Unprecedented Galling Resistance of 316L After Expanite Surface Hardening

AISI 316L Stainless Steel Galling Test G98

Defined in the standard as "a severe form of wear characterized by localized, macroscopic material transfer, removal or formation of surface protrusions when two solid surfaces experience relative sliding under load", galling, or adhesive wear, is a pervasive problem for stainless steels, of which the soft 300 series are notoriously prone. In an on-going effort to increase its database of engineering data, Expanite® recently conducted the ASTM G98 Standard Test Method for Galling Resistance of Materials on AISI 316L, with and without Expanite surface treatment.

ASTM G98 Standard Test for Galling Resistance

This standardized test involves a cylindrical button which is rotated 360° against a stationary block under normal force. After rotation, the contact surfaces are visually inspected for galling. The load is gradually increased until galling is observed, thereby establishing the threshold galling stress. The table below lists the extent of galling observed after a test with increasing load on both, self-mated native surface (untreated) and self-mated SuperExpanite® hardened AISI 316L (1.4404).

 

316L test results

 

Surface Hardening with SuperExpanite

SuperExpanite20 (NC) process was used in this study to demonstrate the excellent improvement in galling 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.

Galling Mitigated Beyond Yield Strength.

The test results are summarised in Table 1. In Figure 2 the photos demonstrate the remarkable galling resistance afforded by SuperExpanite proprietary surface hardening technology. In the picture on the top, untreated 316L surfaces can be seen with severe galling under relatively low normal stresses in the range of 3.4–13.8MPa (0.5–2.0ksi). In a mating couple with two SuperExpanite-treated surfaces of the same alloy (shown in the bottom picture), galling is almost completely mitigated beyond the yield strength of the material (at contact pressure of 276MPa or 40ksi)! The visible marks are simply an effect of polishing of the test surfaces.

G98 316L Galling photoExpanite ASTM G98 test result

Unprecedented Solution to Your Galling Problems.

Steven Budinski, owner of Bud Labs (Rochester, NY), where the test was conducted remarked, "I've never seen such a solution to galling of 316L in my many years of tribology testing". The patented process that produced such results is known as SuperExpanite.

Improve Your Results with SuperExpanite

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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