Plasma machining electrode and plasma machining device

Details Plasma machining electrode and plasma machining device Plasma machining electrode and plasma machining device

2000-11-22

2003-06-24

Van, Quang T. (Department: 3742)

Electric heating

Metal heating

By arc

C219S121390, C219S121500

Reexamination Certificate

active

06583378

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrode for the generation of plasma arcs used in plasma machining devices, and in particular, to an electrode having a heat-resistant insert made of hafnium, zirconium or alloys thereof, and to an improvement of the electrode structure with the purpose of improving the durability of the electrode.
In particular, the present invention prolongs the electrode lifetime in oxygen plasma cutting which is useful for cutting mild steel.
2. Description of the Related Art
Electrodes made of highly heat-resistant metals, such as tungsten (W), hafnium (Hf) or zirconium (Zr) are used in plasma machining devices, especially in plasma cutting devices. When the temperature of the electrode exceeds 3000° C. during the arc generation, it thermally emit electrons and operates as a cathode spot. Such electrodes can be broadly classified into two types, depending on their material. The first type uses tungsten, or tungsten into which small amounts of other elements have been added. The second type uses hafnium or zirconium.
These two types of electrodes are also used with different plasma gases (working gas: the gas that is turned into a plasma by the arc emission). Tungsten electrodes are used in plasma cutting devices using argon (Ar), helium (He), nitrogen (N
2
) or hydrogen (H
2
) either alone or as gas mixtures as the plasma gas. On the other hand, hafnium or zirconium electrodes are used in plasma cutting devices using oxygen or air as the plasma gas. That is to say, tungsten electrodes are used when the plasma gas does not contain oxygen, and hafnium or zirconium electrodes are used when the plasma gas contains oxygen. The reason for this is that tungsten alone has a very high melting point (about 3400° C.) and boiling point (about 5700° C.), but when it oxidizes, the melting point and the boiling point are lowered considerably (the melting point to about 1500° C. and the boiling point to about 2000° C.), so that it cannot be used as an insert anymore. In contrast, the melting point of hafnium and zirconium alone is a little lower (about 2200° C. for hafnium), but the melting point of their oxides is actually higher (about 2800° C. for hafnium), so that they can be used satisfactorily as inserts.
Depending on the material to be cut by plasma cutting, there are optimal plasma gas combinations for attaining a favorable cutting quality. Especially for mild steel cutting, which occupies a large proportion of applications for plasma cutting, oxygen plasma-cutting, in which a plasma containing oxygen is used, attains the best cutting quality, and has an excellent cutting speed.
The thermal conductivity of hafnium (the following explanations relate to hafnium, but the same is true for zirconium), which is the electrode material for oxygen plasma cutting, is very poor and is only one tenth of that of copper, so that if the electrode is made of hafnium alone, it is usually not cooled enough, the temperature of the hafnium rises too much, and the consumption of the hafnium may proceed rapidly. In order to prevent this, in electrodes using hafnium, usually a substantially cylindrical electrode body is made of copper, and a substantially column-shaped small piece of hafnium (referred to as “insert” in the following) is inserted into a tip, which serves as the cathode spot, of the cylindrical copper electrode body (referred to as “holder” in the following). The cylindrical copper holder is cooled by air or by water, so that the hafnium insert in its tip is. cooled due to the thermal conduction with the copper holder.
Thus, for oxygen plasma cutting, electrodes are used that have hafnium or zirconium inserts in their tips. However, since the temperature of the cathode spot exceeds 3000° C. during the plasma arc&generation, it is difficult to reduce the consumption of the hafnium or zirconium to the point where it is negligible, even using materials formed of high melting point oxides, such as hafnium oxide or zirconium oxide. Thus in the past, several techniques have been developed to reduce the electrode consumption and improve the lifetime of electrodes.
For example, the thermal shock to the electrode can be dampened by slowly increasing the arc electric current immediately after the arc ignition, which reduces the electrode consumption right after the arc ignition (see JP H05-104251A). Or, the electrode consumption immediately after the arc ignition is reduced by igniting a plasma arc with nitrogen and then switching to oxygen plasma (see JP H03-258464A). Another method that has been proposed is to reduce the electrode consumption by optimizing the insert diameter with respect to the arc electric current (see JPH07-506772A). A further method that has been proposed is to accelerate the cooling of the insert and improve the electrode lifetime by forming an intermediate layer of a silver alloy between the insert and the holder to improve the thermal conduction between the insert and the holder (see JP H04-167996A).
However, in spite of those technical improvements, the durability of electrodes is limited to a few hours in actual oxygen plasma cutting, and there is great demand for a further increase of their lifetime.
FIG. 1
shows schematically how the electrode is consumed away during the generation of an arc. The arc generation first consumes the insert
11
at the tip of the electrode
10
, until it is shaped like a mortar (FIG.
1
(
b
)). The speed with which the insert
11
is consumed varies with such factors as the current, the cooling of the electrode
10
, the composition of the plasma gas, and the gas pressure. Moreover, as the.arc generation proceeds, the consumption of the insert
11
invades deeper to make a hole in the tip of the electrode
10
(FIG.
1
(
c
)). Then, when the consumption depth d of the insert
11
(that is, the distance from the top surface of the consumed insert
11
to the top surface of the electrode
10
) reaches a limit value d
max
, a stable arc emission from the insert
11
becomes impossible, and arc generation becomes difficult, the arc starts to be emitted from the copper holder
12
, and the copper holder
12
is consumed rapidly, which leads to destruction of the electrode
10
(FIG.
1
(
d
)).
One might think that if the consumption speed of the electrode stays the same, it should be possible to prolong the possible usage time of the electrode by increasing the volume of the insert. However, if the diameter D of the hafnium insert
11
is simply increased, then the thermal conduction of the insert
11
worsens, so that the temperature inside the insert
11
rises and the consumption speed accelerates more than what the volume has been increased, thereby instead rather shortening the electrode lifetime. That is to say, there is an optimal value for the diameter D of the insert, and there is no advantage in simply enlarging it (for an invention related to the optimization of the insert diameter, see JP H07-506772A) Also even when the buried length H of the insert
11
is increased more than the limit depth d
max
, the consumption does not proceed beyond the limit depth d
max
. The limit depth d
max
, which depends on the swirling of the plasma gas stream, the cooling of the electrode and the arc electric current, is usually about 1 mm to 2 mm and does not depend on the buried length H of the insert
11
. Consequently, it is sufficient if the buried length H of the insert
11
is equal to the limit depth d
max
at least. There is no advantage in making the buried length H larger than the limit depth d
max
, but this is uneconomic, because the expensive hafnium is used in excess.
SUMMARY OF THE INVENTION
Consequently, it is an object of the present invention to improve the electrode structure of a plasma machining electrode having a hafnium or zirconium insert, so as to prolong the electrode lifetime.
It is another object of the present invention to improve the arc generation conditions for this improved plasma machining electrode, so as to prolong the electrode lifetime.

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