GB/T 3098.25-2020 PDF English
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GB/T 3098.25-2020: Mechanical properties of fasteners - Guidance for the selection of stainless steels and nickel alloys for fasteners
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GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 21.060.01
J 13
Mechanical properties of fasteners - Guidance for the
selection of stainless steels and nickel alloys for fasteners
ISSUED ON: NOVEMBER 19, 2020
IMPLEMENTED ON: JUNE 01, 2021
Issued by: State Administration for Market Regulation;
National Standardization Administration.
Table of Contents
Foreword ... 3
Introduction ... 5
1 Scope ... 6
2 Normative references ... 6
3 Terms and definitions ... 7
4 Stainless steel categories and groups ... 7
4.1 General... 7
4.2 Class A stainless steel (austenitic structure) ... 8
4.3 Class C stainless steel (martensitic structure) ... 10
4.4 Class F stainless steel (ferritic structure) - Group F1 ... 10
4.5 Class D stainless steel (austenite-ferrite structure)... 11
4.6 Stainless steel and nickel alloy groups at high and ultra-high temperatures ... 12
5 Stainless steel and nickel alloy composition ... 12
6 Resistance to stress corrosion cracking ... 16
7 Resistance to pitting and crevice corrosion ... 16
8 Intergranular corrosion ... 17
9 Sensitivity to the formation of intermetallic compounds ... 19
10 Stainless steel magnetic permeability properties ... 20
Appendix A (Informative) Commonly used designations for stainless steel of fasteners
... 21
Appendix B (Informative) Characteristics of troostitic high-strength stainless steel . 28
Appendix C (Informative) Characteristics of QN1803 high-strength nitrogen-
containing austenitic stainless steel ... 31
References ... 34
Mechanical properties of fasteners - Guidance for the
selection of stainless steels and nickel alloys for fasteners
1 Scope
This Part of GB/T 3098 specifies the selection guide for stainless steel and nickel alloy
fasteners; provides technical information on stainless steel and nickel alloys and their
properties, which is important for the use of other stainless steel fastener mechanical
property standards. This Part includes the technical conditions of corrosion-resistant
stainless steel and nickel alloys suitable for fastener manufacturing.
This Part applies to austenitic stainless steel, martensitic stainless steel, ferritic stainless
steel and duplex (austenitic-ferritic) stainless steel and nickel alloy fasteners.
Appendix A gives the common material grades of stainless steel and nickel alloy
fasteners.
Appendix B gives the characteristics of a new troostite high-strength stainless structural
steel for reference.
Appendix C gives the characteristics of a high-strength nitrogen-containing austenitic
stainless steel for reference.
2 Normative references
The following documents are essential to the application of this document. For the dated
documents, only the versions with the dates indicated are applicable to this document;
for the undated documents, only the latest version (including all the amendments) is
applicable to this standard.
GB/T 3098.1 Mechanical properties of fasteners - Bolts, screws and studs
GB/T 3098.6 Mechanical properties of fasteners -- Stainless steel bolts, screws and
studs;
GB/T 3098.8 Mechanical properties of fasteners - Parts for bolted connections for
use at temperatures from -200 °C ~ +700 °C
GB/T 3098.15 Mechanical properties of fasteners - Nuts made of stainless-steel
GB/T 3098.16 Mechanical properties of fasteners - Set screws made of stainless-
steel
GB/T 3098.21 Mechanical properties of fasteners - Part 21: Tapping screws of
stainless-steel
GB/T 3098.24 Mechanical properties of fasteners - Stainless steel and nickel alloys
bolts, screws, studs and nuts for high temperature applications
GB/T 4334-2020 Corrosion of metals and alloys - Test methods for intergranular
corrosion of austenitic and ferritic-austenitic (duplex) stainless steels
ASTM A923 Standard test methods for detecting detrimental intermetallic phase in
duplex austenitic/ferritic stainless steels
ASTM A1084 Standard test methods for detecting detrimental phases in lean duplex
austenitic/ferritic stainless steels
ASTM G48 Test methods for pitting and crevice corrosion resistance of stainless
steels and related alloys by use ferric chloride solution
3 Terms and definitions
The terms and definitions defined in GB/T 3098.24 apply to this document.
4 Stainless steel categories and groups
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.
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.
Appendix A gives stainless steel materials that are suitable for fasteners but are not
included in the GB/T 3098 stainless steel fastener mechanical properties series standard.
For these materials, the marking system specified in the GB/T 3098 stainless steel
fastener mechanical properties series standard cannot be used.
