Introduction to the GB/T 3098.15-2014 Standard
GB/T 3098.15-2014 specifies the mechanical properties of stainless steel nuts used in fasteners. This standard ensures reliability in various industrial applications by defining requirements for marking, chemical composition, mechanical performance, and testing. It applies to nuts made from austenitic, martensitic, and ferritic stainless steels, covering performance grades suitable for different load-bearing capacities and environmental conditions. Compliance with this standard guarantees that nuts meet stringent criteria for strength, corrosion resistance, and durability, making it essential for engineers in mechanical design and manufacturing.
Marking, Identification, and Surface Finish
The marking system for stainless steel nuts includes the steel group and performance grade, separated by a hyphen. The first part indicates the steel group (A for austenitic, C for martensitic, F for ferritic), followed by a number denoting the chemical composition range. The second part represents the performance grade, expressed as 1/10 of the proof stress in MPa for nuts with height m ≥ 0.8D, or with a leading zero for thinner nuts (0.5D ≤ m < 0.8D).
- Example: A2-70 – Austenitic steel, cold-worked, minimum proof stress 700 MPa (for m ≥ 0.8D nuts).
- Example: C4-70 – Martensitic steel, quenched and tempered, minimum proof stress 700 MPa.
- Example: A2-035 – Austenitic steel, cold-worked, minimum proof stress 350 MPa (for thinner nuts).
Low-carbon austenitic steels (C ≤ 0.03%) may add “L” (e.g., A4L-80). Passivated surfaces per GB/T 5267.4 add “P” (e.g., A4-80P). Nuts with nominal diameter D ≥ 5 mm must be marked clearly on one bearing face or side. Manufacturer identification is mandatory where feasible. Packaging must include manufacturer marks, steel group, performance grade, and batch number per GB/T 90.3. Unless specified otherwise, nuts are cleaned and polished; passivation is recommended for enhanced corrosion resistance.
Chemical Composition
The chemical composition of stainless steels for nuts is detailed in Table 1, aligned with GB/T 3098.6-2014. Manufacturers select compositions within groups unless otherwise agreed. For intergranular corrosion risks, test per GB/T 4334; recommend stabilized A3/A5 or low-carbon (C ≤ 0.03%) A2/A4 steels.
| Category | Group | Chemical Composition (mass fraction)/% | Notes | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ซี | Si | Mn | พี | เอส | เอ็น | Cr | Mo | Ni | Cu | Others | |||
| Austenitic | A1 | 0.12 | 1 | 6.5 | 0.2 | 0.15~0.35 | — | 16~19 | 0.7 | 5~10 | 1.75~2.25 | — | b,c,d |
| A2 | 0.1 | 1 | 2 | 0.05 | 0.03 | — | 15~20 | — e | 8~19 | 4 | — | f,g | |
| A3 | 0.08 | 1 | 2 | 0.045 | 0.03 | — | 17~19 | — e | 9~12 | 1 | — | ชม. | |
| A4 | 0.08 | 1 | 2 | 0.045 | 0.03 | — | 16~18.5 | 2~3 | 10~15 | 4 | — | g,i | |
| A5 | 0.08 | 1 | 2 | 0.045 | 0.03 | — | 16~18.5 | 2~3 | 10.5~14 | 1 | — | h,i | |
| Martensitic | C1 | 0.09~0.15 | 1 | 1 | 0.05 | 0.03 | — | 11.5~14 | — | 1 | — | — | ฉัน |
| C3 | 0.17~0.25 | 1 | 1 | 0.04 | 0.03 | — | 16~18 | — | 1.5~2.5 | — | — | — | |
| C4 | 0.08~0.15 | 1 | 1.5 | 0.06 | 0.15~0.35 | — | 12~14 | 0.6 | 1 | — | — | b,i | |
| Ferritic | F1 | 0.12 | 1 | 1 | 0.04 | 0.03 | — | 15~18 | — j | 1 | — | — | k,l |
Notes: a Maximum values unless specified. b Sulfur may be replaced by selenium. c If Ni < 8%, min Mn = 5%. d If Ni > 8%, no min Cu limit. e Mo optional by manufacturer. f If Cr < 17%, min Ni = 12%. g For max C=0.03%, max N=0.22%. h Ti ≥5×C% to 0.8%, or Nb/Ta ≥10×C% to 1.0%. i For larger diameters, higher C up to 0.12% for austenitic. j Mo optional. k Ti ≥5×C% to 0.8%. l Nb/Ta ≥10×C% to 1.0%.
This composition ensures optimal corrosion resistance and mechanical strength, with appendices providing examples from ISO standards for specialized applications.
