300’s – Chromium-Nickel Stainless Steel this series is austenitic, non-heat treatable, and non-magnetic.

400’s – Chromium Stainless Steel this series is martensitic, heat treatable and magnetic. It also includes types, which are ferrite, non-heat treatable, and magnetic.

“L” at the end of the series number indicates low-carbon content. (Example: 304L)
 
“F” at the end of the series number indicates the addition of a “free-machining” element. (Example: 440F)

Other letters used at the end of the series number signify the symbol of the element added to the alloy. (Example: 440C – the C being the symbol for the Carbon additive)

Stainless Steel Classifications

There are four basic classes of stainless steels, so designated for the metallurgical conditions of the steels:

Class I: Martensitic – Heat treatable, Straight” Chromium

This class is so named for the man, Martens, who first examined metals microscopically. It is referred to as “martensitic” because of its acicular or needle-like microstructure in the hardened condition. Its chief alloying agent is chromium, found in amounts from 11.5 to 18.0%. It contains from 0.08 to 1.10% carbon. It is magnetic and responds excellently to heat treating, producing a hard and strong stainless steel.

Class II: Ferritic – Non-Heat treatable, Straight Chromium

This class name is derived from the Latin word “ferrum” meaning iron. It is so named because its microstructure is very similar to that of low-carbon iron. It also utilizes chromium as its chief alloying agent, being found in amounts from 14.0 to 27.0%. It has a very low carbon content of .08 to .20%. Due to its high chromium and low carbon content, ferritic alloys do not generally harden in high temperatures. It is a magnetic alloy, and is soft and ductile.

Class III: Austenitic – Non-Heat treatable, Chromium-Nickel

The austenitic class derives its name from Roberts-Austen who first observed its charac­teristic banded grain structure. Its chief alloys are: chromium, found in amounts from 16.0 to 26.0%; and a appreciable nickel content from 6.0 to 22.0%. This alloy cannot be heat treated, but responds excellently to cold working. It is generally non-magnetic. In the an­nealed condition, this alloy is tough, strong, and extremely ductile. Austenite itself is soft and tough and remains ductile even at extreme low temperatures.

Class IV: Precipitation-Hardening

This is a relatively new class metallurgists have deemed necessary to group separately be­cause of its increasing popularity. These alloys have low hardening temperatures that produce precipitation hardening. This capability averts problems such as warping, cracking, and scaling. They can be hardened by simple heat treatments, require no stress-relief treat­ment and are available in all forms. These grades are easily fabricated, and are corrosion resistant without added treatment. They are also known for their high strength. An American Iron and Steel Institute (AISI) Type designation has not been issued these grades, as they are patented proprietary.

Extra-Low-Carbon (ELC) Grades

These alloys contain only approximately .03% carbon, which is low enough to permit elimination of carbide precipitation during welding. These grades are not generally recommended for high-temperature utilization.

Effects of Alloying Elements 

Stainless Steel Sheet Finishes 

Type 301 

This grade is an austenitic stainless steel manufactured by-the electric furnace process. Its chromium and nickel content are lower than most other grades, offering the advantage of a high work-hardening rate which combines cold-worked high strength with good ductility. Tensile strength and hardness increase rapidly when the metal is cold rolled, cold drawn, or worked at room temperature. The standards of the aircraft industry are met by requiring adequate discard to be extracted from each ingot. Its application is indicated where low cost is desired and high corrosion resistance is not a primary concern.

Types 302, 304, 304L

Type 302 is the fundamental alloy of the austenitic class. It is commonly known as “18-8”: 18% chromium; 8% nickel; and is the most commonly used of all the stainless grades, Type 302 is non-heat treatable, but cold-working considerably increases both its hardness and tensile strength. Type 302 in the cold state offers great versatility of workability because of its toughness and ductility and can be rigorously spun, rolled, drawn or machined. It offers outstanding weldability. It is extremely resistant to corrosion, and retains an untarnished silvery surface. It is also resistant to heat oxidation at temperatures up to 1500° F. It is non-magnetic in the annealed condition. The principal drawback of Type 302 is that of sensitization-under extreme conditions, carbide precipitation may occur. Type 304 alleviates this problem by decreasing the carbon content and thereby eliminating the possibility of intergranular corro­sion. This low carbon alloy is most often utilized for applications requiring welding. An Extra Low Carbon alloy, Type 304L is also available for especially severe welding applications. Type 304L has the capability to avert any detrimental precipitation in the extreme 800° F. to 1650° F. range.

Types 303S & 303Se

Types 303S and 303Se are both free-machining modifications of Type 302. Sulfur or a combination of selenium and phosphorus are added to this “18-8” chromium-nickel alloy to promote chip formation in machining rather than spindly spirals. Type 303 has uniform machinability and can be machined at speeds of SAE 3120, 3145, and 4615, adapting it very well for automatic screw machine applications. Manufactured by the electric-furnace process, this is a non-heat treatable alloy, the hardness and tensile strength of which may be increased greatly by cold working. It has good corrosion resistance, and is non- magnetic in the annealed state. It concurs with the stringent requirements of the aircraft industry.

