Alloying Elements: Who Does What

The list of alloying elements is the dullest part of any conversation about steel and at the same time the only part that shows why grades behave differently. Before the list it helps to keep two things in mind, without which it turns into a set of incantations.

First: almost no element works alone. The effect depends on the overall composition and on the heat treatment, and the same element gives a different result in different grades.

Second: what matters is not only how much of an element is in the steel, but where it sits. Carbon binds metals into carbides — hard, brittle particles. Chromium that has gone into a carbide no longer protects against rust. Vanadium spent where there is a lot of chromium does not form a carbide of its own. So the number in the composition and the property that actually works are not the same thing.

The basics

Carbon (C):

• increases edge retention and raises tensile strength;
• increases hardness, improves resistance to wear and abrasion;
• reduces ductility as the amount goes up;
• provides hardenability.

Chromium (Cr):

• raises hardness, tensile strength and toughness;
• raises resistance to corrosion, heat and wear;
• more than 11% makes the steel “stainless” by producing an oxide layer;
• carbide inclusions reduce wear, but the material itself becomes softer.

Carbide formers

Vanadium (V):

• raises strength and wear resistance and increases toughness;
• improves corrosion resistance by helping the oxide layer form;
• its carbide inclusions are very hard;
• raises resistance to chipping;
• expensive.

Niobium (Nb):

• limits the growth of carbide grains;
• improves machinability;
• forms the hardest carbide;
• raises strength, heat resistance, corrosion resistance and toughness.

Molybdenum (Mo):

• raises strength, hardness, hardenability and toughness;
• improves machinability and corrosion resistance.

Tungsten (W):

• adds strength and toughness, improves hardenability;
• keeps hardness at elevated temperature;
• raises corrosion and heat resistance.

Titanium (Ti):

• raises strength, toughness, heat resistance and corrosion resistance, and reduces weight;
• raises hardness and wear resistance when nitrogen or carbon is present at the surface of the alloy.

Tantalum (Ta):

• raises corrosion and heat resistance, strength, ductility and toughness.

Instead of carbon

Nitrogen (N):

• substitutes for carbon in the crystal lattice: a nitrogen atom works much like a carbon atom, but gives unusual advantages in corrosion resistance.

The rest of the cast

Cobalt (Co):

• raises strength and hardness, allows hardening at higher temperatures;
• amplifies the individual effects of other elements in more complex steels;
• raises resistance to heat and corrosion.

Nickel (Ni):

• adds stiffness;
• raises corrosion and heat resistance;
• reduces hardness;
• in excess it interferes with hardening during heat treatment.

Manganese (Mn):

• raises hardenability, wear resistance and tensile strength;
• deoxidises and degases, removing oxygen from the molten metal;
• in large amounts raises hardness and brittleness;
• raises or lowers corrosion resistance — depending on the type and grade of steel.

Silicon (Si):

• raises strength, heat resistance and corrosion resistance;
• deoxidises and degases, removing oxygen from the molten metal.

Copper (Cu):

• raises corrosion resistance.

More like impurities

Phosphorus (P):

• improves strength, machinability and hardness;
• increases brittleness at high concentrations.

Sulphur (S):

• improves machinability when added in trace amounts;
• is usually treated as a contaminant.

How to use this

The logic of designing a knife steel falls out of the list by itself. A knife needs a combination of high hardness, toughness and wear resistance, and every extra requirement placed on the alloy takes something away from the others.

High-speed steels, for example, need large amounts of molybdenum or tungsten for “hot hardness” — the ability not to soften when heated by high-speed cutting. The requirement is legitimate, but it constrains: without it the composition could be optimised for toughness and wear resistance. The stainless requirement works exactly the same way — it forces you to put in a lot of chromium.

The modern tricks come from the same place. Niobium is a stronger carbide former than vanadium: it forms its own carbides even where there is plenty of chromium, so replacing part of the vanadium with niobium reduces the amount of chromium carbide. Vanadium leads to more chromium carbide, niobium does not. Nitrogen, in turn, is less inclined to form chromium nitrides than carbon is to form chromium carbides, and it raises hardness just as well; so replacing part of the carbon with nitrogen gives the same level of hardness with better corrosion resistance and better toughness.

Both tricks get combined. S45VN has 0.5% niobium and about 0.17% nitrogen — which is why it has only slightly more chromium carbide than S30V and S35VN, even though its chromium content is 16% rather than 14%.

The practical conclusion from the list is simple: the composition on a steel’s data sheet is not a set of virtues but a set of trade-offs, and it should be read as a list of what the manufacturer decided to sacrifice.