OUTSTANDING
GALLING PREVENTION BY USING SUPEREXPANITE HARDENING ON MARTENSITIC STAINLESS STEEL
Martensitic Galling Test ASTM 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 a standard ASTM test G98 for galling resistance of materials, the galling prevention results on the 1.4021 martensitic stainless steel with the SuperExpanite® surface hardening process are truly outstanding and show that Expanite® offers superior solutions to improve galling resistance.
ASTM G98 standard test for galling resistance
Table 1.: Description of ranks of surfaces in the ASTM G98 test standard
|
Key |
Rank |
|
O = No Damage |
0 |
|
B = Burnished |
1 |
|
W = Wear |
2 |
|
S = Scoring |
3 |
|
I = Incipient Galling |
4 |
|
A = Adhesive Transfer |
6 |
|
G = Galled |
10 |
Table 2.: Ranks given to treated test surfaces after ASTM G98 test
|
Pressure (ksi) |
Pressure (MPa)
|
Rank / Grade
|
Rank / Grade
|
|
10
|
68.9
|
1
|
1
|
|
20
|
137.9
|
1
|
1
|
|
30
|
206.8
|
1
|
1
|
|
40
|
275.8
|
1
|
1
|
|
50
|
344.7
|
1
|
1
|
|
60
|
413.7
|
1
|
1
|
|
70
|
482.6
|
1
|
1
|
|
80
|
551.6
|
1
|
1
|
Galling after SuperExpanite10 (NC Cryo)
The test result shows an outstanding galling resistance afforded by SuperExpanite10 (NC Cryo) surface hardening. The ranks for button and block surfaces at the specific test loads are shown in Table 2. Vacuum hardened 1.4021 surfaces gall under very low normal stresses of 0.7 MPa (0.1ksi), which was shown in more detail in a previous test. In a mating couple with two SuperExpanite-treated surfaces of martensitic stainless steel 1.4021 galling is completely mitigated even at extremely high loads (up to the tested maximum at 551.6MPa).
Notice that when comparing with and without SuperExpanite10 (NC Cryo), even 800-fold increase of contact pressure is not enough to induce galling. The burnishing marks on the test flats and the buttons’ mating surface can be seen in Figures 4-9. No significant difference can be seen between tests with the lowest and highes contact pressure.
During the test the torque required to rotate the buttons was measured (see Figure 3.), and for the highest force used, the value of around 27N.m was measured. The torque for 1.2379 alloy (around 750HV) was referenced in the test at above 200N.m.
Figure 1.: Microphotograph of SuperExpanite®10 (NC Cryo) on alloy 1.4021 (x40 magnification).
Specialized Solution for Demanding Applications
High performance materials require high performing solutions. The SuperExpanite10 (NC Cryo) is a special surface treatment developed for martensitic alloys. The process consists of three steps: ExpaniteHigh-T that provides increased hardness up to 1mm depth, a sub-zero treatment ensuring homogenous martensitic structure and an ExpaniteLow-T, that creates in-situ a hardened 10µm thick layer of expanded austenite (see Figure 1.) with a surface hardness of around 1200HV.
However, this step is also responsible for tempering of the core material. The hardness profile for SuperExpanite10 (NC Cryo) treated alloy 1.4021 can be seen in Figure 2.
The average core hardness after treatment was measured at 625HV1. A drop in hardness of the underlying ExpaniteHigh-T treated material starts to show up above 800µm and reaches the core hardness values at around 1.4mm.
Figure 2.: Hardness profile SuperExpanite10 (NC Cryo) on alloy 1.4021 (fitted power curve to guide the eye).
Figure 3.: Torque measurements (N.m) of SuperExpanite10 (NC Cryo) treated surface of alloy 1.4021 in ASTM G98 test.
Figure 4.: Buttons after test. Left: 10ksi; Right: 20ksi.
Figure 5.: Buttons after test. Left: 30ksi; Right: 40ksi.
Figure 6.: Buttons after test. Left: 50ksi; Right: 60ksi.
Figure 7.: Buttons after test. Left: 70ksi; Right: 80ksi.
Figure 8.: Flats after test with visible burnishing marks. Left to right – 10 to 50 ksi.
Figure 9.: Flats after test with visible burnishing marks. Left to right – 60 to 80 ksi.
