Powder filled tube and a method for the continuous manufacture of such tube

A powder filled tube has a steel outer wall which consists of two layers formed as two complete turns of a spiral made from a single strip and welded together at their interface to form a unitary wall. The method for the continuous manufacture of this tube involves PA0 (i) forming a steel strip into a channel, PA0 (ii) introducing the powder filling into the channel, PA0 (iii) closing the channel by rolling it into a tube in which, as seen in cross section, the steel strip overlaps itself by at least 360.degree. so that the tube wall has two layers around its whole circumference and the powder filling is compacted, PA0 (iv) heating the tube and while hot subjecting it to stretch/reduction rolling in which simultaneous size reduction and extension of length take place and in which the two wall layers are welded together by the effect of the elevated temperature and the pressure applied in rolling.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to powder-filled tube and to a method for the 
continuous manufacture of such tube. The invention is particularly but not 
exclusively applicable to powder-filled tube known as ladle wire which is 
used in metallurgical processes, and to powder-filled tube used as, or in 
the manufacture of, welded rod. 
2. Description of the Prior Art 
Powder-filled tube having a steel outer wall welded to form a closed tube 
and a compacted powder core in the closed tube is well known. Various 
methods, none of them wholly satisfactory, for manufacturing such tube 
have been proposed. 
In GB-A-No. 1,604,144 and EP-A-No. 3370 methods are described for the 
continuous fabrication of a welded tube filled with powder, starting from 
a strip of metal which is formed into an open channel, into which filler 
powder is fed. In this condition the edges of the channel are brought 
together and held together by pressure after which a closed longitudinal 
seam is created by electric welding. Afterwards the diameter of the closed 
tube is reduced so that the powder filler within the tube is substantially 
compressed and thereby cannot shift during further handling. This method 
has the practical difficulty that the nature of the powder can negatively 
influence the quality of the weld made can contaminate the edges of the 
strip through the magnetic ingredients in the powder. At the same time 
there are limitations on the filling ratio (the weight ratio between the 
quantity of powder and the quantity of steel per unit length). Also there 
are limits to the production rates achievable. 
U.S. Pat. No. 3,543,381 shows a process in which a strip is formed into a 
channel, the channel is filled and then the channel edges are brought 
together to form a butt seam or a lapped seam. The seam is not welded. 
Compaction of the powder is poor, and the powder may leak or be 
contaminated because the seam is not an adequate seal. DE-A-No. 2,603,412 
similarly described the production of a filled tube with a lapped, 
non-welded seam, but in this case the powder is first compacted into a 
wire-shape before being inserted into the tube. 
SUMMARY OF THE INVENTION 
The object of the invention is to provide a product in the form of a 
powder-filled tube in which welding of the edges is unnecessary and in 
which nonetheless hermetic sealing of the filler powder is achieved. 
According to the invention this product in one aspect a powder-filled tube 
having a steel outer wall welded to provide a closed tube of uniform 
thickness all around its circumference and a powder filling enclosed 
within the steel outer wall is characterized in that the steel outer wall 
consists substantially all around its circumference of at least two layers 
which are formed as two complete turns of a spiral made from a single 
strip and are welded together at their interface to form a unitary wall. 
The invention also relates to a method for the continuous fabrication of 
this new product. According to the invention, in this second aspect the 
method consists in passing a steel strip continuously through the 
following steps: 
(i) forming the strip into a channel open upwardly, 
(ii) introducing the powder filling into the channel so formed, 
(iii) closing the channel by rolling it into a tube shape in which, as seen 
in cross-section, the steel strip overlaps itself by at least 360.degree. 
so that the tube wall has at least two layers around its whole 
circumference and the powder filling is compacted, 
(iv) heating the tube shape and while it is hot subjecting it to 
stretch/reduction rolling in which simultaneous size reduction and 
extension of length take place and in which the two wall layers are welded 
together by the effect of the elevated temperature and the pressure 
applied in rolling. 
Preferably the tube shape is heated inductively. Preferably the 
stretch/reduction rolling is performed by at least two triple 
stretch/reducer roll sets each having three rolls with axes at 120.degree. 
to each other. 
This method offers a number of notable advantages. There are no more 
problems with welds; the problem of contamination of the strip edges by 
the powder filling especially is avoided. There are fewer strip-shaping 
problems, especially as the method is not so critical in respect of the 
positioning of the strip edges. The formation of the tube is much simpler. 
