Functions and applications of stainless steel


Stainless steel corrosion resistant element

All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, the rust continues to expand and eventually form pores. The surface of carbon steel can be protected by an electroplating coating or an oxidation resistant metal such as zinc, nickel and chromium. However, as is known, this protection is just a film. When the protective layer is damaged, the metal begins to rust.

Chromium is an essential element for obtaining the corrosion resistance of stainless steel. When the chromium content in the steel reaches about 12%, chromium acts on oxygen in the corrosive medium to form a very thin oxide film (self-passivation film) on the steel surface. This prevents further corrosion of the steel substrate. In addition to chromium, commonly used alloying elements are nickel, molybdenum, titanium, tantalum, copper, nitrogen and the like to meet the various requirements of stainless steel’s corrosion protection structure and performance requirements.

Heat-resistant

Heat resistance means that stainless steel retains excellent physical and mechanical properties at high temperatures. Carbon effect: Carbon strongly forms and stabilizes austenite in austenitic stainless steels, expanding the elements in the austenitic region. Carbon’s ability to form austenite is about 30 times that of nickel. Carbon is a type of interstitial element that can significantly increase the strength of austenitic stainless steel by solid solution strengthening. Carbon also improves the stress and corrosion resistance of austenitic stainless steels in high concentrations of chloride (eg, 42% MgCl 2 boiling solution).

However, in austenitic stainless steel, carbon is generally considered to be a harmful element. This is mainly to form a high chromium Cr 23 C 6 type carbon compound in which carbon has chromium in the steel under certain conditions of corrosion resistance of stainless steel (eg welding or heating at 450-850 ° C). This is because it can be done. As a result, a part of chromium is depleted, and the corrosion resistance of the steel, particularly the intergranular corrosion resistance, is lowered. Therefore, newly developed chromium-nickel austenitic stainless steels since the 1960s have a carbon content of 0.03% or 0.02% or less of the ultra-low carbon type. It can be seen that as the carbon content decreases, the intergranular corrosion sensitivity of steel decreases. The most obvious effect is obtained when the carbon content is less than 0.02%. Some experiments have also pointed out that carbon increases the pitting corrosion tendency of austenitic stainless steel chromium. Due to the harmful effects of carbon, not only is the carbon content controlled as low as possible in the austenitic stainless steel refining process, but also carbonization of the stainless steel surface is prevented during subsequent heating, low temperature treatment and heat treatment. , Precipitation of chromium carbide is avoided.

Stainless steel parts

Chemical composition

The corrosion resistance of stainless steel decreases as the carbon content increases. Therefore, most stainless steels have a low carbon content, and some steels have a lower wC instead of 0.03% (eg 00Cr12). The main alloying element of stainless steel is Cr. Steel is corrosion resistant only when the Cr content reaches a certain value. Therefore, stainless steel typically exceeds 13% wCr. Stainless steel also contains elements such as Ni, Ti, Mn, N and Nb. Stainless steels are usually classified according to the state of their structure, such as martensitic steels, ferritic steels, and austenitic steels. Further, it can be divided into chrome stainless steel, chrome nickel stainless steel and chrome manganese nitrogen stainless steel.

1. Ferritic stainless steel:

Contains 12% to 30% chromium. Corrosion resistance, toughness and weldability increase with increasing chromium content, and resistance to chloride stress corrosion is superior to other types of stainless steel. Cr17, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. belong to this category. Ferritic stainless steels have high corrosion resistance and oxidation resistance due to their high chromium content, but their mechanical and processing properties are inferior. It is mainly used for acid resistant structures and oxidation resistant steels and has lower stress resistance. This steel is resistant to atmospheric and saltwater corrosion, has high temperature oxidation resistance, and has a low coefficient of thermal expansion. It is also used in the manufacture of food factory equipment and high temperature parts. As a gas turbine part.

2. Austenitic stainless steel:

It has a chromium content of over 18% and contains about 8% of small elements such as nickel and molybdenum, titanium, and nitrogen.
It has good overall performance and is resistant to various media corrosions. Commonly used austenitic stainless steel grades include 1Cr18Ni9 and 0Cr19Ni9. The wC of 0Cr19Ni9 steel is less than 0.08% and the steel number is marked “0”. This steel contains large amounts of Ni and Cr that austenitize the steel at room temperature. This steel has good plasticity, toughness, weldability and corrosion resistance, and has good corrosion resistance in both oxidation and reduction media. It is used in the manufacture of acid resistant equipment such as corrosion resistant containers and equipment parts and transport pipes. Austenitic stainless steel is usually solution-treated, heated to 1050 to 1150 ° C., and then water-cooled to obtain a single-phase austenitic structure.

3. Duplex Stainless Steel: Ferritic Two-Phase Stainless Steel:

It combines the advantages of austenitic and ferritic stainless steels with superplasticity. Austenite and ferrite are about half that of stainless steel. When the C content is low, the Cr content is 18-28% and the Ni content is 3-10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti and N. These steels combine the properties of austenite and ferritic stainless steels. Compared to ferrite, it has higher plasticity and toughness, is not brittle at room temperature, and has significantly improved resistance to intergranular corrosion and weldability. At the same time, it maintains the brittleness and high thermal conductivity of ferritic stainless steel at 475 ° C and has superplasticity. They have higher strength, intergranular corrosion resistance and chloride stress corrosion resistance than austenitic stainless steel. Duplex stainless steel has excellent pitting corrosion resistance and is also nickel stainless steel.

