Surface Hardening for Austenitic Stainless Steels
Despite some inherent deficiencies such as low base hardness and susceptibility to galling, austenitic stainless steels – through conventional heat treating such as carburizing – have been able to answer the demand in most environments and for most applications.
The issue surrounding conventional heat treatments of austenitic stainless steels lies in the reality that standard treatments generally reduce the material’s corrosion-resistant capabilities. In turn, this causes a bit of a trade off when looking for both a harder surface and the sustainable level of corrosion resistance needed for a particular application. The net result of this trade off has been a restriction on the range of applications for these materials.
A newly introduced heat treatment in North America, however, may open some doors that have previously been closed to the austenitic stainless steel family. The name of the treatment is Kolsterising®, and it has recently become available to North American markets after more than 15 years of commercial availability in Europe.
This low-temperature process has the ability to create a surface hardness on a given component in the range of 68-70 HRc without creating dimensional change to the component. It is also worth noting that the corrosion-resistant capabilities of certain high-nickel austenitic stainless steels – such as AISI 316 or AISI 316L – are not adversely affected and may in certain instances actually be improved through use of this treatment process.
The process creates what is commonly referred to as a “super-saturated” carbon or Kolsterised layer from the surface of the component inward (Fig. 1). This layer is formed through diffusion of large amounts of carbon on the order of 3-5 wt% into the base material at a temperature that precludes the formation of chromium carbides. Chromium carbides formed during the higher temperatures associated with conventional heat treatments result in the material losing some of its corrosion-resistant capabilities.
The treatment process takes its ability to maintain its corrosion-resistant capabilities from the fact that the carbon is diffused interstitially into the austenitic lattice causing residual stresses. It is diffused at a low enough temperature to ensure that the formation of chromium carbides is not possible.
Two things can have a negative effect on the process gaining optimum results. Shown in Figure 2, mitigation occurs through initial alloy selection in the case of delta ferrite and electro-polishing or passivation in the case of deformation martensite.
Kolsterising has the ability to provide a uniformly hard surface to the entire part, and unlike some of the more popular coatings currently available, the process does not suffer from line-of-sight issues. Threaded components, components with small bores or crevices, and thin-walled components are all very viable candidates for the treatment.
It should be noted that high compressive stresses in the surface developed during the treatment led to an increase in the fatigue strength of a treated component as well as an improvement in friction-coefficient capabilities. As mentioned earlier, the process does not change the dimensions of the treated component, nor does the diffused layer chip or peel away during the wear life cycle.
Although optimum results are obtained with austenitic stainless steels, there are other treatable materials. These include duplex stainless steels as well as certain nickel-based alloys such as Inconel®, Hastelloy® and A-286.
Currently, the treatment is offered at two standard case depths – a 22-micron treatment for applications of moderate wear exposure and a 33-micron treatment for applications of severe wear exposure. There is a third treatment option designed primarily for the duplex stainless steel group (AISI 2205, AISI 2507) that generally achieves a case depth in the range of 18-20 microns. Figure 3 depicts the hardness profiles of an AISI 316 sample for both the 22-micron (0.00079”) and 33-micron (0.00157”) treatments.
There are four basic properties that are improved through the use of Kolsterising on components that might otherwise suffer from wear and/or galling-related issues through contact with abrasive medias or sliding motion contact.
The first, of course, is basic surface hardness. Using the 33-micron treatment, surface-hardness levels can range anywhere from 1000 HV0.05 on an AISI 304 sample to 1750 HV0.05 on a PH 13-8Mo test sample. Because of the profile of the hardened case, the hardness can only be measured properly using a Vickers microhardness tester with a load of 50g or less. It should also be noted that the hardness profile gradually degrades as it penetrates into the component and is at its peak hardness at the surface. Figure 4 shows the gradual degradation versus the more pronounced drop off of a comparable coating.
GallingThe second improved property lies in the fact that the treatment has been shown to be effective in eliminating galling issues. In the past, the lack of this ability has been a severe failing of austenitic stainless steels, and this has been widely reported. The photos depicted in Figure 5 show three separate shafts (untreated , nitrided and Kolsterised) that have been rotated at 330rpm while being held by a pair of jaws with a closing force of 707N. The results speak directly to the anti-galling capabilities of the treatment.
WearThe third characteristic is the improved wear capabilities of a treated component. Pin-on-rotating-disc wear tests have been carried out to demonstrate the unique post-treatment wear characteristics. The results shown in Figure 6 were obtained testing both AISI 304 as well as AISI 316 samples. Three separate test media have been used, i.e. air with 60% humidity, SAE 10W oil and artificial seawater. The results are reported as the decrease in height per unit of load and per unit of sliding distance (10-4µm/Nm). The accompanying chart provides evidence for the increase in post-treatment wear resistance.
Lastly is the previously mentioned corrosion resistance of the treated material to include improved pitting, stress and crevice corrosion resistance. Figure 7 shows testing of a 316 sample in a solution of 34% MgCl2 with an applied stress of 80% of the 0.2 % yield strength at room temperature. Once again, the results indicate that resistance to stress and crevice corrosion is enhanced.
The process found its initial niche with European manufacturers of bottling and filling equipment. Since that time, the treatment has spread to a wide variety of industries to include the medical, automotive, food and beverage, and chemical-processing industries. Any component that suffers from wear, galling, pitting or general erosion may be a viable candidate for the treatment.
Because the process is carried out at a temperature below what would allow for the formation of chromium carbides, it is generally recommended that the operational temperature of a treated component should not exceed 400°C (752°F) to avoid the formation of chromium carbides at a later stage. The fact that areas of a treated component cannot be blanked off during treatment should also be taken under advisement. There are also certain component size limitations that must be reviewed prior to consideration.
Examining the data from existing mechanical tests would indicate that certain key mechanical properties of austenitic stainless steels can be improved through the use of this low-temperature treatment process.
- Surface hardness is increased substantially in the range of 1000-1200 HV 0.05 (equivalent to 68-70 HRc).
- Corrosion-resistant properties are maintained if not improved through the lack of formation of chromium carbides.
- Resistance to galling is greatly increased.
- Wear properties are increased.
- Fatigue properties show improvement.
- In AISI 316, resistance to pitting is increased.
- Resistance to stress-corrosion cracking is increased.
- No change in color to the treated component.
- Uniform hardening on sharp edges and inside bores.
- Improved resistance to microscopic impact wear.
As industries continue to find ways to improve equipment uptime, drive down component replacement costs and search for ways to build longer-lasting components, processes similar to this one may find their niche and make the world run just a little more efficiently. IH
For more information: Jim Harbert, Director of Sales, S3P, Bodycote; tel: 330-784-9814; main office: 740-852-4955; fax: 740-852-4956; e-mail: firstname.lastname@example.org; web: www.bodycote.com/s3p
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: austenitic stainless, chromium carbides, surface compressive stress, galling, stress corrosion, crevice corrosion