What is stainless steel? The ABRAMS Steel Guide® explains!

From a technical point of view, stainless steel is a steel with a particularly high purity. Stainless steel may only be called stainless steel if its iron impurities sulphur and phosphorus do not exceed the value of 0.025%, as these have a negative effect on the performance properties of the steel. This means that any steel with a high degree of purity can be described as stainless steel, but it is not the same as stainless.

Corrosion resistant steel is also divided into alloyed and unalloyed stainless steels in the Stahlschluessel – key to steel:

  • Unalloyed stainless steels have the steel groups No. 10 – 18
  • Alloyed stainless steels have the steel groups no. 20 – 89

Alloyed stainless steel can be further divided into low and high alloyed stainless steels.

Corrosion resistant steel

In the industry stainless steels can be used in a variety of applications in the form of tool steel, high speed steel, ball bearing steel, structural steel, engineering steel, chemical resistant steel, tempering resistant steel, high temperature resistance, corrosion resistant steel.

What is the difference between stainless steel and corrosion resistant steel?

Corrosion resistant steel (commonly named and from here on out named “stainless steel”), contains a minimum of 10.5% of chromium and a maximum of 1.2% of carbon. Steel is called corrosion resistant if, the material has specific corrosion- or oxidation properties. Adding alloys like nickel, molybdenum and manganese will increase the corrosion resistance even more.

What groups are there for stainless steel?

Stainless steel can be divided into the following groups: austenitic -, martensitic – and ferritic stainless steel.

Austenitic steels always contain at least 8% of nickel, the so-called chromium nickel steels. With a minimum of 13.5% of chromium, the added nickel advances the corrosion resistance as well as the machinability and mechanical properties.

Adding molybdenum, the so called CrNiMo-steels, the corrosion resistance is even higher and they are resistant to pitting in chlorine containing and acid reducing atmospheres. They can be used in aggressive mediums, for example in the chemical industry, chlorine containing sea water but also in the food industry. Austenitic stainless steels are non-magnetizable.

In this group of stainless steel, you can find steel grades like

Martensitic steels contain between 12% and 18% chromium and have a carbon content of 0.1% to 1.2%. The high carbon content allows these steels to achieve a high hardness. However, the carbon content reduces corrosion resistance and the steel has poorer formability and weldability.

With its special properties, martensitic stainless steel approaches the mechanical properties of engineering steels and also offers medium to high corrosion resistance.

Martensitic steels are divided into four (partially overlapping) subgroups:

1. Fe-Cr-C steels: They represented the first martensitic grades. They are still widely used today in mechanical engineering and in components subject to wear.

For example:

2. Ni-containing grades: In this steel group nickel replaces part of the carbon. Compared to the previously mentioned group, they have a higher toughness, especially at low temperatures. Their higher chromium content also results in a higher corrosion resistance. Adding molybdenum further improve the steels in subgroups 1 and 2. (e.g., 431).

3. Precipitation hardening stainless steels: They represent the best combination between strength and toughness.

4. Creep resistant grades: with a chromium content of about 11%, they differ from the steels of subgroup 1 by alloying additions of cobalt, niobium, vanadium, and bismuth, which increase the high temperature and fatigue strength (up to 650°C).

Is stainless steel corrosion resistant?

A corrosion-resistant stainless steel is a steel that has a special degree of purity and a high alloy content of chromium. Due to the high chromium content of at least 10.5%, a protective passive layer of chromium oxide forms on the surface of the corrosion-resistant stainless steel. As a result of this patina the steel is corrosion resistant.

Although such a chromium content does not necessarily guarantee the complete absence of rust, it does prevent technically problematic pitting corrosion though. The addition of further alloying elements, e.g., nickel, molybdenum, manganese, and niobium, can result in even higher corrosion resistance or improved mechanical properties. The alloying element nickel, with a mass fraction of at least 10%, also makes a corrosion-resistant steel acid-resistant.

What is V2A stainless steel?

The term “V2A” stands for “Versuch (Trial Melt) 2 Austenite” and is widely known under this term in Europe. It refers to a group of stainless steels commonly used in various industries and construction. V2A contains a high chromium and nickel content; the chromium content gives the steel its corrosion resistance, while the nickel content contributes to strength and toughness.

V2A steel is a stainless steel commonly found in everyday life, used for example in the construction of railings, vehicles, or sinks, and has applications in the beverage, pharmaceutical and cosmetics industries (e.g., materials 303 and 304).

What is a V4A stainless steel?

The term “V4A” stands for “Versuch (Trial Melt) 4 Austenite”, it also contains molybdenum in addition to a high chromium content and nickel. The combination of these alloying elements gives these “stainless steels” a higher corrosion resistance and make it particularly resistant to influences such as moisture, salt water or chemical substances. Due to its excellent corrosion resistance, V4A steel is ideal for outdoor use (e.g., materials 316Ti, 316 and 316L).

What is the difference between V2A and V4A stainless steel?

The stainless steel groups V2A and V4A differ only in their respective alloying elements. The alloying elements for the V2A stainless steel are mainly 18% chromium and 8% nickel. Typically, the V4A stainless steels contain 16% chromium, 10% nickel and 2-3% of molybdenum.

Is stainless steel magnetizable?

Not all stainless steels are magnetisable. If a stainless steel is magnetisable depends on its finish and its microstructure. An austenitic microstructure is not magnetisable while a martensitic and ferritic microstructure can be magnetised.

Austenitic steels have a very high nickel content, this is the reason that those steels cannot be magnetised. About 70% of all corrosion steels are austenitic stainless steels. Through forming/shaping the microstructure can be changed to make it marginally magnetic as the forming process will change the austenitic structure into a martensitic structure.

Ferritic stainless steels have a high chromium and low carbon content, which makes this steel magnetisable.

Martensitic stainless steel is magnetisable due to its microstructure and will be even more so after hardening.

 

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