4, Martensitic stainless steel:

High strength, but poor plasticity and weldability. Martensitic stainless steels commonly used due to their high carbon content are 1Cr13, 3Cr13 and the like. As a result, strength, hardness, and wear resistance are improved, but corrosion resistance is reduced. Applying high mechanical performance requirements, general corrosion resistance requires several parts such as springs, turbine blades, and hydraulic valves. This steel is used after quenching and tempering.

5, Precipitation hardening stainless steel:

It has good moldability and good solderability. Suitable for ultra-high strength materials in the nuclear, aerospace, and aerospace industries, and classified into Cr-based (SUS400), Cr-Ni-based (SUS300), Cr-Mn-Ni (SUS200), and precipitation hardening-based (SUS600). increase.

  • 200 Series-Chromium-Nickel-Manganese Austenitic Stainless Steel
  • 300 Series-Chromium Nickel Austenitic Stainless Steel
  • 301 –The molded product has good ductility. It can also be machine cured. Good weldability. Abrasion resistance and fatigue strength are superior to 304 stainless steel.
  • 302 –Corrosion resistance is the same as 304. Due to its relatively high carbon content, the strength is better.
  • 303-Easy to trim by adding a small amount of sulfur and phosphorus.
  • 304-ie 18/8 stainless steel. The GB rating is 0Cr18Ni9.
  • 316-after 304, the second most widely used steel grade. It is mainly used in the food industry and surgical equipment, and molybdenum element is added to obtain a specific structure that is resistant to corrosion. SS316 is commonly used in nuclear fuel recovery equipment because it is more resistant to chloride corrosion than 304.
  • 321-Other properties are 304, except that the addition of titanium reduces the risk of weld corrosion of the material.
    Series 400-Ferrite and martensitic stainless steels
  • 408 –Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
  • 409 –The cheapest models (UK and USA) commonly used for automobile exhaust pipes are ferritic stainless steel (chrome steel).
  • 410 Martensite (high-strength chrome steel), good wear resistance, poor corrosion resistance.
  • 416 –The addition of sulfur improves the workability of the material.
  • 420- “Leave-grade” martensitic steel, like Brinell high chrome steel, is the first stainless steel. It is also used in surgical instruments and can be very bright.
  • 430 Ferritic stainless steel, eg ornaments for automotive accessories. Good moldability can be obtained, but heat resistance and corrosion resistance are inferior.
  • 440 –High-strength cutting tool steel with a slightly high carbon content. After proper heat treatment, a higher yield strength can be obtained with a hardness of 58HRC, which is one of the hardest stainless steels. An example of the most common application is the “razor blade”. There are three commonly used models: 440A, 440B, 440C and 440F (easy to handle).
  • 500 Series-Heat-resistant chrome alloy steel.
  • 600 Series-Martensite Precipitation Hardened Stainless Steel.
  • 630-The most commonly used model of precipitation hardened stainless steel, also commonly referred to as 17-4, with 17% Cr and 4% Ni.

Stainless steel is usually divided into the following matrix structures:

1. Ferritic stainless steel.

Chromium 12% to 30%. Corrosion resistance, toughness and weldability increase with increasing chromium content, and resistance to chloride stress corrosion is superior to other types of stainless steel.

2, austenitic stainless steel.

The chromium content is over 18% and contains about 8% nickel and small amounts of molybdenum, titanium and nitrogen. It has good overall performance and is resistant to various media corrosions.

3 Duplex stainless steel with austenite ferrite.

It combines the advantages of austenitic and ferritic stainless steels with superplasticity.

  • Stainless steel surface treatment grade
  • Stainless steel surface treatment
  • Original: NO.1

After hot rolling, the surface is annealed and pickled. Commonly used for cold rolled materials, industrial tanks, chemical equipment, thickness 2.0MM-8.0MM.

Smooth surface: NO.2D

Heat treatment and pickling after cold rolling. The material is soft and the surface is silvery white, and it is used for deep drawing of automobile parts and water pipes.

Matte surface: NO.2B

After cold rolling, heat treatment, pickling and finish rolling are used to give the surface moderate brightness. Due to its smooth surface, it is easy to re-grind to make the surface brighter, and it is widely used in tableware, building materials, etc. After surface treatment, mechanical properties are improved and almost all applications are satisfied.

Coarse sand NO.3

The product was ground on a 100-120 polishing belt. The discontinuous rough pattern improves the gloss. It is used for interior materials, exterior materials, electrical products, kitchen equipment, etc.

# 320

Sophisticated product, No. 320 grinding belt. It has a better luster, a discontinuous rough pattern, and stripes that are thinner than NO.4. Used in bathrooms, interior and exterior materials, electrical appliances, kitchen equipment, food equipment.

Crushing:

HLNO.4 is manufactured by continuously grinding a polishing belt of appropriate size (subdivision number 150-320). It is mainly used for architectural decoration, elevators, doors, architectural panels, etc.

Shining: BA

After cold rolling, it is brightly annealed to obtain a flat product. Excellent surface gloss and high reflectance. Like a mirror. Suitable for home appliances, mirrors, kitchen equipment, decorative materials, etc.

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