Changes in Pipe Geometry in the Course of Sequential Creation of Stresses on the Inner Surface under External Thermomechanical Impacts

Abstract

In modern high-technology industry, pipe bending technology is widely used. Pipe bends are an integral part of pipeline systems. The most widely used methods are techniques of cold pipe bending accompanied by a number of negative phenomena such as reduction in the wall thickness on the outer side of the bend, ovalization of the cross section, and formation of crimps. The article presents a study of the influence of deforming pipe billets by rolling under high tension on the structure and properties of the billet’s material. The pipe deformation by rolling under tension allows obtaining radially bent billets without destroying them and without causing obvious defects on the surface and microstructure. The tests were conducted using samples manufactured of 3sp (ASTM 284 grade D) and 12Cr18Ni10Ti (UNS S32100) grade steels. The microstructure was examined in accordance with GOST 5639–82 state standard, the mechanical properties were investigated in accordance with GOST 1397–84, and the microhardness—in accordance with GOST 9450–76. The effect of geometry changes of the pipe billets on their structure and properties was studied. It is shown that the pipe deformation by rolling leads to changes in the mechanical properties of the tested materials. The microhardness and strength values increase, while the grain points decrease. In the course of deformation, the microstructure of the material may change as a result of structural transformations (quenching). The plastic flow of the metal during the thermomechanical deformation suggests possible change in the pipe wall structure as a result of recrystallization and heat treatment of the material in the bend area, which requires further study and in-depth analysis of the technology in question.