Why Do Cold-Formed Stainless Steel Heads Show Magnetism?

Why Do Cold-Formed Stainless Steel Heads Show Magnetism?

Under normal conditions, austenitic stainless steel is non-magnetic and is widely used for cold-forming processes. However, after cold working, austenitic stainless steel components—especially those with large deformation, such as vessel heads, elbows, and deep-drawn parts—may exhibit weak or noticeable magnetism.

This is a normal metallurgical phenomenon and does not indicate a material defect.

What Causes Magnetism After Cold Forming?

Most commonly used austenitic stainless steels are classified as metastable austenitic stainless steels. During cold forming, part of the austenitic structure transforms into martensite through a rapid, diffusion-free process known as strain-induced martensitic transformation.

Martensite in stainless steel typically appears in two forms:

• α′ (alpha-prime) martensite
  – Body-centered cubic (BCC) structure
  – Magnetic

• ε (epsilon) martensite
  – Hexagonal close-packed (HCP) structure
  – Non-magnetic

In commonly used stainless steels, the transformation mainly produces α′ martensite, which is why magnetism may occur after cold forming. The formation of ε martensite generally requires higher nickel and chromium content and is less common.

What Factors Affect the Degree of Magnetism?

The amount of magnetism depends on:

• Chemical composition of the stainless steel (austenite stability)

• Severity of cold deformation during forming

• Product geometry, especially deep-drawn or heavily worked components

Greater deformation typically results in stronger magnetic response.

Does Magnetism Affect Performance?

For general industrial applications, including food processing, pharmaceutical equipment, water treatment systems, and storage tanks, the presence of magnetism does not affect corrosion resistance, mechanical performance, or service life.

For this reason, many international standards allow a certain amount of magnetic phase (ferrite equivalent):

• ASME Boiler and Pressure Vessel Code, Section III

• Service temperature < 427°C: 3%–7% ferrite allowed

• Service temperature ≥ 427°C: ≥5% ferrite allowed
  (calculated using the WRC diagram)

• Mechanical Engineering Handbook (China)

• Recommended ferrite content: 4%–12%
  (estimated using the Schaeffler or DeLong diagrams)

When Is Special Treatment Required?

For applications involving:

• Neutron radiation exposure

• Nuclear equipment

• Highly corrosive or special media

It is recommended to select alternative materials or apply solution heat treatment to eliminate or reduce magnetic phases.

Our Experience

Based on these principles,Shandong Risen Imp & Exp Co., Ltd have produced millions of cold-formed stainless steel heads over more than 50 years.
No service issues related to magnetism have ever been reported.

Judy WhatsApp/Wechat: +86 15269864663/ 17662518060