INFLUENCE OF HEAT TREATMENTS ON THE MICROSTRUCTURE OF WELDED API X70 PIPELINE STEEL

Abstract

Welding is one of the most important technological processes used in many branches of industry such us industrial engineering, shipbuilding, pipeline fabrication among others. Generally, welding is the preferred joining method and most common steel are weldable. This investigation is a contribution to some scientific works which have been done on welding of low carbon steel. This work, presents some heat treatments were used to alter the microstructure of base metal (BM), heat affected zone (HAZ) and weld metal (WM) in the welded pipe steel of grade API X70. It presents the microstructures obtained after three heat treatments at 200°C, 400°C and 600°C for 30 min. Scanning electron microscopy and X-ray diffraction have been used as characterization techniques to observe the WM microstructures, in addition the Vickers hardness test are also achieved. The results revealed that the isothermal heat treatment caused grain growth and coarsening reactions in the weld zone and the hardness of weld joints decreased were the main transformations after increasing the temperature of the heat treatment.

Keywords

• heat treatment
• low carbon steel
• microstructure;
• welding