304 stainless steel introduction

1. Definition of 304 stainless steel

304 stainless steel (industry name: 18/8 stainless steel) is a common stainless steel material, density of 7.93g /cm3, high temperature resistance of 800℃, in order to maintain the inherent corrosion resistance of stainless steel, steel must contain more than 18% chromium, more than 8% nickel content.

It has the characteristics of good processing performance, high toughness, widely used in industrial and furniture decoration industry and food and medical industry and production requirements of good comprehensive performance (corrosion resistance and molding) equipment and parts.

Suitable for food processing, storage and transportation. It has good machining performance and weldability. Plate heat exchanger, bellows, household products (Class 1, 2 tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs), auto parts (windshield wipers, mufflers, mold products), medical appliances, building materials, chemistry, food industry, agriculture, Marine parts, etc. 304 stainless steel is food grade stainless steel recognized by the country.

304 stainless steel, with its good heat resistance, is widely used in the production of corrosion resistance and molding equipment and parts. 304 stainless steel has been widely used in food, chemical, atomic energy and other industrial equipment and decoration field and household products, cabinets, indoor pipelines, water heaters, boilers, bathtubs, auto parts, medical appliances, building materials, chemistry, food industry, agriculture, ship parts.

SUS304 is 18/8 stainless steel. GB brand is 0Cr18Ni9(new standard 06CR19NI10).

SUS304 is Japanese JIS standard material, is Japan SUS series austenitic stainless steel. It's equivalent to China's 0Cr18Ni9 and the US's 304.

2. Types of 304 stainless steel

304 stainless steel has two sub-categories - class 304L and class 304H. Grade 304L has a low carbon content and does not require annealing after welding, so it can be used for large parts. The 304H grade has higher carbon content and can be used in applications requiring high temperature.

3. Morphological classification of 304 stainless steel

Stainless steel can be divided into the following five categories according to the shape.

The most important of these are 304 stainless steel coils and plates. Profiles are materials of various shapes made of plates, such as angle steel, flat steel, I-beam, and channel steel.304 stainless steel pipes mainly refer to seamless steel pipe, and the opposite of seamless steel pipe is welded pipe, the difference is whether it is formed at one time.304 stainless steel parts mainly refer to elbows andfittingand other small things.

The coil and the sheet are actually the same, but the form is different from the factory. If it is, it cannot be rolled, generally more than 16 mm can not be rolled.

There are coils, boards, and most of the time they are boards. The price of coils and boards is also different. If it is purchased coils, it is calculated according to actual weight, so the price is high. The board is calculated according to theory, and the price is low, the reason is that there is a difference. For example, the thickness of 10mm may actually be 9.6mm, and there will be a price difference.

4. Features introduction of 304 stainless steel

It has good corrosion resistance, heat resistance, low temperature strength and mechanical properties, good thermal processing property such as punching and bending, no hardening phenomenon of heat treatment, no magnetism.

5. Heat treatment of stainless steel

Solid solution treatment

The purpose is to fully dissolve the carbide and retain it in the austenite at room temperature, so that the single-phase austenite structure can be obtained at room temperature, so that the steel has the highest corrosion resistance.

The heating temperature of solution treatment is generally higher, between 1050-1100℃, and adjusted appropriately according to the carbon content. Due to the poor thermal conductivity of 18-8 stainless steel, not only through preheating and quenching heating, but also in the solution treatment (quenching heating) when the holding time should be long. Solution treatment, special attention should be paid to prevent carburization. Because carburization will increase the intergranular corrosion tendency of 18-8 steel. Cooling medium, generally using water. The microstructure after solution treatment is generally single-phase austenite, but for stainless steel containing titanium, niobium, molybdenum, especially when it is a casting, it also contains a small amount of ferrite. The hardness after solution treatment is generally about 135HBS.

Stress removal annealing

In order to eliminate the residual stress after cold working, the treatment was carried out at a lower temperature. Generally heated to 250-425℃, often used is 300-350℃. For steel without titanium or niobium, the temperature should not exceed 450℃, so as not to precipitate chromium carbide and cause intergranular corrosion.

In order to eliminate the residual stress after welding and eliminate the sensitivity of the steel to stress corrosion, the treatment is generally carried out at a higher temperature. The heating temperature should not be lower than 850℃. Cooling mode, for steel containing titanium or niobium can be directly cooled in the air; Steel without titanium or niobium should be cooled by water to 500℃ and then cooled in air.

