Bolt Grades Complete Guide — Grade 2, 5, 8, B7, B8 & L7 Explained - Blog

What Are Bolt Grades and Why Choosing the Right Grade Matters

Definition of Bolt Grades - Standardized Classification of Strength, Material, and Heat Treatment

Bolt grades are standardized classifications that define the mechanical properties, chemical composition, and heat treatment requirements for bolts. Each grade specifies minimum tensile strength, yield strength, hardness range, and elongation, ensuring predictable performance under defined loading conditions. The grade system allows engineers to select bolts with confidence that they will meet the design requirements without over-specifying or compromising safety.

Cost of Choosing the Wrong Grade - Fracture, Loosening, Equipment Accidents

Selecting a bolt grade below the required strength can lead to thread stripping, bolt fracture, joint separation, and catastrophic equipment failure. Conversely, overspecifying to a higher grade than needed often adds unnecessary cost and can introduce risks such as hydrogen embrittlement susceptibility. The correct grade must match the design load, service temperature, environmental conditions, and applicable code requirements.

Global Mainstream Standard Systems - SAE J429, ASTM A193/A320, ISO 898, GB/T 3098

The major global bolt grade standards include SAE J429 for carbon and alloy steel bolts in imperial sizes (North America), ASTM A193 for alloy and stainless steel bolts in high-temperature/high-pressure service, ASTM A320 for low-temperature service bolts, ISO 898 for metric bolts (international), and GB/T 3098 (Chinese standard). Understanding cross-references between these systems is essential for international projects and global procurement.

SAE J429 Bolt Grades Explained (Grade 2, 5, 8)

Grade 2 Bolts - Low Carbon Steel, Lowest Strength, Suitable for General Non-Critical Applications

SAE J429 Grade 2 bolts are made from low or medium carbon steel without any required heat treatment. They have a minimum tensile strength of 60,000 psi (414 MPa) for sizes up to 3/4 inch and 55,000 psi for larger diameters. Grade 2 bolts have no head markings and are suitable for light-load applications such as furniture assembly, light fixtures, and non-structural brackets where strength requirements are minimal.

Grade 5 Bolts - Medium Carbon Steel Quenched and Tempered, Medium Strength

Grade 5 bolts are made from medium carbon steel that is quenched and tempered to achieve a minimum tensile strength of 120,000 psi (827 MPa) for diameters up to 1 inch. They are identified by three radial lines on the bolt head spaced 120 degrees apart. Grade 5 is commonly used in automotive assemblies, agricultural equipment, machinery, and general construction where moderate to high strength is required without the cost of premium grades.

Grade 8 Bolts - Medium Carbon Alloy Steel Quenched and Tempered, High Strength

Grade 8 bolts are manufactured from medium carbon alloy steel (typically 4140 or similar) quenched and tempered to deliver a minimum tensile strength of 150,000 psi (1,034 MPa). They are marked with six radial lines on the head and are specified for heavy equipment, earthmoving machinery, military applications, and structural connections requiring maximum strength in the SAE system.

Mechanical Properties Data Table

Grade	Material	Min Tensile	Yield (0.2%)	Hardness	Head Markings
Grade 2	Low/Medium Carbon Steel	60,000 psi	36,000 psi	-	None
Grade 5	Medium Carbon Steel Q&T	120,000 psi	92,000 psi	19-30 HRC	3 radial lines
Grade 8	Alloy Steel Q&T	150,000 psi	130,000 psi	33-39 HRC	6 radial lines

ASTM A193 Bolt Grades Explained (B7, B8, B8M, B16)

B7 - Chromium-Molybdenum Alloy Steel, High Temperature and High Pressure

ASTM A193 B7 is the most widely specified grade for bolting in pressure vessels, valves, flanges, and piping systems. Made from AISI 4140 or 4142 chromium-molybdenum steel, B7 bolts are quenched and tempered to achieve minimum 125 ksi (860 MPa) tensile strength with excellent toughness and creep resistance up to 450°C. B7 bolts are marked with the designation "B7" on the head and are typically paired with ASTM A194 Grade 2H nuts.

