304 Stainless Steel

304 stainless steel is a grade of stainless steel produced according to the American ASTM standard. Among all stainless steel materials, 304 stainless steel is almost the most common grade. The density of 304 stainless steel is 7.93 g/cm³. The professional name of 304 stainless steel is also called 18/8 stainless steel, because it contains more than 18% chromium and more than 8% nickel. 304 stainless steel is resistant to high temperature (up to 800 ℃), and has the characteristics of good processing performance and high toughness. It is widely used in industry, furniture decoration industry and food and medical industry. In the ASTM A351 standard, CF8 is the equivalent of 304 stainless steel.

 

Chinese Name	304不锈钢/06Cr19Ni10 (GB/T 3280-2015)	Main Alloy Elements	Ni, Mo
English Name	AISI 304 Stainless Steel, AISI 304 S.S.	Magnetic Performance	Weak magnetism, after cold working such as stretching or stamping, the magnetism will become stronger (can be attracted by magnets)
Other Names	SUS 304 S.S. (JIS G4303) or 18-8 S.S.	Characteristics	Corrosion resistance (not completely immune to corrosion), high temperature resistance, good toughness
Metallographic Structure	Austenite

 

It should be noted that food-grade 304 stainless steel has stricter content indicators than ordinary 304 stainless steel. For example: the international definition of 304 stainless steel is basically 18%-20% of chromium and 8%-10% of nickel, but food-grade 304 stainless steel is 18% of chromium and 8% of nickel, which is allowed to fluctuate within a certain range, and Limit the content of various heavy metals. In other words, 304 stainless steel is not necessarily food grade 304 stainless steel

Common labeling methods on the market include 06Cr19Ni10 and SUS 304, of which 06Cr19Ni10 generally means national standard production (GB/T 3280-2015), AISI 304 generally means ASTM standard production, and SUS 304 means Japanese standard production.

304 is a general purpose stainless steel that is widely used to make equipment and parts that require a good combination of properties (corrosion resistance and formability). In order to maintain its inherent corrosion resistance, it must contain more than 18% chromium and more than 8% nickel.

The American Iron and Steel Institute uses three digits to designate various standard grades of malleable stainless steel. in:
1. Austenitic stainless steels are designated by numbers in the 200 and 300 series. For example, some of the more common austenitic stainless steels are designated by 201, 304, 316 and 310.
2. Ferritic and martensitic stainless steels are represented by 400 series numbers.
3. Ferritic stainless steel is represented by 430 and 446. Martensitic stainless steels are represented by 410, 420 and 440C. ,
4. Duplex (austenitic-ferritic) stainless steels, precipitation hardening stainless steels, and high alloys with less than 50% iron are usually designated by proprietary or trademarked names.

For 304 stainless steel, the Ni element in its composition is very important, which directly determines the corrosion resistance of 304 stainless steel and its value. The most important elements in 304 are Ni and Cr, but they are not limited to these two elements. Specific requirements are specified by product standards. The common judgment in the industry is that as long as the Ni content is greater than 8% and the Cr content is greater than 18%, it can be considered as 304 stainless steel. This is why the industry calls 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. Below are some common product standards and tests. To determine whether a material is 304 stainless steel, it must meet the requirements of each element in the product standard. As long as one does not meet the requirements, it cannot be called 304 stainless steel.

 

Chemical Compositions of 304 Stainless Steel According to ASTM A276
304 S.S.	C	Mn	P	S	Si	Cr	Ni
%	≤0.08	≤2.00	≤0.045	≤0.030	≤1.00	18.0–20.0	8.0-11.0
Chemical Compositions of 304 Stainless Steel According to ASTM A240
304 S.S.	C	Mn	P	S	Si	Cr	Ni
%	≤0.07	≤2.00	≤0.045	≤0.030	≤0.75	17.5–19.5	8.0–10.5
Chemical Compositions of 304 Stainless Steel According to JIS G4303
SUS 304 S.S.	C	Mn	P	S	Si	Cr	Ni
%	≤0.08	≤2.00	≤0.045	≤0.030	≤1.00	18.0–20.0	8.0-10.5

 

Applications of 304 Stainless Steel

304 stainless steel is the most widely used chromium-nickel stainless steel. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; good hot workability such as stamping and bending, no heat treatment Hardening phenomenon (use temperature -196℃～800℃). Corrosion resistance in the atmosphere, if it is an industrial atmosphere or heavily polluted area, it needs to be cleaned in time to avoid corrosion. Suitable for food processing, storage and transportation. Has good processability and weldability. Plate heat exchangers, bellows, household products (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs), auto parts (windshield wipers, mufflers, molded products), medical appliances, building materials, chemicals, food industry , agriculture, ship parts, etc. The 304 stainless steel whose content is strictly controlled can also be called food grade 304 stainless steel.

