GB/T 3098.25-2020Mechanical properties of fasteners—Guidance for the selection of stainless steels and nickel alloys for fasteners (English PDF)

4.1 General

GB/T 3098 stainless steel mechanical properties series standard covers the following categories and groups:

- Austenitic stainless steel A1 ~ A5 and A8;

- Martensitic stainless steel C1, C3 and C4;

- Ferritic stainless steel F1;

- Duplex (austenitic-ferritic) stainless steel D2, D4, D6 and D8;

- High temperature stainless steel and nickel alloys CH0, CH1, CH2, VH/VW, V, SD, SB, 718.

Stainless steel covers a variety of materials, providing different corrosion resistance and functional properties. Special fasteners made of stainless steel shall be carefully selected considering the foreseeable working environment conditions of the bolted connection. A5.

4.2 Class A stainless steel (austenitic structure)

Austenitic steels are usually non-magnetic in the annealed state; however, some residual magnetism may be generated during cold heading, see Chapter 10. When low magnetic permeability is a critical factor, a stainless steel expert shall be consulted.

4.2.2 Group A1

Group A1 stainless steels are designed for machining. Due to the high sulfur content, the corrosion resistance is lower than that of the corresponding standard sulfur content stainless steels. This group of steels is not suitable for use in non-oxidizing acid media or environments with chloride components (such as swimming pools with chloride as a cleaning medium, or marine environments).

4.2.3 Group A2

Group A2 stainless steels are the most widely used stainless steels. This group of steels is not suitable for use in non-oxidizing acid media or environments with chloride components (such as swimming pools with chloride as a cleaning medium, or marine environments).

4.2.4 Group A3

Group A3 stainless steels have similar properties to Group A2 stainless steels, but have higher heat resistance (usually up to 350 °C). By adding Ti or Nb to combine with carbon, titanium carbide or niobium carbide is generated to form a stable stainless steel.

This group of steel is not suitable for non-oxidizing acid media or environments with chloride components (such as swimming pools using chloride as a cleaning medium, or marine environments).

4.2.5 Group A4

Group A4 stainless steel is an "acid-resistant steel" that contains Mo elements and can provide quite good corrosion resistance. This group of steel can be used in some chloride-containing environments, but it is still not suitable for swimming pools or marine environments using chloride as a cleaning medium.

4.2.6 Group A5

Group A5 stainless steel is a stable "acid-resistant steel" with the same performance as Group A4 steel, which can resist a variety of acids, but has high heat resistance (usually up to 350 °C). By adding Ti or Nb to combine with carbon, titanium carbide or niobium carbide is generated to form a stable stainless steel. This group of steel can be used in some chloride-containing environments, but it is still not suitable for swimming pools or marine environments using chloride as a cleaning medium.

4.2.7 Group A8

Group A8 stainless steel is known as "6% Mo" stainless steel and has high corrosion resistance to various forms of corrosion, including pitting, crevice corrosion, stress corrosion cracking. It is suitable for swimming pools using chloride as a cleaning medium and is also suitable for marine environments. However, consultation shall be conducted when used for buildings and structures with special requirements and/or provisions.

4.3 Class C stainless steel (martensitic structure)

4.3.1 General

Group C1, C3, C4 martensitic stainless steels are included in the GB/T 3098 stainless steel series standard and can be strengthened by quenching and tempering. As the carbon content increases, the mechanical properties improve; as the Cr content increases, appropriate corrosion resistance is achieved.

Group C1, C3, C4 martensitic stainless steels are generally less corrosion resistant than austenitic steels. Martensitic steels with improved corrosion resistance can also be used in the manufacture of special fasteners (see Table A.2).

Because martensitic stainless steels have low impact strength and ductility at low temperatures, they shall be used with caution below zero degrees. Martensitic stainless steels are usually very magnetic.

4.3.2 Group C1

The corrosion resistance of Group C1 stainless steels is limited.

4.3.3 Group C3

Although the corrosion resistance of Group C3 stainless steels is better than that of Group C1 steels, it is still limited.

