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Hot Working

MCHN 1319 Ch 9
by

Jared Cammon

on 24 November 2015

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Transcript of Hot Working

From the Ingot...
Bloom-formed from ingots with square cross section 6 x 6 or larger
Strand Casting
Molten metal continuously supplied to a reservoir where it is fed through a water cooled copper mold from which it emerges as a continuous strand of steel
Swaging
What happens during hot working?
Annealing - the heating of metal to soften it or reduce its strength
Forging
Gives metal the highest static, fatigue, and impact strength, out of all hot working processes
-Often used for tools
The basics of Hot Working
-NOT necessarily deforming metal at high temperatures
Hot Working
Hot Rolling
Forging Machines
Common types of Forging...
1. Open - die drop forging
2. impression- die drop forging
3. press forging
4. Swaging
5. Upset forging
Rotating a spindle between a set of cylindrical rollers mounted in a ring around the spindle
Metal worked at high
enough
temperature
(above the recrystalization temperature)
that no strain hardening, or cold working takes place

Key Test...
-measure hardness before and after,
if no strengthening occurs it is hot
working
Before metal cools, hot temperature anneals the metal and removes the cold working that would have occurred
The beginning is ingot casting - steel is poured into
ingot molds. Large dendritic crystals form, weakening the metal
Grains grow parallel to the direction of heat
high elongation = low strength
Ingots are filed between metal rollers. Then
recrystallization occurs forming smaller grains
Annealing then creates larger grains
During forming anisotropy occurs -
-allows metal to have mechanical properties
that vary with the direction
Because of the grain flow machined parts are not as resistant to failure as forgings
Three intermediate forms...
Slab - rolled from an ingot or a bloom with a
rectangular cross section 10in or more wide and
1.5 in or more thick.
Billet - rolled from a bloom 1.5 in
square or larger
Rolling Process
Blooms and ingots are further processed in structural rolling mills
two high
three high
Four high and Cluster
Tandem Mill
-Used for reducing stock by reverse action
-Requires complex mechanism to reverse direction of rotation
-Advantage over two high since rollers don't have to be reversed
-Stock is passed between top two rolls, then lowered and passed between lower tow rolls, then lifted and repeated
-Disadvantage in that it requires an expensive handling system

Use rolls of small diameter to reduce the dimension of stock
-smaller rolls use less energy and force
-need to be reinforced by larger rolls to increase their stiffness
Usually a number of mill stands in series
Requires a complex system for adjusting rotating speeds
to prevent uneven thickness and fracture of the metal
Sprays of water under high pressure cool and harden
the metal even further
Recrystallization
Metal is rolled at or near recrystallization temperature and grains are deformed. Since metal is at an elevated temperature recrystallization occurs where new, smaller grains are created. Leaving the metal at high temperature then anneals the metal (grain growth)
Factors Affecting the rate...
1. Initial grain size
2. Amount of deformation
3. Finishing temperature
4. Rate of cooling
Heating a metal to increase its plasticity
and then forming it.
Drop Hammer
Forging
Steam or air pressure is used to raise the hammer and force it down on heated metal
Open die forging
Metal is squeezed or hammered to increase
its length and decrease its cress-section

-Skill is required by the operator in giving the
hammer just the right amount of force
Impression die forging
- When shaped upper and lower dies are used
This increases production rates
Ensures repeatability
Usually progressive dies are used
Mechanical Press
Have an eccentric shaft or crank that moves the ram down and back
Compared to drop forging machines...
-Problem with jamming of ram at the bottom
-operate more quietly
-allow more accurate control
-produce higher force and energy
(Chosen when part needs closer tolerances)
Fewer blows required per forging
Less operator skill is needed
Motion is to squeeze rather than
strike the metal
Shorter forging-to-die contact
extends life of the die
Vertical Screw Press

Hydraulic Press
Forging Dies
Three Groups...

Open dies
Impression dies
Flashless dies
Uses a vertical screw instead of a crankshaft to transfer flywheel energy to the ram

Provides uniform force through the stroke unlike the drop hammer
The slowest types but also exerts the highest forces (compared to other mechanical presses) needed for large forgings.

Ram is powered by hydraulic cylinders
Extrusion
Forcing metal through a die (like toothpaste from a tube)
Adv - Allows manufacture of intricate cross-sectional shapes
Disadv - It is more expensive than rolling
-Metal is placed into a container
-a ram is pushed
-a work billet is forced to flow through the die
A die is mounted on the ram
As the ram is pushed against the
work billet material flows through
the die opening and through the ram
Disadv- extrusion has to be small enough to go through
opening in the ram
Adv - work imparted to the metal is uniform which makes
dimensional control better
Disadv - there is relative motion b/w billet and container.
Meaning friction between billet and container leads to non-uniform work on the metal
Direct Extrusion
Hot Drawing
Cross section of a bar, rod or wire is reduced by
pulling
it through a die opening
Extrusion = Push though die Drawing = pulled through die
Hot Spinning
Shaping flat or preformed metal discs over a rotating form
Non Destructive Testing
Ultrasonic Inspection - high frequency sound waves
sent through a test piece. A receiver on the other side
picks up the ultrasound waves and displays them
Radiography Inspection - uses x rays to detect cracks
What might be some disadvantages?
Magnetic Particle Inspection - uses magnetic field of
piece to reveal very small surface cracks
Fluorescent penetrants - spray on for visual
examination and use a fluorscent light to spot
cracks
Other mechanical Presses
Full transcript