Patent Application
20190119795
This application claims priority to Australian Provisional Application No. 2017904280 filed on Oct. 23, 2017, which is hereby incorporated by reference in its entirety.
The present invention relates to ductile iron alloys and in particular to ductile iron alloys with at least a partial pearlitic structure.
The invention has been developed primarily for providing vehicle parts, particularly disc brake rotors, and will be described hereinafter with reference to this application. However, it will be appreciated that the invention is not limited to this particular field of use.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Disc brake rotors are arranged in machinery to rotate with a rotating member and assist in the deceleration of such rotation. They are commonly constructed with two oppositely-facing annular friction surfaces which, when engaged by friction-providing surfaces of a brake, provide a frictional force to decelerate the rotating member. Such brake rotors are commonly used in machinery to decelerate components such as wheels or gears—particularly in motor vehicles.
Presently, commercial disc brake rotors are most commonly constructed of grey cast iron, which are characterised by their graphitic microstructure. However, on testing, the inventors have found such brake rotors to be inadequate in longevity, thermal resistance, and their ability to dissipate the heat from braking, particularly when the brake rotors are subject to frequent and/or intense thermal stress—for example, during motor vehicle racing.
Accordingly, the inventors have sought to develop an improved iron alloy which may be used in a disc brake rotor.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
It is an object of the invention in its preferred form to provide an improved ductile iron alloy, which may be utilised in the construction of a disc brake rotor.
According to a first aspect of the invention, there is provided a method for producing a ductile iron alloy, comprising the steps of:
Preferably, the inoculant includes one or more of ferrosilicon, calcium, aluminium, copper, and molybdenum. More preferably, the ferrosilicon is magnesium ferrosilicon.
Preferably, the inoculant is covered by a covering means to provide a controlled release of said inoculant in step (iii). In preferred embodiments, the covering means is a steel covering.
The controlled release preferably occurs after substantially all of the molten steel is transferred into the inoculation ladle. In other preferred embodiments, the controlled release occurs about 20 seconds after step (ii).
In a preferred embodiment, the partial pearlitic structure is about 30% to 50% of the structure. In a more preferred embodiment, the partial pearlitic structure is about 40% to 45% of the structure.
In a particular embodiment, the partial pearlitic structure is about 30% of the structure. In a further particular embodiment, the partial pearlitic structure is about 50% of the structure.
Preferably, the steel composition is provided by a combination of steel scrap, pig iron, and further additives.
More preferably, the further additives include one or more of a ferrosilicon alloy, a ferromolybdenum alloy, and a recarburiser.
In one particular embodiment, copper molybdenum maybe included in the formulation to assist with corrosion resistance.
In a preferred embodiment, the steel composition comprises:
In a more preferred embodiment, the steel composition comprises:
Preferably, the furnace is an induction core furnace.
In a preferred embodiment, in step (ii), the furnace is heated to a minimum internal temperature of about 1400° C. More preferably, the furnace is heated to an internal temperature in the range of about 1400-1700° C., or 1450-1600° C., or 1470-1580° C., or any range therebetween. For example, in a preferred embodiment, the furnace is heated to an internal temperature of about 1400° C., 1420° C., 1450° C., 1500° C., 1550° C., 1600° C., 1650° C., or 1700° C., or any temperature therebetween.
In a particular preferred embodiment, in step (ii), the furnace is heated to an internal temperature of about 1470° C.
In another particular preferred embodiment, in step (ii), the furnace is heated to an internal temperature of about 1580° C.
In further particular preferred embodiment, in step (ii), the furnace is heated to an internal temperature of about 1600° C.
Preferably, in step (ii), cerium is added to the molten steel. More preferably, the cerium is added at about 0.25% to 0.55% by weight of the molten steel. For example, in a preferred embodiment, the cerium is added at about 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, or 0.55%, or any percentage therebetween.
Cerium and magnesium to some extent work cooperatively. Magnesium as it is heated reverts to a vapour. As temperature increases in the furnace, more magnesium will be lost to vapour. Accordingly, cerium is used to prolong the presence of magnesium. Cerium vaporises at a higher temperature but is more potent than magnesium. Accordingly, the reaction is then “taken over” by cerium and the magnesium remains available for the alloy.
Preferably, in step (iii), the predetermined inoculation time is between about 3 minutes and 10 minutes. More preferably, the predetermined inoculation time is between about 5 to 6 minutes.
In a preferred embodiment, the inoculation ladle includes a pocket for housing the inoculant. This pocket is designed to enclose the inoculant, such as to restrict the contact area available to the molten steel, and hence control the inoculation time.
