Martensitic Steel – Properties and Applications
In the vast spectrum of steel grades, martensitic stainless steel holds a unique position due to its distinct properties and versatile applications. Characterized by a body-centered tetragonal structure, martensitic steel grades can be hardened and strengthened through heat treatments, thanks to their carbon content. Martensitic alloys primarily consist of chromium, molybdenum, and carbon, with nickel being absent in most grades. This article delves into the properties of martensitic steel grades, their strengths and hardness, corrosion resistance, magnetic properties, and their various applications in different industries.
Properties of martensitic steel grades
Martensitic stainless steel is defined by a body centered tetragonal structure, which is similar to the structure of ferritic steel. In addition, martensitic grades can be hardened and strengthened by heat treatments, due to the addition of carbon. The alloying mainly consist of chromium (10,5 – 18 %), molybdenum (0,2 — 1%) and carbon (0.1 – 1.2 %). Further, except for two grades, no nickel is included.
Martensitic grades are mostly characterized by average corrosion resistance and good mechanical properties. Their exceptional strength and hardness set them apart, providing durability and resistance to wear. However, while they are corrosion resistant to some degree, it’s important to note that this is not their strongest suit when compared to other steel grades. Additionally, their magnetic properties further broaden their range of potential applications, setting them apart from many other types of steel.
Strength and hardness characteristics
Martensitic stainless steel grades show tensile strengths of up to 275 MPa in the annealed condition. Further, they can be machined, cold formed, or cold worked in this state. It is possible to increase the strength and hardness potential by increasing the carbon content of the alloy. Grades with high carbon content can be heat treated to hardness of 60 HRC. However, this procedure decreases the ductility and toughness of martensitic stainless steel.
Corrosion resistance
Martensitic stainless steels are characterized by the highest strength of all stainless steel grades. However, they possess the lowest corrosion resistance and have a lower corrosion performance than ferritic and austenitic steels with the same chromium content and other alloys. The optimum corrosion resistance of martensitic steels is achieved in a hardened and tempered condition. Further, the addition of up to 0.2 % of nitrogen allows mitigating intergranular corrosion which occurs in martensitic microstructures when the material is exposed to sulfuric acid solutions.
The toughness, weldability and corrosion resistance can be improved through the addition of nitrogen and nickel, but with lower levels of carbon than the traditional grades.
Magnetic properties
Martensitic grades are classified as a hard ferromagnetic group. This means that this steel grades are very hard to magnetize or demagnetize. If the material is magnetized in the hardened condition, it shows permanent magnetic characteristics. Cold working can change the behavior from a soft magnet to a weak permanent magnet. The exposure to a strong magnetic field leads to a permanent magnetization of the cold worked metal. This can cause handling problems and can affect the corrosion resistance due to the attraction of small bits of iron or steel. Therefore, it is crucial to demagnetize parts which have been exposed to a strong magnetic field during the process of fabrication.
Hardening of martensitic stainless steel
The hardening process consists of two steps. First, the metal undergoes a heat treatment with high temperature. The goal is to achieve an austenitic structure with carbon in a solid solution. The structure changes at a temperature range between 925 and 1070 °C. One of the main influencing factors for the reached strength and hardness is the composition of the material and varies vastly with the carbon content. In particular, it should be noted that the time at the austenitizing temperature has an influence on the hardness of the material. The hardness increases to a maximum and then starts to decrease slowly.
The heat treatment is followed by quenching, while the quenching medium depends on the material and is usually performed with air, oil or water. When the material is cooled below the temperature where martensite begins to transform, the so called MS-temperature, the austenite transforms to martensite. The transformation finishes at around 150 to 200 °C below the MS-temperature, which ranges between 300 and 700 °C.
The MS-temperature depends on the composition of the alloy in metal. While all alloying elements lower the MS-temperature, carbon has the most significant effect. Thus, due to the low temperature which is needed to finish the transformation, highly alloyed martensitic grades contain retained austenite.
An even higher surface hardness can be achieved with BORINOX®. In fact, the treatment improves the corrosion resistance and wear protection of martensitic stainless steels. In acidic or chloride environments, martensitic grades are not suitable for long-term use without special treatments. The BORINOX® low pressure process produces an exceptionally wear resistant surface.
Applications
Due to the excellent tensile and fatigue strength combined with mediocre corrosion resistance, martensitic stainless steel is a good fit for application where the material is exposed to both, corrosion and wear and are used in a wide range of industries.
High carbon grades are often used for tool steels, including surgical and dental equipment, but also screws, blades, cutting tools and gears. Further, steam and gas turbines blades and buckets are fabricated of this stainless steels. Therefore, it is an important material for the petrochemical industry. Martensitic stainless steel is also used in aerospace, automotive and the sporting equipment industry.
