Q2. What is the effect of the chromium on the friction coefficient of the contact?
When the contact has totally become iron oxide on iron oxide, a stabilization of friction coefficient is observed corresponding to a chemical and thermal stabilization of the contact.
Q3. What is the effect of the sheared iron oxide scale on the pin surface?
the compactness of the sheared iron oxide scale on the pin surface constitutes of a barrier to the oxygen diffusion towards the metal–oxide interface and slows down the oxidation of inner MC carbides.
Q4. What is the origin of the iron oxide scale at the pin surface?
The iron oxide scale at the pin surface goes on growing during friction and comes in contact with the Nb-oxide grains agglomerated in the disc craters.
Q5. How does the pin surface temperature increase after a few minutes?
When the pin is put on the disc at 950 ◦C, its temperature increases very quickly owing to the important radiation from the disc surface and the thermal conduction by contact.
Q6. What causes the cutting of the excrescences of oxidised carbides?
The roughness of the oxidised disc surface due to the columnar cells of Fe-oxides causes a cutting of the excrescences of oxidised Nb-carbides or (Nb, V)-carbides present at the pin surface.
Q7. What is the tribological behaviour of the oxides in the hot rolling process?
De facto the tribological behaviour of the oxides is very complex in the hot rolling process because it is influenced by the nature and the physical properties of the oxide scales into the contact.
Q8. What is the coefficient of friction between the work rolls and the strip?
the coefficient of friction generated between the work rolls and the strip usually tends to decrease with the increase in thickness of the oxide scale on the rolls.
Q9. What is the role of oxides in the mechanical resistance of the roll?
they can contribute also to the damage of the work rolls according to two mechanisms:• on the one hand, they oxidise differently from the matrix and the formed oxides may have a different influence on friction and wear than the rest of the matrix; • on the other hand, under thermo-mechanical stresses, superficial cracks may occur on the roll and follow the network of primary carbides.
Q10. What is the effect of the creation of an oxide scale on the roll barrel?
In this way, the creation of an oxide scale on the roll barrel has a direct impact on the operating cost of a hot mill by reducing the damage both on the rolls and on the rolled sheet at the same time as the energy consumption.
Q11. What is the effect of the shearing stresses on the pin and disc?
The shearing stresses are so high that phenomena of mechanical fatigue and delamination of the oxide scale appear at the pin contact surface.
Q12. What is the criterion for the thickness of the oxide scale?
The H criterion is defined as the rate between the mean thickness of the oxide scale formed on the strip material and the penetration depth of heat in the oxide scale.