Diffusion-Based Surface Hardening Technology for Stainless Steel and other Specialty Alloys
The patented Expanite surface hardening technology for stainless steel has many advantages compared to traditional surface hardening techniques, including coatings and heat treatment solutions. The main benefits are improved wear and galling performance, and potentially improved corrosion resistance.
Compared to traditional, often costly hardening processes such as traditional diffusion processes, DLC/PVD and hard-chrome coatings, Expanite’s diffusion-based hardening reduces costs, CO2 emissions, and eliminates the use of hazardous chemicals. At the same time, Expanite operates under ISO 9001 and ISO 14001 certified management systems and follows relevant industry requirements such as CQI-9. This combination allows manufacturers to improve product performance while meeting both regulatory and sustainability demands and reduce cost.
The Expanite gas-based processes are taking place in electrical-fired furnaces at lower temperatures, and do not use any hazardous materials or chemicals. Thus, there is no outlet of any chemicals or waste. The Expanite processes are applicable to stainless steel, nickel-based steels, and titanium alloys.
The Expanite approach is what specialists refer to as a “thermo-chemical conversion” or in different terms, a diffusion-based surface hardening process. The product which Expanite offers on for instance austenitic alloys can consist of two process steps. The first step is ExpaniteHigh-T, a vacuum solution nitriding process that anneals the core material and provides limited increase in hardness, in relation to the surface and core. The second step, ExpaniteLow-T creates a conversion layer of expanded austenite. Together, this process example is referred to a SuperExpanite. No material is added or deposited on top of the surface of the parts, but rather a solid solution of interstitially dissolved large amounts of nitrogen and carbon atoms is created, thanks to precise control of process parameters. Those interstitial atoms occupy octahedral holes of the crystal lattice and are responsible for inducing compressive stresses in the affected region. It is thanks to this effect, we see increase in hardness, and with it, improved wear, galling and other mechanical properties.
The SuperExpanite process does not require extensive pre-processing steps, that are traditionally used to remove the passive layer prohibiting absorption of atoms from the process atmosphere. The passive oxide layer is a natural protection barrier on top of the surface of stainless steels and is impenetrable to atoms. In the Expanite process, the passive oxide layer is reduced in-situ during treatment utilizing the same process atmosphere. After the process, the surface is exposed to atmospheric air and re-passivates, keeping the corrosion resistance properties of the material.
Because Expanite is a gas-based diffusion process (and not a coating), the uniformity of coverage is not limited by complicated geometries. Moreover, the lack of post-process finishing like grinding and polishing, makes the overall treatment significantly simpler and shorter, and the final geometry of products is more stable.
Depending on the alloy, what Expanite achieves is a high hardness (up to 1300HV) on the surface.
In standardized tests, Expanite treatment lowers the wear rate of the austenitic stainless steels over 100 times, as compared to the untreated material. This improvement can be attributed to the surface hardness increase of a factor of 5 or more (up to 1300HV0.05). Download test results
The depth of the treatment varies and is dependent on the final application, however, but usual values lie within 10-30µm.
The standard treatment depth of 20µm provides protection against wear even when hard, 3rd body particles are present. This increase in hardness is also responsible for significant improvement of resistance to adhesive wear – both, the risk of cold welding and of its dynamic form known as galling, are mitigated beyond the yield strength of the austenitic stainless steels. This allows for smooth and trouble less operations of valve components, even under high loads and after long periods without operation.
One of the biggest achievements of Expanite treatment that makes it stand out from the crowd is the proven improvement of pitting corrosion resistance of austenitic components treated with SuperExpanite.(Corrosion test results) Thanks to the nitrogen dissolved in the surface, the PREN number goes up, and with it, the breakdown potential – meaning that the surface stays passive beyond what’s possible for the untreated material.
Don´t Just Take Our Word For it!
Expanite has conducted several certified ASTM and/or ISO tests to document wear, galling and corrosion resistance click below for more:
