Introduction to Stainless Steel Screw Performance Standards
Stainless steel screws are essential fasteners in industries requiring corrosion resistance, such as marine, chemical processing, and food equipment. This article details the mechanical performance standards for screws made from austenitic stainless steels SUS304 and SUS316, excluding dimensions which are covered in standards like ISO 3506 or GB/T 3098.6. Key properties include material composition, tensile strength (ability to withstand pulling forces), torque (torsional strength before failure), proof stress (load without permanent deformation), yield strength (point of plastic deformation), and related metrics.
These standards are derived from international norms like ISO 3506 and Chinese GB/T 3098.6, simplified here for engineers, procurement specialists, and newcomers. For precise testing protocols, refer to official documents such as GB/T 3098.6-2014. Note that processing like cold working enhances properties beyond raw material values.
Performance varies by grade and size; for instance, smaller screws (M4 and below) may not achieve higher grades due to manufacturing limits. Always verify with suppliers for custom applications.
Material Compositions: SUS304 and SUS316
SUS304 and SUS316 are the most common austenitic stainless steels for screws, offering excellent corrosion resistance. SUS304 is suitable for general environments, while SUS316 provides superior resistance in chloride-rich settings due to molybdenum addition. Other variants like 304L (low carbon) or 316L are used for weldability but are less common in standard screws.
Avoid misleading materials like 201 or 668 series, which lack the corrosion resistance of true austenitic grades. Martensitic grades like SUS410 are not covered here as they fall under stainless iron categories.
The primary differences: SUS316 has higher nickel (10-14%), chromium (16-18%), and includes 2-3% molybdenum, enhancing pitting resistance.
Chemical Composition of SUS304 (Based on GB/T 1220-2007 and Equivalent Standards)
ElementCarbon (C)Manganese (Mn)Silicon (Si)Phosphorus (P)Sulfur (S)Nickel (Ni)Chromium (Cr)Copper (Cu)Molybdenum (Mo)Standard Range (%)≤0.08≤2.00≤1.00≤0.045≤0.0308.00-11.0018.00-20.00≤1.00-
Typical mechanical properties for processed SUS304 screws: Minimum tensile strength 515 MPa, elongation 40%, hardness up to HRB 92.
| Element | Carbon (C) | Manganese (Mn) | Silicon (Si) | Phosphorus (P) | Sulfur (S) | Nickel (Ni) | Chromium (Cr) | Molybdenum (Mo) | Copper (Cu) |
|---|---|---|---|---|---|---|---|---|---|
| Standard Range (%) | ≤0.08 | ≤2.00 | ≤1.00 | ≤0.045 | ≤0.030 | 10.00-14.00 | 16.00-18.00 | 2.00-3.00 | – |
Typical mechanical properties for processed SUS316 screws: Minimum tensile strength 515 MPa, yield strength 205 MPa, elongation 40%.
Performance Grades and Markings
Stainless steel screws are marked with grades like A2-50, A2-70, A4-70, A4-80 on the head, indicating material and strength class per ISO 3506.
- A2 denotes SUS304 material.
- A4 denotes SUS316 material.
- The number (e.g., 70) represents minimum tensile strength in multiples of 10 MPa (e.g., 700 MPa for 70).
Not all sizes achieve maximum grades; e.g., SUS316 hex bolts are typically A4-70, while nuts can reach A4-80. Industry guidelines for applicability:
MaterialGradeApplicable Sizes and TypesSUS304A2-50Machine screws, bolts M5 and belowA2-70Bolts and nuts up to M24SUS316A4-70Bolts up to M24A4-80Bolts and nuts up to M24
For diameters beyond M24, consult supplier agreements as standards may not specify.
Mechanical Properties: Tensile Strength, Yield Strength, Proof Stress, and Elongation
These properties define screw performance under load. Tensile strength is the maximum stress before fracture; yield strength is the stress at 0.2% offset plastic deformation; proof stress is similar but for non-proportional extension; elongation measures ductility.
Per GB/T 3098.6, minimum values:
- A2-50: Tensile 500 MPa, Proof 210 MPa, Elongation 0.6d (d = diameter).
- A2-70: Tensile 700 MPa, Proof 450 MPa, Elongation 0.4d.
- A4-70: Tensile 700 MPa, Proof 450 MPa, Elongation 0.4d.
- A4-80: Tensile 800 MPa, Proof 600 MPa, Elongation 0.3d.
Cold forming increases these from raw material levels (e.g., raw 304 ~500 MPa to 700 MPa in screws).
Torque Standards and Failure Criteria
Torque (breaking torsion) is critical for installation without failure. Standards from GB/T 3098.6 provide minimum breaking torque values for various sizes and grades.
Example: For an M6 A2-70 hex screw, minimum breaking torque is approximately 13 N·m. Consult detailed tables for precise values, as they vary by thread type and length.
To apply: Identify grade, then reference torque table. Over-torquing risks thread stripping or fracture.
Effective Stress Cross-Sectional Area Calculations
The stress area (As) is used to calculate actual loads: As = π/4 * (d – 0.9382*p)^2, where d is nominal diameter, p is pitch (approximation for metric threads).
Common values (in mm²):
Nominal DiameterM3M4M5M6M8M10M12M16M20Stress Area (mm²)5.038.7814.220.136.658.084.3157245
Use As to compute tensile load: Load = Tensile Strength * As (in N).
Detailed Performance Parameters for Common Sizes
Calculated parameters for A2-70 (example sizes):
SizeStress Area (mm²)Min Tensile Load (kN)Min Proof Load (kN)M620.114.19.0M836.625.616.5M1058.040.626.1
Similar for A4-80: Higher loads due to 800 MPa tensile.
For A4-70 and A4-80, adjust based on grade multipliers.
Applications and Industry Guidelines
In practice, select grades based on environment: SUS304 for indoor/dry; SUS316 for marine/acidic. Ensure mating parts match grades to avoid galvanic corrosion. Preload calculations should account for 70-80% of proof stress for secure joints.
Testing: Use direct tension or wedge tests per standards. Industry rules suggest derating for cyclic loads or high temperatures (austenitic steels soften above 400°C).
For procurement: Specify grade, size, and standard (e.g., ISO 3506 A2-70). Verify certifications for critical applications like aerospace or pressure vessels.
Frequently Asked Questions (FAQ)
Why does raw SUS304 have ~500 MPa tensile strength, but screws reach 700 MPa?
Cold working during manufacturing induces strain hardening, increasing strength and hardness without heat treatment.
What is the main difference between SUS304 and SUS316 for screws?
SUS316 includes 2-3% molybdenum for better corrosion resistance in chloride environments, with slightly higher nickel and adjusted chromium.
How do I calculate the tensile load for a specific screw size?
Multiply the minimum tensile strength (e.g., 700 MPa for A2-70) by the stress area (As) in mm², then convert to kN (divide by 1000).
Are all SUS316 screws rated A4-80?
No; standard hex bolts are A4-70 (700 MPa), while nuts can achieve A4-80 (800 MPa). Check size and type.
What is the source of torque standards for stainless steel screws?
From GB/T 3098.6 or ISO 3506, providing minimum breaking torque values to ensure safe installation.
Can stainless steel screws be used in high-temperature applications?
Yes, up to 800°C for short periods, but strength decreases; use stabilized grades like 321 for prolonged exposure.
