Vacuum Decarburization Zone
A near-surface microstructural region with reduced carbon content resulting from diffusion processes under reduced gas pressure.
The vacuum decarburization zone forms when, during vacuum welding, the carbon migrates out of the weld and the heat-affected zone. This happens because the pressure is so low that the carbon is essentially sucked out. The result is a carbon-poor zone that becomes softer and tougher. This is a double-edged sword in welding: on one hand, it lowers hardness and reduces the risk of cracking; on the other, you lose strength. When welding under vacuum, you must ensure that cooling is not too fast, otherwise you'll still get hard, brittle structure. So adjust the welding parameters so that the heat input is right and the weld cools slowly enough.