Three-dimensional analysis of medium-frequency induction heating of steel pipes subject to motion factor
Introduction
Oil well pipes are cornerstone of petroleum industry, accounting for about 60% of oilfield equipment assets. And among oil well pipes, casing consumption is above 70%, and their service condition is harsh, usually they have to withstand internal and external pressures of hundreds of atmospheric pressures, tensile load of hundreds of tons and so on. As hydrocarbon resource exploration will further develop towards harsh areas in future, such as deep sea, alpine cold areas and remote areas, steel pipes call for higher safety and stability. Meanwhile, more stringent requirements are put forward for material [1], molding [2], welding [3], [4] and heat treatment process of steel pipes [5].
Electromagnetic induction heating technology not only merits in high performance and environment friendliness, but also precise electromagnetic heating is increasingly valued by people with continuous progress being made in computer digitization technique [6], [7], and it, therefore, has been widely applied in a number of sectors [8], [9], [10]. For a welded steel pipe, high-frequency welding is subjected to skin effect, temperature difference between inside and outside surfaces of the steel pipe is very great, prone to plate edge overheat and cold weld core, and the higher the wall thickness, the more pronounced the overheat and cold weld phenomena [11]. High-frequency heating of welds of a steel pipe results in temperature gradient distribution in proximity to edges of the pipe billet, and forms characteristic zones such as molten zone, normalized zone, incompletely normalized zone, and tempered zone. As electromagnetic heating is fast, austenitic grains grow larger drastically, and when cooled, will form hard and brittle coarse-grained zone, besides, existence of large temperature gradient will lead to postweld residual stress [12]. Hence, there comes a circumstance that mechanical properties of a weld zone are inferior to those of the base metal. Using a medium-frequency heat treatment device, one can soften and refine the microstructure of local normalized zone of a weld and lower postweld residual stress to some extent, so as to improve overall mechanical performance of heat-affected zone of a weld, which is very necessary to improve weld quality [13]. In local heating, temperature shall be tightly controlled; too low temperature would fail to achieve the desired heat treatment effect, while too high temperature would make the grains grow larger again, hampering the effect of local heat treatment. Therefore, to improve weld quality of a steel pipe, it is imperative to control heating temperature tightly [14]. To acquire more accurate induction heating temperature distribution has always been one of the objectives pursued by related scholars [15], [16], [17]. This can not only offer theoretical basis for multi-parameter optimization under complex process condition, but also has positive practical implication to further improving safety and reliability of welded pipe quality.
Utilizing ANSYS finite element software, the authors established a three-dimensional electromagnetic-thermal coupling computing model in consideration of pipe billet motion. Through dynamic simulation of medium-frequency heat treatment process for a longitudinally-welded pipe, a three-dimensional “double ellipsoid” shaped temperature field was obtained. And root cause and immediate cause of it were analyzed.
Section snippets
Mathematical model
In dynamic simulation of medium-frequency heat treatment process for a high-frequency induction welded (HFIW) pipe, a sequential electromagnetic-thermal coupling approach was employed, in other words, electromagnetic field analysis was carried out first, then the Joule heat arising from induced eddy current calculated on the basis of electromagnetic field served as the heat source of thermal analysis.
The differential forms of Maxwell equations are:And
Setup of the simulation model
The Φ406.4 × 16.66 mm welded pipe made by a factory was exemplified for analysis. As shown in Fig. 1, high-frequency welding and medium-frequency heat treatment of the steel pipe are a continuous process. After high-frequency welding, the HFIW pipe has to undergo medium-frequency induction heating twice before reaching heat treatment temperature. In this study, medium-frequency heat treatment process for a HFIW pipe was dynamically simulated, i.e., the induction heating process for the weld after
Results and discussion
To facilitate data analysis below, specified paths and position points are illustrated in Fig. 6.
Conclusions
In this study, dynamic simulation of medium-frequency heat treatment process for longitudinally-welded pipe was achieved, and electromagnetic field and temperature field arising from steel pipe movement were analyzed. The following findings were obtained:
- (1)
By setting up a three-dimensional electromagnetic–thermal coupling mathematical model for medium-frequency heat treatment process of welded pipe in consideration of motion factor, it is found that the steady-state temperature field formed at
Acknowledgments
This project is supported by National Natural Science Foundation of China (Grant No. 51505418) and Natural Science Foundation of Hebei Province (Grant No. E2015203032).
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