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Hex nut

Petrochemical hex nuts are core fasteners used in flange joints of petroleum refining and chemical processing units. Manufactured to SH/T 3404, HG/T 20634 (American system) and HG/T 20613 (European system) standards, these heavy hex nuts are designed to pair with corresponding studs and bolts under high-pressure, high-temperature operating conditions.

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Product Specifications：

Specification Model

M10-M105

Material

Carbon Steel

Product details:

Hexagonal Nuts for Petrochemical Service - Product Introduction

Hexagonal nuts for petrochemical service are core fasteners in flange connection assemblies of petrochemical units.

They are manufactured mainly in accordance with standards including SH/T 3404, HG/T 20634 (American System),

and HG/T 20613 (European System). They shall be used together with stud bolts of the same standard for critical

connection positions such as pipeline flanges, valves, pressure vessels, heat exchangers and towers. Such nuts are

classified by material characteristics into "Special Grade" (identified by material grades, e.g., 30CrMoA, 35CrMoA)

and "Commercial Grade" (identified by property classes, e.g., Class 8, Class 10, A2-50). Strict controls over materials,

heat treatment and hardness are implemented to guarantee connection tightness and structural integrity under severe

working conditions such as high temperature, high pressure, hydrogen service, corrosion and low temperature.

I. Product Features

1. Materials and properties closely matched to working conditions: Graded according to temperature, pressure and medium corrosivity.

Special Grade adopts Cr-Mo alloy steel or austenitic stainless steel with quenching and tempering treatment, featuring excellent high-temperature strength,

anti-relaxation property and low-temperature toughness.

2. Hardness gradient design: The nut material is usually one grade lower or slightly softer than the mating stud bolt

(e.g., 35CrMoA stud bolt matched with 30CrMoA nut) to form a hardness gradient, preventing thread seizure under high temperature and high pressure and

facilitating disassembly and maintenance.

3. Standardized dimensions and types: Dimensions of Type I (thin type) and Type II (thick type) hexagonal nuts are produced in strict compliance with standards.

Type II nuts have higher load-bearing capacity and are mostly used for more severe working conditions to ensure load-bearing area and preload transmission.

4. Reliable quality assurance: Hardness tests and proof load tests shall be conducted batch by batch. For critical working conditions, high-temperature tensile tests,

low-temperature impact tests and spectroscopic tests are required to avoid material mixing and ensure stable performance.

II. Product Advantages

1. Wide coverage of extreme working conditions: Matching nut grades are available for environments ranging from cryogenic temperature of -196℃ to high

temperature above 600℃, from normal pressure to high-pressure hydrogenation, strong corrosion and toxic media, enabling one set of standards for multiple scenarios.

2. Excellent anti-seizure performance: Reasonable material and hardness matching between stud bolts and nuts, combined with lubrication treatment such as

molybdenum disulfide, greatly reduces the risk of thread galling and seizure during disassembly after high-temperature service and shortens maintenance cycles.

3. Mature standards with good interchangeability and compliance: Fully aligned with petrochemical industry standards and comparable with the mainstream international

ASTM A194 system. It has standardized procurement specifications, high recognition among owners and design institutes, and can easily pass safety reviews.

4. Long service life and high sealing reliability: Precise control of material tempering stability and anti-relaxation performance effectively maintains bolt preload during

long-term high-temperature operation and prevents flange leakage caused by preload attenuation.

III. Scope of Application

1. Oil refining and chemical units: High-temperature, high-pressure and hydrogen-containing flange connections for atmospheric and vacuum distillation, catalytic cracking,

reforming, hydrocracking, ethylene cracking and other units.

2. Chemical fertilizer and coal chemical units: Sealing connections of key equipment such as ammonia synthesis converters, urea synthesis converters and high-pressure scrubbers.

3. Long-distance oil & gas pipelines and storage & transportation: Connections of pipeline flanges, valve bonnets and station headers, subject to pressure fluctuations and ambient

temperature changes.

4. Power generation and boiler systems: Flange fastening for power boiler drums, superheater headers and high-temperature steam pipelines.

5. Cryogenic and air separation units: Low-temperature connection systems adopting austenitic stainless steel nuts, such as liquefied natural gas (LNG) storage tanks and air separation cold boxes.

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