What are Heavy Hex Bolts & Hex Nuts

What is Heavy Hex Bolt?

Heavy hex bolts are tailored for environments of extreme temperature or pressure, where the heads of these bolts interact directly with flanges. They are commonly produced in diameters starting from 1/2″ and upwards. One of the distinguishing features between a heavy hex bolt and a regular hex bolt is the spacing across the hexagonal flats—where the heavy hex bolts extend an additional 1/8 inch beyond their regular counterparts for the same diameter. The sizing rule for these bolts is to multiply the diameter by 1.5 and then add 1/8 inch.
ASTM Specifications

ASTM A193, a prevalent specification for heavy hex bolts, encompasses a range of material types. Nickel Systems provides bolts conforming to A193 standards in various grades such as B8 (304 Stainless), B8C (347 Stainless), B8M (316 Stainless), B8R (Nitronic® 50), B8S (Nitronic® 60), and B8T (321 Stainless), along with Strain Hardened versions like A193 B8 Class 2 and A193 B8M Class 2. Beyond these specifications, Gulian also stocks heavy hex bolts in materials like Hastelloy® C276 and Carpenter® 20, with availability to supply other materials as required.
Hex Nuts Basics

Hex nuts, recognizable by their six-sided, hexagonal shape, are internally threaded components designed to engage with external threads, such as those found on bolts. Through rotational action, they exert substantial clamping forces, securing the joined elements tightly between the nut’s and bolt’s bearing surfaces. The strength of a hex nut, when matched with a bolt of the same material and heat treatment, is approximately 1.75 times the axial strength of the bolt, assuming the nut’s effective thread length meets or exceeds the bolt’s major diameter. This relationship is crucial as it underscores the role of bolts as pivotal elements in nut/bolt assemblies, emphasizing the importance of selecting nuts with suitable material and heat treatment for rigorous testing scenarios.

Distinctions in Hex Nut Design

Among the varieties of hex nuts, the finished hex nut, often referred to as a standard hex nut, typically features a narrower width across its flats and corners than its counterpart, the heavy hex nut. The heavy hex nuts are noted for their increased thickness and greater mass. These attributes contribute to a higher proof load strength relative to standard hex nuts, offering a larger bearing surface area which is advantageous in applications requiring enhanced load distribution and structural integrity.

Hex Nuts Grades & Markings Overview

Hex nuts come in diverse grades, tailored to specific mechanical and chemical properties suited to their intended use.

  • SAE J995 Grade 2 Hex Nuts: These are budget-friendly, low-strength nuts crafted from low carbon steel, available in sizes ranging from 1/4″ to 1 1/2″. They do not require any material markings and are commonly paired with Grade 2, Grade 5, and A307 bolts. Their strength parallels that of ASTM A563 Grade A nuts.
  • SAE J995 Grade 5 Hex Nuts: Constructed from quenched and tempered medium carbon steel, these medium-strength nuts are offered in the same size range. They feature two distinct circumferential lines 120 degrees apart for identification, making them suitable for use with Grade 5 bolts.
  • SAE J995 Grade 8 Hex Nuts: These high-strength nuts are made from quenched and tempered carbon alloy steel and are available from 1/4″ to 1 1/2″. Marked with two lines 60 degrees apart and the manufacturer’s identification, they are typically used with Grade 8 bolts and A354 Grade BD bolts, matching the strength of ASTM A563 Grade C nuts.
  • 18-8 Stainless Steel Hex Nuts: Made from alloys such as 303, 303Se, 304, and XM7, these nuts contain 17-19% chromium and 8-10% nickel, offering superior corrosion resistance. They are unmarked and widely used in environments needing general atmospheric corrosion resistance, like in chemical and food processing.
  • 316 Stainless Steel Hex Nuts: These are crafted from an austenitic alloy richer in molybdenum (2-3%) and nickel (10-14%) compared to 18-8, providing even greater corrosion resistance and are marked “316”. They are ideal for severe atmospheric conditions.
  • Brass Hex Nuts: Composed of a copper and zinc alloy, these unmarked nuts are favored in electrical, communication, and some marine applications for their corrosion resistance and nonmagnetic properties.
  • Silicon Bronze Hex Nuts: Primarily made of copper and tin with a small silicon content, these nuts are unmarked and excel in marine applications due to their resistance to saltwater and conductivity.
  • ASTM A563 Grade A Heavy Hex Nuts: These carbon steel nuts don’t typically require markings but may be marked with an ‘A’. They conform to ASTM A563 specifications for carbon and alloy steel nuts.
  • ASTM A563 Grade DH Heavy Hex Nuts: Often used in structural bolting, these nuts are identified by a “DH” mark and are known for their high strength in applications that involve structural steel frameworks.
  • ASTM A194 Grade 2H Heavy Hex Nuts: Designed for high pressure or high temperature applications, these nuts are marked with “2H” and are used with various high-strength bolting materials.
  • ASTM A194 Grade 2HM Heavy Hex Nuts: These are quenched and tempered alloy steel nuts, tested for hardness and marked with “2HM”. They are used in settings where hydrogen sulfide could cause embrittlement.
  • ASTM A194 Grade 8 Heavy Hex Nuts: Made from AISI 304 grade stainless steel and marked with an “8”, these nuts are intended for high-pressure or high-temperature settings.
  • ASTM A194 Grade 8M Heavy Hex Nuts: Similar to Grade 8 but made from AISI 316 stainless steel, these nuts are marked “8M” and are used in equally demanding environments as Grade 8.
  • ASTM A194 Grade 7L Heavy Hex Nuts: Quenched and tempered, these alloy steel nuts are marked “7L” and are used for high and low temperature services, adhering to A320/A320M specifications for additional low temperature requirements.

Each type of hex nut is designed to meet specific requirements, ensuring safety, reliability, and performance in their respective applications.

Hex Nuts Finishes & Coating Materials

Hex nuts come in a wide array of finishes and coatings, each selected based on its ability to improve the nut’s performance in various environments. These finishes enhance properties like corrosion resistance and influence factors such as torque retention during installation and the ease of removal after extended service periods.

Black or Plain Finish Hex Nuts

These hex nuts either lack a finish or have only a minimal protective layer. While plain black hex nuts provide little to no resistance against corrosion, those treated with black oxide or oil coatings exhibit improved resistance compared to their unfinished counterparts.

Galvanized Hex Nuts

These nuts are treated with a sacrificial zinc layer, which serves as an anode to protect the underlying metal from corrosion.

  • Hot Dipped Galvanized: This process involves submerging the fasteners in molten zinc, creating a robust coating renowned for its protective qualities.
  • Mechanically Galvanized: Here, hex nuts are agitated in a barrel with zinc powder, water, and other chemicals along with glass beads. This method yields a cleaner appearance but offers less corrosion resistance than hot dipping due to the weaker zinc-steel bond.

Plated Hex Nuts

These are coated with a fine zinc layer, typically applied through electroplating. The zinc or cadmium plating acts as a barrier to corrosion, especially effective until the coating is penetrated.

PTFE or Teflon Coated Hex Nuts

Coated with PTFE (often known by the Dupont tradename “Teflon”), these hex nuts feature enhanced durability and chemical resistance. This coating is particularly valued for its ability to withstand high pressures up to 100,000 psi and tensile strengths up to 4,000 psi. PTFE-coated nuts are optimal for demanding applications like insulating piping and other settings where chemical resistance and ease of maintenance are critical. The non-stick nature of PTFE also ensures that there are no issues with thread locking oxides, facilitating straightforward disassembly when necessary.

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