WO2001032336A1 - A method for coating a casting - Google Patents

A method for coating a casting Download PDF

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Publication number
WO2001032336A1
WO2001032336A1 PCT/FI2000/000950 FI0000950W WO0132336A1 WO 2001032336 A1 WO2001032336 A1 WO 2001032336A1 FI 0000950 W FI0000950 W FI 0000950W WO 0132336 A1 WO0132336 A1 WO 0132336A1
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WO
WIPO (PCT)
Prior art keywords
coating
cast
coating layer
core
metal
Prior art date
Application number
PCT/FI2000/000950
Other languages
French (fr)
Inventor
Jari Koskinen
Pentti Eklund
Petri Jokinen
Original Assignee
Valtion Teknillinen Tutkimuskeskus
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Valtion Teknillinen Tutkimuskeskus filed Critical Valtion Teknillinen Tutkimuskeskus
Priority to AU11507/01A priority Critical patent/AU1150701A/en
Publication of WO2001032336A1 publication Critical patent/WO2001032336A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D19/00—Casting in, on, or around objects which form part of the product
    • B22D19/08—Casting in, on, or around objects which form part of the product for building-up linings or coverings, e.g. of anti-frictional metal

Definitions

  • the present invention relates to a method as defined in the preamble of claim 1.
  • the object of the invention is to eliminate the above-mentioned problems.
  • the method of the invention is characterized by what is presented in claim 1.
  • the mold and/or core are/is coated with a coating layer so that, during the casting operation as the molten metal is solidifying, the coating layer on the mold and/or core is bound to the cast, forming a sur- face layer on it.
  • the molten metal forms a metallurgical joint with the coating on the surface of the mold or core, with the result that a very good adhesion of the coating is achieved.
  • the invention makes it possible to coat the outer and inner surfaces of casts with materials having a good adhesion.
  • the method can be applied to coat areas of the cast that, because of difficult accessibility of the surface, are impossible to coat using traditional spray coating methods.
  • a surface difficult to access is e.g. the inner surface of a hollow body.
  • a layer having a composition and properties differing from those of the base material is formed on the surface of the cast.
  • the composition of the metal used as base material it is possible to produce castings in which e.g. an excellent surface wear resistance is combined with a desired base material property, e.g. a given strength and ductility or a good machineability and weldability.
  • a desired base material property e.g. a given strength and ductility or a good machineability and weldability.
  • the surface layer it is possible to improve the properties of the surface in respect of slide friction, wear resistance and corrosion resistance.
  • Such properties can be utilized e.g. in pumps, engines, valves, gears, etc.
  • the coating layer on the mold or core is formed using a coating method that causes no substantial heating of the mold or core .
  • the coating layer is formed via thermal spraying.
  • the coating layer is formed by arc spraying, flame spraying, plasma spraying or HVOF high-speed spraying. In an embodiment of the method, the coating layer is formed from material whose melting point is higher than that of the metal to be cast .
  • the metal to be cast is steel . In an embodiment of the method, the metal to be cast is aluminum.
  • the coating layer consists of molybdenum.
  • the coating layer comprises ceramic particles.
  • the ceramic material can be applied to the surface of the mold or core e.g. by brushing .
  • the coating layer comprises chromium carbides in a steel matrix.
  • Fig. 1 presents a 50-fold magnification of a coating (in this case, on a metal substrate) on the surface of a core coated by the arc spraying method.
  • Fig. 2 presents 50-fold magnification of a cross-section of the surface of a casting coated according to an embodiment of the method of the invention.
  • Fig. 3 presents a 200-fold magnification of the cross-section shown in Fig. 2 (4 -fold magnifica- tion of a part of Fig. 2) .
  • Fig. 4 presents a 500-fold magnification of the cross-section shown in Fig. 2 (100-fold magnification of Fig. 2) .
