Fastener material properties and use analysis
Low-carbon cold-rolled and hot-rolled wire rods are essential materials used in the production of fasteners with lower strength requirements. The SWRCH6 to 16A series, which are non-heat-treated steels, are specifically designed for cold heading and cold extrusion processes. These materials are commonly used for manufacturing complex-shaped fasteners that require significant deformation during cold heading. Due to the high demand for good pickling and phosphating quality, it's important to ensure the material is suitable for such surface treatments. When the cold heading deformation exceeds one-third, annealing is recommended for certain steel grades. It’s advisable for customers to choose low-carbon grades like SWRCH6A, especially when large deformation is involved. Additionally, selecting similar wire specifications can help reduce drawing deformation and minimize work hardening effects.
Free return cold-rolled steel and hot-rolled wire rods have been a focus of innovation in the steel industry. In 2004, Maanshan Iron & Steel Co., Ltd. initiated the National 863 Program titled "Research on High Performance and Low Cost Cold Rolled Steel" (2003AA331030). This project aimed to develop an advanced, cost-effective technology for producing cold heading steel wire with independent intellectual property rights in China.
High-strength fasteners rated at 8.8 and above typically use medium carbon or alloy steels. The standard production process involves hot rolling, spheroidizing annealing, cold drawing, cold forming, and quenching and tempering. Spheroidizing annealing is crucial as it enhances the material's plasticity, allowing it to withstand up to 70–80% total deformation during cold heading. The hardness must be kept low, generally not exceeding HRB82. However, this step is time-consuming and energy-intensive, often taking 12–24 hours in batch or continuous furnaces. As a result, the industry seeks ways to simplify or even eliminate this process to improve efficiency and reduce costs.
Maanshan Iron & Steel has developed advanced hot-rolling technology, including the production of annealed SWRCH35K-M and CH35A-M wire rods, as well as SWRCH35K-MT and CH35A-MT for U.S. market fasteners. These materials offer a range of sizes from 6.5mm to 20mm and are suitable for manufacturing hex bolts and other components requiring moderate cold heading. For products with larger deformation, spheroidizing annealing is still necessary. Meanwhile, MFM-1 and MFM-3 hot-rolled wire rods have been successfully developed for ordinary high-strength nuts, while MFM-2 eliminates both annealing and quenching/tempering, directly producing 8.8-grade nuts.
Non-tempered cold heading steel is widely used for high-strength bolts such as 8.8, 9.8, and 10.9 grades. Using this type of steel allows manufacturers to skip the annealing before cold drawing and the quenching and tempering after bolting, significantly simplifying the production process. This leads to reduced energy consumption, shorter production cycles, and fewer issues like surface oxidation or decarburization. Japan has already implemented mass production of these materials.
In 2005, Maanshan Iron & Steel participated in the National 863 Program Guided Project titled “Research and Development of Non-Quenched and Tempered Steel Wire for Low Temperature Controlled Rolling and Controlled Cooling Fasteners†(2005AA001110). In 2008, the company further expanded its efforts by undertaking the National Support Program Project “Development of Low-Cost, High-Strength Non-Quenched and Tempered Cold-Rolled Steel Products for Steel Structure Joints†(2007BAE30B04).
The non-tempered steel series developed by Maanshan Iron & Steel utilizes micro-alloying, controlled rolling, and controlled cooling techniques to achieve ultra-fine grain structures. This enables the production of high-strength fasteners without the need for raw material annealing or final heat treatment. The resulting products meet current mechanical performance standards (GB/T 3098.22-2009) and offer significant economic and environmental benefits. It also reduces defects such as quenching cracks and surface decarburization caused by traditional heat treatment methods.
Alloy cold heading steels such as 10B21, ML40Cr, and SCM435 are used for high-grade fastener production, including 8.8 to 12.9 grade bolts. Through precise process control, these materials deliver superior performance and excellent cost-effectiveness, making them ideal for demanding applications in various industries.
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