Justification of the Choice of Metals for Drilling Hammer Components

G. V. Toporov, V. F. Gorbunov

The durability of drilling hammers and the preservation, during operation, of the parameters characterizing their productivity are determined to a considerable degree by the choice of the material from which the parts are made, and by the technology of their manufacture. Of substantial importance is also the correct choice of the mechanical properties of the metal—in particular, hardness—which ensure minimum wear of the parts and the greatest resistance to failure. It is widely known that the service properties of parts depend, in addition to those mentioned, on a number of other factors: the structure of the metal, the cleanliness of the surfaces of the parts, etc.

The above, far from complete, list of the most important factors influencing the service properties of parts testifies to the complexity of the task of selecting their optimal values.

The matter is further complicated by the fact that very often a change in some factor, while favorably affecting some properties of a part, simultaneously worsens others. Thus, for example, high surface hardness of a part, while helping to increase wear resistance, reduces resistance to failure under impact loads.

In the present work an attempt has been made, on the basis of data obtained as a result of investigations of drilling-hammer parts from a number of plants and firms, as well as through the use of literature sources, to substantiate: 1) the advisability of using particular grades of metals for the manufacture of parts of modern drilling-hammer designs; 2) the selection of parameters of the mechanical properties—in particular, hardness—of parts; 3) in some cases, the technology of strengthening treatment.

In carrying out the work we used data on the parts of more than twenty different drilling machines produced by...

both by domestic plants and by foreign firms. These data were borrowed by us from the materials of the Pneumatika plant and the Malakhov Experimental Plant of Giproglemash. In addition, together with employees of the Central Plant Laboratory of the Tomsk Electromechanical Plant—engineers E. F. Filatova and E. S. Anan’eva—we carried out a comprehensive study of the parts of five drilling hammers, including chemical analysis and metallographic examination.

Striker piston

The striker piston is one of the most heavily loaded parts of the hammer. It is subjected to cyclic impact loading, striking the tool with its end face. Its surface is worn by the mating parts: the cylinder, the guide bushing, the rotary coupling, and, in some designs, the ratchet bushing.

Thus, requirements are imposed on this part for, on the one hand, high wear resistance and, on the other, considerable resistance to impact-fatigue failure.

The wear resistance of a part, as a rule, increases with increasing surface hardness [1]. Resistance to impact-fatigue failure is in a complex dependence on hardness and, as our investigations have shown [2], is determined by the ratio of hardness to impact toughness. The presence of stress concentrators associated with the configuration of the part and the cleanliness of machining has a substantial effect on resistance to impact-fatigue failure. The harder the steel, the more strongly its sensitivity to stress concentrators is manifested. Raising the hardness above 55 \(H_{R_c}\) leads, for almost all steels without exception, to a decrease in resistance to impact-fatigue failure. At the same time, to ensure high wear resistance it is desirable to have a hardness of 60–62 \(H_{R_c}\).

To satisfy simultaneously the requirements of high wear resistance and considerable resistance to impact-fatigue failure, it is necessary to adopt such a decision regarding the choice of hardness that, while satisfying the principal requirement, would not lead to a substantial deterioration in the quality of the part from the standpoint of the other requirements. In the present case the principal requirement should be considered resistance to impact-fatigue failure, since the initiation and development of a fatigue crack lead comparatively quickly to the final removal of the part from service.

At domestic plants the striker piston is made from alloy carburizing steels of the grades: 12Kh2N4, 18KhGT, 12KhN3, 20Kh. The depth of carburizing ranges from 1.1–

1.5 to 2.0–2.2 mm. The surface hardness in some cases ranges from 58–62 \(H_{R_c}\), in others from 55–60. The hardness of the core also varies over a wide range, from 29 to 45 \(H_{R_c}\).

Some foreign firms manufacture pistons from carbon steel corresponding in chemical composition to steels of grades U12, U10 (Holman, Demag, Atlas-Diesel), U8, U7 (M-11, DK-60-1). The surface hardness in most cases fluctuates within \(H_{R_c}=59\text{–}62\), more rarely 56–59. As a rule, the parts are not hardened through. The depth of the layer hardened to high hardness is 2–4 mm. The hardness of the core fluctuates within 35–45 \(H_{R_c}\).

Hardenable alloy steels are used in T-10 hammers (Finland). This steel is close in chemical composition to grade 6KhV2S. The hardness of a piston made from this steel fluctuates within 53–58 \(H_{R_c}\).

