The metals that are characterized as steel are a diverse class of materials that are an integral part of modern society. Steel is an alloy that is made by combining iron with carbon as well as other elements that provide it with unmatched strength. Since its beginnings in the early 20th century, steel has proliferated into hundreds of types, each with its own unique material profiles. To help organize these steels, The American Iron & Steel Institute (AISI) and the Society of Automotive Engineers (SAE) have distinguished certain steel types, and this article will explore a steel class from the stainless steels or those which are more resistant to stains, corrosions, and rust. This stainless steel class is known as the 440 steels, and it is a highly popular material thanks to its hardness and resistance to wear. This article will investigate the characteristics, material properties, and applications of 440 steel so that readers can understand where it can be used and how to best apply this metal.
What is 440 Steel?
440 steel is a series of high-carbon martensitic stainless steels, including grades 440A, 440B, and 440C. Known for their excellent hardness and wear resistance, these steels are commonly used in the production of high-performance knives and cutting tools.
Physical Properties of 440 Steel
440 steel is part of the stainless steels or those alloy steels which contain a minimum of 10% chromium, imbuing these alloys with increased resistance to corrosion. The AISI has created a three-digit naming index for the stainless steels (as opposed to the typical four-digit names given to other, non-stainless steels; see our article about the types of steels for more information), where the first digit represents the type of steel and alloying elements. The stainless steels have also been designated based on their molecular structure, where stainless steels can be austenitic, ferritic, martensitic, or duplex types. The differences between these types lie in their crystal lattice structures, which give the stainless steels certain beneficial working characteristics such as increased hardness, malleability, or resilience to fracture. For more information on stainless steel, refer to our related article that describes the types and grades of those metals.
The 4xx class of stainless steels are alloyed with chromium and are ferritic or martensitic in structure. 440 steel actually refers to four types of steel: 440A, 440B, 440C and 440F steel, which only differ based on the level of carbon in their composition. All types of 440 steel are popular choices, but 440C steel is the most popular of the four, as it has the highest hardness. Below are the chemical compositions of each type:
Common elements across all 440 steels:
16-18% chromium
1% Manganese
1% silicon
0.75% Molybdenum
0.04% Phosphorus
0.03% sulfur
Some percentage of Carbon
440A steel
0.6-0.75% Carbon
440B steel
0.75-0.95% Carbon
440C and 440F steels
0.95-1.20% Carbon
All 440 steels are hardenable, meaning they gain additional strength via heat treatment, and they are considered a high carbon alloy steel. When strengthened using this process, 440 steels are the hardest of all the stainless steels, making them both abrasion-resistant as well as corrosion-resistant. When in its soft “annealed” state, 440 steel is easily worked, machined, and manipulated into shape, where it can then be hardened and gain the increased strength for which it is known.
Corrosion resistance & temperature effects
While most stainless steels are resistant to rusting, 440 steels are slightly less corrosion resistant. This is the result of its martensitic structure, which increases its hardness but leaves it more open to corrosion than austenitic and ferritic stainless steels. This is not to say that 440 steel rusts easily, as they are more corrosion-resistant than most steels, but understand that it should not be chosen purely for its rust resilience.
It is also important to know that 440 steels do not work well at temperature extremes. They lose their strength when at high temperatures because they over temper and become softer, and they lose their ductility at a temperature below 0o Celsius. This fact is important to any designer that wishes to implement this steel in particularly harsh environments, as 440 steel would probably not be the best choice.
Mechanical Properties
Table 1, below, shows some important mechanical properties of 440 steel. It is important to note that each alloy blend holds its own unique values, and these values fluctuate based on the strengthening treatment. Table 1 shows values for AISI 440 stainless steel, which is closest to 440C steel:
Table 1: Summary of mechanical properties for 440 steel.
Mechanical Properties
Metric
English
Ultimate Tensile Strength
1750 MPa
254000 psi
Tensile Yield Strength
1230 MPa
186000 psi
Hardness (Rockwell)
58
58
Modulus of Elasticity
200 GPa
29000 ksi
Charpy Impact
19.0 J
14.0 ft-lb
440 steel has impressively high strengths, as can be seen through its ultimate and tensile yield strengths. These measures come from experimental stress tests, where a specimen of 440 steel is pulled apart axially and its strain vs. stress is plotted on a curve. The yield stress is the maximum amount of stress a specimen can experience before plastically (or permanently) deforming, and the ultimate strength is the maximum stress experienced by the specimen before it fractures. These measures are useful in structural applications, where a material must not permanently deform under intense weight, and in high-stress applications where failure must not occur.
Material hardness is a comparative measure of how difficult it is to scratch, etch, or damage the surface of a material. There are many scales such as the popular Brinell, Vickers, and Mohs hardness scales, but steel is most often graded based on the Rockwell hardness scale. 440 steel has a Rockwell hardness of 58, which is considered quite hard. For comparison, hard steels used for blade edges have a hardness of 60, so it is clear that 440 steel is quite resilient to surface deformation.
The modulus of elasticity is a measure of how a material responds while elastically, or non-permanently deforming. A high modulus of elasticity means that is it requires a high level of stress to stretch a material, which can be thought of as having increased stiffness. 440 steel has a high modulus of elasticity, meaning it does not easily yield to stress, and as such makes it a stiff material overall.
Charpy impact testing determines the amount of energy absorbed by a material when under extreme conditions, i.e. under fracture or high strain conditions. A heavy pendulum is swung into a notched test specimen of steel, where a gauge will show how much energy, in Joules, is absorbed through the material. In general, high strength materials will not readily absorb much energy, as they will instead simply fracture (remember; brittleness increases with strength). As a result, most steels have a relatively low Charpy impact score, and 440 steels are no different. It is yet another illustration of the stiffness and strength of 440 steels, as they only absorb less than 20 Joules of energy when struck by the Charpy test pendulum.
How Strong is 440 Stainless Steel?
440 stainless steel is strong, and is known for its strength and hardness. 440 steel has a tensile yield strength of 186000 psi and 58 on the Rockwell (hardness) scale.
Applications of 440 Steel
440 steel can be found in most machine shops, as it is an easy metal to machine when annealed and yet is strong and resilient when heat treated. This material is exceptionally good for components that must resist corrosion relatively well and yet not wear under repeated use. It is a great steel for knife edges, as its hardness yet formability excels when used for kitchen cutlery. There are some other notable applications of 440 steel, listed below:
Mold and dies
Valve components
Instruments/measurement devices
Textile components
Ball bearings
to name but a few.
Summary
This article presented a brief summary of the properties, strength, and applications of 440 steel. For information on other products, consult our additional guides or visit the XTJ Platform to locate potential sources of supply or view details on specific products.