How do the grades of screws differ from each other

This article will explain the numbers that appear on screw or bolt heads, which some may refer to as screw grades. Have you ever wondered what the numbers appearing on screw heads, such as 4.8, 8.8, 10.9, or 12.9 that we see in various industries and general applications, actually mean? Today, BSF will explain these designations for you..

Numbers on Screw and Bolt Heads

550.365x350.851 (Original: 3153x2010) The numbers on screw heads typically appear in metric units according to ISO or JIS standards. For screws using imperial units or other standards such as SAE or ASTM, the numerical markings may differ, possibly featuring numbers with five lines, among others. However, this article will focus specifically on the ISO and JIS numbering systems.

The numbers appearing on screw heads, such as 4.8, 8.8, 10.9, or 12.9, where the first digit (before the decimal point) indicates the tensile strength value, and the second digit (after the decimal point) indicates the yield strength value at the yield point, enabling users to select appropriate screw grades for their specific applications. BSF will explain this in the following sections.

Grade 4.8 Example

The first digit, 4, represents the tensile strength or maximum tensile force that the bolt can withstand. This value is determined by multiplying this number by 100 to obtain the force value that this bolt can handle, with units in MPa (Megapascals), N/mm² (Newton/Square Millimeter), or PSI (Pound/Square Inch), depending on the standard used and the user's country. These values can be converted between different units. Example: Tensile Strength = 4 x 100 = 400 MPa or N/mm²

The second digit, 8, represents the yield strength (some texts may use different Thai terminology) that the bolt can withstand while being able to return to its original form without permanent deformation. This value is determined by multiplying the tensile strength calculated from the first digit by 0.8 (from digit 8; if it were another digit, a zero (0) would be placed followed by a decimal point and the second digit) to obtain the force value that this bolt can handle while still maintaining its ability to return to original form. This value is crucial as it enables users to utilize the bolt at full efficiency while preventing cracking or breaking during use. Example: Yield Strength = 400 x 0.8 = 320 MPa or N/mm²

Therefore, Grade 4.8 bolts have Tensile Strength = 400 N/mm² and Yield Strength = 320 N/mm², while Grade 8.8 bolts have Tensile Strength = 800 N/mm² and Yield Strength = 640 N/mm², and so forth.

Screw and Bolt Hardness Grades According to Numerical Markings

• All types of screws and bolts with hardness grades 4.6, 4.8, and 5.8 are typically manufactured from low carbon steel, such as steel grades 1006, 1010, 1016, 1018, 1022, and 1035, which are classified under "low carbon steel" raw materials. Low Carbon Steel: Generally contains carbon content less than 0.25%, featuring outstanding formability and durability properties, while being well-suited for metal forming and welding applications, and offering more economical pricing compared to other steel material types. Low carbon steel has tensile strength ranging between 60,000 psi to 80,000 psi (410 MPa to 550 MPa).
• Fasteners with hardness grade 8.8 are manufactured from medium carbon steel, including steel grades 1038, 1541, 5132, and 5135. Medium Carbon Steel: Contains carbon content ranging between 0.25% to 0.60%. This steel type can undergo heat treatment processes to increase hardness and has tensile strength ranging between 100,000 psi to 120,000 psi (690 MPa to 830 MPa).
• Alloy steel is used in manufacturing products with hardness grades 10.9 and 12.9.

Alloy Steel: Steel containing additions of boron, manganese, chromium, silicon, and other elements to enhance alloy steel capabilities in terms of hardness, formability, toughness, and high-temperature resistance. This steel type has tensile strength exceeding 150,000 psi (1034 MPa).

Letters and Symbols on Screw and Bolt Heads

Letters on bolt heads, such as KN, TY, or other letters, indicate the manufacturer's name, brand, or trademark of the bolt. Alphabetical markings or various symbols on bolt heads, aside from grade or hardness specifications, may or may not appear on bolt heads, as there are no standard requirements for these markings, depending on the manufacturer's discretion.

In this case, BSF represents the abbreviation of the manufacturing company named Boonsan Fastener LTD., PART. and can be frequently found on "Socket Head Cap Screws Grade 12.9".