Note: Descriptions are shown in the official language in which they were submitted.
~/27677
_ROCESS FOR HARDENING T~IE CUTTING EDGES OF SAWS KNIVES
CUTTING T LS AND PUNCI-IING TOOLS
The invention relates to a process for hardening the cutting
edges of saws, especially for working wood, as well as knives,
cutting tools and punching tools for working wood, paper,
paperboard, plastic, leather or textiles, by means of an energy
beam which is passed over the areas of the tool to be hardened.
Saws, knives or cutting tools and punching tools for the said
area of application demon6trate wear at the cutting edges. The
useful life of these tools depend~s Oil the quality of the cutting
edge (material used, hardening process), on the material cut and
on the cutting output. After the end of their useful life, these
tools are either reground or scrapped. Many types of saws,
knives, cutting tools and punching tools are made of carbon
steel, which can easily be hardened by heating and subsequent
rapid oolinyO But since such hardening is always connected with
a reduc~ion in strength, gre~t hardness is desired only in the
area oP the cutting edges. The other parts of a saw, a knife or
a cutting tool are supposed to demonstrate lesser hardness, but
greater strength.
Known methods ~or partial hardening of the cutting edges use
electron or laser beams as the energy source. The complicated
devices which are required Por carrying out such proc~s~es are a
disadvantage in hardening with electron beams or laser beams.
For this reason, such processes have been used hardly at all in
practical situations.
Another known hardening process is inductive hardening. After
grinding of the cutting edge, the cutting edge area is heated by
an eddy current, generated by a high-frQquency m~gnetic
alternating field, and hardened by rapid cooling.
Furthermore, it is known ~rom WO 83/00051 to carry out surface
hardening of flat areas by means of a plasma beam. ~lardening of
cutting edges by means of plasma beams was not taken into
consideration until now, however, because such plasma beams
demonstrate too little stability.
In saws, welding stellite onto the tooth tips is known. The
stellite material welded on is subsequently ground to the desired
pointed tooth shape. However, this process is very complicated.
It is the task of the present invention to indicate a process for
hardening the cutting edges of saw, knives, cutting tools and
punching tools in which an energy beam which is simple to produce
and cost-e~fective to operate is used.
According to the invention, it is therefore provided that as the
energy source, a plasma beam is used, where the plasma beam i5
guided at a relative velocity of 5 to 100 mm/sec with refer~nce
to the tool, and where the distance of the outlet no~zle of the
plasma torch from the cutting edge lies between ~ and 14 mm, and
where furthermore the power of the plasma beam lies between 1 and
lO kW, and the diameter o~ the outlet nozzle of the plasma torch
lies between 3 and 7 mm.
Surprisingly, it was found that with a precisely coordinate
constellation of parameters, it is certainly possible to use a
plasma beam for hardening the cutting edges of these tool[s],
where it is furthermore possible only at these parameters to
achieve hardening by self-quenching, in other words without
additional cooling, for example by air or water.
The heating and cooling speed is adapted to optimum values at
different material thicknesses and cutting edge angle~ with the
forward velocity v. For thinner blade thicknesses, especially
below 3 mm, i.e. for smaller cutting edge angles, especially
below 25, the ~orward velocity must be selected higher, since
otherwise the cooling rate is too small for sufficiently high
hardening, due to the limited heat conduction into the base
material. For greater blade thicknesses, i.e. cutting edge
angles, the forward velocity can be selected lower, to achieve
larger hardening zones.
Plasma beams are produced by ionization of argon or nitrogen, or
of mixed gases. Ionization takes place by electric arc discharge
or by excitation with a high-frequency electromagnetic field. By
means of a suitable formation of the electrodes or the nozzles, a
beam in which temperatures of up to 15,000 C are reached in the
axis is achieved.
