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Features and Uses of A2 Cold Work Tool Steel
A2 is a medium alloy, air hardening tool steel
which means it possesses excellent non-deforming characteristics and is capable
of being vacuum hardened.
Because of this, where tools of large mass are being produced, A2 is more
suitable than oil or water hardening tool steels.
After heat treatment, it offers good abrasion resistance coupled with medium
toughness positioning it in terms of application suitability between the
tougher, but less abrasion resistant oil hardening grades and the more hard
wearing, but less tough, higher chromium grades of tool steels.
In this regard, due to its lower chromium content, the wear resistant properties
of A2 are not as great as those found in high carbon, high chromium steels such
as D2 and D3 but it is much easier to machine than these qualities, having
machinability factors comparable with most oil hardening tool steels and can be
ground to a cutting edge which is less likely to crumble in service.
Typical Applications
A2 gives good results when used for cold forging dies, coining
dies, cold heading dies, shear blades, knurling tools and blanking and punching
tools.
It is also used widely to produce cutting knives for paper, wood, fibre and
resin bonded material. It also finds wide use as material for machine slides.
This steel is suitable for electro-discharge machining (E.D.M.), either annealed
or heat treated (subject to a maximum heat treated hardness of 60 HRc).
Available Forms
Working and Heat Treatment
Forging
Pre-heat at 650°/680°C before raising the temperature to
1050°/1100°C and soak until the steel is uniformly heated.
Initial hammer blows should be light due to the comparatively high hot strength
of the steel, and the forging temperature must be maintained at about 1000°C
until the material begins to yield. Re-heat as often as necessary to complete
forging. Final forging should not be carried out below 900°C.
Because of the air hardening characteristics of this steel, slow cooling after
forging is essential to avoid the possibility of cracking.
Finished forgings should always be annealed.
Annealing
The steel is normally supplied in the annealed condition.
Re-annealing will only be necessary if the tool maker has forged the steel or if
a hardened stool has to be re-machined or re-hardened.
To anneal this steel, heat slowly to 850°/870°C. After holding at this
temperature for one hour per inch of section, or at least a minimum of two
hours, the furnace should be lowered to 730°/750°C and held there for four to
six hours. Allow to cool very slowly in the furnace to 600°C or below before
removing the work to cool in air.
Stress Relieving
When tools are heavily machined or grinding has been carried out
it is advisable to stress relieve in order to minimise the danger of distortion
or cracking during the subsequent heat treatment.
To stress relieve heat carefully to 670°C/700°C, soak for a minimum of 2 hours
per inch of section and allow to cool in the furnace.
The tools may then be finish-machined, leaving on an allowance
for final grinding after hardening and tempering.
Hardening
Pre-heat slowly at 790°/820°C and thoroughly soak at this
temperature. Continue heating in the same furnace or transfer to another furnace
at the hardening temperature of 950°/980°C.
Allow sufficient time for the tools to become evenly heated through, withdraw
from the furnace and then either air cool or oil quench.
Sizes over 75 mm (3") in smallest section may not achieve full hardness on
air cooling and usually it is necessary to speed up the cooling rate in the
critical temperature range of 480°-200°C, the cooling rate through his
critical range may be increased by air blast or by oil quench.
The steel may be protected against scaling and decarburisation by heating in a
neutral salt bath. Alternatively the steel can be vacuum hardened.
Tempering is always necessary after hardening. And should be carried out as soon
as the tools are hand warm.
Martempering
Martempering is an alternative hardening procedure which may be
used when suitable salt bath equipment is available. By this method scaling,
internal stress, distortion and risk of quench cracking are reduced to a
minimum, especially for intricate tool design.
Preheat at 300°/400°C. Heat in a properly rectified salt bath at 950°/980°C
and hold for 12 minutes per inch of the section.
Marquench into salt held at 230°/250°C until the temperature of the component
has equalised for a time not exceeding 10 minutes and cool in air.
The salt must be washed off before tempering.
Tempering
Temper with the least possible delay after hardening and as soon
as the pieces have reached room temperature.
Refer to the tempering curve to determine the temperature at which to temper to
achieve the required hardness.
Heat the tools slowly to the required tempering temperature, soak for 1 hour per
inch of section (minimum of 1 hour) and allow to cool in air.
It is recommended that this grade is double tempered, especially where a
tempering temperature of 480°C or higher is chosen. The second temper being a
repeat of the first.
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