What is 304 Stainless Steel？
Acier inoxydable 304 est un alliage – c'est-à-dire un métal fabriqué en mélangeant ce qu'on appelle des éléments d'alliage dans un métal de base – et il constitue une véritable colonne vertébrale pour l'industrie moderne. L'acier est principalement composé de carbone et de fer, avec d'autres éléments traces qui peuvent conférer aux aciers des propriétés uniques. Une catégorie d'aciers est connue sous le nom d'aciers inoxydables, qui utilise le chrome pour réduire la corrosion habituelle rencontrée par la plupart des matériaux à base de fer. Cet article explorera l'acier inoxydable le plus courant, l'acier 304, et étudiera ses propriétés physiques, mécaniques et de travail. Les concepteurs comprendront mieux ce qu'est ce matériau, comment il fonctionne et où l'acier 304 est utilisé dans l'industrie afin de pouvoir éventuellement sélectionner ce matériau pour leurs propres projets.
Physical Properties of Acier inoxydable 304
Les aciers inoxydables tirent leur nom de l'American Iron & Steel Institute (AISI) et de la Society of Automotive Engineers (SAE), qui ont chacun créé leur propre système de dénomination pour les alliages d'acier basé sur les éléments d'alliage, les usages et d'autres facteurs. Les noms d'acier peuvent devenir confus, car le même alliage peut avoir différentes identifiants selon le système utilisé ; cependant, il faut comprendre que la composition chimique de la plupart des mélanges d'alliages reste la même à travers les systèmes de classification. Dans le cas des aciers inoxydables, ils sont souvent composés de 10 à 30% pour cent de chrome et sont conçus pour résister à divers degrés d'exposition à la corrosion. Pour en savoir plus sur les différences entre les aciers inoxydables, n'hésitez pas à lire notre article sur le type d'acier inoxydable.
L'acier de type 304 fait partie des aciers inoxydables 3xx ou de ces alliages qui sont mélangés avec du chrome et du nickel. Voici une répartition chimique de l'acier 304 :
<=0,08% pour cent de carbone
18-20% pour cent de chrome
66,345-74% pour cent de fer
<= 2% pour cent de manganèse
8-10,5% pour cent de nickel
<=0,045% pour cent de phosphore
<=0,03% pour cent de soufre
<=1% pour cent de silicium
The density of Acier 304 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.
L'acier de type 304 est austénitique, ce qui est simplement un type de structure moléculaire composée du mélange d'alliage fer-chrome-nickel. Cela rend l'acier 304 essentiellement non magnétique, et lui confère une moindre faiblesse à la corrosion entre les grains grâce au fait que les aciers austénitiques sont généralement à faible teneur en carbone. L'acier 304 se soude bien avec la plupart des méthodes de soudage, avec ou sans matériaux d'apport, et il se façonne, se forme et se tourne facilement en forme.
Résistance à la corrosion & effets de la température
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.
Propriétés mécaniques de l'acier inoxydable 304
Tableau 1 : Résumé des propriétés mécaniques de l'acier 304.
Propriétés mécaniques
Métrique
Anglais
Résistance à la traction ultime
515 MPa
74700 psi
Résistance à la traction
205 MPa
29700 psi
Dureté (Rockwell B)
70
70
Module d'élasticité
193-200 GPa
28000-29000 ksi
Impact Charpy
325 J
240 ft-lb
Le tableau 1 présente quelques propriétés mécaniques de base de l'acier 304. La section suivante détaillera brièvement chacun de ces paramètres, et montrera comment ils sont pertinents pour les propriétés de travail de l'acier 304.
Les résistances à la traction ultime et à la limite d'élasticité sont une mesure de la résilience d’un matériau face aux forces de traction. La limite d'élasticité est inférieure à la résistance à la traction ultime, car elle décrit la contrainte maximale avant que le matériau ne se déforme de façon permanente, tandis que la résistance ultime fait référence à la contrainte maximale avant la fracture. Bien que moins résistant que certains autres aciers disponibles, ces résistances réduites permettent à ce métal d’être facilement façonné et manipulé sans trop de difficulté.
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 Acier 304 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.
Une mesure relativement obscure, mais néanmoins importante, d’un matériau est la quantité d’énergie absorbée lorsqu’il est frappé par une force importante, ce qui montre comment il se fracture sous contrainte. Il est vital de savoir comment un matériau se cassera, car certaines applications privilégient un scénario de défaillance plus ductile plutôt qu’une fracture plus fragile. Le test d’impact Charpy utilise un grand pendule qui balaye une éprouvette fendue d’acier pour simuler ces conditions, où un indicateur montre combien d’énergie est transférée du pendule au métal. Un score d’impact Charpy faible signifie que le matériau est généralement plus dur, où sa structure cristalline rigide préférerait simplement se fracturer sous la force du pendule à haute énergie. L’acier 304 a un score d’impact Charpy élevé, ce qui signifie qu’il est généralement plus malléable et se pliera avant de se casser, absorbant une partie de l’impact. Cette valeur est une preuve supplémentaire que l’acier 304 est facilement travaillé et manipulé, où la fracture est moins probable dans des conditions stressantes.
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.
Quelle est la résistance à la traction de l'acier inoxydable 304 ?
La résistance à la traction de l'acier inoxydable 304 est de 515 MPa ou 74700 psi. La résistance à la traction spécifique de l'acier inoxydable 304 peut varier en fonction de facteurs tels que le traitement thermique, les processus de fabrication et la variante spécifique de l'acier inoxydable 304.
Applications de l'acier inoxydable 304
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:
Équipements de cuisine (éviers, couverts, crédences)
Tubes de différents types
Équipements alimentaires (brasseurs, pasteurisateurs, mélangeurs, etc.)
Équipements de traitement pharmaceutique
Aiguilles hypodermiques
Casseroles et poêles
Équipements de teinture
ainsi que d'autres usages.
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 Usinage CNC.
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