Flame cutting is a small groove cut into the underside of the revolver's topstrap just above the barrel/cylinder gap by the hot gases of burning powder. It's most commonly found on magnums using hot loads.
However, its nothing to worry about. The groove rarely goes deeper than a few thousandths of an inch and then stops getting any deeper all by itself.
I have no knowledge of any particular problems with titanium guns.
S&W adds a steel "blast deflector" to the topstrap of the Scandium revolvers to prevent flame cutting. They also warn against using bullets that weigh less than 125 gr. as the high velocity rounds produce more flame cutting.
