Luyang Geng

Damage analysis and life prediction of a main steam pipeline at elevated temperature based on creep damage mechanics

Main steam pipelines are important components of power plants and chemical plants, which operated at elevated temperature and high pressure for the long term. Creep is a potential mechanism of failure of these pipelines. In this paper, the modified Karchanov-Rabotnov creep damage constitutive equation has been incorporated into finite element program ABAQUS through its user subroutine to predict the creep damage and service life of a serviced steam pipeline made of 10CrMo910 heat resistance steel. Creep tests and short-term tensile tests at serviced temperature of 540°C were carried out to get the material constants in creep damage constitutive equation, which are necessary in the mentioned creep damage and life prediction. Based on these, the analysis on stress and damage of a main steam pipeline serviced for 150,000 hrs was carried out. Stress and damage distribution and maximum damage location of the pipeline were obtained. The damage distribution and maximum damage location of the pipelines were obtained. Furthermore, the local creep damage analysis of a T-type joint serviced for 150,000 hrs was also carried out because T-type joint used in the main steam pipeline is one of weakness in the piping system. According to the results obtained above, the life evaluation of the pipeline and the T-type joint were performed.

Experimental Investigation of Hydrogen Effect on Mechanical Properties of 304L Stainless Steel

Hydrogenation reactor is one of the key equipments in the production process of purified terephthalic acid (PTA). The safe and reliable operation of hydrogenation reactor decides the safety of PTA installation. Service experience has shown that some longitudinal cracks occurred on the internal wall of the 304L stainless steel (SS) hydrogen nozzle. In this paper, a series of electrochemical hydrogen charging tests were carried out for 304L stainless steel at the ambient temperature. The comparison on mechanical properties was investigated before and after hydrogen charging in order to investigate the effect of mechanical properties of 304L SS. Fractography was analyzed by using scanning electron microscope (SEM). The results showed that the mechanical strength of 304L SS was not basically lost after hydrogen charging, but the ductility reduced in a certain extent. The fractography also showed the hydrogen embrittlement effect on the fracture.Copyright

Formation of G-phase in 20Cr32Ni1Nb Stainless Steel and its Effect on Mechanical Properties

A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carried out to investigate the mechanical properties and microstructural evolution of 20Cr32Ni1Nb steel. Experimental results indicate that the as-cast microstructure of the steel typically consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides (NbC and M23C6). In the ex-service samples, large amounts of secondary carbides precipitate within austenite matrix. Besides the growth and coarsening of NbC and M23C6 carbides during service condition, the Ni-Nb silicides known as G-phase (Ni16Nb6Si7) are formed at the interdendritic boundaries. The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitate rate of G-phase, depending in part on Si content, varies greatly for the 20Cr32Ni1Nb steel, which plays a key role in the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time–temperature–transformation curve for the steel is obtained from the aged specimens.

Study on Gradual Change of Corrosion Behaviors in 13MnNiMoR Steel Weld Joint Serviced in EO Reactor

This paper will focus on the gradual change of corrosion behaviors among different zone metal of 13MnNiMoR steel weld joint in the EO reactor service environment. Metallographic analysis and electrochemical corrosion tests in boiler water have been carried out on cylindrical specimens, taken from the 13MnNiMoR steel welded joints at the direction across the weld. Metallographic structures, values of linear polarization resistance (Rp) and potentiodynamic polarization curves on sections normal to the axis of the specimens have been obtained. The results showed that the Rp values and potentiodynamic polarization curves of the weld joint were significantly varied with the microstructure among different zones. Results indicated that the overheating zone in HAZ shows the minimum value of Rp and the maximum value of corrosion current density (icorr), which make it the weakest part of the joint from a corrosion stand point.Copyright

Prediction on Initiation of Hydrogen-Induced Delayed Cracking in High-Strength Steel Based on Cohesive Zone Modeling

This study presents prediction on initiation of hydrogen-induced delayed cracking (HIDC) in hydrogen pre-charged high-strength steel notched bars under a constant load based on hydrogen influenced cohesive zone modeling (CZM). The prediction is implemented by using a three-step sequential coupling finite element procedure including elastic-plastic stress analysis, stress-assisted hydrogen diffusion analysis and cohesive stress analysis with cohesive elements embedded along the potential crack path. Hydrogen influenced linear traction separation law is applied to the cohesive elements. The predicted initiation time of HIDC gives a good agreement with the experimental fracture time reported in a literature. The prediction reproduces the experimental trend that the critical hydrogen concentration for crack initiation is independent of the initial hydrogen concentration, while decreases with increasing load or stress concentration factor of the notch. CZM has a potential to predict HIDC of high-strength steel.© 2014 ASME

Effect of Heat Treatment on Corrosion and Fracture Behavior of SPV50Q High Strength Steel Weldment in H2S-Containing Environment

SPV50Q high strength steel is often used to fabricate liquefied petroleum gas (LPG) spherical tanks with larger capacity, and tanks are expected to free post weld heat treatment (PWHT) for avoiding the possible reduction in strength. Sulfide stress corrosion cracking (SSCC), however, has been found in weldment, especially the heat-affected zone (HAZ), in LPG environment contaminated by wet H2 S. The failure analysis showed that the existence of welding residual stress in weldment is one of the major factors in the occurrence of cracking. Post welding heat treatment (PWHT) is a feasible method of reducing welding residual stress. Therefore, in order to investigate the effect of heat treatment on mechanical properties, corrosion and fracture behavior of SPV50Q steel weldment, the difference in mechanical properties of the weldment with and without heat treatment at 590°C for 160 min after welding was measured using tensile test and impact test. The corrosion behaviors of base metal (BM), weld metal (WM) and HAZ metal in the weldment were investigated by potentiodynamic polarization in H2 S-containing solution. In the same solution, the susceptibility to environmental cracking was evaluated by slow strain rate testing (SSRT). The feature of fracture and the morphologies of cracks were observed by scanning electrode microscope (SEM) and optical microscope (OP). The results indicate that the execution of heat treatment does not greatly change the properties of SPV50Q steel weldment, which can provide technology support for the remanufacturing of the LPG tanks having suffered from SSCC by repair welding following local or integral heat treatment.Copyright