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Ferritic Stainless Steels

• satyendra
• May 4, 2014
• 1 Comment
• Alpha phase, austenitic, chromium, crevice corrosion, Delta phase, ferritic, ferro magnetic, LDR, Molybdenum, nickel, pitting corrosion, PREN, Schaeffer diagram, stainless steel,

Ferritic Stainless Steels

Stainless steels are ‘stainless’ since their chromium (Cr) content (minimum 10.5 %) gives them remarkable resistance to wet corrosion and high temperature oxidation. There are several different grades of stainless-steels meeting a wide range of strength, weldability, and durability requirements, and these can be categorized into five groups namely austenitic, ferritic, martensitic, duplex, and precipitation-hardening grades. Chromium content of austenitic grades is normally kept in the 17 % to 18 % range because of austenitic phase stability considerations. Lower or increased chromium content in AISI (American Iron and Steel Institute) 300-series austenitic grades needs further increase of expensive nickel (Ni) to stabilize the austenitic phase.

Ferritic stainless-steels are a family of utility stainless steels which considerable offer better atmospheric corrosion resistance than carbon steels, as well as having good ductility, formability and impact resistance. Ferritic grades, containing only chromium and possibly other elements such as molybdenum (Mo), titanium (Ti), aluminum (Al), and niobium (Nb) etc., are well known as cost savings materials since majority of them have no expensive nickel additions. Also, the chromium content can be optimized ranging from 10.5 % to 29 % taking into account a very wide range of applications.

Ferritic stainless steels e.g., grades AISI 409, and AISI 430 consist of chromium (typically 12.5 % or 17 %) and iron (Fe). Ferritic stainless steels are essentially nickel-free. These materials contain very little carbon (C) and are non-heat treatable, but show superior corrosion resistance to martensitic stainless steels and possess good resistance to oxidation. They are ferro-magnetic and, although subject to an impact transition (i.e., become brittle) at low temperatures, possess adequate formability. Their thermal expansion and other thermal properties are similar to conventional steels. Ferritic stainless steels are readily welded in thin sections, but suffer grain growth with consequential loss of properties when welded in thicker sections.

Ferritic stainless steels have been used for a range of applications including vehicle frames / chassis, railway wagons, conveyors, chutes, tanks, power generation, mining, water, and walkways in industries such as road and rail transport. These steels are very successfully used in important applications, such as washing-machine drums and exhaust systems. The market share of the ferritic stainless steels has grown in the recent past and these steels represent already around 30 % of total global stainless-steel production. Key advantages of ferritic stainless steels include (i) good atmospheric corrosion resistance, (ii) higher yield strength relative to carbon steel and austenitic stainless steels, (iii) less non-linear yielding behaviour compared with the austenitic grades, (iv) lower cost than other grades of stainless steel of equivalent corrosion resistance, and (v) easier to roll form and achieve flatness, and less weld distortion, compared to austenitic stainless steels.

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 centre. This crystal structure is the same as that of pure iron (alpha iron) at room temperature.

There is increased interest of ferritic stainless steels in structural application since they share several of the attractive properties of austenitic and duplex materials such as strength and durability but are cheaper alloys containing little, or no, nickel. Used in the appropriate applications, ferritic stainless-steels can offer a very competitive and economical solution.

Despite being widely used in the automotive and domestic appliance sectors, structural applications of ferritic stainless steels are scarce owing to a lack of knowledge, performance data, and design guidance. They have been specified for cladding and roofing applications as well as in the transportation sector for load-bearing members, for example for tubular bus frames. They have also been used for a range of structural applications which include (i) structural steelwork for shaft supports in mines (especially deep mines having very aggressive conditions, including high levels of chlorides and temperatures up to 50 deg C), (ii) railway electrification masts along the railway line especially those rail lines which run along the coast and are constantly exposed to sea spray during windy conditions, and (iii) tubular piles and support framework for the building, which are situated in a very corrosive environment.

Standard ferritic grades such as AISI 409, AISI 410, and AISI 430 are readily available all over the world. They actually have much broader application potential, in several fields. More recently developed ferritic grades, such as AISI 439 and AISI 441 meet an even wider range of requirements. They can be formed into more complex shapes and joined using most conventional joining methods, including welding. In material selection decisions, these stainless-steel grades are frequently weighed against AISI 304 austenitic grades.

The addition of molybdenum improves the resistance of ferritic stainless steels to localized corrosion (AISI 434, and AISI 436). Grade AISI 444 is even considered at least equal to austenitic grade AISI 316 in the majority of the cases when considering corrosion resistance properties. Super-ferritic stainless-steel grades have also been developed since several years. Their very high chromium content (25 % to 29 %) with additional nickel and molybdenum alloying makes them well-known highly corrosion resistant steels although restricted to marginal applications. This is because of their high sensitivity to embrittling phase transitions.

