【作者】 韩豫；

【导师】 陈学东； 刘全坤；

【作者基本信息】 合肥工业大学 ， 材料加工工程， 2012， 博士

【摘要】 固溶态奥氏体不锈钢具有较高的抗拉强度和极好的塑性指标,但因其屈服强度较低而使通常设计制造的奥氏体不锈钢制压力容器壁厚较厚,材料利用率低,经济性较差。采用应变强化技术来制造奥氏体不锈钢容器能显著提高材料的屈服强度,减薄容器的设计壁厚,减少容器的用钢消耗,从而可以实现压力容器的轻型化,同时容器的安全性也能得到保证,因此应变强化技术是一种节材降耗的绿色制造技术。本文对应变强化技术应用于奥氏体不锈钢容器的相关问题进行了系统的理论和试验研究。分析了应变强化对奥氏体不锈钢材料的常温、低温和高温力学性能的影响；设计并制造了奥氏体不锈钢试验容器,讨论了容器应变强化的时机、处理方式、压力控制及应变测量方法等问题；通过有限元模拟了容器的应变强化过程,得到了容器关键部位的应力应变规律；探讨了应变强化对奥氏体不锈钢材料在高温氯离子环境中应力腐蚀开裂敏感性的影响及其对材料高温棘轮行为的影响规律。主要研究内容如下：讨论了应变强化过程中的两个关键工艺参数应变速率和应变量对奥氏体不锈钢力学行为的影响。研究结果表明,强化过程中应变速率不宜过慢,否则材料会出现锯齿形屈服效应,从而对材料的使用性能造成不利影响。室温下把应变量控制在10%以下,在显著提高S30408奥氏体不锈钢屈服强度的同时,对材料的常温和低温力学性能影响不大,且强化效果在100～400℃温度区间内仍保持较好。以J积分作为评价指标,测定了应变强化前后含缺陷奥氏体不锈钢焊缝金属的常温断裂韧性,得到了原始态和强化态焊缝金属的启裂韧度J1C的表观值。与原始焊缝性能相比,8%强化态焊缝金属的屈服强度仅提高了19%,而J1C表观值却下降了近33%。这表明焊缝的性能对应变强化效应比较敏感,因此对应变强化容器的焊接工艺、焊接质量和焊后检验要求更加严格。按照现行应变强化技术标准设计并制造了低温操作下的奥氏体不锈钢试验容器,并对其实施了应变强化过程。采用应变片、百分表和周长测量三种方法来测量和控制容器应变强化过程中的应变量,并对比了三种方法的优劣及适用条件。由应变强化容器的爆破试验结果可知,应变强化处理后的容器仍保留了良好的塑性储备,说明容器经应变强化处理后的使用安全性可以保证。基于材料非线性和几何非线性建立了奥氏体不锈钢容器应变强化过程的有限元模型,在验证有限元模型可靠性的基础上,对容器的应变强化过程进行数值模拟,得到了容器关键部位的受力情况和变形规律。针对奥氏体不锈钢容器在腐蚀性介质环境中服役易发生应力腐蚀开裂的情况,采用慢应变速率试验方法,研究了应变强化前后奥氏体不锈钢母材和焊接接头在高温氯离子环境中的应力腐蚀敏感性变化情况。研究发现,随着应变量的增加,S30408奥氏体不锈钢母材的应力腐蚀敏感性呈上升趋势,其中8%强化态母材试样的应力腐蚀敏感性指数比固溶态材料高出近一倍。由于奥氏体不锈钢焊缝中含有一定量的铁素体,因而在焊缝处断裂的试样应力腐蚀敏感性相对较低。研究还发现应变强化有利于改善焊缝中存在的焊接残余应力。开展了应力控制模式下的疲劳试验,研究应变强化对奥氏体不锈钢高温棘轮行为的影响。研究发现,固溶态材料的棘轮行为随着应变幅的增加愈加显著,而材料经应变强化后,棘轮行为可以得到明显的抑制,并且随着预应变量的增加,棘轮受抑制程度愈加明显,且应力控制下的疲劳寿命也逐步提高。在Hull-Rimmer理论的基础上,建立适用于不同预应变强化量条件下的奥氏体不锈钢高温疲劳寿命预测方程,预测结果与试验结果吻合较好。更多还原

