Stainless steel is well-known for its corrosion resistance and durability, but it also has magnetic properties which have not been understood correctly.  The magnetism of stainless steel depends on its chemical composition and heat treatment, that’s why it’s important to know about it, to have the right applications and utilisations.

Stainless steel and magnetism: a complex relation

Stainless steel is an iron alloy which has magnetic properties relating to the addition of specific chemical elements. We usually distinguish stainless steel from steel because of its amagnetism. This information is not totally correct. In fact, several stainless steels are as magnetic as steel, whilst other stainless steels are not magnetic at all. In order to understand this we need to look at the crystal structure.

Myths and misunderstandings

There are several myths and misunderstanding around stainless steel magnetism.

Generally speaking stainless steels are not magnetic. In fact, it depends on the crystal structure and the specific chemical composition of the alloy. Only austenitic stainless steel is amagnetic and this is true 95% of the time.  However they can become magnetic with the addition of certain elements or after a manufacturing process.

We could also think magnetism means an inferior quality for stainless steel and poor corrosion resistance. Magnetism don’t affect the quality of stainless steel but it could point to a corrosion resistance problem. This simple physical characteristic could lead to problems of corrosion and thus impact the usefulness of stainless steel in several applications.

If this phenomenon appears, it is be important to understand why it happens and take it into account when making a choice of materials.

Basic principles of stainless steel magnetism

The understanding of stainless steel magnetism requires an exploration of the scientific principles that govern the composition and crystal structure of this alloy. These factors don’t just determine the magnetic properties but also influence other crucial characteristics of stainless steel.

Crystal structure

The crystal structure and chemical composition of stainless steel determine its magnetic behaviour:

• Chemical composition: Stainless steel is mainly composed of iron, chromium, and for some grades, nickel, manganese, molybdenum, and other elements. The presence and proportion of the so-called "alphagene" elements affect the crystal structure of the alloy. Consequently, these are the magnetic properties insofar as they contribute to promoting the ferritic structure.

• Crystal structure: Stainless steel can have several crystal structures. Austenitic stainless steel has a face-centered cubic structure that is non-magnetic. The centered cubic structure of ferritic and martensitic steels is magnetic. Changes in structure can take place and become magnetic depending on the formed crystal.

• Mechanical treatments: the non-magnetic austenitic crystal can give rise to a phase called "work hardening martensite" during deformation by work hardening. It is the modification of the crystal system that generates magnetism.

Different stainless steel and magnetic form

Magnetic characteristics change with the family of stainless steel: 

Globally, austenitic stainless steel like 304 and 316 are not magnetic. However, they can have a weak magnetism force due to:

• Elaboration which leads to delta ferrit in certain conditions,

• Mechanical transformation with work hardening martensite: When austenitic stainless steel is cold deformed, it can partially transform into a magnetic phase called work hardening martensite. We can have a greater or lesser degree of magnetism according to the mechanical transformation.

Austenitic heat treated microstructure

Austentic phase after work hardening : deformed austenitic phase

Work hardening martensitic

• Ferritic and martensitic: These steels are naturally magnetic due to their centered cubic crystal structure. Their magnetism is stable and does not change with heat or mechanical treatment.

Martensitic micro structure

• Duplex: Duplex steels combine 50% austenitic phase and 50% ferritic structure, resulting in a magnetism that varies according to the proportion of each phase and the specific treatment.

Choosing the right grade of stainless steel, taking into account the ferrite point of view, is very important to optimize performance and safety, especially in sectors where magnetism directly influences the practical application.

Impacts of magnetism

The magnetism of stainless steel is not simply a material characteristic. It has significant practical implications in various industrial applications. To understand how and where the magnetism of stainless steel plays a key role can guide decisions about the selection and use of this special material.

Impact of material choice

The choice of the type of stainless steel must be made carefully, because of its magnetic properties. 

Drawbacks of magnetism with stainless steels: For some applications, magnetism can have an impact, influencing the decision towards non-magnetic austenitic steels. For example, in environments where cleanliness and non-adhesion of metal particles are critical, avoiding magnetic steels can be crucial. In addition, magnetic zones with austenitic steels can lead to problems with ferrous pollution on the surface and therefore the resistance to pitting corrosion.

