Views: 1 Author: Site Editor Publish Time: 2026-02-06 Origin: Site
ASTM A193 B7/B7M bolts, with their excellent strength, toughness, and high temperature resistance, meet the needs of industries such as petrochemicals, power energy, and pressure vessels. The reliability and safety of industrial pipeline and equipment connections are extremely important in high-temperature and high-pressure working environments. Therefore, it is crucial to choose high-performance fasteners that meet the most standard requirements. This article will provide a detailed introduction to the performance characteristics of B7/B7M materials in the ASTM A193 standard system, as well as related products and standards, to assist you in selecting materials for engineering construction.
The full English name of ASTM A193 standard is "Standard Specification for Alloy Steel and Stainless Steel Bolting for High Temperature or High Pressure Service and Other Special Purpose Applications", which means "Standard for Alloy Steel and Stainless Steel Bolting Materials for High Temperature, High Pressure or Special Purpose Applications". This standard is developed by the American Society for Testing and Materials and is a globally recognized authoritative specification for bolt materials used in high-temperature and high-pressure service environments.
It strictly specifies the chemical composition, mechanical properties, heat treatment processes, and testing requirements for alloy steel and stainless steel bolts, bolts, and other threaded fasteners suitable for high temperature, high pressure, or other special working conditions. Fasteners produced in accordance with this standard ensure reliability and safety for long-term use in extreme environments. Among them, B7 and B7M grades are the most widely used high-strength alloy steel bolt materials.
B7/B7M bolts are usually made of chromium molybdenum alloy steel (such as AISI 4140/4142, etc.). Its core chemical components include:
Carbon (C): Provides basic strength and hardness.
Chromium (Cr): significantly improves hardenability, corrosion resistance, and high-temperature strength.
Molybdenum (Mo): enhances high temperature strength and creep resistance, and prevents temper brittleness.
This scientific alloy ratio provides excellent comprehensive performance foundation for bolts.
| Grade | Carbon | Manganese | Phosphorus | Sulfur | Silicon | Chromium | Molybdenum |
| B7, B7M | 0.38-0.48 | 0.75-1.00 | 0.035 | 0.040 | 0.15-0.35 | 0.80-1.10 | 0.15-0.25 |
| Product Analysis Variation | 0.02 | 0.04 | 0.005 | 0.005 | 0.02 | 0.05 | 0.02 |
B7/B7M bolts undergo a heat treatment process of quenching and tempering to achieve their nominal mechanical properties.
High strength: The yield strength of B7 bolts shall not be less than 105 ksi (approximately 724 MPa), and the tensile strength shall not be less than 125 ksi (approximately 862 MPa). B7M maintains similar tensile strength while adjusting the heat treatment process to control its hardness between HRC 22-32, effectively reducing its sensitivity to stress corrosion cracking (SCC).
Good toughness: While ensuring high strength, strict impact energy requirements ensure that the material has good toughness, can withstand impact loads, and prevent brittle fracture.
Excellent anti relaxation performance: able to maintain pre tension at high temperatures, ensuring the sealing reliability of flange connections during thermal cycling.
| Class | Diameter [mm] | Minimum Tempering Temperature, ℃ | Tensile Strength, min, MPa | Yield Strength, min,02% offset, MPa | Elongation in 4D, min,% | Reduction of Area min,% | Hardness max |
| B7 | M64 and under | 593 | 860 | 720 | 16 | 50 | 321 HBW or 35 HRC |
| over M64 to M100 | 593 | 795 | 655 | 16 | 50 | 321 HBW or 35 HRC | |
| over M100 to M180 | 593 | 690 | 515 | 18 | 50 | 321 HBW or 35 HRC | |
| B7M | M100 and under | 620 | 690 | 550 | 18 | 50 | 235 HBW or 99 HRB |
| over M100 to M180 | 620 | 690 | 515 | 18 | 50 | 235 HBW or 99 HRB |
B7: When the connection environment is mainly high temperature and high pressure, but there are no specific corrosive media such as wet H ₂ S, chloride, etc., B7 is an economical and efficient choice.
B7M: When the connection environment contains high temperature, high pressure, and media that may cause stress corrosion cracking, B7M must be selected. This is a common and mandatory safety regulation in industries such as petroleum refining, natural gas processing, and chemical (chlorine containing environments).
The strength of B7 and B7M is comparable, but B7M sacrifices some hardness (lower upper limit) and enforces toughness in exchange for much better stress corrosion cracking safety in corrosive environments than B7M. In critical applications involving sensitive corrosive media, B7M is a safer and more compliant choice.
ASTM A193 specifies materials and properties, while the specific dimensions, tolerances, and mechanical properties of bolts must comply with relevant product standards. For B7/B7M bolts, the following two core ASME standards are mainly followed:
This standard specifies in detail the dimensions, tolerances, mechanical and material requirements for the British series (corresponding to B7) square head bolts and hexagonal head bolts. It is the most universal product standard for British heavy-duty hexagonal bolts.
ASME B18.31.2 is a standard specification developed specifically for Stud Bolts in the B18 series. It systematically specifies the requirements for bolts used for pressure vessels, pipeline flanges, valves, and other critical structural connections.
The bolts that meet these standards ensure precise interchangeability of dimensions, firm bonding of the head rod, and uniformity of installation tools.
Based on ASTM A193 B7/B7M materials, fastener types that meet various installation scenarios and functional requirements can be produced:
Double headed stud: with threads on both ends, used in conjunction with two nuts, it is an important fastener for pressure vessels and pump valve flange connections, making it easy to install and disassemble.
Full thread stud: Full body thread, suitable for situations that require full-length adjustment or nuts that can be locked in any position.
Hexagonal bolt: The most common type, providing standard hexagonal heads that can be installed with a regular wrench and have a wide range of applications.
Hexagonal socket bolt: With a cylindrical head and hexagonal socket, the structure is compact and requires minimal installation space. The head can sink into the interior of the workpiece, resulting in a neat appearance.
Square head bolt: The head is square, which is easy to use a wrench to lock and prevent rotation. It is commonly used to withstand large torque or fix equipment bases.
With its outstanding performance, ASTM A193 B7/B7M bolts have become an indispensable key component in the following high-end industrial fields:
Oil and gas: oil and gas extraction platforms, refinery pipeline flanges, reactors, heat exchangers, high-pressure valves.
Chemical and petrochemical: flange connections for various towers, reactors, high-pressure pipelines, and synthesis equipment.
Electric energy: boilers, turbines, and high-temperature steam pipelines in thermal power plants; Auxiliary systems of nuclear power plants.
Pressure vessels and boilers: all flange and manhole cover connections for pressure vessels and boilers that require ASME certification.
Heavy machinery: mining machinery, large hydraulic equipment, and other applications that require high loads and impacts.
