Dendritic Arm Spacing
The dendrite arm spacing is the distance between the branches of a dendrite and depends on the cooling rate. A small spacing indicates faster solidification.
Physical-Chemical Mechanism
Dendrites are crystal structures that form during solidification. Their branches grow out from the nucleus and form a tree-like structure. The spacing between these branches (dendrite arm spacing) is determined by the local cooling rate: rapid cooling leads to small spacings, slow cooling to larger spacings.
Metallurgical Relevance & Welding Risks
A small dendrite arm spacing can result in a fine and dense crystal structure that offers higher strength and toughness. However, it can also increase susceptibility to cracking and hot cracking, as the crystals are tightly packed and have no room to expand under stress.
Workshop Practice & Action Items
To control dendrite arm spacing, it is important to regulate the cooling rate during welding. This can be achieved by choosing the right shielding gas atmosphere and maintaining correct temperature profiles. For steels with high carbon content, a dendrite arm spacing of about 10-20 µm is typical, while for lower carbon contents a larger spacing of 30-50 µm is preferred. If cooling is too rapid (e.g., due to high thermal conductivity of the shielding gas), the dendrite arm spacing can decrease, posing risks for cracking.