DESCRIPTION

Technical Field of the Present Invention

The present invention relates to a process for the production of a cast iron alloy involving enhanced mechanical properties as well as resistance to high temperatures.

Prior Art/Background of the Present Invention

Cast iron can be grouped under iron-carbon alloys having carbon content greater than 2%. With the exception of malleable cast irons, cast iron is a brittle material. Cast iron having good properties such as low melting point, fluidity, good casting properties, resistance to deformation and wear resistance offers an important manufacturing material with widespread use in engineering applications. Cast irons are subject to widespread use especially in various industries such as machine and automotive industry, for instance particularly in automotive industry in the form of materials out of which different components such as exhaust manifolds, turbocharger casings or brake brackets are made.

Main alloying elements of cast iron is carbon and silicon, with the amount ranging from 2-4 % and 1-3 % by weight respectively. Silicon in the cast iron effectuates the dissociation of iron carbide to iron and graphite at high temperatures. Grey cast iron has graphite flakes reducing the tensile properties. Depending on the application, carbon and silicon contents are adjusted to a preferred level, within the range of 2-4 wt% and 1-3%, respectively. Other elements are then added to the melt by which the final form is obtainable by casting. The properties of cast iron change with addition of different alloying elements. Nickel (Ni) is one of the most used alloying elements providing toughness and also helpful in eliminating hardness differences in terms of normalizing of different section thicknesses. While Sulfur is typically effective in preventing the formation of graphite, thereby increasing hardness, Manganese is conventionally used together with sulfur to form manganese sulfide instead of iron sulfide.

At present, mechanical properties of cast iron at elevated temperatures, typically at temperatures in the range of 950 °C- 1000 °C, reveals that it no longer exhibits the same performance as it does within the range of 700 to 900 °C. It is to be noted that in view of the geometrical complexity of various machine parts and necessity to maintain operational performance for these machine parts in the presence of severe thermal and mechanical conditions, different measures so as to enhance thermal resistance and mechanical properties of cast iron are needed. In automobile industry, components of the engines such as exhaust manifolds and turbine housings are typically expected to withstand elevated temperature conditions and hence function under serious thermal and mechanical stresses.

A further concern originates from the fact that additional elements designed for supporting mechanical parts so as to enhance mechanical strength and robustness may generally cause increase in the system's overall weight, which should in turn be compensated elsewhere. The latter approach further increases overall manufacturing costs. The present invention is therefore devised under the recognition that it is necessary to produce materials with lower production costs while at the same time retaining their mechanical stability and resistance to higher temperatures and oxidation while having lower weights.

A prior art publication in the technical field of the present invention may be referred to as US 4,501,612, which discloses a process for the production of a compacted graphite cast iron which is free of carbides in castings as thin as 0.15 inches, and with nodularities of less than 20% by volume without post-inoculation, which comprises adding aluminum to a compacted graphite cast iron, such that the compacted graphite cast iron consists essentially of about 0.5 to 7% by weight aluminum, about 2.5 to 4% carbon, 0 to about 1.5% silicon, with the remainder iron and inevitable impurities. The compacted graphite cast iron produced thereby is useful in the production of castings with both thick and thin sections.

The present invention on the other hand provides a low weight cast iron having properties of thermal resistance, mechanical strength and lower manufacturing costs.

The present invention provides a vermicular and/or spheroidal cast iron having heat resistance from 950 to 1000 °C.

Additionally, the present invention provides a mechanically enhanced light weight cast iron material by modifying amounts of alloying elements conventionally having higher weights such as nickel, chromium and molybdenum and increasing the amounts of lower weight elements in the melt. The decrease in the nickel, chromium and molybdenum ratios in the melt additionally reduces the manufacturing costs.

Objects of the Present Invention

Primary object of the present invention is to provide a light weight cast iron material to withstand temperature conditions from 950 to 1000 °C and to hence operate under more severe thermal and mechanical stresses.