Recently, newly developed ferritic stainless-steel grades with the aim to replace AISI 304 austenitic grades have been introduced into the market. Their chromium content lies in the range of 20 % to 22 % and they are free of expensive nickel or molybdenum additions. The grades are stabilized by minor additions of titanium, niobium, and copper. The recent volatility of nickel has brought the AISI 400 series under the spotlights. Fig 1 shows family relationships for standard ferritic stainless steels.

Fig 1 Family relationships for standard ferritic stainless steels

Ferritic stainless steels are iron-chromium (Fe-Cr) based alloy steels which are used for their anti-corrosion properties rather than for their mechanical properties (strength, ductility, or toughness). However, these properties are still considered when ‘stress corrosion cracking’ (SCC) is taken into account and for which ferritic stainless-steel grades are considered superior to the austenitic grades.

Formerly, ferritic stainless steels have been used in non-welded constructions (riveted and bolted assemblies etc.). The first generation (those with high chromium) have been supposed to be non-weldable, particularly for thicker sections which have a high propensity to cold cracking and to a drastic drop in ductility in the as-welded condition. The low weldability of these alloy steels is mainly because of a high chromium content and to the difficulty related in getting alloy steels with a low level of impurities. The development of the AOD (argon oxygen decarburization) process in the 1960s has enabled the production of high purity iron-chromium alloy steels containing negligible traces of carbon. Since then, the development of refined grades in thinner sections has led both to a considerable gain in weight and structures maintaining their good integrity during the welding process.

Ferritic stainless steels are used in several fields and applications such as elements and accessories for kitchens and bathrooms, roofs, architectural objects, appliances, safeguards, metallic doors, elevators, and storage tanks etc. Because of their good thermal conductivity and low thermal expansion these alloy steels are also used in the manufacturing of chimneys, mufflers, exhaust systems, and fasteners as well as heating elements used in molten salt baths for heat treatments etc.

Compared to austenitic stainless steels, the low coefficient of thermal expansion (CTE) of the ferritic stainless steel explains the low tendency to scaling of the refractory oxide layer formed at the surface. This feature ensures the good thermal stability of these alloy steels. However, it is important to outline that ferritic stainless steels have a designed temperature normally limited to 400 deg C maximum in order to prevent the fragilization of the ferritic micro-structure which occurs roughly at 475 deg C, mainly in the high chromium grades.

Classes (generations) of ferritic stainless steels – The evolution of ferritic stainless steels has been through three main steps or generations. This progress has been aimed at the development of a fully ferritic single-phase micro-structure which has a considerable corrosion resistance and a good weld integrity in the as-welded condition. The development of these properties has been achieved through (i) a gradual increase of chromium content, (ii) the addition of elements such as niobium, titanium, and aluminum which have a high chemical affinity for the residual interstitial elements such as carbon, nitrogen (N2), or oxygen (O2), and (iii) the addition of other ferrite stabilizing elements such as molybdenum and silicon (Si).

The first generation includes mainly the AISI grades 430, 434, 436, 442 and 446. Besides their high chromium content, those stainless-steels containing 0.12 % to 0.20 % of carbon are vulnerable to a drop in ductility and to corrosion because of the formation of martensite (quenched structure) and inter-metallic precipitates in the ferritic phase in the as-welded condition.

The second generation includes low carbon alloy steels developed from the modification of the first-generation grades. They contain other ferrite forming elements like titanium, niobium, and aluminum. In addition to promoting the ferritic micro-structure, aluminum also improves the oxidation resistance at high temperatures by stabilizing the chromium oxide layer. This generation includes the AISI grades 405, 409, 409Cb, and 439 etc. The weldability of these alloy steels is noticeably better than the one of the first-generation grades.

The third generation is that of the recent alloy steels called super-ferritic grades. The forming process of these alloys experiences some difficulties related to the high chromium levels. These are used for applications subjected to severe corrosion conditions such as in chloride environments. The AISI grades 444, 29-4, and 29-4-2 are the most typical ones in this class. These consist of high purity alloy steels where the interstitial elements, carbon, and nitrogen in particular, are reduced to a minimum in order to improve the ductility of the alloy steel. Among the alloying elements is molybdenum which increases the localized corrosion resistance in addition to its ferritizing effect.

Micro-structure – Ferritic stainless steels have a ferritic single-phase structure which is stable at any temperature. Chromium remains the main alloying element which ensures the stainless character to these ferritic alloy steels. The content of other elements such as nickel and manganese remain marginal while carbon and nitrogen traces are reduced as much as possible.

The delta ferrite phase forms at high temperatures from the liquid, once crystallized, it does no longer transform upon cooling. This fully ferritic micro-structure is hence not hardenable by any heat treatment. This lack of phase transformation explains the tendency of these alloy steels to grain coarsening of the micro-structure upon heating.

Weldability – The different problems inherent to the welding of ferritic stainless steels are mainly cold cracking and loss of ductility, hot embrittlement and hot cracking, and deterioration of corrosion resistance.

Cold cracking and loss of ductility are largely related to the formation of martensite, a hard and brittle phase, between the ferritic grains upon cooling. The phenomenon of cold cracking depends on different factors such as content of interstitial elements (carbon, nitrogen) present in the alloy steel, heat or welding conditions and also of the post-weld heat treatment (PWHT). Indeed, the reduction of the interstitial elements aims both to the improvement of the inter-granular corrosion resistance and to the prevention of the martensite formation upon cooling.

When considering the stress exerted on the welded joint, its effects are relatively low because of the low coefficient of thermal expansion of the ferritic structure on one hand and to the fact that the base metal is normally shaped into thin sections on the other hand. Concerning the effect of hydrogen, its high diffusivity in the ferritic structure implies a preheat and a post-high heat treatment. Taking this into account, sometimes the use of some austenitic filler materials is desired for welding thick or critical joints to prevent these steels from cold cracking.

Hot cracking phenomenon depends particularly on the purity of the alloy steel and the size of micro-structures grains. The purity is related to the content of undesirable residual elements sulphur, phosphorus, and carbon etc. which segregate at the last stage of solidification to form low melting eutectic compounds (a eutectic compound has a specific analysis and the lowest melting point for a mixture these elements). Also, the presence of these compounds serves as starters for hot tearing, which occurs mainly during the contraction of the alloy steel at the last stage of solidification. However, because of their primary ferritic solidification mode, hot cracking is less critical in the ferritic stainless steels than in the austenitic stainless steels because the predominant phase, delta ferrite, is less sensitive to the effect of impurities factor since it can, among others, dissolve sulphur up to a limit of 0.14 %.

The other controlling is the sensitivity of ferritic grains to coarsening under the effect of heat. The more the micro-structure is coarse, the less are the number of grain boundaries, and the longer they are. The segregation of residual impurities at grain boundaries renders them in a liquid or a pasty state at the final stage of solidification. As a result of that, the already solidified grains are prone to slide on each other from the weak plans before causing internal tearing in the micro-structure. Hence, overheating and maintaining the structure of the alloy steel at high temperatures is to be avoided as much as possible. In this regard, a moderate preheat at 150 deg C to 200 deg C helps to reduce the heat input and the holding time at high temperatures during the arc time. In addition, the good thermal conductivity of ferritic stainless steels is another advantageous factor which accelerates the cooling rate, refines the micro-structure, and reduces the risk of hot cracking.

Hot embrittlement consists of different embrittlement modes (loss of ductility) and is associated to the ferritic stainless steels whose three main forms are described below.

Embrittlement at 475 deg C phenomenon is proportional to the chromium content and occurs in the temperature range of 425 deg C to 550 deg C. It is the result of the decomposition of the ferrite into two phases namely a rich-chromium phase (alpha’) and a rich-iron phase (alpha). The more the alloy steel is rich in chromium and ferritizing elements (molybdenum, niobium etc.), as in the alloy steels of the third generation, the faster is the reaction. The results of this reaction consist necessarily in a selective corrosion of the rich-iron phase (alpha). In order to prevent this form of embrittlement, the service temperatures of the part are always to be limited to 400 deg C maximum.

Precipitation of embrittling phases and inter-granular corrosion result because of sigma phase and chi phase. Like for the embrittlement at 475 deg C, sigma phase (a chromium-rich and very brittle phase) is the product of transformation of the ferritic delta phase occurring when the micro-structure is subject to a long exposure time in the temperature range of 540 deg C to 870 deg C. This new phase forms rapidly in high chromium steel grades and molybdenum is also a promoting element for this transformation. The presence of this new phase alters both the corrosion resistance and the ductility of weld joints. Dissolving this phase needs a solutionizing heat treatment at a temperature of around 870 deg C followed by water quenching. Chi phase forms predominantly in molybdenum bearing grades, mainly those of the third generation such as 29-4 (29Cr-4Mo) or 29-4-2 (29Cr-4Mo-2Ni). This phase, stable up to 900 deg C, forms mutually with sigma phase.

High temperature embrittlement is because of the precipitation of inter-metallic phases (carbides, nitrides) at the grain boundaries at temperatures of roughly of 0.7 Tm (Tm is the melting temperature of the alloy steel). Influential factors consist of residual elements or impurities (carbon, nitrogen, and oxygen), grain size, and the chromium content. For this matter, cast alloy steels which frequently contain high levels of carbon are not desired. Titanium or niobium stabilized grades or aluminum bearing grades (denitrifying element) hinder both the grains coarsening and the inter-granular precipitation. Inter-granular precipitation embrittles locally the grain boundaries and alters the ductility of the whole structure. In order to restore the ductility and the corrosion resistance of the steel weld, one is to dissolve these precipitates by a solutionizing heat treatment to 750 deg C to 950 deg C followed by rapid water cooling.

Grain coarsening takes place when the grains of a ferritic structure start to grow from 1,000 deg C to 1,100 deg C since the micro-structure does not undergo any phase transformation. The phenomenon speeds up when the steel is in the cold hardened condition and it favours concurrently the sensitization to inter-granular corrosion of the micro-structure. The grain size and the content in interstitial elements have a combined effect in the loss of ductility. Stabilized grades (titanium or niobium) show a better resistance to grains coarsening than the non-stabilized grades.

Although the Schaeffler diagram (Fig 2) is mainly used for welded structures, it is very useful to show the different areas of stability of stainless-steel micro-structures. The classical austenitic grades (the so called AISI 300 series) contain normally 8 % to 10 % nickel while the more (chromium and molybdenum) alloyed steel grades need even more nickel to stabilize the austenitic phase. The most popular stainless steel, AISI 304, is one of the lowest alloyed steel grades of the austenitic area (not including nitrogen alloyed steel grades). AISI 316 grade having 2 % molybdenum content is considered as the standard alloyed austenitic stainless steel for corrosion resistance properties. With the extreme volatility of alloying element costs, new steel grades have recently been introduced. These steel grades are also austenitic grades, but with partial replacement of nickel by combined additions of manganese (Mn) and nitrogen.

Fig 2 Schaeffler diagram

The ferritic stainless-steel grades – When classifying ferritic stainless steels by their carbon content, the main information relies on iron-chromium equilibrium diagram. By the present-day standards, this kind of the classification is outdated. Modern ferritic stainless steels vary with their interstitial content, formation of martensite, and stabilization attributes. International Stainless-Steel Forum (ISSF) describes ferritic stainless steels by ferritic families or groups

ISSF has classified ferritic stainless-steel grades into five groups, three groups of standard grades and two groups of ‘special’ grades. By far the highest current use of ferritic stainless steels, both in terms of tonnage and number of applications, is centered around the standard grades (Fig 3).

Fig 3 International Stainless-Steel Forum (ISSF) classification of ferritic grades

Group 1 – This group consists of 10 % chromium to 14 % chromium (AISI 409 / AISI 410L types) ferritic stainless steels. It has the lowest chromium content of all stainless steels. When studying the effects of chromium, nickel, and carbon / nitrogen alloying on phase stability, it is clearly seen that the stable austenite domain (gamma loop) which is observed around 1,000 deg C to 1,200 deg C is extended by nickel or carbon (or nitrogen) additions while chromium additions stabilize the ferritic phase. As a result, stainless steel with a minimum of 13 % chromium, no nickel, and extra low interstitial elements (carbon / nitrogen) can present a fully ferritic structure at all temperatures.

When reducing chromium and / or increasing carbon + nitrogen, the steel grade, when heated, undergoes a ferrite / austenite transformation. Grain refining treatments can be performed and the grades having a stable austenitic loop can undergo martensitic transformation when quenched to room temperature. Several studies have been made on the influence of alloying elements on the Ms temperature. In the case of 12 % chromium steels, Tab 1 gives the change in Ms temperature per percent of element added, the value for the base alloy steel being 300 deg C. Tab 1 also gives the effect of alloying elements on the Ac1 temperature (temperature at which the austenite starts to form on heating). Carbon and nitrogen appear to have no significant effect on Ac1 temperature in 12 % chromium steel grades.

The mechanical properties of 12 % to-13 % chromium alloy steels are closely related to the carbon and nitrogen contents. This is particularly the case for quenched products from the gamma loop. Hardness clearly increases with carbon content. Hardness is even higher than that of carbon-manganese steels with the same quantity of carbon because of simultaneous chromium solid-solution hardening effects and lower Ms temperature which reduce the self-tempering effects. Higher quenching temperatures make it possible to further increase the hardness by improving the dissolution of carbides which further contributes to increase the carbon content in solid solution. At higher quenching temperatures, beyond 1,150 deg C, the hardness can fall because of the formation of delta ferrite and for the highest carbon content steel grade, the presence of retained austenite.

Obviously, ferritic stainless steels of 12 % to 14 % chromium 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. AISI 409 type has been originally designed for automotive exhaust system silencers (exterior parts in non-severe corrosive environments). AISI 410L type is frequently used for containers, buses and coaches, and recently for the LCD (liquid crystal display) monitors frames. Fig 4a shows hardness values achieved on austenitized 13 % chromium samples, oil quenched at 0 deg C and stress relieved at 200 deg C.

Fig 4 Effects of carbon content on mechanical properties and Fe-Cr-C phase diagram

Group 2 – This group consists of 14 % chromium to 18 % chromium (type AISI 430 type) ferritic stainless steels. It is the most widely used group of ferritic stainless steels. Majority of the industrial grades have between 16 % chromium and 18 % chromium. AISI 430 is the most widely used ferritic stainless steel. Its typical composition is 16 % to 18 % chromium, and less than 0.08 % carbon. In order to increase the ductility, the actual carbon content, particularly in the case of thin sheet, is frequently much lower, typically in the range 0.02 % to 0.05 %. Nitrogen is normally of the order of 0.03 %, but can be considerably reduced. Fig 4b shows the influence of combined carbon and nitrogen contents on the iron-chromium equilibrium diagram. It can be seen that for normal (carbon + nitrogen) values, (typically 0.08 %), the high temperature structure consists of two phases (austenite + ferrite), the maximum level of austenite being achieved at around 1,100 deg C.

Hence, after fast cooling to room temperature from the high temperature mixed austenite / ferrite region, the micro-structure of the stainless-steel transforms into a mixed ferrite / martensite micro-structure. It is necessary to temper anneal the martensite to restore the ductility. Tempering can be carried out at a temperature below Ac1 temperature. Final heat treatment is closely linked to the chemistry of the grade. The final micro-structure normally presents a mixed ferrite / carbides micro-structure. The highest density of carbides being related to carbon content and the former austenitic grains enriched in carbon when heat treated occurs in the duplex ferrite / austenite region. The carbon enrichment of the austenite against ferrite results from higher carbon solubility in austenite against ferrite.

16 % to 18 % chromium ferritic stainless-steel grades are known to present potentially brittle micro-structures when welded. This is explained by the combined negative effects of grain coarsening at very high temperature in the HAZ (heat affected zone) close to the fusion line, possible martensitic transformation in the austenitized areas and / or inter-granular carbide precipitations. Having a higher chromium content, group 2 grades show higher resistance to corrosion and behave more like austenitic steel grade AISI 304. In situations, where corrosion resistance is less of a concern, these grades are suitable to replace AISI 304 type and are normally sufficiently alloyed for indoor applications. AISI 430 type is frequently substituted for AISI 304 type in household utensils, dish-washers, pots, pans, and decorative panels.

Group 3 – This group consists of ferritic stainless-steels with 14 % chromium to 18 % chromium + stabilizing elements such as titanium, niobium, and zirconium (Zr). It includes AISI 430Ti, AISI 439, and AISI 441 etc. types. During solidification and cooling, titanium, niobium, and zirconium additions in steels tie 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 % chromium stabilized ferritic stainless-steel grade frequently has a fully ferritic micro-structure at all temperatures.

The quantity and nature of stabilization elements can be optimized taking into account the desired in-service properties. Specific improvements in functional properties such as drawability, pitting corrosion resistance, high temperature strength, and 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 (niobium carbide), to TiC (titanium carbide), to ZrC (zirconium carbide), the latter being extremely stable at high temperature. Mixed TiC / NbC are preferred for pitting corrosion resistance. The NbC compound is the carbide of choice in order to achieve the creep resistance properties. The minimum quantity of titanium or niobium is normally in a range of 6 times to 8 times the carbon + nitrogen. Of course, the carbon + nitrogen content is optimized for specific applications. For room temperature applications, carbon content is typically kept at the lowest possible level (taking into account the economic considerations) so that the quantity of expensive titanium, and niobium can be reduced and a fully stabilized micro-structure still be maintained.

Titanium and niobium are the most popular stabilizing elements. They have strong affinities with other residual elements such as oxygen and sulphur and act as intrinsic ferrite forming elements of the steel micro-structure. As a major consequence of this, the steel is fully ferritic at all temperatures and chromium-carbide precipitations are prevented, particularly in the HAZ area (prevention of inter-granular corrosion along depleted chromium areas). Also, the nature of inclusions (oxides, nitrides, and sulphides) and precipitations (carbides, carbo-nitrides, phosphides, and inter-metallic phases etc.) is different from that of the basic non-stabilized 17 % chromium stainless- steel.

