CHROMIUM CONTAINING:
As we mentioned, to be a stainless, the iron base must contain at least 10.5% Cr. and the carbon content is less that 1%. These two things made stainless "Steel" totally different from mild "Steel."

The basic stainless with 12 to 18% chromium are called "Martensitic" (based on the structure) and have the following characteristics:

• Are magnetic
• Can be hardened by "heat treatment"
• Have "poor" welding characteristics

Common Uses:

• Knife blades
• Surgical instruments
• Fasteners
• Shafts
• Springs

Common Grades:

Stainless is designated by three different systems

1. Metallurgical structure - Martensitic
2. Grade: 410 (most used), 420 (cutlery), 440C (for very high hardness)
3. Unified Numbering System UNS: S41000, S42000, S44004

(Note chemistry and properties given for 410)

SECOND BRANCH:

CHROMIUM CONTAINING
The second branch also contain 12 to 18% chromium BUT has a LOWER carbon level (less than 0.2%). Since the carbon is low, these grades have a different metallurgical structure and are called "Ferritic" stainless steels. They have the following characteristics:

• Are magnetic
• CANNOT be hardened by "heat treatment" (always used in the annealed or softened condition)
• Weldability is still poor

Common Uses:

• Automotive exhaust and fuel lines
• Architectural trim
• Cooking utensils
• Bank vaults

Common grades:
Stainless is designated by three different systems

1. Metallurgical structure - Ferritic
2. Grade: 409 (high temperature), 430 (most used)
3. Unified Numbering System UNS: S40900, S43000

(Note chemistry and properties given for 430)

Forging:
Stainless steel forging are also available in several shapes. For information, contact the "Forging Industry Association" (216-781-0102).

Rolled Products:
Angles are available as a hot rolled product. Contact "Slater Steel Corporation (219-432-2561).

Hot rolled and cold rolled shapes, as well as cold-drawn "shapes" can also be made from stainless steel as detailed in the Designer Handbook "The Selection and Use of Stainless Steel."

Tubular Products:
Tubular products are commonly used for structural applications either in round or square sections. Further information is available for the "Specialty Tubular Group" (202-342-8450).

Bar and Rod:
Bar is defined as hot finished or cold finished rounds, square, octagons and hexagons and flats (Table 1).

NICKEL CONTAINING:
When nickel is added and the chromium level is increased, the structure changes again and it is called "Austenitic" and they have the following characteristics:

• Are NOT magnetic
• CANNOT be hardened by "heat treatment" BUT CAN be hardened by cold working
• Have the "BEST" corrosion resistance
• Can be easily welded
• Have excellent cleanability and hygiene characteristics
• Have exceptional resistance to both high and low temperature

Common Uses:

• Kitchen sinks
• Architectural applications such as roofs and gutters, doors and windows, tubular frames
• Food processing equipment
• Restaurant food preparation areas
• Chemical vessels
• Ovens
• Heat exchangers

Common Grades:
Stainless is designated by three different systems

1. Metallurgical structure - Austenitic
2. Grade: 304 (most used), 310 (for high temperature), 316 (for better corrosion resistance), 317 (for best corrosion resistance)
3. Unified Numbering System UNS: S30400, S31000, S31600, S31700 (Note chemistry and properties given for 304 and 316)

Some Limitations:
Austenitic stainless steels have some limitations:

• The maximum temperature under oxidizing conditions is 925ºC (see heat resisting stainless steels)
• They are suitable only for low concentrations of reducing acid (Super Austenitics are available for higher acid levels)
• In cervices and shielded areas, there might not be enough oxygen to maintain the passive oxide film and crevice corrosion might occur (Super Austenitics, Duplex and Super Ferritic are available in these situations)
• Very high levels of halide ions, especially the chloride ion can also breakdown the passive surface film

(Super Austenitics and Duplex are available to withstand these conditions)

Information on Super Ferritic and Super Austenitic material is available from the list of Stainless Producers.

DUPLEX STAINLESS STEELS
When the chromium content is high (18 to 26%) and the nickel content is low (4 to 7%), the resulting structure is called Duplex. In addition most grades contain 2 to 3% molybdenum. This results in a structure that is a combination of both Ferritic and Austenitic (hence the name Duplex), and have the following characteristics:

• High resistance to stress corrosion cracking
• Increased resistance to chloride ion attack
• Very weldable
• Have higher tensile and yield strengths than Ausenitic or Ferritic stainless steels

Common Uses:

• Sea water applications
• Heat exchangers
• Desalination plants
• Food pickling plants

Common Grades:

1. Metallurgical structure - Duplex
2. Grade: 2205
3. Unified Numbering System UNS: S31803

HOW IS IT MADE?
Stainless steel is produced in an electric arc furnace where carbon electrodes contact recycled stainless scrap and various alloys of chromium (and nickel, molybdenum etc. depending on the stainless type). A current is passed through the electrode and the temperature increases to a point where the scrap and alloys melt. The molten material from the electric furnace is then transferred into an AOD (Argon Oxygen Decarbonization) vessel, where the carbon levels are reduced (remember stainless has a much lower carbon level than mild steel) and the final alloy additions are made to make the exact chemistry. It is then melted and cast either into ingots or continually cast into a slab or billet form. Then the material is hot rolled or forged into its final form. Some material receives cold rolling to further reduce the thickness as in sheets or drawn into smaller diameters as in rods and wire.

Most stainless steels receive a final annealing (a heat treatment that softens the structure) and pickling (an acid wash that removes furnace scale from annealing and helps promote the passive surface film that naturally occurs).

LIFE CYCLE
The fact that stainless steel has a great resistance to corrosion means that using stainless will result in a very long life compared to mild steel. Structures made from stainless steel will last many times the normal life (well over 100 years in most cases). So, while stainless steel is probably more expensive to buy in the beginning -- because it lasts a long time, it is usually cheaper in the long run because there is little or no maintenance and repair costs. AND, once the useful life is over, stainless steel is 100% RECYCLABLE. Scrap stainless steel is recharged into the electric furnaces for re-melting back into stainless steel. Stainless steel is a true "full life cycle" material.

BENEFITS OF STAINLESS STEEL

Corrosion resistance
--lower alloyed grades resist corrosion in atmospheric and pure water environments, while high-alloyed grades can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments, properties which are utilized in process plants.

Fire & heat resistance
--special high chromium and nickel-alloyed grades resist scaling and retain strength at high temperatures.

Hygiene
--the easy cleaning ability of stainless makes it the first choice for strict hygiene conditions, such as hospitals, kitchens, abattoirs and other 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, therefore 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, from elevated temperatures 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.