Unique Wear Reduction by Using SuperExpanite® Hardening on AISI 316L (EN 1.4404) Austenitic Stainless Steel

AISI 316L Stainless Steel Wear Test G133

Wear is a pervasive problem for any kind of stainless steel, especially for the austenitic grades. In those alloys, Nickel is added not only to improve corrosion resistance but primarily to stabilize the FCC crystal structure. High ductility and toughness of these alloys, originating from the specific properties of FCC structure, provides good formability at the expense of hardness, and with that, of wear resistance. Based on results of a standardized test for wear resistance of materials, the reduction in wear for austenitic stainless steel with the SuperExpanite® surface hardening treatment is truly remarkable and shows that Expanite® is the leading solution to drastically improve the lifetime of your parts and products!

Ball-on-flat Sliding Wear Test (ASTM G133)

The ASTM G133 test geometry involves a spherical counterpart, sliding in a reciprocating motion with a constant force of 25N, for a total distance of 100m. The resulting wear track is both visually inspected and measured with a profilometer. The latter yields a digital profile – a 2D cross-section of the wear track, from which the wear depth and volume can be measured. In this study, two sets of samples made with AISI 316L austenitic stainless steel, one without hardening process applied and one treated with SuperExpanite hardening, were tested against 100Cr6 counterpart (bearing steel with hardness above 800HV).

Surface hardening with SuperExpanite

SuperExpanite20 (NC) process was used in this study to demonstrate the excellent improvement of wear resistance achievable with Expanite surface hardening solution. This can be achieved thanks to the increase of surface hardness by up to 10 times, while maintaining or even enhancing corrosion resistance of AISI 316L. The SuperExpanite (NC) process family consists of two main steps. First is ExpaniteHigh-T that anneals the core material and provides increased hardness up to 1mm depth, relative to the core. The second step, ExpaniteLow-T creates a conversion layer of expanded austenite (solid solution of interstitially dissolved large amounts of nitrogen atoms that occupy octahedral holes of the FCC lattice) which provides a surface hardness of 1200HV. The hardened case depth can vary from 10μm through 15μm and 20μm up to 30μm, depending on the Expanite product. The hardness profile of an alloy EN 1.4404 (AISI 316L) treated with SuperExpanite20 (NC) is shown in the Figure 1.

 

316L test results

 

Even 125 times lower wear rate

The results obtained from the tests are compared in the graph in Figure 1. The wear track on the untreated reference material (represented as a blue line in Figure 2.) is up to 100 times deeper than the measured depth of a wear mark on the SuperExpanite surface (represented as a green line in Figure 2.). This means that the wear rate – the amount of displaced material described as volume, normalized by the test parameters – is on average, up to 125 times lower for the SuperExpanite treated surface, as compared to the native material (see Table 1).

Expanite test G98 galling graph

Test Results

The tracks visible in Figure 3 were made during the test with an amplitude of 10mm in ambient temperature and without lubrication. The wear marks on the left photograph are superficial and can be seen as polishing of the SuperExpanite surface. The observed wear depth of 2µm to 6µm is not detrimental to the hardened layer. On the right-hand side the untreated test sample with wear tracks numbered 4-6 is presented and shows severe wear.

Improve Your Results with SuperExpanite.

The surface performance and lifetime of AISI 316L parts can be significantly improved. Our unique treatment is suitable for a wide range of austenitic-, ferritic-, martensitic and duplex stainless steels, as well as some Ni-based superalloys. Expanite’s innovative and environmentally conscious surface hardening technology is based on cutting-edge research, therefore, your parts can be treated with extremely short lead times—a few days—which is previously unseen within surface hardening of stainless steel.

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