Temper Rolling,Luders Bands

when the stress strain curves of metals deformed in ension are plotted, two basic types of curves are observed.One curve exhibit sharp yield point,where the stress rises with almost negliible plastic deformation toa point calld upper yield point.At this point the material begin to yield , with a simultaneous drop in flow stressrequired for contnued deformation .This new yield point is called lower yield point and corresponds to an appreciable plastic strain at an almost constant stress.Evenually the metal starts to harden with in incrase in the stress necessary for additional deformation. After this occurs , there is little difference between the appearance of the stress strain curves for metals with a yield point and those without it.

The sharp yield point is an especially important effect because it occurs in iron and in low carbon steels . Its existance is an considerable concern to manufacturers who stamp or draw thin sheets of these material in forming such objects as automobile bodies.Th significance of the yield point is this :once plastic deformation starts in a given area ,the metal at this point is effectively softened and suffers a relatively large plastic deformation . This deformation then spreads into the material adjoining the region which has yielded because of the stress concentration at the boundary between the deformed and undeformed areas.In general,deformation starts at positions of stress concentration as discrete bands of deformed material ,called LUDERS BAND.

The effect of using metals containing a yield point is to develop a roughened surface.This surface results from the uneven spread of the LUDERS BANDS which leaves striation on the surface ,commonly called STRECHER STRAINS.

Annealed steel sheet is often given a slight reduction in thickness by rolling which amounts to about a 1% strain.This is called TEMPER ROLLING(SKIN PASS), and it produces a very large number of luders band neclei in the sheet . When the metals is deformed latter into a finished product , these small bands grow,but because of there small size and close proximity to each other ,the resulting surface roughening is greatly reduced.

Rferences:i)Physical Metallurgy,R.E REEDHILL,
ii)http://www.ccr.buffalo.edu/etomica/app/modules/sites/MaterialFracture/Images/SSPicture3.jpg