6 Resistance to stress corrosion cracking
Austenitic stainless steel A1 ~ A5 is prone to stress corrosion cracking.
The use of duplex steels, especially D6 and D8 or high alloy austenitic A8 groups, can
significantly reduce stress corrosion caused by chloride ions.
Ferritic groups F1 and martensitic groups generally have good resistance to stress
corrosion cracking.
Specific cases of stress corrosion cracking of stainless steel structural components in
swimming pools can be found in EN 13451-1. The only fastener group recommended
for use in this application is A8.
7 Resistance to pitting and crevice corrosion
Pitting and crevice corrosion are localized forms of corrosion that are the result of
exposure to specific environments, especially those containing chlorides. In most
structural applications, the extent of pitting is superficial and the effect on the
dimensions of the parts is negligible. However, the corrosion products will cause
defects in the building. For pipelines, ducts and shell structures, relatively strict
requirements for pitting shall be adopted. If there is a known pitting risk, Mo-containing
stainless steels are required.
The pitting resistance equivalent (PREN) is a theoretical method of comparing the
pitting and crevice corrosion resistance of different types of stainless steel.
Warning: PREN (PRE) numbers are generally used to classify and compare
different groups. However, it cannot be used to predict whether a particular group
is suitable for a particular application where pitting corrosion may be a risk.
The calculation usually takes into account the Cr, Mo, N contents, as calculated by
formula (1):
Where:
w - Mass fraction, expressed in % (e.g., if the stainless steel contains 18 "weight"
percent Cr, then wCr = 18).
When in tungsten-containing super duplex steels, tungsten is also included in the
molybdenum-rating factor to recognize its effect on pitting corrosion, for type 1.4501,
it is adjusted by formula (2) into:
Where:
w - Mass fraction, expressed in %.
Note: Pitting corrosion resistance decreases sharply with increasing sulfur and phosphorus
content.
8 Intergranular corrosion
Fasteners manufactured by hot upsetting (or if welding occurs) may be sensitive to
intergranular corrosion. Intergranular corrosion may occur when the following three
conditions are met at the same time: inappropriate temperature during manufacturing,
carbon content above 0.03%, humid or corrosive application environment.
Where there is a risk of intergranular corrosion, the following stainless steel groups are
recommended:
- A3 or A5, stable type;
- A2 or A4, carbon content not exceeding 0.030%;
- A8;
- All duplex (austenite-ferrite) stainless steel groups.
In this case, the test can be carried out according to GB/T 4334-2020.
Figure 1 shows the approximate time before intergranular corrosion occurs for A2 group
austenitic stainless steels with different carbon contents in the temperature range of
550 °C ~ 925 °C.
The lower the carbon content, the higher the resistance to intergranular corrosion.
The risk of intergranular corrosion exists in the area to the right of the curve.
- ASTM G48 Standard test methods for pitting and crevice corrosion resistance of
stainless steels and related alloys by use of ferric chloride solution.
10 Stainless steel magnetic permeability properties
The magnetic permeability of a material is related to its ability to be attracted by a
permanent magnet or affected by a magnetic field.
Ferritic, martensitic, duplex and non-austenitic precipitation hardened stainless steels
are often classified as "magnetic" stainless steels, because they have a strong response
(or pull) to magnetic fields, such as hand-held magnets.
In contrast, austenitic stainless steels are classified as "non-magnetic" stainless steels;
however, cold work hardening during fastener manufacturing may produce some
residual magnetism. This residual magnetism can be reduced through specific heat
treatment processes.
In some applications, stainless steels with very low magnetic permeability are required.
Paramagnetic materials are usually specified to have a minimum relative permeability
approaching 1.0 (i.e., the material's magnetic response is the same as "free space" - a
space without gravitational and electromagnetic fields or a perfect vacuum).
The best types of austenitic stainless steel for low permeability applications are those
with high austenitic stability and low permeability in both the annealed or cold work-
hardened condition. For example, nitrogen-containing steels such as 4311-304-53-I
(A2L) and 4406-316-53-I (A4L) or high nickel steels such as 1.4845 (see Appendix A)
are considered suitable. When special non-magnetic properties are required, a stainless
steel material expert shall be consulted before entering into a supply and demand
agreement.
Appendix C
(Informative)
Characteristics of QN1803 high-strength nitrogen-containing austenitic stainless
steel
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.
......
Source: Above contents are excerpted from the full-copy PDF -- translated/reviewed by: www.ChineseStandard.net / Wayne Zheng et al.
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