If such a plasma beam is passed over the ground cutting edge of a
saw, a knife or a cutting tool at the parameters according to the
invention, a local area of the cutting edge heats up, at heating
rates of up to 5000 K/sec. After termination of the energy feed,
the cutting edge c0015 by self-quenching, i.e. by heat conduction
into the base material of the tool, at cooling speeds of up to
1000 K/sec. This results in a fine-grain martensite structure
with hardnesses up to 1000 HV (Vickers hardness).
J
However, it i5 critical in such processes that the cutting edge
is not allowed to melt during the heat treatment. Nevertheless,
sufEicient heating must be present in the area o~ the cutting
edge, in order to ensure the desired hardening. This is only
achieved at the parameter constellations indicated above.
Particularly good conditions for hardening result at the
following values:
Power of the plasma beam: 1 to 5 kW
Diameter of the beam at the outlet
nozzle of the plasma torch:4 to 5.5 mm
Distance of the outlet nozzle of the
plasma torch from the cutting edge: 3 to 9 mm
Relative velocity of the plasma beam
with reference to the cutting edge: 15 to 50 mm/sec
Preferably, a knife or cutting tool is guided through the plasma
beam by mechanical movement along the cutting edge, where the
axis of the plasma beam coincides with the axis of symmetry of
the cutting edge. In this manner, the most uniform possible heat
effect is achieved over the flanks of the cutting edge. In the
case of saws, the plasma beam is guided over the back of the
teeth, in the area of the upper cutting edge, by mechanical
movement of the plasma torch perpendicular to the saw blade. In
this manner, the most uniform possible heat effect is achieved
over khe entire length of the cutting edge of the tooth tip. For
cer-tain saw shapes, it is advantageous and simpler technically to
guide the plasma torch along the saw blade without perpendicular
movement. By electromagnetic deflection by means of a coil,
which is arranged in the area between the cathode and the bottom
edge of the nozzle, a defined broadening of the plasma beam and
thus an adaptation to the tooth geometry (e.g. for cross saws) is
possible. The difference from the known method of
electromagnetic deflection of the plasma beam for melt treatment
(hardfacing~ consists of the fact that there, the effect of the
electromagnetic field takes place in the area between the bottom
edge of the nozzle and the workpiece surface. In this method, a
cathode spot of the arc must be located on the workpiece surface.
This known method does not work in plasma hardening, since here,
the arc must burn between the cathode and the bottom edge of the
nozzle.
A reduction in the energy re~uirement in hardening can be
achieved in that the plasma beam functions in pulse operation, at
a pulse frequency f, with f = forward velocity of the saw blade
divided by the distance between teeth, wh~re the pulse duration
lies in the range from 0.2 to 0.8 sec.
For knives, it is furthermore possible that the axis of the
plasma beam covers a certain angle (e.g. goo, 135D or half of the
cutting edge angle) relative to the axis of symmetry of the
cutting edge. In this way, a distribution of the hardening zone
which is asymmetrical to the axis of symmetry, and thus, an
adaptation to special wear situations, can be achieved. For
knife blades with a thickness of more than 5 mm, in particular,
good adaptation of the hardening zone to various cutting edge
geometries is thereby possible.
In the following, the invention is explained in more detail on
the basis of the attached ~igure6:
Fig. 1 shows a schematic representation of Wle basic arrangement
of the plasma system, using the example of saw hardening.
The plasma torch 1 generates a plasma beam 2 from the gas fed to
it, using an electric arc discharge, which beam exits at the
outlet nozzle of the pla~ma torch 1. The distance b~tween the
outlet nozzle and the cutting edge is a. The plasma bea~ is
directed at the -tooth top 5 of a saw tooth 4 and heats this area.
After termination of the energy effect, the heated area cools
rapidly and hardens. Subsequently, the saw blade 3 is moved
forward and the plasma beam 2 is direc~edl at the tooth top 5a oP
the following tooth 4a.
Figure 2 shows the area of the tooth tip of a saw blade in
detail, in an axonometric representationO The plasma beam 2 has
a diameter d and is moved ~ither along the cutting edge 6 or in
the direction of the toothing at a relative velocity v.
