02. Ferritic stainless steels-almost immune to stress corrosion

The second in a series of eight columns throughout 2017 on the topic of the seven families of stainless steels; their characteristics, complementary properties and the wide variety of applications from the smallest of items destined for the human body to large scale constructions in the process industry. This article focuses on ferritic stainless steels which are so resistant to stress corrosion.

While austenitic stainless steels suffer from sensitivity to stress corrosion cracking ferritic stainless steels (FSS) are almost immune. Owing to the absence of nickel they are less expensive than their austenitic counterpart and, therefore, provide a cost-effective alternative when corrosion resistance and oxidation resistance is required. About 10% of all stainless steels are ferritic. Because of the relatively low toughness of thick sections they are preferred as thin sheet or thin wire. Were it not for the limitations regarding dimensions, formability and welding their share would be even larger. The improvement of modern process metallurgy has resulted in FSS of improved toughness and formability, which has led to growing popularity. In fact, in many less demanding applications FSS have replaced the austenites.

Figure 1: The crystal structure of ferrite which is body centered cubic (one atom in the center of the unit cell).FSS derive the name from the crystal structure, which is body centered cubic or ferritic (Figure 1). The structure is not as densely packed as austenite and, therefore, more open. This explains the higher diffusion rates and associated high creep rate at elevated temperature. Despite the low creep resistance, FSS are suitable in high temperature applications in which oxidation and corrosion are more important than mechanical strength. It is seemingly paradoxical that an open structure such as ferrite offer smaller holes than austenite for interstitials. However, this is a fact and explains why only minute amounts of carbon and nitrogen are dissolved in FSS. Even low levels of carbon and nitrogen lead to brittleness and must be reduced to extremely low levels in FSS. Fortunately, this has become possible using modern process metallurgy, a circumstance which has led to dramatic improvement in toughness beginning in the 1970’s.

At least three groups of ferritic steels may be identified. Group I (10-14% Cr) represented by type 409 and type 410 ferritic steels suitable in non-severe applications where water is absent. Typical applications of group I steels include automotive exhaust systems, where low alloys steels have dominated for a long time. The work horse among ferritic stainless steels has for a long time been 430, here representing Group II (16-19%Cr).

Figure 2: Cross-section of an automotive exhaust catalyst rolled into a spiral to increase the effective surface area.Examples of applications are washing drums, decorative trim on vehicles, domestic appliances such as ovens, cookware, pots, pans, kitchen sinks and cutlery. A disadvantage of traditional 430 is the poor weldability in combination with the fact that some of the chromium needed for corrosion resistance is tied to carbon. Hence the corrosion potential is not fully utilized. These problems have been satisfactorily solved in 439 containing 17-19% chromium and in which titanium is used for stabilization. This makes 439 a suitable replacement for the austenitic grade 304, leading to significant cost reductions. Typically, 439 is half as expensive as 304 (based on a nickel price of 15$/kg). Therefore, ferritic steels have become an increasingly attractive alternative to low alloy austenites.

Figure 3: Si-based solar cells entering a diffusion furnace. The process tube is heated with electric resistance elements made from the ferritic dispersion strengthened alumina former Kanthal APM®. This alloy tolerates temperatures up to 1400°C in oxidizing atmosphere. Courtesy B. Jönsson, Sandvik Heating Technology.Bright annealed ferritic steels often show shiny surfaces. An important application sector is therefore applications where the surface lustre is important. One example is decorative trims in automotive applications. A third group may be identified including stabilized FSS such as 439 stabilized with titanium and 444 stabilized with niobium and titanium in combination. Such FSS show better weldability than those in groups I and II. The pitting corrosion resistance of FSS may be further improved by adding molybdenum as in 444 or chromium beyond 18% as in 446. Ferritic steels alloyed with aluminium are used in high temperature applications where oxidation resistance is desired.

The creep resistance under extreme conditions is often improved by dispersion strengthening as in type APMT ferritic steels. Furthermore, aluminium contributes to very good high temperature corrosion resistance. This has led to a series of Fe-Cr-Al grades used in high temperature applications such as in automotive exhaust catalysts (Figure 2) and for resistance heating in various types of furnaces (Figure 3). Service temperatures are often 1200°C or above. Thanks to a stable layer of aluminium oxide on the surface catastrophic oxidation is avoided (Figure 4) and satisfactory performance is obtained. A selection of commercial FSS is shown in Table 1.

Figure 4: Scanning electron micrograph of surface oxide of type Al2O3 formed at 1300°C for 1100h. The oxide has remained stable despite the harsh conditions. Courtesy B. Jönsson, Sandvik Heating Technology.

Alloy C N Cr Ni Mn Si Mo P S Other Applications
409 0.03 0.03 10.5–11.7 0.5 1 1 - 0.04 0.02 ≤0.5 Ti Automotive exhaust systems
410 0.04 0.1 10.5–12.5 0.6–1.1 1.0 1.0 - 0.04 0.03
430 0.12 - 16–18 - 1 1 - 0.04 0.03 Washing drums, decorative trim, domestic appliances
439 0.07 0.04 17–19 0.5 1 1 - 0.04 0.03 ≤1.1 Ti Washing drums, decorative trim, domestic appliances
444 0.025 0.035 17.5–19.5 1 1 1 0.75–1.25 0.04 0.03 Ti+Nb
446-1 0.17 0.17 23–27 - 0.8 0.5 - 0.03 0.015 Recuperators, soot blower tubes, thermocouple tubes, muffle tubes in furnaces
UNS K 91500 20–24 5–6 Al Automotive exhaust systems, resistance heating in ovens and furnaces

In my next column we will find that synergies are obtained in duplex stainless steels when austenite and ferrite co-exist and interact.

This article was first published in Stainless Steel World Magazine in April 2017.

Jan-Olof Nilsson

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