Fault detection during remelt of electrodes into blocks

Apparatus for the detection and evaluation of process parameters arising during the remelting of an electrode (31) to a metallic block (32) in a vacuum arc furnace detects deviations of at least one process parameter from a predetermined course and uses them to locate faults in the electrode and/or in the metal block (32).

BACKGROUND OF THE INVENTION 
The invention concerns apparatus for the detection and evaluation of 
process parameters arising during the remelting of an electrode to a 
metallic block in a vacuum are furnace. 
During the remelting of electrodes into blocks various faults arise. The 
best-known of these faults are voids or pipes, `tree-rings`, segregations, 
white spots, `freckles`, non-metallic inclusions and structural disorders. 
Should one or more of these faults be discovered at certain locations, the 
whole block is usually thrown away, because the assumption is made that in 
addition to the discovered locations similar faults are also present at 
undiscovered positions. This conclusion is, however, often incorrect 
because the fault arose by a single disturbance of a process parameter 
which no longer arises again during the subsequent stages of melting. If 
the site of the fault that has occurred were known, then only the 
frequently very small faulty location of the block would require excision. 
The rest of the block could then still be wholly delivered for its proper 
purpose. 
There is already known a process for regulating the course of melting away 
of self-consuming electrodes, wherein the instantaneous actual weight of 
the electrode as well as the electrical properties of the section between 
the electrode and the surface of the melt bath influence the regulation 
(DE-AS-1,934,218). Here not only are the electric data relating to the arc 
gap utilized in the regulation, but on this regulation a further 
regulation of the power input of a metallurgical furnace is superimposed. 
Thus one is concerned here with a regulation having several influencing 
parameters. A correlation of faults arising in the melting process to a 
defined site of the finished block is not possible with this process. 
Furthermore, a process for the automation of the electrode melt processes 
is known, wherein the state of the melting process is determined according 
to a complex criterion, which takes into account the energy consumed, the 
harmonic content and fluctuations of the arc current, the spatial position 
of the electrodes and the temperature of the hottest zone of the delivery 
(D. A. Gitgarz: The Use of Microprocessors for the Control of Arc and 
Induction Melting Furnaces, Elektrie 35, 1981, pp. 545-547). A positional 
correlation of faults is not possible with this process either. 
SUMMARY OF THE INVENTION 
The invention accordingly concerns the task of providing apparatus for 
detecting the site of a fault in a block arising during remelting of 
electrodes to form the block. This is done by detecting the deviations of 
at least one process parameter from a predetermined course and correlating 
the deviation to faults in the electrode and/or in the block. 
The advantage achieved by the invention consists particularly in that the 
customers of metallic blocks produced by electrode melting are able to 
undertake e.g. ultrasonic tests at precisely designated positions of the 
block in order to sense faults. The rejection rate of finished parts, e.g. 
turbine discs, is considerably reduced thereby. In addition, the risk of 
damage to blocks during forging is lowered.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 the dependence of the weight of the electrode G.sub.E on the 
position P.sub.E of the electrode support is represented, where at the 
same time the cast or molten electrode 1 with included faults 3, 4 is 
shown within a vessel 2. At the beginning of the melting process the 
electrode still has its full weight G.sub.EA, and moreover in the position 
P.sub.EA, while the block is not present at all yet and thus the weight 
G.sub.BA is zero. The reduction in the electrode weight G.sub.E is 
connected during the melting process with the increase in the weight 
G.sub.B of the block. If one assumes that the electrode 1 is melted 
extensively void-free, one can very precisely determine from the weight of 
the electrode 1 and the geometric dimensions of the mould the 
corresponding height of the block for signalling. From the position 
P.sub.E of the electrodes and the instantaneous weight G.sub.E of the 
electrodes the total density of the residual electrode may be determined 
at each point in time of the melting process. When there is a void in the 
electrode, a linear correlation beween the electrode position or reduction 
and the electrode weight loss can no longer be expected. 
The theoretical course of weight loss of a void-free electrode is 
designated in FIG. 1 with the straight line 5. When a significant 
deviation of the weight loss from the linear course occurs, it may be 
assumed that e.g. a piece of the electrode has fallen off. 
