Martensitic Stainless Steel: Properties, Grades, and Applications

Martensitic stainless steels are one of the four main types of stainless steels (Austenitic, Ferritic, Duplex, Martensitic). They were developed mainly to satisfy the property requirements for hardness, high strength, wear resistance, and corrosion resistance. They are also ferromagnetic, meaning that they can retain their magnetic properties after the magnetic field is withdrawn. Unlike ferritic and austenitic stainless steels, they can be hardened by heat treatment. However, due to their relatively lower chromium content, martensitic stainless steels are not as corrosion resistant as ferritic or austenitic stainless steels [1].

Martensitic stainless steels make up the 400 series of stainless steels. The 410 grade is the base grade and also the most commonly used one. It typically contains 11.5 – 13% chromium, 0.15% carbon, 0.1% manganese and is used in applications such as gas turbines blades and cutlery. 416 is another popular grade. It contains more chromium and manganese with an addition of molybdenum and sulphur/selenium, and it is used to make screws and gears.

In this article, you will learn about:

• The properties of martensitic stainless steel
• Heat treatment of martensitic stainless steel
• The applications of martensitic stainless steel
• Grades & standards

Properties of martensitic stainless steel

Martensitic stainless steels typically contain between 11.5 to 18% chromium, up to 1% carbon, and other elements, such as nickel, selenium, phosphorus, vanadium, and sulphur are added in different grades to achieve specific properties. These steels have a face-centred cubic (FCC) structure at high temperatures, but when quenched during heat treatment, the austenite transforms into martensite with a body-centred cubic (BCC) structure.

Martensitic stainless steels are very amenable to heat treatment as they can be quenched and tempered to achieve improved mechanical properties, such as higher hardness and tensile strength. They are also among the group of stainless steels that are precipitation-hardenable to satisfy certain mechanical property requirements. Although martensitic stainless steels can be hot worked, they do not possess good formability or weldability, but the addition of sulphur can improve their machinability. If the carbon content is low, these steels undergo cold working with relative ease.

Table 1. Properties of selected hardened martensitic stainless steels.

 

	AISI 410	AISI 420	AISI 431	AISI 440A	AISI 440B	AISI 440C
Yield strength at 20 °C	540 MPa	760 MPa	1070 MPa	1650 MPa	1860 MPa	1900 MPa
Poisson ratio at 20 °C	0.3	0.3	0.3	0.3	0.3	0.3
Elong. at 20 °C	16 %	12 %	15 %	5 %	3 %	2 %
Tensile str. at 20 °C	740 MPa	980 MPa	1390 MPa	1790 MPa	1930 MPa	1970 MPa
Elec. cond. at 20 °C	1.68*107 S/m	1.68*107 S/m	1.45*107 S/m	1.57*107 S/m	1.28*107 S/m	1.28*107 S/m
CTE at 20 °C	1.1*10-5 1/K	1*10-5 1/K	1.2*10-5 1/K	1*10-5 1/K	1*10-5 1/K	1*10-5 1/K
Thermal cond. at 20 °C	30 W/(m·K)	30 W/(m·K)	25 W/(m·K)	30 W/(m·K)	15 W/(m·K)	15 W/(m·K)
Melting point at 20 °C	1480 °C	1450 °C	1450 °C	1370 °C	1370 °C	1370 °C
Specific heat capacity at 20 °C	460 J/(kg·K)	460 J/(kg·K)	460 J/(kg·K)	460 J/(kg·K)	430 J/(kg·K)	430 J/(kg·K)

Heat treatment of martensitic stainless steel

The heat treatment of martensitic stainless steels undergoes three processes, namely austenitising, quenching and tempering.

Austenitising 

Austenitising involves heating the steel to a temperature between 980 °C and 1050 °C, which puts the steel in its austenitic phase with an FCC crystal structure.

Quenching 

Quenching occurs when the steel is rapidly cooled in air or other media, such as oil, which converts most of the austenite into martensite with a new BCC crystal structure. This newly created martensite is very hard compared to the austenite.

Tempering

Tempering is when the mechanical properties of the steel are customised. It is a more intricate process than austenitising and quenching. Quenched martensite is not ready for most applications, and so, further heat treatment is required. Tempering involves heating the steel to about 500 °C and maintaining it at that temperature before cooling it in air. Tempering must be done at specific temperature ranges as they affect the mechanical properties of the finished steel, such as tensile strength, elongation and impact resistance.

Figure 1. Steel phase diagram showing the relationship between carbon content and temperature. Martensitic steel takes place at the lower end of the graph. (Fractory) 

The applications of martensitic stainless steel

Martensitic stainless steels have high thermal conductivity, which makes them suitable for applications that require good heat distribution, such as heat exchangers. Furthermore, their low coefficient of thermal expansion (CTE) makes them more likely to retain their shape at high temperatures. They are also used in aerospace applications, where a high degree of stiffness is required, thanks to their high Young’s modulus. Below are some applications for some of the common grades of martensitic steels.

Table 2. Typical applications of martensitic stainless steels [1].

AISI Grade	Applications
410	General-purpose martensitic steel. Used for applications where corrosion is mild. Applications include cutlery, steam and gas turbine blades, bushings, etc.
416	Contains additional sulphur and phosphorus to improve its machinability. Its variant 416Se replaces sulphur with selenium. Applications include screws, gears etc.
420	Contains increased carbon for improved mechanical properties. Applications include dental and surgical instruments.
431	Contains increased chromium for better corrosion resistance. Applications include valves and pumps.
414	Contains added nickel for improved corrosion resistance. Applications include springs.
440	Contains increased chromium and carbon for improved hardness and corrosion resistance. Applications include measuring instruments, ball bearings, gauge blocks, moulds and dies, etc. It has sub-grades, 440A, 440B and 440C which have varying amounts of carbon to increase/reduce its hardness and toughness.

Grades and Standards of Martensitic Stainless Steel

The chart below shows the various grades of martensitic stainless steels. Grade 410 is the basic martensitic grade, and it contains the least alloying elements of the martensitic family. It has a variant, 410S wherein the carbon content has been lowered to improve its weldability.  Most martensitic grades contain no nickel except for the 414 grade, which has some nickel added for better corrosion resistance. Grade 440 has three main variants 440A, 440B and 440C with increasing carbon content respectively for higher hardness but reduced toughness.

Figure 2. The martensitic stainless grades [2].

[1] Garrison Jr, W. M., & Amuda, M. O. H. (2017). Stainless Steels: Martensitic.

[2] SERIES, A. D. H. DESIGN GUIDELINES FOR THE SELECTION AND USE OF STAINLESS STEELS.