A Brief Discussion on The Material Grade of Stainless Steel Bolts

A Brief Discussion on the Material Grade of Stainless Steel Bolts

Stainless steel bolts play a critical role in various industries due to their durability, corrosion resistance, and strength. The materials of stainless steel bolts are primarily categorized into austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and precipitation hardening stainless steel. These material differences significantly impact their performance across diverse applications. To select the appropriate bolt material and specifications, one can analyze and choose based on the following considerations.

1. Ferritic Stainless Steel

Ferritic stainless steel is known for its good corrosion resistance and heat resistance, but it has relatively lower strength and hardness. Key examples include:

• 430 Type Ordinary Chromium Steel: This material is suitable for applications requiring moderate corrosion and heat resistance but with lower strength requirements. It is magnetic and cannot be strengthened through heat treatment.

2. Martensitic Stainless Steel

Martensitic stainless steel offers high strength and hardness, making it suitable for applications requiring strong and corrosion-resistant materials. These materials can be heat-treated to enhance their properties:

• 410 and 416 Types: These have a hardness range of 35–45HRC and excellent machining properties, making them suitable for general purposes.

• 420 Type: With high sulfur content, this type achieves a maximum hardness of 53–58HRC after heat treatment, ideal for high-strength applications.

3. Precipitation Hardening Stainless Steel

Precipitation hardening stainless steel is prized for its high strength and good corrosion resistance:

• 17-4PH and PH15-7Mo: These grades offer superior strength compared to standard 18-8 stainless steel and are used in high-strength, corrosion-resistant environments.

• A-286: This non-standard stainless steel exhibits excellent corrosion resistance and mechanical properties at high temperatures, functioning effectively in environments up to 650–700°C.

4. Austenitic Stainless Steel

Austenitic stainless steel is the most commonly used category and includes grades like 302, 303, 304, and 305:

• Type 302: Used for machining screws and self-tapping bolts.

• Type 303: Enhanced with sulfur for better cutting performance, suitable for bar processing nuts.

• Type 304: Ideal for hot upsetting processes, especially for larger diameter or longer bolts.

• Type 305: Suitable for cold upsetting processes, commonly used for cold-formed nuts and hexagonal bolts.

For high-temperature and high-corrosion resistance applications:

• Type 309 and 310: Suitable for high-temperature environments.

• Type 316 and 317: With higher molybdenum content, these grades exhibit stronger corrosion resistance.

• Type 321 and 347: Stabilized with elements like titanium or niobium, they improve intergranular corrosion resistance and are suitable for post-weld applications without annealing.

Material Grade and Representation of Stainless Steel Bolts

Stainless steel bolts are classified based on material and strength grades. Some common representations include:

• Austenitic Materials: A1, A2, and A4 correspond to 302, 304, and 316 stainless steels, respectively. Among these, A2 and A4 are the most widely used.

• Strength Grades: Grades like 45, 50, 60, 70, and 80 represent 1/10 of the tensile strength in MPa. For example, A2-70 indicates that the material is A2 and the tensile strength is 700MPa.

The Difference Between Carbon Steel Bolts and Stainless Steel Bolts

• Carbon Steel Bolts: These bolts have more than 10 grades, such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9. For instance, a grade 8.8 bolt indicates a tensile strength of 800MPa and a yield strength ratio of 0.8, giving a yield strength of 640MPa.

• Stainless Steel Bolts: Typically made from austenitic materials, their strength is not enhanced by heat treatment but through cold working hardening.

Key Grades of Carbon Steel Bolts:

1. Common bolt materials include Q215, Q235, and steels such as 25 and 45. For special-purpose connectors, higher mechanical property alloy steels like 15Cr, 20Cr, 40Cr, and 30CrMrSi are used.

2. SUS304 (A2): Commonly used for standard bolts, screws, washers, and nuts, with a strength grade of 70.

3. SUS316 and SUS316L (A4): Ideal for high-corrosion resistance applications, with a strength grade of 70 or 80.

4. Material Representation: A represents austenitic stainless steel, 2 refers to 304, and -70 indicates a tensile strength of 700MPa.

Grades of carbon steel bolts

The grades of carbon steel bolts are divided into more than 10 grades, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. The numbers before and after the decimal point represent the nominal tensile strength and yield strength ratio of the bolt material. For example, the bolt marked with grade 8.8 means that the tensile strength of the material reaches 800MPa, and the yield strength ratio is 0.8, that is, its yield strength reaches 800X0.8=640MPa.

1. Ordinary bolt materials Q215, Q235, 25 and 45 steels. For important or special-purpose threaded connectors, alloy steels with higher mechanical properties such as 15Cr, 20Cr, 40Cr, 15MnVB, 30CrMrSi can be selected.

2. SUS304 (0Cr18Ni9 - also known as A2, is mainly used for the manufacture of ordinary stainless steel bolts, screws, washers, and nuts. The strength grade of bolts is generally 70.

3. SUS316 (0Cr18Ni12Mo2) and SUS, 316L (00Cr17Ni14Mo2). Also known as A4, it is mainly used for the manufacture of high corrosion-resistant stainless steel bolts, screws, washers, and nuts. The strength grade of bolts is generally 70, and it can also reach a high strength grade of 80.

4. A represents austenite, 2 represents the second material (actually 2 refers to 304), and -70 represents the strength grade of 700MPa. The common materials are A2 and A4, which are 304 and 316 respectively.

5. There is no difference between stainless steel bolts A4-70 and A4-80 in terms of the material itself. The 70 and 80 after A4-70 and A4-80 represent 1/10 of the minimum tensile strength of the fastener. There is no difference between A4-70 and A4-80 in terms of the material itself. Since they are austenitic stainless steel, the tensile strength cannot be improved by heat treatment. The improvement of its tensile strength is through dislocation barrier after deformation (commonly known as cold hardening). A4-70 bolts are more popular in the market.

6. A1, A2, A4 represent 302, 304, 316; 45, 50, 60, 70, 80 represent 1/10 of the minimum tensile strength of the fastener. .

Group 7 and 4 are austenitic stainless steels, which were developed for boiling sulfuric acid, hence the name acid-resistant steel. They are metastable acid-resistant steels, with SUS316 as a typical representative.

Summary

The selection of stainless steel bolts requires comprehensive consideration of material properties, application scenarios, and performance requirements. From the economical choice of ferritic materials to the high-strength needs of martensitic grades, and from the versatile austenitic category to the extreme performance of precipitation hardening options, various materials provide tailored solutions for industrial and civil applications.

Jiaxing Haina Fasteners Co., Ltd. specializes in delivering high-quality stainless steel bolt products and comprehensive support for customers' fastening requirements. Whether addressing high-strength demands or special environmental conditions, we offer customized fastening solutions to meet your specific needs.