B8 / B8M - 304 / 316 Stainless Steel, Corrosion-Resistant Environments

ASTM A193 B8 bolts are manufactured from Type 304 stainless steel, while B8M bolts use Type 316 stainless steel with added molybdenum for enhanced pitting resistance. These grades deliver moderate strength (approximately 75-80 ksi minimum tensile for strain-hardened condition) combined with excellent corrosion resistance suitable for chemical processing, food and beverage, pharmaceutical, and marine environments. B8 bolts are marked "B8" and B8M bolts "B8M" on the head.

B16 - Chromium-Molybdenum-Vanadium Alloy Steel, Higher Temperature (≤593°C)

ASTM A193 B16 bolts are made from chromium-molybdenum-vanadium alloy steel with controlled vanadium content for enhanced high-temperature strength. B16 bolts maintain useful strength up to 593°C (1100°F), making them the preferred choice for power generation turbines, refinery heaters, and high-temperature process piping. They are marked "B16" on the head and matched with ASTM A194 Grade 7 nuts for complete high-temperature assemblies.

B7 vs B8 Selection Boundaries - Operating Temperature, Media Corrosiveness

The choice between B7 and B8 depends primarily on the service environment. B7 is selected for non-corrosive, high-temperature applications up to 450°C where strength is the priority, such as steam systems and hydrocarbon processing. B8 is chosen when the media is corrosive (acids, chlorides, caustic solutions) even at lower temperatures, or when product contamination from rust must be avoided. In the temperature range of 300-450°C with mild corrosivity, B8 may be preferred despite its lower strength.

ASTM A320 Low-Temperature Bolt Grades (L7, L7M, L43)

L7 - Low-Temperature Impact Toughness, Suitable for -101°C and Above

ASTM A320 L7 is the standard low-temperature bolt grade, made from chromium-molybdenum alloy steel with specified Charpy V-notch impact toughness at -101°C. L7 bolts are quenched and tempered to achieve the same strength level as B7 (125 ksi minimum tensile) but with guaranteed impact properties that prevent brittle fracture in cold service. They are used extensively in cryogenic storage tanks, LNG facilities, and cold climate infrastructure.

L7M - Same as L7 but Lower Hardness, Prevents Hydrogen Embrittlement

A320 L7M is a modified version of L7 with a maximum hardness requirement of 22 HRC (versus approximately 35 HRC for standard L7). This lower hardness reduces the risk of sulfide stress cracking and hydrogen embrittlement in sour service environments containing hydrogen sulfide. L7M bolts are specified for NACE MR0175/ISO 15156 compliant applications in oil and gas production where both low-temperature toughness and sour service resistance are required.

L43 - Lower Temperature, -150°C

A320 L43 is made from nickel-chromium-molybdenum alloy steel (AISI 4340) with enhanced low-temperature toughness certified to -150°C. This grade is specified for the most demanding cryogenic applications including LNG processing at -162°C and liquid nitrogen service. L43 bolts have the highest strength level of the low-temperature grades with minimum 125 ksi tensile and guaranteed impact values at extreme low temperatures.

Low-Temperature Impact Test Requirements - Charpy V-Notch

All A320 low-temperature grades require Charpy V-notch impact testing at the specified temperature. L7 requires minimum 20 ft-lb (27 J) average at -101°C, L43 requires the same at -150°C. The specimens are taken longitudinally from the bolt material and tested to verify ductile behavior at the minimum design temperature. These impact requirements are critical for preventing catastrophic brittle fracture in cold-service pressure vessels and piping systems.

