Phase Boundary Hardening
Phase boundary precipitation hardening increases resistance to plastic deformation through coherent or semi-coherent interfaces between phases that hinder dislocations. This is typical for two-phase alloy systems.
Physico-Chemical Mechanism
Grain-boundary hardening occurs when different phases (e.g., crystal structure or chemical composition) meet in a metal. These boundaries can be coherent or semi-coherent and prevent dislocations from sliding within the material. This increases resistance to plastic deformation.
Metallurgical Relevance & Welding Risks
This hardening can lead to increased hardness and reduced ductility in the weld, raising the risk of cracking. This effect is especially strong in two-phase alloy systems like martensite or ferrite-austenite structures. High hardness can make the weld more prone to hot cracking and fracture.
Shop Practice & Action Items
To minimize grain-boundary hardening, take these steps:
• Control melt time: Too short a melt time can lead to incomplete deformation and higher hardness.
• Regulate cooling rate: Fast cooling can create coherent boundaries, while slow cooling creates semi-coherent ones. The optimal rate depends on the specific metal and desired properties.
• Use suitable welding methods: With high-hardening processes like TIG or MIG, control deformation and cooling carefully to get even hardness.
• Post-weld treatment: Normalizing or tempering after welding can reduce hardness and increase ductility, preventing cracks.