What are the principles for selecting stainless steel wire for fasteners?

Fasteners are mechanical foundation components with a high demand. In order to ensure safety or generally do not need to consider the impact of temperature, harsh environments or other dangerous working conditions, commonly used materials include carbon steel, low alloy steel, and non-ferrous metals. However, in specific situations, fastener materials require severe corrosion or high strength, and many stainless steel or ultra-high strength stainless steel have emerged. What are the requirements for selecting stainless steel materials for fasteners?

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

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

2. The requirements for the corrosion resistance of materials under operating conditions.

3. The requirements of working temperature on the heat resistance performance (high-temperature strength, oxidation resistance) of materials.

4. The requirements for material processing performance in terms of production technology.

5. Other factors, such as weight, price, and procurement, need to be considered.

Austenitic stainless steel

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

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

In order to improve the cutting performance of 303 stainless steel, a small amount of sulfur is added to it, which is used to machine nuts using bar stock.

Type 304 is suitable for processing fasteners using hot heading processes, such as longer specification bolts and larger diameter bolts, which may exceed the scope of cold heading processes.

The 305 model is suitable for processing fasteners using cold heading processes, such as cold formed nuts and hexagonal bolts.

The 309 and 310 types have higher Cr and Ni content than 18-8 stainless steel, making them suitable for fasteners working at high temperatures.

316 and 317 types both contain alloy element Mo, so their high-temperature strength and corrosion resistance are higher than those of 18-8 type stainless steel.

Types 321 and 347 contain relatively stable alloying element Ti, while Type 347 contains Nb, thereby improving the intergranular corrosion resistance of the material. Suitable for fasteners that are not annealed after welding or are in service at 420~1013 ℃.

Ferritic stainless steel

430 type ordinary chromium steel has better corrosion resistance and heat resistance than 410 type. It has magnetism, but it cannot be strengthened by heat treatment. It is suitable for stainless steel fasteners with slightly higher corrosion and heat resistance performance and general strength requirements.

Martensitic stainless steel

The 410 and 416 models can be heat treated and strengthened, with a hardness of 35-45HRC and good machining performance. They are used for heat-resistant and corrosion-resistant fasteners for general purposes. The 416 type has a slightly higher sulfur content and is a free cutting stainless steel.

420 type, with a sulfur content of ≥ 0.15%, improved mechanical properties, and can be heat treated for strengthening. The maximum hardness value is 53-58HRC, used for fasteners that require high strength.

17-4PH, PH15-7Mo, they can achieve higher strength than typical 18-8 stainless steel, and are therefore used for high-strength, corrosion-resistant stainless steel fasteners.

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