Stainless Steel vs. Aluminum Alloy: The Differences Explained

Introduction to stainless steel

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, expanding the corrosion and finally forming holes.

The carbon steel surface can be guaranteed by electroplating with paint or oxidation resistant metals (e.g. zinc, nickel and chromium), but as is known, this protection is only a thin film.

If the protective layer is damaged, the steel below begins to rust.

Steel resistant to weak corrosive media such as air, steam and water and chemical corrosive media such as acid, alkali and salt, also known as stainless acid resistant steel.

In practical application, the steel resistant to weak corrosion medium is often called stainless steel, while the steel resistant to chemical medium is called acid resistant steel.

Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical medium corrosion, while the latter is generally stainless.

The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Chromium is the basic element for stainless steel to obtain corrosion resistance.

When the chromium content in the steel reaches about 1.2%, chromium reacts with oxygen in the corrosive medium to form a thin oxide film (self passivation film) on the steel surface, which can prevent the further corrosion of the steel matrix.

In addition to chromium, the commonly used alloy elements include nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., so as to meet the requirements of various uses on the microstructure and properties of stainless steel.

Stainless Steel

Stainless steel is generally divided into:

1. Ferritic stainless steel.

Chromium content 12% ~ 30%. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its chloride stress corrosion resistance is better than other kinds of stainless steel.

2. Austenitic stainless steel.

It contains more than 18% chromium, about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and can resist the corrosion of various media.

3. Austenitic ferritic duplex stainless steel.

It has the advantages of both austenitic and ferritic stainless steel, and has superplasticity.

4. Martensitic stainless steel.

High strength, but poor plasticity and weldability.

Stainless steel grade grouping

Precipitation hardening stainless steel has good formability and weldability.

It can be used as ultra-high strength material in nuclear industry, aviation and aerospace industry.

According to the composition, it can be divided into Cr system (SUS400), Cr Ni system (SUS300), Cr Mn Ni (SUS200) and precipitation hardening system (SUS600).

200 series – chromium nickel manganese austenitic stainless steel;

300 series – chromium nickel austenitic stainless steel;

301 – good ductility, used for molding products. It can also be hardened by machine speed. Good weldability. The wear resistance and fatigue strength are better than 304 stainless steel.

302 – the corrosion resistance is the same as 304, because the carbon content is relatively high, so the strength is better.

303 – make it easier to cut by adding a small amount of sulfur and phosphorus.

304 – 18 / 8 stainless steel.

The GB brand is 0Cr18Ni9.

309-304 has better temperature resistance than 304.

316-the second most widely used steel after 304, which is mainly used in food industry and surgical equipment.

Adding molybdenum makes it obtain a special corrosion-resistant structure.

Because it has better chloride corrosion resistance than 304, it is also used as “marine steel”.

SS316 is usually used in nuclear fuel recovery units.

Grade 18 / 10 stainless steel usually meets this application grade.

[1] Model 321 – other properties are similar to 304 except that the risk of weld corrosion of the material is reduced due to the addition of titanium.

400 series – Ferritic and martensitic stainless steel;

408 – good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.

409 – the cheapest model (British and American), usually used as automobile exhaust pipe, is ferritic stainless steel (chromium steel).

410 – martensite (high strength chromium steel), with good wear resistance and poor corrosion resistance.

416 – adding sulfur improves the processability of the material.

420 – “cutting tool grade” martensitic steel, similar to the earliest stainless steel such as Brinell high chromium steel. It is also used for surgical tools, which can be made very bright.

430 – ferritic stainless steel for decoration, such as automotive accessories.

Good formability, but poor temperature resistance and corrosion resistance.

440 – high strength cutting tool steel, with slightly higher carbon content, can obtain higher yield strength after appropriate heat treatment, and the hardness can reach 58hrc.

It is one of the hardest stainless steels. The most common application example is “razor blade”.

There are three common models: 440A, 440B, 440C, and 440F (easy to process).

500 series – heat resistant chromium alloy steel.

600 series – Martensitic precipitation hardening stainless steel.

