Grain Coarsening
Grain coarsening is the growth of crystallites in metals at high temperatures or long holding times, which reduces toughness and strength.
Physico-Chemical Mechanism of Action
Grain coarsening occurs when metals remain at elevated annealing temperatures or during long holding times in the weld zone. This leads to uncontrolled growth of crystallite sizes (grain sizes) because the total grain boundary area is reduced and fewer interfaces are available for atom diffusion.
Metallurgical Relevance & Risks in Welding
Grain coarsening leads to a drastic deterioration of the material's toughness and strength. The material becomes softer and loses its mechanical properties, endangering the service life of components and weld joints. It also increases the risk of cracking and fracture.
Workshop Practice & instructions
To avoid grain coarsening, the following measures should be taken:
• Use an optimal annealing temperature that stays below the melting temperature limits.
• Keep the workpiece in the weld zone as short as possible (maximum 10 seconds).
• Use appropriate shielding gas mixtures or deoxidizers to prevent corrosion and control grain size formation.