METHOD OF PRODUCING COMPOSITE STEEL BODY SHAFT

Abstract: A method of producing a composite steel body shaft by using at least one cooling metal mold (29, 31) having a cavity (45) of a diameter and a steel body (30) having another cavity (47) of another diameter both of which cavities (45, 47) are coaxially disposed to define a through-hole in which the remelting and solidi­ fying of electrode (27) is effected to provide a shaft portion (41) connected to the hollow body (30). At a contact position defined by both the metal mold (29, 31) and the hollow steel body (30) at which contact position the diameter of the through-hole is reduced step-wise, a space is provided so that the diameter of the through hole is reduced gradually without the step-wise reduction of diameter of the through hole so as to prevent the confining of slag (39) from occurring at the contact position.

Description: The invention relates to a method of producing a composite steel body shaft, comprising the steps of vertically disposing a cylindrical steel body having a coaxial cavity with a circular cross section and upper and lower axial end surfaces, vertically disposing an upper and/or lower cylindrical metal mold having a coaxial cavity with a circular cross section in a coaxially contacting relation to the upper and/or lower axial end surface of the cylindrical steel body, thereby forming a through-hole defined by said cavities of said cylindrical steel body and said cylindrical metal mold(s), inserting a consumable electrode into said through-hole, and effecting a vertically upward electroslag remelting- and-solidifying of the electrode by supplying power to said consumable electrode so as to form a shaft portion in said through-hole and so as to connect said shaft portion and said cylindrical steel body to each other. With such a method known from DE 26 13 535 A1, the shaft portion of the composite steel body shaft is formed from the bottom in vertically upward direction with increasing diameters, the transition areas shaped as diverging trucated cones. Because of the upwardly increasing diameters, there is no confinement of slag in solidifying regions of the metal forming the shaft portion. Upwardly from the cylindrical steel body the mold sections have decreasing diameters, the transition areas shaped as converging truncated cones. As the diameter of the bottom of the truncated cone having the greater diameter and facing the cylindrical steel body corresponds to the diameter of the cylindrical steel body, the upward movement of the slag on the top of the molten metal from the cavity of the hollow steel body to the upper cooling metal mold results in the occurrence of slag confining which means occurrence of a notched portion in the solidified slag-confining portions. Due to this confined slag and the occurrence of the notched portions, there is the risk of serious defects during use of the composite steel body shaft. It is the object of the present invention to provide a method of the generic kind for producing a composite steel body shaft of high quality by use of electroslag remelting. This object is achieved with the method of the generic kind in that a diameter reduction of the cavities is provided between the cylindrical steel body and the upper cylindrical metal mold and/or between the lower cylindrical metal mold and the cylindrical steel body, in that the section of the cavity of the upper cylindrical metal mold facing the cylindrical steel body and/or the section of the cavity of the cylindrical steel body facing the lower cylindrical metal mold is formed as a truncated cone the end face of which having the greater diameter faces the cylindrical steel body and the lower cylindrical metal mold, respectively, and the greater diameter of the truncated cone is greater than the diameter(s) of the opposing cavity of the cylindrical steel body and/or of the lower cylindrical metal mold. With the method according to the invention any slag confinement in the sections, where the diameter of the shaft portion to be formed is reduced in the vertical upward production direction, is prevented resulting in the exclusion of any notched portions in the surface of the composite steel body shaft in transition areas including a diameter reduction. Therefore, the composite steel body shaft produced in accordance with the invention has a high quality. It is convenient that the inclination of the chamfer of the truncated cone is in a range of 5° to 40° with respect to the axis of the through-hole. It is preferred that