Why Stainless Steel? Facts, Applications and Compositions by Eagle Stainless Tube

by Eagle Stainless Tube & Fabrication, Inc.

What is Stainless Steel?

The many unique values provided by stainless steel make it a powerful candidate in materials selection. Engineers, specifiers and designers often underestimate or overlook these values because of what is viewed as the higher initial cost of stainless steel. However, over the total life of a project, stainless is often the best value option.

Stainless steel is essentially a low carbon steel which contains chromium at 10% or more by weight. It is the addition of chromium that gives the steel its unique stainless, corrosion resisting

The chromium content of the steel allows the formation of a tough, adherent, invisible, corrosion-resisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, provided that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen.

There are more than 60 grades of stainless steel. However, the entire group can be divided into four classes. Each is identified by the alloying elements which affect their microstructure and for which each is named.

Grades / Applications of Stainless Steel

400 Series Martensitic – Typical grade: 410
Straight chromium (12 – 18%); magnetic and can be hardened by heat treatment.
Typical use: Fasteners, pump shafts.

400 Series Ferritic – Typical grade: 430
Straight chromium (12 – 18%); low carbon, magnetic, but not heat treatable.
Typical use: Appliance trim, cooking utensils.

300 Series Austenitic – Typical grade: 304
Chromium (17 – 25%), Nickel (8 – 25%); nonmagnetic, not heat treatable. Can develop high strength by cold working. Additions of molybdenum (up to 7%) can increase the corrosion resistance.
Typical use: Food equipment, chemical equipment, architectural applications.

Precipitation Hardening – Typical grade: 17-4
Chromium (12 – 28%), Nickel (4 – 7%); martensitic or austenitic. Develop strength by precipitation harden reaction during heat treatment.
Typical use: valves, gears, petrochemical equipment.

Stainless Steel Tubes

Benefits of Stainless Steel

Corrosion resistance – Lower alloyed grades resist corrosion in atmospheric and pure water environments; high-alloyed grades can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments making their properties useful in process plants.

Fire and Heat Resistance – Special high chromium and nickel-alloyed grades resist scaling and retain high strength at high temperatures.

Hygiene – The easy cleaning ability of stainless makes it the first choice for strict hygiene conditions, such as hospitals, kitchens and food processing plants.

Aesthetic Appearance – The bright, easily maintained surface of stainless steel provides a modern and attractive appearance.

Strength-to-weight advantage – The work hardening property of austenitic grades, that results in a significant strengthening of the material from cold working alone, and the high strength duplex grades, allow reduced material thickness over conventional grades yielding considerable cost savings.

Ease of fabrication – Modern steel making techniques mean that stainless can be cut welded, formed, machined, and fabricated as readily as traditional steels.

Impact resistance – The austenitic microstructure of the 300 series provides high toughness at elevated temperatures ranging to far below freezing, making these steels particularly suited to cryogenic applications.

Long term value – When the total life cycle costs are considered, stainless is often the least expensive material option.

Types of Stainless Steel

Eagle Stainless Stainless Steel

Chemical Compositions and Major Characteristics

Type Cr Ni CMax Other Significant Elements Major Characteristics
303 17.00- 19.00 8.00-10.00 0.15 S 0.15 min †Free-machining modification of Type 302
303 Se 17.00-19.00 8.00-10.00 0.15 Se 0.15 min †Free-machining version of Type 302. Produces better surface finish than Type 303.
304 18.00-20.00 8.00-10.00 0.08 †Low carbon variation of Type 302, minimizes carbide precipitation during welding.
304 L 18.11-20.00 8.00-10.00 0.03 †Extra-low carbon content eliminates harmful carbide precipitation due to welding.
309 22.00-24.00 12.00-15.00 0.20 †High strength and resistance to scaling at high temperatures
310 24.00-26.00 19.00-22.00 0.20 †Higher alloy content improves basic characteristics of Type 309
316 16.00-18.00 10.00-14.00 0.08 Mo 2.00-3.00 †Mo improves general corrosion and pitting resistance and high temperature strength over that of Type 302.
316 L 16.00-18.00 10.00-14.00 0.03 Mo 2.00-3.00 †Extra-low carbon version of Type 316. Eliminates harmful carbide precipitation due to welding
317 18.00-20.00 11.00-15.00 0.08 Mo 3.00-4.00 †Higher alloy content improves basic advantages of Type 316.
321 17.00-19.00 9.00-12.00 0.08 Ti 5 x C, min †Stabilized to permit use in 800-1500° F temperature range without harmful carbide precipitation.
347 17.00-19.00 9.00-13.00 .008 Ch Ta 10xC, min †Characteristics similar to Type 321. Stabilized by Cb and Ta.
348 17.00-19.00 9.00-13.00 0.08 Ta 0.10 max Cb 0.20 maxCb Ta 10 x C, min †Similar to Type 347 but Ta reduced for atomic energy applications
410 11.50-13.50 0.15 ¥ Lowest cost general purpose stainless steel. Wide use where corrosion is not severe.
416 12.00-14.00 0.15 S 0.15 min ¥ Free-machining version of Type 410.
420 12.00-14.00 0.15min ¥ Similar variation of Type 410 but higher carbon produces higher strength and hardness.
430 14.00-18.00 0.12 *Most popular of the chromium types. Combines good corrosion and heat resistance and mechanical properties.
430 F 14.00-18.00 0.12 S 0.15 min *Free-machining version of Type 430.
440 A 16.00-18.00 0.60-0.75 ¥ Series of high carbon types. Same basic composition with varying carbon content. Higher carbon produces higher strength and hardness but lower toughness. All Type 440 versions are corrosion resistant only in the hardened condition.
440 B 16.00-18.00 0.75-0.95
440 C 16.00-18.00 0.95-1.20
446 23.00-27.00 0.20 *Similar to Type 442 but Cr increased to provide maximum resistance to scaling.
15.5 PH 14.00-15.50 3.50-5.50 0.07 Cu 2.50-4.50 • Similar in properties and characteristics to 17-4 PH but has superior transverse ductility and toughness.
PH 15-7 Mo 14.00-16.00 6.50-7.75 0.09 Mo 2.00-3.00Al 0.75-1.50 ••Special type similar to 17-7 PH but with higher strength.
17-4 PH 15.50-17.50 3.00-5.00 0.07 Cu 3.00-5.00 •Special type that combines excellent corrosion resistance high strength and hardness, low temperature hardening and good fabrication characteristics.
17-7 PH 16.00-18.00 6.50-7.75 0.09 Al 0.75-1.25 ••Special ultra-high strength type with good formability, excellent fabricating characteristics.

* Feretic, non-hardenable
** Ferritic, hardened by aging
†Austenitic, non-hardenable by heat treatment
†† Martensitic, essentially non-hardenable
•Martensitic, precipitation hardened
•• Semi-Austenitic, precipitation hardened
°Austenitic, precipitation hardened
°°Austenitic, hardened by aging
¥Martensitic, hardenable

Eagle Stainless Tube & Fabrication, Inc., 10 Discovery Way • Franklin, MA 02038
Phone (800) 528-8650 • Fax (800) 520-1954 • www.eagletube.com