This page contains a Flash digital edition of a book.
84
circulation flow. The quenching be taken for setting and fine-tuning of the depths in the liquid of around one metre.
UIforwardUIforwardUIforward
technique supposes that the furnace parameters. Another important In most cases the desired surface property
fasteners’ alloy content is high enough to quality determining factor is the uniform for fasteners is still blackened/oiled. The
reach the critical quenching speed in the distribution of the fasteners on the belt. If pores in the Fe3O4 oxide layer of 1-3µm
fasteners’ total section. For big bolts up to they pile up, heating-up and soaking times thickness are being filled with oil residues.
M48 alloys like 34CrNiMo6 have to be used can vary, so that final mechanical properties Condition for this blackening is a tempering
to also harden the core. The risk with big scatter strongly.
temperature of minimum 400°C. The
bolts is cracking, due to the high inner The tempering furnaces are also executed
resulting surface protects sufficiently against
tensions caused by heat transfer during as wire mesh belt or cast link belt conveyor
corrosion during storage of the fasteners.
quenching. The known hardness of the types, matched to the throughput of the
Common problems arising with the heat
fasteners depending on their carbon hardening furnace. The throughput is again a
treatment of fasteners
percentage with 99.9% martensite can
function of the bulk density. Since the heat
The mass bulk production of fasteners
easily be used to confirm the completeness
transfer in the typical tempering range from
entails specific problems that must be taken
of austenite transformation by taking
350°C to 570°C depends mainly on the
samples during production.
care of and minimised during design and
convection imparted by the axial cyclones or
operation of the continuous furnace plants.
Figure 4 Rev.xls Dia 850 kg-h Most important are:
• Damage on threads due to handling of the
1000 20
bulk flow, especially in the container
900 18
loading station and pre-washing machine,

when fasteners are still soft. Fine-threading
800 16
might be done only after the heat-
soaking austenitizin
treatment.
700 14
heating up • Parts mixing of different heat treatment
600 12
lots on the belts.
residence time with bandwidth dT <
• Bending of fasteners with a length/
500 10
diameter ratio of approximately greater
400 8 than 12. These fasteners must be
temperature [°C] _
straightened afterwards or heat-treated on
300
T belt right
6
temperature dT [K] _
a rack in batch type furnaces.
T belt left
200 4
• Contamination by alkaline deposits of the
T belt center
high temperature furnace proper and its
100
T scatter
2
influence on the heat treated fasteners.
This can be reduced by a double-rinsing
0 0
0 5 10 15 20 25 30 35 40 stage after dephosphating in the
time [min]
pre-washing machine and employing a
20/6/08
triple water cascade with fresh water
Fig 4
Thermocouple drag test of temperature uniformity
directly sprayed on the fasteners in the
in a hardening furnace with 1,000kg/h
final washing stage.
Tempering
Conclusion
The tempering determines the final
fans, a lower bulk density (i.e. slim
As discussed above the challenge in the
properties of the fasteners. The tempering
fasteners) allows for higher bulk heights
heat-treatment of fasteners lies chiefly in
temperature and time of approximately 60 to
with fasteners’ final properties remaining
120 minutes are the parameters for final
constant. To achieve the powerful the mass – production, which requires that
tensile strength and toughness. As a rule of
atmosphere recirculation the furnace is each single piece exits the process with the
thumb the higher the temperature and the
divided into multiple zones and precisely
same values of final tensile strength and
longer the dwelling time of the fasteners in
controlled.
toughness. Today’s furnace developments
the tempering furnace the lower will be the
As an alternative to the air or nitrogen
are aimed at reducing the statistical
tensile strength and the higher the
atmosphere the so-called bright tempering
tolerance bands of these properties.
toughness values. The disintegration speed of
process with protective gas is being used.
the martensite is also an exponential
The aim is to reduce the risk of hydrogen
function of the still non- transformed
embrittlement. Since the modern zinc
martensite, so that the times both for
lamellar surface coatings require a bright
heating up and dwelling on temperature are
surface, i.e. not even permitting tempering
summed up. To get results within narrow
colours, the pickling normally done to reduce
tolerance bands it is not sufficient to rely on
the oxide layer as a surface preparation may
set recipes for carbon / alloy content and
be reduced or even left out. This article was first published in
fastener grade. The metallurgic analysis (18
After tempering, the fasteners are cooled HEAT PROCESSING Issue 1/2008,
elements) for each heat treatment lot has to
down in a soluble oil bath with falling page 22-24.
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140
Produced with Yudu - www.yudu.com