What is 304 Stainless Steel？
304 Stainless Steel on seos – eli metalli, joka on valmistettu sekoittamalla niin kutsuttuja seosaineita perusmetalliin – ja se tarjoaa kirjaimellisesti selkärangan modernille teollisuudelle. Teräs koostuu pääasiassa hiilestä ja raudasta, sekä muista hivenaineista, jotka voivat antaa teräksille ainutlaatuisia ominaisuuksia. Yksi teräslaji tunnetaan ruostumattomana teräksenä, jossa käytetään kromia vähentämään useimpien rautapohjaisten materiaalien tavallista korroosiota. Tämä artikkeli tutkii yleisintä ruostumatonta terästä, 304 terästä, ja selvittää sen fysikaalisia, mekaanisia ja työstöominaisuuksia. Suunnittelijat saavat paremman käsityksen siitä, mikä tämä materiaali on, miten se toimii ja missä 304 terästä käytetään teollisuudessa, jotta he voivat mahdollisesti valita tämän materiaalin omiin projekteihinsa.
Physical Properties of 304 Stainless Steel
Ruostumattomat teräkset ovat saaneet nimensä American Iron & Steel Institute (AISI) ja Society of Automotive Engineers (SAE) -järjestöiltä, jotka ovat erikseen luoneet omat nimeämisjärjestelmänsä terässeoksille seosaineiden, käyttökohteiden ja muiden tekijöiden perusteella. Terästen nimet voivat olla hämmentäviä, koska samalla seoksella voi olla eri tunnisteet riippuen siitä, mitä järjestelmää käytetään; ymmärrä kuitenkin, että useimpien seosten kemiallinen koostumus pysyy samana luokitusjärjestelmien välillä. Ruostumattomien terästen tapauksessa ne koostuvat usein 10–30 % kromista ja ne on tehty kestämään vaihtelevia määriä korroosiota. Jos haluat lisätietoja ruostumattomien terästen eroista, voit lukea artikkelimme ruostumattoman teräksen tyypistä.
Tyypin 304 teräs on osa 3xx ruostumattomia teräksiä tai niitä seoksia, jotka on sekoitettu kromin ja nikkelin kanssa. Alla on 304 teräksen kemiallinen erittely:
<=0,08 % hiiltä
18-20 % kromia
66,345-74 % rautaa
<= 2 % mangaania
8-10,5 % nikkeliä
<=0,045 % fosforia
<=0,03 % rikkiä
<=1 % piitä
The density of 304 teräs 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.
Tyypin 304 teräs on austeniittista, joka on yksinkertaisesti rauta-kromi-nikkeliseoksesta valmistettu molekyylirakenne. Se tekee 304 teräksestä olennaisesti ei-magneettista ja antaa sille pienemmän heikkouden rakeiden väliseen korroosioon, koska austeniittiset teräkset ovat yleensä vähähiilisiä. 304 teräs hitsautuu hyvin useimmilla hitsausmenetelmillä, sekä täyteaineilla että ilman, ja se on helppo vetää, muotoilla ja pyörittää muotoon.
Korroosionkestävyys ja lämpötilavaikutukset
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.
304 Ruostumattoman teräksen mekaaniset ominaisuudet
Taulukko 1: Yhteenveto 304-teräksen mekaanisista ominaisuuksista.
Mekaaniset ominaisuudet
Mitta
Englanti
Lopullinen vetolujuus
515 MPa
74700 psi
Myötölujuus
205 MPa
29700 psi
Kovuus (Rockwell B)
70
70
Joustavuuden moduli
193-200 GPa
28000-29000 ksi
Charpy-iskuvoima
325 J
240 ft-lb
Taulukko 1 näyttää joitakin 304-teräksen perusmekaanisia ominaisuuksia. Seuraavassa osiossa esitellään lyhyesti jokainen näistä parametreista ja näytetään, kuinka ne liittyvät 304-teräksen työominaisuuksiin.
Loppuvetolujuus ja myötölujuus ovat mitta materiaalin kyvystä kestää vetovoimia (vetäviä voimia). Myötölujuus on alhaisempi kuin loppuvetolujuus, koska myötölujuus kuvaa suurinta jännitystä ennen pysyvää muodonmuutosta, kun taas loppuvetolujuus viittaa suurimpaan jännitykseen ennen murtumista. Vaikka se ei ole yhtä vahva kuin jotkut muut saatavilla olevat teräkset, vähäisemmät lujuudet mahdollistavat tämän metallin helpon muotoilun ja käsittelyn ilman suuria vaikeuksia.
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.
Tyyppi 304 teräs 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.
Vähemmän tunnettu, mutta silti tärkeä mitta materiaalin kyvystä absorboida energiaa, kun siihen kohdistuu suuri voima, mikä kertoo, miten se murtuu rasituksen alla. On tärkeää tietää, miten materiaali murtuu, koska joissakin sovelluksissa halutaan taivutettavampi murtumistapa kuin hauraampi halkeama. Charpy-iskutesti käyttää suurta heiluripendeliä, joka heiluu uritetun näytteen päälle simuloiden näitä olosuhteita, ja mittari näyttää, kuinka paljon energiaa heiluri siirtää metalliin. Matala Charpy-iskuvoima tarkoittaa, että materiaali on yleensä kovempi, ja sen jäykkä kristallirakenne haluaa murtua helposti suuren energian heilurin vaikutuksesta. 304-teräksellä on korkea Charpy-iskuvoima, mikä tarkoittaa, että se on yleensä taivutettavampi ja taipuu ennen murtumista, absorboiden osan iskusta. Tämä arvo on vielä yksi todiste siitä, että 304-teräs on helposti työstettävissä ja muokattavissa, ja murtuma on epätodennäköisempää rasituksen alla.
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.
Mikä on 304 ruostumattoman teräksen vetolujuus?
304 ruostumattoman teräksen vetolujuus on 515 MPa tai 74700 psi. 304 ruostumattoman teräksen erityinen vetolujuus voi vaihdella tekijöiden, kuten lämpökäsittelyn, valmistusprosessien ja tietyn 304 ruostumattoman teräksen variantin mukaan.
304 Ruostumattoman teräksen sovellukset
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:
Keittiölaitteet (altaat, ruokailuvälineet, suojakiskot)
Erilaiset putket
Elintarvikelaitteet (panimot, pastöroijat, sekoittimet jne.)
Lääketieteellisen prosessoinnin laitteet
Narkoosineulat
Kannut ja padat
Värjäyslaitteet
sekä muita käyttötarkoituksia.
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-koneistus.
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