Stainless Steel Bolts

Product Description

When moving from carbon or alloy steels to stainless steel for bolting, the primary trade-off is corrosion resistance versus mechanical strength. While stainless steel is the gold standard for harsh environments, it behaves very differently under load and heat compared to high-strength alloys like 4340.

1. Common Grades and Classifications

Stainless fasteners are generally categorized by their crystalline structure, which dictates their performance:

• Austenitic (300 Series): The most common for general industrial use.

  • 304 (A2): The "workhorse" grade. Excellent corrosion resistance for most applications but susceptible to chlorides (saltwater).

  • 316 (A4): Contains molybdenum, making it significantly more resistant to "pitting" in marine or chemical environments.

• Martensitic (400 Series): These are heat-treatable.

  • 410: Used when you need higher strength and hardness than the 300 series can provide, though it offers less corrosion protection.

• Precipitation Hardening:

  • 17-4 PH: Used in high-performance aerospace and chemical applications. It offers a rare combination of high tensile strength (up to 190,000 psi) and excellent corrosion resistance.

2. Strength Comparison

One of the most common misconceptions is that stainless bolts are "stronger" than standard steel. In reality, a standard 304 stainless bolt often has a lower yield strength than a Grade 8 or Metric 10.9 carbon steel bolt.

3. The "Galling" Phenomenon

The biggest technical hurdle with stainless steel fasteners is thread galling (or cold welding). Because stainless steel has a protective oxide layer, the friction of tightening can strip that layer away, causing the high spots on the threads to lock together.

• Prevention: Always use a dedicated anti-seize lubricant (like nickel or moly-based compounds).

• Design: Using different grades for the bolt and the nut (e.g., a 316 bolt with a 304 nut) can slightly reduce the risk of galling due to the difference in hardness.

4. International Standards & Designations

When cross-referencing for global procurement, stainless fasteners use a specific coding system:

• A2-70: "A2" indicates 304 grade; "70" indicates a tensile strength of 700 $N/mm^2$.

• A4-80: "A4" indicates 316 grade; "80" indicates a higher tensile strength of 800 $N/mm^2$ (usually achieved through cold-working).

5. Selection Criteria

• Use Stainless when: You face moisture, chemicals, or food-grade requirements.

• Avoid Stainless when: You require the extreme clamp loads found in engine internals (like connecting rods) or structural steel frames, unless using specialized high-strength alloys like B8M Class 2.