ASTM A193 B7 vs B8 vs B16: Complete Selection Guide for Stud Bolts

Introduction to ASTM A193 Grades

ASTM A193 is the most widely used specification for high-strength alloy steel and stainless steel bolting materials for pressure vessels, valves, flanges, and fittings. The standard covers several grades, each designed for specific service conditions. Among them, B7, B8, and B16 are the most common choices for stud bolts in critical applications. Selecting the correct grade is essential to ensure joint integrity, corrosion resistance, and safety under operating conditions.

This guide provides a detailed comparison of these three grades, including chemical composition, mechanical properties, temperature limits, and typical applications. Whether you are a procurement engineer or a project manager, understanding these differences will help you make informed decisions and avoid costly failures.

Chemical Composition and Mechanical Properties Comparison

The table below summarizes the key chemical and mechanical requirements per ASTM A193.

Note: B8 Class 1 is solution-annealed only; Class 2 is strain-hardened (higher strength). B16 contains vanadium for improved high-temperature strength.

Temperature and Pressure Limits

Each grade has a recommended temperature range based on its material properties and industry codes.

B7 is suitable for moderate temperatures and general service. B8 offers excellent low-temperature toughness and corrosion resistance. B16 is designed for high-temperature, high-pressure applications such as steam turbines and boiler feedwater systems.

Application Scenarios

B7: General Service

ASTM A193 B7 is the workhorse of the bolting industry. It is used in refineries, chemical plants, and power generation for flanged connections, valve bonnets, and pressure vessel closures. B7 offers high strength at a relatively low cost, making it the default choice for non-corrosive environments up to 750°F.

B8: Corrosive Environments

Grade B8 (typically Type 304 stainless steel) provides excellent resistance to atmospheric corrosion, mild chemicals, and hydrogen sulfide (H2S) environments when properly heat treated. It is commonly specified for sour service per NACE MR0175/ISO 15156 (with hardness control). B8 is also used in cryogenic applications due to its impact toughness at low temperatures.

B16: High-Temperature Service

B16 is a chromium-molybdenum-vanadium alloy steel that retains strength at elevated temperatures up to 1100°F. It is the preferred material for high-pressure steam systems, turbine casings, and boiler feedwater pumps. B16 bolts are often used with B16 nuts (ASTM A194) to match thermal expansion characteristics.

Cost and Availability Considerations

B7 is the most economical and widely stocked grade. B8 (stainless) is more expensive due to alloy content and processing. B16 is the most costly because of vanadium addition and tighter quality control. Lead times for B16 may be longer, especially for large diameters or special lengths. LOKRON maintains inventory of all three grades in common sizes (1/2" to 4" diameter) and can expedite custom orders.

How to Order from LOKRON

When ordering stud bolts, specify the following: ASTM A193 grade (B7, B8, or B16), diameter, thread pitch (UNC/UNF or metric), length, nut grade (e.g., A194 2H for B7, A194 8 for B8), and any additional requirements such as NACE MR0175, PED 2014/68/EU, or EN 10204 3.1 certification. LOKRON provides full traceability and documentation. Contact our sales team for a quote.

Frequently Asked Questions

1. Can B7 be used in sour service?

B7 is not recommended for sour service unless specially heat treated to meet NACE MR0175 hardness limits (≤22 HRC). Standard B7 hardness is typically higher. For sour service, consider B7M (modified) or B8.

2. What is the difference between B8 Class 1 and Class 2?

B8 Class 1 is solution-annealed only, with minimum tensile strength of 75 ksi. Class 2 is strain-hardened to achieve 125 ksi minimum tensile strength. Class 2 is used when higher strength is needed, but it may have reduced corrosion resistance in some environments.

3. Can B16 be used at subzero temperatures?

B16 is not recommended below -20°F due to potential embrittlement. For low-temperature service, use B8 or L7 (ASTM A320).

4. What nuts should be used with B7, B8, and B16?

Per ASME B16.5, use A194 Grade 2H nuts with B7 bolts, Grade 8 nuts with B8 bolts, and Grade 8M nuts with B8M bolts. For B16, use A194 Grade 2H or Grade 16 nuts (if specified).

5. Does LOKRON provide third-party inspection?

Yes, LOKRON offers PED 2014/68/EU Category IV inspection by a notified body, as well as API 20E and 20F certification. We can also arrange customer or third-party witness testing.

Summary

Selecting the right ASTM A193 grade is critical for safe and reliable bolting. B7 is the cost-effective choice for general high-temperature service. B8 provides corrosion resistance and low-temperature toughness. B16 delivers superior strength at extreme temperatures. LOKRON has 20+ years of experience supplying certified stud bolts to global industries. Contact us for expert guidance and competitive pricing.