When considering the advisability of using one or another grade of steel, it should be borne in mind that in many cases the choice of grade is determined, in addition to the general properties of the metal, by the configuration of the part and the manufacturing technology.

The use of carbon steel for the manufacture of striker pistons is not the best solution to the problem of selecting a steel grade for the following reasons. Quenching to obtain the high hardness to which pistons are hardened must be carried out in water (or through water into oil). With a complex configuration of the part and the presence of notches, this entails the possibility of quench cracks [3]. Steel U10, U12, hardened to high hardness, has fairly good wear resistance, but at the same time also considerable sensitivity to notches under impact-cyclic loading.

Alloy case-hardening steel of the 12Kh2N4 type has proved itself well in the manufacture of striker pistons, providing good wear resistance and resistance to impact-fatigue failure. A significant drawback in the use of these steels is the complication of the manufacturing technology of the parts due to the introduction of the carburizing operation.

It should be noted that the use of complete carburizing of the entire surface, rather than only those areas subject to wear, is in our opinion inexpedient. The carburized layer in the places of greatest stresses, which most often are not subject to wear, will resist impact-fatigue failure less well than the hardened base metal.

The use of hardenable alloy structural steels is in many cases a more successful solution to the question. The use of alloy steels of the 6KhV2S, 5KhV2S, 40KhN, etc., types provides, at hardnesses on the order of 50–55 \(H_{R_c}\), high resistance to impact-fatigue failure and sufficient resistance to wear.

In those cases where the parts of a component that are subject to wear do not experience high stresses under the action of impact cyclic loading, it is most expedient to use chromium-nickel steels with a carbon content of 0.3–0.5%. The parts should be carburized, protecting from carburization the areas not subject to wear. After quenching and low tempering, we obtain a hardness of the carburized areas of 60–64 \(H_{R_c}\), and of the non-carburized areas about 46–52 \(H_{R_c}\). The hardness of the non-carburized areas will be optimal from the standpoint of resistance to impact-fatigue failure, while the hardness of the carburized areas will be the best from the standpoint of wear resistance.

The latter variant of selecting the steel grade and heat-treatment technology is used in the “Medon” hammer (France), where the striking piston is made of a steel corresponding to grade 37KhN3. The piston was subjected to carburizing, subsequent quenching and, judging by its hardness, low tempering. The hardness of the surface is 58–59, and of the core 51 \(H_{R_c}\).

To reduce the influence of stress concentrators on resistance to impact-fatigue failure, it becomes necessary to have a surface finish of the component on the order of \(\nabla\nabla\nabla 9\).

Cylinder

During operation, the cylinder of a drilling hammer is subjected mainly to wear on the piston side. The vibrational load experienced by the cylinder is transmitted through the mating parts, which are subjected to cyclic impact loading. The magnitude of this load is small, since the principal reason for failure of the component is wear.

In most cases, cylinders for hammers are produced by casting from low-carbon steels, with a carbon content of 0.15–0.20%. Very often the steel is alloyed with chromium up to 1%, or with chromium and molybdenum.

To increase wear resistance, the cylinders are carburized to a depth of 0.8–1.5 mm, followed by quenching and low tempering. The hardness of the carburized layer ranges from 58 to 62 \(H_{R_c}\).

Alloying with chromium (and molybdenum) is dictated mainly by the fact that in this case it becomes possible to obtain a high hardness of the carburized layer when quenching in oil. To obtain the same hardness in carbon steel, it must be quenched in water.

The introduction of molybdenum in small quantities promotes the formation of an inherently fine-grained steel [4], which makes it possible to quench directly after heating during carburizing. Taking into account the comparatively low impact load on the cylinder and the scarcity of molybdenum, its introduction into the composition of the steel,

intended for the manufacture of cylinders, cannot at present be recommended.

In the T-10 hammers the cylinder is made of gray cast iron of approximately the following chemical composition: 3.30% C; 1.80% Si; 0.88% Mn; 0.039% S; 0.12 P. The metallic matrix has a pearlitic structure with a uniform distribution of inclusions of phosphide eutectic in the form of a network. The graphite inclusions are small and are arranged in the form of columns with a considerable degree of isolation. Judging by its structure, the cast iron may presumably be assigned to grade SCh24-44.

From the standpoint of antifriction properties, cast iron has an advantage over steel under operating conditions involving low specific pressures and limited lubrication [5, 6].

