FOCUS ON MATERIALS
The ridging index ( RI ) is a repeatable and easy-to-use method of measuring ridging for users and manufacturers of ferritic stainless steel .
“ The creation of a new ridging index ( RI ) is an important development for the forming of ferritic stainless steel ( FSS ).”
By
Suresh Kodukula , Researcher , Outokumpu ( R & D Centre )
Manufacturers seeking a material that provides their finished products with high corrosion resistance and an excellent surface finish often turn to ferritic stainless steel ( FSS ). It offers excellent forming properties in both deep drawing and stretch-forming processes .
However , forming FSS can sometimes result in a surface defect known as “ ridging ”, the small parallel elevations and valleys which develop in the rolling direction ( RD ) when the sheet material is elongated ( see Figure 1 ).
Ridging can mar the aesthetic appeal of the finished product , requiring polishing to remove it . Until now , the only way to assess the severity of ridging was the visual examination of test specimens , a method which is both subjective and lacks repeatability . This prompted Outokumpu to undertake a research programme to create a new ridging index ( RI ) based on surface measurements made by profilometers .
The main intention when introducing the RI methodology is to offer customers a practical way to assess the impact of their forming operations on the particular FSS that they are planning to use . The possibility of ridging is influenced not just by the composition of the FSS . Many other factors are also involved , such as the way the sheet has been processed . In fact , apparently identical samples of FSS produced by different manufacturers can exhibit significantly different levels of ridging .
The Ridging Index ( RI ) is also useful for researchers to determine how modifications made at the various processing
Figure 2 : Photographs and 3D surface profiles of an 80mm specimen of grade EN 1.4016 FSS ( a ) before and ( b ) after 15 % elongation parallel . The scale of the 3D profiles varies between -50μm ( black ) and + 50μm ( white ). stages - from liquid metal to sheet production - influence the occurrence of ridging .
How does ridging occur ?
Figure 1 : Ridging appears in the rolling direction ( RD ) 180 ° apart ( both inside and out ) when ferritic stainless steel is formed .
Ridging is the result of anisotropic plastic deformation caused by colonies of different crystal orientations in the rolled sheet . Its likelihood is largely pre-determined during the casting process , when the crystal structure is formed .
The maximum level of ridging occurs when FSS sheet is strained along its rolling direction ( RD ) and is minimal or absent when elongated along its transverse direction ( TD ).
The distance between two ridges is usually in the range of a few millimetres . The additional profile height that is introduced by the ridging may be up to ± 50μm , depending upon the applied strain . See Figure 2 .
18 | sheetmetalplus . com | ISMR June 2021