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球形储罐局部消应力热处理的机理与效果评价研究

A Study on the Mechanism and Efficiency Evaluation of Local Stress-relief Heat Treatment on Spherical Tanks

【作者】 王泽军

【导师】 荆洪阳;

【作者基本信息】 天津大学 , 材料加工工程, 2007, 博士

【摘要】 球形储罐是应用广泛的大型压力容器。使用中发现裂纹等缺陷后通常需要进行补焊和局部消应力热处理。目前各国标准中均未给出有效的关于球形储罐局部热处理的规定。公开报道的案例表明,对局部热处理远未达到有效控制水平。本文采用有限元数值模拟和试验相结合的方法对球形储罐局部热处理问题进行了研究。研究中提出了考察区域的概念以及以此为基础的局部加热过程热跟踪的算法,建立了球形储罐多层多道补焊过程数值模拟模型和三种局部加热过程分析模型,并开发出了对应的运算程序。为分析局部热处理的效果,提出了补焊区域表面平均残余应力、熔敷金属平均残余应力和结构最大残余应力三种评价方法。通过大规模系列运算,得出了许多有价值的结论。研究发现,若采用小面积集中加热,局部加热过程本身会带来足以引起材料屈服的残余热应力。虽然残余热应力在冷却过程中产生,但造成残余热应力的真正原因是加热、恒温过程形成的塑性应变和蠕变应变,而冷却速度对残余热应力基本无影响。当同时考虑焊接残余应力时,局部加热过程本身带来的热应力仍是影响热处理效果的主要控制因素。增加加热面积可以有效降低局部加热带来的残余热应力,提高残余应力消除率。分析表明,在球形容器局部热处理中,焊接残余应力的消除机制包括高温屈服和高温蠕变。材料的高温蠕变起到两个相反的作用,它有利于焊接残余应力的消除,但会增加局部热处理过程本身带来的残余热应力。在材料屈服、蠕变和温度梯度共同作用下,局部热处理存在最优的恒温温度和恒温时间。对十种可能的影响因素进行了逐一分析,表明球形储罐体积、壁厚、加热区域弧长半径和恒温温度是影响局部热处理效果的主要因素;环形保温带宽、加热升温速度、恒温时间和考察区域尺寸对热处理效果影响不显著,为次要因素;冷却速度(包括加速冷却)和加热面布置对补焊残余应力消除效果几乎没有影响。基于合于使用的原则,得出球形储罐局部热处理加热区域弧长半径的推荐准则为Rheat≥3.4(Rit)1/2。这时,可以将考察区域的残余应力平均值控制在30%σs左右,最大残余应力可控制在50%σs左右,对应的焊接残余应力消除率可分别达到75%和67%。

【Abstract】 Spherical tanks are widely used large-scale pressure vessels. Repair welding and local stress-relief heat treatment (LSRHT) is usually employed in case defects such as cracks are found. Effective specifications on LSRHT are not yet given in the standards presently available. It shows from the analysis on the published cases that the LSRHT is far away from effectively controlling.Researches on LSRHT of spherical tanks are performed in this paper based on the methodology of combination of numerical simulation and experiments. The concept of observed region and based on that, the arithmetic for the thermal tracing of LSRHT process are put forward. A model simulating the multi-layer multi-bead repair welding process and three models for the LSRHT process are founded, and the corresponding programs are developed in which especially the thermal tracing program is included. In order to analysis the efficiency of LSRHT, three evaluation methods are employed, viz. the maximum residual stress, the average residual stresses on the weld surface and in the deposit metal. Through large-scale series analysis, many valuable conclusions are obtained.It is found that the LSRHT process itself does cause obvious residual thermal stress that is high enough to cause yield when concentrated heating on small region is adopted. The residual thermal stress is formed phenomenally in the cooling period, however, the plastic and creep strain formed in the heating and holding periods are found to be the real reason, and the cooling rate can hardly affect the final stress. In the analysis in which the weld residual stress is included, the thermal stress induced by local heating is still the main control factor. Sufficiently enlarged heated region can lower effectively the residual thermal stresses, and also results in desirable relief efficiency of weld residual stress.The analysis shows that the relief mechanism of weld residual stress includes high temperature yield and creep. In the LSRHT process of spherical tank, the creep mechanism has two opposite effects, it is helpful for the relief of weld residual stress, but can enlarge the residual thermal stress induced by LSRHT process itself. Under the combination effect of yield, creep and thermal gradient, best holding temperature and holding time exist. It can be found through one by one analysis on ten factors which are possibly influential on stress relief ratio, arc radius of heated region, holding temperature, volume and wall thickness of spherical tanks are primary effective factors. Band width of annular insulated region, heating rate, holding time and the size of observed region are secondary factors. Heating pattern and cooling rate (include accelerated cooling) can hardly affect the relief ratio of weld residual stress.Based on the principle of“Fitness for Purpose”, the recommended criterion about arc radius of heated region is R heat≥3.4Rit. In this case, the average residual stress in observed region can be controlled approximately at 30%σs, and the maximum residual stress at 50%σ_s, the corresponding relief ratios of weld residual stress are 75% and 67%, respectively.

  • 【网络出版投稿人】 天津大学
  • 【网络出版年期】2009年 04期
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