High Alloy Iron, Use of the Material for Structural Components that are Subject to High Thermal Stress and Corresponding Structural Component

Abstract

The invention relates to a high alloy iron that has an austenitic structure and a spherical graphite configuration and comprises the following components in percent by weight: carbon<2.0%, silicon 2.0-6.0%, nickel 29-36%, chrome 1.0-2.5%, niobium 0.1-1.0% and molybdenum 0.1-2.5%. The invention also relates to the use of the cast iron material for structural components that are subject to high thermal stress, especially for exhaust manifolds and turbocharger housings of internal combustion engines of the individual or integrated type of construction. The invention finally relates to a structural component that is subject to high thermal stress, especially the exhaust system of an engine consisting of the aforementioned cast iron material.

Description

The present invention refers to a high-alloy cast iron material with an austhenitic structure and a spherical graphite configuration. The structure according to the invention is especially suitable for use in those parts of an engine that are under great thermal stress, particularly the exhaust system of an engine, but is also suitable for other structural components under high thermal stress.

The thermal stresses of exhaust manifolds and turbocharger housings have increased a great deal due to the introduction of new burning processes. The exhaust temperatures of modern Otto engines, in particular, currently reach well over 1000° C. Structural components subject to such thermal stresses must have, apart from high-temperature resistance, high scaling resistance, good temperature fluctuation resistance, and a low temperature expansion coefficient. Therefore, it is known from the state of art that structural components under high thermal stress are made of cast steel. Although the latter satisfies the requirements listed above, it is very expensive and therefore not very suitable for producing such structural components in series.

Furthermore, austhenitic cast iron alloys with spherical graphite are known from technical advances under the trade name Ni-Resist. These alloys, described in EN 13835 as EN-GJSA XNi 35 (or according to ASTM 439 as D5), have high thermal resistance, good scaling resistance and very high temperature fluctuation resistance in addition to their good mechanical properties. That is why in the state of art they are used for series production of engine parts under high thermal stress, particularly for exhaust manifolds and/or turbocharger housings of highly compressed or supercharged engines.

These alloys are economical and can be easily cast. They already have very high thermal resistance of up to 1050 degrees centigrade, as indicated in EN 13835. However, in thin-walled structural components with wall thicknesses lower than 8 mm, thermal resistance falls to only about 950° C. In order to lower temperatures to this maximum permissible rate, injection and burning are carefully regulated by expensive engine control, which in turn uses up once again more fuel. Therefore, there is a need to develop materials that will have higher resistance under high temperature conditions even if their components have thin walls.

The task of the present invention is to suggest an austhenitic cast iron material with very high thermal resistance in spite of having thin-walled structural components.

The task is solved with a cast iron material, an application thereof, and with structural components in accordance with the features of the independent claims.

According to the invention, a high-alloy cast iron material having an austhenitic structure and a spherical configuration contains, among other things, the following elements (in % of weight): carbon<2%, silicon 2.0-6.0%, nickel 29-36%, chrome 1.0-2.5%, niobium 0.1-1.0%, and molybdenum 0.1-2.5%. Compared to known austhenitic alloys containing spherical graphite based on nickel, molybdenum and niobium, in particular, have been added to the material in alloyed form in the concentrations given above. It has been shown that by adding molybdenum together with niobium, one can increase the temperature fluctuation resistance of such alloys, without causing the material to become brittle by the formation of carbide. In addition, the resistance of the material under higher temperatures and the scaling resistance can be improved with an alloy according to the invention. In this case, the alloy according to the invention has a relatively low thermal expansion coefficient, thus reducing the cracking risk that can occur under temperature fluctuation conditions.

The alloy's carbon concentration has been limited to a value of 2% to prevent carbide formation. Here, the carbon concentration is adjusted in such a way that in spite of this, the molten mass still has good flowing and pouring properties.

It is known that the alloy's silicon concentration has a deoxidizing effect and improves here the resistance against hot gas corrosion. Nevertheless, the material according to the invention has an advantageously low thermal expansion coefficient.

Chrome improves oxidation resistance under high temperatures. Here, the proportion of chrome has been limited to 2.5%.

A particularly advantageous configuration of the invention provides the cast iron material to have a manganese concentration of 0.5-1.5%. The manganese concentration in an alloy influences on the one hand the pouring properties, but on the other hand reduces graphite precipitation as well, so that the manganese concentration in this case has been preferably limited to 1.5%.

In accordance with another advantageous configuration of the invention, the material has a phosphorous concentration lower than 0.1%, since a higher proportion of phosphorous could also lead to the material's brittleness. Furthermore, it is advantageous for the material to have a copper concentration of less than 0.5%.

An especially preferred configuration of the invention foresees the cast iron material to have a nickel concentration of 34 to 36%. This proportion of nickel has the function of creating an austhenitic basic structure.

Another advantageous configuration of the invention provides the material to have preferably a chrome concentration of 1.5-2.5% in order to improve both high-temperature resistance and oxidation resistance.

It is particularly advantageous for the cast iron (in accordance with the present invention) to have a higher temperature resistance compared with the conventional, austhenitic cast iron alloys that use spherical graphite and nickel as main carriers (D5). Therefore, the cast iron according to the invention is also highly suitable for use in extremely thin-walled materials having wall thicknesses of 3-6 mm under very high temperatures.

The material according to the invention is therefore extremely suitable for use in structural components under high thermal stress. The thin-walled structural components made from the cast iron material according to the invention are resistant to about 985 degrees centigrade according to current knowledge. According to the invention, this high-alloyed cast iron material is therefore used for parts that make up the exhaust system of an engine. The material is especially suitable for exhaust manifolds and turbocharger housings. The material can be used with special advance in parts of the exhaust system of highly compressed and supercharged engines, in which exhaust temperatures of up to 1100° C. occur.

The material is particularly suitable for use in integral housings (in other words, housings that integrate exhaust manifolds and turbine housings of exhaust turbochargers).

According to the invention, structural components under high thermal stress (especially those of an engine's exhaust system) are composed of a cast iron material having the properties described above. Therefore, the cast iron used in the present invention can replace the materials that are conventionally used for exhausts systems. By and large, the cast iron can be manufactured and processed in accordance with the usual methods. The structural components can be subject to an annealing treatment for homogenizing the structure, which in turn achieves better carbide distribution.

Compared to conventional austhenitic cast iron alloys for use in vehicle exhaust systems it was possible to achieve an increase of thermal resistance by about 35 degrees centigrade. The increase in the exhaust or burning temperature achieved as a result of this also allows one to increase displacement-related power and reduce contaminant emissions, thus conserving fuel.

In addition, the cast iron material according to the invention had an 8% higher temperature fluctuation resistance in the shear-crack test compared to conventional alloys belonging to the D5 class (ASTM 439). In the autobahn permanence test, improvements of up to 30% could be seen with the material according to the invention.

The invention is especially suitable for structural components of a vehicular exhaust system, but not limited to them. Modifications and combinations within the scope of the invention also fall under the patent claims.

Claims

1. High-alloy cast iron material having an austhenitic structure and spherical graphite configuration with higher temperature fluctuation resistance and advantageous use for engines characterized in that the material expressed in percent of weight contains the following ingredients: Carbon <2.0% Silicon 2.0-6.0% Nickel 29-36% Chrome 1.0-2.5% Niobium 0.1-1.0% Molybdenum 0.1-2.5%

2-13. (canceled)