Decarb

Google decarburization and you will find a lot of information; including the answers to your questions. Decarburization results from a loss of carbon on the surface of carbon steels (and other materials) usually due to exposure to an oxidizing atmosphere in a heat-treating furnace. The decarb layer is soft and is generally evaluated by depth.

Decarburization is essentially the opposite of carburization.

Carbon is waht helps steel transform into higher strength microstructure.

Decarb layer means that the carbon in that area has been deleted due to improper thermal treatment. this layer will not come up to the required properties.

It is easily seen under metallurgical microscope.

It would be problematic on 10.9 as it would be softer and promote lower tensile strength in that layer.

Basically, it is a sign of poor process control of heat treatment.

milo