There is higher welding residual stress in welded structure. It will endanger the safety of the structure. So it is a hot item to research the rule of the evolvement of welding stress and how to control or eliminate it. In this article, the research on some problem about welding residual stress and explosion treatment process was introduced. Those works were based on numerical simulation method mostly.
Firstly, the theory, hypothesis, specific concept involved in the model of welding temperature field, stress strain field and the method how to carry out the simulation employed in Ansys software were introduced. Based on the C-J model of detonation wave, it was explained to how to determine the parameters of state equation of detonation product of condensed explosive. The constitutive and state equation of 16Mn steel and Air were interpreted also. The algorithm, solution to hourglass problem and control of time step employed by LS-DYNA software were commented, too.
To further comprehend the effect of explosion treatment on structure safety, the impact properties of 16Mn steel, treated by single-side and double-side explosion treatment, were researched based on the initiation energy and expansion energy of the crack revealed by scillometric impact tester. It was inferred that single-side explosion treatment could improve impact toughness but double-side explosion treatment could deteriorate it.
It is difficult to control the deformation of thin sheet when welded. A new welding method, welding process with trailing heating (WPTH), was introduced in this article. With a rigid cover plate to confine the deformation induced by transient stress, the residual stress of the thin sheet welded by WPTH was close to zero and the buckle deformation could be ignored. In contrast with normal welding process, WPTH could be considered as a stress-free and non-deformation welding method.
The hypothesis of “planar section” is usually employed to analyze the distribution of stress and strain of any section of weldment. The result of simulation in this article indicated that the hypothesis didn’t accord with the fact. There was a “hidden” hypothesis of “planar section” when data of section by initial coordinate was drawn in the post treatment of simulation. A wrong conclusion about the section would be put forward if the “hidden” hypothesis was ignored. It was argued that the relative longitudinal displacement of nodes on the same section didn’t change following to the change of physical variable of nodes on adjacent sections. So it was regarded as low risk that the status of any section at any time was represented by data drawn by initial coordinate of the section.
This article put forward a problem how to give right judgment on tensile or compressive status of metal when heated or cooled. It was argued that the compressive and tensile strain would be defined according to free thermal expansion and contraction. The two strains couldn’t be remarked by sign of actual strain which is calculated according to node’s real displacement. When the increment of actual strain is lower than the increment of thermal strain between two following time points, it is considered that metal is compressed, otherwise stretched. The development of strain of points on the weldment is not only determined by itself thermal strain, but also is affected and confined by adjacent nodes. The absolute value of maximal compressive strain of metal located in welded joint exceeds to the maximal thermal strain and the maximal tensile strain after cooled is also greater than the maximal thermal strain. The sum of the maximal compressive strain and the maximal tensile strain is equal to residual strain. It was indicated in this article that the width of area in which residual stress is greater than yield strength at room temperature is same to that of area in which the maximal actual compressive strain exceeds to 0.2% and the residual actual compressive strain remains 0.2% at the same area.
Different welding process parameters were designed according to the thickness, 8mm, 16mm, 32mm, of the 16Mn steel. The development of temperature, stress and strain was simulated by Ansys software. The residual stress distribution of the weldment was gained and founded the basis of simulation of explosion treatment. The double ellipsoid heat source model was selected to simulate the heat input of SMW. The heat absorption and release of liquid-solid phase transformation was calculated as effective specific heat. The technologic of “birth-death” element was employed to simulate the filling of weld element for more agreement of the simulation result on real world. The simulation result indicated that the parameters of welding process were reasonable and the residual stress distribution was perfect.
A series of parameters of explosion treatment process were designed and their affection on elimination of welding residual stress was analyzed. The simulation tests showed that the expansion wave propagating on the weld before front wave of detonation wave induced the relief of longitudinal tensile stress because of its higher amplitude and that it stretched the weld elements so much that there was compressive stress in those elements before detonation wave passed them. The explosion treatment status of the metal under the explosive was regarded as “excessive” status. That of the metal with residual compressive stress far away from the explosive, where stress was close to zero after explosion treatment, belonged to moderate status. And that of the metal more far away was classed as “soft” status. It was argued that excessive compress would convert compressive stress into tensile stress and that it would deteriorate structure safety if the detonation pressure was too high. The longitudinal stress of inner weld elements could be changed into compressive stress. The single-side explosion treatment was suited to be employed on the 8mm-thickness and 16mm-thickness steel. The ideal detonation pressure was 0.6GPa and 1.5GPa for two steel respectively. The double-side explosion treatment could be designed for 32mm-thickness steel. The double-side explosion treatment was divided into two processes: coinstantaneous explosion and one-by-one explosion. The ideal detonation pressure for two processes was same, 2GPa.
修改评论