Load Bearing Capacity and Safety Analysis for Strain-hardening Austenitic Stainless Steel Pressure Vessels

By increasing the yield strengths of austenitic stainless steels for pressure vessels with strain hardening techniques,the elastic load bearing capacity of austenitic stainless steel pressure vessels can be significantly improved.Two kinds of strain hardening methods are often used for austenitic stainless steel pressure vessels：Avesta model for ambient temperature applications and Ardeform model for cryogenic temperature applications.Both methods are obtained from conventional design rules based on the linear elastic theory,and only consider the hardening effect from materials.Consequently this limits the applications of strain hardening techniques for austenitic stainless steel pressure vessels because of safety concerns.This paper investigates the effect of strain hardening on the load bearing capacity of austenitic stainless steel pressure vessels under large deformation,based on the elastic-plastic theory.Firstly,to understand the effect of strain hardening on material behavior,the plastic instability loads of a round tensile bar specimen are derived under two different loading paths and validated by experiments.Secondly,to investigate the effect of strain hardening on pressure vessels strength, the plastic instability pressure under strain hardening is derived and further validated by finite element simulations.Further,the safety margin of pressure vessels after strain hardening is analyzed by comparing the safety factor values calculated from bursting tests,finite element analyses,and standards.The researching results show that the load bearing capacity of pressure vessels at ambient temperature is independent of the loading history when the effects of both material strain hardening and structural deformation are considered.Finite element simulations and bursting tests results show that the minimum safety factor of austenitic stainless steel pressure vessels with 5% strain hardening is close to the recommended value for common pressure vessels specified in the European pressure vessel standard.The proposed study also shows that in the strain hardening design of austenitic stainless steel pressure vessels,the calculation for plastic instability pressure could use theoretical formula or finite element analyses based on geometrical dimensions and material property parameters before strain hardening,but a 5%strain should be employed as a design limit.The proposed research can be used for the strain hardening design of austenitic stainless steel pressure vessels safely.

Research on reasonable winding angle of ribbons of Flat Steel Ribbon Wound Pressure Vessel

Flat Steel Ribbon Wound Pressure Vessels (FSRWPVs) are used in many important industry areas. There is no such kind of pressure vessel exploding on operation for its reasonable structure design. Many explosion experiments on Flat Steel Ribbon Wound Pressure Vessel showed that their limited load pressure is related to the winding angle of the steel ribbons. FSRWPVs with reasonable winding angle have better security and lower cost. Reasonable angels given at the end of this paper facilitate engineering design.

Recent development in standards of steels used for pressure vessels in China

In this paper,five items of standards of steels used for pressure vessels which are issued recently or about to be issued and put into effect have been introduced.The differences of standards mentioned above and the former standards and the primary qualifications have been formulated.These qualifications indicate that,some qualifications of national standards of steels used for pressure vessels are on the international advanced level.

Microstructure and properties of S30403+Q345R roll-bonded clad steel plate for pressure vessels

To meet the demand of the domestic pressure vessel industry for roll-bonded clad steel plates, Baosteel has developed an S30403 ＋ Q345R roll-bonded clad steel plate. Comprehensive inspections of the composition, microstructure, and properties are made to systematically evaluate the steel plate in the normalized and normalized ＋ stress relieved states. The results show the cladding interface of the S30403 ＋ Q345R roll-bonded clad steel plate has high shear strength, the base metal has good properties, and the mechanical properties of the steel plate head and tail are uniform. The performance is fully consistent with the technical requirements of the roll-bonded clad steel plate for pressure vessels.

ESTIMATE OF BURSTING PRESSURE OF MILD STEEL PRESSURE VESSEL AND PRESENTATION OF BURSTING FORMULA

In order to get more precise bursting pressure formula of mild steel, hundreds of bursting experiments of mild steel pressure vessels such as Q235（Gr.D） and 20R（1020） are done. Based on statistical data of bursting pressure and modification of Faupel formula, a more precise modified formula is given out according to the experimental data. It is proved to be more accurate after examining other bursting pressure value presented in many references. This bursting formula is very accurate in these experiments using pressure vessels with different diameter and shell thickness. Obviously, this modified bursting formula can be used in mild steel pressure vessels with different diameter and thickness of shell.

