Diffusion Grain Boundary
The diffusion grain boundary is the region at grain boundaries where the atomic structure is disturbed and atoms preferentially diffuse at elevated temperatures.
Physicochemical Mechanism of Action
At elevated temperatures, atoms in metals move more freely. At grain boundaries, where the atomic structure differs, there is a disturbed region with less orderly atomic packing. This disturbed zone is considered a preferential diffusion path for atoms, as they can move and exchange more easily there.
Metallurgical Relevance & Welding Risks
The diffusion grain boundary is critical for the hardness and toughness of the weld. If cooling is too fast, the material can harden excessively at these boundaries, leading to hot cracks. Conversely, if cooling is too slow, atoms may exchange too much in this zone, making the structure uneven and reducing strength.
Workshop Practice & Action Guidelines
To handle the diffusion grain boundary optimally, ensure a controlled cooling time. For high-carbon steels, the cooling time should be between 5 and 20 seconds to achieve balanced hardness and toughness. Avoid fast cooling (t8/5 < 5 s) as it can cause hot cracks, and prevent too slow cooling (t8/5 > 25 s) to avoid uneven structure.