ASTM A194 Nut Grades Comparison: 2H, 8, 8M, and More

Introduction to ASTM A194 Standard

ASTM A194 is the standard specification for carbon steel, alloy steel, and stainless steel nuts intended for high-pressure or high-temperature service, or both. It covers a range of grades designed to match with corresponding bolts and studs under standards like ASTM A193 and ASTM A320. Selecting the correct nut grade is critical for ensuring joint integrity, preventing galling, and meeting code requirements such as ASME B16.5, ASME PCC-1, and PED 2014/68/EU.

This article provides a systematic comparison of the most common ASTM A194 grades: 2H, 8, 8M, 7, 7M, and 16. We will examine their chemical composition, mechanical properties, hardness ranges, temperature limits, and typical applications. A selection guide will help procurement engineers and project managers choose the right grade for their service conditions.

Grade 2H: Properties and Applications

Grade 2H is the most widely used carbon steel nut grade under ASTM A194. It is intended for general-purpose high-strength bolting and is typically used with ASTM A193 Grade B7 stud bolts and other carbon or low-alloy steel bolts.

Chemical Composition

Grade 2H nuts are made from carbon steel with a carbon content of 0.40% minimum (for sizes up to 4 in.) and may contain manganese, phosphorus, sulfur, and silicon. The exact composition is per ASTM A194 Table 2.

Mechanical Properties

• Hardness: 248–352 HB (Brinell) or 24–38 HRC (Rockwell C)
• Proof load: 175,000 psi (for sizes 1/4 to 4 in.)
• Temperature range: -20°F to 1000°F (-29°C to 538°C) for carbon steel; however, for elevated temperatures above 750°F, the user should consider the reduction in strength.

Applications

Grade 2H nuts are used in pressure vessels, heat exchangers, piping flanges, valves, and general structural bolting in refineries, chemical plants, and power generation. They are suitable for non-corrosive or mildly corrosive environments when coated or galvanized.

Grade 8: Properties and Applications

Grade 8 is an austenitic stainless steel nut grade (Type 304) with excellent corrosion resistance and good strength at elevated temperatures. It is commonly used with ASTM A193 Grade B8 bolts.

Chemical Composition

Grade 8 nuts are made from Type 304 stainless steel (18% Cr, 8% Ni). The composition meets the requirements of ASTM A194 Table 2.

Mechanical Properties

• Hardness: 183 HB maximum (for sizes up to 1 in.)
• Proof load: 80,000 psi (for sizes 1/4 to 1 in.)
• Temperature range: -425°F to 1500°F (-253°C to 816°C) for many environments, but limited by the bolt material.

Applications

Grade 8 nuts are used in corrosive environments such as chemical processing, food processing, pharmaceutical, and marine applications. They are also suitable for cryogenic service down to -425°F.

Grade 8M: Properties and Applications

Grade 8M is an austenitic stainless steel nut grade (Type 316) with molybdenum for enhanced pitting and crevice corrosion resistance. It is used with ASTM A193 Grade B8M bolts.

Chemical Composition

Grade 8M nuts are made from Type 316 stainless steel (16-18% Cr, 10-14% Ni, 2-3% Mo).

Mechanical Properties

• Hardness: 183 HB maximum (for sizes up to 1 in.)
• Proof load: 80,000 psi (for sizes 1/4 to 1 in.)
• Temperature range: Similar to Grade 8, but with improved resistance to chlorides and reducing acids.

Applications

Grade 8M nuts are preferred in environments with chlorides, seawater, and other aggressive media. They are common in offshore oil and gas, chemical plants, and pulp and paper mills.

Less Common Grades: 7, 7M, and 16

Grade 7

Grade 7 is a low-alloy steel nut grade with higher strength than 2H. It is typically used with ASTM A193 Grade B7 or B16 bolts for high-temperature and high-pressure service.

• Hardness: 248–352 HB
• Proof load: 175,000 psi (same as 2H but with better elevated temperature properties)
• Temperature range: Up to 1000°F (538°C)

Grade 7M

Grade 7M is a low-alloy steel nut grade with controlled hardness for sour service (NACE MR0175/ISO 15156). It is used with ASTM A193 Grade B7M bolts.

