Study on hydrogen embrittlement susceptibility of pipeline steel under stress
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摘要:目的 氢脆现象是限制输氢管道安全运行的一个重大安全问题。然而，氢气工况下氢是否会进入管线钢并引发氢脆仍然存在争议。一方面，众多研究结果表明临氢环境下钢材的塑性、韧性将有显著损失。另一方面，众多输氢管道工程在长时间运行后未发现明显的氢脆危险。氢脆实验过程中试样的极端服役状态可能是导致实验室研究结论与实际工程经验差异的关键，因此有必要研究氢脆实验过程对材料氢脆敏感性的影响。方法 通过应力下高压气相原位拉伸实验，以慢应变速率拉伸实验为例，研究了不同充氢方式下X52钢试样的力学性能及氢脆程度，探究了试样内部氢的来源，梳理了不同弹性应力下实验过程对试样氢脆敏感性的影响。结果 实验结果显示，应力是影响试样内部氢含量、导致氢脆敏感性差异的关键因素。通过应力充氢实验确定了导致X52钢产生氢脆的临界氢浓度，发现产生的氢效应大都来自拉伸环境中进入试样的氢，预充氢阶段进入试样内部的氢含量十分有限，并且氢在弹性阶段就已经充分进入试样。结论 无预充氢、直接在氢环境拉伸的试样出现了屈服阶段的氢致硬化效应，说明氢在时间极短的弹性阶段内已经进入钢材内部；应力充氢状态下，试样内部氢含量明显上升；应力促进气相氢环境下的氢表面吸附及内部氢溶解度是导致应力促进氢效应的机制。在今后的研究中，应考虑慢应变速率拉伸实验对材料氢效应的影响。Abstract:Objective Hydrogen embrittlement poses a significant safety risk that restricts the safe operation of hydrogen pipelines. However, there is ongoing debate regarding the extent to which hydrogen penetrates pipeline steel and induces hydrogen embrittlement under service conditions. While numerous studies indicate a substantial loss of plasticity and toughness in steel within a hydrogen environment, many hydrogen pipelines show no evident hydrogen embrittlement hazards after long-term operation. The extreme service conditions experienced by specimens during hydrogen embrittlement testing may explain the discrepancies between laboratory findings and practical engineering experiences. Therefore, it is essential to investigate how the hydrogen embrittlement testing process affects the susceptibility of materials to hydrogen embrittlement.Methods The mechanical properties and hydrogen embrittlement of X52 steel specimens under different hydrogen charging methods were studied by high-pressure gas-phase in-situ tensile testing under stress, with a focus on the slow strain rate tensile test. The source of hydrogen inside the specimens was explored, and then the influence of the testing process under different elastic stresses on the susceptibility of the specimens to hydrogen embrittlement was analyzed.Results The experimental results indicated that stress is a critical factor influencing hydrogen content in the specimen and its susceptibility to hydrogen embrittlement. Consequently, the critical hydrogen concentration that leads to hydrogen embrittlement in X52 steel was determined through stressed hydrogen charging tests. The results indicated that most of the hydrogen effect originated from hydrogen penetration into the specimens under tensile conditions, with minimal penetration during the hydrogen pre-charging stage and sufficient penetration occurring during the elastic stage.Conclusion The hydrogen-induced hardening effect occurred during the yield stage of specimens subjected to direct tensile testing in a hydrogen environment without hydrogen pre-charging, indicating that hydrogen penetrated the steel during the brief elastic stage. With stressed hydrogen charging, the hydrogen content in the specimens significantly increased. The mechanisms leading to the stress-promoted hydrogen effect include stress-enhanced hydrogen surface adsorption and internal hydrogen solubility in the gas-phase hydrogen environment. Future studies should consider the impact of slow strain rate tensile testing on hydrogen effects in materials.