Type 310

Type 310 is the grade of stainless steel containing the highest chromium-nickel content of all grades – 25% chromium, 20% nickel, It is chiefly known for its superior scaling and corrosion resistance and it excels all other grades in its high temperature physical properties. At extremely high temperatures, its creep strength and resistance to brittling far surpasses all other austenitic grades. In the annealed state it is non-magnetic. Type 310 is a non-heat treatable alloy pro­duced by the electric-furnace process, which concurs with the exacting standards of the aircraft industry.

Types 316 & 316L

Type 316 is an electric furnace processed modification of Type 302: it contains 18% chromium; 8% nickel, and; 2-3% molybdenum. This addition of molybdenum increases both the corrosion resistance and the high temperature strength of this alloy. The most outstanding advantage of this addition is the increased corrosion resistance to reducing acids and pitting or pin hole corrosion. In general, Type 316 is known as the major all-around corrosion resistant austenitic stainless steel available. Under extremely elevated temperatures, Type 31 6 proves itself to possess remarkable creep and rupture strength, This non-heat treatable, non-magnetic alloy possesses excellent cold forming and drawing properties, making it suitable for a wide range of applications. Type 316L is an extra low carbon modification of Type 316 recommended for use during welding operations. The low carbon factor eliminates the possibility of harmful carbide precipitation in the 800° F. to 1500° F. range.

Type 321

Type 321 is an electric-furnace processed austenitic stainless steel. It is non-heat treatable and non-magnetic in the annealed condition. This alloy contains 18% chromium, 8% nickel and a substantial addition of titanium. The titanium forms insoluble and stable carbide, which ties up all the carbon in the alloy and therefore prevents it from precipitating as chromium carbides. This leaves the chromium in solution to resist corrosion to a very high degree. This is extremely beneficial in applications of temperatures ranging from 800° F, to 1600° F., as it eliminates the necessity for re-annealing after fabrication.

Type 347

Type 347 is a non-heat treatable, austenitic, electric furnace processed grade of stainless steel very similar in composition to Type 321. The major difference between Type 321 and Type 347 is that rather than having an addition of titanium, Type 347 has columbium added to it. Tantalum occurs in nature in conjunction with columbium and therefore it may be said that both are additives to this alloy. Type 347 can withstand more severely elevated temperatures than Type 321, as the resulting columbium carbide is more stable and insoluble than the titanium carbide. The only drawback in the use of Type 347 as opposed to Type 321 is that it is not recommended for use in radioactive services as radioactive tantalum has a much longer half-life than columbium. It is non-magnetic in the annealed state.

Type 410

Type 410 is a magnetic, martensitic, heat treatable alloy that is 12% straight chromium. It has excellent creep strength and corrosion resistance. Heat treatments may be applied to develop a very wide range of mechanical properties and hardness. It is popularly used for parts operating at temperatures up to 850° F.

 Type 416

Type 416 is an electric furnace processed, magnetic, free-machining grade of the martensitic stainless steels. It is a modification of Type 410 with approximately .30% sulfur added for excellent machinability. Like Type 410, it has an exceptionally wide range of mechanical properties obtainable through heat treating, This grade of stainless steel has the highest machinability of all grades now developed, and can often be used in the “as machined” condition without heat treatment.

15-5 PH Precipitation Hardening

15-5 PH is a chromium-nickel alloy containing a 5% copper additive, which permits it to be hardened by low temperature heat treatments. The high percentages of chromium and nickel give-15-5 PH excellent corrosion resistance, transverse toughness and forgeability. This alloy is produced by the vacuum arc re-melt method, which enhances ductility and toughness. 15-5 PH has excellent physical and mechanical properties and may be deep drawn, forged, welded and formed.

17-4PH Precipitation Hardening

17-4 PH is a chromium-nickel alloy that has a 4% copper additive, which enables it to be hardened by very low-temperature heat treatments known as precipitation hardening. The high percentages of chromium and nickel give this alloy excellent corrosion resistance, physical properties, and a high level of strength at temperatures up to 800° F. The major advantage of low temperature heat treatments is the elimination of distortion and scaling. 17-4 PH has excellent mechanical properties and may easily be welded, deep drawn, forged, and severely formed.

17-7PH Precipitation Hardening

17-7 PH contains 17% chromium, 7% nickel and 1% aluminum. It is a precipitation hardening steel capable of reaching very high strength and hardness without any loss of corrosion resist­ance. Surface scaling and distortion in heat treatment is eliminated by its ability to be treated at very low temperatures. In the annealed condition, it has excellent ductility and machine­ability. It retains remarkable mechanical and physical properties at temperatures up to 800° F.