Treatment of stabilization

In order to prevent titanium and niobium austenitic stainless steel in welding or solution treatment, due to the reduction of TiC and NbC caused by intergranular corrosion resistance, the stainless steel needs to be heated to a certain temperature (the temperature makes the chromium carbide completely dissolved in austenite, while TiC and NbC only partially dissolved) and then slowly cooling. During the cooling process, the carbon in the steel is fully combined with titanium and niobium, and stable TiC and NbC are precipitated, while chromium carbide is not precipitated, so as to eliminate the intercrystalline corrosion tendency of 18-8 austenitic stainless steel. This process is called stabilization.

18-8 stainless steel stabilization annealing, generally heated to 850-880℃, holding 2-6h, followed by air cooling or furnace cooling.

6. The advantages of 304 stainless steel

(1). Environmental health

304 stainless steel put an end to hidden water problems, and the surface is smooth, no smell, will not be contaminated by bacteria, so green environmental protection, harmless to human body.

(2). Wear resistance

304 stainless steel surface smooth, not easy to produce scratches, and not never rust and fracture, at the same time, the surface has rich chromium oxide film, even if put in water for a long time also corrosion resistance.

(3). Low temperature resistance

304 stainless steel made of water pipes or taps, not easy to break, can withstand a certain low temperature, and the wall is not easy to scale, so the water is clean. At the same time, stainless steel has the net benefit of thermal expansion and contraction, so the insulation is also good.

(4). Authoritative certification

Finally 304 stainless steel after relevant tests that the standard value is lower than the European drinking water usage, and can reduce pipeline pressure, so as to reduce transmission costs, whether used for hot water or cold water, can be recycled.

7. The disadvantage of 304 stainless steel

Although 304 stainless steel is widely used, its main disadvantage is that it cannot be used in saline environments near coastlines (316 stainless steel is more suitable for such conditions). Chlorine in these environments produces "pitting," a small area of localized corrosion that spreads beyond protective chromium. In this environment, level 316 is used. It contains 16% chromium and 10% nickel, and about 2-3% molybdenum, providing corrosion resistance, especially against chloride.

8. Specific application of 304 stainless steel

Chemical industry - Large storage tanks and containers for liquids and solids

Food Industry - Food processing equipment, such as milking machines, storage tanks, transport tanks, pipes, valves, milk trucks, etc. It is also used in breweries, brewing and the juice industry.

Household use - 304 stainless steel resists the corrosive effects of acids found in fruits, milk, meat and vegetables and is quite resistant to abrasion. Therefore, it is widely used in sinks, table tops, stoves, refrigerators, cutlery, utensils and various other utensils.

Building uses - paneling, trim strips, balustrades, trim.

Industrial applications - Tools, bolts, nuts, screws, etc

Automotive and aerospace industries - Fuel tanks, various structural components.

Hospital - Surgical equipment.

Business -- Sanitary fittings, plumbi

9. How to distinguish 304 stainless steel

(1)Through the stainless steel detection agent to test 304 stainless steel true and false, different stainless steel products meet stainless steel detection agent will show different colors, so through different colors to distinguish 304 stainless steel is true, this is also more accurate and standard practice.

(2)The use of magnetic to determine whether 304 stainless steel is true, 304 stainless steel is weak magnetic, so using a magnet to attract magnetic 304 stainless steel will show magnetic is not strong.

(3)With copper sulfate to detect, with copper sulfate wipe to remove the oxide layer of steel, such as not discoloration after wiping, generally can judge stainless steel, but if it becomes purple, if there is no magnetic high manganese steel, magnetic is generally ordinary steel or low alloy steel.

10. Difference between 304 and 316

304 stainless steel is the most widely used stainless steel, used in food equipment, general chemical equipment, atomic energy industry, etc.

316 stainless steel in seawater and other media, corrosion resistance is better than 06Cr19Ni0. Mainly used for pitting resistant materials.The biggest difference between 304 stainless steel and 316 stainless steel is the market application, 304 stainless steel price is slightly lower than 316 stainless steel, widely used, such as kitchen utensils, building decoration, food industry, agriculture, Marine parts, bathroom, auto parts and so on.Although a small part of 316 stainless steel is used in daily life, most of it is still used in seawater equipment and corrosive production equipment, because the 2-3% molybdenum content is added, consolidating the structure of 316 stainless steel, making it more resistant to wear and oxidation, and at the same time the corrosion resistance is also greatly increased.