【作者】 崔好选；

【导师】 王威强； 范志超；

【作者基本信息】 山东大学 ， 过程装备工程， 2011， 博士

【摘要】 优质碳素结构钢(如：10、20、20G等)韧性优良、冷变形塑性好,在正常工作条件下不会发生脆性破坏,因此,碳钢高压管广泛使用于合成氨和石油裂解等装置中。临氢(H2)环境碳钢高压管设计时,通常参考Nelson曲线选材,并考虑一定的安全系数,高压管实际工作温度一般不会高于200℃,多数情况下在常温和低温运行。通常认为,上述工况环境下,碳钢高压管可以安全运行,不会造成损失恶劣的安全事故,特别是脆性断裂事故。但是,该类高压管发生脆性爆炸的脆性断裂事故竟时有发生,造成巨大的经济损失和人员伤亡。脆性断裂失效事故虽然是小概率事件,但其后果严重程度往往超出了社会、企业和个人的承受限度。为了实现在用和再建的临氢碳钢管的长周期低风险运行,作者做了如下研究：(1)采用泛化分析方法研究了近年来的失效案例,指出和论证了临氢碳钢高压管中失效的关键性(或普遍性)问题—“应变时效脆化”。以其中一起典型的临氢碳钢高压管失效案例的化学成分(特别是N、H含量)、力学性能、微观组织及宏微观缺陷等试验结果为基础,并结合其它失效案例的试验结果,分析了材料劣化和裂纹萌生及扩展的原因和机理,研究发现材料韧性下降是应变时效脆化所致,环境中的氢(H)会进一步加剧材料劣化；裂纹萌生表面上看是氢致开裂,根本原因是应变时效脆化导致了氢脆的门槛值降低。(2)采用透射电镜(TEM)观测碳化物和氮偏析物的微观组织结构,通过分析从微观机理上证明了间隙原子氮(N)是主导碳钢应变时效脆化的化学成分。详细分析观察了应变时效脆化和氢脆协同作用后的微观结构,研究表明应变时效脆化后氢脆的机理是在珠光体和铁素体的交界处大量产生微孔洞、微裂纹。分析了应变时效脆化下通过夏比冲击吸收能(KV2)估算断裂韧度(KIC或JIC)下限的方法,对我国现行标准方法提出了改进建议。对检测碳钢应变时效脆化敏感性的分析表明KV2方法具有很高的可靠性。研究了应变时效脆化对失效评定图(FAD)方法的影响,结果表明应变时效脆化会使评估点向左上方偏移。(3)为了降低应变时效脆化对临氢碳钢高压管安全的影响,以KV2做为材料力学性能评价指标,分析了预制中避免应变时效脆化导致高压管失效的方法及其作用机理,以及已发生劣化的临氢碳钢高压管性能恢复的方法和作用机理。研究表明：轧制过程中已发生应变时效脆化,后续的预制会加重管子的应变时效脆化程度；明确了冷变形中正火处理和去应力退火处理对于长周期避免脆化倾向有极高的可靠性；应变时效脆化消除的热处理方法对初始组织结构极为敏感,性能恢复的评价须结合使用环境(特别是使用温度)；正火态高压管在冷弯成型中产生的应变时效脆化,通过再结晶退火或去应力退火即可有效消除；在制造中已发生应变时效脆化,并在服役过程中发生了氢脆的碳钢厚壁管,采用正火处理恢复其力学性能比较可靠,微孔洞和微裂纹愈合以及晶粒细化是该材料脆化消除及性能恢复的关键作用机理。(4)以前期失效分析、加工预防、性能恢复等研究为基础,根据失效可能性风险划分准则,构架了基于应变时效脆化失效机制的风险评估方法。根据各种可能的失效和后果提出了相应的失效可能性计算方法,即：基于失效机制的风险评估方法由一套不同情况下的失效可能性分析及风险判定准则的子方法构成。在失效可能性分析研究的基础上,分微小裂纹和宏观裂纹两种情况确立了风险划分准则,提出了等值线风险FAD方法。基于分级和分类的风险评估理念,根据临氢碳钢高压管风险评估中数据的特点,采用区间值描述基本数据单元和偏序关系将基本信息单元有层次的组织起来。采用Pawlak粗糙集理论,分别按均匀分布区间值和概率分布区间值建立了失效模式识别的算法,包括基于等值线风险FAD各种情况下的算法。总之,通过作者的工作,明确了应变时效脆化是临氢碳钢高压管的普遍失效模式,为临氢碳钢高压管存在冷加工情况的失效机理、评估方法的研究和为材料选择、定期检验和结构完整性评估提供了理论依据和数据支持;通过避免和消除应变时效脆化倾向研究提出的具体措施和作用机理可以直接用于降低失效可能性的工程实际：基于失效机制的风险评估方法和风险等级评定程序化的研究有助于加快规范化、实用化进程。由于临氢碳钢高压管在制造和服役过程中导致材料劣化进而失效的问题是一个多维度、多尺度、非平衡的结构强度问题,为了更好适应各种碳钢高压管的制造工艺和服役环境,特别是对弥散微裂纹和应变时效脆化的无损检验,还需要进一步在试验分析中开展工作,采集适合我国实际服役条件的比较全面数据满足风险评估的需要。更多还原

【Abstract】 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.更多还原