630 – the most commonly used precipitation hardening stainless steel model, usually also known as 17-4; 17%Cr,4%Ni。

Characteristics and uses of stainless steel:

1. Austenitic 1Cr17Mn6Ni5N nickel steel, instead of 1Cr17Ni7, is magnetic after cold processing. For railway vehicles.

2. 1Cr18Mn8Ni5N nickel steel instead of 1Cr18Ni9

3. 1Cr17Ni7 has high strength after cold processing. Railway vehicle, conveyor belt, bolt and nut

4. 1Cr18Ni9 has high strength after cold processing, but the elongation is slightly worse than 1Cr17Ni7. Decorative components for construction.

5. Y1Cr18Ni9 improves cutting and ablation resistance. Most suitable for automatic lathes. Bolt nut

6. Y1Cr18Ni9Se improves cutting and ablation resistance. Most suitable for automatic lathes. Rivets, screws

7. 0Cr19Ni9 is the most widely used stainless heat-resistant steel for food equipment, general chemical equipment and atomic energy industry

8. 00Cr19ni11 steel with lower carbon content than 0Cr19Ni9 has superior intergranular corrosion resistance. It is a component without heat treatment after welding

9. 0Cr19Ni9N add n to the brand 0Cr19Ni9, the strength is improved and the plasticity is not reduced. Reduce the thickness of the material. As a structural strength component

10. 0cr19ni10nbn adds N and Nb to the brand 0Cr19Ni9, which has the same characteristics and uses as 0Cr19Ni9N

11. 00cr18ni10n adds n to the brand 00Cr19Ni11, which has the same characteristics and uses as 0Cr19Ni9N, but has better intergranular corrosion resistance

Compared with nicr19Cr

12. Hardenability. Spinning, special drawing, cold heading

13. 0Cr23Ni13 has better corrosion resistance and heat resistance than 0Cr19Ni9

14. 0Cr25Ni20 has better oxidation than 0cr23ni13. In fact, it is mostly used as heat-resistant steel

15. 0Cr17Ni12Mo2 has better corrosion resistance than 0Cr19Ni9 in seawater and other media. It is mainly used as pitting corrosion resistant material

16. 0Cr18Ni12Mo2Ti is used for equipment resistant to sulfuric acid, phosphoric acid, formic acid and acetic acid, and has good intergranular corrosion resistance.

17. 00Cr17Ni14Mo2 is an ultra-low carbon steel of 0Cr17Ni12Mo2, which has better intergranular corrosion resistance than 0Cr17Ni12Mo2

18. 0Cr17Ni12Mo2N add N to the brand 0Cr17Ni12Mo2 to improve the strength, not reduce the plasticity and reduce the thickness of the material.

Aluminium alloy

Aluminium alloy

General term for aluminum based alloys.

The main alloy elements are copper, silicon, magnesium, zinc and manganese, and the secondary alloy elements are nickel, iron, titanium, chromium, lithium, etc.

Aluminum alloy has low density, high strength, close to or higher than high-quality steel, good plasticity, and can be processed into various profiles.

It has excellent conductivity, thermal conductivity and corrosion resistance.

It is widely used in industry, and its use is only second to that of steel.

Aluminum alloy is divided into two categories: cast aluminum alloy, which is used in as cast state; Deformed aluminum alloy can withstand pressure processing, and its mechanical properties are higher than those of as cast.

Wrought aluminum alloys are divided into non heat treatable strengthened aluminum alloys and heat treatable strengthened aluminum alloys.

The non heat treatable reinforced type can not improve the mechanical properties through heat treatment, but can only be strengthened through cold working deformation.

It mainly includes high-purity aluminum, industrial high-purity aluminum, industrial pure aluminum and antirust aluminum.

Heat treatable strengthened aluminum alloy can improve mechanical properties by heat treatment means such as quenching and aging.

It can be divided into hard aluminum, forged aluminum, superhard aluminum and special aluminum alloy.

Some aluminum alloys can be heat treated to obtain good mechanical properties, physical properties and corrosion resistance.

Cast aluminum alloys can be divided into aluminum silicon alloy, aluminum copper alloy, aluminum magnesium alloy, aluminum zinc alloy and aluminum rare earth alloy according to their chemical composition.

Among them, aluminum silicon alloy also includes simple aluminum silicon alloy (unable to be strengthened by heat treatment, low mechanical properties and good casting properties), special aluminum silicon alloy (able to be strengthened by heat treatment, high mechanical properties and good casting properties).

The material of “Xiangyun” torch of 2008 Beijing Olympic Games is aluminum alloy.

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