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cracked inside the furnace proper might be
used. The control of the carbonpotential to
adapt it to the fastener steels’ carbon content
quality is done for both processes
continuously with sampling tools like Lambda
or oxygen probes.
The time for austenitising and carbide
dissolving with cold formed low-alloy
fasteners varies between approximately 10
and 15 minutes, with previous good
annealing for easier cold forming prolonging
this time. Add to this the heating up and
soaking times with bigger diameters and
higher bulk heights on the belt requiring
more time. This implies that the throughput
Fig 2 Cast link belt furnace for a production rate of 3,000kg/h
≈ bulk height / furnace residence time
with opened maintenance front door.
depends on the fastener dimension, bulk
density and temperature distribution in the
station, a prewashing and dephosphating Further characteristics: appropriate also for
furnace proper. An example of a measured
machine, the hardening furnace, an oil smaller fasteners, the belt charging can be
temperature curve of fasteners and derived
quench, a postwasher, the (bright) tempering monitored easily from the outside; the belt
resident times in a hardening furnace is
furnace with subsequent soluble oil cooling drive is made as a moving honeycomb
shown in Fig. 4.
and a container refilling station. hearth plate or on support rollers, electric
The drop chute for the transfer of the
Continuously operating furnace lines for heating for smaller throughputs.
fasteners into the quench bath is heated to
fasteners of the belt type can be divided The continuous washing machines are
prevent the austenitised fasteners from
into: available as drum and conveyor version for
cooling prematurely and remaining soft. Its
• Cast link belt type furnaces employed for alkaline cleaning of the fasteners before and
quenchant curtain hinders rising vapour from
the hardening and tempering of more rugged after hardening. The pre-washing also
entering the furnace proper. Above it, on the
fasteners from M5 to M48 with throughputs comprises a dephosphating stage required
inside the drop shaft, a gas suction device
from 500 to 3,000 kg/h, see Fig. 2. Further for grade 12.9 by EN ISO 898-1, since the
further shields off the protective atmosphere.
characteristics of these furnaces are: the phosphate stemming from the wire coating
The martensite hardening of high-grade
rugged belt can be charged approximately may result in dangerous surface
fasteners from boron or higher alloyed steel
twice as much as a wire mesh belt and the decarburisation. The postwasher, apart from
is done in mineral quench oil to achieve the
furnace entrance may be closed by a cleaning off the quench oil, should also
critical cooling speed necessary for the
protective gas lock, both of which reduce guarantee that the fasteners are properly
austenitic transformation and on the other
considerably the heat and protective gas dried before entering the
hand to avoid bending or even cracking. The
consumption. The furnace is usually tempering furnace.
falling depth depends primarily on the
gas- heated with recuperative ceramic or cast
fasteners’ diameter, then on its alloy content.
steel radiation tubes. It is economically best Hardening
The falling speed in the quenchant varies
suited for large-scale production with big Hardening temperatures for fasteners
from 1 to 2 metre/second, nearly
heat treatment lots. range normally from 880 to 900°C. The
independently of the fasteners’ size.
purposes of the
The discharge is almost always done by a
furnace is to
belt conveyor with bolted-on flights, with
homogenously
smaller fasteners also being extracted by
austenitise and
means of a bucket-chain or magnetic
dissolve the
conveyor.
ferrous and alloy
Since most fasteners, due to heat transfer
carbides in the
physics, need rather a long time to cool
steel, as carbides
down, they hit the belt being still in their
or lower carbon
soft, red-incandescent, austenitic state. The
content in the
time, for example, for a M16 screw of
austenitic phase
20MnB4 steel to cool down to 500°C is
deteriorate the
approximately 10 seconds. This means that
hardness of the
the minimum quenching speed according to
Fig 3 Section of a wire mesh belt hardening
fasteners upon the
the steel quality must be fulfilled when
furnace with integrated oil quench for a
subsequent quenching.
hitting the belt by sufficient delivery of
throughput of 500kg/h, (courtesy of SAFED).
The process is carried
quenchant into the impact area. If the belt is
out under a neutral
running too slowly, with the aim to extract
• Wire mesh belt type continuous furnace endothermic protective atmosphere, enriched
the fasteners rather cold to avoid oil smoke,
lines for fasteners from approximately M2 to with either natural gas or propane as
the fasteners might amass on the flights and
M24 with smaller throughputs from correctives.
worsen the quenching speed for the
approximately 10 to 1,000 kg/h, see Fig. 3. Alternatively, nitrogen with methanol
single fastener then lacking
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