Note on the distinction of chromium steel and chromium-nickel steel. The most common chromium steel grades also contain nickel. But colloquially, chromium steel is generally referred to, so that the widespread steel AISI 304, for example, is also actually a chromium-nickel steel.
What are the Specific Characteristics of Chromium in Steel Alloys?
As an austenite former, chromium lowers the critical cooling rate, increases the wear resistance, the scaling resistance and high-temperature strength. Chromium is most frequently used to increase the resistance to corrosion. From a dissolved proportion of 10.5 % in the mixed crystal with austenite or ferrite, the corrosion resistance increases. Steel grades, which meet this requirement, are by definition also called “Stainless Steel”. In this case, chromium forms a passive layer above the materials underneath. This layer is thin but strongly adheres. Again, it heals independently in case of attacks. Stainless, however, does not mean that chromium steel is immune to corrosion in all areas of application. In case of the mentioned value of 10.5 %, chromium steel is only protected against corrosion under normal atmospheric conditions. With an increasing proportion of chromium in the alloy, the resistance to corrosion also increases. But in practice, chromium steel is therefore only referred to as such from a proportion of 12 % onwards only. The widely used steel AISI 304 has a chromium proportion of at least 17 %. An increased proportion of chromium has a positive effect on the resistance to oxidizing acids such as sulfuric acid.
Nickel as an alloying element extends the protective function against corrosion processes to non-oxidizing acids. Nickel is also an austenite former, like chromium.
Can Chromium Steel Be Hardened?
Chromium steel is versatile. It is true that chromium increases the corrosion resistance, but not the hardness properties of steel alloys. To this end, further treatment steps are required. Heat treatments are for instance frequently performed. Some of these processes have a negative effect on corrosion resistance. The BORINOX® process for stainless steel hardening is another way to harden chromium steel. This method is a gas-based low-pressure diffusion process. It is particularly suitable for use in chromium alloys, since BORINOX®´s own process execution effectively prevents the precipitation of chrome carbide nitrides. This can guarantee the maintenance of corrosion protection.