The present invention refers to iron-based powder metallurgical combinations and to methods for preparing sintered powder metallurgical components therefrom. More specifically the invention refers to the production of sintered components including copper, nickel and molybdenum by using these combinations.
Within the powder metallurgical field, copper, nickel and molybdenum has since long been used as alloying elements in the production of high strength sintered components.
Sintered iron-based components can be produced by mixing alloying elements with the pure iron powders. However, this may cause problems with dust and segregation which may lead to variations in size and mechanical properties of the sintered component. In order to avoid segregation the alloying elements may be pre-alloyed or diffusion alloyed with the iron powder. In one method molybdenum is pre-alloyed with iron powder and this pre-alloyed iron powder is subsequently diffusion alloyed with copper and nickel for production of sintered components from iron-based powder compositions containing molybdenum, nickel and copper.
It is however obvious that, when producing a sintered iron-based component, from a powder wherein molybdenum is pre-alloyed and wherein copper and nickel are diffusion alloyed, the content of the alloying elements in the sintered iron-based component will be substantially identical with the content of alloying elements in the used diffusion alloyed powder. In order to reach different contents of the alloying elements in the sintered component, yielding different properties, iron-based powders having different contents of the alloying elements have to be used.
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The present invention provides a method of eliminating the need of producing a specific powder for each desired chemical composition of the sintered iron-based component having alloying elements from molybdenum, copper and nickel. The invention also offers the advantage of providing a method for controlling the dimensional change and the tensile strength to predetermined values. In a specific embodiment the dimensional change is independent of the carbon content and the density.
In brief the invention concerns a powder metallurgical combination of three different iron-based powders as defined in claim 1. The first of these iron-based powders consisting of core particles of iron, pre-alloyed with molybdenum, which is additionally diffusion alloyed with copper and the second iron-based powder consisting of core particles of iron, pre-alloyed with molybdenum, which is diffusion alloyed with nickel. The third iron-based powder essentially consists of particles of iron pre-alloyed with molybdenum.
A method according to the invention is defined in claim 6 and comprises the steps of combining these three iron-based powders in predetermined amounts, mixing the combination with graphite, compacting the obtained mixture and sintering the obtained green body to provide a sintered component having a predetermined strength and a predetermined dimensional change during sintering.
Specifically the iron-based powder metallurgical combination according to the invention comprises:
The amount of copper and nickel which is diffusion alloyed to the core particles is limited in the upper range to 15% copper and 12% nickel. The lower limit of copper and nickel which is diffusion alloyed to the core particles should be substantially higher than the amount required in the sintered component to achieve the advantages of the invention. Thus, for practical reasons an iron-based powder essentially consisting of core particles pre-alloyed with molybdenum and comprising at least 6% copper diffusion alloyed to the core particles and an iron-based powder having core particles pre-alloyed with molybdenum and comprising at least 4.5% nickel diffusion alloyed to the core particles are of special interest.
The powders A, B and C, respectively, essentially consist of particles of iron pre-alloyed with molybdenum, but other elements, except unavoidable impurities, may be pre-alloyed to the particles. Such elements may be nickel, copper, chromium and manganese.
In order to produce a sintered component from the powder combination according to the present invention, the respective amounts of powder A, B and C are determined and mixed with graphite in the amount required for the predetermined strength. The obtained mixture may be mixed with other additives before compaction and sintering. The amount of graphite which is mixed in the powder combination is 0.3-0.7%.
Other additives are selected from the group consisting of lubricants, binders, other alloying elements, hard phase materials, machinability enhancing agents.
In accordance with one embodiment of the powder metallurgical combination, powder C is essentially free from Cu and Ni.
The relation between powder A, B and C is chosen so that the copper content will be 0.2-2% by weight, the nickel content will be 0.1-4% by weight and the molybdenum content is preferably 0.5-1.5% by weight of the sintered component.
When the copper content is 0.2-2%, preferably 0.4-0.8% and the nickel content is 0.1-4%, it has been unexpectedly been found that the dimensional change during sintering is independent of the carbon content and sintered density.