the shaft portion is made of carbon steel for machine structural use, the cylindrical steel body integrally connected onto the shaft portion being made of a high Nickel ductile cast iron. With such a composite steel body the high Nickel ductile cast iron consists essentially by weight of 32 to 46 % Nickel and the balance iron. The composite steel body shaft produced according to the invention is preferably used to form a shaft member, such as a screw rotor for a screw compressor, which screw rotor has tooth portions and a shaft portion. The invention is further explained with reference to the accompanying drawings, in which Fig. 1 shows schematically in an axial section an arrangement for producing a composite steel body shaft, Fig. 2 is an axial section of a composite steel body shaft produced with the arrangement of Fig. 1, Fig. 3 is an axial section of the lower mold and of the steel body showing melting of the shaft portion and Fig. 4 is an axial section of the steel body and the upper mold during melting of the shaft portion. The arrangement shown in Fig. 1 is used to produce a composite steel body shaft to be formed into a rotor for use in an oil-free screw compessor, the composite steel body shaft being in the form of a stepped round bar. A center shaft portion thereof is made of carbon steel for machine structural use such as S45C defined in JIS G4051 which is a material of the consumable electrode 27. An outer steel body in form of a hollow round bar 30 of high nickel ductile cast iron consisting of 32 to 46 wt% Ni and the balance Fe and incidental impurities was connected to a part of the outer periphery of the center shaft portion. In this arrangement, a lower cooling mold 31 made of Cu which has an internal cavity 45 and which as both a columnar shape having a diameter of 39 mm and an water jacket 43 were placed on a molding board 38 made of Cu which is disposed at the lowermost position. A hollow round bar 30 made of hig Ni ductile cast iron and which bar 30 has a columnar cavity 46 of 23 mm in diameter and a truncated-cone-shape space 47 was coaxially placed on the lower cooling mold 31. An upper cooling mold 29 made of Cu which has both an internal cavity 45 and an water jacket 42 was placed in an end-to-end contact coaxial relation to the hollow round bar 30. The truncated-cone-like space 47 was defined by a chamfer having an inclination of 5.2° and was provided with a lower bottom of 40 cm in diameter. The cooling metal molds were formed of copper because copper has a high thermal conductivity. Cooling water was supplied to the water jackets of the upper and lower cooling metal molds by a pump 34 which draws cooling water from a water tank 36. Cooling water was first supplied from the pump 34 to the water jacket 43 via a pipe 33, then to the water jacket 42 via a pipe 32, and was finally returned to the water tank 36 via a pipe 35. An consumable electrode 27 was inserted in the through-hole so that the lower end thereof was in the vicinity of the board 38, and electroslag remelting was started from the position immediately above the molding board 38. At this time, electric power of 500 - 600 A at 35 - 45 V was supplied from power source equipment 28 by connecting one of the terminals thereof to the molding board 38 through a brush 37 and by connecting another terminal to the consumable electrode 27 through an electrode-lifting device 26. In this state, electroslag remelting was continuously performed successively from the lower cooling metal mold 31 to the hollow round bar 30 then to the upper cooling metal mold, therby obtaining a composite shaft member bar for producing a composite rotor used in an oil-free screw compressor. In this melting process, a chamfer 44 having an inclination of 5,2° with respect to the axis of the through-hole was provided at the lower end of the hollow round bar 30 in order to prevent the confining of slag 39 from occurring at any intermediate portion, with the result that no occurrence of a notched or recessed portion at the end portions of the hollow round bar 30 was ensured because no confining of slag occurs during the upper movement of the slag and remelting metal. The shaft member as shown in Fig. 2 and produced by this method had a center shaft portion 18 made of the material S45C and an outer peripheral portion 30 made of the high nickel ductile cast iron connected to a part of the center shaft portion 13 by a connecting portion 16 without any notched portions in the corner part 47'. In the case shown in Fig. 3 the size of the