Material Selection And Heat Treatment Of High Strength Fasteners

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. This article will examine material selection for high-strength fasteners and the effect of heat treatment on their performance.

Material selection

The material selection of high-strength fasteners is related to the performance and reliability of the product. One common material is 45-gauge steel, which is one of the most commonly used materials for high-strength fasteners. After quenching and tempering, the steel can meet the requirements of tensile strength and hardness. However, 45 steel has poor hardenability and improper quenching can easily lead to cracking, so careful heat treatment is required.

Another common material is 40Cr, which is an alloy structural steel with good hardenability. It is commonly used to make high-strength bolts and hex nuts. However, 40Cr steel has difficulty meeting low-temperature impact requirements, so careful consideration is needed in certain applications.

20MnTiB is another excellent material choice, especially suitable for making high-strength hexagonal bolts. It has good machining properties and a fine-grained structure, which can maintain grain stability when heated at high temperatures. However, its hardenability is poor and it is only suitable for small-sized products.

​Importance of heat treatment

Heat treatment is an indispensable step in the manufacturing of high-strength fasteners. Tempering is an often overlooked link that has three important functions. First, it helps eliminate thermal stress and prevent crack expansion caused by stress under working conditions. Secondly, tempering helps to eliminate organizational stress and prevent the remaining austenite from transforming into martensite, thereby improving toughness. Finally, tempering can lead to the re-precipitation of alloying elements and carbides, making them more evenly distributed, finer and more dispersed, thereby improving material properties.

The types of tempering mainly include low temperature tempering, medium temperature tempering, high temperature tempering, aging tempering and dehydrogenation tempering. Each type of tempering has its own unique application areas and is selected based on the product’s specifications and performance requirements.

In addition, temper brittleness is a phenomenon that requires vigilance. It is divided into low temperature temper brittleness and high temperature temper brittleness. Low temperature temper brittleness usually occurs in the temperature range of 250-400℃, while high temperature temper brittleness is between 500-650℃. These phenomena may lead to the generation of grain boundary microcracks, thereby affecting the performance of the product. Therefore, proper tempering process is crucial to avoid these problems.

​Heat-resistant steel and heat treatment

Heat-resistant steel is an alloy steel with high strength and good chemical stability at high temperatures. It includes two categories: oxidation-resistant steel and heat-strength steel. Oxidation-resistant steel requires better chemical stability, while heat-strength steel requires higher high-temperature strength and corresponding oxidation resistance.

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

Finally, precipitation hardening stainless steel is a special type of heat-resistant steel that requires solution treatment and aging treatment. These processes help to separate out the precipitated phase and increase the hardness and strength of the material.

In summary, material selection and heat treatment of high-strength fasteners are key factors to ensure product performance. Different application areas require different types of materials, and appropriate heat treatment processes can tailor the material’s properties to meet specific needs. Understanding the composition of materials and the principles of heat treatment is key to successfully manufacturing high-strength fasteners.