Selection of Stainless Steel Bolts

Fasteners are mechanical basic parts, which are in great demand. Usually bolts, screws, rivets, etc. are used to ensure safety or generally do not need to consider the influence 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, fastener materials need to meet the conditions of severe corrosion or high strength, and many stainless steels and ultra-high strength stainless steels have emerged.

This paper briefly introduces the excellent stainless steel used in the production of fasteners. Most stainless steel materials can be made into steel wire or bar for fastener production, there are austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, precipitation hardening stainless steel.

1. selection principle

The choice of stainless steel material is mainly considered from the following five aspects.

1. Requirements for mechanical properties of fastener materials, especially strength;
2. Requirements of working conditions on corrosion resistance of materials;
3. The requirements of the working temperature on the heat resistance of the material (high temperature strength, oxidation resistance);
4, the production process of material processing performance requirements;
5, other aspects, such as weight, price, procurement factors should be considered.

Through a comprehensive and comprehensive consideration of these five aspects, the brand, variety, specification and material standard of the fastener are finally determined.

2. austenitic stainless steel

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

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

Type 303 In order to improve the cutting performance, type 303 stainless steel is added with a small amount of sulfur, which is used to process nuts with bar stock.

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

Type 305 is suitable for processing fasteners with cold heading process, such as cold forming nuts and hexagonal bolts.

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

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

321 type and 347 type, 321 type contains a more stable alloying element Ti,347 type contains Nb, thereby improving the material's intergranular corrosion resistance. It is suitable for fasteners that are not annealed after welding or serve at 420~1013 ℃.

Three, ferritic stainless steel

430 type ordinary chromium steel, its corrosion resistance and heat resistance is better than 410 type, magnetic, but it can not be heat treatment strengthening, suitable for corrosion resistance and heat resistance slightly higher, strength requirements of the general stainless steel fasteners.

Four, martensitic stainless steel

410 type and 416 type can be heat treatment strengthening, hardness in 35 ~ 45HRC, good machining performance, for general purpose heat and corrosion resistant fasteners. Type 416 has a slightly higher sulfur content and is free-cutting stainless steel.

Type 420, sulfur content ≧ 0.15%, mechanical properties improved, heat treatment strengthening, the maximum hardness value of 53 ~ 58HRC, for fasteners requiring higher strength.

5. precipitation hardening stainless steel

17-4PH,PH15-7Mo, they can obtain higher strength than the usual 18-8 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 the commonly used 18-8 stainless steel, and still has good mechanical properties when the temperature is increased. Used for high strength, heat resistance, corrosion resistance fasteners, can be used to 650~700 ℃.