Effect of Pre-Deformation Enhanced Thermal Aging on Precipitation and Microhardness of a Reactor Pressure Vessel Steel

Microstructure evolution in neutron irradiated Reactor Pressure Vessel （RPV） steels was experimentally simulated through an improved degradation procedure in this study. The degradation procedure includes austenitizing at 1 150℃ and water quench, deformation 10% and 30% respectively, and then thermal aging at 500℃ for different period of time. The microstructure of the specimens was analyzed in details using transmission electron microscopy （TEM）. The micro-hardness test results showed that all the hardness curves of undeformed, 10% pre-deformed and 30% pre-deformed specimens have two micro-hardness peaks with the first peak value corresponding to different thermal aging time of 1 hour, 5 hours and 10 hours, respectively. It was revealed that the hardness curves were influenced by the precipitation of Cu-rich precipitates （CRPs） and carbides, deposition of martensite and work hardening.

Development of S11306+SA516Gr70 roll-bonded clad steel plate for pressure vessels

To meet the demand of the pressure vessel industry, Baosteel has developed an S11306 ＋ SA516Gr70 roll-bonded clad steel plate. Comprehensive inspections of the composition, microstructure, and properties are performed to systematically evaluate the steel plate in normalized state and normalized ＋ stress relieved state. The results show the cladding interface of the Sl1306 ＋ SA516Gr70 roll-bonded clad steel plate has high shear strength, and the base metal has good mechanical properties. The performance is fully consistent with the technical requirements of the roll-bonded clad steel plate for pressure vessels.

INVESTIGATION OF INFLUENCE OF HYDROGEN ATTACK ONFATIGUE PROPERTIES OF PRESSURE VESSEL STEELS

Influence of hydrogen attack on mechanical and fatigue properties of 1020, 15CrMo and 2.25Cr-1Mo steels has been studied, which were exposed to gaseous hydrogen at pressure of 18MPa and 300-480℃ for up to 480h. The results of fatigue test showed that hydrogen attack (HA) resulted in an obvious change in fatigue properties of 1020 and 15CrMo steels, however, 2.25Cr-1Mo steel showed a lower susceptibility to hydrogen attack due to the addition of Cr and Mo. Fatigue crack behaviour for 1020 steel is attributed to hydrogen attack damage and the crack closure induced by the rough cracking surface, which lead to a valley on △Kth～HA% plot. The change in the fatigue behaviour of 15CrMo steel is possibly caused by severe surface decarburization.

Stress Corrosion Cracking of Pressure Vessel Steel and Countermeasures by Titanium-Based & Cerium-Based Inhibitors in Subcritical Aqueous Environment

An overview of a severe kind of environmentally-assisted cracking-stress corrosion cracking (SCC) of pressure vessel steel (PVS),such as stainless steel 304, alloy 600,690 and other nickel-based alloys in subcritical (～300 ℃) aqueous environment was given. The mechanisms of SCC of metals under this inclement surrounding were briefly generalized. Herein,some pragmatic solutions to mitigate the SCC susceptibility and retard its propagation were presented. The titanium and cerium-based inhibitors addition countermeasure was highlighted.

Development of a new irradiation-embrittlement prediction model for reactor pressure-vessel steels

Predicting the transition-temperature shift(TTS)induced by neutron irradiation in reactor pressure-vessel(RPV)steels is important for the evaluation and extension of nuclear power-plant lifetimes.Current prediction models may fail to properly describe the embrittlement trend curves of Chinese domestic RPV steels with relatively low Cu content.Based on the screened surveillance data of Chinese domestic and similar international RPV steels,we have developed a new fluencedependent model for predicting the irradiation-embrittlement trend.The fast neutron fluence(E>1 MeV)exhibited the highest correlation coefficient with the measured TTS data;thus,it is a crucial parameter in the prediction model.The chemical composition has little relevance to the TTS residual calculated by the fluence-dependent model.The results show that the newly developed model with a simple power-law functional form of the neutron fluence is suitable for predicting the irradiation-embrittlement trend of Chinese domestic RPVs,regardless of the effect of the chemical composition.

EMBRITTLEMENT OF A Cr-Mo LOW-ALLOY STEEL DUE TO LOW-TEMPERATURE NEUTRON IRRADIATION

Embrittlement of a Cr-Mo2.25CrlMo steel stemming from neutron irradiation at 270℃ is studied by virtue of small punch testing in conjunction with scanning electron microscopy. The ductile-brittle transition temperature determined by the small punch test is much lower than that determined by the standard Charpy test. There is some irradiation-induced embrittlement effect after the steel is irradiated for 46 days with a neutron dose rate of 1.05×10^18dpa/s （displacement per atom per second）.

