Influence of the different alloy components in stainless steels.

1. Chromium may be classified as an alpha element. The curl that shows, called gamma curl, in the low chromium content area is due to this property.

We see that up to 12-13% chromium content, steel shows an alpha-gamma change point, and from this value on, there is no other transformation point.

This is strictly valid for theoretic, non-Carbon alloys only.

By adding Carbon, we move the gamma curl towards higher Chromium contents, while widening the mixed alpha-gamma area. The maximum movement of the curl which limits alloys that become completely austenitic is obtained by a Carbon content of 0.6%. Higher quantities of this element lead to an increase in free carbide particles. The Chromium content for this maximum is of 18%. As from this point, and up to a 26-27% chromium content, alloys present mixed alpha-gamma structures, and above 27%, alloys are definitely Ferritic.

This is due to the gamma influence of Carbon, which widens the stability field of Austenite.

2. At lower temperatures, and at Chromium contents of 45%, we find the stability area of a highly damaging and thus important phase called sigma, an inter-metallic Iron-Chromium compound, soluble over 850º. This is a paramagnetic phase, which shows remarkable hardening and extreme brittleness.

Nickel action

Nickel belongs to the gamma element group.

When steels contain Chromium as well, its opposed action combines to produce multiple structured alloys, the most representative of which is the 18-8 type, or classic austenitic steel.

The diagram in fig.2 shows the balance of an 18-8 steel for different Carbon contents. The curb indicates Carbon solubility in the gamma phase, according to temperature.

1. Austenite-gamma curl non-carbon Fe-Cr alloys.
2. Austenite-gamma curl Fe-Cr-C C- 0, 60% alloys.
3. Austenite + Ferrite – biphasic area –Fe-Cr-C C = 0, 60% alloys.%

The diagram explains why the hot annealing treatment of Stainless Steels in performed through quick cooling.

If we quickly cool this type of steel, previously heated to 1,100ºC, we will keep this alloy in an unstable balance and produce homogeneous Austenite, which will contain the Carbon in the solution. Otherwise, the Carbon will precipitate as Chromium carbide, preferably on grain boundaries, thus leading to Chromium level descents below the stainless limit, and leaving the alloy subject to corrosive attack. This is the theory behind intergranular corrosion.

Nickel, therefore:
1°- Broadens the Austenite formation range and increases its stability. With 18% Chromium and 8% Nickel contents, we may obtain alloys which are austenitic at room temperature.
2°- Nickel moves sigma phase stability to lower Chromium contents and higher temperatures.

Other alloy components

Silica: Increases Stainless Steels’ resistance before Oxygen, air, and hot oxidizing gasses. It is an alpha element, employed in heat-resistant alloys.

Aluminium: Used in heat-resistant alloys, it shows similar patterns to Silica.

Molybdenum: It affects passivation and resistance of Stainless Steels before reducing acids and chloride ions. It is an alpha element, and therefore this action must be considered if we look for completely austenitic steel, type 316. It also increases resistance of austenitic steels in hot temperatures.

Manganese: It is a gamma element, but with no appreciative influence regarding stainless properties.

Nitrogen: It is a gamma element which acts analogically to Carbide. It is used to turn steels of low Nickel content into austenitic steels, or to increase mechanical resistance of ELC steel types. Its content percentage must be strictly monitored, because although it is a great austenitizing agent and considerably increases mechanical resistance, it has negative effects line decreasing toughness, among others.

Copper: It increases corrosion resistance in non-oxidising environments; hence it is added to certain types of steels. It presents no influence on the structure.

Titanium- Niobium: They are alpha elements, used due to their chemical affinity with Carbon to avoid Carbide precipitation during low cooling processes of Austenitic stainless steels.

Sulphur, Selenium, and Phosphorus: Added to facilitate machinability.

When steel presents several elements, its structure depends on the sum of each element’s actions. This may be appreciated in the SCHAEFFER diagram on fig. 3.

According to what we’ve seen on balance diagrams and the properties of the above mentioned elements in relation to corrosion resistance and structure, an infinite number of compositions is possible, depending on different uses.

We may summarize them in:
1) Martensitic Stainless Steels:	They contain Chromium (12 to 17%) and Carbon (0,1 to 0,5%). This material can be austenitic at high temperature and tempered in the cooling process (as is the case in some commercial alloys). They rarely present other alloy components, save Silica in order to avoid rust formation at high temperatures. They reach a mechanical resistance of 145 to 200 kg/mm2 after tempering, and 80 to 130 kg/mm2 after annealing, depending on the Carbon content of the final values. They present good corrosion resistance before certain weak organic and inorganic acids, and some food products in case there is no enzyme-linked fermentation processes. They are known as “stainless in water”.
2) Ferritic Stainless Steels:	They contain 16 to 30% Chromium. Carbon content must remain low, but may reach 0,35% in the case of Chromium contents of 30%. The C content is normally below 0,1%. These steels have no change point, and can therefore not be hardened by tempering. It presents grain growth when heated at high temperatures, and show substantial brittleness. Their mechanical resistance is approximately of 50 Kg/mm2 and they present elongation ratios of 22%. Generally considering, they present better chemical resistance than martensitic steels, but not as good as austenitic steels.
3) Austenitic Stainless Steels:	They contain up to 18 - 25% Chromium and up to 20% Nickel. Their composition is balanced so that they preserve an austenitic structure at room temperature. As they have no change point, they present grain growth at high temperatures, but this does not render them as brittle as ferritic steels. Mechanical characteristics are very good. They present great ductility, mechanical resistance ranging from 56 to 60 Kg/mm2, and elongation ratios of 60%. It is highly strengthened by cold-working. They also present high resilience, and an extremely low cracking temperature point (up to -200ºC), which makes them ideal for cryogenic processes.
4) Austenitic-Ferritic Stainless Steels:	They are analogical to the latter, whose composition was balanced so they would contain a certain amount of Ferrite. They contain Chromium (20 to 25%) and Nickel (8%). Their mechanical resistance is of approximately 70 Kg/mm2. They present the advantage of being immune to intercrystalline corrosion, and tensile stress corrosion.