The output speed of the manufactured product can be very high, whilst 
because of the great size reduction which may be employed, thicker 
starting material can be used, which has an economic advantage. 
It is a major advantage that any type of powders can be used as the filler, 
naturally provided that it can be readily dispensed into the channel. It 
is useful if the powder particles can withstand a short period of heating 
without disadvantage. Because the tube heating does not occur until the 
tube exterior is fully closed, there is only a capillary gap which extends 
over 360.degree. of the circumference, no poisonous gases can escape into 
the atmosphere. It has now therefore become possible to use dangerous 
materials for the filler powder, such as gas-evolving calcium carbide, 
inflammable magnesium or poisonous tellurium. 
Another advantage of the invention is that welding is accomplished by a 
rolling step, which can be carried out easily at a high process speed. 
Because the filling is compacted to a solid core before the welding step, 
it is possible to perform the stretch-reduction rolling of the heated tube 
which simultaneously effects the welding as well as the elongation and 
reduction of diameter. In prior art processes, e.g. that of EP-A-No. 3370, 
reduction has followed welding, which has been carried out by heating of 
the butting edges. In the present invention, of course, further 
stretch/reduction may follow the welding step. 
It is mentioned that formation of steel tube having a welding multi-layer 
wall has in itself been proposed, in U.S. Pat. No. 3,470,605 and DE-A-No. 
1,602,232, but these tubes are not filled tubes and consequently cannot be 
subjected to stretch/reduction. U.S. Pat. No. 3,470,605 proposed that the 
tube is heated to achieve welding of the layers in an oxygen-free 
atmosphere to a temperature in the softening range between the solidus and 
the liquidus lines, which is impractical for a powder filled tube. 
DE-A-No. 1,602,232 mentions heating to an unspecified temperature below 
the solidus line.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the method of the invention illustrated in the drawings, a narrow flat 
single ply strip 1 of uncoated steel in continual longitudinal motion is 
formed into a powder-filled tube whose steel covering is a wall of overall 
even thickness and is formed from two layers welded together. The width of 
the strip 1 is chosen according to the diameter of the tube to be 
fabricated and in dependence on the process performed. 
The strip 1 is drawn from a rotatable coil, not shown in FIG. 1. When so 
called "slit strip" is used, the edges of the strip are square, as can be 
seen in FIG. 2. This makes for certain difficulties in further cold 
working. Because of this the strip 1 undergoes a preliminary stage of 
working, for example by rolling under pressure, to chamfer both 
longitudinal edges so that the strip 1" in FIG. 2 is formed. As will be 
described later, the strip 1" is bent over about 720.degree. so that the 
chamfered edges 4 and 5 (FIG. 2) overlap each other. The forming of the 
strip to the two-ply walled tube by bending through about 720.degree. or 
even more is carried out to a double overlap practically the same as the 
chamfer width. This principle is in itself known, from U.S. Pat. No. 
2,014,983, for example. 
As can be seen in FIG. 1, the chamfered strip 1" is led between the first 
set of rolls 35 of a complete line of tube forming stations comprising 
various sets of driven rolls 35 to 41, which in successive stages 
progressively turn the strip into a closed multi-ply tube. In FIG. 1, 
there are schematically shown always two opposed rolls per set, but these 
tube forming stations can be of a more complex design. The specific design 
for rolls 35 to 41 which form the strip during its passage through them, 
will be known to those skilled in this art, and these rolls need be 
neither drawn nor described in detail. 
Firstly the strip 1" is folded by the rolls 35 and 36 to a U shaped channel 
2 (FIG. 2) open upwards. The channel has a height greater than its width. 
This channel is shown as being symmetrical in FIG. 2, but may be 
asymmetric. The designations "upper" and "lower" are naturally for 
descriptive purposes and the terms imply no restriction in the absolute 
sense. The bending radius of the lower half is approximately (but somewhat 
greater than) that of the radius of the tube when closed up later (10, 
FIG. 2) but before reduction. 
