IRON-CHROMIUM-BORON ALLOY FOR GLASS MANUFACTURING TOOLS

Abstract: An iron-chromium-boron alloy, suitable for the production of tools for the manufacture of glass articles, and a tool made from the alloy, has a composition of from 1 to 20 wt.% chromium and from 0.5 to 3 wt.% boron. The composition optionally contains carbon subject to carbon in excess of 1.0 wt.% being bound by at least one strong carbide forming element in a carbide and/or carbo-boride phase, with the alloy otherwise optionally including the at lest one carbide forming element. The composition also optionally contains one or more of silicon up to 3 wt.%, aluminium up to 0.2 wt.%, manganese up to 2 wt.%, nickel up to 3 wt.%, copper up to 3 wt.% and molybdenum up to 5 wt.%. The balance, apart from incidental impurities, is iron.

Description: IRON-CHROMIUM-BORON ALLOY FOR GLASS MANUFACTURING TOOLS This invention is concerned with improved tooling used in the manufacture of glass articles, such as glass containers, and with an alloy for such tooling. Glass containers are made by forming molten glass, at temperatures above the glass softening temperature. The glass may be made from either recycled glass or from a mixture of raw materials that includes lime, soda ash, silica sand and other additives. During the forming operation, an amount of molten glass at a temperature over 1200°C is placed into a metal die and shaped by blowing air through a metal nozzle. In most manufacturing operations the shaping process is repeated at high rates and significant wear is experienced by tools such as the metal "blow nozzle" or plunger which is in contact with what becomes the neck of the glass container and through which the air is blown. In glass shaping machinery the metal components or tools such as nozzles and other tools, that come into contact with the hot glass, must have high hardness and high wear resistance as well as high resistance to oxidation and scaling. Because of the rapid thermal cycling that occurs through repeated contact with molten glass and removal of the glass from the die, metal components or tools should have a high resistance to thermal cracking. In most types of glass container manufacture, the metal tooling experiences many thousands of thermal cycles before being re-machined or replaced. In conventional tooling used in the manufacture of glass bottles, the "blow- blow plunger" forms the inside of the neck of the bottle, as well as enabling air to be blown to press the molten glass against the wall of the die. Such blow-blow plungers and other glass moulding components are conventionally made from nickel boron alloys containing 2 to 3 wt% boron, usually 98 wt% nickel and 2 wt% boron. These alloys produce a microstructure of nickel dendrites and a eutectic mixture of nickel and nickel boride. This material has a good thermal shock resistance and a reasonable hardness of about 40 to 45 Rockwell C. However, the nickel matrix is relatively soft and cannot be hardened by heat treatment. The soft matrix contributes to two problems: (a) the matrix wears relatively rapidly compared to the eutectic borides and the total wear resistance is lowered by this mechanism; and (b) wear of the nickel matrix often results in microscopic wear particles becoming imbedded in the inside of the glass container. In most glass bottle manufacturing operations, wear is a major problem; the wear of blow-blow plungers, for example, necessitate their removal after between 7 and 14 days of continuous operation in order for it to be re-machined. Also, particles which become embedded in the glass of containers results in a reduction of the shatter strength of a typical bottle by a factor of 4 to 5. Several defective bottles in a batch can result in wholesale rejection of a run due to the possibility of failure in the hands of a consumer with possible legal or recall consequences. Resistance to thermal shock, combined with resistance to wear caused by repeated contact with molten glass, is of paramount importance for materials used for tools such as blow-blow plungers and push-blow plungers that are used in the manufacture of glass containers. The present invention is directed to providing improved ferrous alloys suitable for manufacture of tools, which have high resistance to both wear and thermal shock, for the manufacture of glass articles such as containers. The invention also provides such tools cast from such alloys. A tool according to the invention may comprise a blow nozzle or plunger, such as a blow-blow plunger or a push-blow plunger. However, the tool may comprise any other component of machinery for use in the manufacture of glass articles, such as containers, which component is used in forming the article by contact with molten glass. The component thus may comprise a die or die section used for shaping a glass article. An alloy according to the invention is an iron-chromium-boron alloy. The alloy has from 1 to 20 wt% chromium, from 0.5 to 3 wt% boron, up to 1.0 wt% carbon or higher if substantial amounts of strong carbide forming elements such as molybdenum, vanadium, titanium, niobium