304 Stainless Steel

304 Stainless Steel

304 stainless steel is a common material in stainless steel with a density of 7.93 g/cm³; it is also called 18/8 stainless steel in the industry, which means it contains more than 18% chromium and more than 8% nickel; it is resistant to high temperatures of 800°C and has good processing performance , with the characteristics of high toughness, it is widely used in industrial and furniture decoration industries and food and medical industries. However, it should be noted that compared with ordinary 304 stainless steel, food-grade 304 stainless steel has stricter content indicators. For example: The international definition of 304 stainless steel is that it mainly contains 18%-20% chromium and 8%-10% nickel, but food-grade 304 stainless steel contains 18% chromium and 8% 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 marking methods on the market include 06Cr19Ni10 and SUS304. Among them, 06Cr19Ni10 generally indicates production according to national standards, 304 generally indicates production according to ASTM standards, and SUS 304 indicates Japanese standards. Production.

304 is a versatile stainless steel that is widely used to make equipment and parts that require good comprehensive properties (corrosion resistance and formability). In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 18% chromium and more than 8% nickel. 304 stainless steel is a brand of stainless steel produced in accordance with American ASTM standards.

Physical properties of 304 stainless steel

Tensile strength σb (MPa)≥515-1035
Conditional yield strength σ0.2 (MPa)≥205
Elongation δ5 (%)≥40
Reduction of area 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
Linear expansion coefficient (10-6·K-1): (0~100℃) 17.2, (0~500℃) 18.4
Resistivity (20℃, 10-6Ω·m2/m): 0.73
Longitudinal elastic modulus (20℃, KN/mm2): 193

Product composition of 304 stainless steel

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

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. A 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 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 on 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 every element in the product standard. As long as one of them does not meet the requirements, it cannot be called 304 stainless steel.

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

304	C	Mn	P	S	Si	Cr	Ni
Require,%	≤0.08	≤2.00	≤0.045	≤0.030	≤1.00	18.0–20.0	8.0-11.0

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

304	C	Mn	P	S	Si	Cr	Ni	N
Require,%	≤0.07	≤2.00	≤0.045	≤0.030	≤0.75	17.5–19.5	8.0–10.5	≤0.10

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

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

4、JIS G4303 （Stainless steel bars）

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

The above four standards are just a few common standards. In fact, ASTM and JIS mention more than these 304 standards. Each standard actually has different requirements for 304, so if you want to determine whether a material is 304, the accurate way of expression should be to say whether it meets the 304 requirements of a certain product standard. Material certificates generally require the following types of reports:
(Detailed comparison of standard specifications: as of 2015-12-08)

NO.	Standard/S30400	C	Mn	P	S	Si	Cr	Ni	N	Cu
1	ASTM A276/A276M-15	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0
2	ASTM A959-11	0.07	2.00	0.045	0.030	1.00	17.5-19.5	8.0-11.0
3	ASTM A240/A240M-15a	0.07	2.00	0.045	0.030	0.75	17.5-19.5	8.0-10.5
4	ASTM A182/A182M-15	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0	0.10
5	ASTM A193/A193M-15	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0
6	ASTM A269/A269M-15	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0
7	ASTM A312/A312M-15a	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0
8	ASTM A320/A320M-15a	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0
9	ASTM A403/A403M-15	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0
10	ASTM A493-09(2013)	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-10.5	0.10	1.00
11	ASTM A554-15a (MT-304)	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0
12	JIS G4303:2012	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-10.5
13	JIS G4304:2012	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-10.5
14	JIS G4305:2012	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-10.5
15	GB/T 20878-2007	0.08	2.00	0.045	0.030	1.00	18.0-20.0	8.0-11.0

Application range of 304 stainless steel

System description: 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; it has good hot workability such as stamping and bending. , no heat treatment hardening phenomenon (use temperature -196℃～800℃). It is resistant to corrosion in the atmosphere. If it is an industrial atmosphere or a heavily polluted area, it needs to be cleaned in time to avoid corrosion. Suitable for food processing, storage and transportation. Has good processing performance and weldability. Plate heat exchangers, corrugated tubes, household products (Class 1 and 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs), auto parts (windshield wipers, mufflers, molded products), medical equipment, building materials, chemistry, 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 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 corrosive nature of the local atmosphere, and the cleaning regime to be adopted. Increasingly, however, other applications are simply seeking structural integrity or impermeability. For example, roofs and side walls of industrial buildings. In these applications, the cost of construction to the owner may be more important than aesthetics, and the surface may not be very clean. The effect of using 304 stainless steel in dry indoor environments is quite good. However, maintaining its appearance outdoors in the countryside and cities requires frequent cleaning. In heavily polluted industrial areas and coastal areas, the surface will be very dirty and even rusty.

But to achieve aesthetic results in outdoor environments, nickel-containing stainless steel is required. Therefore, 304 stainless steel is widely used in curtain walls, side walls, roofs and other construction purposes. However, in severely corrosive industries or marine atmospheres, it is best to use 316 stainless steel. Stainless steel sliding doors, people have fully realized the advantages of using stainless steel in structural applications. There are several design guidelines that include 304 and 316 stainless steel. Because "duplex" stainless steel 2205 has integrated good atmospheric corrosion resistance with high tensile strength and elastic limit strength, this steel is also included in the European standards. 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 medium and thick plates are also used to produce special products, such as the production of hot-rolled structural steel and extruded structural steel. There are also round, oval, square, rectangular and hexagonal welded pipes or seamless steel pipes and other forms of products, including profiles, bars, wires and castings.

