A Study On The Mechanism And Efficiency Evaluation Of Local Stress-relief Heat Treatment On Spherical Tanks

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.