A BRIEF OVERVIEW OF STAINLESS STEEL

Stainless steel is primarily when corrosion or oxidation are a problem. The function that

they perform cannot be duplicated by other materials for their cost. Over 50 years ago, it

was discovered that a minimum of 12% chromium would impart corrosion and oxidation

resistance to steel. Hence the definition “Stainless Steels”, are those ferrous alloys that

contain a minimum of 12% chromium for corrosion resistance. This development was the

start of a family of alloys which has enabled the advancement and growth of chemical

processing and power generating systems upon which our technological society is based.

Subsequently several important sub-categories of stainless steels have been developed.

The sub-categories are austenitic, martensitic, ferritic, duplex, precipitation hardening and

super alloys.

Austenitic Grades

Austenitic grades are those alloys which are commonly in use for stainless applications.

The austenitic grades are not magnetic. The most common austenitic alloys are ironchromium-

nickel steels and are widely known as the 300 series. The austenitic stainless

steels, because of their high chromium and nickel content, are the most corrosion resistant

of the stainless group providing unusually fine mechanical properties. They cannot be

hardened by heat treatment, but can be hardened significantly by cold-working.

 

 

 

 

Straight Grades

The straight grades of austenitic stainless steel contain a maximum of .08% carbon. There

is a misconception that straight grades contain a minimum of .03% carbon, but the spec

Stainless Plate Products

does not require this. As long as the material meets the physical requirements of straight

grade, there is no minimum carbon requirement.

“L” Grades

The “L” grades are used to provide extra corrosion resistance after welding. The letter

“L” after a stainless steel type indicates low carbon (as in 304L). The carbon is kept to

.03% or under to avoid carbide precipitation. Carbon in steel when heated to temperatures

in what is called the critical range (800 degrees F to 1600 degrees F) precipitates out,

combines with the chromium and gathers on the grain boundaries. This deprives the steel

of the chromium in solution and promotes corrosion adjacent to the grain boundaries. By

controlling the amount of carbon, this is minimized. For weldability, the “L” grades are

used. You may ask why all stainless steels are not produced as “L” grades. There are a

couple of reasons. First, the “L” grades are more expensive. In addition, carbon, at high

temperatures imparts great physical strength

Frequently the mills are buying their raw material in “L” grades, but specifying the

physical properties of the straight grade to retain straight grade strength. A case of having
your cake and heating it too. This results in the material being dual certified 304/304L;
316/316L, etc.

“H” Grades

The “H” grades contain a minimum of .04% carbon and a maximum of .10% carbon and

are designated by the letter “H” after the alloy. People ask for “H” grades primarily when

the material will be used at extreme temperatures as the higher carbon helps the material

retain strength at extreme temperatures.

You may hear the phrase “solution annealing”. This means only that the carbides which

may have precipitated (or moved) to the grain boundaries are put back into solution

(dispersed) into the matrix of the metal by the annealing process. “L” grades are used

where annealing after welding is impractical, such as in the field where pipe and fittings

are being welded.

Type 304

The most common of austenitic grades, containing approximately 18% chromium and 8%

nickel. It is used for chemical processing equipment, for food, dairy, and beverage industries,

for heat exchangers, and for the milder chemicals.

Type 316

Contains 16% to 18% chromium and 11% to 14% nickel. It also has molybdenum added

to the nickel and chrome of the 304. The molybdenum is used to control pit type attack.

Type 316 is used in chemical processing, the pulp and paper industry, for food and

beverage processing and dispensing and in the more corrosive environments. The molybdenum

must be a minimum of 2%.

Type 317

Contains a higher percentage of molybdenum than 316 for highly corrosive environments.

It must have a minimum of 3% “moly”. It is often used in stacks which contain

scrubbers.

Type 317L

Restricts maximum carbon content to 0.030% max. and silicon to 0.75% max. for extra

corrosion resistance.

Type 317LM

Requires molybdenum content of 4.00% min.

Type 317LMN

Requires molybdenum content of 4.00% min. and nitrogen of .15% min.

Type 321, Type 347

These types have been developed for corrosive resistance for repeated intermittent exposure

to temperature above 800 degrees F. Type 321 is made by the addition of titanium

and Type 347 is made by the addition of tantalum/columbium. These grades are primarily

used in the aircraft industry.

