TESTING OF BITS REINFORCED WITH HARD ALLOY FOR HEAVY-DUTY COAL-CUTTING MACHINES

V. I. Strokopytov

As is known, the productivity of a coal-cutting machine is determined by the area of the kerf produced by the machine over a certain period of time. Depending on the unit adopted in calculating time, the hourly, shift, and daily productivity of the machine are distinguished; while daily productivity is influenced mainly by the system of organization of work at the given mine, shift (hourly) productivity is to an enormous extent a function of the general adjustment of the machine’s operation, the degree to which it is adapted to the given local conditions, and the ability of the operating personnel to find the optimum operating regime for the machine under the given conditions. Naturally, only its shift productivity can serve as an indicator of the operating qualities of machines. Discarding all auxiliary operations connected with servicing the machine, one can also determine its shift productivity.

\[ A = hv_1 (T - t_1 - t_2), \]

where \(A\) is the shift productivity in \(m^2\); \(h\) is the depth of the kerf in meters; \(v_1\) is the feed speed of the machine in m/hour; \(t_1\) is the duration of the operation of replacing worn bits, in hours (downtime); \(T\) is the total operating time in a shift; \(t_2\) is other downtime of the machine (resetting the post, re-tensioning the cable, etc.).

When cutting in hard coal seams, wear of the bits is so considerable that operations for replacing dulled bits take up to 50% of the time \(T\), and this naturally leads to a corresponding decrease in productivity. In addition, wear of the bits during operation is the main cause that worsens the operating regime of all parts of the coal-cutting machine.

Wear of a bit \(\lambda\) (Fig. 1), i.e., the linear shortening along its axis (which, for a given bit profile, determines the increase in the end area \(F_T\)), is proportional to the strength of the coal \(K\), to the length of the path traveled by the bit \(L\) (friction path), and inversely proportional to the quality of the bit material (resistance to abrasion \(K_1\)).

Thus: \(\lambda=\dfrac{kL}{k_1}\), and the length of the path traversed by the bit will be expressed by the formula:

\[ L=\frac{Hh}{b}\cdot\frac{l}{l_1}, \]

where \(H\) is the length of the cut in linear meters, \(h\) is the depth of the undercut, \(b\) is the chip thickness, \(l_1\) is the length of the cutting chain, and \(l\) is the distance between bits of the same position in the cutting chain. Then

\[ \lambda=\frac{kHh}{bk_1}\cdot\frac{l}{l_1}=\frac{c}{bk_1}. \]

It follows from this that, other conditions being equal, bit wear can be reduced by increasing the chip thickness \(b\). This can be accomplished in two ways: by increasing the feed speed of the machine \(v_2\) and by reducing the speed of the cutting chain \(v_1\), for

\[ b=\frac{lv_2}{v_1}. \]

Another way to reduce bit wear is to improve the quality of the bit material, i.e., to increase its resistance to abrasion. In this respect, the use of hard alloys played a decisive role.

Numerous tests of bits with hard alloys have shown that the life of a bit protected by hard alloy is many times higher than the life of steel bits, increasing in individual cases up to 50-fold. Bits with hard alloys, being resistant to abrasion, operate much longer and thereby greatly reduce the amount of downtime \(t_1\), increasing the net operating time of the machine \((T-t_1-t_2)\). Owing to the special conditions under which a coal-cutting machine operates and to the design features of this machine, the indicated property of hard alloys gives the following results:

1. Reduction of friction losses in the cutting chain of the coal-cutting machine.

2. Reduction of the tension of the drive rope and of the power consumed for feeding the machine.

3. Improvement in the quality of the machine’s energy balance.

4. Reduction of downtime for replacing bits in a set, which, in addition to increasing productivity, leads to an easier operating regime for the motor, to a reduction in short-term motor starts that cause burning of the controller contacts, and to the elimination of a number of causes producing general disorder of the machine.

5. Reduction of stresses in all parts of the machine, reduction of forces causing breaks in the cutting chain, wear

cams, bar guides, etc., which leads to an increase in the machine’s depreciation period.

1. Work with hard-alloy bits proves to be far more economical than work with steel bits, since, owing to the reduced consumption of bits, it requires less metal and improves the working conditions of the crew servicing the machine by eliminating the need to transport large quantities of bits to the work site.

Recently, hard alloys of the stellite type have been used as surfacing for the bits of coal-cutting machines; these are based on chemical compounds of carbon with tungsten, chromium, manganese, iron, and other elements (carbides), which possess high resistance to abrasion. The hard alloys that have found the widest application are stalinite, sormite, pobedit, vokar, and others.

