Oxygen-induced decarburization
Reaction of atmospheric oxygen with carbon at the workpiece surface during high-temperature annealing. Carbon diffuses outward and oxidizes to carbon monoxide, resulting in a soft surface layer.
Carbon steel loses carbon at the surface when it gets hot and oxygen is present. The carbon burns away, leaving a soft, ferritic edge layer. This often happens during annealing or when you stay too long with the flame on one spot while welding. This soft layer is a problem because it kills the hardness and wear resistance at the surface. If you want to harden after welding, the edge won't harden properly. On components under load, the soft layer can even lead to cracks. To avoid this, make sure no oxygen reaches the hot workpiece. Weld with shielding gas, and when annealing, put the part in an inert gas chamber or in a bed of activated carbon. And generally: don't make it hotter or longer than necessary.