Microstructure/Property Examination of Weld HAZ in Grade 100 Microalloyed Steel

The weld thermal cycle in microalloyed pipeline and structural steels results in significant changes in microstructure and, consequently, mechanical properties of the weld heat-affected zone (HAZ). To have better control of the properties of the HAZ, knowledge of these changes and correlation with mechanical property variations is required. The first step in achieving this is to construct a methodology to examine different regions of the HAZ thoroughly, so that important characteristics such as grain size, microstructure, precipitate type and distribution, and mechanical properties are determined. The next step would be using this methodology to examine different regions of the HAZ under different welding conditions (the most important of which is heat input) and therefore to understand the effects of the welding process. In this paper, a methodology for studying HAZ microstructure/property relationships is demonstrated for a Grade 100 microalloyed steel welded autogenously (no filler metal). Microhardness measurements are used to assess the mechanical property variation across the HAZ. Optical microscopy (OM) and transmission electron microscopy (TEM) are used for grain structure examination and precipitate analysis. The relationship between microstructure and mechanical properties, with emphasis on precipitate reactions, is presented for different regions of the HAZ.