Improving the mechanical properties of stainless steel for food and beverage applications
The soft surface characteristics of the typical materials used in food and beverage designs sometimes lead to galling, premature wear and insufficient edge retention. Austenitic stainless steels, such as 304 and 316, are used extensively in food and beverage applications where corrosion resistance is of extreme concern. However, the material’s other mechanical properties related to durability and part life are not as stellar. Consequently, chrome plating and other coatings are often used to address these issues, but can lead to other side effects such as cracking, delamination and subsequent part failure.
In the Netherlands, a low-temperature diffusion process called Kolsterising was developed to increase the wear resistance capability and life of SS components. The technology can be applied to SS valves, bulk fasteners, blades, knives, slitters, nuts, washers, fixtures, pump components and other types of machined parts manufactured from corrosion-resistant austenitic stainless steels, duplex stainless steels and nickel-based alloys.
In 1998, Bodycote PLC—a UK-based supplier of heat treatment, metal joining, hot isostatic pressing and coating services—acquired the technology and subsequently commercialized the surface treatment process. Dave Montesano, market development engineer in North America for Bodycote Specialty Stainless Steel Processes (S3P), is part of the company’s efforts to increase industry awareness for the process in North America. The surface treatment has been offered by Bodycote in Europe for almost 30 years, but did not have a US presence until 2006 when the company responded to domestic market needs and opened a facility in London, OH. FE asked Montesano to fill in the blanks on this technology.
FE: Why does SS need wear resistance?
Montesano: Austenitic stainless steels deliver corrosion resistance and high toughness, but they fall short when it comes to other desirable mechanical properties. These materials are relatively soft, so they tend to wear quickly. Austenitic SS is also susceptible to galling, in which the adhesive friction between two components causes material to smear from one surface to the other, eventually resulting in a cold weld between the two moving parts.
FE: Why is S3P a better alternative to coating or thermal treatment?
Montesano: A coating adds material to the part’s surface, changing the part’s dimensions, which can introduce problems in assemblies that use high-precision parts. Coating also increases the risk of delamination or peeling. Since S3P does not apply a coating, there is no risk of the material cracking or peeling and potentially contaminating products processed on the machinery. And, since the surface hardness is relatively equivalent to chrome plating, Kolsterised material provides similar longevity.
In addition, traditional through-hardening with conventional heat treatment is not possible with austenitic SS, so mechanical designers who want both high wear resistance and corrosion resistance often specify an SS such as 17-4 PH as an alternative, a precipitation-hardened steel that is hardened to a certain spec, such as H900. A 17-4 PH SS can be hardened to 40-45 Rockwell (HRc) but is inferior to austenitic stainless steels in terms of corrosion resistance. By comparison, components treated with S3P can achieve the equivalent of 72 HRc on the surface. Since hardness and wear resistance are generally directly proportional, SS parts with a higher surface hardness typically exhibit superior wear resistance characteristics.
FE: What are the other benefits of Bodycote S3P?
Montesano: S3P can increase component life by resisting the potential effects of both surface abrasion and cavitation erosion. Some materials can be surprisingly abrasive, and the growing trend of using extremely caustic cleaning agents at food manufacturing facilities seems to be driving more SS specifications to 304 and 316, often in direct conflict with durability. Materials such as strawberries, nuts and toothpaste that are run through pumps and valves in food processing machinery can contribute to premature wear, and pressure differences caused by high-speed fluids can cause cavitation erosion on components such as impellers.
Further, the S3P surface treatment does not affect the lubricity or corrosion resistance of the SS, and since there is no dimensional change or distortion, post-process machining is not required after treatment. Another benefit of the compressive stresses within the hardened surface is considerably improved component fatigue strength.
FE: How hard a surface is attainable?
Montesano: S3P treatments can increase the surface hardness of a base material by four or five times, to a level of 900-1300 Vickers (comparable to 72 HRc) and a case depth from 20 to 50 microns. Part surfaces are uniformly hardened at sharp edges and inside bores and cavities; surface roughness is usually not affected.
FE: How does the S3P process work?
Montesano: The parts are placed in a proprietary heat treatment furnace, where they are subjected to a low-temperature carbon diffusion process. During the process, carbon atoms are forced into the interstitial spaces in the microstructure of the metal that are smaller than the diameter of the carbon atom. This creates compressive stresses on the part surface, which results in extremely high surface hardness and wear-resistance properties. Since the process is performed at such a low temperature, chromium carbides do not form, so the corrosion resistance of the base material is maintained.
FE: What stainless steels can be treated currently?
Montesano: The process primarily targets 300 series austenitic stainless steels, including 304, 304L, 316 and 316L, as well as CF8M and 904L. Precipitation-hardened stainless steels (A286, 17-4, 17-7 and 15-5), duplex stainless steels (2205, 2304 and super duplex) and nickel alloys such as Hastelloy C22 and C276 and Inconel 625 and 718 are also treatable.
FE: You mentioned that this process could be applied to pumps and slicers. Can it also be applied to grinders and hammer mills?
Montesano: Absolutely! Screens and rollers in hammer and roller mills are perfect examples of components that are currently treated and benefit substantially from S3P hardening, and I am aware of applications in Europe where we’ve successfully addressed premature wear issues in various dry grinder product lines. Further examples are SS screens and impellers used in granular sifting applications. In all of these examples, we’ve increased system performance and subsequent time between required machine maintenance and component replacement.
FE: What approvals have been granted by agencies/standards bodies for the resultant parts that might come into contact with food?
Montesano: In most cases, no approval standard is required since we are not adding anything new to the component, and the sensitivity to bacterial growth does not change from the original SS material. We also have test data demonstrating there is no release of substances in cytotoxic concentrations, risk of contamination or release of material into foodstuff. However, if a customer does need to reference our process during a submission to the FDA, we have a master file on record with the FDA that can be referenced by the submitting party to assist with the approval process.
For more information, visit www.bodycote.com.