Compared with group 2 steel grades, these grades show better weldability and formability than AISI 430 grade. Their behaviour, in majority of the cases, is equivalent to that of AISI 304 austenitic grades. Typical applications include sinks, heat exchanger tubes, exhaust systems (longer life than with type AISI 409 grade) and the welded parts of washing machines. Group 3 grades can even replace AISI 304 type in applications where this grade is over-specified. The best in-service wet corrosion resistance properties are observed for the highest chromium content (17 % to 18 % chromium) and a mixed niobium / titanium stabilization effect.

Group 4 – This group consists of 10 % chromium to 18 % chromium and molybdenum content higher than 0.5 % and includes types AISI 434, AISI 436, and AISI 444 etc. These grades are molybdenum alloyed, for extra corrosion resistance. Chromium content is mainly in the range of 17 % to 18 %. Because of the increase of ferrite forming elements (molybdenum), these steel grades present a fully ferritic micro-structure and majority of them are fully stabilized by titanium and / or niobium additions. The grades are also more sensitive to inter-metallic phase precipitations (chi, sigma) when these steels are heated to high temperatures. Brittle behaviour can occur if improperly heat treated or after long period use at high temperatures. However, since chromium content is kept at a relatively low level, these 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 micro-wave oven elements, automotive trim, and outdoor panels etc. The corrosion resistance of AISI 444 type can be similar to that of AISI 316 type.

Group 5 – This group consists of steel grades with chromium content higher than 18 % and not belonging to other groups. It includes AISI 446, AISI 445, and AISI 447 types etc. These steel grades traditionally have molybdenum additions, for extra wet corrosion resistance. Having most frequently 25 % to 29 % chromium and 3 % molybdenum, these steel grades are superior to AISI 316 type with respect to this property. These steels are very sensitive to embrittlement because of the inter-metallic phase precipitations and are very difficult to weld. Their uses are restricted to thin gauges (mainly below 2 mm). Extra low levels of carbon + nitrogen is needed for ensuring sufficient structure stability. Nickel additions are considered (2 % to 4 %) for increasing the toughness properties. Nickel has controversial effects since nickel simultaneously reduces the brittle / ductile transition temperature and improves phase precipitation kinetics which decrease the ductility.

The high chromium and molybdenum containing steel grades are called super-ferritic stainless steels. The new generation of super-ferritic stainless-steels is designed to have an extra low interstitial content because of specific melting procedures. The steel grades are designed to replace titanium in the most severe corrosion resistance applications (including nuclear power station condensers and sea-water exchanger tubes, geo-thermal, and desalination etc.).

More recently, a new family of ferritic stainless-steel grades has been developed. These steels are designed to replace AISI 304 grade and normally contain around 20 % chromium. Since these steels are molybdenum-free, they can be considered as the best alternative to nickel and molybdenum price volatility. For corrosion resistance properties and weldability, the steel grades are fully stabilized by mixed titanium / niobium / copper additions. The steel grades show attractive properties for an extremely wide range of applications.

Group 5 steels also contains a family of grades developed for exhaust applications, including grades containing exotic additions such as high aluminum (2 % to 5 %), cerium (Ce), and yttrium (Y) etc. but also a 19Cr-2Mo-Nb steel grade designed for high temperature applications. Because of its high resistance to scaling, this grade is particularly well designed for exhaust manifold applications.

Physical properties – The most obvious difference between the properties of ferritic and austenitic stainless-steels is their ferro-magnetic behaviour 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.

Magnetism has nothing to do with corrosion resistance which is closely linked to chemical composition (chromium and molybdenum etc.). Moreover, corrosion resistance is almost independent from the micro-structure (not considering the specific case of stress corrosion cracking where ferritic structure is an advantage or crevice corrosion propagation rate where nickel plays a beneficial role). The popular link between magnetism and poor corrosion resistance results from an inappropriate comparison i.e. comparing a ferritic stainless-steel grade having lower chromium content (13 % to 16 %) with the austenitic AISI 304 stainless-steel grade (18 %).

In fact, the magnetism of ferritic stainless-steel grades is one of the major assets of the material in some applications. This includes advantages ranging from the ability to stick memos on the refrigerator door to storing knives and other metallic implements. Indeed, it is also a necessary property for ferritic stainless-steel uses in applications dealing with induction heating such as the familiar pans and other cook-ware for induction cooking. In these applications, magnetic materials are needed to generate heat from magnetic energy.