The widespread introduction of cast iron for the manufacture of cylinders may be restrained only by the danger of breakage from external impacts during operation of the hammers or during their transportation. In this connection, it should be considered expedient for certain hammer designs to manufacture cylinders from malleable or high-strength cast irons [7, 8, 9].

Of the cast steel cylinders, it seems to us most advisable to make them from steel L20Kh. After carburizing, as indicated, a hardness of 60–62 \(H_{R_c}\) can be obtained in the surface layer when quenched in oil. Oil quenching makes it possible to avoid an increased rejection rate due to quench cracks, especially if the cylinder has a complex configuration.

Ratchet Bushing

The ratchet bushing works in pair with the ratchet pawl and, in some designs, mates with the splined portion of the piston rod. The principal requirements imposed on this part are high wear resistance and good resistance to destruction from impacts that arise between the bushing and the mating parts. The bushing must have high hardness—high resistance to plastic deformation, since a change in the shape of the surfaces mating with the pawl as a result of wear or plastic deformation will lead to unreliable operation of the rotating unit.

The principal steel from which the ratchet bushing is made by most plants in this country and abroad is steel 20Kh. Chromium-nickel steel 12KhN2 and steels of the 18KhGT and 18KhGM types are used considerably less often. In American hammers, in addition to steel of the 18KhGM type, a low-carbon steel corresponding to grade 15 is used. The bushing in the RH-754 hammer is made of hypereutectoid steel of the ShKh15 type, and in the French “Medon” hammer it is made of steel grade 37KhN3.

The hardness of the carburized layer ranges from 58 to 63 \(H_{R_c}\); in some cases plants manufacture bushings with

with a hardness of 55–58 \(H_{R_c}\). The carburizing depth of the bushing is set by most plants at 1.0–1.2 mm. In some cases, however, the carburizing depth is specified lower, 0.7–1.0, and sometimes, conversely, higher, 1.2–1.5 mm.

Taking into account that the ratchet bushing is subjected to high specific pressures, especially in those cases where the force is taken up by one of two or four pawls, it is necessary to ensure sufficient hardness of the metal lying beneath the carburized layer. Therefore it is more expedient to use steel 20Kh or, better still, 18KhGT.

When steel ShKh15 is used for the ratchet bushing, a hardness after oil quenching of 61–63 \(H_{R_c}\) can be obtained with through hardenability. The shape of the part makes it necessary to take precautions during quenching in order to avoid the formation of cracks. Therefore bushings made of ShKh15 steel should be subjected to step quenching in order to reduce internal stresses.

Steel ShKh15 has considerably poorer machinability than steel 20Kh, which will lead to some increase in the cost of machining.

The comparatively low cost and non-deficit nature of steel 20Kh, together with satisfactory technological and service properties, serve as the basis for recommending this steel as the principal one for the manufacture of ratchet bushings.

The requirement of high wear resistance and high resistance to plastic deformation in the presence of considerable specific pressures makes it necessary to obtain high hardness in the surface layers of the ratchet bushing. A hardness of not less than 59 \(H_{R_c}\) should be recommended. When steels 20Kh or 18KhGT are used for the bushing, such hardness will be guaranteed by oil quenching.

When assigning the carburizing depth, it is necessary to take into account the dimensions of the parts. It is quite natural that, under the corresponding load, larger parts should have a greater carburizing depth.

Ratchet pawl

This part is required, above all, to have high wear resistance, although it is also simultaneously subjected to impact loading.

The pawl is made from hypereutectoid alloy steel of the KhVG, ShKh15, KhG type, or from carburizing alloy steels: 12KhN3, 12KhN2, 12Kh2N4, 20KhG, 20Kh.

When pawls are made from steels of the first group, they are hardened to a hardness of 56–62 \(H_{R_c}\); sometimes the hardness is reduced to 50–55 \(H_{R_c}\).

When carburized steels are used, the carburizing depth is specified within the range of 0.7–1.2 or 1.4–1.6 mm. The hardness ranges from 55 to 63 \(H_{R_c}\).

From the standpoint of wear resistance, the use of carburized steel has no advantages over the above-mentioned types of hardening steels. At the same time, the use of carburized steel somewhat complicates the technological process of treatment because of the additional operation—carburizing.

The resistance to impact failure of parts made of carburized steels depends on the strength of the carburized layer and on the properties of the core.

