Case hardening steels

A new hardening process called Kolsterizing offers some tangible benefits over earlier techniques

Austenitic stainless steels, like 316 and 304, are known to offer the benefits of corrosion resistance. However, disadvantages include a limited wear resistance and increased risk of galling. Galling, or fretting, occurs when two contacting surfaces, normally at rest, undergo minute oscillatory tangential relative motion. The danger is not so much the removal of material and loss of dimensional control, but the reduction in fatigue life.

Austenitic stainless steels also suffer from local corrosion problems, such as crevice pitting and stress corrosion cracking in chloride environments. Hardening the surface of stainless steel is the way of overcoming these problems. Common treatments include Tuftriding, Sulphinizing, and Nitriding, but these processes change the shape and colour of the steel, lead to peeling and separation of the surface layer and add foreign elements into the surface. What is more, they can decrease the corrosion resistance of the steel.

Now, a new process called Kolsterizing, a derivative of an earlier process called Hardcor, claims to eliminate some of these disadvantages. Recently introduced to the UK by a Dutch company, Hardiff, no coating is applied during the process and even holes and crevices measuring just a few microns can achieve a surface which is equally hard all over.

The surface hardness of a Kolsterised stainless steel is between 1000 and 1200 Vickers (HV). Theoretically, any depth of hardening can be achieved, but cost considerations lead to two standard treatments being offered. One has a thickness of 22 micro m and the other 33 micro m. Each of these treatments results in a different hardness profile. The hard surface cannot peel off, because the hardness gradually decreases from the surface towards the core, thus eliminating the risk of delamination.

Components to be treated must fit into a space with a diameter of 485mm and a length of 500mm. Thin products having a diameter less than 40mm can be Kolsterised in lengths up to 700mm.

In principle, any type of austenitic stainless steel can be Kolsterised. However, the preferred type is one containing molybdenum and which is ferrite-free, since this results in greatly increased resistance due to pitting, crevice corrosion and stress corrosion. Kolsterizing can also be applied to precipitation-hardened stainless steels, for applications where a core material is needed which is stronger and tougher than 316. Werkstoff 1.4980 alloy is recommended, whereby Kolsterizing can produce a surface hardness of 1300 Vickers with the core about 400HV.

In situations where austenitic stainless steel components suffer from wear and/or corrosion in an aggressive medium, Kolsterizing can be one solution, providing that the maximum temperature in service does not exceed 300 degrees C.

Figure 1: Each of the Kolsterized treatments results in a different hardness profile. Hardness profile of the Kolsterized layer with (a) 33 micro m treatment (b) 22 micro m treatment

Figure 2: The names of steels relates to their crystal structure. The chemical composition determines which category a stainless steel belongs

{{ C% Cr% Ni%Ferritic <0.15 3 – 18 NoneMartensitic 0.20 – 1.10 12 – 18 0 – 2.5Austenitic <0.1 16 – 26 8 – 22Duplex(austenite and ferrite) 18 – 27 4 – 7 2 – 4Mo, Cu, N}}

Figure 3: Components to be Kolsterized must fit into a space with a diameter of 485mm and a length of 500mm. Thin products having a diameter less than 40mm can be Kolsterised in lengths up to 700mm