Ferritic Stainless Steels
Ferritic Stainless Steels
Ferritic stainless steels are high chromium (Cr), magnetic stainless steels that have low carbon (C) content. The chromium content of these stainless steels varies from 10.5 % to 29 % taking into account a very wide range of applications. Ferritic stainless steels can also contain other elements such as molybdenum (Mo), titanium (Ti), aluminum (Al), and niobium (Nb) etc. It is a cost saving material since most of the grades do not have expensive nickel (Ni) additions. Their market share has grown in the recent past and they represent already about 30 % of total global stainless steel production.
In comparison to austenitic stainless steels, which have a face-centered cubic (fcc) grain structure, ferritic stainless steels are defined by a body-centered cubic (bcc) grain structure. In other words, the crystal structure of such steels is comprised of a cubic atom cell with an atom in the center. This crystal structure is the same as that of pure iron (? iron) at room temperature.
Although the Schaeffler diagram (Fig. 1) is mainly used for welded structures, it is very useful to illustrate the different areas of stability of stainless steel microstructures.
Fig 1 Schaeffler diagram
The ferritic grades
Ferritic grades are usually classified into five groups (Fig 2) consisting of three families of standard grades and two of ‘special’ grades. By far the greatest current use of ferritic stainless steels, both in terms of tonnage and number of applications, is centered on the standard grades.
Fig 2 – Classification of ferritic stainless steel grades
- Group 1 – These have the lowest chromium content (10 % to 14 %, grades 409 and 410L) of all stainless steels and are ideal for slightly corrosive environments where localized rust is acceptable. These stainless steels have no Ni and extra low interstitial elements (carbon and nitrogen) and represents a fully ferritic structure at all the temperatures. The mechanical properties of group 1 ferritic stainless steels are closely related to the carbon and nitrogen contents. These grades with sufficient ductility can only be produced by an optimum heat treatment and a stringent control of chemistry including interstitial elements (carbon/nitrogen) or in the fully annealed condition. This group can be ideal for non corrosive or lightly corrosive environments or applications where slight localized rust is acceptable. Type 409 was originally designed for automotive exhaust system silencers (exterior parts in non-severe corrosive environments). Type 410L is often used for containers, buses and coaches and, recently, LCD monitors frames.
- Group 2 – This is the most widely used family of ferritic stainless steels and has 14 % to 18 % Cr (grade 430). Most of the industrial grades have Cr content in the range of 16 % to 18%. Their typical composition, by weight, is 16 % -18 % Cr and < 0.08 % C. In order to increase the ductility, the actual carbon content is often much lower, typically in the range 0.02 % to 0.05 %. Nitrogen is generally of the order of 0.030 %, but can be significantly reduced. Since this stainless steel has a higher chromium content and is therefore, more resistant to corrosion by nitric acids, sulphur gases and many organic and food acids. In some applications, this grade can be used as a replacement for austenitic grade 304. Grade 430 is often found in interior appliances, including washing machine drums, kitchen sinks, cutlery, indoor decorative panels, dishwashers, pots, pans and other cooking utensils. 16 % -18 Cr ferritic grades are known to present potentially brittle microstructures when welded. This is explained by the combined negative effects of grain coarsening at very high temperature in the HAZ close to the fusion line, possible martensitic transformation in the austenitized areas and/or intergranular carbide precipitations.
- Group 3 – These steels have 14 % to 18 % Cr along with stabilization elements as Ti, Nb, and Zr. These steels include types 430Ti, 439, and 441 etc. During solidification and cooling, Ti, Nb, Zr additions in steels ties up carbon and/or nitrogen in the form of highly stable compounds. Carbides and nitrides are precipitated leaving the ferritic structure with much lower carbon / nitrogen contents in solid-solution. As a result, the 16 % to 18 % Cr stabilized grade often has a fully ferritic microstructure at all temperatures. The amount and nature of stabilization elements can be optimized taking into account the in-service properties. Specific improvements in functional properties such as drawability, pitting corrosion resistance, high temperature strength, creep resistance, can be achieved by adding the appropriate alloying elements and selection of one or more stabilization elements. Typically, stability of the carbides increases from NbC, TiC to ZrC, the latter being extremely stable at high temperature. Mixed TiC/NbC is preferred for pitting corrosion resistance. The NbC compound is the carbide of choice in order to obtain creep resistance properties. The minimum amount of Ti or Nb is generally included in a range of 6 to 8 times the content of C+N. Of course the C+N content is optimized for specific applications. For room temperature applications carbon content is typically kept at the lowest possible level so that the amount of expensive Ti, Nb can be reduced and a fully stabilized microstructure still be maintained. Ti and Nb are the most popular stabilization elements. They have strong affinities with other residual elements such as oxygen and sulphur and act as intrinsic ferrite forming elements of the steel microstructure. As a major consequence of this, the steel is fully ferritic at all temperatures and Cr-carbide precipitations are inhibited, particularly in the HAZ (prevention of intergranular corrosion along depleted Cr areas). Compared with Group 2, these grades show better weldability and formability than 430 grades. Their behaviour is, in most cases, equivalent to that of 304 austenitic grades. Typical applications include sinks, heat exchanger tubes (the sugar industry, energy, etc.), and exhaust systems (longer life than with type 409) and the welded parts of washing machines. Group 3 grades can even replace type 304 in applications where this grade is over specified. The best in-service wet corrosion resistance properties are observed for the highest Cr content (17 % to18 % Cr) and a mixed Nb / Ti stabilization effect.