According to L. S. Moroz and S. S. Shurakov [11], in carburized specimens having a sufficiently hard core, the resistance to failure under impact is considerably higher than in specimens having the same carburized layer (in depth and properties), but a soft core. In specimens with identical core properties, the specimen with the higher strength of the carburized layer will have the greater impact resistance.

Proceeding from these considerations, the use of a carburized steel alloyed simultaneously with chromium and nickel, which ensure increased strength of the carburized layer and of the core, may be justified. The use of steels of grades 20Kh and 20KhG for manufacturing ratchet pawls is less advisable than steels of grades ShKh15, KhG, and KhVG, since in resistance to abrasion and resistance to impact failure they have no advantages over the latter, while the technology for processing parts made from them is considerably more complicated.

Helicoidal (ratchet) rod

The nature of the load on the helicoidal rod during operation of the drilling hammer is in many respects similar to the load experienced by the ratchet bush. In addition, the rod is subjected to torsional stress. The complexity of the configuration of this part and its heavy loading have necessitated the use, for its manufacture, of an alloyed carburized steel possessing high strength.

Chromium-nickel steels of the 12KhN2 and 12Kh2N4 types have come into wide use both in domestic and foreign practice. Steels of the 18KhGT, 18KhGTM, and 20Kh types are used rather often for making rods. In the USA, steels containing 3% Ni and about 0.25% C are used, as well as steels alloyed with manganese.

The carburizing depth of rods in almost all drilling machines ranges from 0.8 to 1.2 mm. Some plants allow the replacement of carburized steels by hardening steels, for example, 40Kh, with a surface-layer hardness of the part of 49–50 \(H_{R_c}\).

The hardness of the carburized layer of the rod in most drilling machines is 58–63 \(H_{R_c}\). In some machines the hardness of the rods is reduced to 55 and even to 51–55 \(H_{R_c}\) (JB-4).

The use of non-carburized (hardenable) structural steels of the 40Kh type is inexpedient. After hardening they do not provide the necessary hardness and therefore do not ensure the required wear resistance.

The most suitable steels for manufacturing helical rods should be considered steels of the 12KhN2 (12Kh3N4) or 18KhGT type, since they satisfy the requirements of high wear resistance and strength. A fully sufficient carburizing depth when using the recommended steels (12KhN2, 18KhGT) should be considered to be 0.7–0.9 mm.

The hardness of the carburized layer, satisfying the requirements of high wear resistance and sufficient resistance to impact fracture, should be within the range 59–62 \(H_{R_c}\).

The technology of carburizing and subsequent heat treatment must be designed with a view to obtaining a hypereutectoid carbon concentration in the surface zones, but with a uniform distribution of carbides. The presence of a cementite network sharply reduces the resistance of the part to fracture under impact.

Upper Part of the Rotation Nut

The upper part of the rotation nut is subjected to a load analogous to that on the helical rod. In the inner part there are splines mating with the piston rod. At the end face, by means of projections, the nut is joined to the lower part, which is often called the rotation-nut sleeve. The upper part of the nut works mainly in wear, but at the same time is also subjected to impact loading at the places of jointing, and in individual elements the part works in shear.

For manufacturing the rotation nut, the most diverse materials are used. In most drilling machines the nuts are made of carburized alloy steels: 20Kh, 12KhN2, 12Kh2N3, 18KhGT, 20KhG, and in the RH-754 hammer of steel 20 and bronze (BrOTs4-3).

Very rarely are the parts made of steel U10. In English and Finnish hammers the upper part of the rotation nut is made of bronze. In Finnish hammers a tin bronze corresponding to grade BrO-10 is used.

The carburizing depth in various hammers, when carburized steels are used, varies over a wide range. In hammers produced by the Pneumatika plant, the carburizing depth is 0.5–1.0 mm. In the RH-754 hammer it is 0.6–0.7 mm. The nut of the JB-4 hammer has the same carburizing depth. The nuts of hammers from other plants and firms have a carburizing depth of 1–1.5 mm.

The hardness of the surface layer of the bushing in most hammers is specified within the range of \(58\text{–}62\,H_{R_c}\). According to data from the “Pneumatika” plant, the firm Demag manufactures hammers with bushing hardness on the order of \(52\text{–}53\,H_{R_c}\).

