Ferritic Stainless Steels
Ferritic stainless steels are iron-carbon alloys that do not undergo treatments for hardening and, at room temperature, it possesses the ferritic structure (α) that distinguishes them (Figure 1).
Figure 1 microscopy (SEM) of pearlite phase (P) distributed in the ferrite matrix (α)
The ferritic steels are divided into 5 macro groups, 3 families standard (Group 1,2,and 3) and 2 “special” (Group 4 and 5).
Standard ferritic stainless steels are very satisfactory and entirely appropriate for many demanding applications.
They are divided into three subgroups:
Group 1: content of Cr 10%-14% AISI 409, 410, 420
Figure 2 automotive exhaust system silencers made of AISI 409
Group 2: content of Cr: 14%-18% AISI 430
Figure 3 Cutter made of AISI 430
Group 3: content of Cr: 10%-14% with stabilizing elements as Ti, Nb AISI 430Ti, 439, 441
Figure 4 The basket of a washing machine
The Special Ferritic Stainless Steel are divided in two subgroup:
Group 4: content of Mo>0,5% AISI 434, 436, 444
Figure 5 solar water heater
Group 5: content of Cr: 18%-30% or not belonging to one of the other groups
Figure 6 Steel Pipes AISI 447
The Group 1 (type 409/410 L) has the lowest chromium content of all stainless steels and is also the least expensive. This group is ideal for non- or lightly-corrosive environments or applications where there is a slight and acceptable localized rust. The type 409 was originally designed for automotive exhaust system silencers (exterior parts in non-severe corrosive environments). This type provides an adequate proportion of titanium which, in addition to stabilize the carbon and nitrogen, has an ferritic effect. Type 410L is often used for containers, buses and coaches and, recently, LCD monitor frames.
Group 2 (type 430) is the most widely used family of ferritic grades. Having a higher chromium content it is considered as “17% Cr”. The steels of this group show a higher corrosion resistance and behave most like AISI 304 Stainless Steel for indoor applications. This steel can resist in oxidizing environments at temperatures of 800°C or 850° C according to whether it uses a continuous or intermittent. Typical uses include washing-machine drums, indoor panels, etc.
Group 3 includes (types 430Ti, 439, 441 etc..) Compared to the materials of the Group 2, these grades show better weld-ability and formability. Their behavior is, in some cases, even better than that of austenitic type 304. These qualities are enhanced by the use of stabilizing substances such as titanium, niobium and zirconium which have the function of:
Make the fully ferritic structure
Inhibit the precipitation of chromium carbides
Change the nature and shape of the precipitates and non-metallic inclusions
Typical applications include sinks, pipes for heat exchangers (the sugar industry, energy etc.), Exhaust systems (they have a longer life than the 409) and the welded parts of washing machines.
Group 4 includes (types 434, 436, 444 etc..) These grades have added molybdenum for increased corrosion resistance. Typical applications include hot water tanks, solar water heaters, visible parts of exhaust systems, electric kettle and microwave oven elements, automotive trim, skirts etc. The level of corrosion resistance of the type 444 can be compared to that of type 316.
The materials of the group 5 (types 446, 445/447 etc..) containing higher amounts of chromium and molybdenum, for a better resistance to corrosion and hot oxidation. In this way, these materials are superior to type 316. Typical uses include applications in coastal and other highly corrosive environments. The corrosion resistance of JIS 447 is equal to that of titanium metal. Ferritic steels are those less suitable to be used at low temperatures because of their structural composition (body-centered cubic lattice). At lower temperatures the steel passes by broken tenacious to brittle failures. This behavior increases with increasing thickness of the pieces. The other factors that influence this behavior are:
Dimensions of wheat
The content of interstitial elements
So all the steps, both during the processing phase of the liquid metal, both during the machining, are of significant importance to increase the toughness of this steel.
At high temperatures, these steels have a resistance to oxidation as greater as higher is the content of chromium in the alloy. They may be subject to embrittlement when remain at a specific temperature for a specific period of time and cooled to ambient temperature because there is the formation of undesirable phases that lead to decrease of tenacity. From the commercial point of view, ferritic steels have a lower and stable price that can replace some applications for austenitic steels. In fact, it does not contain nickel, the rate of the chromium, the element that makes the “stainless” steel particularly resistant to corrosion, is historically relatively stable.
In conclusion, it can declare that:
- Ferritic steels are magnetic;
- Ferritics have low thermal expansion (they expand less than austenitics when heated);
- Ferritics have excellent resistance to oxidation at high temperatures;
- Ferritics have high thermal conductivity;
- Ferritics stabilized with niobium have excellent resistance to “creep (creep hot)”;
- Ferritics are easier to cut and work than austenitics;
- Ferritics are significantly less prone to springback than austenitics, during cold forming;
- Ferritics have higher yield strength;
- Ferritics have higher yield strength (similar to that of popular carbon steels) than type 304 austenitics;
- Ferritic steels are not susceptible to stress corrosion cracking.