Classes and Types
19.2.2 Classes and Types
There are fi ve main classes of stainless steels, designated in accord with their crystallographic structure. Each class consists of several alloys of somewhat dif- fering composition having related physical, magnetic, and corrosion properties. These were given type numbers by the American Iron and Steel Institute (AISI). Although AISI stopped issuing designations for new stainless steels several decades ago, the AISI numbers remain in use today. The Unifi ed Numbering System (UNS) was introduced in the 1970s to include all alloys, including stain- less steels. A listing of many stainless steels that are produced commercially is given in Table 19.2 .
The fi ve main classes of stainless steels are martensitic, ferritic, austenitic, precipitation - hardenable, and duplex.
1. Martensitic. The name of the fi rst class derives from the analogous mar- tensite phase in carbon steels. Martensite is produced by a shear - type phase transformation on cooling a steel rapidly (quenching) from the austenite region (face - centered cubic structure) of the phase diagram. It is the characteristically hard component of quenched carbon steels, as well as of the martensitiic stainless steels. In stainless steels, the structure is body - centered cubic, and the alloys are magnetic. Typical applications include cutlery, steam turbine blades, and tools.
2. Ferritic. Ferritic steels are named after the analogous ferrite phase, or relatively pure - iron component of carbon steels cooled slowly from the austenite region. The ferrite, or alpha, phase for pure iron is the stable
phase existing below 910 ° C. For low - carbon Cr − Fe alloys, the high - temperature austenite (or gamma) phase exists only up to 12% Cr; above this Cr content, the alloys are ferritic at all temperatures up to the melting point. They can be hardened moderately by cold working, but not by heat treatment. Ferritic stainless steels are body - centered cubic in structure and are magnetic. Uses include trim on autos and applications in synthetic nitric acid plants.
3. Austenitic. Austenitic stainless steels are named after the austenite, or γ, phase, which, for pure iron, exists as a stable structure between 910 ° C and 1400 ° C. This phase is face - centered cubic and nonmagnetic, and it is readily deformed. It is the major or only phase of austenitic stainless steels at room temperature, existing as a stable or metastable structure depend- ing on composition. Alloyed nickel is largely responsible for the retention of austenite on quenching the commercial Cr – Fe – Ni alloys from high temperatures, with increasing nickel content accompanying increased sta- bility of the retained austenite. Alloyed Mn, Co, C, and N also contribute
T A B L E 19.2. Types and Compositions of Stainless Steels
AISI Type UNS
Composition (%)
or Common No. Name
P (max)
S (max)
Class: Martensitic — Body - Centered Tetragonal, Magnetic, Heat - Treatable
403 S40300
1.0 0.04 0.03 0.5 Turbine quality 410
Easy machining, nonseizing
O YIN
416Se S41623
0.15 max
1.25 0.06 0.06 1.0 0.15 Se min
Easy machining, nonseizing G
1.0 0.04 0.03 1.0 OR C 440A
O R 440B S S44003 16.0 – 18.0 — 0.75 – 0.95 1.0 0.04 0.03 1.0 0.75 Mo ION
max
440C S44004
Class: Ferritic — Body - Centered Cubic; Magnetic, not Heat - Treatable
; ST
405 AINL S40500 11.5 – 14.5 — 0.08 max 1.0 0.04 0.03 1.0 0.1 – 0.3 Al. 430
1.0 0.04 0.03 1.0 E 430F
Easy machining,
S S STE
nonseizing 430FSe
1.25 0.06 0.06 1.0 0.15 Se min
Easy machining,
nonseizing
AISI Type UNS
Composition (%)
or Common No. Name
P (max)
S (max)
1.5 0.04 0.03 1.0 0.25 N max
Resistant to high -
Class: Austenitic — Face - Centered Cubic, Nonmagnetic, not Heat - Treatable