【Abstract】 Solid solution austenitic stainless steel has high tensile strength and good plasticity, but exhibits low yield strength, thus causing thicker wall thickness of austenitic stainless pressure vessel designed by conventional method with lower material utilization and poorer economic benefit. Cold stretching technique can improve the yield strength of material and thereby reduce the wall thickness of vessel to achieve lightweight, while the security can be guaranteed, so it is a kind of green manufacturing technology.In this paper, systematic theoretical and experimental studies were carried out for cold stretching technique applied to austenitic stainless steel pressure vessel. The effects of cold stretching on mechanical properties of austenitic stainless steel at room temperature, cryogenic and elevated temperature were investigated respectively. Test vessels were designed and manufactured, and the time, treatment mode, pressure controlling and strain measurement methods of cold stretching were discussed. The distributions of stress and strain on the key parts of the vessel during cold stretching were obtained by finite element simulations. The influence of cold stretching on stress corrosion cracking susceptibility at high-temperature chloride environment for austenitic stainless steel was discussed, and its effect on ratcheting behavior of the material at high-temperature was also summarized. All investigations can be concluded as follows:Strain rate and strain level were two key parameters in the process of cold stretching and their effects on the mechanical behavior of austenitic stainless steel were investigated. The results show that the strain rate should not be too slow, otherwise the serrated yielding effect will occur which have an adverse effect on the properties of the material. When strain level was limited below10%at room temperature, it has little effect on the mechanical properties of S30408austenitic stainless steel while its yield strength can be improved significantly and the stretching effect in the temperature range of100～400℃will still be maintained.J-integral was taken as evaluation index to determine the fracture toughness of austenitic stainless steel weld metal with defect before and after cold stretching, and their apparent values of initiation toughness J1C at room temperature were obtained. Compared with the original weld metal, The yield strength of stretched one (8%level) just increased by19%, whereas its apparent value of J1C dropped nearly33%, which indicate that the properties of weld metal are sensitive to the cold stretching effect, and thus more strictly requirements should be implemented for welding process, welding quality and post welding inspection.The austenitic stainless steel cryogenic test vessels were designed and manufactured according to the current standard of cold stretching. The stretching processes were enforced and three measurements such as resistance strain gauge, dial indicator and circumference measurement were used to measuring and controlling hoop strain of the vessel. The merits and demerits of three methods were compared, and their respective applicable conditions were also discussed. The burst tests for cold stretched vessels demonstrate that the vessel still maintain good plastic reservation and the safety of the cold stretched vessel can be guaranteed.The finite element model of cold stretching process for austenitic stainless steel pressure vessel was established on the basis of material and geometric nonlinearity. The reliability of the FE model was validated before simulation, and then the variation of stress and deformation on the key parts of the vessel during cold stretching were analyzed.In view of the phenomenon that stress corrosion cracking is apt to occur for austenitic stainless steel vessel under corrosive media environment, slow strain rate tests (SSRT) were adopted to investigate the change of stress corrosion cracking susceptibility in high-temperature chloride environment for base metal and welded joint of austenitic stainless steel. The research indicates that the stress corrosion cracking susceptibility of base metal show rising trend with the increase of strain level and the stress corrosion cracking susceptibility index for stretched (8%level) base metal is nearly twice as solid solution one. Due to a certain amount of ferrite in weld metal, the stress corrosion cracking susceptibility of it is relatively low. Moreover, cold stretching is beneficial to improve the welding residual stress.The influence of cold stretching on ratcheting behavior at high-temperature for austenitic stainless steel was discussed by developing fatigue test under stress controlling mode. The results show that the ratcheting behavior of solid solution material is more notable with the increasing of strain amplitude while the ratcheting effect of stretched material is inhibited under the same condition. The degree of inhibition is increasingly obvious with the increase of strain level and also the fatigue life is gradually improved under stress controlling. Based on the Hull-Rimmer theory, a life prediction equation applicable to different pre-strain in high temperature was developed, and the predicted lives coincided well with the test results.更多还原