Crystal Structure Evolution of the Cu-rich Nano Precipitates from bcc to 9R in Reactor Pressure Vessel Model Steel

The crystal structure evolution of the Cu-rich nano precipitates from bcc to 9R during thermal aging was studied in nuclear reactor pressure vessel （RPV） model steels. The specimens, contained higher copper and nickel contents than commercially available one, were heated at 890 ~C for 0.5 h and then water quenched followed by tempering at 0（50 ~C for I0 h and aging at 400 ~C for 1000 h. It was observed that bcc and 9R orthogonal structure, as well as 9R orthogonal and 9R monoclinic structure, coexist in a single Cu-rich nano precipitate. Further analyses pointed out that Cu-rich nano precipitates of bcc structure were not stable, it may preferentially transform to 9R orthogonal structure and then to 9R monoclinic structure. This results showed that the crystal structure evolution of the Cu-rich nano precipitates was complex.

Investigation of low-cycle fatigue behavior of austenitic stainless steel for cold-stretched pressure vessels

Cold-stretched pressure vessels from austenitic stainless steels （ASS） are widely used for storage and transportation of liquefied gases, and have such advantages as thin wall and light weight. Fatigue is an important concern in these pressure vessels, which are subjected to alternative loads. Even though several codes and standards have guidelines on these pressure vessels, there are no relevant design methods on fatigue failure. To understand the fatigue properties of ASS 1.4301 （equivalents include UNS $30400 and AISI 304） in solution-annealed （SA） and cold-stretched conditions （9% strain level） and the response of fatigue properties to cold stretching （CS）, low-cycle fatigue （LCF） tests were performed at room temperature, with total strain amplitudes ranging from ：~0.4% to ＂0.8%. Martensite transformations were measured during the tests. Comparisons on cyclic stress response, cyclic stress-strain behavior, and fatigue life were carried out between SA and CS materials. Results show that CS reduces the initial hardening stage, but prolongs the softening period in the cyclic stress response. Martensite transformation helps form a stable regime and subsequent secondary hardening. The stresses of monotonic and cyclic stress-strain curves are improved by CS, which leads to a lower plastic strain and a much higher elastic strain. The fatigue resistance of the CS material is better than that of the SA material, which is approximately 1 - 103 to 2 - 104 cycles. The S-N curve of the ASME standard for ASS is compared with the fatigue data and is justified to be suitable for the fatigue design of cold-stretched pressure vessels. However, considering the CS material has a better fatigue resistance, the S-N curve will be more conservative. The present study would be helpful in making full use of the advantages of CS to develop a new S-N curve for fatigue design of cold-stretched pressure vessels.

Effect of rare earth cerium on brittleness of simulated welding heat-affected zones in a reactor pressure vessel steel

The welding coarse-grained heat-affected zones（CGHAZs） in the undoped and Ce-doped samples of SA508CL-3 reactor pressure vessel steel were simulated using a Gleeble 1500 D thermomechanical simulator with a peak temperature of 1320 oC at the heat inputs of 30, 50 and 100 kJ /cm, respectively. The ductile-to-brittle transition temperature（DBTT） of the simulated CGHAZs was evaluated along with microstructural and microchemical characterizations. The results indicated that Ce could substantially lower the DBTT of the CGHAZs by its microstructural and microchemical effects. After the thermal cycling of welding, the microstructure in the Ce-doped samples was apparently finer than that in the undoped samples, regardless of the lath bainite obtained at the heat inputs of 30 and 50 kJ /cm or the granular bainite acquired at the heat input of 100 kJ /cm, leading to lower DBTTs for the Ce-doped samples. Moreover, grain boundary segregation of Ce occurred apparently in the Ce-doped samples and exhibited a non-equilibrium characteristic. The segregation of Ce could play an important role in lowering the DBTT of CGHAZs or toughening the CGHAZs.

Research into tritium and helium-3 contents distributions in steel wall of spherical pressure vessel

The tritium quantity stored in a steel pressure vessel decreases with time because of helium-3 decay and permeation of tritium into the steel wall of the pressure vessel.Meanwhile,the tritium quantity permeating into the steel wall also decreases with time due to helium-3 decay and diffusion in and out of the wall of tritium.Tritium and helium-3 in the steel wall will cause hydrogen and helium embrittlement of the wall material,respectively,and thereby change the carrying capacity of the vessel.Taking contemporarily both decay and permeation of tritium within the vessel and decay and diffusion of tritium having permeated into the wall into consideration,the governing equations of tritium and helium-3 contents in the wall were established and solved,and relevant formulas were deduced.Through analytical calculations,curves of tritium and helium-3 contents versus radius and time were theoretically plotted,the contents spatio-temporal distributions laws were obtained,and a law about helium-3 contents distribution in steel wall of a spherical pressure vessel was discovered which was called the law of double helium-3 content.

