I. Technical Background and Product Overview

8.8 Grade Carbon Steel External Hex Nuts are key components in the field of mechanical connections, directly affecting the stability and safety of engineering structures. This product uses medium carbon steel materials (such as 35#, 45# steel) and achieves the 8.8 strength grade through heat treatment (tensile strength ≥800MPa, yield strength ≥640MPa), suitable for high-load scenarios such as bridges, buildings, and heavy machinery. Handan Guangxuan Fastener Manufacturing Co., Ltd., with years of technical accumulation, has formed a full-process standardized production system from raw material selection to finished product delivery, and its products have gained market recognition for their high precision and consistency.

Chapter II: Core Technology Analysis: Synergistic Optimization of Materials and Processes

1. Material Selection and Pretreatment
Guangxuan Company selects high-quality carbon steel that meets the GB/T 699 standard, strictly inspecting the composition deviation (C content 0.32%-0.45%, Mn content 0.50%-0.80%) through a spectrometer, ensuring the fundamental material properties. The raw material is shot-peened to remove the surface oxide layer, with roughness controlled within Ra3.2μm, providing a stable base for subsequent processing.

2. Cold heading forming process
Using a multi-station cold heading machine (such as the Asahi machine from Japan) for one-time forming, the hexagonal profile is gradually extruded through 6 molds. Key parameters include: upsetting force of 1200kN, forming speed of 80 pieces per minute, and mold temperature of 150-180°C. This process improves efficiency by 5 times compared to traditional turning processing and achieves material utilization rate of over 98%.

3. Heat Treatment Strengthening Technology
Quenching and tempering treatment through a continuous belt furnace: quenching temperature 850±10°C, holding time 45 minutes; tempering temperature 420±15°C, holding time 2 hours. This process ensures uniform tempered sorbite structure in the nut core, surface hardness of HRC28-32, core hardness of HRC22-25, balancing strength and toughness.

3. Production Practice: The Three Pillars of Quality Control System

1. Full-process detection mechanism
Guangxuan Company establishes a four-level quality inspection system: chemical composition analysis (OES detection) and mechanical property tests (tensile testing machine) before raw material入库; hexagon side size inspection (tolerance ±0.1mm) with an image measuring instrument after cold heading process; surface cracks screening with a magnetic particle inspection instrument after heat treatment; 100% torque test (standard value ≥45N·m) before the finished product leaves the factory.

2. Equipment Precision Assurance
Key equipment regular calibration: Cold heading die wear checked monthly (allowable value ≤0.05mm); heat treatment furnace temperature uniformity verified quarterly (compliant with AMS2750E standard); hardness tester calibrated daily with standard test blocks (error ≤±1HRC).

3. Non-standard customization capability
For special working condition requirements, the company can provide customized services such as material upgrade (e.g., 40Cr alloy steel), surface treatment (Dacromet, electroplated zinc-nickel alloy), and structural improvement (adding anti-loosening slots, non-standard dimensions). Typical cases include the development of a high-strength M36×3 nut for a wind power enterprise, which achieves a low-temperature impact energy of 27J by optimizing the heat treatment process under -40℃ conditions.

Four, Industry Application Case: Practice of a Large Steel Structure Project

In a certain cross-sea bridge project, Guangxuan Company's 8.8-grade nuts are used to connect the main beam joints. The project requires the nuts to maintain a service life of over 10 years in marine corrosive environments and to pass a fatigue test of 100,000 cycles (load amplitude ±50kN). By adopting the solution of 45# steel base material + Dacromet coating (film thickness 8μm), the final product achieved 1,000 hours of salt spray test without red rust and a fatigue life of 123,000 cycles, exceeding the standard requirements by 23%, and winning high praise from the project party.

5. Technical FAQ: Common Questions and Answers

Q1: What is the main difference between an 8.8 grade nut and a standard nut?
A: The core difference lies in the material and heat treatment process. 8.8-grade nuts require tempering treatment to achieve tempered sorbite structure, whereas ordinary nuts usually only undergo annealing treatment, resulting in essential differences in strength indicators (tensile strength ≥800MPa vs ≥400MPa) and application scenarios.

Q2: How to judge if the nut quality meets the standard?
A: Verification can be conducted in three aspects: 1) Appearance inspection (hexagonal symmetry, thread integrity); 2) Hardness test (surface HRC28-32); 3) Torque test (standard bolt fit requires reaching specified torque value). It is recommended to prioritize products certified by ISO 898-1.

Q3: How to prevent rust on carbon steel nuts in a humid environment?
A: Guangxuan Company offers three solutions: 1) Electroplated zinc (thickness 5-8μm, neutral salt spray test ≥48 hours); 2) Dacromet (chrome-free coating, salt spray test ≥500 hours); 3) Hot-dip galvanized (thickness 45-55μm, salt spray test ≥1000 hours). Users can choose according to cost and protective cycle requirements.