Case hardening stainless steel surfaces using the Kolsterising process

Introduction

The Kolsterising process is marketed in the UK by Bodycote S3P Group, (www.bodycote.com, Speciality Stainless Steel Processes).
Kolsterising improves the wear resistance of stainless steel part surfaces, without degrading their corrosion resistance. There are no additions of chemical elements, (other than carbon), to the steel during the process.
It is claimed that austenitic stainless steels which contain molybdenum, such as 1.4401, (316), can also have enhanced corrosion resistance after Kolsterising.

The Kolsterising process

Kolsterising does not apply a coating on the surface, but is a low temperature surface carbon diffusion treatment. Although large quantities of carbon are diffused into the surface, (known generically as ‘colossal supersaturation’), visible chromium carbides are not formed. The avoidance of such carbide formation by using low treatment temperatures whereby significant substitutional element diffusion, (i.e. Chromium), cannot take place, is why the treated steel’s corrosion properties are not adversely affected by this process.
The resulting surface treated layers can have hardnesses in the range of 1000 to 1200 VPN, (approx 72 HRC). The increase in hardness is due to the parent matrix, lattice strains caused by these high levels of interstitial carbon. The thickness of the hardened layer is dependent on the process conditions used, but includes 22 or 33 micron effective case depths. Complex shapes can be effectively hardened by this process.

Surface property enhancements and benefits associated with Kolsterising

The main property enhancements from the Kolsterising process, claimed by the developers of the process include

Improvement of wear and galling resistance
Improvement of cavitation erosion resistance
Increased fatigue strength
Good dimensional stability

There should be no degradation of corrosion resistance and the magnetic properties of austenitic steels, (low magnetic permeability), are not affected by the process. The pitting and crevice corrosion resistance of molybdenum containing grades like 1.4404, (316), in chloride environments is claimed to be improved by the process.
The surface hardened layer also has good toughness properties and there is no risk of delamination or peeling of the surface hardened layer.
The improvement in fatigue resistance is due to the formation of compressive stresses in the treated surface layer, which also helps enhance the SCC resistance of Kolsterised parts.
There is little or no change in dimensions or shape when stainless steels are Kolsterised and no change in steel colour, (appearance). The process can be applied to finished parts with a high standard of dimensional accuracy. These high levels of dimensional accuracy also mean that seals may not be needed, enhancing the operational temperature ranges of components with possible reductions in maintenance costs.

Range of stainless steels that can be Kolsterised

A wide range of austenitic and duplex steels can be treated. These include
1.4301, (304)
1.4307, (304L)
1.4401, (316)
1.4404, (316L)
1.4462, (2205)
Nickel based alloys including Hastelloy C22 and C276 and Inconel 625 and 718 can also be Kolsterised.

Applications for Kolsterising

The fields of application for Kolsterised surfaces includes automotive, valve manufacturing, marine, oil and gas, fastener, food/beverage, pharmaceutical and medical industries.

Specifically, valve parts, (balls, needles, seats and housings), in petrochemical industries, parts for NACE MR 0175 sulphide stress corrosion cracking sour gas applications and beverage filling machine pistons and seals.

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