0.06 0.03 1.0 0.25 N max
0.06 0.03 1.0 0.25 N max
302B S30215
Resistant to high - temperature
oxidation 303
Easy machining, nonseizing 303 Se
Easy machining, nonseizing 304
2.0 0.20 0.06 1.0 0.15 Se min
0.03 1.0 Extra - low carbon 305
304L S30403
0.03 1.0 Lower rate of work hardening than 302 or 304
309 S S30908
310 S S31008
Extra - low carbon 317
316L S31603
Stabilized grade
C min
T A B L E 19.2. Continued
AISI Type UNS
Composition (%)
or Common No. Name
P (max)
S (max)
0.03 1.0 Cb + Ta: 10
Stabilized grade
0.03 1.0 Cb + Ta: 10
Stabilized grade
when radiation
max
conditions require low Ta ALL
O YIN
Class: Precipitation - Hardenable
15 – 5 PH FOR S15500 14.00 – 15.5 3.50 – 5.50 0.07 1.00 0.04 0.03 1.00 2.5 – 4.5 Cu 0.15 – 0.45
Nb
OR
17 – 7 PH S17700
Class: Daplex — Austenitic/Ferritic
Zeron 100 S32760
W 0.20 – 0.30 N
STAINLESS STEELS
to the retention and stability of the austenite phase. Austenitic stainless steels can be hardened by cold working, but not by heat treatment. On cold working, but not otherwise, the metastable alloys (e.g., 201, 202, 301, 302, 302B, 303, 303Se, 304, 304L, 316, 316L, 321, 347, 348; see Table 19.2 ) transform in part to the ferrite phase (hence, the descriptor, “ metastable ” ) having a bond - centered - cubic structure that is magnetic. This transforma- tion also accounts for a marked rate of work hardening. Alloys 305, 308, 309, and 309 S, on the other hand, work harden at a relatively low rate and become only slightly magnetic, if at all. Alloys containing higher chromium and nickel (e.g., 310, 310 S, 314) are essentially stable austenitic alloys and do not transform to ferrite or become magnetic when deformed. Uses of austenitic stainless steels include general - purpose applications, architectural and automobile trim, and various structural units for the food and chemical industries.
4. Precipitation Hardenable. The precipitation - hardening stainless steels achieve high strength and hardness through low - temperature heat treat- ment after quenching from high temperatures. These Cr – Fe alloys contain less nickel (or none) than is necessary to stabilize the austenite phase, and, in addition, they contain alloying elements, such as aluminum or copper, that produce high hardness through formation and precipitation of inter- metallic compounds along slip planes or grain boundaries. They are applied whenever the improved corrosion resistance imparted by alloyed nickel is desirable, or, more important, when hardening of the alloy is best done after machining operations, using low - temperature heat treatment [e.g., 480 ° C (900 ° F)] rather than a high - temperature quench as is required in the case of the martensitic stainless steels.
5. Duplex. The duplex stainless steels contain both austenite and ferrite, typically with the austenite:ferrite ratio ∼ 60 : 40. The mixed austenite – ferrite microstructure, consisting of a continuous ferrite matrix with aus- tenite islands (Fig. 19.1 [10] ), is achieved by including in the composition
a balance of elements that stabilize austenite (e.g., nickel and nitrogen) and those that stabilize ferrite (e.g., chromium and molybdenum). The 22% chromium duplex stainless steel was developed in the late 1970s and was later modifi ed and designated as Alloy 2205 (UNS designation S32205). Because of good mechanical properties, high resistance to chlo- ride stress - corrosion cracking, good erosion and wear resistance, and low thermal expansion, duplex stainless steels are used in many applications, including pressure vessels, storage tanks (e.g., for phosphoric acid), and heat exchangers [10] . For seawater service, duplex stainless steels of higher molybdenum content (e.g., Zeron 100) have been developed [11] .