Most of the usage requirements are to maintain the original appearance of the building for a long time. When determining the type of stainless steel to choose, the main considerations are the required aesthetic standards, the corrosiveness of the local atmosphere, and the cleaning system to be used. However, other applications increasingly seek only structural integrity or water impermeability. For example, roofs and side walls of industrial buildings. In these applications, the owner's construction cost may be more important than aesthetics, and the surface is not very clean. 304 stainless steel works reasonably well in dry indoor environments. However, to maintain its appearance outdoors in rural and urban areas, frequent cleaning is required. In heavily polluted industrial and coastal areas, the surface can be very dirty and even rust.

However, for an aesthetic effect in an outdoor environment, nickel-containing stainless steel is required. Therefore, 304 stainless steel is widely used in curtain walls, side walls, roofs and other construction purposes, but in severely corrosive industrial or marine atmospheres, 316 stainless steel is preferred. Stainless steel sliding doors, people have fully realized the advantages of using stainless steel in structural applications. 304 and 316 stainless steels are included in several design guidelines. Because the "duplex" stainless steel 2205 combines good atmospheric corrosion resistance with high tensile and elastic strengths, this steel is also included in European guidelines. Product Shapes Stainless steel is actually manufactured in a full range of standard metal shapes and sizes, and many special shapes are also available. The most commonly used products are made from sheet and strip, but also special products are produced from medium and heavy plates, for example, in the production of hot-rolled structural steel and extruded structural steel. There are also round, oval, square, rectangular and hexagonal welded or seamless pipes and other forms, including profiles, bars, wires and castings.

 

Physical Properties of 304 Stainless Steel		Machanical Properties of 304 Stainless Steel
Density（20℃，g/cm³）	7.93	Tensile Strentgh σb (MPa)	≥515-1035
Melting Temperature（℃）	1398 - 1454	Yield Strentgh σ0.2 (MPa)	≥205
Specific Heat Capacity（0~100℃，KJ·kg-1K-1）	0.5	Elongation δ5 (%)	≥40
Thermal Conductivity（W·m-1·K-1）	(100℃)16.3, (500℃)21.5	rate of reduction in area ψ (%)	40 - 60
Coefficient of Linear Expansion（10-6·K-1）	(0-100℃)17.2, (0-500℃)18.4	Hardness	≤201HBW;≤92HRB;≤210HV
Resistivity（20℃，10-6Ω·m2/m）	0.73
Longitudinal Elastic Modulus（20℃，KN/mm2）	193

 

Reasons for rusting of 304 stainless steel

The rust phenomenon of 304 stainless steel material may have the following reasons:

1. Chloride
Chloride ions exist widely, such as salt/sweat/seawater/sea breeze/soil and so on. In the presence of chloride ions, stainless steel corrodes rapidly, even more than ordinary low carbon steel. Therefore, there are requirements for the use environment of stainless steel, and it needs to be wiped frequently to remove dust and keep it clean and dry. There is an example in the United States: a company uses an oak container to hold a solution containing chloride ions. The container has been used for nearly a hundred years. It was planned to be replaced in the 1990s. Because the oak material is not modern enough, the stainless steel container was used for 16 days after replacement. Leak due to corrosion.

2. Solution treatment
Alloy elements are not dissolved into the matrix, resulting in low alloy content in the matrix structure and poor corrosion resistance.

3. Intergranular corrosion
This titanium- and niobium-free material is prone to intergranular corrosion. The addition of titanium and niobium, together with stabilization treatment, can reduce intergranular corrosion. A high-alloy steel that can resist corrosion in the air or in chemically corrosive media. Stainless steel has a beautiful surface and good corrosion resistance. It does not need to undergo surface treatment such as color plating, but instead exerts the inherent surface properties of stainless steel. It is used in many A type of steel, commonly referred to as stainless steel. The representative performance is high alloy steel such as 13 chrome steel and 18-8 chrome nickel steel. From the metallographic point of view, because the stainless steel contains chromium, a very thin chromium film is formed on the surface, which isolates the oxygen from the steel and plays a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12% chromium. For occasions where welding is required. The lower carbon content minimizes carbide precipitation in the heat-affected zone near the weld, which can lead to intergranular corrosion (weld erosion) of stainless steel in some environments.