cross section of the cavity of the hollow steel body 4 having the diameter d₄ is smaller than that of the cross section of the cavity of the lower cooling metal mold 5 having the diameter D₅. At the position of a contact interface defined between the lower cooling metal mold 5 and the hollow steel body 4, that is, at a position of the through-hole where the size of the cross section thereof is reduced with respect to a direction in which the melding portion 14 of the solidified shaft portion 13 proceeds, a sudden reduction in the size of the cross section is prevented by providing a chamfer 20 in the hollow steel body 4 which chamfer 20 defines a truncated cone shape having a lower bottom with a diameter d'₄ slightly larger, for example by 1 to 3 mm than the diameter D₅ of the metal mold cavity, therby preventing the confining of the slag 15 from occurring and allowing manufacture of a composite steel body shaft having no notched portion. It is preferred that an inclination of the chamfer 20 defined with respect to the axis of the through-hole is in a range of 5 to 45°. Fig. 4 illustrates the case in which slag 15 moves from the cavity of the hollow steel body 4 to the upper cooling metal mold 6 and in which the size of the cavity cross section of the upper cooling metal mold 6 having the diameter D₆ is smaller than that of the cavity cross section of the hollow steel body 4 having the diameter d₄. For achieving smooth upper movement of the slag 15 and the melting portion 14 through the through-hole having the diameter-reducing portion, a chamfer 23 is formed at the lower edge of the cooling metal mold 6, which chamfer 23 defines a space of a truncated cone shape having a lower bottom facing the hollow steel body 4, which bottom is slightly larger in size than the cavity of the metal mold 6. The inclination of the chamfer 23 is in a range of 5° to 45°.

Claims: A method of producing a composite steel body shaft, comprising the steps of vertically disposing a cylindrical steel body (4, 30) having a coaxial cavity (46) with a circular cross section and upper and lower axial end surfaces, vertically disposing an upper and/or lower cylindrical metal mold (5, 6; 31, 29) having a coaxial cavity (45) with a circular cross section in a coaxially contacting relation to the upper and/or lower axial end surface of the cylindrical steel body (4, 30), thereby forming a through-hole defined by said cavities (46, 45) of said cylindrical steel body (4, 30) and said cylindrical metal mold(s) (5, 6; 31, 29), inserting a consumable electrode (7, 27) into said through-hole, and effecting a vertically upward electroslag remelting-and-solidifying of the electrode (7, 27) by supplying power to said consumable electrode (7, 27) so as to form a shaft portion (13) in said through-hole and so as to connect said shaft portion (13) and said cylindrical steel body (4, 30) to each other, characterized in that a diameter reduction of the cavities (45, 46) is provided between the cylindrical steel body (4, 30) and the upper cylindrical metal mold (6, 20) and/or between the lower cylindrical metal mold (5, 31) and the cylindrical steel body (4, 30), the section of the cavity (45) of the upper cylindrical metal mold (6, 29) facing the cylindrical steel body (4, 30) and/or the section of the cavity (46) of the cylindrical steel body (4, 30) facing the lower cylindrical metal mold (5, 31) is formed as a truncated cone (23, 20, 47) the end face of which having the greater diameter (D'₆, d'₄) faces the cylindrical steel body (4, 30) and the lower cylindrical metal mold (5, 31), respectively, and the greater diameter (D'₆, d'₄) of the truncated cone (23, 20, 47) is greater than the diameter(s) (d₄, D₅) of the opposing cavity (46, 45) of the cylindrical steel body (4, 30) and/or of the lower cylindrical metal mold (5, 31). A method according to claim 1, wherein the inclination of the chamfer of the truncated cone (23, 20, 47) is in a range of 5° to 40° with respect to the axis of the through-hole. A method according to claim 1 or 2, wherein the shaft portion (13) is made of carbon steel for machine structural use, the cylindrical steel body (4, 30) integrally connected onto the shaft portion (13) being made of a high Nickel ductile cast iron. A method according to claim 3, wherein the high nickel ductile cast iron consists essentially by weight of 32 to 46 % Nickel and the balance iron.

Inventors: KURAGANO, MITSURU, SAKAI, TAKAAKI
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: B22D27
IPC codes: C22B9, B22D19, B22D23