Technicals: Fastener Material Class/ Grades

Fastener Performance Class or Grade

A metal's grade/ class is established by a combination of numbers, letters, or both, and is based on its chemical composition, physical characteristics and mechanical properties. Numerous organizations have set standards for the various metal grades. The grading of steel serves to categorise a fastener and material/ finish is assigned a grade, to facilitate evaluation of its suitability for various uses.

Metric Steel Class/ Grade

Material grade is most often a reference to it's 'tensile resisistance', otherwise known as 'tensile strength'. It is defined as the utmost amount of stress a material can withstand when stretched or pulled before breaking/ failing. The tensile testing process yields critical information about the strength, yield strength, and ductility of a material, providing key insights into its performance. It involves measuring the amount of force needed to break a composite or plastic specimen and the degree to which the specimen stretches/ elongates before breaking. The fastener will experience permanent stretching if we exceed its elastic limit and reach its yield strength. Be aware that loading past the ultimate load and maximum tension will lead to necking and failure of the bolt. Use caution not to exceed this point.

Different Metric Steel Tensile Strengths Available

UNC/ UNF/ UNS American Inch Steel Class/ Grade

Different grade options are available for standardised, inch measured bolts and screws made of carbon and alloy steels. These bolts are usually labeled on the head with the bolt grade and the manufacturer's identification, as below:

BSW (Whitworth), BSF British Inch Steel Class/ Grade

Bolts and screws made to English specifications (BSW and BSF) are marked with an 'R' and this represents 'railway' class. The mechanical properties are just lower than 8.8 metric high tensile steel. Grade 'S' is very similar to metric 8.8.

English European and American Material Fastener Grade Comparison Chart

KEY

Grade

Material classification

ton/ in²

Tonne per Square Inch - Unit of Pressure

Hardness - Brinell

The Brinell hardness test requires a specific load or force to be applied for a designated time period, typically between 10-30 seconds, and using a tungsten carbide ball/ sphere with a diameter of 2.5 or 10mm. The load is usually set between 187.5 and 3000Kgf. The size (diameter) of the indentation determines it's hardness.

Tensile Strength

Pressure needed to cause the material to fail/ rupture.

Yield Strength

Yield strength is the resistance point beyond the elasticity of the material whereby the material will elongate

P.S.I.

Pounds Per Square Inch - Unit of Pressure

Proof (PSI)

The maximum pressure that can be applied to a product without causing permanent damage to its operational function is known as proof pressure. This measurement is also referred to as "over-range capacity". Proof is the last point at which the material can reform once pressure is released and is therefore the information you need to know when designing for function.

ASTM

The ASTM steel grades are defined by the American Society for Testing and Materials to meet strict criteria for mechanical properties and chemical composition. These criteria are backed by scientific research and specify precise testing methods.

SAE

SAE standards were created for experts working in the fields of engineering, specifically in the realm of transportation, including the automotive, aerospace, and commercial vehicle industries.

ASTM vs SAE Standards

The application usage for ASTM fastener standards typically consists of construction projects, whereas SAE grades are commonly implemented in the automotive and equipment industries.

Stainless Steel Material Class/ Grades 

Metric Stainless Steel Material Class/ Grades

Stainless steel fasteners and bolts made from austenitic materials, marked with the designation letter ‘A’, boast exceptional corrosion resistance and are widely utilized in their production - accounting for around 75% of all stainless steel production. These materials can only be strengthened through cold working, not heat treatment. The three main alloy classes for austenitic steels are A2, the most common, A4 for increased corrosion resistance, and A1 for free machining applications such as dowels and pins. The final identification is the strength class/grade, measured in Mega Pascal’s (MPa) and divided by 10. Hence, A2-70 has a strength of 700 MPa, while A2-80 has a strength of 800 MPa.
Type A4/ 316 stainless steel is the second most widely used type of stainless, just behind Type A2/ 304. Although it has a lower chromium content of 16%, compared to 18% for Type 304, its nickel content is increased to 10% and 2% molybdenum is added to improve its resistance to corrosive elements like chlorides. This modification in the chromium/nickel ratio and the inclusion of molybdenum make Type 316 ideal for applications in highly corrosive environments such as marine construction and exposure to chemical, solvent, or salt water corrosion.

Stainless Imperial ASTM - F593

ASTM F593 outlines the necessary chemical and mechanical specifications for stainless steel bolts, hex cap screws, and studs that range in diameter from 1/4″ to 1-1/2″.

Metric Nut Material Class/ Grade

The property class of hexagon nuts is denoted by a single-digit symbol, with each numeral representing approximately 1/100 of the minimum tensile strength in MPa. For instance, a property class 8 nut has a minimum tensile strength of 800 MPa and should be paired with a grade 8 bolt. A grade 10 nut is the proper match for bolts with a property class of 10.9.

Imperial Nut Material Class/ Grade

Hexagon nuts manufactured to meet SAE J995, have 3 grades: grade A providing low tensile strength and being suitable for use with grade 2 bolts, while grades B and C are designed for use with Grade 5 bolts. Additionally, similar grades are available under ASTM A563, and various other standards exist for gas and oil-specific applications.

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