The operation of bits surfaced with hard alloy has shown their great advantage. But alongside this, practice has shown that the use of the hard alloys stalinite and pobedit at present does not fully solve the problem of obtaining bits of proper quality.

The rapidly developing Stakhanovite movement in the coal industry and the transition of mines to a strictly cyclic method of work have encountered quite a few obstacles. One such obstacle is the bits of coal-cutting machines. Frequent stoppages of the coal-cutting machine to change blunted bits disrupt the order of work in the longwall, throw crews off schedule, etc.

To overcome this obstacle, Gormashproekt proposed manufacturing bits reinforced with pobedit crumb. This proposal is still only being tested under production conditions and has not found mass application. And it is unlikely that the mines of the Union will soon be able to use these bits in operation, since manufacturing bits reinforced with pobedit crumb will require a large number of contact welding machines at the mines, which they do not currently possess.

The author, together with engineer V. M. Grozina, proposed for the mines of Kuzbass a new type of surfacing, which differs from all methods currently in use and has a whole series of advantages over them.

A substantial feature of the proposed type of surfacing is an alloy that, in its physical and mechanical properties, is similar to Hadfield steel. This alloy is obtained after melting iron electrodes coated with SG flux, consisting of components which, after melting in the zone of the electric arc, together with the wire and the metal of the bit, produce a high-manganese metal.

Electrodes with SG coating outwardly consist of metal rods 400 mm long and 3–4 mm in diameter,

Fig. 1.

Labels in the figure: \(c\), \(F_{\mathrm{t}}\), \(\lambda\).

Fig. 2.

Labels in the figure: electrode; graphite; stop; bit.

Fig. 3.

having along a length of 370 mm a continuous coating layer 0.8–1.2 mm thick.

Wire from the Beloretsk plant or another plant, grades I and II according to OST-2407, is used as the rods. The coating of these electrodes consists of the following components:

1. Ferromanganese — 800 parts by weight, or 63.2%
2. Silver graphite — 145 parts by weight, or 11.5%
3. Fluorspar — 50 parts by weight, or 4.1%
4. White marble — 120 parts by weight, or 9.4%
5. Liquid glass — 150 parts by weight, or 11.8%

The SG-type coating is a gas- and slag-forming coating of the acid type. An alloying material—ferromanganese—has been introduced into the coating.

The chemical composition of the metal deposited with SG electrodes is presented in Table 1.

Table 1.

The hardness and wear resistance of the layer deposited with these electrodes is provided in it by up to 1.35% carbon and 10–15% manganese. The hardness of the deposited metal, determined on a Brinell apparatus, is 460–480 units. In the process of abrasion work, the surface layer of the deposited metal, becoming work-hardened, obtains a hardness of up to 500 Brinell units.

With regard to microstructure, deposition with electrodes having an SG coating is a combination of an austenitic matrix, complex carbides of iron and manganese, and, in some cases, products of decomposition of ledeburitic eutectic.

Electrodes with an SG coating are melted by the Slavyanov method. When working with them, it is preferable to use direct-current welding machines. However, the possibility is not excluded of using alternating-current transformers, especially when an oscillator is connected into the welding circuit.

Depending on the diameter of the electrode wire, when working on a direct-current machine the following current values should be observed: for an electrode diameter of 4 mm—160–180 A, and for a diameter of 5 mm—190–200 A. When working on alternating current, the current of the voltaic arc should be equal to:

\[ A = 50d, \]

where \(A\) is the current in amperes, \(d\) is the diameter of the electrode wire in millimeters.

A bit for a heavy coal-cutting machine of the Gorlovka Plant type is hardfaced with SG electrodes. The bit to be hardfaced must be sharpened on a Sullivan sharpening machine. The surface of the end of the bit intended for hardfacing must be cleaned of dirt, scale, oil, etc., down to a metallic luster.

During hardfacing, the bit must be positioned so that the surface of the upper face assumes a horizontal position. For this purpose, the cleaned bits are placed in multi-seat carbon or graphite molds, Fig. 2. The recesses in the molds are made with such calculation that the surface of the upper face assumes a horizontal position.

Hardfacing should be begun from the sharp part and proceed toward the thickened part of the bit. In this process the electrode is set at an angle of 10–15° to the vertical, with its upper end inclined from left to right (if viewed from the welder’s right side).

The speed of movement of the electrode during hardfacing must be such that a pool of molten metal is constantly maintained under the electrode, and also such as to provide the necessary depth of penetration and thickness of the deposited layer. The speed of movement of the electrode near the nose of the bit must be somewhat greater than in the thickened part of the bit. The hardfacing of the bit must be carried out as indicated in Fig. 3.

