What are the two main classes in forging?
open and closed die forging
How is the transferred temperature from workpiece to die calculated in closed die forging? What is the aproximate heat transferral rate for hot forging?
alpha=15000 W/m^2K
What is the relation for static recristallization after hot forging?
What are typical operations in open die forging?
Cogging
for changing shape and reducing diameter
Upsetting
for disc production
Heading
upsetting of a bar end
What are the main application of open die forging?
obtaining large parts in a low number
pre shaping parts for subsequent closed die forging
What are the stages of closed die forging?
Heating
Preshaping
Forging
Flash removal
possible heat treatment
sand blasting
possible final machining
Explain the importance of lubrication in Forging and name 2 examples each.
used to reduce metal/die friction and favour the metal flow into cavities
reduces forging force
Hot-working: spray of graphite particles, glass particles
cold working: mineral oils, MoS2
What is the function of the flash gutter?
acts as a safety valve for excess material
ensures that metal filles all the recesses of the die cavity
What is the effect of shrinkage in forging?
Forging cavities need to have dimensions 1% greater then the product due to shrinkage after cooling.
What is meant by the term machine allowance in forging?
In general it is necessary to add a stock of 1-2 mm on the surface to be machined.
What should be considered when designing the corners of a forging die?
The corners should not be too sharp to allow an easy material flow.
What is the maximum force of a hydraulic press?
2.2-145 MN
How is the pressure for open die forging with friction calculated?
a=radius probe
h=height
x_2=cordinate of way
Sigma ef=effective stress after mises
Describe the stress distribution of open die forging with friction
How is the effective stress in cold deformation for open die forging calculated?
How is the effective stress in hot deformation for open die forging calculated?
How is the forging load calculated in the simplified approach?
What are typical values of µ for hot and cold forging?
cold: µ=0.05-0.1
hot: µ=0.1-0.2
What are typical lubricans for cold forging?
steels: mineral oil, phosphatin with soaps or MoS2
stainless steels: soaps and graphite
Cu: soaps, wax
Al: soaps, wax, phospahyting+ soaps
What is the parameter of setting and how can the effective strain be calculated with it?
For planar strain (w=depth>>2a):
What shape changing effect appears in real open die forging?
Barreling
Describe the forging load over the stages in closed die forging?
How is the forging load calculated for closed die forging?
Which quantities are used to calculate the forging energy?
forging load F
stroke height t
What increases barreling?
friction coefficient
decreasing the initial height to diameter ratio (aspect ratio=h_0/d_0)
What is described by the parameter M?
M describes the triaxiality factor
It is the hydrostatic stress divided throught the mises stress:
(the mises stress describes the multiaxial load)
What are the most important types of cracks in forging?
Surface and internal ductile cracks
What is the reason surface cracks form under 45° in Aluminium
Due to low m and n values the Al is prone to shear localizations which leads to a fast fraccture by reaching the critical effective strain fast. the shear leads to the characteristic angle.
Which phenomenas can lead to barrelling cracks during hot forming?
Hot shortness
red shortness
What favours the formation of internal shear cracks?
high strain rates
low thermal conductivity
high metal strength
low ambient temperature
How can surface cracks form in closed die forging?
Similar to barreling in regions of large triaxiality, with a combination of low ductility (due to for example hot shortness from increased temp through process)
What is the result of the inhomogenous distribution of strain in closed die forging?
Microstructural differences in different sections of the forged parts, which lead to different properties
What is the process window?
The process window is the material specific strain (or strain rate) and temperature area where a work piece can be deformed without defects.
What are adiabatic shear bands?
Due to high rates of deformation there is no heat transfer (=adiabatic), which makes the affected regions enter the region of hot shortness, which can lead to ductile failure.
For what stands HSLA?
High-Strength Low-Alloy steel <=> microalloyed steels
What is special about forging microalloyed steels?
no dynamic recristallization and almost no grain growth
=> very fine and good microstructure
subsequent normalizing treatment necessary (to avoid dissolution of alloyed carbo nitrides)
What damages a forging die?
Oxidation
Thermo-mechanical fatigue
Sliding wear (tribo-oxidation)
How does thermo mechanical fatigue work?
Oxides form on the surface, which are more prone to cracking. They crack and the oxidized part grows further into the material.
How can the sliding wear of a die be calculated? (describe)
The change in the workpiece diameter can be calculated by the deformation. With the maximum sliding for the original radious of the piece.
the wear h (as a unit of removed material) can be calculated by
What are surface treatments used for treating dies?
Nitriding
Hard chromium
Hardfacing of high alloy steels
Hardfacing of Ni and Co alloys
Thin films (PVD, CVD)
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