Martensitic Grades

Martensitic grades were developed in order to provide a group of stainless alloys that

would be corrosion resistant and hardenable by heat treating. The martensitic grades are

straight chromium steels containing no nickel. They are magnetic and can be hardened by

heat treating. The martensitic grades are mainly used where hardness, strength, and wear

resistance are required

 

 

 

 

Type 410

Basic martensitic grade, containing the lowest alloy content of the three basic stainless

steels (304, 430, and 410). Low cost, general purpose, heat treatable stainless steel. Used

widely where corrosion is not severe (air, water, some chemicals, and food acids. Typical

applications include highly stressed parts needing the combination of strength and corrosion

resistance such as fasteners.

Type 410S

Contains lower carbon than Type 410, offers improved weldability but lower

hardenability. Type 410S is a general purpose corrosion and heat resisting chromium

steel recommended for corrosion resisting applications.

Type 414

Has nickel added (2%) for improved corrosion resistance. Typical applications include

springs and cuttlery.

Type 416

Contains added phosphorus and sulfer for improved machinability. Typical applications

include screw machine parts.

Type 420

Contains increased carbon to improve mechanical properties. Typical applications include

surgical instruments.

Type 431

Contains increased chromium for greater corrosion resistance and good mechanical

properties. Typical applications include high strength parts such as valves and pumps.

Type 440

Further increases chromium and carbon to improve toughness and corrosion resistance.

Typical applications include instruments.

Ferritic Grades

Ferritic grades have been developed to provide a group of stainless steel to resist corrosion

and oxidation, while being highly resistant to stress corrosion cracking. These steels

are magnetic but cannot be hardened or strengthened by heat treatment. They can be cold

worked and softened by annealing. As a group, they are more corrosive resistant than the

martensitic grades, but generally inferior to the austenitic grades. Like martensitic grades,

these are straight chromium steels with no nickel. They are used for decorative trim,

sinks, and automotive applications, particularly exhaust systems.

 

 

 

 

Type 430

The basic ferritic grade, with a little less corrosion resistance than Type 304. This type

combines high resistance to such corrosives as nitric acid, sulfur gases, and many organic

and food acids.

Type 405

Has lower chromium and added aluminum to prevent hardening when cooled from high

temperatures. Typical applications include heat exchangers.

Type 409

Contains the lowest chromium content of all stainless steels and is also the least expensive.

Originally designed for muffler stock and also used for exterior parts in non-critical

corrosive enviornments.

Type 434

Has molybdenum added for improved corrosion resistance. Typical applications include

automotive trim and fasteners.

Type 436

Type 436 has columbium added for corrosion and heat resistance. Typical applications

include deep-drawn parts.

Type 442

Has increased chromium to improve scaling resistance. Typical applications include

furnace and heater parts.

Type 446

Contains even more chromium added to further improve corrosion and scaling resistance

at high temperatures. Especially good for oxidation resistance in sulfuric atmospheres.

Duplex Grades

Duplex grades are the newest of the stainless steels. This material is a combination of

austenitic and ferritic material. This material has higher strength and superior resistance

to stress corrosion cracking. An example of this material is type 2205. It is available on

order from the mills.

Precipitation Hardening Grades

Precipitation hardening grades, as a class, offer the designer a unique combination of

fabricability, strength, ease of heat treatment, and corrosion resistance not found in any

other class of material. These grades include 17Cr-4Ni (17-4PH) and 15Cr-5Ni (15-5PH).

The austenitic precipitation-hardenable alloys have, to a large extent, been replaced by

the more sophisticated and higher strength superalloys. The martensitic precipitationhardenable

stainless steels are really the work horse of the family. While designed primarily

as a material to be used for bar, rods, wire, forgings, etc., martensitic precipitationhardenable

alloys are beginning to find more use in the flat rolled form.

While the semiaustenitic precipitation-hardenable stainless steels were primarily designed

as a sheet and strip product, they have found many applications in other product forms.

Developed primarily as aerospace materials, many of these steels are gaining commercial

acceptance as truly cost-effective materials in many applications.

Superalloy Grades

Superalloys are used when 316 or 317 are inadequate to withstand attack.They contain

very large amounts of nickel and/or chrome and molybdenum. They are usually much

more expensive than the usual 300 series alloys and can be more difficult to find. These

alloys include Alloy 20 and Hastelloy.

 

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