Austenitic Stainless Steels

General Characteristics and Applications

The basic austenitic stainless is UNS S30400 (type 304), sometimes called “18-8 stainless”. It is an iron-based alloy containing nominally 18% chromium and 8.5% nickel, minor amounts of carbon, nitrogen, manganese, and silicon. This alloy had its origins in the second decade of this century when it was simultaneously discovered in England and Germany that an addition of about 12% chromium to iron would prevent the formation of rust that normally would develop in moist air.

This effect, illustrated in Figure 4.1, was subsequently found to hold in many environments and provides the basis for the outstanding corrosion resistance characteristic of all modern stainless steels. Shortly after, it was discovered that an addition of nickel would stabilize the high temperature allotrope of iron, austenite, at room temperature. Austenite, with a face-centered cubic crystal structure, provides highly desirable mechanical properties in terms of strength, ductility, and toughness. Thus, through a fortuitous combination of discovery and thermodynamics, this family of steels was born.

Figure 4.1  Corrosion rates of chromium-iron alloys in intermittent water spray, room temperature.

From this beginning a family of dozens of variations on the original “18-8” alloy have evolved. These are known as the “standard austenitic stainless steels” and a representative list is shown in Table 4.1.

Many modifications of these standard alloys have been developed for special applications, and their evolution continues as a result of technological progress and the demands of the marketplace. Technological progress has been based on an improved understanding of factors which control mechanical properties, corrosion resistance, fabricability, and cost.

Mechanical property development has largely been based on defining optimum combinations of chromium, nickel, carbon, and nitrogen in relation to the work hardening and toughness characteristics of austenite. Corrosion resistance development has used additions of molybdenum and nitrogen for better performance in aqueous environments, and silicon and rare earth elements for resistance at high temperatures. Improvements in fabricability and cost have come from new steel refining techniques that cost effectively eliminate impurities such as carbon and sulfur, and from the use of continuous casting and other advanced production techniques. Thus a continuing specialization of these alloys exists, and they are being used in an ever wider range of applications.

Major market segments for the austenitic stainless steels include:

• consumer products,
• transportation,
• architecture,
• food and beverage,
• chemical and petrochemical,
• paper,
• pharmaceutical and biotech,
• semiconductor,
• energy,
• environmental, and
• aerospace.

Table 4.1  Chemical Compositions of Austenitic Stainless Steels (Weight %)a

UNS No.	Grade	EN	DIN	C	Cr	Ni	Mo	Cu	N	Other
Corrosion Resistant
S20100	201	1.4372	---	0.15	16.0-18.0	3.5-5.5	---b	---	0.25	5.5-7.5 Mn
S20200	202	---	---	0.15	17.0-19.0	4.0-6.0	---	---	0.25	7.5-10.0 Mn
S20400	Nitronic 30c	---	---	0.03	15.0-17.0	1.50-3.00	---	---	0.15-0.30	7.0-9.0 Mn
S30100	301	1.4310	1.4310	0.15	16.0-18.0	6.0-8.0	---	---	0.10	---
S30200	302	---	---	0.15	17.0-19.0	8.0-10.0	---	---	0.10	---
S30430	302HQ	---	---	0.03	17.0-19.0	8.0-10.0	---	3.00-4.00	---	---
S30400	304	1.4301	1.4301	0.08	18.0-20.0	8.0-10.5	---	---	0.10	---
S30403	304L	1.4307	---	0.030	18.0-20.0	8.0-12.0	---	---	0.10	---
S30451	304N	1.6907	1.6907	0.08	18.0-20.0	8.0-10.5	---	---	0.10-0.16	---
S30500	305	1.4303	1.4303	0.12	17.0-19.0	10.5-13.0	---	---	---	---
S31600	316	1.4401	1.4401	0.08	16.0-18.0	10.0-14.0	2.00-3.00	---	0.10	---
S31603	316L	1.4404	1.4404	0.030	16.0-18.0	10.0-14.0	2.00-3.00	---	0.10	---
S31703	317L	1.4438	1.4438	0.030	18.0-20.0	11.0-15.0	3.00-4.00	---	0.10	---
S31726	317LMN	1.4439	1.4439	0.030	17.0-20.0	13.5-17.5	4.0-5.0	---	0.10-0.20	---
S32100	321	1.4541	1.4541	0.080	17.0-19.0	9.0-12.0	---	---	0.10	5 x (C+N) Ti min. 0.70 max.
S21800	Nitronic 60c	---	---	0.10	16.0-18.0	8.0-9.0	---	---	0.08-0.18	7.0-9.0 Mn; 3.5-4.5 Si
S20161	Gall-Toughe	---	---	0.15	15.0-18.0	4.0-6.0	---	---	0.08-0.20	4.0-6.0 Mn; 3.00-4.00 Si
S30300	303	1.4304	1.4304	0.15	17.0-19.0	8.0-10.0	---	---	---	0.15 S min.
S30323	303Se	---	---	0.15	17.0-19.0	8.0-10.0	---	---	---	0.15 Se min.
S30800	308	---	---	0.08	19.0-21.0	10.0-12.0	---	---	---	---
S30883	308L	---	---	0.03	19.5-22.0	9.0-11.0	0.05	0.75	---	0.30-0.65 Si

