304 Steel is an alloy – that is, a metal made from blending so-called alloying elements into a base metal – and it provides a quite literal backbone for modern industry. Steel is composed of primarily carbon and iron, with other trace elements that can give steels unique properties from each other. One class of steels is known as the stainless steels, which utilizes chromium to reduce the usual corrosion experienced by most iron-based materials. This article will explore the most common stainless steel, 304 steel, and will investigate its physical, mechanical, and working properties. Designers will gain a better understanding of what this material is, how it works, and where 304 steel is applied in industry so that they can potentially select this material for use in their own projects.
Table of Contents
Physical Properties of 304 Stainless Steel
Stainless steels get their names from the American Iron & Steel Institute (AISI) and the Society of Automotive Engineers (SAE), who have separately created their own naming systems for steel alloys based on alloying elements, uses, and other factors. Steel names can get confusing, as the same alloy can have different identifiers depending on which system is used; however, understand that the chemical composition of most alloy blends remains the same across classification systems. In the case of stainless steels, they are often composed of 10 to 30% chromium and are made to withstand varying degrees of corrosion exposure. To learn more about the differences among stainless steels, feel free to read our article on the type of stainless steel.
Type 304 steel is part of the 3xx stainless steels or those alloys which are blended with chromium and nickel. Below is a chemical breakdown of 304 steel:
<=0.08% carbon
18-20% chromium
66.345-74% iron
<= 2% manganese
8-10.5% nickel
<=0.045% phosphorus
<=0.03% sulfur
<=1% silicon
The density of 304 steel is around 8 g/cm3, or 0.289 lb/in3. Type 304 steel also comes into three main varieties: 304, 304L, and 304H alloys, which chemically differ based on carbon content. 304L has the lowest carbon percentage (0.03%), 304H has the highest (0.04-0.1%), and balanced 304 splits the difference (0.08%). In general, 304L is reserved for large welding components that do not require post-welding annealing, as the low carbon percentages increase ductility. Conversely, 304H is most used in elevated temperatures where the increased carbon content helps preserve its strength while hot.
Type 304 steel is austenitic, which is simply a type of molecular structure made from the iron-chromium-nickel alloy blend. It makes 304 steel essentially non-magnetic, and gives it a lower weakness to corrosion between grains thanks to austenitic steels being generally low carbon. 304 steel welds well using most welding methods, both with and without fillers, and it easily draws, forms, and spins into shape.
Corrosion resistance & temperature effects
Type 304 steel, being the most popular stainless steel, is naturally chosen for its corrosion resistance. It can resist rusting in many different environments, only being majorly attacked by chlorides. It also experiences increased pitting in warm temperatures (above 60 degrees Celsius), though the higher carbon grades (304H) mitigate this effect considerably. This means that 304 steel mainly rusts not in high temperatures, but in aqueous solutions where continuous contact with corrosive materials can wear down the alloy. 304 steels are not readily hardened by thermal treatment, but can be annealed to increase workability and cold worked to increase strength. If corrosion resistance is of high priority to a project, 304L is the best choice as its decreased carbon content reduces intergranular corrosion.
Mechanical Properties of 304 Stainless Steel
Table 1: Summary of mechanical properties for 304 steel.
Mechanical Properties
Metric
English
Ultimate Tensile Strength
515 MPa
74700 psi
Yield Strength
205 MPa
29700 psi
Hardness (Rockwell B)
70
70
Modulus of Elasticity
193-200 GPa
28000-29000 ksi
Charpy Impact
325 J
240 ft-lb
Table 1 shows some basic mechanical properties of 304 steel. The following section will briefly detail each of these parameters, and show how they are pertinent to the working properties of 304 steel.
The ultimate tensile and yield strengths are a measure of a material’s resilience to tensile (pulling) forces. The yield strength is lower than the ultimate tensile strength, as the yield strength describes the maximum stress before the material will deform permanently, whereas the ultimate strength refers to the maximum stress before fracture. While not as strong as some other steels available, the decreased strengths allow this metal to be easily worked into shape and manipulated without much difficulty.
The hardness test is one of the various hardness tests used to describe a material’s response to surface deformation. A harder material will not scratch easily and is typically more brittle, while a softer material will deform under local surface stress and is generally more ductile. The higher the Rockwell hardness, the harder the material, but to what degree depends on how it compares to other metals on the same scale. 304 steel has a Rockwell B hardness of 70; for reference, the Rockwell B hardness of copper, a soft metal, is 51. Simply put, 304 steel is not as hard as some of its stainless steel brothers such as 440 steel (see our article on 440 steel for more information), but still holds its own as a tough general purpose steel.
Type 304 steel has a range of elastic moduli, depending upon what type is used, but they all lie within 193-200 GPa. The modulus of elasticity is a good measures of a material’s ability to retain shape under stress, and is a general indicator of strength. As with most steels, the elastic modulus of 304 steel is quite high, meaning it will not easily deform under stress; however, note that a lower elastic modulus makes it easier to machine, so 304 is often fabricated to have a lower elastic modulus to allow for easy machining.
A relatively obscure, but nevertheless important measure of a material is how much energy is absorbed when it is stuck by a large force, which will show how it fractures under stress. It is vital to know how a material will break, as some applications will desire a more ductile failure scenario over a more brittle fracture. The Charpy impact test uses a large pendulum that swings into a notched specimen of steel to simulate these conditions, where a gauge will show how much energy is transferred from the pendulum into the metal. A low Charpy impact score means that the material is generally harder, where its rigid crystal structure would rather simply fracture under the high energy pendulum force. 304 steel has a high Charpy impact score, meaning it is generally more malleable and will bend before it breaks, absorbing some of the impact. This value is yet more proof that 304 steel is easily worked and manipulated, where fracture is less likely under stressful conditions.
What is the Strength of 304 Stainless Steel?
The yield strength of 304 stainless steel is 205 MPa or 29700 psi. The yield strength can vary based on factors such as the specific heat treatment and manufacturing processes applied to the stainless steel.
What is the Tensile Strength of 304 Stainless Steel?
The tensile strength of 304 stainless steel is 515 MPa or 74700 psi. The specific tensile strength of 304 stainless steel can vary based on factors such as heat treatment, manufacturing processes, and the specific variant of 304 stainless steel.
Applications of 304 Stainless Steel
304 steel is often referred to as “food-grade” stainless steel, as it is unreactive with most organic acids and is used in the food processing industry. Its excellent weldability, machinability, and workability suits these stainless steels to applications that require a level of corrosion resistance as well as complexity. As a result, 304 has found many uses, such as:
Kitchen equipment (sinks, cutlery, splashbacks)
Tubing of various types
Food equipment (brewers, pasteurizers, mixers, etc.)
Pharmaceutical processing equipment
Hypodermic needles
Pots and pans
Dyeing equipment
as well as other uses.
Through this list, it is clear that 304 steel is effective in many different areas. Its excellent working characteristics, combined with its extensive history and availability make it a great first choice when choosing a stainless steel.
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