The lower thermal expansion coefficient of ferritic stainless steels combined with their improved thermal conductivity frequently provides a key advantage to ferritic stainless-steels over austenitic stainless-steels when considering applications involving heat transfer. Typical physical properties of ferritic stainless-steels compared to austenitic stainless-steels are given in Tab 2.

Mechanical properties – Mechanical properties of ferritic stainless-steel grades are given in Tab 3. Ferritic stainless steels have normally lower elongation and strain hardening properties than austenitic stainless steels. As for plain carbon steels, ferritic stainless steels in the annealed state have a yield point followed by a stress drop on the stress / strain curves. This behaviour is caused by the break-away of pinned dislocations and enables a ‘true yield stress’ to be defined. It is accompanied by the formation of localized deformation bands named ‘Piobert-Luders’ bands. As a result, after plastic deformation on annealed samples, surface defects can be observed. In the case of deep drawing, they are called stretcher strains or worms. It can be avoided partially by stabilization or by a skin pass operation which introduce fresh dislocations in the structure. Tab 3 gives typical mechanical properties of ferritic stainless-steel grades.

Ferritic stainless steels show a non-uniform texture which leads to heterogeneous mechanical behaviour. Phenomena such as ‘earing’ as well as ‘roping’ (sometimes called ‘ridging’) are observed. Roping normally occurs during deep drawing and involves the formation of small undulations elongated in the tensile direction. Those defects are to be eliminated during finishing. The stabilized ferritic stainless-steels are less sensitive to roping than the basic AISI 430 grade. In practice, optimization of process parameters makes it possible to considerably weaken this phenomenon.

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 steel, which makes them particularly suitable for deep drawing applications. The main stress ratio can be optimized in ferritic stainless steel by process cycle parameters including slab micro-structure control and cold rolling parameters preceding the final heat treatment.

In industrial practice, for a single cycle cold rolling process, values of 1.8 LDR to 1.9 LDR are achieved for a conventional AISI 430 grade. The LDR can reach values higher than 2.1 for optimized process including a two-step cold rolling process (Fig 5). Stabilization (by titanium, and niobium addition etc.) of ferritic stainless steel induces a considerable modification in the crystalline texture leading to a sharp improvement of the strain ratio and hence, improved LDR values are achieved. The performance regarding pure deep drawing aside, ferritic stainless-steel grades are inferior to austenitic stainless steels in pure stretch forming. ‘Dome height’ refers to the maximum degree of deformation, of a blank undergoing stretching, before ‘necking’. Dome height (K50, in mm) values of ferritic and 304 austenitic stainless-steel grades are shown in Fig 5.

Fig 5 LDR and dome height values of several ferritic and 304 austenitic grades

In practice, industrial forming operations involve a combination of both drawing and stretch forming deformation, in a series of passes or steps. Forming limit curves (FLCs) are a useful guide for assessing maximum deformation before failure, in both deep drawing and stretching processes. These curves define local deformations during and after forming in terms of two principal ‘true strains’ namely longitudinal (major) strain and transverse (minor) strain. The curves plot the effects of the different combinations of these two strains, up to the point of fracture. Typical results achieved for ferritic stainless-steels and AISI 304 steel grade are shown (Fig 6). Ferritic stainless-steels clearly have less combined forming properties than austenitic stainless-steels. For the most severe forming conditions, the switch from austenitic stainless-steels to ferritic stainless-steels can need some design optimization with shape modifications of the most critical areas.

Fig 6 Forming limit curves of ferritic and 304 stainless grades

Wet corrosion resistance properties – Wet corrosion resistance properties are described below.

Localized corrosion resistance – Pitting and crevice corrosion resistances are one of the major issues regarding material selection in aqueous solutions. Pitting corrosion resistance is one of the key properties for material selection in neutral, oxidizing conditions typically observed in halogen (chlorine, and fluorine etc.) containing solutions. Sea-water and brine solutions even with few additions of salt (cooking) are the most common in-service conditions related to pitting corrosion.

Pitting corrosion resistance is clearly linked to the PREN (pitting resistance equivalent number) value (% chromium + 3.3 % molybdenum + 16 % nitrogen). In the case of ferritic stainless-steels nitrogen additions are kept to minimum values in order to avoid nitride precipitations. Only chromium and molybdenum play a positive role. Typical data are shown in Fig 7a (pH 6.6, 0.02 M NaCl, and 23 deg C). Of course, an increase of temperature or salinity reduces the pitting corrosion resistance. No effect of micro-structure, ferritic or austenitic, on the pitting corrosion resistance properties can be observed. Chemical composition and cleanliness are the most important parameters when considering pitting corrosion resistance. Sulphur content, particularly, is to be kept at a very low level to get sufficient pitting corrosion resistance properties.