Mechanical Properties
Mechanical properties must meet Tables 2 and 3 for acceptance, including hardness (for quenched/tempered C1, C3, C4) and proof load tests. Not all grades apply to all nuts; refer to product standards.
Table 2: Mechanical Properties – Austenitic Steel Groups
| Category | Group | Performance Grade | Proof Stress SP/MPa min | ||
|---|---|---|---|---|---|
| Nuts m≥0.8D | Nuts 0.5D≤m<0.8D | Nuts m≥0.8D | Nuts 0.5D≤m<0.8D | ||
| Austenitic | A1, A2, A3, A4, A5 | 50 | 025 | 500 | 250 |
| 70 | 035 | 700 | 350 | ||
| 80 | 040 | 800 | 400 | ||
Table 3: Mechanical Properties – Martensitic and Ferritic Steel Groups
| Category | Group | Performance Grade | Proof Stress SP/MPa min | Hardness | ||||
|---|---|---|---|---|---|---|---|---|
| Nuts m≥0.8D | Nuts 0.5D≤m<0.8D | Nuts m≥0.8D | Nuts 0.5D≤m<0.8D | HB | HRC | HV | ||
| Martensitic | C1 | 50 | 025 | 500 | 250 | 147~209 | — | 155~220 |
| 70 | — | 700 | — | 209~314 | 20~34 | 220~330 | ||
| 110 a | 055 a | 1100 | 550 | — | 36~45 | 350~440 | ||
| C3 | 80 | 040 | 800 | 400 | 228~323 | 21~35 | 240~340 | |
| C4 | 50 | — | 500 | — | 147~209 | — | 155~220 | |
| 70 | 035 | 700 | 350 | 209~314 | 20~34 | 220~330 | ||
| Ferritic | F1 b | 45 | 020 | 450 | 200 | 128~209 | — | 135~220 |
| 60 | 030 | 600 | 300 | 171~271 | — | 180~285 | ||
a Quenched and tempered, min tempering 275°C, D≤24 mm. b Softened.
These properties guide selection for applications requiring specific strength levels, with considerations for heat treatment and nut dimensions.
Testing Methods
Hardness tests for martensitic and ferritic steels follow GB/T 231.1 (HB), GB/T 230.1 (HRC), or GB/T 4340.1 (HV), with Vickers as referee. Procedures align with GB/T 3098.2; values must fall within Table 3 ranges. Proof load tests per GB/T 3098.2 ensure load-bearing capacity without failure.
- Conduct hardness on prepared surfaces.
- Apply proof load gradually and hold to verify integrity.
- Use calibrated equipment for accuracy.
These methods provide reliable verification of nut performance in real-world assemblies.
Appendices: Additional Technical Details
Appendices offer in-depth information:
- Appendix B: Stainless steel composition from ISO 683-13, with examples for ferritic, martensitic, and austenitic types.
- Appendix D: Materials reducing chloride-induced stress corrosion cracking, e.g., X2CrNiMoN17-13-5 with specific compositions for high-risk environments like swimming pools.
- Appendix E: High-temperature yield strength ratios (e.g., A2/A4 at 400°C: 70%) and low-temperature applicability (A2/A3 to -200°C).
- Appendix G: Relative magnetic permeability (e.g., A2 ≈1.8, non-magnetic in solution-annealed state but may become magnetic after cold working).
Consult these for specialized applications, ensuring material selection mitigates risks like corrosion or magnetic interference.
คำถามที่พบบ่อย
- What is the difference between A2 and A4 stainless steel nuts?
- A2 is general-purpose austenitic steel with good corrosion resistance, while A4 includes molybdenum for enhanced resistance in chloride environments, per Table 1 compositions.
- How do I select the appropriate performance grade for a nut?
- Choose based on proof stress requirements and nut height; e.g., grade 70 for 700 MPa in m ≥ 0.8D nuts, considering application loads and Tables 2/3.
- What tests are required for acceptance of stainless steel nuts?
- Hardness for martensitic/ferritic groups and proof load for all, per section 3 methods, ensuring compliance with specified mechanical properties.
- Can these nuts be used in high-temperature applications?
- Yes, but consult Appendix E for reduced yield strength at elevated temperatures; e.g., A2 retains 70% at 400°C under non-cyclic loads.
- How to mitigate intergranular corrosion in austenitic nuts?
- Use low-carbon variants (e.g., A4L) or stabilized A3/A5, tested per GB/T 4334, as recommended in the chemical composition section.
- What marking is required on packaging?
- Include manufacturer identifier, steel group/performance grade per Figure 1, and batch number per GB/T 90.3 for traceability.
