[ Surface Hardening ]
[ Surface Hardening ]
Figure 5 . Untreated AISI 316L test specimens after ASTM G98
Figure 4 . Wear measurement according to ASTM G133
procedure ( Figure 4 ), the test specimen is subjected to the reciprocating motion of 100Cr6 counterpart , under a contact pressure of 25N . The wear volume is determined after a sliding distance of 100 meters . The results are shown in Figure 2 . The 316L sample with this process is 125 times more abrasion-resistant than the untreated sample . Following the ASTM G133 , an ASTM G98 galling test ( cold wear ) was carried out with test specimens made of AISI 316L , the results of which are shown in Figures 5 and 6 . In comparison to the unhardened test specimens , which already exhibit galling at a contact pressure of 35 bar , the test specimens hardened with SuperExpanite ® do not show any galling even at a contact pressure of 2758 bar . This is even though the first plastic deformations occur at this pressure due to the yield strength being exceeded . The risk of galling is therefore eliminated in the case of both friction partners being hardened .
Corrosion resistance Although the basic intention of conventional surface hardening processes is to produce harder surfaces , this is usually at the expense of corrosion resistance . However , tests have shown that 316L samples hardened with SuperExpanite ® can spend up to 1,000 hours in a salt spray chamber without showing signs of corrosion ( Figure 7 ). In some cases , the pitting corrosion resistance can be significantly increased by the surface hardening process , even beyond the level of the unhardened base material ( Figure 8 ). This effect is caused mainly by the large amount of nitrogen dissolved in the surface layer . A common formula for calculating the pitting resistance equivalent number ( PREN ) for alloys not containing tungsten , is given below ( Eq . 1 ). It is easy to notice a relatively large 16x modifier in front of the nitrogen content expressed in weight percent ( wt . %). This means that even small amounts of nitrogen have a significant positive influence on pitting resistance .
Eq . 1 : PREN = % Cr + ( 3.3 x % Mo ) + ( 16 x % N )
With the assumption of applicability of Eq . 1 for large amounts of dissolved nitrogen 1 usually between 5 and 13 wt % 2 the PREN number can be calculated . For the AISI 316L alloy the calculated PREN numbers then range from around 100 even up to 230 . A minimum of 4-fold increase over untreated material which has a PREN number of around 24 . The underlying ExpaniteHigh-T zone can be characterized with a PREN number of around 30 ,
Figure 6 . SuperExpanite treated AISI 316L test specimens after ASTM G98
providing additional improvement even if the case hardening has been damaged or worn down . A potentiodynamic polarization curve for SuperExpanite ® treated and reference AISI 316 material is shown in Figure 8 . This test is a common tool used to evaluate the pitting corrosion potential and characteristics of a metal . There are several highlights that can be derived from the graph , showing the undeniable positive influence of the treatment on the
Figure 7 . Results of the salt spray test on SuperExpanite hardened 316L alloy part ( far right ) as compared to competitors ’ solutions
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