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Stainless Steel Fasteners: Tightening and Breaking Torques
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Understanding the correct tightening torques for stainless steel bolts is crucial for ensuring optimal fastener performance, safety, and longevity in your applications. This comprehensive guide provides essential torque settings for stainless steel bolts across all common metric sizes and grades.
Why Proper Stainless Steel Torque Settings Matter
Stainless steel fasteners are widely used in applications requiring high corrosion resistance, strength, and durability. However, achieving proper performance depends entirely on applying the correct stainless steel bolt torque specs during installation. Incorrect torque application can lead to:
- Fastener failure from over-tightening
- Joint loosening from under-tightening
- Reduced corrosion resistance
- Compromised structural integrity
Understanding Torque Fundamentals
Tightening torque is the rotational force required to achieve proper preload in a fastener. Preload is the tension that holds the fastener securely in place and prevents loosening under service loads. The correct torque depends on several factors including thread size, material grade, surface conditions, and the coefficient of friction between mating surfaces.
Breaking torque represents the maximum torque a fastener can withstand before failure occurs through shearing, stripping, or stretching. This value is typically much higher than the recommended tightening torque and indicates the ultimate strength limit of the fastener.
Stainless Steel Grades Explained: Property Class Numbers
Stainless steel fastener grades use a two-part designation system that indicates both material composition and strength properties:
Understanding the Dash Numbers
- A1-50: The "50" indicates a minimum tensile strength of 500 N/mm² (50 kgf/mm²). This is an austenitic stainless steel similar to 303 grade.
- A2-70: The "70" indicates a minimum tensile strength of 700 N/mm² (70 kgf/mm²). This corresponds to 304/316 grade stainless steel.
- A4-80: The "80" indicates a minimum tensile strength of 800 N/mm² (80 kgf/mm²). This is a higher strength austenitic grade with enhanced corrosion resistance.
Higher numbers indicate greater strength but may come with trade-offs in ductility and corrosion resistance depending on the specific application.
What is Bumax and How Does it Compare?
Bumax is a super-austenitic stainless steel alloy developed for extreme environments requiring both exceptional strength and superior corrosion resistance. Unlike standard stainless steel grades:
- Strength: Bumax offers significantly higher tensile strength (up to 1000+ N/mm²) compared to standard A2-70 or A4-80 grades
- Corrosion Resistance: Superior performance in seawater, chloride environments, and acidic conditions
- Temperature Performance: Maintains properties across wider temperature ranges
- Trade-offs: Higher cost and limited availability compared to standard grades
The key trade-off between standard stainless steel and Bumax is cost versus performance - while Bumax offers superior properties, standard grades like A2-70 provide excellent performance for most applications at a more economical price point.
Complete Torque Values for Stainless Steel Bolts - Metric Sizes
The following table provides comprehensive torque values for stainless steel bolts metric sizes from M1.6 to M39. Values are given for three coefficient of friction levels to account for different surface conditions and lubrication states.
Thread Size Coeff Friction Preload (kN)
Property ClassTightening Torque (Nm)
Property ClassMin Breaking Torque (Nm)
Property ClassA1-50 A2-70 A4-80 A1-50 A2-70 A4-80 A1-50 A2-70 A4-80 M1.6 0.1 0.4 0.55 0.55 0.1 0.1 0.2 0.15 0.2 0.24 0.2 0.3 0.35 0.35 0.1 0.2 0.35 0.3 0.2 0.3 0.3 0.2 0.25 0.45 M2.0 0.1 0.5 0.6 0.6 0.15 0.2 0.3 0.3 0.4 0.48 0.2 0.4 0.5 0.5 0.25 0.3 0.4 0.3 0.25 0.36 0.36 0.4 0.4 0.55 M2.5 0.1 0.65 0.9 0.9 0.25 0.45 0.6 0.6 0.9 0.96 0.2 0.4 0.5 0.5 0.45 0.6 0.65 0.3 0.3 0.3 0.3 0.6 0.75 0.8 M3 0.1 0.9 1 1 0.85 1 1.3 1.1 1.6 1.8 0.2 0.6 0.65 0.65 1 1.1 1.6 0.3 0.4 0.45 0.45 1.25 1.35 1.85 M4 0.1 1.08 2.97 2.97 0.8 1.7 2.3 2.7 3.8 4.3 0.2 1.12 2.4 2.4 1.3 2.6 3.5 0.3 0.9 1.94 1.94 1.5 3 4.1 M5 0.1 2.26 4.85 4.85 1.6 3.4 4.6 5.5 7.8 8.8 0.2 1.83 3.93 3.93 2.4 5.1 6.9 0.3 1.49 3.19 3.19 2.8 6.1 8 M6 0.1 3.2 6.85 6.85 2.8 5.9 8 9.3 13 15 0.2 2.59 5.54 5.54 4.1 8.8 11.8 0.3 2.09 4.49 4.49 4.8 10.4 13.9 M8 0.1 5.86 12.6 12.6 6.8 14.5 19.3 23 32 37 0.2 4.75 10.2 10.2 10.1 21.4 28.7 0.3 3.85 8.85 8.85 11.9 25.5 33.9 M10 0.1 9.32 20 20 13.7 30 39.4 46 65 74 0.2 7.58 16.2 16.2 20.3 44 58 0.3 6.14 13.1 13.1 24 51 69 M12 0.1 13.6 29.1 29.1 23.6 50 67 80 110 130 0.2 11.1 23.7 23.7 34.8 74 100 0.3 9 19.2 19.2 41 88 117 M14 0.1 18.7 40.6 53.3 37.1 79 106 210 290 330 0.2 15.2 32.6 32.6 56 119 159 0.3 12.3 26.4 26.4 66 141 188 M16 0.1 25.7 55 73.3 56 121 161 - - - 0.2 23.2 49.9 66.6 93.1 199.6 266.1 0.3 19.8 42.4 56.5 115.3 247.1 329.5 M18 0.1 32.2 69 92 81 174 232 - - - 0.2 28.3 60.7 80.9 129.1 276.7 369 0.3 24 51.4 68.5 159.2 247.1 329.5 M20 0.1 41.3 88.6 118.1 114 224 325 - - - 0.2 36.4 78 104 182.4 390.9 521.2 0.3 30.9 66.2 88.2 226 484.3 645.7 M22 0.1 50 107 143 148 318 424 - - - 0.2 45.5 97.4 129.9 247 529.3 705.7 0.3 38.6 82.8 110.4 307.2 658.3 877.8 M24 0.1 58 142 165 187 400 534 - - - 0.2 52 112 150 314 674 898 0.3 45 95 127 389 835 1113 M27 0.1 80 170 - 289 619 - - - - 0.2 69 148 - 468 1003 - 0.3 59 126 - 583 1249 - M30 0.1 97 208 - 394 843 - - - - 0.2 71 153 - 635 1360 - 0.3 59 126 - 789 1691 - M33 0.1 121 - - 531 506 - - - - 0.2 105 - - 864 - - 0.3 89 - - 1079 - - M36 0.1 142 - - 684 651 - - - - 0.2 123 - - 1108 - - 0.3 105 - - 1380 - - M39 0.1 170 - - 883 842 - - - - 0.2 148 - - 1441 - - 0.3 126 - - 1802 - - Note: This is a representative sample of the complete torque table. The full table includes all metric sizes from M1.6 to M39 with corresponding values for each friction coefficient and property class.
Understanding Preload in Fastener Applications
Preload is the clamping force generated when a fastener is tightened, creating tension within the bolt and compression in the clamped materials. This prestress is crucial for:
- Joint Reliability: Maintains constant clamping force under varying loads
- Fatigue Resistance: Reduces stress variations that can cause failure
- Vibration Resistance: Prevents loosening in dynamic applications
- Sealing: Maintains consistent pressure in gasketed joints
Proper preload is achieved by applying the correct tightening torque, which must account for friction in the threads and under the fastener head. The coefficient of friction values in our torque table (0.1, 0.2, 0.3) represent different surface conditions from well-lubricated to dry conditions.
Factors Affecting Stainless Steel Torque Requirements
Coefficient of Friction
The friction coefficient significantly impacts required torque values. Use our Coefficients of Friction reference table to determine appropriate values for your specific application conditions.
Surface Conditions
- Clean, dry surfaces: Higher friction coefficient (0.3)
- Lightly oiled surfaces: Medium friction coefficient (0.2)
- Well-lubricated surfaces: Lower friction coefficient (0.1)
Environmental Considerations
- Temperature effects on material properties
- Corrosive environment impact on surface conditions
- Galvanic compatibility with mating materials
- Long-term relaxation and creep effects
Best Practices for Stainless Steel Fastener Installation
Torque Application Guidelines
- Use calibrated torque tools appropriate for the size range
- Apply torque gradually and evenly in multiple stages
- Follow proper tightening sequences for multi-fastener joints
- Consider retorquing after initial loading for critical applications
Quality Control Measures
- Regular calibration of torque tools
- Documentation of torque values applied
- Visual inspection for signs of over-torquing
- Periodic re-tensioning in service as required
Frequently Asked Questions
What happens if I exceed the recommended torque values?
Exceeding recommended torque can lead to fastener failure, thread stripping, or material yielding. Always stay within the specified range and use the breaking torque values as absolute maximum limits.
How do I choose between A2-70 and A4-80 grades?
A2-70 is suitable for most general applications, whilst A4-80 offers higher strength and enhanced corrosion resistance for demanding environments such as marine or chemical processing applications.
Can I use these torque values for imperial sized fasteners?
No, these values are specifically for metric fasteners. Imperial fasteners require different torque specifications based on their thread pitch and material specifications.
How often should I re-check torque values in service?
This depends on the application criticality and service conditions. Critical structural joints may require periodic checks, especially after initial loading cycles or exposure to temperature variations.
Related Products and Services
At Vital Parts, we supply a comprehensive range of stainless steel fasteners in all standard grades and sizes. Our technical team can assist with fastener selection, torque specifications, and application-specific requirements.
Conclusion
Proper application of tightening torques for stainless steel bolts is essential for achieving reliable, long-lasting fastened joints. By following the guidelines and using the comprehensive torque values for stainless steel bolts metric data provided in this guide, you can ensure optimal performance in your applications.
Remember that these values are guidelines based on standard conditions. Always consider your specific application requirements, environmental conditions, and safety factors when determining final torque specifications.
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