discussed. Two assessment methods are reviewed (1) metallographic examination of ex-service material to characterize microstructure and creep damage and (2) modeling of creep damage accumulation using special-purpose finite-element software (WinTUBE^TM).     

The effect of thermo-mechanical loading on fracture-related parameters of austenitic steel

Failure analysis shows that 80–85% of emergency boiler shutdowns at power plants result from heating surface damage. When in service, the tubes are exposed to alternating thermo-mechanical loads, which trigger phase change affecting the individual service life. The service life sometimes differs significantly from its estimated value. This research is based on the hypothesis about the role and influence of internal structural stresses on the actual strength and long-term performance of tube products made of steels and alloys. The purpose of this work is to determine the limit state region of internal stresses of the first kind, in which microdamage will not lead to fracture. For that, we did a set of experiment studies to model the accelerated aging processes by thermal cycling and cold cyclic deformation. We chose a tube made of austenitic steel 10Cr13Mn12Si2Ni2Cu2Nb (Di59) as the object of research. The methods used were XRD analysis, microhardness testing, X-ray spectroscopy, and microstructure analysis. Due to redistributing properties, steel has no stable states: all its states are short-term and dependent on external disturbances. In accordance with the suggested hypothesis, the research findings make it possible to forecast the trends and direction of changes in the material properties. This allows us to evaluate the achievement of the limit state by the object and to use relaxations of internal stresses as a sign determining the conditions of crack initiation and propagation. The results are confirmed by the data obtained from the microstructure analysis of fractured tubes of a superheater from a functioning boiler.     

Effect of long time service exposure on microstructure and mechanical properties of gas turbine vanes made of IN939 alloy

In the present study, the effects of long-term service exposure have been investigated on microstructure and mechanical properties of gas turbine vanes made of IN939 superalloy. The major microstructural changes for the investigated service-exposed vanes include the formation of continuous grain boundary carbides and the transformation (degeneration) of MC carbides located at the grain boundaries. The brittle σ phase, which is predicted to be stable on the basis of thermodynamic calculations, is not observed in the microstructure of service-exposed vanes. The microstructural changes during service lead to a loss in room temperature ductility as well as in creep properties of the alloy.     

Influence of precipitated phases on properties during prolonged aging in TP347H steel at 700 °C

The aging of austenitic stainless steel TP347H (18% Cr-12% Ni-1% Nb) was performed at 700 °C for 500, 800, 1500, 2500 and 3650 h. Microstructure, precipitates and mechanical properties were examined on aged materials to analyze the impact of microstructure on mechanical properties. These tests showed that the main precipitate of the TP347 specimen was Nb(C,N) while M₂₃C₆ carbides precipitated at the aging time of 500 h, with the coarsening of M₂₃C₆ and MX phases during prolonged aging. The fine and dispersive Nb(C,N) particle precipitation up to 1500 h aging is a benefit for hardness and creep resistance. After aging for 3650 h, σ phase precipitated. Meanwhile, coarsening of Cr₂₃C₆ and Nb(C,N) led to creep cavity and brittle intergranular fracture. No clear change in tensile properties at room temperature during aging were observed. A distinct decline in creep properties was caused by an average diameter increase and precipitation of σ phase and bulky Cr₂₃C₆.     

HK-40钢制氢转化炉炉管失效分析

采用宏微观观察和X射线分析方法对服役近4年并已爆裂的HK-40钢制氢转化炉炉管进行了失效分析。结果表明，炉管的开裂是从内壁诱发，裂纹特征为多裂纹脆性断裂，高温蠕变是炉管失效的主要原因，提出了炉管在使用过程中应注意的问题。     

Microstructure and mechanical properties of the T23 steel after long-term ageing at elevated temperature

The paper presents the results of research on the microstructure and mechanical properties of T23 steel after long-term ageing up to 70 000 h at the temperature of 550 and 600 °C. It has been shown that the main mechanisms of degradation of the T23 steel microstructure were: recovery of the matrix, disintegration of the bainitic microstructure, growth of carbide diameter and precipitation of M6C carbides. These processes were more advanced in the steel aged at higher temperature. The changes in the microstructure resulted in a 10–15% decrease in mechanical properties (YS, TS) and hardness HV10, and a 30–40% decrease in the impact energy kV. A greater fall of impact strength occurred in the steel aged at the lower temperature. This was ascribed to the segregation of phosphorus to grain boundaries, which has a well-known adverse effect on ductility of low-alloy steels.     

Effect of heat treatment on microstructure and mechanical properties of Cr–Ni–Mo–Nb steel

Abstract

The microstructure and mechanical properties of a medium carbon Cr–Ni–Mo–Nb steel in quenched and tempered conditions were investigated using transmission electron microscopy (TEM), X-ray analysis, and tensile and impact tests. Results showed that increasing austenitisation temperature gave rise to an increase in the tensile strength due to more complete dissolution of primary carbides during austenitisation at high temperatures. The austenite grains were fine when the austenitisation temperature was <1373 K owing to the pinning effect of undissolved Nb(C,N) particles. A tensile strength of 1600 MPa was kept at tempering temperatures up to 848 K, while the peak impact toughness was attained at 913 K tempering, as a result of the replacement of coarse Fe rich M₃C carbides by fine Mo rich M₂C carbides. Austenitisation at 1323 K followed by 913 K tempering could result in a combination of high strength and good toughness for the Cr–Ni–Mo–Nb steel.