  • Fig. 5 presents a magnification of a cross- section of a casting coated with molybdenum according to another embodiment of the method of the invention.
  • the task was to cast a nozzle for a shot- creting gun, i.e. a hollow body whose inner surface is subject to intensive wear during use. Therefore, the aim was to form a wear resistant coating on the inner surface of the object.
  • the coating was formed according to the in- vention by spraying the surface of the core to be used in the casting operation with coating material by the arc spraying method.
  • the coating material used was SM 8222 FeCrCMnSi flux-cored electrode (manufactured by Sulzer Metco) , which has been designed for the formation of a hard coating on machine components and which has a high chromium and carbon content.
  • SM 8222 is a flux- cored electrode with a filling of chromium carbide granules and a sheath of manganese-alloyed steel.
  • the composition of SM 8222 iron alloyed with chromium 28 %, carbon 5.0 % and manganese 1.0 %.
  • the surface of the core was sprayed with the coating material SM 8222 by the arc spraying method. Arc spraying is one of the methods used in hot spraying.
  • a d.c. current is used to melt metal rods serving as electrodes .
  • the metal rods are supplied continuously from nozzles at an oblique angle to each other.
  • An electric arc set up between the rods causes the rods to melt.
  • compressed air is blown to atomize the molten metal drops formed at the ends of the rods, thus producing a fine particle distribution.
  • the compressed air causes the melt particles to dash onto the substrate, where they hit the surface and form a coating on it.
  • the core was a bar-like, resin- impregnated sand core body about 40 cm long and having a diameter of 10 cm. It was fastened to a turning table. During the spraying, the table with the core on it was rotated at a speed of 100 rpm while at the same time spraying the surface of the core with the coating ma- terial using a hand-held spraying device, which was passed over the core about 400 times.
  • Fig. 1 which presents a magnified cross- sectional view of the coating on the core, shows that when hitting the surface of the core, the melt drops have formed flat cakes on top of each other.
  • the coating is mechanically held fast on the surface of the core, adhering to the roughness of the surface.
  • the core was set into a mold and casting was performed using martensite steel.
  • Fig. 2 shows a 50-fold magnification of a NITAL-corroded cross-section of the surface.
  • Fig. 3 shows a 200-fold magnification of the area of the boundary between the coating and the base material.
  • the dark gray area at the lower edge of the figure is cast steel.
  • the light gray area in the middle is the coating.
  • the thickness of the coating layer is about 1 mm.
  • Dendritic solidification means that sharp tines branching out in a regular manner extend from the solidified metal into the melt, so that the growing solid metal forms a structure resembling a fir tree.
  • Fig. 4 the surface has been corroded by picric acid.
  • the dark areas are corroded martensite.
  • a carbide structure which can be seen as light spots in the figure.
  • the hardness of the coating was also measured. According to the hardness measurement, the coat- ing is somewhat harder than the cast material . The hardness of the coating was 585 (HV 1kg) while that of the cast material was 508 (HV 1kg) .
  • Fig. 5 presents a cross-sectional view of the surface of the casting.
  • the coating molybdenum and the cast metal have been mixed with each other.
  • the coating changed gradually toward the base material without a sharp boundary as in the previous example.
  • the surface had a higher Mo content than the rest of the casting. It can be assumed that such an alloyed surface also improves the surface properties of the casting.

Landscapes

  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Coating By Spraying Or Casting (AREA)

Abstract

A method for coating a metal cast, in which method a mold and/or a core are/is formed. The mold and/or core are/is coated with a coating layer having a composition and properties differing from those of the metal in the object to be cast. The molten metal is then cast into the mold and/or brought into contact with the core. During the casting operation, as the molten metal is solidifying, the coating layer on the mold and/or core is bound to the cast, forming a surface layer on it.