A possible real course of the weight reduction of the electrode is 
represented by the measurement curve 6. At the beginning of the melting 
process the weight of the electrode is G.sub.EA and this is moreover at 
position P.sub.EAR. In the region 7 the weight loss runs linearly at 
first, and then in region 8 deviates significantly from a linear run. This 
significant deviation infers a thickening or change in density of the 
electrode. Should the measurement curve 6 change now via a reversed 
deviation 9 to a linear region 10, a thinning or change in density is 
indicated. A short jump-back of the measurement curve 6 towards the 
initial position, designated with region 11, refers to a short-circuit 
with electrode retraction. From this juncture the measurement curve 6 
returns to a linear region 12 again, from where it passes to a region 13, 
which indicates the appearance of a large horizontal void. Then the curve 
6 runs along a further linear region 14 which jumps into the disturbance 
region 15 that refers to the falling down of a piece of electrode. The 
further regions 16-19 of the curve 6 signify a nearly normal burn-off of 
the electrode. 
In FIG. 2 electrode 1 is shown again, wherein the fault 4 is again 
recognized, the fault concerned being a void. If one considers this 
electrode as having a length l.sub.EA divided into several identical 
slices 70-82, and if one further assumes that the electrode is melted off 
from below upwardly, then it will be recognized that in the region of 
slices 70-74 there is no close relationship between the height of the 
melted-off part and its weight, since the weight of the slice 70 is 
considerably smaller than the weight of the slice 73 of equal weight. Only 
in the upper region of the electrode 1 characterised by slices 76-82 does 
a close relation exist again between the weight of the melted-down 
electrode part and the height, insofar as the electrode--as is in general 
the case--melts off horizontally in a uniform manner. 
The deviation of the relationships of the weight of the electrode 1 to the 
length of the electrode 1 from a predetermined function thus permits an 
inference to be made about a fault, especially a void, in the electrode. 
It is therefore important to monitor this weight/length ratio. The weight 
of the not-yet-melted off residual electrode is in general detected by a 
measuring device connected to the electrode with an electrode rod. As 
against this, the measurement technology is difficult for detecting the 
residual length of the electrode 1, because at the position where the 
electrode 1 is melted off, very high temperatures arise. Preferably, 
therefore, the electrode length is computed. 
In FIG. 3A an electrode 1 is represented at the beginning of the melt-off 
procedure and provided with geometrical reference magnitudes. The length 
of the electrode 1 at the beginning of the melting process is designated 
with l.sub.EA, while d indicates the diameter of the electrode and 
l.sub.STA is the distance between the attachment of the electrode rod 20 
and the bottom of the vessel 2. 
The distance between the bottom of the vessel 2 and the underside of the 
electrode 1 is indicated by c. The spacing l.sub.STA is as a rule given by 
a position indicator 21 connected to the electrode rod 20 and which 
indicates how far the electrode 1 has already sunk. 
FIG. 3B shows the same arrangement as FIG. 3A, but where a major portion of 
the electrode 1 has already melted off and become block 22. The specific 
weight of the electrode 1 is designated with .rho..sub.E, while the 
specific weight of the block is designated with .rho..sub.B. Assuming a 
cylindrical vessel 2, the desired residual length l.sub.Ex of the 
electrode 1 may be obtained by the following equations. 
##EQU1## 
Hx substituted into equation (a) results in: 
##EQU2## 
However, 
##EQU3## 
V.sub.Block x is the volume of the block of height Hx and G.sub.Block x is 
the weight of this block. 
V.sub.Block x is thus G.sub.Block x .multidot..rho..sub.B. Substituting 
this into equation (c), one obtains 
##EQU4## 
But G.sub.Block x corresponds to the weight of the electrode 1 at the 
beginning of the melting-off process less the weight G.sub.Ex of the 
residual electrode: 
EQU G.sub.Block x =G.sub.EA -G.sub.Ex 
Substituting this into equation (d), one obtains 
##EQU5## 
This equation contains only known magnitudes--c, D, .pi., .rho..sub.B, 
G.sub.EA --or those that are continuously measured or indicated--G.sub.Ex, 
l.sub.STx. 
In FIG. 4 it is shown schematically that the magnitudes l.sub.Ex and 
G.sub.Ex are fed to a comparator 83 in which the ideal G.sub.Ex /l.sub.Ex 
correlation is stored, i.e. a correlation for a fault-free electrode 1. 