and tungsten are present, optional alloying additions as detailed in the following, and a balance apart from incidental impurities of iron. A tool according to the invention is cast from a melt of the alloy of the invention, and can achieve a high hardness and resistance to wear, coupled with a high resistance to thermal shock. The tool also has a high resistance to oxidation. The alloy and the tool of the invention have the further benefit of being able to be softened by annealing and re-hardened to high hardness levels. Also, particularly with alloys with a high chromium content, such as at least 8 wt%, they have a high chromium content matrix which can be hardened to martensite with heat treatment and is hard and corrosion resistant. This matrix, coupled with the presence of hard iron-chromium eutectic borides, provides a material with much better wear resistance under conditions of elevated temperatures and rapid thermal cycling or heat shock. The iron-chromium-boron alloy and tools cast therefrom withstand wear for much longer periods than the nickel-boron alloy discussed above, and with their excellent oxidation resistance, the tools produce bottles which do not suffer from alloy metal contamination and thus give a more reliable product. The iron-chromium-boron alloy and tools cast therefrom can be softened to 35 Rockwell C by annealing, enabling machining, then re-hardening by heating to above 900°C and air cooling. Tempering can then further adjust hardness if needed. The alloy and the tool also can be provided with a high level of surface finish, achievable by polishing. The iron-chromium-boron alloy and tool according to the invention can be substantially free of carbon, with carbon present effectively only as an incidental impurity. However, as indicated, carbon can be present at up to 1.0 wt%. Preferably carbon does not exceed 0.6 wt%, and may for example be present at from 0.1 to 0.6 wt%, such as from 0.1 to 0.3 wt%. The boron content is not less than 0.5 wt%, and most preferably is from 0.5 to 2.5 wt%, such as from 1 to 2.5 wt%. For most applications, the preferred chromium content is from 3 to 18 wt%, such as from 8 to 18 wt%. If strong carbide forming elements, such as molybdenum, vanadium, titanium, tungsten and niobium, are included in the alloy composition then the carbon level may be in excess of 1.0 wt% provided that the level of strong carbide forming elements is such that the excess carbon is bound by these elements in carbide or carbo-boride phases. The carbon content of the matrix would then remain low. The fracture toughness, thermal shock resistance and wear resistance of the cast ferrous alloys is largely determined by the volume fraction of hard phases, which in turn is a function of the content of both boron and carbon and carbide and boride forming elements, and also the interstitial boron and carbon content of the matrix. The boron content of the matrix is always low because the solubility of boron in ferrite and austenite is low. However the solubility of carbon in austenite and therefore the carbon content of the martensitic matrix can be as high as approximately 2 wt% unless the carbon is bound in some other phase. It is overriding in the design of satisfactory alloy compositions for the present invention that the carbon content of the matrix is kept to a low enough level for sufficient fracture toughness or thermal shock resistance to be achieved for the application in question. The preferable level of carbon in the matrix is less than 0.3 wt% and can be significantly lower in some applications. The iron-base alloy can contain sufficient alloying additions for enhancement of oxidation properties and hardenability. Suitable alloying elements for these purposes include silicon, aluminium, manganese, nickel, copper and molybdenum, either separately or in combination. Preferred additions for these purposes are of silicon at up to 3 wt% such as from 0.5 to 3 wt%, aluminium at up to 0.2 wt%, manganese at up to 2 wt% such as from 0.2 to 1.5 wt%, nickel at from 0.2 to 3 wt% such as from 0.2 to 2 wt%, copper at up to 3 wt%, and/or molybdenum at up to 5 wt% such as from 0.5 to 5 wt%. The presence of silicon and/or aluminium in a melt of the iron-base alloy also is beneficial in keeping the melt in a de-oxidised condition. The addition of molybdenum also increases hardness and improves resistance to softening at high temperatures, due to its action as a strong carbide and/or boride forming element. For the same purposes, sufficient amounts of other strong carbide and/or boride elements, such as vanadium, titanium, tungsten and/or niobium, can be added to the iron-base alloy. Preferred additions for enhancing resistance to softening are molybdenum as indicated above, vanadium at up to 8 wt%, titanium at up to 5 wt%, niobium at up to 6 wt% and/or tungsten at up to 7 wt%. The iron-base alloy required for the present invention can be prepared as a melt for casting by melting suitable constituent materials in an electric induction furnace. This most preferably entails melting mild steel scrap, low