Description

A METHOD FOR COATING A CASTING
The present invention relates to a method as defined in the preamble of claim 1.
In prior art, the properties of the surface of a cast are improved by coating the object after the casting. However, this requires a separate operation after the casting. A further problem is that previously known coating methods are impracticable for the coating of especially the inner surface of a hollow body.
The object of the invention is to eliminate the above-mentioned problems.
The method of the invention is characterized by what is presented in claim 1. According to the invention, before the casting, the mold and/or core are/is coated with a coating layer so that, during the casting operation as the molten metal is solidifying, the coating layer on the mold and/or core is bound to the cast, forming a sur- face layer on it.
During the casting, the molten metal forms a metallurgical joint with the coating on the surface of the mold or core, with the result that a very good adhesion of the coating is achieved. The invention makes it possible to coat the outer and inner surfaces of casts with materials having a good adhesion. The method can be applied to coat areas of the cast that, because of difficult accessibility of the surface, are impossible to coat using traditional spray coating methods. A surface difficult to access is e.g. the inner surface of a hollow body.
During casting, at least part of the coating remains undissolved in and unmixed with the metal being cast. Therefore, a layer having a composition and properties differing from those of the base material is formed on the surface of the cast. By appropriately selecting the composition of the metal used as base material, it is possible to produce castings in which e.g. an excellent surface wear resistance is combined with a desired base material property, e.g. a given strength and ductility or a good machineability and weldability. Again, by appropriately selecting the surface layer, it is possible to improve the properties of the surface in respect of slide friction, wear resistance and corrosion resistance.
Such properties can be utilized e.g. in pumps, engines, valves, gears, etc.
In an embodiment of the method, the coating layer on the mold or core is formed using a coating method that causes no substantial heating of the mold or core . In an embodiment of the method, the coating layer is formed via thermal spraying.
In an embodiment of the method, the coating layer is formed by arc spraying, flame spraying, plasma spraying or HVOF high-speed spraying. In an embodiment of the method, the coating layer is formed from material whose melting point is higher than that of the metal to be cast .
In an embodiment of the method, the metal to be cast is steel . In an embodiment of the method, the metal to be cast is aluminum.
In an embodiment of the method, the coating layer consists of molybdenum.
In an embodiment of the method, the coating layer comprises ceramic particles. The ceramic material can be applied to the surface of the mold or core e.g. by brushing .
In an embodiment of the method, the coating layer comprises chromium carbides in a steel matrix. In the following, the invention will be described in detail by the aid of an example with reference to the attached drawings . Fig. 1 presents a 50-fold magnification of a coating (in this case, on a metal substrate) on the surface of a core coated by the arc spraying method.
Fig. 2 presents 50-fold magnification of a cross-section of the surface of a casting coated according to an embodiment of the method of the invention.
Fig. 3 presents a 200-fold magnification of the cross-section shown in Fig. 2 (4 -fold magnifica- tion of a part of Fig. 2) .
Fig. 4 presents a 500-fold magnification of the cross-section shown in Fig. 2 (100-fold magnification of Fig. 2) .
Fig. 5 presents a magnification of a cross- section of a casting coated with molybdenum according to another embodiment of the method of the invention.
EXAMPLE 1
The task was to cast a nozzle for a shot- creting gun, i.e. a hollow body whose inner surface is subject to intensive wear during use. Therefore, the aim was to form a wear resistant coating on the inner surface of the object.
The coating was formed according to the in- vention by spraying the surface of the core to be used in the casting operation with coating material by the arc spraying method.
