What is Martensitic Stainless Steel and What Can It Do for Your Business?
Stainless steel is known for its corrosion resistance, heat resistance, high strength, cleanliness and more. There are five different types of stainless steel, the difference between them lies in the chemical composition of each. Martensitic steel is a type of stainless steel that is known for its strength, corrosion resistance, and durability. These qualities make martensitic steel a good choice for a variety of applications. Below, we will discuss the properties of Martensitic steel, its composition, its advantages over other types of steel, and its applications throughout industry.
What is Martensitic Steel?
Martensitic steel is a type of stainless steel that, because of its chemical composition, can be hardened and strengthened through heat and aging treatments. These methods make Martensitic steel stronger than other types which makes it a good choice for the fabrication of medical instruments, mechanical valves, turbine parts, mechanical instruments, and other various applications.
Composition of Martensitic Steel
Like all stainless steels, the main component of martensitic steel is chromium, which usually accounts for 11.5-18% of its composition. Other common components include up to 1.2% carbon, and nickel. The high amount of carbon gives this type of steel a strong molecular structure but the lack of nickel makes it less corrosion resistant than other types of stainless steel. Small amounts of other alloying elements such as manganese, molybdenum, and nickel are also added.
Martensitic Stainless Steel Processing
Rapid Cooling Martensitic Grades
The most common types of martensitic stainless strip grades are 410 Stainless Steel, 420 Stainless Steel, and 440A. These martensitic stainless steels react to heat treatment much like high carbon steel alloys. The maximum quenched hardness depends primarily on the carbon content.
They are hardened by heating to high temperatures followed by rapid cooling. Since the hardenability of martensitic alloys is very high, this is frequently referred to as “air hardening”.
Because the as-hardened martensitic structure is quite brittle, the material is typically reheated at low temperatures to stress-relieve the microstructure or reheated to slightly higher temperatures in order to soften (temper) the material to intermediate hardness levels. Process annealing is accomplished by heating just below the alloys’ critical temperature; full annealing is achieved by heating just above the critical temperature with slower cooling.
Types of Martensitic Steel
Martensitic steel can further be divided into two distinct types based on its carbon content.
Low Carbon Martensitic Steel
Low carbon martensitic steel has a carbon content between from 0.05% to 0.25%. Low carbon versions of Martensitic steel are stronger, provide a higher corrosion resistance, and enhanced potential for fabrication.
High Carbon Martensitic Steel
High carbon martensitic steel usually has a carbon content between 0.61% and 1.50%. An increased carbon content makes the steel stronger because carbon strengthens the molecular structure. However, it also makes the metal more brittle and it cannot be welded or easily formed into other shapes.
Type 410 Stainless Steel
Type 410 stainless steel is a martensitic stainless steel grade that is regarded as a general purpose martensitic. Applications include fasteners, springs, pins, cutlery, hardware, gun clips, micrometer parts, turbine blades, coal screens, pump rods, nuts, bolts, fittings, ball bearings, shafts, impellers, pistons, and valves. Hardness levels can be changed with slight variations in hardening and tempering heat treatments.
Normally, Type 410 is supplied in the annealed condition, however Ulbrich can also supply Type 410 with a minimum hardness of RC35 for gauges less than .040”. Another option is cold rolled, with a minimum tensile strength of 110,000 psi.
Type 420 Stainless Steel
Type 420 stainless steel covers a wide carbon range of 0.15% to 0.45% carbon content, and therefore has a relatively wide range of hardness levels in both the hardened and tempered condition.
We can provide Type 420 Stainless at various carbon levels to meet specific hardness or mechanical property requirements after heat treating, including cold rolled with a minimum tensile strength of 120,000 psi.
This steel alloy is also hardenable to a between RC40-50. Versatile hardnesses can be obtained via heat treat cycles which makes 420 desirable where tempered products are necessary for specific applications. Some applications of 420 stainless steel include fasteners, cutlery, machine parts, bushings, surgical tools, firearms, and valve trim.
Type 440 Stainless Steel
Type 440A stainless steel has a greater hardening capability than Type 410 or Type 420, but limited formability in the annealed condition. This grade of stainless steel is hardenable to over RC50 which makes it very attractive for blanking into blade applications. The high hardness of this grade means that blades remain sharper longer.
Typical uses include:
- Surgical Instruments
Type 440A is also used in other applications where high hardness is important alongside corrosion resistance
Other Martensitic Grades
The martensitic grades listed above are the most common grades that are available and used. Other martensitic grades with special chemical composition requirements and/or mechanical properties are also available on the market, but when talking about martensitics, 410, 420, and 440 are the grades that are most commonly referenced.
