Study On The Failure Analysis And Risk Assessment Of Carbon Steel High-Pressure Pipe Under Hydrogen Environment

Generally, brittle failure impossibly happen of the pipe under normal operating conditions in the design life, for quality carbon structural steel (such as 10,20,20G, etc.) have excellent toughness, well plastic deformation of cold-work and high welding performance. So, many pipes are used in the high-pressure hydrogen (H2) environment of ammonia synthesis, petroleum cracking and other units. Reference to Nelson curve and taken into account adequate safety factor, the actual temperature of carbon steel pressure pipe is generally not higher than 200℃, in most cases used at room or low temperature. However, the brittle fracture occurs unexpectedly of the carbon steel high-pressure pipe under hydrogen environment, the severity of its consequences often beyond the acceptable limits of society, enterprises and individuals. In order to achieve the target of long-period low risk running of high-pressure carbon steel pipe under hydrogen environment both in-service now and be fabricated in future, research and conclusions are as follows:(1) Research was finished on the failure cases happened in recent years by generalization analysis method. Strain aging embrittlement (SAE), is the root sourse of the failue for carbon steel high-pressure pipe under hydrogen environment. Based on the experimental results, the chemical compositions (especially the element of N, H), mechanical properties, microstructure, macro-micro defects etc., of a type failure case, and combination with the experimental results of other cases, analysis was done for the causes and mechanisms of material degradation and crack. The further research make it clear that the decrease of material toughness due to SAE, environment hydrogen will further aggravate the degradation of material; phenomenonally crack initiation and propagation is caused by HIC (hydrogen induced cracking), however, the root cause is SAE that lowering the threshold value of HIC.(2) According to the C, N segregation microstructure using TEM (Transmission Electron Microscope), it is clearly that the diffusion of interstitial N is of a competitive advantage during the SAE of carbon steel. And, TEM observation was carried out for the microstructure of the interaction between hydrogen embrittlement damage and SAE of the pipe after a long term service. Results show that the main location of the hydrogen damage occurs is the pearlite and the form of damage is micro-voids, micro cracks generate along the boundary of pearlite and ferrite. Considering the case of SAE, improvement is proposed for the existing method of estimating KIC by KV2 (Charpy-V north absorded energy).KV2-based assessment method was established of SAE, its sensitivity analysis show a high credibility of KV2 method. The study was completed about the effect SAE on the failure assessment diagram approach, the results show that evaluation point will shift to the upper left cause of SAE.(3) For the engineering methods of lowering the possibility of SAE and other failure modes due to SAE, the mechanical property evaluated by KV2 research was finished of the method and mechanism of avoiding failure caused by SAE during high-pressure pipe fabrication, of properties recovery of the deteriorated pipe after a long term service. The results show that:the SAE come into being in rolling process and is enhanced during the bend-forming process of the pipes. Different heat treatments have different effects to eliminate or prevent the SAE, among which normalizing plus high temperature tempering is most effective. The deformation degree has a significant effect on recovering the impact properties of materials and whether aging or not after pre-deformation does not have significant effect on the heat treatment efficiency; it is of extremely sensitivity to the original structure that the heat treatment method for eliminating the SAE of the pipe, and, its effect judgment could be make well if the service condition (especially service temperature) is considered, the SAE as a result of cold bending for normalized carbon steel can be eliminated effectively by stress relief annealing or recrystallization annealing; the SAE, not only as a result of cold-worked bending but also combined with hydrogen embrittlement during in service, can be eliminated using normalizing. The healing of microvoid and microcrack play a key role for the mechanism of the eliminating, so is it the crystal grain fining.(4) Focused on the mechanical properties evaluation, based on the results of previous research on failure analysis, processing prevention, performance recovery, etc., the risk assessment technical approach based on the failure mechanism is proposed and established, especially for SAE. The assessment of risk criteria are established considering two cases of short-micro cracks and macro cracks. In other words, the risk assessment method is composed by a set of sub-methods of the failure possibility and risk analysis for all cases of the pipe. Isoline risk FAD is established for the carbon steel high-pressure pipe under hydrogen environment. According to the characteristics of the data of carbon steel high-pressure pipe under hydrogen environment risk assessment, based on the concept of level and classification risk assessment, an interval values information system was proposed for the risk level assessment of high-pressure pipe. Based on Pawlak's rough set theory study was finished on the expression of knowledge of risk assessment, and using of partial order to organize the equivalent classes in a hierarchy way. Study was done on the algorithms of interval-valued, both uniform and probability distribution, for risk level assessmen based on iso-line rsik FAD.In summary, the results show that it is clear the root cause of the brittle fracture accident is SAE; SAE, the long-neglected failure mode, and its interaction with hydrogen damage should be handled as a new typical failure mode in risk assessment nowadays; failure mechanism and evaluation methods provide a theoretical foundation and data source for the material choice, regular inspections and fitness for service assessment of the pipe in case of cold worked; the specific measures and proposed mechanism can be directly used to reduce the failure possibility in engineering practice; the study on engineering approach to risk assessment and risk level evaluation will help accelerate the standardization of practical engineering risk assessment methods and laid a foundation for other process units. For all, the contents presented in the paper make some contribution to establish and improve the long cycle safe operation technical system of a large number of the pipe, however, it need further work, especially about the Non-destructive inspection on dispersion micro-cracks and SAE, for the test analysis and collection data of actual service conditions to meet the needs of a more comprehensive risk assessment, because the problem is a multi-dimensional, multi-scale and non-equilibrium structural strength issue on material degradation and then failure during the manufacturing process and in-service of the pipe.