Mechanical and fatigue properties of SA508-Ⅳ steel used for nuclear reactor pressure vessels

The mechanical and fatigue properties of SA508-Ⅳ steel with martensite and granular bainite, respectively, were studied. The mechanical tests results showed that the ultimate tensile strength and impact toughness of the specimen with martensite were 830 MPa and 158 J, respectively, and those of the specimen with granular bainite were 811 MPa and 115 J, respectively. The former had higher tensile strength and impact toughness than the latter. The impact tests results showed that the former belonged to typical dimple fracture, while the latter belonged to brittle fracture. The fatigue tests results showed that the fatigue life of the specimen with martensite was 2717 cycles, and that of the specimen with granular bainite was 1545 cycles under the strain amplitude of ± 0.45%. The specimen with martensite had fewer crack initiation points, narrower fatigue striations separation, and larger volume fraction of high-angle grain boundaries than the latter. The fewer crack initiation points meant fewer fatigue cracks, the narrower fatigue striations separation meant slower crack propagation rate, and the larger volume fraction of high-angle grain boundaries could more effectively hinder fatigue crack propagation. Based on these facts, the fatigue life of the specimen with martensite was higher than that of the specimen with granular bainite.

On the heterogeneous microstructure development in the welded joint of 12MnNiVR pressure vessel steel subjected to high heat input electrogas welding

Microstructure features of 12 MnNiVR pressure vessel steel welded joint deposited by the high heat input electrogas welding have been systematically investigated. It is revealed that the welded joint is featured by a heterogeneous juxtaposition. The coarse grained heat-affected zone(CGHAZ) primarily consists of lath bainites and minor granular bainites. The fine grained heat-affected zone(FGHAZ) is dominated by polygonal ferrites, pearlites, and fine cementite particles. Moreover, electron backscatter diffraction results further demonstrate that the CGHAZ is populated by coarse prior austenite grains(PAGs) with high frequency(61.3%) of low angle grain boundaries(LAGBs). On the other hand, the FGHAZ is filled with fine PAGs with a lower frequency(19.6%) of LAGBs. Such microstructural differences may likely contribute to differed mechanical properties for samples tested at designed positions.

三代核电反应堆压力容器低合金钢焊缝性能对比分析

通过模拟国内三代核电压水堆反应堆压力容器环焊缝,分别采用国产焊接材料和进口焊接材料,在焊接工艺性能、焊缝金属化学成分、拉伸、冲击韧性、落锤和低周疲劳试验等方面进行了对比分析。结果表明,国产和进口焊接材料的焊接工艺性能和焊缝金属力学性能均相当;焊接试件不同厚度取样位置的焊缝金属-28.3℃冲击吸收能量平均值都大于90 J,有较大的设计裕量。该国产焊接材料已应用于国内某三代核电机组反应堆压力容器,为进一步推广应用提供技术支撑。

大壁厚不锈钢复合板压力容器建造关键技术及应用

在大壁厚不锈钢复合板压力容器的建造过程中,通过深入研究其技术要点和难点,优化焊接接头结构设计,成功形成了一套完整的新型建造关键技术。这套技术不仅优化了焊接接头结构,还创新了一系列施工方法,包括筒体加热卷制、接头坡口机加工、接管内壁耐蚀层TIG/FCAW自动堆焊、不锈钢衬管液氮冷套、球形封头内壁耐蚀层带极埋弧堆焊、纵环缝窄间隙埋弧自动焊以及管口接头马鞍型埋弧焊等。这些新型建造关键技术的应用,在提高压力容器质量效果的同时,也带来了显著的经济效益,为行业同类型设备的建造提供借鉴。在未来的研究中,可以进一步探索这些技术在其他领域的应用,以实现更广泛的技术交流和合作。同时,对于大壁厚不锈钢复合板压力容器的建造,仍需不断深入研究其材料特性、工艺参数和施工方法,以不断提高设备的性能和安全性。

抗压容器钢在模拟工业码头大气环境下的腐蚀行为研究

为探究抗压容器钢在模拟工业码头大气环境下的腐蚀行为,利用失重分析、EDS、XRD和电化学方法研究了不同模拟加速试验周期下抗压容器钢Q235和16MnNiVR钢的腐蚀行为。结果表明:抗压容器钢Q235和16MnNiVR钢在模拟工业海洋大气环境试验中,其腐蚀速率先呈现上升趋势,随着锈层由疏松逐渐变致密,腐蚀速率会发生不同程度地下降。在此加速试验下2种钢的腐蚀产物化学性质一致,所发生的腐蚀行为差异是由金属基材表面腐蚀产物致密性所致,在东南沿海工业海洋大气环境腐蚀因子作用下,16MnNiVR钢的耐蚀性可达到Q235钢的1.6倍。