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.

Expanite test G98 galling graph

Download Test Result

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