304 stainless steel product standards

International stainless steel labeling method
The American Iron and Steel Institute uses three digits to designate various standard grades of wrought stainless steel. in:
①Austenitic stainless steel is marked with 200 and 300 series numbers. For example, some common austenitic stainless steels are marked with 201, 304, 316 and 310.
② Ferritic and martensitic stainless steel are represented by 400 series numbers.
③ Ferritic stainless steel is marked with 430 and 446, martensitic stainless steel is marked with 410, 420 and 440C.
④ Duplex (austenite-ferrite), stainless steel, precipitation hardened stainless steel and high alloys with an iron content of less than 50% are usually named after patented names or trademarks.
4． Standard classification and grading
4-1 Grading and classification: ①National standard GB ②Industry standard YB ③Local standard ④Enterprise standard Q/CB
4-2 Classification: ① Product standards ② Packaging standards ③ Method standards ④ Basic standards
4-3 Standard level (divided into three levels): Level Y: International advanced level Level I: General international level Level H: Domestic advanced level
4-4 national standard
GB1220-2007 stainless steel bar (Grade I) GB4241-84 stainless steel welded disk (Grade H)
GB4356-2002 Stainless steel welded disc garden (Grade I) GB1270-80 Stainless steel pipe (Grade I)
GB12771-2000 Stainless steel welded pipe (Grade Y) GB3280-2007 Stainless steel cold plate (Grade I)
GB4237-2007 Stainless steel hot plate (Grade I) GB4239-91 Stainless steel cold strip (Grade I)

304 stainless steel processing method

1. Etching method
The etching processing method uses a screen printing acid-resistant protective film on the stainless steel surface, and then etches it with ferrous chloride liquid to form an artistic pattern.

2. Spray color method
The spray color method is to spray color particles after screen printing to form a pear skin-like surface and form an artistic pattern.

3. Process
The process of spray color art processing of stainless steel products is: stainless steel products → silk screen → etching → alkali treatment → silk screen → oxidation coloring → alkali treatment → finished product.
The process of etching art processing of stainless steel products is: stainless steel products → silk screen → etching → alkali treatment → oxidation coloring → finished product.
The chemical coloring method of stainless steel does not use pigments and dyes. Instead, the stainless steel is soaked in a heated concentrated chromium sulfate solution for chemical coloring. It is characterized by good food resistance. The ink used in this processing method must have very strong acid resistance. Generally, UV vulcanized ink with special properties that is compatible with the processing technology is used.

4. Stamping processing
For stainless steel products, especially stainless steel strips, many times the original processing method in factories is to use a punch machine to stamp products. Stamping includes two methods: straight punching and stretching. Generally, if the hardness is lower than 1/2, stretching and bending are used. If the hardness is higher than 1/2, it is straight punching.

Reasons for rust of 304 stainless steel

304 stainless steel materials may rust due to the following reasons:

1.Chloride ion
Chloride ions exist widely, such as table salt/sweat stains/sea water/sea breeze/soil, etc. Stainless steel corrodes very quickly in the presence of chloride ions, even more than ordinary low carbon steel. Therefore, there are requirements for the environment in which stainless steel is used, 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 used an oak container to hold a solution containing chloride ions. The container had been in use for more than a hundred years. It planned to replace it in the 1990s. Because the oak material was not modern enough, the container was replaced with a stainless steel container 16 days later. Leakage due to corrosion.

2. Solid solution treatment
The alloy elements are not dissolved into the matrix, resulting in a 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. Adding titanium and niobium, coupled with stabilization treatment, can reduce intergranular corrosion. A kind of high-alloy steel that can resist corrosion in the air or 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 can utilize the inherent surface properties of stainless steel and is used in many applications. A type of steel, often called stainless steel. Representative performance include 13 chromium steel, 18-8 chromium-nickel steel and other high alloy steels. From a metallographic point of view, because stainless steel contains chromium, a very thin chromium film is formed on the surface. This film isolates the oxygen that intrudes into 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. Used where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone close to the weld, and the precipitation of carbides may cause intergranular corrosion (welding erosion) of stainless steel in certain environments.

4. Surface marks
Surface mark phenomenon refers to the occurrence of line or point-like concavities on the surface of the product after deep processing of 304 stainless steel or grinding of processed products.

5. Point mark defects
Reason: There are foreign objects on the surface and mold surface, and there are scars on the surface of the edge pad.
Countermeasures: Remove foreign matter and scars before processing, and also remove orange peel by sufficient grinding.
The orange peel phenomenon refers to the phenomenon that the surface of 304 stainless steel molded products develops a shape like orange peel during grinding or other circumstances.
Reasons: Insufficient grinding, the larger the processing, the more serious the orange peel, the raw material grains are coarse, and the heat treatment softens it.
Countermeasures: Fully grind deeply processed products, select suitable heat treatment conditions, and control the grain size of raw materials.