The work hardening of austenitic steels can induce a phase transformation, work hardening martensite, making this steel slightly magnetic and more brittle. To understand this transformation is important for choosing the material for applications that require corrosion resistance, stable mechanical properties and structural integrity: it is possible to have delayed fractures related to work hardening martensite with or without additional stresses.

A knowledge of the impact of magnetism on stainless steel is a necessity to have performance, safety, or regulatory compliance. It allows you to choose the right grade of stainless steel for each application context, maximizing the efficiency and safety of industrial applications.

Treatment and properties modifications

To adjust the magnetic properties of stainless steel can be achieved through various treatments and modifications. These methods allow the material to be "customized" for specific applications, improving its performance and functionality.

Heat treatment and effects of magnetism

Heat treatments can significantly modify the magnetic behaviour of stainless steel:

• Effects of heat treatments: Annealed treatment can decrease or eliminate the magnetism induced by work hardening in austenitic steels, to restore the face-centered cubic crystal structure.

• Effects of the treatment called "Anti-ferrite": some austenitic stainless steels contain a small part of ferrite from processing. This treatment aims to eliminate this ferritic phase to make them a 100% non-magnetic structure. With higher temperatures than heat treatment, strain stresses limit their use.

• Ferrite stabilization: Specific treatments can be applied to balance the ferritic/austenitic phase in duplex steels, thus to modify their magnetic behaviour.

• Work hardening martensite formation: several mechanical treatments can transform more or less work hardening martensite in austenitic steels, which increases their magnetism. Their magnetization depends on the amount of martensite thus formed. If the magnetism reveals the deformed area, the presence of work hardening martensite is synonymous with a decrease in corrosion resistance and the risk of failure, as work hardening martensite is a fragile structure.

• Heat treatments are valuable tools to change the magnetic characteristics in austenitic steels, and allow the magnetism to be adapted to the specific requirements of each application.

Magnetic properties modification

The addition or modification of alloying elements can influence the magnetic properties of stainless steel:

• To add elements: when we introduce elements like nickel, manganese in alloys, you can make an impact of magnetism. Austenitic stainless steel with addition of these elements will for example, limit the work hardening martensite and therefore reduce their magnetism. 

• To adjust the element ratios: to change the proportion of the constituent elements, the ratio of alphagenic and gammagenic elements can refine the crystal structure and consequently, modify the magnetic properties.

Measurement and characterisation of magnetism

Measuring and characterizing the magnetism of stainless steel is essential to understand its properties and ensure its proper use in various applications. Magnetism can be a warning of a specific behaviour of an austenitic grade as a function of the heat with the result of work hardening.

According to some metallurgists such as Pickering, Angels and others, the work hardening martensite of austenitic stainless steels is formed according to the temperature and chemical composition. Their research led them to determine empirical formulas that define the temperature Md30. This temperature defines the temperature from which 50% of work hardening martensite is obtained for 30% elongation.

Based on the contents of each addition element, it allows the deformation behaviour of 2 castings to be compared. Of course, this Md30 is between Ms (Martensite starting) and Mf (Martensite finishing). Then, detection of this work hardening martensite can be done with the X-ray diffraction technique or with observation and analysis in EBSD of the phases. It is also possible to measure magnetic permeability and interpret this data in an informative way.

Measurement process of magnetic permeability

The magnetic permeability of stainless steel indicates how the material reacts in a magnetic field. To measure it, several methods are used:

• Measurement of relative permeability: Comparison of the magnetic permeability of stainless steel is often used to determine whether steel is austenitic and ferritic/martensitic.

• Gaussmeter test: Use of a gaussmeter to measure the magnetic field on the surface of stainless steel.

• Hysteresigraph measurement: Measurement of hysteresis loops to characterize the magnetic response of stainless steel which include its maximum permeability and residual magnetization.

These methods help us to quantify the magnetism of stainless steel and identify the right grades for specific applications, whether they require magnetic materials or not.