4.3.4 Group C4

Group C4 stainless steels are similar to Group C1 steels, but due to the different sulfur content, the corrosion resistance is worse than that of Group C1 steels, but it is conducive to machining.

4.4 Class F stainless steel (ferritic structure) - Group F1

Ferritic stainless steel Group F1 is included in the GB/T 3098 stainless steel series standard. The mechanical properties of Group F1 steels can be strengthened by work hardening (cold working), but the cold working efficiency is not as high as that of austenitic stainless steels. Group F1 steels are magnetic.

For environments with lower corrosion than Group A2 or A3 steels, it is more economical to use Group F1 steels. However, due to the low impact strength and ductility of ferritic steels at low temperatures, Group F1 steels are not suitable for environments below -20 °C.

4.5 Class D stainless steel (austenite-ferrite structure)

4.5.1 General

Duplex stainless steel has a dual-phase structure of ferrite and austenite, with the volume fraction of ferrite ranging from 40% ~ 60%.

In the annealed condition, the strength of duplex stainless steel is significantly higher than that of austenitic stainless steel and can be further improved by cold work hardening, but the ductility may be reduced.

Duplex stainless steel groups D2, D4, D6, D8 are included in the GB/T 3098 stainless steel series standard. The larger the number, the better the corrosion resistance. The duplex stainless steel family is described as:

- Low duplex (D2, D4), low alloy content (especially Ni and Mo);

- Standard duplex (D6);

- Super duplex (D8), high alloy content.

Compared with austenitic stainless steel groups A1 ~ A5, duplex steel has greatly improved resistance to stress corrosion cracking.

4.5.2 Groups D2 and D4

D2 and D4 groups are called low duplex steels because the Mo content is less than 2%, or even less than 1%.

Regarding pitting and crevice corrosion, D2 is equivalent to A2 and D4 is equivalent to A4.

4.5.3 Groups D6 and D8

The Mo content of D6 group is higher than 2.5%, which is called standard duplex steel.

Compared with A1 ~ A5 and D4 groups, it has better corrosion resistance, especially pitting and crevice corrosion resistance.

D8 group is called high duplex steel; its corrosion resistance is equivalent to A8.

C.1 Groups of high-strength nitrogen-containing austenitic stainless steel

QN1803 high-strength nitrogen-containing austenitic stainless steel is a nickel-saving Cr-Mn-N type austenitic stainless steel. Compared with A2 group austenitic stainless steel, it has the characteristics of high tensile strength and torsional strength, wear resistance and weak magnetism; it has good toughness, pitting resistance and corrosion resistance. Its Cr content is about 18.1%, the nickel content is about 3.05%, the Cu content is about 2.30%. It has good cold processing performance and can maintain weak magnetism when the drawing reduction rate is below 60%. It is an ideal material for producing high-strength and weakly magnetic austenitic stainless steel fasteners.

QN1803 high-strength nitrogen-containing austenitic stainless steel is A2 group, generally delivered in hot-rolled state, which can obtain better cold processing performance through solid solution treatment. It is non-magnetic after solid solution treatment. The pitting corrosion resistance, sulfur dioxide and neutral salt spray corrosion resistance of QN1803 high-strength nitrogen-containing austenitic stainless steel are comparable to other austenitic stainless steels in A2 group; its uniform corrosion resistance in dilute hydrochloric acid, dilute sulfuric acid, acetic acid and salt is better than other austenitic stainless steels in A2 group.

QN1803 high-strength nitrogen-containing austenitic stainless steel has excellent comprehensive mechanical and physical properties at -60 °C ~ +300 °C.

Compared with A2 group austenitic stainless steel, QN1803 high-strength nitrogen-containing austenitic stainless steel is used to make high-strength, weakly magnetic austenitic stainless steel fasteners. The fastening torque coefficient is less discrete; it is not easy to "seize" during installation and fastening.

C.2 Composition of QN1803 high-strength nitrogen-containing austenitic stainless steel

The composition of QN1803 high-strength nitrogen-containing austenitic stainless steel is shown in Table C.1. […]