The coating material used was SM 8222 FeCrCMnSi flux-cored electrode (manufactured by Sulzer Metco) , which has been designed for the formation of a hard coating on machine components and which has a high chromium and carbon content. SM 8222 is a flux- cored electrode with a filling of chromium carbide granules and a sheath of manganese-alloyed steel. The composition of SM 8222 : iron alloyed with chromium 28 %, carbon 5.0 % and manganese 1.0 %. The surface of the core was sprayed with the coating material SM 8222 by the arc spraying method. Arc spraying is one of the methods used in hot spraying. In arc spraying, a d.c. current is used to melt metal rods serving as electrodes . The metal rods are supplied continuously from nozzles at an oblique angle to each other. An electric arc set up between the rods causes the rods to melt. From behind the arc, compressed air is blown to atomize the molten metal drops formed at the ends of the rods, thus producing a fine particle distribution. The compressed air causes the melt particles to dash onto the substrate, where they hit the surface and form a coating on it.
The core was a bar-like, resin- impregnated sand core body about 40 cm long and having a diameter of 10 cm. It was fastened to a turning table. During the spraying, the table with the core on it was rotated at a speed of 100 rpm while at the same time spraying the surface of the core with the coating ma- terial using a hand-held spraying device, which was passed over the core about 400 times.
Fig. 1, which presents a magnified cross- sectional view of the coating on the core, shows that when hitting the surface of the core, the melt drops have formed flat cakes on top of each other. The coating is mechanically held fast on the surface of the core, adhering to the roughness of the surface.
After the coating operation, the core was set into a mold and casting was performed using martensite steel.
After the cast had cooled down, a sample was cut off it for examination under a microscope.
Fig. 2 shows a 50-fold magnification of a NITAL-corroded cross-section of the surface. Fig. 3 shows a 200-fold magnification of the area of the boundary between the coating and the base material. In Fig. 2, the dark gray area at the lower edge of the figure is cast steel. The light gray area in the middle is the coating. The thickness of the coating layer is about 1 mm. During the casting, the coating has melted, an indication of which is the resolidified dendritic structure of the coating. Dendritic solidification means that sharp tines branching out in a regular manner extend from the solidified metal into the melt, so that the growing solid metal forms a structure resembling a fir tree. In figures 2 and 3, it can be clearly seen that the boundary surface between the coating and the cast material is continuous and solid. The boundary is a metallurgical joint, and consequently the coating has an excellent adherence. The smoothness of the boundary also indicates that the coating has melted, because the surface of the coating on the core before the casting was very rough (approximately Ra 100) .
In addition, it was established that, in the microstructure of the coating, the areas between the dendrites contain ledeburite formed by martensite and primary carbides, so the microstructure of the coating resembles that of white cast iron, which has a good wear resistance. Thus, it is possible to produce a casting that combines the good strength and ductility properties of steel used as base material with the excellent wear resistance of white cast iron used as coating, by providing between the coating and the base material a metallic joint that guarantees a good adherence . In the 500-fold magnification presented in
Fig. 4, the surface has been corroded by picric acid. The dark areas are corroded martensite. In the areas between the dendrites there has been formed a carbide structure, which can be seen as light spots in the figure.
The hardness of the coating was also measured. According to the hardness measurement, the coat- ing is somewhat harder than the cast material . The hardness of the coating was 585 (HV 1kg) while that of the cast material was 508 (HV 1kg) .
EXAMPLE 2
In another test, a mold was spray-coated with molybdenum (Mo) , whereupon a tube was cast into the mold.
Fig. 5 presents a cross-sectional view of the surface of the casting. As can be seen from the figure, the coating molybdenum and the cast metal have been mixed with each other. The coating changed gradually toward the base material without a sharp boundary as in the previous example. The surface had a higher Mo content than the rest of the casting. It can be assumed that such an alloyed surface also improves the surface properties of the casting.
The invention is not restricted to the above- described example of its embodiments; instead, many variations are possible within the scope of the inventive idea defined in the claims .

