Austenitic and Ferritic Stainless Steels in Practical Applications: Part One

The commonest austenitic steel is so-called 18/8 containing around 18% Cr and 8% Ni. It has the lowest nickel content concomitant with a fully austenitic structure. However in some circumstances, e.g. after deformation, or if the carbon content is very low, it may partially transform to martensite at room temperature. Greater stability towards the formation of martensite is achieved by increasing the nickel content, as illustrated in the 301 to 310 types of steel. 18/8 stainless steel owes its wide application to its excellent general resistance to corrosive environments. However, this is substantially improved by increasing the nickel content, and increasing the chromium gives greater resistance to intergranular corrosion.

Austenitic steels are prone to stress corrosion cracking, particularly in the presence of chloride ions where a few ppm can sometimes prove disastrous. This is a type of failure which occurs in some corrosive environments under small stresses, either deliberately applied or as a result of residual stresses in fabricated material. In austenitic steels it occurs as transgranular cracks which are most easily developed in hot chloride solutions. Stress corrosion cracking is very substantially reduced in high nickel austenitic alloys.

Type 316 steel contains 2-4% molybdenum, which gives a substantial improvement in general corrosion resistance, particularly in resistance to pitting corrosion, which can be defined as local penetrations of the corrosion resistant films and which occurs typically in chloride solutions. Recently, some resistant grades with as much as 6.5% Mo have been developed, but the chromium must be increased to 20% and the nickel to 24% to maintain an austenitic structure.

Corrosion along the grain boundaries can be a serious problem, particularly when a high temperature treatment such as welding allows precipitation of Cr₂₃C₆ in these regions. This type of intergranular corrosion is sometimes referred to as weld-decay. To combat this effect some grades of austenitic steel, e.g. 304 and 316, are made with carbon contents of less than 0.03% and designated 304L and 316L. Alternatively, niobium or titanium is added in excess of the stoichiometric amount to combine with carbon, as in types 321 and 347.

The austenitic steels so far referred to are not very strong materials. Typically their 0.2% proof stress is about 250 MPa and the tensile strength between 500 and 600 MPa, showing that these steels have substantial capacity for work hardening, which makes working more difficult than in the case of mild steel. However, austenitic steels possess very good ductility with elongations of about 50% in tensile tests.

The Cr/Ni austenitic steels are also very resistant to high temperature oxidation because of the protective surface film, but the usual grades have low strengths at elevated temperatures. Those steels stabilized with Ti and Nb, types 321 and 347, can be heat treated to produce a fine dispersion of TiC or NbC which interacts with dislocations generated during creep. One of the most commonly used alloys is 25Cr20Ni with additions of titanium or niobium which possesses good creep strength at temperatures as high as 700°C.

To achieve the best high temperature creep properties, it is necessary first to raise the room temperature strength to higher levels. This can be done by precipitation hardening heat treatments on steels of suitable composition to allow the precipitation of intermetallic phases, in particular Ni₃(Al Ti).

The importance of controlling the γ-loop in achieving stable austenitic steels was emphasized. Between the austenite and δ-ferrite phase fields there is a restricted (α+γ) region which can be used to obtain two-phase or duplex structures in stainless steels. The structures are produced by having the correct balance between α-forming elements (Mo, Ti, Nb, Si, Al) and the γ-forming elements (Ni, Mn, C and N). To achieve a duplex structure, it is normally necessary to increase the chromium content to above 20%. However the exact proportions of α+γ are determined by the heat treatment. It is clear from consideration of the γ-loop section of the equilibrium diagram, that holding in the range 1000-1300°C will cause the ferrite content to vary over wide limits.

The usual treatment is carried out between 1050 and 1150°C, when the ferrite content is not very sensitive to the subsequent cooling rate The duplex steels are stronger than the simple austenitic steels, partly as a result of the two-phase structure and also because this also leads normally to a refinement of the grain size. Indeed, by suitable thermomechanical treatment between 900°C and 1000°C, it is possible to obtain very fine microduplex structures which can exhibit superplasticity, i.e. very high ductilities at high temperatures, for strain rates less than a critical value.

A further advantage is that duplex stainless steels are resistant to solidification cracking, particularly that associated with welding. While the presence of δ-ferrite may have an adverse effect on corrosion resistance in some circumstances, it does improve the resistance of the steel to transgranular stress corrosion cracking as the ferrite phase is immune to this type of failure.

There is another important group of stainless steels which are essentially ferritic in structure. They contain between 17 and 30% chromium and, by dispensing with the austenite stabilizing element nickel, possess considerable economic advantage. These steels, particularly at the higher chromium levels, have excellent corrosion resistance in many environments and are completely free from stress corrosion.

These steels do have some limitations, particularly those with higher chromium contents, where there can be a marked tendency to embrittlement. This arises partly from the interstitial elements carbon and nitrogen, e.g. a 25% Cr steel will normally be brittle at room temperature if the carbon content exceeds 0.03%. An additional factor is that the absence of a phase change makes it more difficult to refine the ferrite grain size, which can become very coarse after high temperature treatment such as welding. This raise still furthers the ductile/brittle transition temperature, already high as a result of the presence of interstitial elements. Fortunately, modern steel making methods such as argon-oxygen refining can bring the interstitial contents below 0.03%, while electron beam vacuum melting can do better still.