Bolt Grade Cross-Reference (SAE / ASTM / ISO / GB)

Grade	Material	Tensile (min)	Yield (min)	Hardness	Temp Range
SAE Gr 2	Low Carbon Steel	60 ksi	36 ksi	-	-20 to 150°C
SAE Gr 5	Medium Carbon Q&T	120 ksi	92 ksi	19-30 HRC	-20 to 150°C
SAE Gr 8	Alloy Steel Q&T	150 ksi	130 ksi	33-39 HRC	-20 to 150°C
A193 B7	4140 / 4142 Cr-Mo	125 ksi	105 ksi	~35 HRC	-20 to 450°C
A193 B8	304 SS	75-80 ksi	30-40 ksi	~20 HRC	-200 to 800°C
A193 B16	Cr-Mo-V	125 ksi	105 ksi	~37 HRC	-20 to 593°C
A320 L7	4140 / 4142	125 ksi	105 ksi	~35 HRC	-101 to 450°C
A320 L43	4340 Ni-Cr-Mo	125 ksi	105 ksi	~35 HRC	-150 to 450°C
ISO 8.8	Medium Carbon Q&T	116 ksi (800 MPa)	92 ksi (640 MPa)	22-32 HRC	-20 to 150°C
ISO 10.9	Alloy Steel Q&T	145 ksi (1000 MPa)	130 ksi (900 MPa)	32-39 HRC	-20 to 150°C

How to Identify Bolt Grades

Head Marking System - Grade 2 (No Marks), Grade 5 (3 Radial Lines), Grade 8 (6 Radial Lines)

The SAE head marking system provides quick visual identification of bolt grade. Grade 2 bolts have no identifying marks on the head. Grade 5 bolts feature three equally spaced radial lines on the head surface. Grade 8 bolts are identified by six radial lines. Additionally, the manufacturer's mark is typically stamped on the head for traceability. These markings allow rapid grade verification during installation and maintenance inspections.

ASTM Bolt Markings - B7, B7M Designations on the Head

ASTM bolts are marked with alphanumeric grade designations on the head. A193 B7 bolts are stamped "B7" or "B7M" for the lower hardness variant. B8 bolts are marked "B8," B8M with "B8M," and B16 with "B16." Low-temperature A320 grades are marked "L7," "L7M," or "L43" as applicable. The manufacturer's symbol is typically adjacent to the grade marking. These markings provide definitive identification when visual grade confirmation is needed.

Metric Bolt Markings - Head Numbers 8.8, 10.9, 12.9

Metric bolts use a two-number property class marking on the head, such as 8.8, 10.9, or 12.9. The first number multiplied by 100 gives the minimum tensile strength in MPa, and the second number after the decimal gives the ratio of yield to tensile strength as a percentage. For example, 10.9 indicates 1000 MPa tensile and 900 MPa yield. Higher property classes up to 14.9 are available for specialized applications.

Bolt Grade Selection Guide (by Application)

Structural connections typically require Grade 8 or ASTM A490 bolts with specified preload and installation procedures. Flange piping systems in petrochemical plants rely on ASTM A193 B7 or B16 bolts for high-temperature, high-pressure integrity. Food and pharmaceutical processing favors ASTM A193 B8 (304) or B8M (316) stainless steel for corrosion resistance and cleanability. Low-temperature tanks and cryogenic vessels mandate ASTM A320 L7 or L43 with certified impact toughness. General machinery and equipment installation is well served by Grade 5 or ISO 8.8 bolts for a balance of strength and economy.

Common Misconceptions in Bolt Grade Selection

One common myth is that Grade 8 is always better than Grade 5 - in reality, the higher hardness of Grade 8 increases hydrogen embrittlement risk in corrosive or galvanized environments. Another misconception is that B7 can replace Grade 8 - these grades are designed for different standard systems and service conditions. A critical misunderstanding is that high-grade bolts do not require matching high-grade nuts - the nut must always be at least as strong as the bolt to achieve the full joint capacity. Finally, some believe galvanizing does not change bolt performance, but the hot-dip process can affect thread fit and hydrogen embrittlement in high-strength bolts.

ManufacturerPipe's Bolt Product Capabilities

ManufacturerPipe supplies bolts across the full range of SAE Grade 2/5/8, ASTM A193 B7/B7M/B8/B8M/B16, and A320 L7/L7M/L43 grades. Our size range covers M6 through M100 in metric and 1/4 inch through 4 inches in imperial systems. We offer custom capabilities including special lengths, non-standard materials, and custom head markings. Every shipment includes original material test certificates (MTCs) with full traceability from melt through final testing, ensuring compliance with project specifications and quality assurance requirements.