Chromium Depletion
Local decrease in chromium concentration at the grain boundaries of austenitic steels due to the precipitation of chromium carbides during improper heat treatments. This leads to the loss of passive protection in these zones and promotes intergranular corrosion.
Chromium depletion occurs when austenitic steel stays too long in the critical temperature range. Then chromium binds at the grain boundary as carbide and the spot loses its corrosion protection. The result is intergranular corrosion, which in the worst case you see as fine cracks. When welding, you avoid this by keeping heat input low and cooling fast enough. If you are unsure which material you have, use a stabilized steel or one with low carbon content. Then you'll hardly have this problem in practice.