During open die forging process using two flat and parallel dies, a solid steel disc of initial radius (R_IN) 200 mm and initial height (H_IN) 50 mm attains a height (H_FN) of 30 mm and radius of R_FN,
Along the die-disc interfaces,
(i) The coefficient of friction (μ) is:
μ = 0.35 [1 + e–R_IN/R_FN]
(ii) In the region Rss ≤ r ≤ R_FN sllding friction prevails and P = √3 K.e^2μ(R_IN–r)/H_FN and ι = μp,
sllding friction prevails and
Where p and ι are the normal and the shear stresses respectively; K is the shear yield strength of steel and r is the radial distance of any point.
(i) In the region 0 ≤ r ≤ R_IN, sticking condition prevails.
The value of Rss (in mm), where sticking condition changes to sliding friction is