Fasteners are mechanical basic parts and are in great demand. Usually, bolts, screws, rivets, etc. are in order to ensure safety or generally do not consider the impact of temperature in harsh environments or other dangerous working conditions. Commonly used materials are carbon steel, low alloy steel and non-ferrous metals. However, in certain occasions, the fastener material needs to meet the conditions of severe corrosion or high strength, and many stainless steels and ultra-high strength stainless steels emerge as the times require.

This article briefly introduces the high-quality stainless steels used in fastener production. Most stainless steel materials can be made into wire or bar for fastener production, including austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and precipitation hardening stainless steel.

Material selection principle

The selection of stainless steel materials is mainly considered from the following five aspects.

1. Requirements for fastener materials in terms of mechanical properties, especially strength;

2. The requirements of working conditions on the corrosion resistance of materials;

3. The requirements of the working temperature on the heat resistance (high temperature strength, oxidation resistance) of the material;

4. Requirements for material processing performance in terms of production technology;

5. Other aspects, such as weight, price, and purchasing factors should be considered.

After comprehensive and comprehensive consideration of these five aspects, the grades, varieties, specifications and material standards of the selected fasteners are finally determined.

Austenitic stainless steel

The commonly used grades are 302, 303, 304, and 305, which are the four grades of the so-called "18-8" type austenitic stainless steel. Both corrosion resistance and mechanical properties are similar. The starting point for selection is the production process method of the fastener, which in turn depends on the size and shape of the fastener, as well as on the quantity produced.

Type 302 is used for machined screws and self-tapping bolts.

Type 303 In order to improve machinability, Type 303 stainless steel is added with a small amount of sulfur and is used to machine nuts from bar stock.

Type 304 is suitable for hot heading processing of fasteners, such as longer gauge bolts, large diameter bolts, which may be beyond the scope of the cold heading process.

Type 305 is suitable for cold heading processing of fasteners, such as cold formed nuts, hex bolts.

Type 309 and Type 310, their Cr content and Ni content are higher than 18-8 type stainless steel, suitable for fasteners working at high temperature.

Types 316 and 317, both of which contain the alloying element Mo, have higher high temperature strength and corrosion resistance than 18-8 type stainless steel.

Type 321 and Type 347, Type 321 contains a relatively stable alloying element Ti, Type 347 contains Nb, which improves the intergranular corrosion resistance of the material. It is suitable for fasteners that are not annealed after welding or serve at 420-1013°C.

Ferritic stainless steel

Type 430 ordinary chrome steel, its corrosion resistance and heat resistance are better than 410 type, and it is magnetic, but it cannot be strengthened by heat treatment. firmware.

Martensitic stainless steel

Types 410 and 416 can be strengthened by heat treatment, with a hardness of 35 to 45HRC, and good machinability. They are used for general-purpose heat-resistant and corrosion-resistant fasteners. Type 416 has a slightly higher sulfur content and is a free-cutting stainless steel.

420 type, sulfur content? R0.15%, improved mechanical properties, can be strengthened by heat treatment, maximum hardness value of 53 ~ 58HRC, used for fasteners requiring higher strength.

Precipitation hardened stainless steel

17-4PH, PH15-7Mo, they can get higher strength than the usual 18-8 type stainless steel, so they are used for high-strength, corrosion-resistant stainless steel fasteners.

A-286, a non-standard stainless steel, has higher corrosion resistance than commonly used Type 18-8 stainless steels, as well as good mechanical properties at elevated temperatures. Used as high-strength, heat-resistant, corrosion-resistant fasteners, can be used to 650 ~ 700 ℃.