What Is 304 Stainless Steel?

 

What is 304 stainless steel?

304 stainless steel is a common material in stainless steel, with a density of 7.93 g/cm³; the industry is also called 18/8 stainless steel, meaning that contains more than 18% chromium and more than 8% nickel; high-temperature resistance of 800 ℃, with good processing properties, high toughness characteristics, widely used in industry and furniture decoration industry and food medical industry. But it should be noted that food-grade 304 stainless steel compared with ordinary 304 stainless steel, its content indicators are more stringent.

 

For example, the international definition of 304 stainless steel is basically mainly containing chromium 18%-20% nickel 8%-10%, but food grade 304 stainless steel is containing chromium 18% and nickel 8%, allowing a certain range of fluctuations, and limiting the content of various heavy metals. In other words, it is 304 stainless steel is not necessarily food-grade 304 stainless steel

 

The common labeling methods in the market are 06Cr19Ni10, and SUS304, where 06Cr19Ni10 generally indicates the production of Chinese standards, 304 generally indicates the production of ASTM standards, SUS 304 indicates the production of Japanese standards.

 

304 is general-purpose stainless steel, which is widely used to make equipment and machine parts that require good overall performance (corrosion resistance and formability). In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 18% chromium and 8% nickel. 304 stainless steel is a grade of stainless steel produced in accordance with ASTM standards.

 

Physical Properties

Tensile strength σb (MPa)≥515-1035

Conditional yield strength σ0.2 (MPa)≥205

Elongation δ5 (%)≥40

Fractional shrinkage ψ (%)≥?

Hardness: ≤201HBW;≤92HRB;≤210HV

Density (20℃, g/cm³): 7.93

Melting point (℃): 1398~1454

Specific heat capacity (0~100℃, KJ-kg-1K-1): 0.50

Thermal conductivity (W-m-1-K-1): (100℃) 16.3, (500℃) 21.5

Coefficient of linear expansion (10-6-K-1): (0~100℃) 17.2, (0~500℃) 18.4

Resistivity（20℃，10-6Ω-m2/m）：0.73

Longitudinal modulus of elasticity (20℃, KN/mm2): 193

 

Product standards

For 304 stainless steel, the element Ni in its composition is very important and directly determines the corrosion resistance of 304 stainless steel and its value.

 

The most important elements in 304 are Ni and Cr but are not limited to these two elements. Specific requirements are specified by product standards. The industry commonly determines that as long as the Ni content is greater than 8% and Cr content is greater than 18%, it can be considered 304 stainless steel. This is why the industry will call this type of stainless steel 18/8 stainless steel. In fact, the relevant product standards have very clear regulations for 304, and these product standards have some differences for different shapes of stainless steel. Here are some common product standards and tests.

 

To determine whether a material is 304 stainless steel, must meet the requirements of each element in the product standards, as long as one does not meet, it can not be called 304 stainless steel.

 

1、ASTM A276 (Standard Specification for Stainless Steel Bars and Shapes profiles)

304 Requirements, %

C≤ 0.08 Mn≤ 2.00 P≤ 0.045 S≤ 0.030 Si≤ 1.00 Cr 18.0-20.0 Ni 8.0-11.0

 

2、ASTM A240 (Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure vessels and for General Applications)

304 Requirements, %

C≤0.07 Mn≤2.00 P≤0.045 S≤0.030 Si≤0.75 Cr 17.5-19.5 Ni 8.0-10.5 N≤0.10

 

3、JIS G4305（cold-rolled cold-rolled stainless steel plate, sheet, and strip)

304 Requirements, %

SUS 304 C≤0.08 Mn≤2.00 P≤0.045 S≤0.030 Si≤1.00 Cr 18.0–20.0 Ni 8.0-10.5

 

4、JIS G4303 （Stainless steel bars stainless steel bar)

SUS 304 Requirements, %

C≤0.08 Mn≤2.00 P≤0.045 S≤0.030 Si≤1.00 Cr 18.0-20.0 Ni 8.0-10.5

 

The above four standards are just a few of the common standards, in fact, ASTM and JIS have mentioned 304 more than a few standards. Each standard actually has different requirements for 304, so to determine whether a material is 304, the exact expression should say whether it meets the requirements of 304 in a product standard. Material certificates are generally issued with the following types of reports.

 

Detailed standard specification comparison: As of 2015-12-08

 

 

Application range

System description: 304 stainless steel is the most widely used chromium-nickel stainless steel, as a widely used steel, has good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties; stamping, bending and hot workability, no heat treatment hardening phenomenon (use temperature -196 ℃ ~ 800 ℃). Corrosion resistance in the atmosphere, if it is an industrial atmosphere or heavy pollution areas, it needs to be cleaned in time to avoid corrosion. Suitable for food processing, storage, and transportation.

