Stainless Steel - The Material

History

The first "stainless steel" was patented by Krupp in Essen in 1912. Alloys that could be grouped under the general term "special steels" had already been developed much earlier (around 1740 with crucible steel).

Definition

It is important to clarify here that these steels are not simply "special steels", but "stainless steel", a specific subgroup.

So what does "stainless steel" actually comprise?

While it is, of course, largely made of iron (FE)! -
as a rule this element tends no longer to be specified in the material description. Many material descriptions only specify the alloying elements that are key to the characteristics of the stainless steel, e.g. Cr = chromium, Ni = nickel, Mo = molybdenum, etc. ...

What makes stainless steel so resistant to corrosion?

As one of the most important alloying elements, representing a minimum of approx. 12%, chromium (Cr) is essential for the formation of a protective oxide layer on the surface of the steel when exposed to oxygen. This process is known as "passivation" and may take several days. However, this process can be speeded up considerably by using nitric acid (an oxidizing acid), thus reducing the time required to repair surface damage, e.g. due to mechanical processing. Adding other alloying metals, such as Mo=molybdenum, further improves the corrosion resistance considerably.

What points need to be taken into account?

If exposed to environments that do not contain oxygen, stainless steel can corrode just like every other corroding steel – unless a protective layer has already formed. For example, weld seams that have not been properly treated with a passivating agent can corrode very rapidly in swampy or muddy waters with very low levels of oxygen. The effects of heat, such as those caused by incorrect welding or heat treatment (annealing/hardening), can also change the structure of the material to such a degree that the stainless steel can corrode or even become brittle.

What are the most common designations?

These days, we are likely to encounter a whole range of designations, such as V4A, V2A or 18/10.
Short designations, such as "V4A", originated from the Krupp experimental laboratory and refer to a test series. In this context, you will also frequently find the designation "A4" for screw parts made of stainless steel. The numbers 18/10 used, for example, on many household cutlery items, stand for the alloying constituents chromium (18%) and nickel (10%). Names like Cromargan, Remanit and Nirosta are proprietary brand names. These examples are very general and not specific enough to be used as a material designation. Because there are many types of stainless steel with very different properties and areas of application, it is important to always specify the material number for certain applications.

Take, for example, the Mat. No. 1.4301 (generally V2A or A2) In this classification system, the "1" that prefixes the dot indicates that the metal in question is steel. The "4" after the dot indicates that the metal is a stainless steel. There are also standard short designations, which facilitate identification of the chemical composition.

In our example, the composition is: X5CrNi18 -10
The components of this designation can be broken down as follows:

• X5 = carbon content in hundredths of a percent by weight (for example: 0.05%
• Cr = chromium
• Ni = nickel
• 18 = 18% chromium
• 10 = 10% nickel

Key "stainless steel" materials commonly used in the construction industry are:

Influence of alloying elements on the properties of stainless steels

Wesentliche Legierungsbestandteile

Physical properties

Both the chemical composition of the material and cold-working have an effect on the ability of the material to be magnetised. Depending on these factors and the type of microstructure, the stainless steel may react magnetically.