双相大应变管线钢及热影响区循环应力塑性损伤及疲劳行为研究

Holding the plastic deformation capacity and increasing the fatigue life of pipeline steel pipe are key factors to insure operation safety of the high-pressure natural gas transmission pipeline worked in complicated geological condition regions. In this project, the ferrite+bainite X80 grade pipeline steels with excellent deformability are selected, the effects of cycle stress caused by pressure fluctuation on plastic-damage behavior of the large strain pipeline steels with different microstructures will be investigated. The pile-up of dislocation at the grain boundary between ferrite and bainite, the interaction mechanism of two phase of ferrite/bainite, the conditions of interfacial crack initiation, and the effects of the interfacial crack on fatigue crack initiation and propagation will be researched, respectively. Consequently, the effects of the microstructure of two phase pipeline steels with excellent deformability on plastic-damage and fatigue property under cycle stress can be illuminated. The effects of microstructure and mechanical properties of heat affected zone, the mechanical properties and weld geometry sizes of welded matel, and residual stress on stress concentration will be studied, the effect of stress concentration on fatigue crack initiation, and the fatigue crack propagation in different heat affected zones will be studied. Consequently, the key factors to affect on fatigue property and the forecasting method for residual life of welded joint will be proposed. the works will provide the experimental and theory basis for controlling the microstructures of large strain pipeline steels, optimizing the weld processes, and evaluating the service safety and life of the pipeline.

保持管线钢管塑性变形能力、提高钢管的疲劳寿命是确保复杂地质条件的高压天然气输送管线运营安全的关键。本申请将以铁素体+贝氏体双相X80大应变管线钢为对象，研究不同组织状态大应变管线钢在运行压力波动引起的循环应力作用下的塑性损伤行为；研究铁素体、贝氏体两相界面位错塞积及两相协调变形机制，界面裂纹萌生条件，以及界面裂纹对疲劳裂纹萌生、扩展的影响，从而阐明大应变管线钢组织对循环应力塑性损伤及疲劳性能影响规律。研究大应变管线钢焊接热影响区的组织和性能、焊缝金属的性能和几何尺寸、残余应力对焊接接头应力集中位置的影响，应力集中对疲劳裂纹萌生的影响，以及不同热影响区域的疲劳裂纹扩展规律，找出影响管线钢管焊接接头疲劳性能关键因素和焊接接头残余寿命的预测方法。该研究为铁素体+贝氏体双相大应变管线钢的组织控制、焊接工艺优化以及管线运行安全和寿命评估提供理论基础和实验参数。

在长输管线建设中，铁素体/贝氏体双相高应变管线钢的应用已成为确保复杂地质条件的高压天然气输送管线运营安全的关键。本项目针对管线钢在长期服役过程中的交变应力作用所引起的塑性损伤及疲劳失效，研究了双相钢的组织和性能的关系，钢管焊接接头疲劳失效及影响机制，取得如下成果：.控轧控冷铁素体/贝氏体双相钢轧制过程中形成的拉长贝氏体有利于降低铁素体、贝氏体两相间应变差，提高铁素体、贝氏体变形协调均匀性和强度，但降低铁素体对塑性的贡献；当贝氏体组织为40~48%时，双相钢可获得良好的强塑性配合；提高贝氏体的强度，使双相钢的强度升高，但塑性降低。.疲劳应力循环显著降低双相高应变管线钢塑性，产生疲劳塑性损伤。但当应力循环超过一定周次后，损伤程度降低。在应变循环过程中，残余应力、应变在铁素体中集中是导致双相高应变管线钢塑性损伤的原因。.X80管线钢焊接细晶区因强度最低，疲劳寿命最低；粗晶区强度最高，但韧性差，疲劳裂纹萌生寿命长，但显著降低疲劳裂纹扩展寿命；而临近两相区强度介于粗晶区和细晶区之间，韧性高于粗晶区，疲劳寿命最高。.管线钢管焊缝，去除焊缝余高可提高焊接接头的疲劳寿命。焊缝余高引起的应力集中，加速疲劳裂纹沿粗晶区萌生并扩展；而去除焊缝余高，因焊接细晶区强度最低，导致疲劳裂纹萌生，沿细晶区和两相临界区扩展，疲劳寿命有所提高。.在钢管成型过程中的扩径变形引起焊接接头强度升高，提高焊接接头的总疲劳寿命，但降低疲劳裂纹扩展寿命，缩短了从疲劳裂纹萌生到最终断裂的时间，影响管线运行安全性。.原始X80焊管焊接接头处表面的残余应力均为压缩残余应力，扩径变提高焊缝强度，改变应力分布，而退火则降低焊接接头处残余应力，从而显著提高焊接疲劳寿命。.焊接几何尺寸对钢管的承载能力有一定影响，当焊缝宽度与板厚相当，内焊缝宽度略大于外焊缝，钢管的承载能力最大。这些成果对提高钢管的运营安全设计具有重要的参考价值。