Thermomechanical- Thermal Paths and History Flashcards
What are the important effects of thermal activation?
Low working forces
Extended ductility
Microstructure modification
Example features of microstructure modification
Recovery
Recrystallisation
Grain coarsening
Dissolution/precipitation
Phase transformation
What is the most important process variable and why?
Temperature. Due to the thermal activation of the processes given in previous cards
Graph of temperature vs distance from surface for different contact times with rolls
Very steep concave curve up then flattens to a constant internal temperature. Increased contact time means less steep curve so further distance in from surface until constant temperature (which is same for all contact times).
What does penetration distance depend on?
This is for heating through the surface or cooling by a tool. Depends on thermal diffusivity and contact time
Formula for penetration distance
dp proportional to rt(D tc)
Where d sub p is penetration distance
D is thermal diffusivity
t sub c is contact time
What 5 things does heat transfer during a metal forming operation involve?
Convection (natural and/or forced, air or water).
Radiation (increasingly important at higher temperatures).
Conduction (witching workpiece and tool/roll).
Interface heat transfer coefficient (e.g conduction through surface oxide and maybe coolant or lubricant).
Heat released during plastic deformation.
Heat released during plastic deformation
Virtually all the work done during plastic deformation under hot working conditions is released as heat within the metal
Fourier’s first law for heat flow
dQ/dt=kAdT/dx
W is heat flow
k is thermal conductivity
A is CSA through which flow is taking place
T is temperature
x is distance and t is time
Stock issues to be considered when modelling deformation through a roll gap
Inhomogeneous elastic-plastic deformation.
Temperature gradients.
Microstructure evolution.
Surface oxide scale.
Surface finish.
Profile.
Grade chemistry.
Rolling schedule.
Roll issues to be considered when modelling deformation through a roll gap
Elastic deformation.
Thermal cycle.
Surface oxide scale.
Wear.
Fatigue.
Profile.
Surface finish.
Roll metallurgy.
Rolling schedule.
Interface issues to be considered when modelling deformation through a roll gap
Friction and heat transfer between stock and roll under wide range of load, shearing, T differences, lubrication conditions, with an oxide scale which can deform plastically or fracture.
How can oxide scale fragments affect rolling?
Where the oxide scale fragments are present, their top surface will exit the roll at the same level as the top surface of the bare metal and therefore press the metal below them down. The rolled surface is therefore uneven?
Methods of removing or accounting for oxide scale
Hydraulic descaling: partly detached scale is first to be part removed by combination of thermal shock and water pressure.
Mechanical descaling: scanning electron microscope image, FE model prediction with multi-layer oxide scale
