Carbon will migrate in steel when heated to above the critical point and the carbon goes into solution. The depth of migration will depend on the time at temp. In damascus or pattern welded steel, the layers get pretty thin and you have migration from both sides. This will enable the carbon to be completely diffused and uniform through out the billet. That said, on a san mia billet that has not been above critical very long, the carbon will only be able to migrate a few thousandths and the core will not loose much of the carbon, percentage wise.
On etching the billet, all steels will be differently affected by the acid. This is what causes the different layers to show up as bands. The composition of the steels will determine what the steel looks like after it has been etched. Even the boundries will show if they have been exposed to the air. Cable and a stacked billet of a singel steel will show boundry lines. The boundries etch different because they have a different composition than the subsurface of the steel.
Something you might try is to use 4 to 5 different steels for a damascus billet and after sanding smooth, slowly heat the steel. You will notice that they will all produce different oxidizing colors. This is due to the different elements in the steel.
The importance of understanding carbon migration is to make sure you know the carbon content of the end product. If it is to be used for hardware, the amount may not be too important, if it is to be used for a blade, it may be very important
