How composition affects stainless steel screening

Shift a point of molybdenum or a fraction of carbon and the screening numbers move, sometimes across a category boundary. Knowing which elements pull which way is what makes a screening result readable.

Stainless screening works because a handful of elements control the behaviours that matter most, and the correlations are organized ways of tracking those elements.

For corrosion, the relevant grouping is the pitting resistance equivalent. Chromium, molybdenum, and nitrogen all raise it, with molybdenum weighted heavily and nitrogen more heavily still, which is what separates 316L from 304.

For phase tendency, the relevant grouping is the balance of two equivalents. The ferrite-forming elements, chromium, molybdenum, silicon, and niobium, are summed into a chromium equivalent, while the austenite-forming elements, nickel, carbon, manganese, and nitrogen, are summed into a nickel equivalent, and it is the ratio between the two that indicates whether a composition leans toward austenite or ferrite. This is why nitrogen and carbon have an outsized effect.

Carbon carries a second consequence. Above roughly 0.03 percent it raises the risk of chromium-carbide precipitation and sensitization, which is the metallurgical reason low-carbon grades exist.

When a screening result changes as elements are adjusted, it is following these relationships. The Materials Simulation Lab shows the formula and the inputs behind each result so a user can trace the output back to the chemistry that produced it.