Further, the present invention aims at reducing manufacturing costs while at the same time maintaining high performance under thermal and mechanical stresses. The cast iron alloy of the present invention particularly aims at manufacturing cast iron components for applications in automobile industry where the components such as exhaust manifolds, turbocharger casings etc. are to be manufactured.

Summary of the Present Invention The present invention relates to a process for the production of a cast iron alloy involving enhanced mechanical properties as well as resistance to high temperatures. The present invention more particularly relates to a heat-resistant vermicular or spheroidal graphite cast iron comprising 4.0% to 4.50% by weight Si, 2.70% to 3.10% by weight C, 4.50% to 4.80% by weight Al and 0.10% to 0.50% by weight Mo.

Aluminum in an amount more than 5% by weight forms kappa carbide within the structure of face centered cubic (FCC) structure therefore causing an undesirable brittle structure. Aluminum in an amount of 4.50% to 4.80% by weight accompanied by molybdenum in the amount of 0.10 to 0.5% by weight effectuates enhancing of the mechanical strength of the solidified structure and resistance thereof to higher temperatures in the range of 900 to 1000 °C.

Further, molybdenum in the amount up to 0.5% by weight together with Aluminum in an amount of 4.50 % to 4.80 % by weight allows both achieving of the above- mentioned technical effects and additionally lowers manufacturing costs.

Brief Description of the Figures of the Present Invention Accompanying drawings are given solely for the purpose of exemplifying a light weight cast iron material with improved mechanical properties and resistance to heat, whose advantages over prior art were outlined above and will be explained in brief hereinafter. The drawings are not meant to delimit the scope of protection as identified in the claims nor should they be referred to alone in an effort to interpret the scope identified in said claims without recourse to the technical disclosure in the description of the present invention.

Fig. 1 demonstrates a phase transition diagram of AlSiMo alloy cast iron at different temperatures according to the present invention.

Detailed Description of the Present Invention

The present invention relates to a heat resistant vermicular/spheroidal graphite cast iron. More specifically, the present invention provides a vermicular or spheroidal cast iron with improved mechanical properties at high temperatures, typically from 900 to 1000 °C.

The cast iron of the present invention is a heat resistant vermicular or spheroidal graphite cast iron containing aluminum, silicon and molybdenum (AlSiMo). Based on the composition of the cast iron of the invention, the graphite can be spheroidal and/or vermicular. The casting of the alloy is achieved at room temperature without requiring control of atmospheric pressure and/or temperature. According to the present invention, addition of aluminum to the structure shifts the temperature of phase transition of the material to approximately 1000 °C. The phase transition of iron from ferrite to austenite, in other words, transition from body centered cubic (BCC in Fig. 1) structure to face centered cubic (FCC) structure is shifted to 1000 °C. The phase transition temperature of a conventional cast iron alloy with high SiMo in the structure typically occurs at 840 °C. In view of the phase transition diagram of cast iron alloy of the present invention as shown in Fig. 1, the phase transition starts at 1000 °C and completes at 1180 °C, body centered cubic structure transforming to face centered cubic structure. According to the vermicular or spheroidal graphite cast iron of the present invention, aluminum is present in the cast iron in the amount of 0.50 to 4.80 % by weight. Silicon is present in the amount of 4.00 - 4.50 % by weight. Carbon is present in the range of 2.70 - 3.10% by weight.

The invention's heat-resistant spheroidal graphite cast iron comprises 4.00% to 4.50% by weight Si, the concentration by weight of Al lying in the range of 4.50% to 4.80%, and the concentration of Mo in the range 0.10% to 0.50%. Carbon in the cast iron being present in the form of graphite flakes are transformed into nodular form by means of magnesium as a graphite- spheroidizing agent, also providing increased strength and ductility.