In general, the highest resistance to uniform corrosion is obtained with the nickel - bearing austenitic types, and, in general, the highest nickel - composition alloys in this class are more resistant than the lowest nickel compositions. For
342 ALLOYING FOR CORROSION RESISTANCE; STAINLESS STEELS
Figure 19.1. Typical microstructure of a duplex stainless steel. The grains are elongated in the rolling direction ( ∼300×) [10]. (Reprinted with permission of John Wiley & Sons, Inc.)
optimum corrosion resistance, austenitic alloys must be quenched (rapidly cooled by water or by an air blast) from about 1050 – 10 ° C (1920 – 2000 ° F). The molyb- denum - containing austenitic alloys (316, 316L, 317) have improved corrosion resistance to chloride - containing environments, dilute nonoxidizing acids, and crevice corrosion.
The ferritic stainless steels have optimum corrosion resistance when cooled slowly from above 925 ° C (1700 ° F) or when annealed at 650 − 815 ° C (1200 − 1500 ° F). * Recently developed ferritic stainless steels of controlled purity contain molybde- num additions and low carbon and nitrogen. Typical analyses include 26% Cr, 1% Mo, < 0.01% C, < 0.02% N; 18% Cr, 2% Mo, < 0.025% C, < 0.025% N; 28% Cr, 4% Mo, < 0.01% C, < 0.02% N. Alloyed titanium or columbium is sometimes included in the fi rst two alloys in order to raise the tolerable carbon and nitrogen contents. All these alloys can be quenched from above 925 ° C without loss of corrosion resistance. Their corrosion properties are generally better than those of the conventional ferritic, and some austenitic, stainless steels listed in Table
19.2 — for example, in NaCl solutions, HNO 3 , and various organic acids, provided they retain passivity. Should they lose passivity, locally or generally, their corro- sion rates tend to be higher than those of the active nickel - containing austenitic stainless steels of equivalent chromium and molybdenum contents [12] .
The martensitic stainless steels, on the other hand, have optimum corrosion resistance as quenched from the austenite region. In this state, they are very hard and brittle. Ductility is improved by annealing (650 – 750 ° C for 403, 410, 416,
* For additional details on heat treating stainless steels, see ASM Handbook , Vol. 4, Heat Treating , ASM International, Materials Park, OH, 1991, pp. 769 − 792.
STAINLESS STEELS
416Se; 650 – 730 ° C for 414; 620 – 700 ° C for 431; 680 – 750 ° C for 440 A, B, C, and 420), but at some sacrifi ce of corrosion resistance. Resistance to pitting and rusting in 3% NaCl at room temperature reaches a minimum after tempering a
0.2 – 0.3% C, 13% Cr stainless steel at 500 ° C, as well as 650 ° C for a similar steel containing 0.06% C [13] . In general, the tempering range 450 – 650 ° C (840 – 1200 ° F) should be avoided if possible. Decrease of corrosion resistance probably results, in part, from the transformation of martensite containing interstitial carbon into
a network of chromium carbides attended by depletion of chromium in the adjoining metallic phase. Cold working any of the stainless steels usually has only minor effect on uniform corrosion resistance if temperatures are avoided that are high enough to permit appreciable diffusion during or after deformation. Phase changes brought about by cold working the metastable austenitic alloys are not accom- panied by major changes in corrosion resistance. * Furthermore, quenched aus- tenitic stainless steel (face - centered cubic) of 18% Cr, 8% Ni composition has approximately the same corrosion resistance as quenched ferritic stainless steel (body - centered cubic), of the same chromium and nickel composition but with lower carbon and nitrogen content [14] . However, if a similar alloy containing a mixture of austenite and ferrite is briefl y heated at, for example, 600 ° C, composi- tion differences are established between the two phases setting up galvanic cells that accelerate corrosion. Composition gradients, whatever their cause, are more important in establishing uniform corrosion behavior than are structural varia- tions of an otherwise homogeneous alloy. This statement is true for metals and alloys in general.