• Hardness: 248–352 HB (but with maximum hardness of 22 HRC for sour service)
• Proof load: 150,000 psi (reduced to avoid hydrogen embrittlement)
• Temperature range: -20°F to 1000°F

Grade 16

Grade 16 is a low-alloy steel nut grade with improved toughness and impact resistance at low temperatures. It is used with ASTM A320 Grade L7 bolts for low-temperature service.

• Hardness: 248–352 HB
• Proof load: 175,000 psi
• Temperature range: -150°F to 1000°F (-101°C to 538°C)

Comparison Table: Mechanical Properties, Hardness, Temperature Range

Selection Guide for Different Service Conditions

General High-Temperature Service

For non-corrosive environments up to 1000°F, Grade 2H nuts with A193 B7 bolts are the standard choice. If higher strength is needed, Grade 7 may be used.

Corrosive Environments

For corrosive service, select Grade 8 (304) or Grade 8M (316) depending on the corrodent. Grade 8M is preferred for chloride-containing environments.

Sour Service (NACE MR0175)

For oil and gas applications with H₂S, use Grade 7M nuts with A193 B7M bolts. Hardness must be controlled to ≤22 HRC to prevent sulfide stress cracking.

Low-Temperature Service

For temperatures below -20°F, use Grade 16 nuts with A320 L7 bolts. These materials have improved impact toughness.

Cryogenic Service

For temperatures below -150°F, austenitic stainless steel grades (8 or 8M) are suitable due to their excellent low-temperature toughness.

LOKRON Solutions

LOKRON (Suzhou Fulida) supplies a full range of ASTM A194 nuts, including Grades 2H, 8, 8M, 7, 7M, and 16, with full traceability and EN 10204 3.1 certification. Our nuts are manufactured to precise dimensional standards (ASME B18.2.2) and are available in heavy hex and other configurations. We also offer custom fasteners per ASTM, DIN, EN, and ISO specifications. All products are backed by our ISO 9001 and IATF 16949 quality systems, and we comply with PED 2014/68/EU and NACE MR0175/ISO 15156 requirements.

Frequently Asked Questions

1. What is the difference between ASTM A194 Grade 2H and Grade 7?

Grade 2H is carbon steel, while Grade 7 is low-alloy steel with better elevated temperature strength. Both have the same proof load, but Grade 7 is often used for higher temperature applications or when improved creep resistance is needed.

2. Can I use Grade 8 nuts with Grade B7 bolts?

It is not recommended because Grade 8 nuts have lower proof load (80,000 psi) compared to Grade B7 bolts (125,000 psi minimum). The nut may fail before the bolt reaches its full capacity. Always match nut grade to bolt grade per standard practice.

3. What does the 'M' in Grade 7M and 8M stand for?

The 'M' indicates a modified grade. For Grade 7M, it means controlled hardness for sour service. For Grade 8M, it indicates the addition of molybdenum for improved corrosion resistance.

4. Are ASTM A194 nuts interchangeable with other standards like DIN or ISO?

No, ASTM A194 nuts have specific dimensions and mechanical properties. For metric applications, use ISO 898-2 or DIN 934. However, LOKRON can supply nuts to both imperial and metric standards.

5. Do I need special documentation for PED compliance?

Yes, for pressure equipment applications under PED 2014/68/EU, nuts must be supplied with an EN 10204 Type 3.1 inspection certificate. LOKRON provides full documentation for all certified fasteners.

Summary

Selecting the correct ASTM A194 nut grade is essential for safe and reliable bolted joints. Grade 2H is the workhorse for general service, while Grades 8 and 8M offer corrosion resistance. Grades 7, 7M, and 16 address specific needs for high temperature, sour service, and low temperature. Always match the nut grade to the bolt grade and consider the operating environment. LOKRON offers a complete range of certified nuts with expert guidance to help you make the right choice.