Yes, martensite is magnetic. It is ferromagnetic, meaning it is strongly attracted to magnets and can be magnetized itself. This property is one of the most reliable ways to distinguish martensite from its parent phase, austenite, which is typically non-magnetic.
Why Martensite Is Ferromagnetic
Martensite forms when steel is rapidly cooled (quenched) from high temperature. During this process, the crystal structure shifts from the face-centered cubic arrangement of austenite to a body-centered tetragonal structure. Carbon atoms get trapped inside this distorted lattice, which is what makes martensite so hard. But the change in crystal geometry also changes how the iron atoms’ magnetic fields interact with each other.
In the body-centered tetragonal structure, iron atoms are spaced and oriented in a way that allows their individual magnetic moments to align in the same direction. This cooperative alignment is what produces ferromagnetism. Austenite’s face-centered cubic structure, by contrast, arranges iron atoms so their magnetic moments tend to cancel out, making it paramagnetic (essentially non-magnetic in practical terms).
Martensitic vs. Austenitic Stainless Steel
This difference has real consequences for stainless steel. The 400-series martensitic stainless steels (like 410 and 420) are strongly magnetic. The International Stainless Steel Forum classifies all martensitic grades as ferromagnetic, and some are specifically engineered as soft magnetic grades for electrical and sensor applications.
The 300-series austenitic stainless steels (like 304 and 316) are chosen partly because they are non-magnetic. However, that non-magnetic behavior isn’t always permanent. When 304 stainless steel is cold-worked, bent, or deformed, some of the austenite transforms into martensite right at the deformation site. This strain-induced martensitic transformation converts the structure from paramagnetic austenite to ferromagnetic martensite, and the part starts attracting magnets in those areas.
This is why a stainless steel kitchen sink might stick to a magnet in spots where it was stamped or bent during manufacturing, even though the base alloy is a “non-magnetic” austenitic grade. The amount of martensite that forms depends on how much plastic strain the metal experienced and the temperature during deformation. Higher strain rates and lower temperatures both promote more transformation.
What Affects How Magnetic It Is
Not all martensite is equally magnetic. The strength of its magnetism depends on composition. Alloying elements like carbon, chromium, nickel, and manganese each reduce the magnetic saturation of martensite to varying degrees. Pure iron has a magnetic saturation of about 274 emu/g. Every alloying element dissolved in the martensite lattice lowers that value by an amount proportional to its concentration and its specific “attenuation coefficient,” a measure of how strongly that element disrupts magnetic alignment.
Carbon is a key factor. It sits in interstitial positions along the c-axis of the tetragonal unit cell, and higher carbon content slightly reduces saturation magnetization. Chromium, present in all stainless steels, also lowers it. This is why a high-carbon, high-chromium martensitic steel won’t be quite as strongly magnetic as plain carbon steel martensite, even though both are clearly ferromagnetic.
Retained Austenite Weakens the Signal
In practice, quenched steel rarely converts 100% to martensite. Some austenite remains trapped in the structure, called retained austenite. Because austenite is non-magnetic, it dilutes the overall magnetic response of the part. Engineers actually use this relationship to measure how much retained austenite is present: they compare the magnetic saturation of a sample to the theoretical saturation of a fully martensitic (austenite-free) version of the same alloy. The gap between the two values reveals the fraction of retained austenite.
This technique is widely used in quality control for hardened steel components like gears and bearings, where too much retained austenite can compromise performance.
When Martensite Loses Its Magnetism
Like all ferromagnetic materials, martensite has a Curie temperature, the point above which thermal energy overwhelms the alignment of magnetic moments and the material becomes paramagnetic. For plain carbon steel martensite, this is close to iron’s Curie temperature of about 770°C (1043 K). In more complex alloys, the Curie temperature varies significantly with composition. In certain cobalt-based Heusler alloys studied for their magnetic shape-memory properties, the martensite Curie temperature ranged from as low as 185 K (-88°C) to as high as 923 K (650°C), depending on aluminum content.
For conventional carbon and stainless steels, the Curie temperature is high enough that martensite remains ferromagnetic across all normal service temperatures. You would need to heat the steel to a dull red glow before it lost its magnetic response.
Practical Uses of Martensite’s Magnetism
The ferromagnetism of martensite isn’t just a curiosity. It has several practical applications:
- Non-destructive testing: Magnetic particle inspection relies on martensite being ferromagnetic to detect surface cracks in hardened steel parts.
- Sorting scrap metal: Magnets can separate martensitic and ferritic stainless steels from austenitic grades in recycling operations.
- Measuring cold work: A magnetometer or permeability gauge can detect how much strain-induced martensite has formed in an austenitic stainless component, revealing whether it has been over-stressed or improperly processed.
- Retained austenite measurement: Magnetic saturation testing provides a fast, non-destructive way to quantify retained austenite in heat-treated parts without needing X-ray diffraction.
If you’re working with steel and wondering whether a particular piece is martensitic, a simple refrigerator magnet is a surprisingly useful first test. Strong attraction means the structure is ferromagnetic, pointing to martensite, ferrite, or a mix of both. No attraction points to austenite.