Figure 3 shows a schematic repreæentation of the basic
arrangement of the plasma system, using the example of knife
hardening. The plasma beam is directed at the cuttiny edge 9 of
the knife at an angle ~, and is moved along this edge at the
velocity v, where this edge is heated. After termination of the
energy efEect, the heated area cools rap~dly by self-quenching
and hardens.
Fiyure ~ shows a schematic representation of a cross-section
through the plasma torch in the area of the outlet nozzle. An
~ 3 .~3 ~J
electromagnet 10, arranged in the area between the cathode 8 and
the bottom edge 11 of the nozzle, causes widening of the plasma
beam 2 by high frequency deflection of the arc within the nozzle
area.
The following embodiments are intended to explain the use o~ the
process in more detail:
Example 1: Hardening of a reciprocating saw
Material: Band steel B412 (alloy steel with 0.8g% C, 0.3% Si,
0.3% Mn, 0.5% Cr; 0.4% Ni, 0.25% V), 45 teeth, distance between
teeth 30 mm,
Width b of the cutting edge: 3.5 mm,
Hardness in the untreated state 420 HV.
Plasma power (kW) ¦ 2.5 ¦3.5 ¦ 2.0
Beam diameter (d in mm~ ¦ 4.0 ¦ 4.0 ¦ 4.0
Distance (a in mm) ¦ 5.0 ¦6.0 ¦ 4.0
Forward velocity ¦ 25 ¦30 ¦ 20
(v in mm/sec)
Gas through-flow (l/min~ ¦ 7 ¦ 10 ¦ 7
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Maximum hardness (HV) ¦ 920 ¦940 ¦ 900
Practical cutting tests in saw mills resulted in an increase ln
useful life by a factor of 5.
s~
Example 2: Hardening of a circular saw
Material: Saw steel B412, 50 teeth, distance between teeth
30 mm,
Width b of the cutting edge: 4.0 mm,
Hardness in the untreated state 410 HV.
Plasma power (kW) ¦ 3.0
Beam diameter (d in mm) ¦ 4.0
Distance ~a in mm) ¦ 5.0
Forward velocity ¦ 30
(v in mm/sec~
Gas through-flow (l/min) ¦ 8
.'
Maximum hardness (HV) ¦ 900
Example 3: Hardening of a band saw
Material: saw steel B412, band length 6 m, distance between
teeth 15 mm,
Width b of the cutting edge: 1.5 mm,
Hardness in the untreated state 410 HV.
Plasma power (kW) I 1.5
Beam diameter ~d in mm) ¦ 3.0
Distance (a in mm) ¦ 5.0
Forward velocity 1 20
(v in mm/sec)
Gas through-flow (l/min) ¦ 7
Maxlmum hardness (HV) ¦ 900
A ~ J`J
Example 4~ Hardening of a punch knife for leather and textiles:
Material~ Band steel CK60 (material No. 1.1221)
Thickness: 2 mm
Hardness in the untreated state: 300 HV (Vickers)
Plasma power (kW) 1 2 4
Beam diameter (d in mm) 4 4
Distance (a in mm) 4 6 8
Angle between plasma axis and
axis of cutting edge (degrees) 0 0 0
Forward velocity
(v in mm/sec~ 25 35 50
Gas through-flow (l/min) , 5 5 5
Maximum hardness (HV) 860 890 9~o
-3 ~ "~
Example 5: I~ardening of a planing kniEe for woodworking
Materlal: 80 CrV 2 (material No. 1.2235)
Thickness: 8 mm
Hardness in the untreated state: 280 ~IV (Vickers)
Plasma power (kW) 2 3 5
Beam diameter (d in mm) 4 4 4
Distance (a in mm) 4 6 8
Angle between plasma axis and
axis of cutting edge (degrees~ 60 90 120
Forward velocity
(v in mm/sec) 20 30 40
Gas through-flow (l/min) 5 5 6
Maximum hardness (HV~ 840 880 905
