FOCUS ON MATERIALS
Figure 5 : Ridging measurements on seven samples cut from a 1250mm wide sheet ( a ) Positions of the samples ( b ) result of six parallel ridging measurements and their average value ( dashed line ) of sample C0 , ( c ) variation of RI across the seven samples .
The RIs of all seven specimens are shown in Figure 5 ( c ). Four of them are similar to C0 , the centre sample . The two samples furthest from the centre of the sheet showed RI values of about one unit higher . This could possibly indicate a locally different texture in the FSS sheet .
To examine the influence of the profilometry technique used , samples were measured with both the 2D stylus and the optical 3D profilometer . The resulting RIs are plotted against each other in Figure 6 . The good correlation shows that the RI method is sufficiently independent of the technique used .
RI compared to traditional visual assessment
The final check on the RI was to compare it to the commonly applied visual inspection and rating . A set of five samples from different batches was rated by five people familiar with the visual inspection of stainless steel surfaces against an arbitrary scale of 0 to 10 , as shown in Table 1 .
The variation in ratings given by different people was huge , in some cases . However , the average result produced by the reference group and the RI measurements gave the same ranking between the specimens . The average rating numbers correlated well with the RIs .
Establishing the ridging index ( RI ) as a repeatable and easy-to-use method of measuring ridging is a critical step for users and manufacturers of ferritic stainless steel . Because of its importance , Outokumpu is making the RI methodology freely available to all interested parties . n
EDITOR ’ S NOTE
In ferritic stainless steel ( FSS ), the undesirable surface defects of ridging can appear during deep drawing . A stripe pattern , parallel to the rolling direction , can manifest as narrow banded areas where the steel strip or sheet is severely etched .
Figure 6 : RIs of 36 FSS sheet samples measured with a 2D stylus profilometer ( RI2D ) and the optical 3D profilometer ( RI3D ). Plotted against each other they demonstrate good correlation .
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Sample Person 1 Person 2 Person 3 Person 4 Person 5 Average Ra ( μm ) Rz ( μm ) Pc RI A 4 3 3 2 2 2.8 0.46 9.6 0.22 2.1 B 9 7 5 9 5 7.0 0.35 25.5 0.30 7.5 C 1 0 1 1 1 0.8 0.11 4.2 0.28 1.2 D 8 5 3 7 4 5.4 0.48 13.7 0.27 3.7 E 7 5 8 3 8 6.2 0.35 11.1 0.40 4.4
Table 1 : Results of the visual inspection and rating of five sheets from different batches strained to 15 % along RD , ( scale between 0 and 10 ) with different intensity of the surface defect . The measured RI is given for comparison .
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