In manufacturing the swivel bushing from steel, preference should be given to case-hardening alloy steels rather than hardenable steels of the U10 type. In order to obtain the hardness required in this case, an article made of U10 steel must be quenched in water or through water into oil. In either case, owing to the rather complicated configuration of the part, quench cracks and warping may occur. The use of unalloyed case-hardening steels is inadvisable for the same reasons. Among case-hardening alloy steels, chromium-nickel steels or steels of the 18KhGT type should be considered the best. An obstacle to their use may be their higher cost in comparison with the cost of 20Kh steel and their somewhat poorer machinability.

The use of bronze for manufacturing swivel bushings (their upper parts) may be justified by its higher antifriction properties. Considering that friction of the bushing against the mating parts occurs under conditions of insufficient lubrication, the use of bronze bushings may be expedient from the standpoint of antifriction properties.

The use of bronze for manufacturing the upper part of the swivel bushing causes some complication in the design of the joint with the lower part of the bushing.

Taking into account the scarcity of nonferrous metals, especially tin, there is no urgent necessity to recommend bronze as the material for manufacturing these parts. If, in order to reduce the coefficient of friction, bronze is nevertheless used, then tin bronze for such parts can quite well be replaced by aluminum-iron-manganese bronze.

When alloy case-hardening steels (chromium-nickel steels or steels of the 18KhGT type) are used for manufacturing bushings, a case depth of about \(0.7\text{–}1.0\) mm should be recommended. With a smaller case depth, in the event of slight wear, plastic deformation of the metal layers lying below the case-hardened layer is possible, which almost always leads to destruction of the case-hardened layer and then to intensive wear.

It is inexpedient to specify a hardness of the case-hardened layer below \(58\,H_{R_c}\), especially since obtaining such hardness does not cause technological difficulties and has a favorable effect on wear resistance.

Lower part of the swivel bushing (chuck of the swivel bushing)

The lower part of the swivel bushing is a highly loaded component. The inner surface of the bushing, mating with a...

with the drill shank, is subject to wear under considerable pressures in certain places and under conditions in which abrasive particles may get between the rubbing surfaces. Pressures of great magnitude arise mainly at the ends of the bushing when the alignment of the hammer and drill is disturbed.

These pressures are transmitted in part also to the outer surface of the bushing. The bushing also experiences a considerable dynamic load when its end face strikes the drill collar.

At domestic plants, for manufacturing the lower part of the swivel bushing, mainly case-hardening chromium-nickel steels are used (12KhN2, 12Kh2N4, 12KhN3). An exception is the Kommunist plant, which in the PRS-3 hammer makes the bushing from 18KhGT steel.

In machines made by foreign firms, bushings are made both from case-hardening alloy steels corresponding to grade 12Kh2N4 (“Holman”), and from carbon tool steels of the U10 type (USA).

In Gardner-Denver machines the bushing is made from steel corresponding to grade 5KhGM. The bushing of the T-10 hammer is made from steel corresponding to high-speed steel grade R18. In the RH-754 hammer, a carbon case-hardening steel corresponding to grade 20 is used.

When the lower part of the swivel bushing is carburized, different plants specify far from identical carburizing depths. The Pneumatika plant makes bushings with a carburized depth of 0.8–1.3 mm; the Kommunist plant, 1.4–1.7 and 1.1–1.4 mm. In some cases the carburizing depth reaches 2 mm (KTsM-4, JB-4, RH-754).

The surface hardness of the bushing in almost all hammers is 58–62 \(H_{R_c}\), and in the PM-508 hammer up to 55 \(H_{R_c}\).

The use, by most plants, of chromium-nickel case-hardening steels for manufacturing the lower part of the bushing is fully justified by machine operating practice. These steels provide sufficient wear resistance of the carburized layer and high resistance to fracture under impact.

Of great interest is the use of high-speed steel R18 for making the bushing in the T-10 hammer. The presence of a large quantity of carbides in this steel ensures high resistance to wear, considerably greater than that of case-hardening steels. Experience in the use of high-speed steel indicates the possibility of using steels that are widely employed for making blanking dies, of the Kh12F and Kh12 types.

Steel of the 5KhGM type, used in the USA, provides high resistance to impact fracture, but it is less wear-resistant because of its reduced carbon content.

Taking into account the high pressures at individual locations of the bushing, a case-hardening depth of not less than 1 mm should be recommended. For powerful machines it is advisable to have a case-hardening depth in the range of 1.5–2 mm, with a hardness on the order of 58–62 \(H_{R_c}\).

