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Why is my Stainless steel spring Magnetic?

by Mark Hayes

IST regularly receives questions regarding property changes which occur in spring materials during simple stress relieving treatments.  For example, a recent IST training course attendee wanted to know why stainless steel spring wire behaved differently than music wire.  He knew that the outside diameter of music wire springs decreased when heat treated, while the outside diameter increased when stainless steel springs are heat treated. He had also been told to use the same value of the torsional modulus (G) for music wire both before and after heat treatment, but to use a different value of G for stainless steel after heat treatment.

To understand the mechanism causing the increase in outside diameter for stainless steel springs on stress relieving, the metallurgy of the wire drawing process needs to be considered.  When stainless steel spring wire is drawn, the microstructure of the rod stock material is austenite, but in every die a little of the austenite is mechanically transformed to martensite as a consequence of the cold reduction.  After several reductions in area, the wire will acquire the tensile strength required for springs, and the microstructure will be a mixture of austenite and martensite (it is the latter constituent in the microstructure that makes stainless steel spring wire slightly magnetic).

The spring manufacturer then coils the wire into springs, and thereby imparts a residual stress at the inside surface of the coil.  When the spring is heat treated, some more of the austenite will transform to martensite, and the transformation will occur most at the position where the residual stress is at a maximum.  A volume expansion is associated with this transformation, and the overall outcome is that the spring diameter becomes larger.

This article is part of a discussion about the changes to the structure of Stainless steel during heat treatment, the full article can be seen here.

Mark Hayes is technical advisor to the Institute of Spring Technology (IST) in Sheffield, England.