Characteristics of Martensitic Steel
Besides its strength, martensitic steel has multiple properties that differentiate it from other types of stainless steel. The martensitic grades of stainless steels cover a wide range of applications, from combating comparatively mild corrosive conditions to creating maximum strength and stiffness for cold formed parts. Martensitic steel grades are grouped together because they share many of the same characteristics that manufacturers look to when specifying steel alloys.
Martensitic steel is typically brittle, and most forms do not react favorably to welding. However, quenched and tempered martensitic steel decreases its brittleness and increases its applications. The quenching and tempering process involves heating the metal and then rapidly cooling it to set it in place quickly. High carbon martensitic stainless steels are generally not recommended for welded applications, although Type 410 Stainless can be welded with relative ease.
Many types of martensitic steel are magnetic. The crystal-like molecular structure can be magnetic if there is iron present in the alloy. Magnetism can make metals easier to sort but it can make welding and other fabrication processes more difficult. Martensitic steels are magnetic in both the annealed and hardened condition.
Formability is a metal's ability to be made into different shapes without breaking or cracking. Martensitic steel's formability decreases as carbon content increases. Low carbon forms are not ideal for shaping, but it is possible.
Martensitic stainless steels are used primarily, but not always, where high mechanical properties are necessary. Their degree of corrosion resistance is more of a limiting factor in their application than is that of other alloys in the stainless steel family.
Frequently, some superficial rust stain will appear on their surfaces. When only limited corrosion resistance or resistance to elevated temperature scaling is required, they can be used in the annealed condition, but their highest corrosion resistance is attained in the hardened or tempered condition.
Annealed Martensitic Stainless Steel
The martensitic grades are typically supplied to manufacturers in the annealed condition because this state provides the best forming characteristics. A hardening heat treatment generally follows forming operations.
Grades 410 and 420 can also be attained in the cold worked condition at relatively low tensile strength levels when compared to 300 Series, Austenitic stainless steels.
How is Martensitic Steel Different From Other Kinds of Stainless Steels?
As stated earlier, the types of stainless steel differ in their chemical structure and components. These factors determine the steels behavior and possible applications.
Austenitic Stainless Steel
This type of steel has a high chromium content compared to other stainless steel types. They are also composed of nitrogen, manganese, and nickel. This makes them highly resistant to corrosion and one of the most used types of stainless steels. Unlike martensitic steel, austenitic steel is weldable, formable, typically non-magnetic, and are not heat treatable – they can only be hardened through cold work.
Ferritic Stainless Steel
Ferritic steel has a high chromium content and a low carbon content. As a result of the low carbon content, ferritic steel is not as strong as martensitic steel but it is very resistant to corrosion and it is magnetic. These steels are often used in the auto industry, for kitchenware, and for the construction of industrial machinery. Ferritic steels are also not heat-treatable, and processed almost exclusively in the annealed temper.
Precipitation Hardened (PH) Stainless Steel
PH stainless steel is made by adding copper, molybdenum, aluminium, and titanium (by themselves or in any combination). These metals can be three or four times stronger than austenitic steel and has a relatively low toughness. PH steel is typically used in the aerospace, oil and gas, and nuclear industries because it is a unique blend of strength and good formability.
Hardened Martensitic Stainless Steel
One of the benefits of Martensitic steel is that it becomes stronger and harder after heat treatment. When this type of steel is heated and rapidly cooled, the atoms become stuck in a distorted position known as a body centered tetragonal, this makes the steel harder and stronger. There are numerous processes that can be employed in order to harden martensitic steel.
This process hardens steel by heating it in order to form precipitates which prevent the movement of defects in the steel's molecular structure. The containment of these defects makes the metal harder and stronger. After heating, it is then stored for hours at an elevated temperature until the process is complete. This process is often used to increase the strength of martensitic steel.
Annealing is the process of heating steel to try and make the steel's molecular structure more homogeneous and remove stress. This makes the steel softer and easier to manipulate.
Quenching and Tempering
Quenching and tempering is a process that hardens steel by heating, rapidly cooling, and then heating the metal again. After the initial heating and cooling, the metal is hard but very brittle. The second heating is designed to bring the steel back to a temperature at which it is ductile.
Stainless steel's many unique features make it ideal for a number of different applications. There are multiple types of stainless steel that fit into five main categories. Martensitic stainless steel is a versatile steel that has many practical uses for various industries. This type of steel is very responsive to multiple forms of heat treatment that can increase strength, hardness, and corrosion resistance. Martensitic stainless steel's strength and corrosion resistance can be ideal for marine, industrial, and medical applications.
Martensitic stainless steel's versatility can be the solution to a number of problems. If you want to learn more about our Martensitic Stainless Steel Strip & Wire offerings, or would like to speak with an expert in specialty metals who can help you find the right alloy that will meet your precise needs, Contact a Specialist. We know the metal industry and our job is to help you get your job done.