Many problems faced by engineers tend to be skin deep.
Provided a component is properly designed, it will meet the main stresses imposed on it, but the surface of the component is where the problems begin. This is the interface with the outside world, open to attack from corrosion, chemicals, and abrasion. Furthermore it is where stresses from machining or manufacturing are likely to be concentrated, and where fatigue cracking tends to begin. Fortunately, engineers now have a range of surface treatment techniques to build up defences against external attacks or internal weakness. The carbon is absorbed into the surface of the steel, increasing its hardness, but leaving the interior of the steel relatively soft, a combination which remains easy to work, but with very high resistance to both stress and fatigue, for applications like gears and bearings.