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我国黄土地区分布广泛，主要集中在陕甘宁、青海、内蒙古等地区，兰郑长成品油管道、西气东输管道、陕京天然气管道、涩宁兰输气管道、马惠宁输油管道、石兰输油管道等途经该地区。管道施工在不同程度上改变了原有的环境状况，同时运营中的管道也普遍受到黄土病害的困扰，尤其是黄土高陡边坡，给管道安全运行造成了较大威胁[1]。
油气管道途经高陡边坡时通常采取大开挖直埋铺设，作业带宽度约5~20 m，边坡高度一般8~30 m，最高甚至可达百米。大开挖施工方法对坡面环境和植被破坏严重，黄土(尤其是湿陷性黄土)具有大孔隙、散体状及遇水湿陷特性，强降雨容易导致管沟水土流失甚至坡面失稳[2]。兰郑长成品油管道、涩宁兰输气管道及石兰输油管道的专项地质灾害调查结果表明，黄土区油气管道沿线水毁灾害和黄土湿陷灾害分布密度约为每千米0.51处，灾害数量比例超过40%。在管道建设期，一般利用草袋进行边坡防护，但受草袋自身结构特点和施工质量的限制，管道运行不久后草袋往往损坏严重并失去护坡作用，导致管沟水土流失、露管甚至斜坡失稳；在管道运营期，则普遍采用砌体或灰土、水泥土、混凝土结构的防护形式，但仍存在易损和成本过高的弊端[3]。为此，以兰郑长成品油管道陕西段干线、庆阳支线为例，通过对其途经地区的黄土斜坡变形破坏特征进行分析，提出了加筋干打垒护坡方法。
1. 病害特征
管道大开挖处黄土斜坡水毁病害特征主要体现在：①斜坡高度大、坡度陡，对于斜长超过100 m的冲沟斜坡，坡面往往上缓下陡，下部坡度超过60°；②管道埋深一般小于2 m，部分管沟夯填欠密，易塌陷；③建设期修建的草袋损坏殆尽，坡面水土流失严重；④坡面降雨汇流通道一旦形成，发展迅速，尤其对于植被覆盖稀少的管沟部位，易成为坡面汇水冲刷的主路径。
产生上述病害的原因主要包括两个方面：①黄土斜坡自身特性。黄土本身孔隙比大、天然含水量低、可塑性强、具有较高的黏聚强度，常呈硬可塑、坚硬状态，力学强度较高。但黄土被水浸泡后，土粒间的胶结作用立即遭到破坏，黏聚强度急剧降低，力学强度也随之大幅度下降[4]。天然的黄土斜坡以及斜坡上经原土回填夯实的黄土管沟一般处于稳定状态，一旦浸水，其土体强度将迅速降低。此外，对于高陡斜坡而言，坡脚土体的稳定性也至关重要，稳定的坡脚土体能够在一定程度上减缓坡面水土的流失和管沟冲刷作用。②外部因素影响。坡面管沟的开挖在一定程度上造成了原坡面的扰动，如土体的松动、坡表植被的破坏；同时有可能改变坡面原有排水路径，夯填不密实的管沟极易发展成为新的坡面汇流通道。黄土(尤其是湿陷性黄土)一旦被大量浸水或冲刷而得不到有效遏制，那么坡面水土流失、管沟塌陷、土坎垮塌甚至斜坡失稳则极易发生[5]。
2. 治理原则
油气管道开挖范围内高陡黄土斜坡病害治理的关键原则是阻渗水和稳坡脚。阻渗水要求护坡所用材料必须结构密实、水稳性好，具备良好的抗冲刷和抗渗透能力。稳坡脚则要求坡脚构筑物能够保证护坡材料稳定地附着在坡体上，不因失稳而开裂或滑塌。同时，治理措施还需满足以下3个要求：①应用的广泛性，即要求治理措施能够在黄土地区普遍适用且效果良好；②施工的便捷性，即要求治理措施取材方便、施工简单；③低成本特性，即要求治理措施成本低廉。
3. 防护方法
基于上述治理原则及要求，结合多年的坡面防护经验，提出了油气管道途经地区的黄土斜坡坡面加筋干打垒护坡防护方法(图 1，其中H为拟处理边坡的高度；α为边坡坡度)：在坡脚设置护脚矮墙，坡面采用三七灰土添加草筋进行夯实，坡顶布设截排水沟。
(1) 灰土拌置。在夯填土和石灰拌置前，需严格按照以下要求做好准备工作：①夯填土首选砂质黏土，可以采用新鲜黄土或黏土，不得采用含淤泥、腐植土、冻土、膨胀土及有机物质土作为填土材料。②夯填土含水率以15~20%为宜。③石灰需经过充分“消化”。④采用人工或机械拌置方式，将夯填土和石灰以3:7的体积比充分拌合，拌置后的灰土静置时间不得超过6 h。将拌置好的三七灰土作为干打垒的夯填料可以有效改善黄土的大孔隙特性，并阻断坡体渗水；同时，黄土中含有易溶性碳酸盐，石灰中Ca(OH)2缓慢与黄土中的CO2作用后合成CaCO3，形成坚硬的土体，可以提高土的力学性质，达到加固土层的作用[6-8]。
(2) 护脚矮墙施工。为稳定干打垒夯实体，在坡脚设置护脚矮墙。矮墙基础埋深不小于0.5 m，基底挖除0.5 m原土并用三七灰土夯实换填，矮墙地面高度以0.5 m为宜。墙身采用素混凝土、毛石混凝土或浆砌石砌筑，当墙身强度达到设计强度的70%后，方可进行干打垒夯筑。
(3) 干打垒夯筑。操作要点：①夯筑前需清除坡面上的植被，并根据拟处理边坡的高度H确定夯体级数、单级夯体高度以及夯体外侧坡度(表 1)。②根据夯体级数、夯体厚度及夯体外侧坡度的情况来预留施工马道宽度，通常施工马道宽度不小于1.2 m。③为了增加夯体抗拉强度，夯筑过程中应加筋。筋体可以采用稻草编织的草绳，草绳具有一定的耐腐蚀性，其纤维组织能承受较大的拉力，在受拉过程中状态稳定，与素土夯体相比，其抗拉强度可提高30%。因此，草绳可以作为夯体的主要加筋材料，从而延长夯体使用寿命。草绳应该编织紧密，直径不小于3 cm。筋体应该沿干打垒斜坡顺向和纵向均匀布置，间距为50 cm，每夯筑20~25 cm布置一层。④夯体厚度一般为1.5~2 m，采用机械或人工分层夯实，分层厚度宜为20 cm，夯实后的夯体压实系数不小于0.93。
表 1 夯体级数、单级夯体高度以及夯体外侧坡度取值情况