After completion of the hardfacing of the bit, it should be placed for slow cooling in dry sand or ash. With very rapid cooling, the hardfacing proves less hard and fine cracks, invisible to the eye, form, which impair the quality of the deposited layer.

In order to obtain a sharp end of the bit and cutting faces after hardfacing, its rear part is subjected to mechanical treatment (grinding) with alundum or carborundum wheels. The upper face of the bit is not machined. The sharpened bit has the appearance shown in Fig. 4.

With one SG-coated electrode, 30 bits are hardfaced. In one shift, a welder of average qualification hardfaces up to 1,000 bits. The cost of one ton of SG-coated electrodes, when they are made by the artisanal method, is 1,256 rubles. From one ton, 22,600 electrodes of 4 mm diameter are obtained. In all, 67,800 bits can be hardfaced with these electrodes.

Preliminary experiments under laboratory conditions and at mines 9/15 and 5/7 of the Anzherougol and Kuzbassugol trusts showed that bits with SG hardfacing are at least 8 times more wear-resistant than bits of the Gorlovka Plant, and three times more than bits of the Gorlovka Plant hardfaced with stalinit.

At the assignment of the Chief Coal Directorate of the East, we, jointly with the Scientific Research Coal Institute of Kuzbassugol, conducted prolonged tests of bits with SG hardfacing and, for comparison, bits hardfaced with stalinit, cast iron, reinforced with pobedit plates, as well as unreinforced ones. The tests

were conducted at the mines: S. M. Kirov, Zhurinka 3, Em. Yaroslavsky, of the Leninugol trust, Kuzbass Combine.

More detailed tests were carried out at the S. M. Kirov mine on the Serebrennikov and Meyer seams. The coal was of increased hardness, with individual inclusions of sulfur pyrite.

The bits were installed on DTK-2 machines, with a 22 kW motor; the rate of travel of the machine was set at 54 and 72 meters/hour. The cutting speed was from 2.15 to 2.40 meters/sec. The length of the longwalls where the tests were conducted was from 135 to 195 meters; the thickness of the coal seam was from 1.15 to 1.85 m; the dip angle was from 10° to 15°. The work of the machines was carefully timed by specially assigned timekeepers. In all, the following bits were tested:

1. Ordinary bits of the Gorlovka Plant named after S. M. Kirov — 150 pcs.
2. Bits of the Gorlovka Plant, surfaced with stalinite on one side — 400 pcs.
3. Bits of the Gorlovka Plant, surfaced with stalinite on two sides — 406 pcs.
4. Bits of the Gorlovka Plant, surfaced with cast iron — 380 pcs.
5. Bits of the Gorlovka Plant with SG surfacing — 876 pcs.
6. Bits reinforced with pobedit plates — 28 pcs.

The bits of the Gorlovka Plant were made from grade 6 steel and quenched in oil.

The bits reinforced with pobedit plates were obtained from Plant No. 3; their principal dimensions and shape are shown in Fig. 5.

The cast iron for surfacing the bits was specially selected by us (1). Its chemical composition is given in Table 2.

Table 2.

Both the rear and the front side were built up with cast iron. The shape and dimensions of the bits with SG surfacing, with two-sided stalinite and cast-iron surfacing, are shown in Fig. 6.

The scope of the present article does not permit us to dwell on the separate results of the comparative tests of all the tested types of bits, diagrams, and other data; therefore we may confine ourselves to the data presented in Table 3, where the following indicators are summarized:

1. Number of bits tested.
2. Specific consumption of bits per 1 m² of cut area, which is obtained by dividing the number of bits worn out during cutting by the undercut area (under the same conditions).

Fig. 4.

Fig. 5.

Fig. 6.

1. Specific consumption of bits, in percent. The operation of bits with brazed-on Pobedit plates is taken as 100%.

2. The average rate of advance of the machine, obtained by dividing the number of linear meters undercut by the net cutting time.

3. The total, or commercial, rate of advance of the machine, obtained by dividing the number of linear meters of face undercut by the net cutting time plus the time for changing bits.

Table 3

Our tests showed that the unreinforced bits from the Gorlovka plant are very unstable in operation and give a high specific consumption per one undercut meter of coal. In addition to wearing out quickly, these bits also break very often. This occurs because, when the tip has worn down, unreinforced bits take on higher specific pressures, and also because the existing shape of the bits has not been chosen quite correctly.

Rapidly wearing bits do not cut a sufficiently wide undercut slot, which causes the bar to jam and worsens the operating regime of the machine. As a result, the productivity of the coal-cutting machine falls.

