Table of Contents
What is 304 Stainless Steel？
304 Stainless 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.
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 Rockwell B 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.
Stainless steel is an iron-chromium alloy that contains anywhere from 10 to 30% chromium which gives the metal high resistance to corrosion. Although there are many grades of stainless steel only a dozen or so are used with any regularity. For example, AISI Type 304 SS, having a chromium-nickel constituent and low carbon, is popular for its good corrosion resistance, cleanability, and formability, making it popular for many everyday items such as kitchen sinks. AISI Type 316 SS, containing the alloying element molybdenum, is even more resistant to chemical attack than Type 304, making it useful for exposure to seawater, brine, sulfuric acids, and other corrosives found in the industrial environment. This article briefly discusses some of the popular grades of stainless steel as well as the settings in which these grades excel.
The principal types of stainless steels include:
Ferritic
Martensitic
Austenitic
Duplex
Ferritic Stainless Steel
The addition of chromium (>17%) to a steel alloy stabilizes the ferritic phase of the alloy, making a material that is highly corrosion-resistant, if not exceptionally strong. It cannot be hardened through heat treatment but can be cold-worked to increase hardness. It is an inexpensive grade and is often used for kitchen equipment, architectural/ornamental applications, etc. where corrosion resistance, ductility, formability, and cost are important, and strength is not a concern.
Martensitic Stainless Steel
Adding carbon (up to 2%) to the chromium-iron alloy increases the alloy’s hardenability. Although unable to be hardened to the level of iron-carbon martensite, martensitic stainless steel can be sufficiently hardened to produce rust-resistant cutlery, surgical instruments, ball valves and seats, for example. Martensitic stainless steels tend to be used in specialty applications. AISI Type 410, for example, is used for making food-machine parts, pump shafts, etc., while Type 403 is used in high-heat applications such as turbines. Type 416 is considered free-machining and has the best machining characteristics of all the stainless steels; it is used for many turned SS parts. Martensitic stainless steel is magnetic and, with a high carbon content, difficult to weld.
Austenitic Stainless Steel
Adding nickel (8-20%) to the chromium-iron alloy produces a steel that is austenitic at room temperature, with a face-centered cubic structure that resists corrosion, and whose magnetic field is one of a soft magnet (ie, it can be magnetized in an electric field, but not permanently). These steels have relatively low carbon content, which makes them weldable. This group is the most commonly used of all the stainless steels, notably Type 302. The economical 304, sometimes called food-grade, is used for general-purpose corrosion-resistant applications where welding-related corrosion is of concern. The improved corrosion-resistant 316 is used for industrial applications and is considered the most corrosion-resistant of the austenitic stainless steels. An “L” after the grade indicates improved weldability under the harshest of welding conditions. Temperature resistance is increased by adding titanium, as in Type 321, a popular material in aerospace applications.
A relatively new grade of stainless steels, sometimes called PHSSs and carrying identifiers such as 15-5, 17-4, and 17-7 PH, are precipitation hardened. This special heat-treating process increases the metal’s resistance to stress corrosion cracking. Some of these PHSSs are austenitic, some are martensitic, and some fall somewhere in between. A-286 alloy was one of the first of the so-called superalloys.
Duplex Stainless Steel
Duplex steels have structures that combine both ferritic and austenitic phases, giving them almost twice the strength of austenitic varieties. With good corrosion resistance and weldability akin to that of austenitic stainless steel, they are used in a variety of special applications–on offshore platforms and in pressure vessels, for instance, where strength is imperative.
Stainless Steel Grades Summary
Table 1 below describes many of the AISI stainless steels, their strengths, and typical applications. Some of the steels with suffixes (L, S, etc.) have not been included, nor have many of the specialty PHSSs.
Grade Reference
Stainless Steel Type
Description of strengths, characteristics, and applications
201
Austenitic
Low nickel equivalent of 301, used in flatware
202
Austenitic
Low nickel equivalent of 302, used for kitchenware
205
Austenitic
Low work hardening, for spin forming
301
Austenitic
Higher work hardening, for trailer bodies, fasteners
302
Austenitic
General purpose grade
303
Austenitic
Free machining version of 302, for screw machining
304
Austenitic
Low carbon, economical grade, not seawater resistant but weldable
304L
Austenitic
Extra-low carbon improves resistance to post-weld corrosion
305
Austenitic
Low work hardening, for spin forming
308
Austenitic
Higher alloy content for corrosion/heat resistance, for welding rod/wire
309
Austenitic
High temperature, scale resistant, for heat exchangers
310
Austenitic
High temperature, scale resistant, for furnaces
314
Austenitic
High resistance to scale, for radiant tubes
316
Austenitic
Increased molybdenum for improved corrosion resistance in seawater
316L
Austenitic
A low carbon version of 316 for improved post-weld corrosion resistance
317
Austenitic
Improved corrosion and creep resistance over 316
321
Austenitic
High titanium version of 304 for better high-temperature performance
329
Aust-Ferritic
General corrosion resistance, like 316, with improved stress-crack resistance
330
Austenitic
Resistant to carburization, oxidation, thermal shock, for heat-treating fixtures
347
Austenitic
A higher creep-strength version of 321, for jet engine components
348
Austenitic
Low retentivity version of 321, for nuclear service
384
Austenitic
Low cold work hardening, for bolts, screws
403
Martensitic
Turbine grade, for steam turbine blading
405
Ferritic
Non-hardenable grade of 403
409
Martensitic
General purpose, for constructions not requiring heat treatment
410
Martensitic
General purpose, for machine parts such as shafting, auto exhausts
414
Martensitic
High hardenability, for springs
416
Martensitic
Free machining version of 410
420
Martensitic
High carbon modification of 410, for surgical instruments
422
Martensitic
High strength for temperatures to 1200°F, for turbine blades
429
Ferritic
Exhibits better weldability than 430
430
Ferritic
Chromium type, non-hardening, for annealing baskets, dishwashers
431
Martensitic
Special purpose, hardenable, for beater bars
434
Ferritic
Modified 430, for high resistance to road salts
436
Ferritic
General corrosion and heat resistant grade, for automotive trim
440A, B, C
Martensitic
Highest hardenability of the stainless steel grades, for use to create bearing balls
442
Ferritic
High temperature and scale resistance, for furnaces
446
Ferritic
High temperature and scale resistance, for intermittent use, pyrometer tubes
501
Martensitic
Heat resistant with high strength, for petrochemical equipment
502
Ferritic
Heat resistant with high ductility, for petrochemical equipment
What is the Yield 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. As always, contact your supplier to determine how your specifications can be met, and to see if 304 steel is the right metal for CNC Machining.
XTJ is a leading OEM Manufacturer that is dedicated to providing one-stop manufacturing solutions from prototype to production. We are proud to be an ISO 9001 certified system quality management company and we are determined to create value in every customer relationship. We do that through collaboration, innovation, process improvements, and exceptional workmanship.
