FERRITIC S TAINLESS S TEEL S Ferritics account for approximately 25% of stainless steel use worldwide. The name arises because these alloys have similar properties to carbon steels when they are bent or cut and, unlike the well-known 304 and 316 austenitic grades, ferritics are strongly attracted to a magnet. There is a major misconception that ferritic stainless steels are less corrosion resistant than austenitic alloys. On the contrary, for any required level of corrosion resistance (or Pitting Resistance Equivalent [PRE]), you can select a specific stainless steel from either the austenitic or ferritic family depending on the physical properties desired. Another similarity of these two families of stainless steel is that neither can be hardened by heat treatment. However, a significant difference is that, in common with carbon steels, ferritic stainless steels become brittle when used in sub-zero temperatures. The actual transition temperature depends on the specific alloy, but it increases for welded fabrications. Often regarded as the simplest stainless steel alloy, ferritics are steels (iron and a small addition of carbon) with at least 11% chromium added to produce the passive chromium oxide film. This self-repairing chromium oxide layer gives stainless steel its corrosion resistance. The first stainless steels developed in 1913 were ferritics with a high carbon content. Today, those alloys are called martensitics and are used for high hardness blades or wear resistant surfaces. The alloys now known as ferritic stainless steels have been used commercially for many decades, primarily as sheet cladding up to about 3mm that do not require welding. The Fujitsu building in Brisbane for example is clad in profiled ferritic stainless steel sheet, and the use of perforated and solid ferritic stainless steel sheeting is featured in the ceiling and fascia paneling in Sydney’s Wynyard Walk. Apart from the 12% chromium utility alloys, the sheet thickness limits for the supply and welding of ferritics are due to its metallurgical structure. Unlike austenitic stainless steels, the microstructure does not transform during welding, and so the initially microscopic ferrite grains can grow and embrittle the metal. Ferritics have gained wider acceptance since changes in its alloy design and production methods allowed welding. The adoption of the Argon Oxygen Decarburisation (AOD) refining process in the 1970s also assisted, allowing both the reduction of impurity levels and, critical for welding, good control of both carbon and nitrogen content.
AVAILABLE FERRITIC ALLOYS AND APPLICATIONS The Ferritic Solution (TFS), published by the International Stainless Steel Forum, lists 71 ferritic alloys in ASTM, EN and JSA standards, although most are in sheet form. For example, A240 lists 26 alloys as flat product while ASTM A276 only has nine alloys listed as bar or shape. TFS classifies ferritic alloys into five groups based on chromium content: • Chromium (10.5% to 14%) • Chromium 14% to 18%) • Titanium and/or niobium added to avoid sensitisation with welding • Molybdenum additions for corrosion resistance • Weldable group of alloys with higher corrosion resistance and chromium >18%, added molybdenum and low impurity content. Table 1 lists common names, UNS numbers, typical compositions and applications of representative alloys. There are also families of alloys derived from the same root UNS numbers. In addition, a growing number of proprietary ferritic alloys have been and are being developed especially in Japan. The PRE column is a measure of corrosion resistance based on composition, i.e. PRE = %Cr + 3.3% Mo. The 16%N term used for austenitic and duplex grades is omitted because nitrogen is virtually insoluble in ferritic alloys.
CORROSION AND HEAT RESISTANCE These are not the same. Oxidation (or scaling) resistance of stainless steels in air depends on the stability of the oxide layer (or scale) on the surface. This is not the thin (nanometres) passive film formed in water but the thicker, high temperature oxide formed above about 250oC. Its protective properties depend on its bond to the metal surface below. In turn, this depends on the relative expansion of the oxide and the metal surface.
TABLE 1: SELECTED FERRITIC ALLOYS Common name
UNS
C%
Cr%
Mo% Others
PRED Main uses
409
S40900
0.03
11
-
0.3Ti
11
Car exhausts
4003, 3/5Cr12A
S40977
0.02
11
-
0.5Ni
11
Rail wagons, noncosmetic structures
430B
S43000
0.03
17
-
-
17
Cladding (not marine)
444
S44400
0.02
18
2
0.4(Ti+Nb)
25
Instant hot water units
446
S44600
0.15
24
-
-
24C
High temperature
447
S44700
0.01
29
3.8
0.1Cu, 0.1Ni
42
Seawater tubing
NOTES: A. Balance of composition important to avoid welding corrosion issues B. Also derivative grades with low carbon and Ti/Nb to allow welding C. Not a good indicator of corrosion resistance especially if welded because of high carbon D. For compagrison, the PRE of 304 is ~18.5 and 316 ~23.5
4 AUSTRALIAN STAINLESS 62 www.assda.asn.au
As shown in Table 3, ferritic alloys have low thermal expansion compared to austenitics, which means the adhesion of their protective scale is better in thermal cycling conditions. In practical terms, this means that ferritic alloys have higher scaling temperature limits for intermittent service than in continuous service, whereas the reverse is true for austenitic alloys. At temperatures in the high hundreds (oC), the relatively low strength of most ferritic alloys limits their use, although the niobium-treated ferritics have similar strength to the austentic alloys. Ferritic (and duplex) grades should not be used in the band around 475oC as metallurgical phase transformations cause embrittlement during extended exposures.