In order to produce a sintered component with a predetermined dimensional change and strength, the amounts of copper, nickel and carbon, respectively, in the sintered component is determined by means of diagrams, e.g. from
The powders are mixed with graphite to obtain the final desired carbon content. The powder combination is compacted at a compaction pressure between 400-1000 MPa and the obtained green body is sintered at 1100-1300°C for 10-60 minutes in a protective atmosphere. The sintered body may be subjected to further post treatments, such as heat treatment, surface densification, machining etc.
The exemplifying diagrams in
According to the present invention sintered components containing various amounts of molybdenum, copper and nickel may be produced. This is achieved by using a combination of three different powders, which are mixed in different proportions to achieve a powder having the required chemical composition for the actual sintered component.
To summarize a particular advantage of the invention is that the dimensional change during sintering as well as the strength of the sintered component can be controlled. The advantage of being able to control the dimensional change will facilitate the use of existing pressing tools. When producing sintered parts a certain scatter in carbon content and density may be unavoidable. By utilising the combinations having a dimensional change independent of the density and carbon content the scatter in dimensions after sintering will be reduced hence subsequent machining and machining costs can be decreased.
The invention is illustrated by the following nonlimiting examples:
This example demonstrates how to choose an alloying composition having a desired strength of about 600 MPa and three levels of dimensional change (-0.1%, 0.0% and +0.1%). This was done for two carbon levels, 0.5% C and 0.3% C, respectively, in the powder combinations according to table 1, where the lower carbon content yields better ductility as can be seen in table 2.
The powder combinations according to the present invention were prepared from a powder A with 10% of copper diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum, a powder B with 5% of nickel diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum and a powder C of an iron-based powder pre-alloyed with 0.85% of molybdenum.
The powder combinations were mixed with 0.8% amide wax as a lubricant and graphite, to yield a sintered carbon content of 0.3 % and 0.5 %, respectively. The obtained mixtures were compacted to tensile test specimen according to ISO 2740.
The compaction pressure was 600 MPa and the sintering conditions were: 1120°C, 30 min, 90% N2/10% H2. In table 2 other mechanical properties from the powder combinations according to the invention are presented. It can be clearly seen that the powder combinations according to the invention have the predetermined dimensional change according to
This example illustrates powder combinations according to the invention, comprising 0.6% Cu and 2% Ni and a specific embodiment having dimensional change independent of carbon content and sintered density as shown in table 3. The results obtained with these combinations are compared with the results obtained with Distaloy AB (available from Höganäs AB, Sweden) as well as with a powder having the same chemical composition as the powder combination according to the invention but wherein iron-based powder pre-alloyed with molybdenum has both copper and nickel diffusion alloyed to the surface, in table 3 designated as "fixed composition".
The powder combinations according to the present invention were prepared from a powder A with 10% of copper diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum, a powder B with 5% of nickel diffusion alloyed to the surface of an iron-based powder pre-alloyed with 0.85% of molybdenum and a powder C consisting of an iron-based powder pre-alloyed with 0.85% of molybdenum.
Table 3 shows a specific example where a mixture of powder A, powder B and powder C having a total content of 0.6% copper, 2% of nickel and 0.83% of molybdenum is compared with a known powder, Distaloy AB, and an iron-based powder having 0.83% of pre-alloyed molybdenum, 0.6% of copper and 2% of nickel diffusion alloyed to the surface of the iron-based powder. As disclosed in table 3 the dimensional change of sintered samples, produced from the powder combination according to the invention, is essentially independent of the carbon content and density compared with the known powder Distaloy AB or the iron-based powder diffusion alloyed with both copper and nickel.
The powder combinations were mixed with 0.8% amide wax as a lubricant and graphite, to yield a sintered carbon content according to table 3. The obtained mixtures were compacted to tensile test specimen according to ISO 2740 at different compaction pressures according to table 3. The tensile test specimens were sintered at 1120°C for 30 minutes in an atmosphere of 90 % nitrogen and 10 % of hydrogen. In table 4 further mechanical properties are presented.
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