To prevent formation of iron carbides during solidification of nodular cast iron, which adversely effects machining of the casting, it is particularly established that aluminum in an amount more than 5% by weight forms kappa carbide within the structure of face centered cubic structure therefore causing a brittle structure. It is established that aluminum in an amount of 0.50% to 4.80% by weight accompanied by molybdenum in the amount of 0.10 to 0.5% by weight is effective in enhancing the mechanical strength of the solidified structure and resistance thereof to higher temperatures in the range of 900 to 1000 °C. Further, use of molybdenum in the amount up to 0.5% by weight together with aluminum in an amount of 4.50 % to 4.80 % by weight instead of 0.50 % to 4.50 % by weight is particularly found to be more prominent in terms of achieving the technical effects of enhancing both mechanical and thermal stability.

It is also to be noted that the present invention enables obtaining a cast iron alloy with lowered manufacturing costs as no more expansive or heavier materials such as nickel or chromium are used and amount of molybdenum is limited to only a critical range to ensure presence of the desired mechanical and thermal characteristics. It is to be noted that molybdenum in the amount of more than 0.5% by weight accompanied by aluminum in an amount of 4.50% to 4.80% is found to produce no further improved effects in terms of mechanical and thermal characteristics. The cast iron alloy of the present invention therefore affords decreasing of the manufacturing costs by avoiding use of more expansive materials and achieves an alternatively acceptable performance in the produced structure. The reduced weight of the cast iron is also critical in automobile industry in that it allows manufacturing flexibility when designing components such as for instance exhaust manifolds.

Example

The detailed chemical composition of the cast iron in which content of different alloy constituents lies in specific ranges and manufactured by conventional cast iron casting process is presented in Table 1.

The cast iron alloy according to the present invention is found to provide at least 5% to 10% weight decrease in comparison to conventional SiMo cast iron. Additionally, high temperature oxidation resistance is increased six times compared to the conventional SiMo cast alloy. Aluminum and molybdenum addition to the alloy respectively in the amounts of 0.50% to 4.80% and 0.10 to 0.5% by weight achieves an equally effective solution compared to conventional up to 4.00 Al AISi structures supported by heavier materials by enhancing the mechanical strength of the solidified structure and resistance thereof to higher temperatures in the absence of heavier and more expansive materials. Further, Aluminum in the specified amounts is also found to be equally effective in increasing thermal and mechanical stability of the oxide film, thereby protecting the material against oxygen diffusion.

AlSiMo cast iron alloy of the present invention oxidized at a higher temperature as discussed above is found to provide electrochemical corrosion resistance up to 180 hours at room conditions, thereby presenting the protective characteristics of the oxide film forming on the material surface.

In a nutshell, the present invention proposes a heat-resistant vermicular or spheroidal graphite cast iron comprising 4.0% to 4.50% by weight Si and 2.70% to 3.10% by weight C.

In one embodiment of the present invention, the heat-resistant vermicular or spheroidal graphite cast iron further comprises 0.50% to 4.80% by weight Al and 0.10% to 0.50% by weight Mo.

In a further embodiment of the present invention, the vermicular or spheroidal graphite cast iron further includes 0.10% to 0.20% by weight of Mn.

In a further embodiment of the present invention, the vermicular or spheroidal graphite cast iron further includes up to 0.10% by weight of Cu.

In a further embodiment of the present invention, the vermicular or spheroidal graphite cast iron further includes up to 0.04% by weight of P.

In a further embodiment of the present invention, the vermicular or spheroidal graphite cast iron further includes up to 0.10% by weight of Cr. In a further embodiment of the present invention, the vermicular or spheroidal graphite cast iron further includes up to 0.10% by weight of Ni.

In a further embodiment of the present invention, the vermicular or spheroidal graphite cast iron further includes up to 0.10% by weight of Cr in addition to 0.10% by weight of Ni.

In a further embodiment of the present invention, vermicular or spheroidal graphite cast iron includes 4.50% to 4.80% by weight Al.