Helicoidal nut (spiral nut)

During operation of the hammer, the helicoidal nut is subject mainly to wear along its splined part, which mates with the helicoidal rod or with the rod of the striker piston.

In drilling machines made by domestic plants, steel 45 is widely used for manufacturing the nut. In this case the nut is hardened and tempered, as a result of which a hardness is obtained that differs for different machines. The Pneumatika plant produces machines in which the hardness of the nuts ranges from 24 to 28 \(H_{R_c}\). The nuts of machines from the Kommunist plant have a hardness of 42–48 \(H_{R_c}\).

In a number of machines, case-hardening steels are used for manufacturing the nut: in the PT-30, 20Kh (case-hardening depth 1.1–1.4 mm, hardness 44–48 \(H_{R_c}\)); the Demag firm uses steel of the 15Kh type (case-hardening depth 0.8 mm, hardness 58–60 \(H_{R_c}\)).

A large group of plants and firms uses bronze of various grades for manufacturing the helicoidal nut. The Kyshtym plant uses BrOF10-1 bronze, and Izhevsk uses BrO-10. Nuts are made from the same grades in Finland and the USA. In the RH-754 hammer, BrOTs4-3 bronze is used, while in the Holman hammer bronze with a tin content of about 8% is used.

The use of case-hardening steels for manufacturing the helicoidal nut, provided they are hardened to a hardness below 50 \(H_{R_c}\), is inadvisable.

If the reduced hardness of the helicoidal nut is specified for the purpose of decreasing wear of the mating part, it should be pointed out that in most cases, on the contrary, lowering the hardness of one of the rubbing parts causes increased wear of the other. Proceeding from this consideration, the manufacture of helicoidal nuts from steel 40–45 with a hardness of 24–28 \(H_{R_c}\), and even 42–28 \(H_{R_c}\), may also be regarded as insufficiently justified. It is more rational to use case-hardening steels with a surface hardness of the part of not less than 58 \(H_{R_c}\).

Bronzes, especially those with a high tin content, of the BrO-10 or BrOF10-1 type, possess very high antifriction properties and, in operation, ensure a low coefficient of friction and minimal wear of the mating parts. Bronzes of the BrOTs4-3 type have poorer antifriction properties, but are more ductile in comparison with high-tin bronzes. Since they lack hard constituents in their structure,

type \((\alpha + \delta)\) eutectic, characteristic of high-tin bronzes, their wear resistance is considerably lower.

Instead of scarce high-tin bronze, aluminum bronzes of the BAZhM type [10] may be recommended for manufacturing the helical nut.

Valve

High wear resistance and resistance to failure under the action of impact cyclic loading are required of the valve. In operation, the valve must maintain a precise fit to the mating parts. Violation of this condition may entail a sharp reduction in the technical characteristics of the hammer or its failure.

Domestic plants use mainly alloyed case-hardening steels 12KhN2, 12KhN3, and 18KhGT for manufacturing the valve. Alloyed tool steel KhVG is used for valves in two hammers: PR-30LB and KTsM-4.

Among foreign firms, KhVG-type steel is used in the JB-4 hammer.

Valves made of steels of the ShKh15 and KhG types are used in the RH-754 and Gardner-Denver hammer. The Demag firm manufactures valves from 30KhG-type steel, hardening them to a hardness of \(45 H_{Rc}\). Other foreign firms use case-hardening chromium or nickel-molybdenum steels.

When case-hardening steels are used for manufacturing the valve, in most hammers the case depth ranges from 0.5–0.8 mm. In the RPM-17 and OM-506l hammers the case depth is 0.3–0.5 mm.

The hardness of the carburized layer after heat treatment of valves in hammers produced by the Kommunist plant is within the range \(56—59 H_{Rc}\), and by the Pneumatika plant, \(56—62 H_{Rc}\).

The choice of steel grade for the valve is determined not only by the conditions under which this part operates, but also by its design. It is expedient to manufacture plate-type valves from tool alloyed hypereutectoid steels of the KhVG, Kh, etc., type. In manufacturing flanged or cylindrical valves, the use of alloyed case-hardening steels reduces the labor intensity of machining owing to the better machinability of these steels compared with hypereutectoid steels; however, heat treatment is thereby complicated by the introduction of an additional labor-intensive operation—case hardening.