UNS No.	Grade	EN	DIN	C	Cr	Ni	Mo	Cu	N	Other
Heat Resistant
S30409	304H	1.4948	1.4948	0.04-0.10	18.0-20.0	8.0-10.5	---	---	---	---
S34700	347	1.4450	1.4450	0.080	17.0-19.0	9.0-13.0	---	---	---	10 x C Cb min. 1.00 max.
S31609	316H	---	---	0.04-0.10	16.0-18.0	10.0-14.0	2.00-3.00	---	0.10	---
S30815	253MAd	1.4835	---	0.05-0.10	20.0-22.0	10.0-12.0	---	---	0.14-0.20	1.00-2.00 Si; 0.03-0.08 Ce
S30900	309	---	---	0.20	22.0-24.0	12.0-15.0	---	---	---	---
S30908	309S	1.4833	1.4833	0.08	22.0-24.0	12.0-15.0	---	---	---	---
S30909	309H	---	---	0.04-0.10	22.0-24.0	12.0-15.0	---	---	---	---
S31000	310	---	---	0.25	24.0-26.0	19.0-22.0	---	---	---	---
S31008	310S	1.4845	1.4845	0.08	24.0-26.0	19.0-22.0	---	---	---	---
S31009	310H	---	---	0.04-0.10	24.0-26.0	19.0-22.0	---	---	---	---

1. Maximum unless range or minimum is indicated.
2. None required in the specification.
3. Nitronic is a trademark of Armco Inc.
4. 253MA is a trademark of Avesta Sheffield AB.
5. Gall-Tough is a trademark of Carpenter Technology.

The consumer market is very broad and includes items such as appliances, sinks, and pots and pans. Transportation consists of automotive trim and emission control applications, truck components, and siding and structural applications in rail and rapid transit cars. Architectural applications include roofing, curtain walls, elevators, and interior and exterior hardware items. The food segment extends from food processing equipment to restaurant counters and cooking stoves and ovens.

Most chemical plants and refineries use tanks, pressure vessels, piping and heat exchangers constructed of austenitic stainless steels, and this is also true of the paper industry. Much of the equipment used to produce pharmaceuticals benefits from the hygienic and cleanability properties of these steels. The semiconductor industry relies on vacuum equipment constructed of stainless steel. In the energy sector, stainless steels are used in piping and heat exchanger applications.

Growing environmental applications range from equipment used in sewage treatment plants to scrubbers to remove polluting gases and particulates from flue gases. Aerospace applications range from aircraft hydraulic lines to pressure vessels handling gases in missiles.

From these applications, it seems clear that growth in the use of austenitic stainless steels will keep pace with the growing world population and especially the expanding economy as third world countries improve their standard of living. New technological developments related to the steels themselves will also contribute to growth. These include developing a continuous steelmaking process from melting to finished coil, which will reduce production costs; thin strip casting, which is near to commercialization, represents further potential cost savings.

Also, technical developments in other areas such as fuel cells and advanced power plants will also further stimulate demand. Fortunately, ample supplies of chromium and nickel exist, and capital markets seem ever ready to provide expanded production capacity. There appears to be no limit to the bright future of the austenitic stainless steels, with more than 25 new grades added to ASTM standards or a 40% increase in the past 25 years.

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