(4) 截排水施工。在I、II级湿陷性黄土地区，若存在冲刷下切的软土地区或边坡坡比大于1的情况下，应在边坡坡顶布设截排水沟。为了保证加筋干打垒护坡效果，其施工质量控制要点如下：①控制夯填土含水率。由于含水率对灰土的强度影响极大[9]，在施工中应尽量接近最优含水率(22%~23.5%)[10]，现场简易试验方法是用手抓一把灰土，要求能捏成团，但又不粘手，将手松开，落地散开。②控制夯体密实度。夯体是否密实是加筋干打垒护坡效果好坏的关键，不密实的灰土极易流失，因此在夯填灰土过程中应该严格控制夯填厚度和压实度。③设置加筋体及护脚矮墙。灰土本身强度低，抗冲击能力弱，在夯体中均匀布置加筋体将极大提高夯体的整体强度[11]。普通三七灰土和加筋三七灰土的对比试验[12]表明，加筋后的三七灰土黏聚力和内摩擦角均有大幅度的提高，而护脚矮墙的设置则提高了夯体的整体稳定性。
4. 工程应用效果
自2012年初在我国黄土区油气管道应用加筋干打垒护坡方法以来，完成了黄土坡面防护93处，防护土方工程量超过8 000 m3。与常规黄土坡面防护措施对比，浆砌石的综合单价为600~900元/m3，混凝土的综合单价为700~1 200元/m3，而干打垒的综合单价仅为150~200元/m3。依据文献[13]，利用水工效能评价技术，对其中35处典型工程进行了效能评价，其风险减缓能力(防护效果)和工程可用性(工程质量和持续发挥作用的能力)均处于较高的水平。可见，该技术防护效果好、持续作用强，在一定程度上避免了黄土坡面短期内再次出现水工病害而进行二次投资治理的情况。
5. 结论
加筋干打垒护坡方法具有适用范围广、施工便捷、成本低廉、防护效果持续时间长等优点，能够有效降低黄土区斜坡坡面水毁带来的管道安全风险。夯填土含水率、夯体压实度和加筋体及护脚矮墙设置是该技术质量控制的关键点，接近最优含水率的密实夯体阻止了水的渗入和土的流失，适当加筋提高了夯体的整体强度，而护脚矮墙则增强了夯体的整体稳定性。采用干打垒护坡治理管道开挖范围内边坡的防治方法具有较高可行性、经济性及安全性，为湿陷性黄土地区类似工程的水土病害防治提供了参考。
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图 2 高压气相慢应变速率拉伸试验机装置示意图
Figure 2. Schematic diagram of high-pressure gas-phase slow strain rate tensile testing machine
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图 4 不同气相氢环境下X52钢力学性能统计图
Figure 4. Statistical charts of mechanical properties for X52 steel in different gas-phase hydrogen environments
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图 5 不同应力环境下充氢X52钢的力学性能统计图
Figure 5. Statistical charts of mechanical properties for X52 steel with hydrogen charging under different stress environments
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图 6 预充H2、H2下拉伸X52钢试样断口形貌图
Figure 6. Fracture surface morphology of X52 steel specimen tensioned in hydrogen environment with hydrogen pre-charging
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图 7 无预充H2、H2下拉伸X52钢试样断口形貌图
Figure 7. Fracture surface morphology of X52 steel specimen tensioned in hydrogen environment without hydrogen pre-charging
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图 8 预充H2、无H2下拉伸X52钢试样断口形貌图
Figure 8. Fracture surface morphology of X52 steel specimen tensioned in hydrogen-free environment with hydrogen pre-charging
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图 9 11 000 N下预充H2拉伸X52钢拉伸断口形貌图
Figure 9. Fracture surface morphology of X52 steel specimen tensioned under 11 000 N with hydrogen pre-charging
表 1 X52钢化学成分表（质量分数）
Table 1 Chemical composition of X52 steel (in mass fraction)
C Si Mn P S Cr Mo Ni Nb V Cu Al Fe 0.0873 ％0.151％ 1.15％ 0.0133 ％0.0095 ％0.0257 ％0.0033 ％0.0133 ％0.0206 ％0.000095 ％0.011％ 0.0251 ％余量 
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