Preferably, the inoculant is added to the inoculation ladle prior to step (ii). More preferably, the inoculant is added to the inoculant ladle about 15-20 seconds prior to transferring the molten steel to the inoculant ladle.
In alternative embodiments, the inoculant may be added to the inoculation ladle after step (ii).
Preferably, the inoculation ladle is dimensioned to have a height about 1.5 times the length of its diameter. More preferably, the inoculation ladle has an insulating layer of about 6 mm between an internal lining and an external steel shell. In a more preferable embodiment, the internal lining consists of about 80% alumina and has a thickness of up to 75 mm.
In an embodiment, the method further comprises a step between steps (iii) and (iv) wherein said inoculated molten steel is transferred from said inoculation ladle to a pouring ladle for pouring in step (iv). In a preferred embodiment, a second inoculant is added to said inoculated molten steel during said transferring from said inoculation ladle to said pouring ladle. This second inoculant may assist in maintaining the pearlite structure during transfer process and reduce impurities in the resultant ductile iron alloy.
Preferably said second inoculant comprises:
Alternatively, said second inoculant comprises:
In a further preferred embodiment, the resultant cast material can undergo suitable heat treatment. If the resultant cast material is for so-called light duty, it preferably undergoes a “normalising” or “low level annealing” treatment. In this embodiment, the material is removed from the mould and maintained at a temperature of 500° C. to 700° C. for up to 6 hours preferably 550° C. to 600° C. for 5 hours. The heat treatment is preferably conducted in the furnace. After the stabilisation at elevated temperature, the material undergoes a slow controlled cooling down to ambient temperature. Again, this preferred slow controlled cooling is conducted within the furnace.
In an alternative heat treatment, the cast material undergoes a high temperature “annealing” step. This is particularly useful for cast material which is intended to undergo heavy use such as disk brake rotors or the like for trucks, racing cars, etc. In this embodiment, the rough cast material is allowed to cool down to ambient temperature in the mould. This is a slow controlled cooling. The material is then placed in the furnace and held at a temperature of 750° C. to 1000° C. for up to 10 hours. Preferably, the temperature is 750° C. to 820° C. and is stabilised for 9 hours. After this “annealing” heat treatment, the material then undergoes a slow controlled cooling within the furnace down to ambient temperature.
In a preferable embodiment, the mould is for a vehicle part. Preferably, the vehicle is selected from the group comprising motor vehicles, heavy vehicles, railed vehicles, ships, and boats. In particular preferable embodiments, the vehicle part is selected from the group comprising disc brake rotors, turbo housings, camshafts, crankshafts, engine blocks, wheels, freight containers or barges, blocks, and carriages. More preferably, the vehicle part is a disc brake rotor.
In preferred embodiments of the invention, the disc brake rotor comprises about: 3.4% carbon, 2.4% silicon, and 1.15% (maximum) molybdenum. It was found that this preferred composition has soft nodules of graphite surrounded by relatively soft and ductile ferrite. These soft parts are strengthened and hardened by the pearlite phase and a very hard complex carbide phase.
According to a second aspect of the invention, there is provided a ductile iron alloy produced by the method according to the invention.
According to a third aspect of the invention, there is provided a vehicle part produced by the method according to the invention.
According to a fourth aspect of the invention, there is provided a disc brake rotor produced by the method according to the invention.
According to a fifth aspect of the invention, there is provided a ductile iron alloy comprising at least a partial pearlitic structure.
Preferably, the partial pearlitic structure is about 30% to 50% of the structure. More preferably, the partial pearlitic structure is about 40% to 45% of the structure.
In a particular embodiment, the partial pearlitic structure is about 30% of the structure.
In a further embodiment, the partial pearlitic structure is about 50% of the structure.
According to a sixth aspect of the invention, there is provided a disc brake rotor comprised of a ductile iron alloy with at least a partial pearlitic structure.
Preferably, the partial pearlitic structure is about 30% to 50% of the structure. More preferably, the partial pearlitic structure is about 40% to 45% of the structure.
In a particular embodiment, the partial pearlitic structure is about 30% of the structure.
In a further embodiment, the partial pearlitic structure is about 50% of the structure.
According to a seventh aspect of the invention, there is provided a method of improving the resistance to thermal stress cracking of a ductile iron alloy, comprising providing said ductile iron alloy with at least a partial pearlitic structure.
Preferably, the partial pearlitic structure is about 30% to 50% of the structure. More preferably, the partial pearlitic structure is about 40% to 45% of the structure.
In a particular embodiment, the partial pearlitic structure is about 30% of the structure.
In a further embodiment, the partial pearlitic structure is about 50% of the structure.
According to an eighth aspect, the present invention provides an inoculant for use in the production of a ductile iron cast alloy wherein as the iron alloy composition is being heated in a furnace it is contacted with a magnesium ferrosilicon-based inoculant, comprising about:
Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.
Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:
The partial pearlitic structure of the ductile iron alloy of the invention can be seen in the microscope (×50) images of
The partial pearlite structure produced by the method of the invention has been found to provide a number of improvements over the conventional flake graphite structure of grey cast iron. In particular, the iron alloy with the pearlite structure improved the dissipation of heat across the iron alloy structure, and hence provided an increased resistance to thermal stress cracking. Through further testing, it was also found that this structure provided for a reduced friction coefficient.
Hence, in implementation, the improved iron alloy structure may provide for an improved longevity of the iron alloy when subjected to regular thermal loads, such as in disc brake rotors, which may lead to both economic and safety advantages. This may be particularly useful in logistics, transport and vehicle racing applications, in particular in braking products for cars, buses, trucks, trains, etc.
Furthermore, it will be appreciated that the pearlite composition of the structure is correlated to the Brinell hardness of the structure. As a specific example, a partial pearlitic structure of 50% correlated to a Brinell hardness of about 200 in the resulting ductile iron alloy.
In light of this correlation and the provided benefits of the present invention, the person skilled in the art would be led to recognise that there exists a trade-off between the hardness of the ductile iron alloy and the improvements to thermal dissipation and stress cracking, and that the partial pearlitic structure must be adjusted for particular applications.
For example, in testing, it has been found that a partial pearlitic structure of about 30% would be useful in common motor vehicle applications, such as a disc brake rotor in a car. Alternatively, a partial pearlitic structure closer to about 50% would be useful in heavier duty applications, such as a brake disc rotor for a mining vehicle or truck. Further uses of the invention in heavier duty applications can include uses in the production of turbo housings, camshafts, crankshafts, engine blocks, wheels, freight containers or barges, blocks, and carriages, particularly for use in heavy vehicles (such as mining vehicles), railed vehicles, ships, and boats.
In the preferred embodiments of the invention, the steel composition is provided by a combination of steel scrap, pig iron, and further additives. The steel scrap, pig iron, and further additives are preferably clean of any rust. In a further preferred embodiment, they are also dry and free from oil.
Ideally, the steel scrap, pig iron, and further additives are in a form having a thin cross-section, such that they melted and dissolved faster into the molten steel form.
The steel composition preferably comprises of at least 40% steel scrap.
The steel scrap provides for the pearlitic matrix density of the structure, and also assists in nucleating austenite during the production process. The steel scrap composition preferably has a low level of manganese, alloy, and phosphorus impurities, and has a low oxygen activity (rust). The used steel scrap preferably does not have any zinc or tin coatings.
The pig iron provides for diluting and reducing the manganese and alloy impurities in the base composition to an acceptable level and provides a source of carbon that is easily dissolvable. The pig iron preferably is in the form of sorelmetal, and further preferably comprises low levels of phosphorus (0.02% max), sulphur (0.015% max) and titanium (0.04% max), and only trace levels of manganese and silicon.
Furthermore, it will be appreciated that any manganese, phosphorus (or phosphates), and sulphur (or sulphites) impurities in the steel composition can function as denodulising agents which assist in encapsulating the molybdenum and silicon in the pearlite structure produced by the invention. It is believed that these components in the pearlite structure promote the advantageous thermal conductivity of the ductile iron alloy.
In a preferred embodiment, in step (ii), the furnace is heated to a minimum internal temperature of about 1400° C.; however further embodiments may require the furnace heated to internal temperatures in the range of about 1400-1700° C., or 1450-1600° C., or 1470-1580° C., or any range therebetween.
It will be appreciated that these embodiments of the method of the invention involve a trade-off between the temperature of molten steel in the furnace and the loss of chemical components in the steel. In particular, the higher temperature provides a higher fluidity in the molten steel and reduces impurities in the ferritic structure. These impurities can form with surrounding air pockets, which may form cracks in the iron alloy when put under stress. However, at these higher temperatures, magnesium is vaporised and lost to the atmosphere, and magnesium is necessary for the formulation of the pearlite structure.
Accordingly, in some embodiments, cerium can be added to the steel composition in step (ii). The cerium acts as a stabilising agent for the magnesium at higher temperatures, and results in less vaporisation of the magnesium. In preferred embodiments, the cerium is added at about 0.25% to 0.55% by weight of the molten steel. For example, in a preferred embodiment, the cerium is added at about 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, or 0.55%, or any percentage therebetween. It will be appreciated that the addition of more cerium will result in a harder iron alloy.
Although the invention has been described with reference to specific embodiments, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.