- Group 4 – This group includes grades 434, 436, and 444 Etc. These ferritic stainless steels contain 10 % to 18 % Cr and molybdenum (Mo) content higher than 0.5 %. These steels are Mo alloyed, for extra corrosion resistance. Cr content is mainly in the range of 17 % to 18%. Due to the increase of ferrite forming elements Mo, these grades present a fully ferritic microstructure and most of them are fully stabilized by Ti and/or Nb additions. The grades are also more sensitive to intermetallic phase precipitations (?, ?) when heated to high temperatures. Brittle behaviour can occur if improperly heat treated or after long term use at high temperatures. However, since Cr content is kept at a relatively low level, those grades show satisfactory structural stability and welding properties. Typical applications include hot water tanks, solar water heaters, visible parts of exhaust systems, electric kettle and microwave oven elements, automotive trim and outdoor panels, etc. The corrosion resistance (PRE, pitting resistance equivalent) of grade 444 can be similar to that of type 316, which allows it to be used in more corrosive outdoor environment.
- Group 5 – In this group of ferritic stainless steels Cr content is higher than 18 %. This group includes grades 446, 445, and 447 etc. These grades traditionally have Mo additions, for extra wet corrosion resistance. Having most often 25 % to 29 % Cr and 3 % Mo, these grades are superior to grade 316 with respect to this property. They are very sensitive to embrittlement due to intermetallic phase precipitations and are very difficult to weld. Their uses are restricted to thin gauges (mainly below 2 mm). Extra low carbon and nitrogen are required to ensure sufficient structure stability. Ni additions are considered (2 % to 4 %) to increase toughness properties. Nickel has controversial effects since Ni simultaneously reduces the brittle/ductile transition temperature and enhances phase precipitation kinetics which decreases the ductility. The high Cr and Mo containing grades are called superferritics. The new generation of superferritics is designed to have an extra low interstitial content thanks to specific melting procedures. The grades are designed to replace titanium in the most severe corrosion resistance applications (including nuclear power station condensers and seawater exchanger tubes, geothermal, desalination etc.). Only marginal worldwide production quantities are reported. More recently, a new family of ferritic grades has been developed. They are designed to replace grade 304 and generally contain about 20 % Cr. Since they are Mo free, they can be considered as the best alternative to Ni and Mo price volatility. For corrosion resistance properties and weldability the grades are fully stabilized by mixed Ti/Nb/Cu additions. The grades present attractive properties for an extremely wide range of applications. Group 5 also contains a family of grades developed for exhaust applications, including grades containing exotic additions such as high Al (2 % to 5 %) and Ce etc. but also a 19Cr-2Mo-Nb grade designed for high temperature applications. Due to its high resistance to scaling, this grade is particularly well designed for exhaust manifold applications.
Properties of ferritic stainless steels
The most obvious difference of properties between ferritic stainless steels and austenitic stainless steels is their ferromagnetic behavior at room temperature and up to a critical temperature known as the Curie point, temperature typically in the range of 650 deg C to 750 deg C at which the magnetic order disappears. The magnetism of ferritic grades is one of the material’s major assets in some applications.
Ferritic stainless steels lower thermal expansion coefficient combined with their improved thermal conductivity often provides a key advantage to ferritic stainless steel over austenitic stainless steel when considering applications involving heat transfer.
Ferritic stainless steels have good mechanical properties, sitting broadly in the middle between the other stainless steel families. They typically have higher yield strength than austenitic stainless steels. Elongation and forming properties are equivalent to those of high strength carbon steels
Ferritic stainless steels have generally lower elongation and strain hardening properties than stainless steel. Ferritic stainless steels in the annealed state present a yield point followed by a stress drop on the stress/strain curves.
Ferritic stainless steels exhibit a non-uniform texture which leads to heterogeneous mechanical behaviour. Phenomena such as ‘earing’ as well as ‘roping’ (sometimes called ‘ridging’”) are observed.
Deep drawing performance is determined by the limit drawing ratio (LDR), which is well correlated with the mean strain ratio. Ferritic stainless steels have higher LDR values than austenitic stainless steels, which makes them particularly suitable for deep drawing applications.
Stabilization (by Ti, Nb addition etc.) of ferritic stainless steel induces a significant modification in the crystalline texture leading to a sharp improvement of the strain ratio. Improved LDR values are observed. In comparison to austenitic stainless steels, the ferritic stainless steels are generally considered to resist better to stress corrosion cracking in chloride containing environments. However, their resistance is not limitless. Their cracking potential is generally higher than the free corrosion potential.
Despite their lower mechanical properties at high temperature compared to those of austenitic grades, ferritic grades exhibit a better resistance to the cyclic oxidation and thermal fatigue and present lower coefficients of thermal expansion. Niobium addition improves high temperature mechanical properties significantly; an addition of half a percent or more of molybdenum allows reaching a good resistance in severe internal or external corrosion conditions. Consequently, ferritic grades are well adapted to exhaust system applications.
Modern ferritic stainless steels are readily weldable with conventional welding methods, including shielded metal arc welding, gas tungsten arc welding, gas metal arc welding, plasma arc welding, laser welding, resistance welding and high resistance welding.