Fasteners are mechanical basic parts and 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, on certain occasions, fastener materials need to meet severe corrosion or high-strength conditions, and many stainless steels and ultra-high-strength stainless steels have emerged.
This article briefly introduces the excellent stainless steel used in fastener production. Most stainless steel materials can be made into steel wires or bars for fastener production, including austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and precipitation hardening stainless steel.
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 of the material (high temperature strength, oxidation resistance);
4. Requirements for material processing performance in terms of production technology;
5. Other aspects, such as weight, price, and purchasing factors must be considered.
After comprehensive and comprehensive consideration of these five aspects, the grades, varieties, specifications and material standards of the fasteners are finally determined.
Austenitic stainless steel
The commonly used grades are 302, 303, 304, and 305, which are the so-called “18-8” austenitic stainless steels. Both the corrosion resistance and the mechanical properties are similar. The starting point for selection is the production process method of the fastener, and the method depends on the size and shape of the fastener, as well as the quantity of production.
Type 302 is used for machined screws and self-tapping bolts.
In order to improve the cutting performance of Type 303, a small amount of sulfur is added to Type 303 stainless steel, which is used to machine nuts from bar stock.
Type 304 is suitable for processing fasteners using hot heading technology, such as longer bolts and large diameter bolts, which may be beyond the scope of cold heading.
Type 305 is suitable for processing fasteners by cold heading technology, such as cold-formed nuts and hexagonal bolts.
Type 309 and Type 310 have higher Cr content and Ni content than 18-8 type stainless steel, which are suitable for fasteners that work at high temperatures.
Types 316 and 317, they both contain alloying element Mo, so their high temperature strength and corrosion resistance are higher than 18-8 type stainless steel. 321 type and 347 type, 321 type contains relatively stable alloying element Ti, 347 type contains Nb, thus improving the material’s resistance to intergranular corrosion. Suitable for fasteners that are not annealed after welding or are in service at 420～1013℃.
Ferritic stainless steel
Type 430 ordinary chromium steel has better corrosion resistance and heat resistance than type 410. It is magnetic, but it cannot be strengthened by heat treatment. It is suitable for stainless steel with slightly higher corrosion resistance and heat resistance and general strength requirements. firmware.
Martensitic stainless steel
Types 410 and 416 can be strengthened by heat treatment, with a hardness of 35-45HRC, good machining performance, and are used for heat-resistant and corrosion-resistant fasteners for general purposes. Type 416 has a slightly higher sulfur content and is a free-cutting stainless steel.
Type 420, the sulfur content is ≧0.15%, the mechanical properties are improved, and it can be strengthened by heat treatment. The maximum hardness is 53～58HRC. It is used for fasteners that require higher strength.
Precipitation hardening 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 the commonly used 18-8 type stainless steel, and still has good mechanical properties when the temperature increases. Used as high-strength, heat-resistant, corrosion-resistant fasteners, and can be used at 650-700°C.