Materials

Materials

Carbon Steel

Over 90% of fasteners manufactured use carbon steel. Steel has excellent workability, offers a broad range

of attainable combinations of strength properties, and in comparison with other commonly used fastener materials, is less expensive.

The mechanical properties are sensitive to the carbon content, which is normally less than 1.0%. For fasteners, the more common steels are generally classified into three groups: low carbon, medium carbon and alloy steel.

Low Carbon Steels

Low carbon steels generally contain less than 0.25% carbon and cannot be strengthened by heat-treating; strengthening may only be accomplished through cold working. The low carbon material is relatively soft and weak, but has outstanding ductility and toughness; in addition, it is machinable, weldable and is

relatively inexpensive to produce. Typically, low carbon material has a yield strength of 40,000 psi, tensile strengths between 60,000 and 80,000 psi and a ductility of 25% EL. The most commonly used chemical analyses include AISI 1006, 1008, 1016, 1018, 1021, and 1022.

SAE J429 Grade 1, ASTM A307 Grade A are low carbon steel strength grades with essentially the same properties. ASTM A307 Grade B is a special low carbon steel grade of bolt used in piping and flange work. Its properties are very similar to Grade A except that it has added the requirement of a specified maximum tensile strength. The reason for this is that to make sure that if a bolt is inadvertently overtightened during installation, it will fracture prior to breaking the cast iron flange, valve, pump, or expensive length of pipe. SAE J429 Grade 2 is a low carbon steel strength grade that has improved strength characteristics due to cold working.

Medium Carbon Steels

Medium carbon steels have carbon concentrations between about 0.25 and 0.60 wt. These steels may be heat treated by austenizing, quenching and then tempering to improve their mechanical properties. The plain medium carbon steels have low hardenabilities and can be successfully heat-treated only in thin sections and with rapid quenching rates. This means that the end properties of the fastener are subject to size effect. Notice on the SAE J429 Grade 5, ASTM A325 and ASTM A449 specifications that their strength properties “step down” as the diameters increase.

On a strength-to-cost basis, the heat-treated medium carbon steels provide tremendous load carrying

ability. They also possess an extremely low yield to tensile strength ratio; making them very ductile. The popular chemical analyses include AISI 1030, 1035, 1038, and 1541.

Alloy Steels

Carbon steel can be classified as an alloy steel when the manganese content exceeds 1.65%, when silicon

or copper exceeds 0.60% or when chromium is less then 4%. Carbon steel can also be classified as an alloy if a specified minimum content of aluminum, titanium, vanadium, nickel or any other element has been added to achieve specific results. Additions of chromium, nickel and molybdenum improve the

capacity of the alloys to be heat treated, giving rise to a wide variety of strength to ductility combinations.

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SAE J429 Grade 8, ASTM A354 Grade BD, ASTM A490, ASTM A193 B7 are all common examples of alloy steel fasteners.

Stainless Steel

Stainless steel is a family of iron-based alloys that must contain at least 10.5% chromium. The presence of chromium creates an invisible surface film that resists oxidation and makes the material “passive” or corrosion resistant. Other elements, such as nickel or molybdenum are added to increase corrosion

resistance, strength or heat resistance.

Stainless steels can be simply and logically divided into three classes on the basis of their microstructure; austenitic, martensitic or ferritic. Each of these classes has specific properties and basic grade or “type.” Also, further alloy modifications can be made to alter the chemical composition to meet the needs of

different corrosion conditions, temperature ranges, strength requirements, or to improve weldability,

machinability, work hardening and formability.

Austenitic stainless steels contain higher amounts of chromium and nickel than the other types. They are not hardenable by heat treatment and offer a high degree of corrosion resistance. Primarily, they are non- magnetic; however, some parts may become slightly magnetic after cold working. The tensile strength of austenitic stainless steel varies from 75,000 to 105,000 psi.

18-8 Stainless steel is a type of austenitic stainless steel that contains approximately 18% chromium and

8% nickel. Grades of stainless steel in the 18-8 series include, but not limited to; 302, 303, 304 and XM7.

Common austenitic stainless steel grades:

• 302: General purpose stainless retains untarnished surface finish under most atmospheric conditions and offers high strength at reasonably elevated temperatures. Commonly used for wire products such

as springs, screens, cables; common material for flat washers.

• 302HQ: Extra copper reduces work hardening during cold forming. Commonly used for machine screws, metal screws and small nuts

• 303: Contains small amounts of sulfur for improved machinability and is often used for custom-made nuts and bolts.

• 304: Is a low carbon-higher chromium stainless steel with improved corrosion resistance when compared to 302. 304 is the most popular stainless for hex head cap screws. It is used for cold heading and often for hot heading of large diameter or long bolts.

• 304L: Is a lower carbon content version of 304, and therefore contains slightly lower strength characteristics. The low carbon content also increases the 304L corrosion resistance and welding capacity.

• 309 & 310: Are higher in both nickel and chromium content than the lower alloys, and are

recommended for use in high temperature applications. The 310 contains extra corrosion resistance to salt and other aggressive environments.

• 316 & 317: Have significantly improved corrosion resistance especially when exposed to seawater and many types of chemicals. They contain molybdenum, which gives the steel better resistance to surface pitting. These steels have higher tensile and creep strengths at elevated temperatures than other

austenitic alloys.

Austenitic stainless steel limitations:

• They are suitable only for low concentrations of reducing acids.

• In crevices and shielded areas, there might not be enough oxygen to maintain the passive oxide film and crevice corrosion might occur.

• Very high levels of halide ions, especially the chloride ion can also break down the passive surface film.

Martensitic stainless steels are capable of being heat treated in such a way that the martensite is the prime microconstituent. This class of stainless contains 12 to 18% chromium. They can be hardened by heat treatment, have poor welding characteristics and are considered magnetic. The tensile strength of

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