Austenitic and Ferritic Stainless Steels in Practical Applications: Part Two

The ferritic stainless steels are somewhat stronger than austenitic stainless steels, the yield stresses being in the range 300-400 MPa, but they work harden less so the tensile strengths are similar, being between 500 and 600 MPa. However, ferritic stainless steels, in general, are not as readily deep drawn as austenitic alloys because of the overall lower ductility. However, they are suitable for other deformation processes such as spinning and cold forging.

Welding causes problems due to excessive grain growth in the heat affected zone but, recently, new low-interstitial alloys containing titanium or niobium have been shown to be readily weldable. The higher chromium ferritic alloys have excellent corrosion resistance, particularly if 1-2% molybdenum is present.

Finally, there are two phenomena which may adversely affect the behavior of ferritic stainless steels. Firstly, chromium-rich ferrites when heated between 400 and 500°C develop a type of embrittlement, the origins of which are still in doubt.

The most likely cause is the precipitation of a very fine coherent chromium-rich phase arising from the miscibility gap in the Fe-Cr system, probably by a spinodal type of decomposition. This phenomenon becomes more pronounced with increasing chromium content, as does a second phenomenon, the formation of sigma phase. The latter phase occurs more readily in chromium-rich ferrite than in austenite, and can be detected below 600°C. As in austenite, the presence of sigma phase can lead to marked embrittlement.

Some austenitic steels are often close to transformation, in that the M_s temperature may be just below room temperature. This is certainly true for low-carbon 18Cr8Ni austenitic steel, which can undergo a martensitic transformation by cooling in liquid nitrogen or by less severe refrigeration. The application of plastic deformation at room temperature can also lead to formation of martensite in metastable austenitic steels, a transformation of particular significance when working operations are contemplated.

In general, the higher the alloying element content the lower the M_s and M_d temperatures, and it is possible to obtain an approximate M_s temperature using empirical equations. Useful data concerning the M_d temperature are also available in which an arbitrary amount of deformation has to be specified. The martensite formed in Cr-Ni austenitic steels either by refrigeration or by plastic deformation is similar to that obtained in related steels possessing an M_s above room temperature.

Manganese can be substituted for nickel in austenitic steels, but the Cr-Mn solid solution then has much lower stacking fault energy. This means that the fee solid solution is energetically closer to an alternative close-packed hexagonal structure, and that the dislocations will tend to dissociate to form broader stacking faults than is the case with Cr-Ni austenites. Manganese on its own can stabilize austenite at room temperature provided sufficient carbon is in solid solution. The best example of this type of alloy is the Hadfields manganese steel with 12 % Mn, 1.2 % carbon which exists in the austenitic condition at room temperature and even after extensive deformation does not form martensite.

However, if the carbon content is lowered to 0.8%, then M_d is above room temperature and transformation is possible in the absence of deformation at 77°K. Both ε and α’ martensites have been detected in manganese steels. Alloys of the Hadfields type have long been used in wear resistance applications, e.g. grinding balls, railway points, excavating shovels, and it has often been assumed that partial transformation to martensite was responsible for the excellent wear resistance and toughness required. However, it is likely that the very substantial work hardening characteristics of the austenitic matrix are more significant in this case.

In general, fee metals exhibit higher work hardening rates than bee metals because of the more stable dislocation interactions possible in the fee structure. This results in the broad distinction between the higher work hardening of austenitic steels and the lower rate of ferritic steels, particularly well exemplified by a comparison of ferritic stainless steels with austenitic stainless steels.

The advantages obtainable from the easily fabricated austenitic steels led naturally to the development of controlled transformation stainless steels, where the required high strength level was obtained after fabrication, either by use of refrigeration to take the steel below its M_s temperature, or by low temperature heat treatment to destabilize the austenite. Clearly the M_s - M_f range has to be adjusted by alloying so that the M_s is just below room temperature. The M_r is normally about 120°C lower, so that refrigeration in the range -75 to -120°C should result in almost complete transformation to martensite.

Alternatively, heat treatment of the austenite can be carried out at 700°C to allow precipitation of M₂₃C₆ mainly at the grain boundaries. This reduces the carbon content of the matrix and raises the M_s so that, on subsequent cooling to room temperature, the austenite will transform to martensite. Further heat treatment is then necessary to give improved ductility and a high proof stress; this is achieved by tempering in the range 400-450°C.

Another group of steels has been developed to exploit the properties obtained when the martensite reaction occurs during low temperature plastic deformation. These steels, which are called transformation induced plasticity (TRIP) steels, exhibit the expected increases in work hardening rate and a marked increase in uniform ductility prior to necking. Essentially the principle is the same as that employed in controlled transformation steels, but plastic deformation is used to form martensite and the approach is broader as far as the thermomechanical treatment is concerned.

In one process, the composition of the steel is balanced to produce an M_d temperature above room temperature. The steel is then heavily deformed (80%) above the Md temperature, usually in the range 250-550°C, which results in austenite which remains stable at room temperature. Subsequent tensile testing at room temperature gives high strength levels combined with extensive ductility as a direct result of the martensitic transformation which takes place during the test.

For example, a steel containing 0.3% C, 2% Mn, 2% Si, 9% Cr, 8.5% Ni, 4% Mo after 80% reduction at 475°C gives the following properties at room temperature:

Higher strength levels (proof stress ~2000 MNm²) with ductilities between 20-25% can be obtained by adding strong carbide forming elements such as vanadium and titanium, and by causing the M_d temperature to be below room temperature. As in the earlier treatment, severe thermomechanical treatments in the range 250-550°C are then used to deform the austenite and dispersion strengthen it with fine alloy carbides. The M_d temperature is, as a result, raised to above room temperature so that, on mechanical testing, transformation to martensite takes place, giving excellent combinations of strength and ductility as well as substantial improvements in fracture toughness.