Material selection and heat treatment processes are crucial factors when manufacturing high strength fasteners. These fasteners play an important role in many critical applications such as automotive, construction, aerospace and mechanical engineering. In this article, we will discuss the material selection of high-strength fasteners and the effect of heat treatment on their performance.

**Material selection**

Material selection for high-strength fasteners is related to product performance and reliability. A common material is No. 45 steel, which is one of the most commonly used materials for high-strength fasteners. This steel is tempered to meet tensile strength and hardness requirements. However, No. 45 steel has poor hardenability and improper quenching can easily lead to cracking, so careful heat treatment is required.

Another common material is 40Cr, an alloy structural steel with good hardenability. It is commonly used in the manufacture of high-strength bolts and hexagonal nuts. However, 40Cr steel struggles to meet low-temperature impact requirements, so it needs to be considered carefully in some applications.

20MnTiB is another excellent material choice and is particularly suitable for the manufacture of high-strength hexagonal bolts. It has good machinability and a fine grain structure that can maintain grain stability when heated at high temperatures. However, its hardenability is poor and it is only suitable for small sizes.

**Importance of heat treatment**

Heat treatment is an indispensable step in the manufacture of high-strength fasteners. Tempering is an often overlooked part of the process that serves three important functions. First, it helps eliminate thermal stresses and prevents crack expansion caused by stresses in the working condition. Second, tempering helps to eliminate organizational stresses and prevent the transformation of austenitic residues into martensite, thereby improving toughness. Finally, tempering can lead to the re-precipitation of alloying elements and carbides, so that their distribution is more uniform, finer and more diffuse, thus improving the material properties.

The main types of tempering include low-temperature tempering, medium-temperature tempering, high-temperature tempering, aging tempering, and dehydrogenation tempering. Each type of tempering has its own unique area of application and is selected based on product specifications and performance requirements.

In addition, temper embrittlement is a phenomenon to be wary of. It is categorized into low temperature temper brittleness and high temperature temper brittleness. Low-temperature temper embrittlement usually occurs at temperatures ranging from 250-400°C, while high-temperature temper embrittlement occurs between 500-650°C. The tempering embrittlement phenomenon can occur at temperatures ranging from 1.5 to 2.5°C. These phenomena may lead to the generation of grain boundary microcracks, which in turn affects the performance of the product. Therefore, a proper tempering process is essential to avoid these problems.

**Heat-resistant steels and heat treatment**

Heat-resistant steels are alloy steels with high strength and good chemical stability at high temperatures. It includes two major categories, oxidation-resistant steels and heat-strengthened steels. Oxidation-resistant steel requires better chemical stability, while the heat-strengthened steel requires higher high-temperature strength and corresponding oxidation resistance.

The alloying elements in heat-resistant steels play a key role. Elements such as chromium, aluminum and silicon generate a dense oxide film at high temperatures, thus preventing further oxidation. Nickel and manganese form and stabilize austenite and increase high-temperature strength. Elements such as vanadium, titanium and niobium form fine carbides that increase high temperature strength. Carbon and nitrogen help expand and stabilize austenite, increasing high-temperature strength.

Finally, precipitation-hardening stainless steels are a special type of heat-resistant steel that undergoes solution treatment and aging. These processes help precipitate phases to precipitate out, increasing the hardness and strength of the material.

To summarize, material selection and heat treatment of high-strength fasteners are key factors in ensuring product performance. Different applications require different types of materials, and the proper heat treatment process can adjust material properties to meet specific needs. Understanding the composition of materials and the principles of heat treatment is critical to the successful manufacture of high-strength fasteners.