This correlation is compared with the actual correlation and when the 
deviation of the actual correlation from the ideal correlation exceeds a 
predetermined amount, a signal S.sub.F is generated and displayed or 
processed. It is also possible to differentiate the signal S.sub.F by a 
differentiating unit 84, to render the deviation clearly recognizable. 
Apparatus for detecting and processing a plurality of data is schematically 
represented in FIG. 5, the apparatus enabling the most important events 
during the melting of an electrode to be recorded and, on the basis of an 
event diagram or catalogue, to characterise the fault inclusions in the 
`block` end product. The representation of FIG. 5 is kept very general and 
is not restricted to vacuum arc furnaces, which is the preferred context 
of use of the invention. A melting crucible is designated by 30 and in it 
is collected the melting charge 32 coming from the electrode 31. The 
melting crucible 30 is surrounded by an electric heating device 33 powered 
via conductors 34, 35 from a non-illustrated power source. The electric 
heat output is detected by means of a KW counter which has a current 
measuring device 37 and a voltage measuring device 38. The instantaneous 
heat output is inputted to a microcomputer 39 via a data line 40. In 
addition to the power it is also possible to input directly the current 
and voltage into the microcomputer 39, which is signified by data lines 
41, 42 and 43, 44 respectively. 
The crucible 30 together with the heating device 31 rest on a weighing 
device represented by two pressure cells (load cells) 45, 46. The 
electrical measurementvalues of these load cells are also reported via a 
further data line to the microcomputer 39. In this way the actual weight 
at any time of the crucible 30 with the melting charge 32 is known. 
The weight of the electrode 31 is detected by a weight measuring device 48 
which reports the actual weight of the electrode 31 via a data line 49 to 
the microcomputer 39. A voltage is applied between the electrode 31 an the 
crucible 30, this voltage being supplied from a power source 50 via 
respective conductors 51 and 52 to the electrode 31 and the crucible 30. 
This voltage is reported via data lines 53, 54 to the microcomputer 39 
which also receives information concerning the current via another data 
line 55. 
The height of the surface of the melting charge 32 is detected by a 
measuring device 56 based e.g. on the principle of wave reflection from 
the surface of the melting charge 32. The instantaneous actual melt 
surface is reported to the microcomputer 39 over a data line 57. 
In a corresponding manner the instantaneous position of the underside of 
the electrode 31 may be detected via a measurement device 59 and reported 
to the microcomputer via a data line 59. 
As has already been explained above, since the measurement technology of 
detecting the position of the surface of the melting charge 32 and the 
underside of the electrode 31 is very difficult, these positions may also 
be computed. 
The temperature of the melt is detected by means of a thermocouple device 
60 which reports the instantaneous actual temperature to the microcomputer 
39 over a line 61. 
The microcomputer 39 evaluates all the input data on the basis of a program 
to the effect that it assigns a fault type to a predetermined height of 
the block. This correlation may be represented on a terminal screen 62 or 
by means of a printer 63. 
In addition it is also possible to display particularly interesting data, 
for example the temperature of the melting charge 32, on an indicating 
device 64. 
Should particularly significant faults arise, they may be indicated by 
means of an acoustic signal 65. 
The records of the microcomputer reproduced on the terminal screen 62 or 
the printer 63 may look something like the following: 
______________________________________ 
z-coordi- y-coor- 
Type nate of 
x-coordi- 
dinate 
Melt time 
Disturbance 
of the block 
nate of 
of the 
Interval 
Events Fault (height) 
the block 
block 
______________________________________ 
1-2 mins 
-- -- -- -- -- 
2-3 mins 
-- -- -- -- -- 
3-4 mins 
Mains volt- 
Void 2 cm 20 cm 15 cm 
age inter- 
ruption 
______________________________________ 
It is inferred from the measurement records that in the first three minutes 
no disturbances arose. After these three minutes a line voltage 
interruption occurred for one minute, which typically causes a void 
(pipe). This void was correlated to a defined location of the block on the 
basis of the other data fed to the microcomputer 39, this location being 
captured by three coordinates x, y, z. If one is concerned more with a 
point-like fault, then in place of the x and y coordinates the respective 
regions .DELTA.x and .DELTA.y are indicated. The x, y, and z coordinates 
may be gained by the evaluation of load cell signals. 
It is understood that process parameters other than those mentioned may be 
evaluated. For example, the detection of gas evolution is important 
particularly in vacuum arc furnaces.