Technique Reference for Bolts, Studs and Screws

Bolts, screws and other external threaded fasteners could be made of various materials, including steels, brass, copper, aluminium and plastics. As we mostly produce metal fasteners, here we list some basic technique information on metal fasteners. The main points for a bolts or similar fasteners are material selection, mechanical properties, forming process, heating treatment (if needed), surface treatment, marking and packing.

Material Options for Bolts & Screws

Over 90% of bolts are made of steels due to its low cost, workability and durability. To select the right material, users need to consider it's strength, temperature, corrosion, vibration, fatigue and may be more. For a general usage of bolts, it can use a low carbon steel, while for a heavy duty fastening, like bridge, construction, it need a higher strength material, for example middle carbon steel or various alloy steels.

Mechanical Properties

Before we decide which material to use, we must clarify the characters of different materials by their mechanical properties (tensile strength, yield strength, proof load, hardness)

1. Tensile Strength: The most widely associated mechanical property associated with standard threaded fasteners is tensile strength. Tensile strength is the maximum tension-applied load the fastener can support prior to or coinciding with its fracture.

2. Proof Load: The proof load represents the usable strength range for certain standard fasteners. By definition, the proof load is an applied tensile load that the fastener must support without permanent deformation. In other words, the bolt returns to its original shape once the load is removed.

3. Hardness: Its a measure of a material’s ability to resist abrasion and indentation. For carbon steels, Brinell and Rockwell hardness testing can be used to estimate tensile strength properties of the fastener.

Material Selections

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.

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.

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.

Stainless Steels
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 nonmagnetic; 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.

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
5 martensitic stainless steel is approximately 70,000 to 145,000 psi. This type of stainless steel should only
be used in mild corrosive environments.
Common martensitic stainless steel grades:
• 410: A straight chromium alloy containing no nickel. General-purpose corrosion and heat resisting,
hardenable chromium steel. It can be easily headed and has fair machining properties. Due to their
increased hardness, are commonly used for self-drilling and tapping screws. These are considered
very inferior in corrosion resistance when compared with some of the 300.
• 416: Similar to 410 but has slightly more chromium, which helps machinability, but lowers corrosion
resistance.

Ferritic stainless steels contain 12 to 18% chromium but have less than 0.2% carbon. This type of steel is magnetic, non-hardenable by heat treatment and has very poor weld characteristics. They should not be
used in situations of high corrosion resistance requirements.
Common ferritic stainless steel grades:
• 430: Has a slightly higher corrosion resistance than Type 410 stainless steel.

Heat Treatments

Heat-treating is performed to change certain characteristics of metals and alloys in order to make them
more suitable for a particular kind of application. In general, heat treatment is the term for any process
employed, by either heating or cooling, to change the physical properties of a metal. The goal of heat
treatment is to change the structure of the material to a form, which is known to have the desired
properties. The treatments induce phase transformations that greatly influence mechanical properties such
as strength, hardness, ductility, toughness, and wear resistance of the alloys. The large number of service
requirements and amount of alloys available make for a considerable amount of heat-treating operations.

Heat Treatment of Carbon Steels and Carbon Alloy Steels
After being formed, the middle and high grade carbon steels and alloy steels bolts usually quenched and tempered to improving properties such as hardness, tensile and yield strength. The desired results are accomplished by heating in temperature ranges where a phase or combinations of phases are stable, and/or heating or cooling between temperature ranges in which different phases are stable.

The reference of SAE standard requirement on the middle and high grade steel bolts as follows,

SAE J429 Grade 5 for thread size 1/4" up to 1" (equals to DIN/EN/ISO Metric Class 8.8)
• Tensile Strength: 120,000 PSI min (826N/mm2)
• Proof Strength: 85,000 PSI
• Yield Strength: 92,000 PSI min
• Hardness: HRC 25-34

SAE J429 Grade 8 for thread size 1/4" up to 1.1/2" (equals to DIN/EN/ISO Metric Class 10.9)
• Tensile Strength: 150,000 PSI min (1033N/mm2)
• Proof Strength: 120,000 PSI (826N/mm2)
• Yield Strength: 130,000 PSI min
• Hardness: HRC 33-39