Fig 7 Pitting and crevice corrosion resistance properties

Crevice corrosion is specific to confined zones, such as under a joint or under deposits. The acidity can increase locally triggering the destruction of the passive film. Test results performed in a 2M NaCl solution at room temperature with different pH values show that depassivation of the stainless-steel grades is directly related to their composition i.e., chromium and molybdenum content. No clear effect of the micro-structure, ferrite or austenite, is reported.

Electro-chemical examination shows that when the pH value drops to the levels lower than the depassivation pH value, current density increases. Clearly ferritic stainless-steel shows higher current density than austenitic stainless-steels. This confirms the in-service properties, where initiated, crevice corrosion propagates very quickly in ferritic micro-structures. Repassivation mechanisms almost never occur in ferritic stainless-steel grades in such acidic conditions. Clearly, standard ferritic stainless-steel grades are not to be used in acidic solutions and crevice-like configurations are to be avoided. Optimum design of equipment is of utmost importance. Fig 7b shows crevice corrosion resistance data for the tests performed in a 2M NaCl solution at room temperature.

Inter-granular corrosion resistance – The most sensitive micro-structure to inter-granular corrosion is the HAZ area of the welded micro-structures. Carbides normally precipitate at grain boundaries and consequently in the case of chromium carbides, chromium depletion areas can form. This is a well-known mechanism in austenitic stainless-steels. For ferritic stain-less steels, diffusion mechanisms are improved and as solubility limits of interstitial elements are very low compared to austenitic stainless-steels, carbides, and nitrides precipitate when the steel is cooled down. In case of non-stabilized grades, chromium diffuses quickly to re-enrich the depleted zones. This is the case in majority of the annealed industrial products. However, in several cases the as-welded micro-structure, particularly the HAZ area, of non-stabilized stainless-steels remains sensitive to inter-granular corrosion. For ferritic welded micro-structures, titanium or niobium stabilized stainless-steel grades are strongly desired as shown in Fig 8a.

Fig 8 Inter-granular corrosion resistance and HT temperature tensile strength and creep sag resistance

In comparison to austenitic stainless-steels, the ferritic stainless steels are normally considered to resist better to stress corrosion cracking in chloride containing environments. However, their resistance is not limitless. Their cracking potential is normally higher than the free corrosion potential. This is related to their deformation mode and relatively poor capacity to repassivate. The risk of cracking mainly appears in concentrated acidic environments. In neutral media, the ferritic stainless-steels can normally be used.

Development of high temperature ferritic stainless-steel grades – Despite their lower mechanical properties at high temperature compared to those of austenitic stainless-steel grades, ferritic stainless-steel grades show a better resistance to the cyclic oxidation and thermal fatigue and show lower coefficients of thermal expansion. Niobium addition improves high temperature (HT) mechanical properties considerably, an addition of half a percent or more of molybdenum allows to reach a good resistance in severe internal or external corrosion conditions. Hence, ferritic stainless-steel grades are well adapted to exhaust system applications.

The increase of the exhaust gas temperature beyond 800 deg C has made the use of titanium stabilized 12 % chromium grades (AISI 409, EN 1.4512) impossible and has led to the use of high temperature resistant ferritic stainless-steel grades containing 17 % chromium and stabilized by both titanium and niobium (AISI 441, EN 1.4509). In such a grade, an excess of niobium improves the mechanical properties at high temperatures, in particular its creep resistance and its thermal fatigue resistance.

A maximum service temperature of 950 deg C can be reached. On the other hand, the ferritic stainless-steel grades are known to have a lower forming capacity, frequently shown by their moderate elongation (maximum elongation normally around 35 %). The hardening coefficient (n) and the anisotropy r-value are in fact more valuable parameters to characterize the formability. Ferritic stain-less steel grades normally show higher r-mean values. The latter parameter is exactly the one which well controls the deep-drawing of clam-shell made from ferritic stainless-steels.

New ferritic grades for exhaust manifolds – Requirements related to severe forming operations, especially for hydro-forming and tube bending, has originally led to the development of a new 14 % chromium (1.4595) steel which combines an improved formability compared to the EN 1.4509 (AISI 441), while still keeping its high temperature resistance. This steel grade can be used in replacement of austenitic stainless -steel grades in several situations. On the other hand, the Euro V norm is going very soon to require higher durability (160,000 km) and ability to be used up to 1,000 deg C. ASME (American Society of Mechanical Engineers) has developed a new 19 % chromium grade (modified 1.4521) to meet this new demand. The HT mechanical properties are considerably improved as shown in Fig 8b.