When the strip has been formed into a "U" shaped cross section 2 (FIG. 2), 
a desired, calculated quantity of powder 3 is introduced to a certain 
level, which is shown in FIG. 1 by an arrow. The introduction of powder 
into a channel is known in itself and described for example in patents 
GB-A-No. 1,313,292 and SE-A-No. 312,388. The channel is filled to less 
than half its height. 
The following sets of rolls 37-41 (FIG. 1) turn the edges 4 and 5 of the 
channel into overlap (FIG. 2) and then the tube is rolled further tightly 
around the powder 3, thus compacting the powder. First the edges 4 and 5 
are bent towards each other to give a tube with overlapping edges; during 
further rolling the walls slide over one another to the stage in FIG. 2 
where the tube wall, shown in section and marked with the number 10, has 
an overall two ply wall. The powder 3 is then already highly compacted and 
forms a solid core. 
In the following stage the formed and filled tube passes through an 
induction coil 11, which has the effect of inductively heating the 
interface of the two layers of the tube wall to about 700.degree. C. 
Induction heating is preferred, because the skin effect produces the 
heating at the outside surface of the tube. Heat flows inwardly to heat 
the welding interface and finally the filling. The filling is therefore 
heated as little as possible. The preferred temperature range achieved at 
the welding interface is 650.degree.-750.degree. C. 
Immediately after the heating the tube while still hot is passed through 
two sets of triple stretch/reducer rolls, which in FIG. 2 are only shown 
schematically. A suitable set of such rolls (see FIG. 3) consists of three 
concave rolls 20, 21 and 22, which have axes at 120.degree. intervals 
around the circumference of the tubular product 23, and bring about a 
further reduction of the diameter from d1 to d2. Two successive sets of 
these triple rolls may be employed, one closely behind the other. 
Not only do these two stretch/reducer rolls reduce the tube diameter from 
d1 to d2 (FIG. 2); they also reduce the thickness of the two ply wall 
24,25 to the same extent down to those of walls 26,27. The diameter of the 
100% compacted core is likewise reduced in proportion to the tube 
diameter. The size reduction is accompanied by a corresponding length 
extension of the tube, since the solid core filling cannot be further 
compacted. This stretch-reduction process is similar to the well-known 
process of hot rolling a wire to reduct its diameter and increase its 
length. 
The most important consequence of the inductive heating of the steel tube 
and the immediate subsequent high pressure stretching/reduction of the 
tubular product, with its two-ply wall, is that the two wall layers 
(26,27) weld together under the high temperature and applied pressure, so 
that in the walls 26,27 the capillary gap, which was initially present 
between the parts of the wall 24, 25 over approximately 360.degree., 
completely disappears. 
This may be seen by comparing FIGS. 4a and 4b. In FIG. 4a the tubular 
product is reproduced before heating and stretch/reduction, and in FIG. 4b 
immediately after. 
To achieve this effect, the strip does not need to be especially clean or 
de-oxidized; it is generally preferred that the strip is previously 
degreased. However the steel surfaces that are to come together should not 
bear any previous noticeable contamination from the filler powder. 
After the welding step the tube is allowed to cool down and thereafter the 
seamless filled tube so obtained is made ready by means of coiling for 
transport to possible further processing departments or to the consumer. 
Because an originally double walled tube is transformed by the application 
of rolling forces at high temperature, a new product has been produced, 
being a seamless unitarily walled tube, without making use of a solder or 
other joining material. By reducing the diameter and the wall thickness by 
rolling at high temperature the two walls are permanently, hermetically 
and indivisibly bonded to one another. 
Compared to other methods for manufacturing powder filled tubes, the method 
according to the invention presented has the advantages that there are no 
influences upon quality stemming from a weld, that the production rate can 
be faster than those achievable with present day welding methods, and that 
it is simple to use a shielding gas when filling with powder. In this way 
no oxidation of the filler during later heating through the influence of 
oxygen from any enclosed air can occur. 
A numeric example of the method according to the invention may serve as an 
illustration: starting from a degreased steel strip with width of 400 mm 
and a thickness of 0.7 mm and an entry speed of 60 m/min (=1 m/sec), a 
closed powder filled tube of 13 mm diameter and a wall thickness of 0.3 mm 
was manufactured, which after heating and reduction had an exit speed of 
1400 m/min (23.3 m/sec). 
This material is intended as ladle wire for metallurgical uses. It is also 
especially suited as a raw material for the preparation of filled welding 
rods.