The design of valves is such that, as a rule, they have thin walls, and the carburized layer sometimes amounts to half their thickness. With such a ratio of thicknesses of the carburized—

of the layer and the core, the latter does not have a positive influence on increasing the impact toughness of the part.

From the standpoint of wear resistance, carburized parts have no advantages in comparison with parts made of hardenable alloy tool steels.

Thus, in terms of service properties, valve spools made of carburized or hardenable steels may be regarded as equivalent. The question of choosing between carburized and hardenable steels is therefore decided mainly by the technological convenience of manufacturing the part.

It should be considered irrational to specify a spool hardness below \(57\text{—}58 H_{Rc}\). With an uncomplicated spool shape and in the absence of stress concentrators, a hardness of \(62\text{—}63 H_{Rc}\) is quite permissible.

Housing of the Valve-Spool Box

The housing of the valve-spool box, during operation of the hammer, is subjected to a load analogous to that on the spool. In many hammer designs this part has a complex shape and is of considerable labor intensity in manufacture.

For the manufacture of the housing of the valve-spool box, both in domestic and in foreign hammers, carburized alloy steels 12KhN3, 18KhGT, 20Kh, and 18KhGM are used. However, the firm “Demag” uses a carbon steel of type 20. The carburized depth ranges within 0.5—0.8, 0.8—1.2 mm (in RH-754 and PR-30LB hammers) or even from 1—1.5 to 2 mm (in the KTsM-4 hammer).

The surface hardness of the housings of valve-spool boxes of hammers produced by the “Kommunist” plant is \(56\text{—}59 H_{Rc}\). The “Pnevmatika” plant, which produces RPM-17, OM-506L, and PR-30LB hammers, manufactures housings with a hardness of \(65\text{—}62 H_{Rc}\). In most foreign firms the hardness of the housings ranges within \(57\text{—}63 H_{Rc}\).

For hammers whose valve-spool-box housing has a complex configuration, the use of carburized alloy steels for its manufacture is advisable.

Housings of simple shape may be manufactured from hardenable tool steels alloyed with chromium (or chromium, tungsten, and manganese), of the Kh or KhVG type.

When carburized steels are used, taking into account the need to obtain high hardness of the carburized layer when quenching in oil, alloy steels should be recommended. Fully satisfactory operating results are shown by valve-spool-box housings made of steel 20Kh. This steel may be recommended for the manufacture of similar parts.

The carburizing depth of housings for almost all designs may be taken as 0.5–0.7 mm. The best results, from the standpoint of wear resistance and other service properties, are obtained by nitrocarburizing, since in this case both wear resistance and the fatigue limit are increased.

The hardness of the slide-valve box housing should be within 58–62 \(H_{Rc}\). A lower hardness will lead to fairly rapid wear at the points of mating with the slide valve.

Barrel

The barrel is subjected mainly to wear from the side of the rotation bushings and to cyclic impact loading, which is transmitted through the mating parts.

Barrels are manufactured predominantly by casting from low-carbon steels containing about 0.2% C and about 1% Cr. Unalloyed low-carbon steels of the L15 type are used by the firms Atlas-Diesel, Demag, Medon, and in domestic PR-35 and TP-4 hammers.

Some plants and firms use steels alloyed with chromium and nickel, or with nickel and molybdenum, for barrels. Thus, in the KTsM-4 hammer a steel corresponding to grade 12KhN2 is used. In Holman hammers the barrel is made of a steel corresponding to grade 20NM.

The carburizing depth of the barrels of most hammers ranges from 0.8 to 1.3 mm. The hardness of the carburized surface of the barrels, in almost all hammers for which we have data, is within 58–62 \(H_R\).

The use of carburizing steels for barrels is dictated by the need to have high hardness of the inner surface of the barrel and a generally high toughness that prevents barrel breakage under the action of external impacts. To obtain the required hardness of unalloyed carburizing steels, the parts must be quenched in water, which can cause defects to arise (cracks, warping).

Alloying with chromium makes it possible to obtain high hardness of the carburized layer when quenching in oil.

There is no great need to use steels alloyed with elements other than chromium; therefore, the principal grade that can be recommended is L20Kh.

Considering that misalignment between the hammer and the drill causes high loading of individual areas of the rotation bushing, which is transmitted to the barrel, the carburizing depth of the barrel should be no less than 1 mm. A rather large carburizing depth is also necessary because, owing to the relative massiveness of the barrel, the hardness of the core after quenching is obtained as insuffi-

high. Plastic deformation of the core beneath the case-hardened layer may lead to destruction of the cementite layer.