Claims

1. Method for coating a metal cast, in which method a mold and/or a core are/is formed and molten metal is cast into the mold and/or molten metal is brought into contact with the core, charact er i zed in that the mold and/or core are/is coated with a coating layer having a composition and properties differing from those of the metal in the object to be cast, so that during the casting operation as the molten metal is solidifying, the coating layer on the mold and/or core is bound to the cast, forming a surface layer on it.
2. Method as defined in claim 1, charac t eri z ed in that the coating layer is formed by a coating method that causes no substantial heating of the mold or core .
3. Method as defined in claim 1 or 2, charac t eri zed in that the coating layer is formed via thermal spraying.
4. Method as defined in claim 3, charac t er i z ed in that the coating layer is formed by arc spraying, flame spraying, plasma spraying or HVOF high-speed spraying.
5. Method as defined in any one of claims 1 - 4, charac t eri z ed in that the coating layer is formed from material having a melting point higher than that of the metal to be cast .
6. Method as defined in any one of claims 1 - 5, charac t e ri zed in that the metal to be cast is steel.
7. Method as defined in any one of claims 1 - 5, chara c t eri z ed in that the metal to be cast is aluminum.
8. Method as defined in any one of claims 1 - 7, chara c t eri zed in that the coating layer consists of molybdenum.
9. Method as defined in any one of claims 1 - 8, characteri zed in that the coating layer comprises ceramic particles.
10. Method as defined in any one of claims 1 - 9, characteri zed in that the coating layer comprises chromium carbides in a steel matrix.
PCT/FI2000/000950 1999-11-05 2000-11-01 A method for coating a casting WO2001032336A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU11507/01A AU1150701A (en) 1999-11-05 2000-11-01 A method for coating a casting

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FI992392 1999-11-05
FI19992392 1999-11-05

Publications (1)

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WO2001032336A1 true WO2001032336A1 (en) 2001-05-10

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Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3878880A (en) * 1973-06-25 1975-04-22 Curtiss Wright Corp Composite casting method
US3945423A (en) * 1973-09-06 1976-03-23 Mahle Gmbh Method for the manufacture of a compound casting
US4034464A (en) * 1975-08-27 1977-07-12 Ford Motor Company Method of aluminum cylinder head valve seat coating transplant
EP0101948A2 (en) * 1982-07-30 1984-03-07 Toyota Jidosha Kabushiki Kaisha Method of manufacturing a surface composite reinforced member
FR2632976A1 (en) * 1988-06-21 1989-12-22 Ugine Savoie Sa Process and device for manufacturing a polymetallic composite product by casting in an enclosure placed in an ingot mould
DE4107416A1 (en) * 1990-04-25 1991-11-28 Gisag Ag Giesserei Masch Producing partially wear-resistant iron or steel castings - by coating cores with hard material which becomes dispersed in the casting surface during casting
US5404639A (en) * 1980-07-02 1995-04-11 Dana Corporation Composite insulation for engine components
EP0807479A1 (en) * 1996-05-17 1997-11-19 Bayerische Motoren Werke Aktiengesellschaft, Patentabteilung AJ-3 Method of manufacturing a light-metal casting, especially a cylinder block for an internal combustion engine

Patent Citations (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3878880A (en) * 1973-06-25 1975-04-22 Curtiss Wright Corp Composite casting method
US3945423A (en) * 1973-09-06 1976-03-23 Mahle Gmbh Method for the manufacture of a compound casting
US4034464A (en) * 1975-08-27 1977-07-12 Ford Motor Company Method of aluminum cylinder head valve seat coating transplant
US5404639A (en) * 1980-07-02 1995-04-11 Dana Corporation Composite insulation for engine components
EP0101948A2 (en) * 1982-07-30 1984-03-07 Toyota Jidosha Kabushiki Kaisha Method of manufacturing a surface composite reinforced member
FR2632976A1 (en) * 1988-06-21 1989-12-22 Ugine Savoie Sa Process and device for manufacturing a polymetallic composite product by casting in an enclosure placed in an ingot mould
DE4107416A1 (en) * 1990-04-25 1991-11-28 Gisag Ag Giesserei Masch Producing partially wear-resistant iron or steel castings - by coating cores with hard material which becomes dispersed in the casting surface during casting
EP0807479A1 (en) * 1996-05-17 1997-11-19 Bayerische Motoren Werke Aktiengesellschaft, Patentabteilung AJ-3 Method of manufacturing a light-metal casting, especially a cylinder block for an internal combustion engine

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