Types of Stud Bolts: A Comprehensive Guide for Industrial Applications

Introduction

Stud bolts are one of the most critical fasteners in industrial applications, used to create secure, leak-proof joints in flanged connections, pressure vessels, and structural assemblies. Unlike hex bolts, stud bolts are threaded on both ends (or fully threaded) and are typically used with two nuts. They offer superior clamping force and ease of installation in confined spaces. This comprehensive guide covers the various types of stud bolts, their material specifications, applicable standards, and selection criteria for different industries.

What is a Stud Bolt?

A stud bolt is a threaded rod with threads on both ends (or fully threaded) designed to be used with nuts. The middle section may be unthreaded (body) or fully threaded. Stud bolts are commonly used in flange connections where one end is screwed into a tapped hole or a nut is used on each end. They provide high clamping force and are essential in high-pressure and high-temperature environments.

Full-Thread vs Half-Thread Stud Bolts

Full-Thread Stud Bolts

Full-thread stud bolts have threads running the entire length of the bolt. They are used when the entire length needs to be engaged with nuts or when the bolt is used in a through-hole application. Full-thread studs provide maximum adjustability and are common in general-purpose fastening.

Half-Thread (or Reduced-Thread) Stud Bolts

Half-thread stud bolts have threads only on both ends, with an unthreaded middle section (body). The body diameter is typically larger than the thread root diameter, providing higher shear strength and better alignment. These are often used in flange bolting where the unthreaded portion passes through the flange bolt holes, reducing stress concentration.

Double-End Stud Bolts

Double-end stud bolts have threads of equal length on both ends, with an unthreaded middle section. They are the most common type for flange connections. The ends are typically chamfered to facilitate nut engagement. They are specified in standards like ASME B16.5 and are used with heavy hex nuts.

Tapered-End Stud Bolts

Tapered-end stud bolts have one end tapered (usually with a smaller diameter) to allow easier insertion into tapped holes. The tapered end acts as a pilot, reducing the risk of cross-threading. These are often used in applications where one end is screwed into a blind hole, such as in engine blocks or hydraulic systems.

Material Types

Carbon Steel

Carbon steel stud bolts (e.g., ASTM A193 Grade B7) are the most common for general industrial use. They offer good strength at moderate temperatures. B7 is quenched and tempered, with a minimum tensile strength of 125 ksi.

Alloy Steel

Alloy steel grades like ASTM A193 B16 provide higher strength and better performance at elevated temperatures (up to 600°C). B16 is also quenched and tempered, with a minimum tensile strength of 125 ksi and improved creep resistance.

Stainless Steel

Stainless steel stud bolts (e.g., ASTM A193 B8, B8M) offer excellent corrosion resistance. B8 is made from Type 304 stainless steel, while B8M is from Type 316. They are used in corrosive environments and low-temperature applications. For low-temperature service, ASTM A320 L7 (alloy steel) or B8 (stainless) are common.

Standards and Specifications

Stud bolts must conform to industry standards to ensure reliability and safety. Key standards include:

• ASTM A193: Standard specification for alloy-steel and stainless steel bolting for high-temperature or high-pressure service.
• ASTM A320: Standard specification for alloy-steel and stainless steel bolting for low-temperature service.
• API 20F: Specification for corrosion-resistant bolting for use in the petroleum and natural gas industries.
• ASME B16.5: Pipe flanges and flanged fittings, which includes stud bolt dimensions for flange connections.
• EN 10204: Type of inspection documents (e.g., 3.1 certificate) for metallic products.

Selection Criteria for Different Industries

Oil & Gas

Requires high-strength, corrosion-resistant materials. ASTM A193 B7 or B16 with NACE MR0175 compliance for sour service. Stud bolts are often half-thread for flange connections per ASME B16.5.

Chemical Processing

Stainless steel (B8, B8M) or exotic alloys for corrosive media. Full-thread may be used for non-flange applications.

Power Generation

High-temperature alloys like B16 for steam turbines. Low-temperature grades (A320 L7) for cryogenic applications.

Marine

Corrosion-resistant materials like stainless steel or duplex stainless steel. Often require third-party certification (EN 10204 3.1).

LOKRON Solution

At LOKRON, we manufacture a full range of stud bolts to ASTM A193 (B7, B8, B8M, B16), ASTM A320 (L7), and other international standards. Our products are available in full-thread and half-thread configurations, with custom lengths and coatings. We provide EN 10204 3.1 certification and comply with PED 2014/68/EU, NACE MR0175, and ASME PCC-1 guidelines. Whether you need standard flange bolting or custom fasteners for demanding environments, LOKRON delivers precision and reliability.

Frequently Asked Questions

1. What is the difference between a stud bolt and a hex bolt?

A stud bolt is threaded on both ends (or fully threaded) and used with two nuts, while a hex bolt has a head on one end and threads on the other. Stud bolts are often used in flange connections where bolts pass through both flanges.

2. When should I use full-thread vs half-thread stud bolts?

Full-thread studs are used when maximum thread engagement is needed or for through-hole applications. Half-thread studs are preferred for flange bolting because the unthreaded body provides higher shear strength and reduces stress concentration.

3. What does ASTM A193 B7 mean?

ASTM A193 is the standard for high-temperature bolting, and B7 is a grade of quenched and tempered chromium-molybdenum alloy steel with a minimum tensile strength of 125 ksi (860 MPa).

4. Are stainless steel stud bolts suitable for high temperatures?

Stainless steel grades like B8 (304) and B8M (316) are suitable up to about 425°C. For higher temperatures, alloy steel grades like B16 are recommended.

5. What certifications should I look for when purchasing stud bolts?

Look for EN 10204 3.1 material certificates, PED compliance for pressure equipment, and NACE MR0175 for sour service. Also ensure compliance with ASTM or API standards.

Summary

Choosing the right type of stud bolt is critical for the safety and performance of industrial equipment. Understanding the differences between full-thread and half-thread, material grades, and applicable standards helps engineers and procurement professionals make informed decisions. LOKRON offers a comprehensive range of certified stud bolts to meet the most demanding requirements.