carbon ferro- chromium and low carbon ferro-boron. Other commercial foundry alloys can be added to provide alloy additions required in the iron-base alloy. For re-melt charges, return scrap with about 2 wt% boron can be readily melted with mild steel scrap and ferro-alloys. The melt may be kept in a de-oxidised condition by the use of ferro-silicon or aluminium. The iron-base alloys of the invention have a melting point close to 1300°C. In general, a melt pouring temperature of from 1400°C to 1450°C is desirable, depending on the nature of the casting. Subsequent to casting the iron-base alloys may be hardened by heat treating at temperatures in the range 950 to 1150°C to form austenite and air cooling to room temperature in order to form a martensitic microstructure in the matrix of the alloy. A typical hardness after such a hardening treatment is 50 on the Rockwell C scale. If desired the iron-base alloys or a tool cast therefrom can be softened for machining by sub-critical annealing at temperatures in the range 700 to 750°C in order to decompose the matrix to a mixture of ferrite and carbide. Such a heat treatment results in a decrease of hardness to 30 to 35 Rockwell C. The alloys can be rehardened for service by heat treating at 950 to 1150°C and air cooling to provide a typical hardness of about 50 Rockwell C. The iron-base alloy tool may be cast to near net shape by a range of casting methods depending upon the dimensional accuracy that is required and the extent to which the amount of machining to final dimensions is to be minimised. The invention now is further illustrated by reference to the following Examples: Example 1. Blow-blow plungers come into direct contact with the molten glass as it falls into the mould, and then forces the glass into an initial shape by air pressure. A small ledge on the tip of the plunger forms an inside rim on the neck of the bottle. This ledge on the plunger must remain sharp so that a thin blade of glass does not form on the inside neck of the bottle. A blow-blow plunger tool for the manufacture of glass containers was manufactured by investment casting an iron-base alloy, within the compositional range specified above, and final machining to the exact shape required for the tool. The composition of the alloy used in this example was: carbon 0.2 wt% chromium 17 wt% boron 2 wt% silicon 0.9 wt% manganese 0.8 wt% molybdenum 0.5 wt% iron remainder. After casting and prior to rough machining the plunger was sub-critically annealed at 700°C for three hours to reduce the hardness to 35 Rockwell C. Following rough machining, the plunger was heated to 950°C for one hour, then cooled in air to room temperature, followed by tempering at 300°C for three hours. The final hardness was 50 Rockwell C. Final machining and grinding was then done. The blow-blow plunger manufactured in this way showed considerably improved performance over the conventional tooling made from nickel-boron alloy. Conventional blow-blow plungers in this particular application have to be removed from service after 1 to 2 weeks continuous operation because of wear and loss of dimensional accuracy. They are then remachined before returning to service. The blow-blow plunger manufactured according to the invention remained in continuous service for 10 weeks without having to be removed for remachining. This represents an improvement in the service life of the component by a factor of 5 to 10. Example 2 Twenty-four plungers were made from an iron-chromium-boron alloy according to the invention, by investment casting. These were used in the production of glass containers. The plungers stayed in production for 10 to 12 weeks without having to be re-sharpened, giving a 5 to 7 times improvement in production life compared with plungers of a conventional nickel-boron alloy. The particular composition for these plungers was as follows: Carbon 0.23% Silicon 1.07% Manganese 1.11% Phosphorous 0.017% Sulphur 0.017% Chromium 14.78% Molybdenum 0.40% Nickel 1.50% Copper 0.14% Aluminium 0.096% Niobium 0.175% Boron 2.0% Balance mainly iron A small amount of niobium was added to precipitate small niobium carbides, to improve wear resistance. Example 3 Guide plate castings are solid flat discs with a central hole through which a plunger will pass and retract. The clearance between the plunger and the central hole is critical and should not be allowed to become large enough to allow molten glass to penetrate between the plunger and the guide plate. Wear between the plunger and the guide plate at temperatures up to 500°C is the main cause of replacement of the guide plate. Twenty guide plates made from the iron-chromium-boron alloy by investment casting have been trialed in production of a 500 ml bottle for 13 days continuously with no measurable change in dimension. Guide plates made from nickel-boron alloy or tool steel require measurement after seven days and generally approximately half are rejected because tolerances are exceeded because of wear. The composition of the iron-chromium-boron guide plates was as follows: Carbon 0.28% by weight Silicon 