Industrial electric heating furnaces – Electroslag remelting device – Power supply system
Reexamination Certificate
2001-04-03
2002-12-17
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Electroslag remelting device
Power supply system
C373S105000
Reexamination Certificate
active
06496530
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention (Technical Field)
The present invention relates to electroslag remelting (“ESR”) electrode immersion depth control systems and methods.
2. Background Art
As shown in
FIG. 1
, ESR furnaces
10
have been utilized for over 40 years to refine metals and produce fully dense homogeneous ingots
22
. The remelting takes place by immersing a consumable metal electrode
14
into a molten slag bath
18
that is resistively heated through applied power
24
to a temperature above the melting point of the metal. The electrode gradually melts, forming metal droplets that fall through the slag and collect in a pool
20
under the slag. The molten pool is contained within a water-cooled mold
16
that has a slightly larger diameter than the electrode. As the electrode melts, it must be translated downward by an electrode drive
12
at a rate related to the fill ratio and the melt rate, as specified by the system controller
26
. A complicating factor is that a small amount of slag solidifies on the surface of the mold, changing the amount of metal needed to fill the mold, and changing the thickness of the molten slag on top of the ingot.
To produce a high quality homogeneous ingot with good surface quality, the deviations in the process—specifically immersion depth—need to be minimized. To optimize process efficiency and surface quality, the immersion depth must be maintained at a constant level, as shallow as possible. However, the shallower the immersion depth, the more sensitive the process is to input or external variables, hence, the more difficult it is to control. If the immersion depth is allowed to get too shallow, gaps can form between portions of the electrode surface and the slag, leading to arcing, atmospheric exposure, and deleterious oxidizing reactions. Conversely, too large an immersion depth, or too much variability in depth, can lead to poor surface and metallurgical quality in the ingot.
Again, the ESR process is used to refine metal, remove inclusions, and produce ingots having a uniform solidification grain structure and good surface quality. The immersion depth is an important parameter to control since it has a major effect on the thermal conditions governing melting and solidification. Deviations in immersion depth will alter the thermal environment of the process, inducing changes in the melting process (rate, efficiency, configuration, droplet location and size) and on solidification parameters (rate, direction, molten metal flow). As a result, immersion depth fluctuations will result in changes to the ingot's solidified grain structure, compositional homogeneity, and properties, and affect subsequent processing operations and final product quality.
Existing control methods drive the electrode in response to an error between the system voltage (which is related to immersion depth as described below) and a voltage set point. They utilize bi-directional electrode drive to oscillate around the set point, inherently resulting in constant fluctuation of the immersion depth. Shallower immersion depths have been shown to result in improved surface quality, hence improved process yields. The voltage of an ESR furnace system is most sensitive to immersion depth changes near the surface. Thus, it is more difficult to control electrode position at shallow immersions. As a result, existing control systems are only stable at a deeper immersion depth than would be associated with optimum surface quality.
No system currently exists to measure the depth directly, so it must be inferred from measured parameters of the process. At present, the ESR immersion depth is controlled in most systems by using the voltage and voltage swing, which is measure of the variation in the voltage. These methods are referred to as swing controllers.
The voltage is used because ESR furnaces primarily operate with a constant current power supply. At a simplified level, the slag can be viewed as a resistor, so the voltage is given by Ohm's Law:
V=I[d/
(
Ak
)]
where V is the voltage, I is the current, and the resistance of the slag is approximated by the expression in the brackets where d is the distance between the electrode and the molten metal pool, A is the area of the electrode in contact with the slag, and k is the slag conductivity. However, there are numerous simplifications inherent in this treatment, so voltage is only a rough indicator of the electrode immersion. Additionally, the slag thermal environment and chemistry will change over the course of a melt, hence its conductivity is not constant. The amount of molten slag will also change during a melt due to slag plating out on the cold crucible walls, further altering the above relationship.
Consequently, while voltage is an effective immediate indicator of relative electrode position with respect to the surface of the slag, voltage alone has not been adequate to indicate or maintain a constant average immersion depth over time. Voltage swing cannot be directly related to the immersion depth via an equation such as the one presented above, nor can it be used as an instantaneous indicator. On the other hand, voltage swing is less sensitive to the factors that can change during the course of a melt. Regardless of slag amount, conditions, or properties, the isopotential lines within the slag will be compressed near the surface of the slag. As a result, increases in voltage swing can be reliably, but not quantitatively, related to a reduction in immersion depth.
Existing control systems utilize changes in voltage swing to adjust the voltage set point in response to changing process conditions. The basic method shown in
FIG. 2
shows a schematic of an existing control system. Over the short term, the drive speed is determined by multiplying the voltage error (V
rms
−V
sp
) by a proportionality constant, K
e
. This can be expressed by the equation: Drive Speed=K
e
(V
rms
−V
sp
) where V
rms
is the system voltage applied to the electrode and V
sp
is the voltage set point, a voltage indicative of desired electrode immersion depth. In the long term, the voltage swing is measured over a period of time and compared to a voltage swing set point. If the measured voltage swing is greater than the voltage swing set point, the immersion is taken to be too small, and the voltage set point is decreased. Conversely, if the measured voltage swing is smaller than the set point, the immersion depth is assumed to be too large, and the voltage set point is increased.
A more recently developed ESR control system was described in U.S. Pat. No. 5,737,355, to Damkroger, titled “Directly Induced Swing for Closed Loop Control of Electroslag Remelting Furnace”. In this system, the electrode drive is the combination of a set unidirectional motion and a superimposed periodic fluctuation. This system then superimposes a periodic fluctuation of known amplitude (rather than electrode motion in response to a voltage error) to provide electrode motion relative to the isopotential lines in the slag, and thus generate the voltage swing signal. In the long term, positive deviations of voltage swing from the set point indicate too shallow immersion, and are used to increase the basic unidirectional drive speed. Negative deviations are used to do the opposite.
This directly induced swing system eliminated the confounding effect of the system's own drive response on voltage swing. However, it incorporates no short-term response to an error, which limits its ability to operate very near the slag surface. Later modifications of the directly induced swing sought to address this shortcoming by incorporating a voltage error response as was used in the original swing controllers. The average is usually a long term average of the drive speed. Over the long term, the voltage swing is measured and deviations from its set point are used to adjust the voltage set point, usually with a linear gain factor. To some extent these modifications mitigated the problem but the immersion depth wa
Damkroger Brian K.
Melgaard David K.
Shelmidine Gregory J.
Hoang Tu Ba
Libman George H.
Sandia Corporation
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