Bits faced with Stalinit on one side also give results in operation that are not entirely satisfactory. These bits have the following substantial shortcomings:

1. The amount of undercut coal reaches (at the average cutting speed) 50–60 linear meters, after which the stalinite surfacing is worn away and the bit must be replaced with a new one;

2. Very often, when undercutting coal, the stalinite surfacing of the bit breaks away from the body of the bit, and then the operator of the coal-cutting machine is forced to stop the machine, since the motor overheats severely, because the machine is operating, in essence, with blunt bits;

3. The welding-on of stalinite is carried out in such a way that it does not make it possible to obtain a proper cutting edge. Nor can bits with such surfacing be sharpened, since stalinite loses its physical properties during sharpening (chiefly its hardness). Therefore, when working with such bits, the coal is not undercut but crushed. As a result, the coal-cutting machine is forced to work under very heavy conditions, and this has an especially strong effect on the operation of the motor.

4. As our investigations show, and also the experiments of other authors who carried out tests in the Donbass, the bits work mainly with their rear and end portions; the front face does not take part in the work; consequently, stalinite surfacing of the front face of the bit does not protect it from rapid wear.

Fairly good results in operation are given by bits surfaced with stalinite on two sides. Their specific consumption is \(0.20\) bits/\(\text{meter}^2\). The rate of advance of the machine in this case is \(0.495\) meters per minute. It should be noted that the manufacture of bits with double-sided surfacing is rather troublesome. A highly qualified welder can surface no more than 200 such bits per shift, which greatly increases the cost of such bits.

Very good results in operation are shown by bits reinforced with pobedit plates. Their specific consumption is only \(0.12\) bits/\(\text{meter}^2\). This is 11 times less than for unreinforced bits and more than twice less than for bits with double-sided stalinite surfacing. But, despite the fact that bits reinforced with pobedit plates show such results, their use at present cannot take place because these bits have a whole series of shortcomings. These bits have the following principal shortcomings:

1. In hard coal seams, the brazed-on pobedit plates crumble out very quickly.

2. Very often the pobedit plates separate from the body of the bit, because the solder does not provide sufficient strength of the joint.

3. When the cutting chain moves in the direction opposite to the direction of cutting, in the event that the coal-cutting slot becomes clogged, there is an almost 100% occurrence of the hard-alloy plates breaking off.

1. The manufacturing technology for bits with brazed-on plates is fairly complex and cannot be mastered quickly by individual mines.

2. The cost of bits with brazed-on Pobedit plates is very high (a bit reinforced with a Pobedit plate costs 15 rubles), which is economically unprofitable. In our tests, a batch of 28 bits was not sufficient to undercut completely a coal face 175 meters long.

Bits with SG hardfacing performed very well in operation. The specific consumption of these bits is only 0.15 bit/meter³. In view of the fact that the bits are considerably more durable than bits hardfaced with Stalinit and cast iron, the productivity of the coal-cutting machine increases substantially. In the tests, very often a coal face 175 meters long was undercut with one set of bits. In general, bits with SG hardfacing, in comparison with other bits, have the following advantages:

1. Simplicity of producing the hardfacing.
2. The cost of hardfacing a set of bits (30 pcs.) does not exceed 75 kopecks.
3. No special capital expenditures are required for mines to introduce bits with SG hardfacing.
4. Undercutting coal of medium and increased hardness without changing bits reaches 150–200 running meters.
5. A hardfaced bit, after becoming dull, can be resharpened at least three times and put back into operation.
6. The time required for undercutting coal is reduced, since the time spent on frequent bit changes and motor cooling is eliminated.
7. The cutting speed of coal is increased, since bits with SG hardfacing, when deformed (during cutting), become more durable and resist wear well.

The good performance of bits with SG hardfacing gave the mines of the Leninugol Trust, the Kuzbass Combine, grounds to switch completely to operating coal-cutting machines with bits of this type. In addition to the fact that bits with SG hardfacing have a low specific consumption, they are also very advantageous economically. This is indicated by the following fact: at the S. M. Kirov mine, where up to 20 heavy coal-cutting machines operate, the use of only bits with SG hardfacing yields up to 25,000 rubles in savings on direct costs (the cost of the bit material, the cost of hardfacing, labor, and production costs for manufacturing the bit).

Conclusion.

From the above results of tests of bits with hard-alloy deposited metal for coal-cutting machines, it is clear that at present the best results have been shown by bits with double-sided deposition using SG electrodes; second place is occupied by bits hardfaced with Stalinit. The worst results are given by ordinary—

...the forged bits of the Gorlovka Plant named after S. M. Kirov, which must be replaced by reinforced ones.

Bits with brazed Pobedit plates, in their existing form, cannot be used for coal cutting.

Literature:

1. V. I. Strokopytov. Surfacing of cast-iron bits of a cutter-loader. Fuel Engineering, 1939, No. 7.