Hot-Dip Galvanized Bolts for Tower Structures

Hot-Dip Galvanized Bolts for Tower Structures — Product Introduction

Hot-dip galvanized bolts for tower structures are high-strength fasteners specially designed for the connection of tower steel structures such as transmission towers, communication towers, and radio and television towers. Through the hot-dip galvanizing process, a dense zinc-iron alloy anti-corrosion layer is formed on the bolt surface, meeting the long-term anti-corrosion requirements of tower structures in outdoor atmospheric environments. The main reference standards include DL/T 284-2021 “Hot-Dip Galvanized Bolts and Nuts for Transmission Line Towers and Electric Power Fittings,” AS 1559:2018 “Hot-Dip Galvanized Steel Bolts and Related Nuts and Washers for Tower Structures,” and ASTM A394 “Standard Specification for Steel Transmission Tower Bolts.” Thread specifications cover M10–M64, and property classes include 4.8, 5.6, 5.8, 6.8, 8.8, and 10.9.

I. Product Features

• Wide coverage of high-strength grades: Common property classes are 6.8 (M16 and M20 specifications) and 8.8 (M24 and above). Grade 8.8 bolts have a tensile strength ≥800 MPa, and grade 10.9 ≥1000 MPa, meeting the connection requirements of steel towers under different load levels.

• Heavy-duty hot-dip galvanizing for corrosion protection: Bolts, nuts, and washers must all be hot-dip galvanized. The local coating thickness shall not be less than 40 μm, and the average thickness shall not be less than 50 μm. Salt spray testing can achieve 1,000 hours without corrosion, adapting to high-humidity, industrial atmosphere, and coastal corrosive environments.

• Thread process after galvanizing: External threads of bolts are manufactured by rolling, and 6g limit dimensions are used before galvanizing. Internal threads of nuts are tapped after galvanizing and conform to 6AZ limit dimensions to accommodate the zinc layer and ensure fitting accuracy.

• Hydrogen embrittlement prevention treatment: Grade 8.8 and 10.9 bolts are mechanically descaled before hot-dip galvanizing. After galvanizing, magnetic particle inspection is required, focusing on stress concentration areas such as the fillet under the head and the thread root.

• Complete materials and specifications: Materials include low carbon steel, medium carbon steel, and alloy steel. M16/M20 commonly use grade 6.8, while M24 and above should use grade 8.8 to reduce the number of gusset plates and lighten the tower weight.

II. Product Advantages

• Ultra-long weather resistance and service life: The hot-dip galvanized layer acts as a sacrificial anode to protect the substrate. Iron accessories with a design service life of more than 20 years should adopt hot-dip galvanizing with a zinc layer thickness not lower than the upper limit requirement, and can remain maintenance-free for decades in outdoor atmospheric environments.

• Strong load-bearing capacity: Grade 6.8 bolts have a tensile strength ≥600 MPa, grade 8.8 ≥800 MPa, and grade 10.9 ≥830 MPa. Combined with an enlarged tapped internal thread design, the connection strength meets the force requirements of transmission towers under various meteorological conditions.

• Strong installation adaptability: All bolts are delivered after hot-dip galvanizing. Nuts are tapped after galvanizing and have oil film lubrication, which can reduce the torque coefficient and its dispersion, facilitating on-site tightening during construction.

• Good standardized interchangeability: As standardized tower fasteners, they have uniform dimensions and standardized marking. Qualified products from different suppliers can be used interchangeably, facilitating engineering procurement and spare parts management.

• Strict quality control: They must pass multiple inspections such as tensile strength tests, hardness testing, coating thickness testing, coating adhesion testing, and salt spray testing to ensure stable and reliable product quality.

III. Scope of Application

• Transmission line towers: Bolted connections for tower legs, tower bodies, cross arms, diagonal members, and other components of angle steel towers and steel tube towers. This is the most core application field of hot-dip galvanized bolts.

• Communication and radio and television towers: Steel structure connections for communication towers, microwave towers, and radio and television transmission towers, which need to meet the anti-corrosion requirements of long-term outdoor service.

• Substations and electric power fittings: Substation structures, electric power fittings, and other steel fastening connection parts.

• Similar tower-type steel structures: Fastening connections for wind turbine towers, monitoring towers, observation towers, and other tower steel structures.