1.13% Manganese 1.15% Phosphorous 0.017% Sulphur 0.014% Chromium 14.6% Molybdenum 0.73% Nickel 1.83% Copper 0.14% Aluminium 0.026% Boron 2.0% Balance mainly iron Example 4 Thimbles are castings shaped like a top hat which are part of the support mechanism of the bottle mould. The main reason for replacement of the thimble is wear on the top horizontal face and wear in the bore. The thimbles are usually made from nickel-boron alloy or tool steel and after two to three weeks of continuous operation are removed and checked for wear. If the two dimensions are worn more than 0.005 inches (0.13 mm), then they are scrapped. Most thimbles of conventional nickel-boron alloy last four to six weeks. A trial with iron-chromium-boron thimbles manufactured by investment casting showed 0.08 mm (0.003 inches) wear on the flat horizontal face and 0.06 mm (0.0025 inches) wear in the bore after eleven weeks continuous service. There is considerable improvement in wear resistance of the new invented alloy compared with presently used alloys. The composition of the iron-chromium-boron thimbles was as follows: Carbon 0.29% by weight Silicon 1.03% Manganese 1.13% Phosphorous 0.009% Sulphur 0.012% Nickel 2.08% Chromium 17.38% Molybdenum 1.12% Copper 0.15% Aluminium 0.026% Boron 2.0% Balance mainly iron In this case, molybdenum was increased to over one per cent in order to improve hot hardness. Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims: CLAIMS: 1. An iron-chromium-boron alloy, suitable for the production by casting of tools for the manufacture of glass articles such as containers, wherein the alloy has from 1 to 20 wt.% chromium and from 0.5 to 3 wt% boron, the alloy optionally containing carbon subject to carbon in excess of 1.0 wt.% being bound by at least one strong carbide forming element in a carbide and/or carbo-boride phase and the alloy otherwise optionally including said at least one carbide forming element, and wherein the alloy further contains: 0 to 3 wt.% silicon, 0 to 0.2 wt% aluminium, 0 to 2 wt.% manganese, 0 to 3 wt% nickel, 0 to 3 wt% copper, 0 to 5 wt.% molybdenum and a balance apart from incidental impurities of iron. 2. An alloy according to claim 1 , wherein the carbon content does not exceed 0.60 wt%. 3. An alloy according to claim 2, wherein the carbon content is from 0.1 to 0.3 wt%. 4. An alloy according to any one of claims 1 to 3, wherein the boron content is from 1 to 2.5 wt%. 5. An alloy according to any one of claims 1 to 4, wherein the chromium content is from 3 to 18 wt.%. 6. An alloy according to claim 5, wherein the chromium content is from 8 to 18 wt%. 7. An alloy according to any one of claims 1 to 6, wherein the chromium content is such that the alloy has a high chromium content matrix which is hardenable to martensite with heat treatment. 8. An alloy according to any one of claims 1 to 7, wherein said alloy contains at least one of silicon, aluminium, manganese, nickel, copper and molybdenum at a respective content of: silicon 0.5 to 3 wt% aluminium up to 0.2 wt% manganese 0.2 to 1.5 wt.% nickel 0.2 to 2 wt% copper up to 3 wt% molybdenum 0.5 to 5 wt% 9. An alloy according to any one of claims 1 to 8, wherein said alloy contains at least one of molybdenum, vanadium, titanium, tungsten and niobium to improve resistance to softening at high temperatures, due to action as a strong carbide and/or boride forming element, at a respective content of: molybdenum 0.5 to 5 wt% vanadium up to 8 wt% titanium up to 5 wt% tungsten up to 7 wt% niobium up to 6 wt%. 10. An alloy according to any one of claims 1 to 9, having less than 0.3 wt.% carbon in the matrix thereof. 11. A tool for use in contact with molten glass in the manufacture of glass containers, wherein said tool is produced by casting an alloy according to any one of claims 1 to 10. 12. A tool according to claim 11 , wherein the tool in its as cast condition, following air-cooling, has a matrix exhibiting a martensitic structure. 13. A tool according to claim 11 , wherein the tool has a matrix with a martensitic structure produced by heat treatment at from 950 to 1150° to form austenite, followed by air cooling to provide the martensitic microstructure. 14. A tool according to claim 11 , wherein the tool in an as cast condition has been softened by annealing at from 700 to 750°C, then machined to final shape, then heat treated at from 950 to 1150°C to form austenite, and then air cooled to provide a martensitic microstructure. 15. A tool according to claim 11 , wherein said tool has a hardness of about 50 on the Rockwell C scale. 16. A tool according to any one of claims 12 to 15, wherein the matrix has less than 0.3 wt% carbon.

Inventors: CHRISTODOULOU PERIKLIS, LAKELAND KENNETH DONALD, CHRISTODOULOU, PERIKLIS, LAKELAND, KENNETH, DONALD
Applicants: CHRISTODOULOU PERIKLIS, COMMW SCIENT IND RES ORG, LAKELAND KENNETH DONALD, UNIV QUEENSLAND, CHRISTODOULOU, PERIKLIS, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR, LAKELAND, KENNETH, DONALD, THE UNIVERSITY OF QUEENSLAND
Application year: 1995
Main classification: C22C38
IPC codes: C03B9, C21D6, C22C38