The inventors have found that the waste remnants of the invention in the furnace and/or the inoculation ladle may be recycled, reheated, remelted, and repoured to produce a further iron alloy product with a pearlitic structure.
It will be appreciated that this the produced recycle product has similar metallurgy as the first product; however, as the inoculants and magnesium are reacted in the first melt, the produced recycle product is typically harder than the first ductile iron alloy product. In some tests, the inventors found the Brinell hardness of the recycle product to be up to 20% harder. Accordingly, the recycle product should only be used in applications where hardness is required, such as in turbo housings, camshafts, crankshafts, engine blocks, wheels, freight containers or barges, blocks, and carriages. The recycle product may also be rolled out for use in ballistic plating.
A ductile iron product with a partial pearlitic structure was produced in accordance with the present invention and detailed as follows.
Into a 500 kg induction furnace, the following charge components were added as an initial steel composition:
The steel composition was then heated to 1470° C. in the furnace in order to form a molten steel and overcome the sluggish fluidity caused by the addition of the molybdenum alloy.
As the steel composition was being heated, a magnesium ferrosilicon-based inoculant was prepared, comprising about:
The inoculant was in the form of 9 mm by 8 mm mesh and covered by a steel covering. The covered inoculant was then added to an inoculation ladle in the ratio of 1 kg inoculant per 500 kg molten steel.
Immediately after the inoculant was added to the inoculation ladle, the molten steel was poured into the inoculation ladle to be inoculated. The steel covering acts as a buffer to prevent the inoculant from immediately inoculating the molten steel.
In a preferred embodiment, the impurities in the induction furnace are preferably removed e.g. by scooping/skimming the surface prior to the malt and steel being poured.
Once the entirety of the molten steel was poured into the inoculation ladle, the mixture is left to inoculate for a period of about 6 minutes. After this time period, the inoculated molten steel was transferred to a pouring ladle for casting into a mould.
On analysis of the produced iron alloy, it was found to be a ductile iron with a partial pearlite structure.
A disc brake rotor with a partial pearlitic structure was produced in accordance with the present invention and its performance was compared against a standard baseline grey iron rotor.
In particular, this comparison was conducted to assess any differences in the friction performance and the wear and crack resistance of the rotors. The methodology and results of this study are considered as below.
The baseline and invention disc brake rotors were similarly set up with the following parameters:
For the purposes of this comparison, it is important to note that the produced invention rotor was considerably lighter than the baseline rotor (−23%).
All tests were conducted using a low-metal friction material BM987S. Each test was conducted with new brake pads and rotor embedded thermocouples.
In this test, the disc brake rotors were first burnished and then subjected to a number of stops from an initial speed of 80 km/h to a full stop at a deceleration of 0.3 g. Full details of the test schedule are as below:
During this test, the average pad wear and rotor wear were measured:
In terms of reduction mass (i.e. wear of the rotor and pad), it can be seen that overall rotor wear of the invention rotor is reduced by 28% compared to the baseline rotor; however, pad wear increased with the use of the invention rotor by 44%.
Following the Wear vs Temperature test, the baseline and invention disc brake rotors were then subject to a further Thermal Shock test, where the disc rotors were thermally cycled to determine the effects of thermal shock from sudden braking. Full details of the test schedule are as below:
The results of this test, shown with regards to the development of any cracks on the rotor faces, are shown as below:
It was found that baseline rotor exhibited greater cracking, with the crack initiation beginning earlier in the test and the crack length being greater at the end. The invention rotor showed relatively few cracks when compared with the baseline.
It is noted that the invention rotor displayed considerable hot-spotting during the Thermal Shock test. The invention rotor also experienced greater in-stop temperature rise and peak temperatures, due largely to the lower mass (shown in Table 2).
The higher thermal loads in the invention rotor (likely due to the lower weight) combined with the lower rate of crack development indicates the invention rotor offers greater resistance to thermal cracking.
The friction levels were also measured throughout the Wear vs Temperature test, and the Thermal Shock test, summarised as follows:
In the Wear vs Temperature test, the invention rotor demonstrated a slightly lower average friction level overall with mu=0.46 compared to mu=0.47 for the baseline. The friction level difference was greater during the thermal shock with mu=0.26 for the invention rotor compared to mu=0.30 for the baseline rotor.
Further differences were also noted, within the different sections of the tests. For example,
The invention rotor was also found to have lower friction levels throughout the Thermal Shock test, as shown in
When compared to the baseline rotor, the invention rotor demonstrated a reduced rotor wear (28%, during Wear vs Temperature test), a higher pad wear (44%, during Wear vs Temperature test), an improved crack resistance, and lower friction levels; however, more burnishing required for friction levels to stabilise.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017904280 | Oct 2017 | AU | national |