 

Good processability and weldability. Plate heat exchangers, bellows, household goods (1 and 2 types of tableware, cabinets, indoor piping, water heaters, boilers, bathtubs), automotive parts (windshield wipers, mufflers, molded products), medical appliances, building materials, chemicals, food industry, agriculture, ship components, etc. Which content strictly controlled 304 stainless steel, can also be called food-grade 304 stainless steel

 

Most of the user requirements are to maintain the original appearance of the building for a long time. In determining the type of stainless steel to be selected, the main considerations are the aesthetic standards required, the corrosiveness of the atmosphere where it is located, and the cleaning regime to be used. However, other applications are increasingly just seeking structural integrity or impermeability.

 

Examples include roofs and sidewalls of industrial buildings. In these applications, the cost of construction to the owner may be more important than aesthetics, and surfaces that are not very clean are fine. The use of 304 stainless steel works quite well in dry indoor environments. However, to maintain its appearance outdoors in rural and urban areas, it needs to be cleaned frequently. In heavily polluted industrial and coastal areas, the surface can be very dirty and even rusty.

 

But to get the aesthetic effect in the outdoor environment, it needs to use nickel-containing stainless steel. Therefore, 304 stainless steel is widely used for curtain walls, sidewalls, roofs, and other architectural purposes, but in the aggressive industrial or marine atmosphere, 316 stainless steel is preferable. Stainless steel sliding doors, the superiority of using stainless steel in structural applications has been well recognized.

 

There are several design guidelines that include 304 and 316 stainless steel. Because the "duplex" stainless steel 2205 has good atmospheric corrosion resistance and high tensile strength and elastic limit strength in one, so, the European guidelines also include this steel. Product shape, In fact, stainless steel is manufactured in full standard metal shapes and sizes, and there are many special shapes. The most commonly used products are made of thin plates and strips, but also with medium and thick plates to produce special products, for example, the production of hot-rolled structural sections and extruded structural sections. And there are also round, oval, square, rectangular, and hexagonal welded or seamless tubes and other forms of products, including profiles, bars, wire rods, and castings.

 

Product Standards

International Stainless Steel Labeling Method

The American Iron and Steel Institute is using three digits to label various standard grades of malleable stainless steel. Among them.

① Austenitic stainless steels are marked with numbers in the 200 and 300 series, for example, some of the more common austenitic stainless steels are marked with 201, 304, 316, and 310

② Ferritic and martensitic stainless steel are marked with 400 series numbers.

③ Ferritic stainless steel is marked by 430 and 446, martensitic stainless steel is marked by 410, 420 as well as 440C.

④ Duplex (austenitic-ferritic), stainless steels, precipitation-hardening stainless steels, and high-alloys containing less than 50% iron are usually named using a patented name or trademark.

 

Rust causes

Rusting of 304 stainless steel materials can occur for several reasons.

 

Chlorine ions

Chlorine ions are widely present, such as salt/sweat/salt/seawater/sea air/soil, etc. Stainless steel corrodes quickly in the environment under the presence of chloride ions, even more than ordinary mild steel. So there are requirements for the use of stainless steel environment, and it needs to be wiped frequently to remove dust and keep it clean and dry. The United States has an example: an enterprise with an oak container containing a solution containing chloride ions, the container has been used for nearly a hundred years, the last century in the 1990s plans to replace, it because the oak material is not modern enough, the use of stainless steel replacement after 16 days of container leakage due to corrosion.

 

Solid solution treatment

Alloying elements are not dissolved into the matrix, resulting in the matrix organization alloy content being low and poor corrosion resistance.

 

Intergranular corrosion

This material does not contain titanium and niobium has a tendency to intergranular corrosion. The addition of titanium and niobium, together with stabilization treatment, can reduce intergranular corrosion. In the air or chemically corrosive media to resist corrosion of high-alloy steel, stainless steel is a beautiful surface and has good corrosion resistance, without the need for surface treatment, such as color plating, and play the inherent surface properties of stainless steel, used in a variety of steel, commonly known as stainless steel.

 

The representative performance of 13 chromium steel, 18-8 chromium-nickel steel, and other high-alloy steel. From the metallographic point of view, because stainless steel contains chromium and the surface the formation of a very thin chromium film, this film separates the invasion of oxygen with the steel to play the role of corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium. For applications requiring welding. The lower carbon content minimizes the precipitation of carbide in the heat-affected zone close to the weld, which can lead to intergranular corrosion (weld erosion) of stainless steel in some environments.

 

Surface trace phenomenon

Surface trace phenomenon refers to 304 stainless steel deep processing or processing products after grinding, in the product surface occurs inline or dotted dent marks defects.

Cause: surface, mold surface foreign matter, crimp pad surface scars.

Countermeasures: remove foreign objects, and scars before processing, but also by fully grinding to remove the orange peel.

Orange peel phenomenon refers to 304 stainless steel formed products in the grinding or other circumstances, the surface produces a phenomenon like orange peel shape.

Causes: Insufficient grinding, the greater the processing of orange peel more serious, coarse grains of raw materials, heat treatment softening.

Countermeasures: deep processing products fully grind, choose suitable heat treatment conditions, and control the grain size of raw materials.

 

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