It is advisable for the hardness of the case-hardened layer of rods to be not less than \(58 H_{Rc}\).

CONCLUSION

The variety of forms of parts bearing the same name, the differences in the magnitude of their loads and, to some extent, in the conditions under which they operate, did not make it possible in all cases to specify recommendations for the choice of material and hardness for them. In a number of cases only general propositions were stated, which to one degree or another may help resolve the question of selecting material for specific parts.

It should be noted that, when deciding on the choice of material and on the technology of strengthening treatment for a part, one should not always strive for the greatest durability of the given part; it is more important to ensure the maximum service life of the assembly or of the machine as a whole. In doing so, it is sometimes necessary to reduce the service life of the given part in order to increase the durability of the mating parts.

When considering the strength of parts on the basis of mechanical properties, it should be borne in mind that the characteristics of the mechanical properties of various metals are obtained on standard specimens. In order to pass from the characteristics of the material properties obtained on specimens to the characteristics of these properties in the manufacture of a specific part, it is necessary to know a number of transition relationships that take into account the influence of various factors (scale factor, temperature factor, and others).

When recommending hardness values for parts bearing the same name and the depth of their carburizing, it is not always possible to indicate a general treatment technology that ensures obtaining the required parameters. Consideration of the technology of strengthening treatment for each part would be a very particular solution; therefore we considered it more expedient to confine ourselves to general recommendations.

When developing the technology, it is necessary to take into account that, in the process of heating for hardening or annealing of parts that are not subjected to subsequent machining, the quality of the parts may be reduced owing to decarburization of the surface layer. The presence of a decarburized layer is especially dangerous in parts subjected to cyclic impact loading or wear. Consequently, the technology must provide for heating such parts under conditions of the least possible oxidation. The oxidation process, in addition to decarburization, by causing scale formation, worsens the cleanliness of the surface, which also to a considerable degree reduces resistance to fatigue failure.

In developing the technological process of carburizing, preference should be given to gas carburizing over carburizing in a solid carburizer. However, for almost all parts still better results can be obtained by using nitrocarburizing. Nitrocarburizing makes it possible to accelerate the carburization process and to carry it out at lower temperatures (on the order of 840°) than in the case of carburizing. In addition, during nitrocarburizing the surface layer is saturated with nitrogen, which, in turn, has a very beneficial effect on corrosion resistance and wear resistance.

Studies of a considerable number of parts have shown that many plants manufacturing drilling hammers either do not specify, or only weakly control, the presence of a carbide network in the carburized layer. For parts subjected to impact and especially impact-cyclic loading, a carbide network or a network-like orientation of carbides in the carburized layer is extremely undesirable. The presence of a carbide network and a network-like orientation of carbides in the surface layer sharply reduces impact toughness and resistance to impact-fatigue failure.

Not all plants take into account the importance of the surface finish of parts for increasing their durability. The importance of surface finish is especially great for parts operating under conditions of variable cyclic loading. In many cases, the service life of parts can be increased severalfold solely by improving the surface finish.

LITERATURE

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2. G. V. Toporov. The influence of texture and initial mechanical properties on the impact fatigue of steel 40KhN. Proceedings of the Siberian Physico-Technical Institute, issue 34, 1955.

3. Yu. Ya. Fridman. Mechanical Properties of Metals. Oborongiz, 1952.

4. N. F. Bolkhovitinov. Grain Size and Properties of Steel. Metallurgizdat, 1943.

5. D. V. Konvisarov. Wear of Metals. ONTI, 1938.

6. M. S. Baranov. Investigation of the wear resistance of cylinder liners made of modified and low-alloy cast irons. Publishing House of the Communal Services of the RSFSR, 1953.

7. E. A. Sukhodolskaya. Materials for Piston Rings. Mashgiz, 1953.

8. B. N. Seredenko, V. I. Stetsenko, E. A. Markovskii. Increasing the Wear Resistance and Service Life of Machines. Mashgiz, 1956.

9. Ya. A. Lifshits. Foundry Production, No. 10, 1955.

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11. L. S. Moroz, S. S. Shurakov. Mechanical nature of the strength of carburized steel. Collection of papers: Problems of Structural Steel, Mashgiz, 1949.