Objectives * To obtain experience in the metallographic preparation of metallic specimens. * To observe the various microstructures in a welded mild steel joint.
Overview of 5 different microstructures in welded steel
Detail microstructures of 5 different zones
Discussion and analysis of results * The heat affected zone is a composition of parent metal which was heated to a high enough temperature for a sufficient period but doesn’t melt so that grain growth occurred. The heat affected zone is also the region whose mechanical properties and microstructure have been altered due to the heat of welding. The heat affected zone undergoes sudden heating followed by rapid cooling, thereby containing a series of microstructures. These microstructures within the heat affected zone are basically categorized into three different zones, namely the grain growth zone, grain refinement zone and transition zone. * * The fusion zone exists between the grain growth zones. The parent metal in this zone was heated to the melting point followed by cooling process. * The grain growth zone is right adjacent to the fusion zone. The parent metal has been heated to a temperature well above the upper critical temperature in this zone. Subsequently, this results in the grain growth or coarsening of the structure. Coarse ferrite grains, Widmanstatten ferrite and pearlite are found in this region. * The grain refinement zone is next to the grain growth zone. The parent metal has been heated to a temperature which is just above the upper critical temperature in this zone, where grain refinement is finished, resulting in the existence of the finest grain structure. Owing to the relatively lower temperature, the ferrite and pearlite, which are represented by the white and dark areas respectively are both much finer. This shows complete recrystallization. * the transition zone is the region where