Magnetic properties of stainless steel fasteners, magnetic issues with stainless steel screws

Debunking the Magnet Myth in Fastener Quality

In the fastener industry, there is a deep-seated misconception that a magnet is a reliable tool for testing the quality of stainless steel. Many believe that if a screw is magnetic, it must be “fake” or low-grade 201 stainless steel. As specialists at Ever Power, we can confirm: this is a myth.

Magnetism is not a definitive indicator of material grade. In fact, under certain manufacturing conditions, a high-quality 304 stainless steel screw can be more magnetic than a lower-grade 201 screw. To understand why, we must look at the science of residual stress and atomic structures.

The Root Cause: Residual Stress and Cold Working

Austenitic stainless steels (like 304 and 316) are essentially non-magnetic in their raw, annealed wire state. However, the process of manufacturing a fastener involves Cold Working—including cold heading (forming the head), thread rolling, and bending.

During these high-pressure deformations, the internal crystalline structure of the metal is stressed. This physical stress causes a partial transformation of the Austenite phase (non-magnetic) into the Martensite phase (magnetic). This is why a stainless steel screw is typically magnetic at the head and the tip of the threads, where the deformation is most extreme, but remains weakly magnetic in the center of the shank.

Can Magnetism Be Removed?

Yes, magnetism can be eliminated through a process called Solution Annealing. This involves heating the fasteners to a high temperature (around 1050°C) and holding them there before rapid cooling to release internal stresses and revert the structure back to pure austenite.

The Trade-off: While this makes the screw non-magnetic, it significantly reduces mechanical performance. The hardness, tensile strength, and yield strength all drop. At Ever Power, we rarely recommend solution treatment for standard fasteners unless they are specifically required for sensitive equipment like MRI machines or high-precision valves, as structural integrity is usually the priority.

Technical Standards: Relative Permeability ($mu_r$)

According to international standards such as ISO 3506 and GB/T 3098.6, austenitic stainless steel fasteners are considered “non-magnetic” in their annealed state, but it is acknowledged that cold working creates magnetism. In physics, this is measured by Relative Permeability ($mu_r$). A value of 1.0 is a perfect vacuum (completely non-magnetic).

• A4 (316 Stainless): $mu_r approx 1.015$ (Extremely low magnetism, highly stable)
• A4L (316L Stainless): $mu_r approx 1.005$ (Near-perfect non-magnetism due to low carbon)
• A2 (304 Stainless): $mu_r approx 1.8$ (Noticeable weak magnetism after processing)
• F1 (Ferritic Steel): $mu_r approx 5$ (Strong magnetism)

The MD30 Formula: Predicting Magnetic Stability

For high-end engineering, we calculate the stability of the austenite using the MD30 Formula. This value predicts the temperature at which 50% of the austenite will transform to martensite under a specific amount of cold deformation. A lower MD30 value indicates better non-magnetic stability.

$MD30 = 551 – 462 times (C + N) – 9.2 times Si – 8.1 times Mn – 13.7 times Cr – 29 times (Ni + Cu) – 18.5 times Mo$

As seen in the formula, increasing elements like Nickel (Ni), Copper (Cu), and Carbon (C) drastically reduces the potential for magnetism. This is why 316 stainless steel, with its higher Nickel and Molybdenum content, is much less magnetic than 304 after thread rolling.

Universal Phenomena in Other Metals

This magnetic transformation isn’t exclusive to stainless steel. It is a fundamental characteristic of metals under stress:

• Rebar: When a steel reinforcement bar is snapped during a tensile test, the point of fracture becomes magnetic.
• Sheet Metal: When a galvanized or stainless sheet is bent, the bend radius will attract iron filings while the flat surface remains inert.
• Permalloy: High-nickel alloys can become magnetic just by being twisted or deformed by hand.