Remarkable Galling Prevention by Using ExpaniteHigh-T Hardening on Martensitic Stainless Steel.

Martensitic Galling Test G98

Galling is a pervasive problem for stainless steel – even on martensitic stainless steel treated with a standard vacuum hardening – the challenges still exist. Based on the results of standard ASTM test G98 for galling resistance of materials, the galling mitigation on the 1.4021 martensitic stainless steel achieved with the ExpaniteHigh-T surface hardening process are truly outstanding and show that Expanite® offers superior solutions to improve galling resistance.

 
Test Galling HT Expanite G98

ASTM G98 Standard Test for Galling Resistance

The ASTM G98 test geometry involves a cylindrical button which is rotated 360° against a stationary block under increasing normal force. For the test conducted with samples manufactured in alloy 1.4021 and treated with ExpaniteHigh-T, the load was varied from 100psi to 30ksi (0.7 MPa to 207 MPa). In the test the load is gradually increased until galling is observed, thereby establishing the threshold galling stress. After the test the surfaces are visually inspected and a descriptive grade is given to characterise the mating pair’s susceptibility to galling.

 
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Heat Treatment - the Expanite Way

Expanite’s solution heat treatment process for martensitic stainless steels such as EN 1.4021 or American standard AISI 420A used in this study, consists of three distinct steps. First is high temperature solution nitriding of around 1mm depth, providing increased surface hardness (even 100 HV above core) and improved pitting resistance, owing to up to 1.0 wt% of diffused nitrogen. Second step is a sub-zero deep cooling treatment, ensuring homogenous martensitic structure throughout the part. Lastly, low temperature tempering is conducted to improve core toughness. Such treatment provides excellent results in trials conducted under laboratory conditions such as ASTM G98 test, and can be readily applied to martensitic stainless steel parts to vastly improve their applicability.

Test Results, Standard Vacuum Hardening Against ExpaniteHigh-T

The test result shows a remarkable galling resistance afforded by ExpaniteHigh-T surface hardening. The results are summarised in Table 1. Two standard vacuum heat treated 1.4021 surfaces gall even under very low normal stresses of 0.7MPa (100psi). Galling threshold can be therefore defined below this contact pressure. Figures 1 and 2 show severe galling of vacuum hardened surfaces that originated in ASTM G98 test under a contact pressure of 5ksi (34MPa). In a mating couple with two ExpaniteHigh-T-treated surfaces of martensitic stainless steel 1.4021 shown in Figures 3 and 4, galling is completely mitigated until very high loads are applied (even at 10ksi or 69MPa of contact pressure). The different shade of the circular mark on the flat counterpart can be attributed to surface polishing. The galling threshold has been crossed between the loads of 69MPa and 103MPa. When comparing results on surfaces with and without ExpaniteHigh-T hardening treatement, a 150-fold increase of contact pressure is required to induce galling.

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