Accurate measurement and characterization of the magnetism of stainless steel is an essential step to use this material. 

Challenges and practical solutions

Understanding and managing the magnetism of stainless steel is essential in an industrial context, where precise and reliable performance is required. This section examines the common challenges associated with stainless steel magnetism and offers practical strategies for overcoming them, as well as real-world examples of problem-solving.

Managing the effects of magnetism in practice

Stainless steel magnetism can present challenges in specific applications, but with the right approach, these challenges can be managed effectively:

• Appropriate grade selection: to select the stainless steel grade and define the right chemical adaptation with the required magnetic properties is a crucial first step.

• Use of magnetic shields: In some cases, the use of magnetic shields or barriers can help minimize the impact of magnetism.

• Predict effects on sensitive components: Understand

• how the magnetism of stainless steel can affect sensitive electronic or mechanical components and plan accordingly.

• Adaptation of manufacturing processes: Adjust manufacturing processes, such as welding or shaping, to minimize unwanted changes in magnetic properties. For example, to manufacture austenitic stainless steel wires, hot drawing can be used, linking the plastic deformation of the metal to the heat treatment.

Applications and problem solving

Here are a few examples of how the challenges of stainless steel magnetism have been solved in industrial projects:

• Medical engineering project: The use of non-magnetic stainless steel is crucial in the manufacture of medical devices for magnetic resonance imaging (MRI). By selecting specific austenitic grades and controlling the manufacturing process, engineers ensured non-magnetic interference.

• Shipbuilding: For ships with strict magnetic signature requirements, engineers used specific stainless steel grades and construction techniques to minimize magnetic impact, ensuring seaworthy safety and performance.

• Food industry: Ferritic stainless steels are magnetic, unlike most austenitic steels. This characteristic makes them detectable by metal detection equipment often used in the food industry. The detection of metal fragments in food is crucial for food safety.

• By adopting strategic planning, meticulous selection of materials and adaptation of manufacturing processes, the challenges of stainless steel magnetism can be effectively addressed and the success of all industrial projects can be guaranteed.

Developments and future prospects

The industry continues to advance in the understanding and use of stainless steel magnetism, with research and innovations that are opening up new horizons. This overview focuses on current developments and expectations for the future of stainless steel magnetism.

Current research and innovations

Current work in the field of stainless steel magnetism focuses on several future areas:

• Development of low-magnetic alloys: for specific needs or uses where non-magnetism is essential.

• Control of magnetism: thanks to heat or mechanical treatments.

• Exploring new applications: the use of magnetic or non-magnetic stainless steel in cutting-edge fields such as sensors and biomedical.

Future trends and new materials

The future of stainless steel magnetism is marked by exciting trends and innovations:

• Environmental applications: the use of magnetic stainless steel in green technologies.

• Nanotechnological advances: to precise customization of magnetic properties at the atomic scale.

• Composite materials: the integration of stainless steel with other materials to create composites with unique properties.

Thanks to these advancements, the industry is moving into an era where stainless steel will become even more prevalent, with the emergence of innovative materials and revolutionary applications. These perspectives underline the crucial role of stainless steel in future technologies and illustrate the dynamic and evolving nature of its potential to serve diverse markets.

Conclusion and additions research

This exploration of the magnetism of stainless steel reveals the complexity and importance of this phenomenon in a variety of industrial applications. Now, it’s time to consolidate this knowledge and consider ways to continue learning and innovating.

Synthesis of knowledge on the magnetism of stainless steel

We have discovered how the crystal structure, chemical composition and treatments of stainless steel influence its magnetic behaviour. Stainless steel grades have different magnetic properties that are essential to understand for their proper application. With the importance of heat treatments to the selection of grades for specific applications, every aspect of stainless steel magnetism has significant practical implications.

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For those who want to expand their knowledge or solve specific challenges related to stainless steel magnetism, Easy Inox offers specialized resources and training. Whether you are looking for technical advice, details on the latest innovations or practical training, Easy Inox can help you to understand and improve your skills. Explore these opportunities to stay on top of the use of stainless steel and ensure excellence in your projects.