LSAW steel pipe heat treatment technology form the temperature difference in the heating and cooling process because of the central portion of the cooling surface and the speed and time of inconsistencies, volume expansion and contraction will cause uneven stress, the thermal stress. Under the effect of thermal stress, because the surface temperature is below the core part begins, shrinkage core portion by the central portion is greater than the tension, when the end of cooling, since the volume shrinkage of the cooling core section last leaving the surface pressure can not freely central portion tension. That is, in the final thermal stress under the action of the workpiece surface pressure while the heart of tension. This phenomenon is cooled rate, material composition and heat treatment process and other factors.
When cooled, the faster, the higher the carbon content and the alloy composition, the cooling process under thermal stress generated in the non-uniform plastic deformation larger, the final form of the residual stress it gets. The other hand, during the heat treatment of steel changes in the organization that the austenite to martensite transition, will be accompanied by an increase in specific volume due to the volume expansion of the workpiece, the workpiece successively all parts of the phase change, resulting in inconsistent grown produce tissue volume stress.
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