Abstract:
A cast iron alloy for cast iron products characterized by a high oxidation stability at surface temperatures of 800° C. to 950° C., comprising the chemical components 2.8 to 3.6% C by weight; 2.0 to 3.0% Si by weight; 2.5 to 4.3% Al by weight; up to 1.0% Ni by weight; up to 0.8% Mo by weight; up to 0.3% Mn by weight; 0.002 to 0.1% Ce by weight; 0.023 to 0.06% Mg by weight; up to 0.01% S by weight, residual Fe, and the usual impurities.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a cast iron alloy for cast iron products with a high oxidation resistance at high surface temperatures. 
         [0002]    Automobile manufacturers are required to comply with the new exhaust emission standards. The catalytic converters operate better when the exhaust gas temperatures are higher. Palladium can be used instead of platinum as a catalyst material, and the maximum exhaust gas temperature will increase from currently 850° C. to 950° C. At these temperatures, the hitherto known cast iron alloys entail problems with scaling resistance. In the previous ferritic alloys, a phase transition from a ferritic lattice to an austenitic lattice takes place at temperatures above about 860° C. The expansion behavior of a ferritic lattice differs from the expansion behavior of an austenitic lattice. Because the thermal expansion coefficient of the austenitic lattice is greater and changes more strongly than the thermal expansion coefficient of the ferritic lattice, a change in volume takes place at the transition temperature. This volume change leads to a nonuniform expansion behavior and microcracking of the cast parts. The cast parts, which are subjected to a frequent temperature change, are mechanically stressed by this nonuniform expansion and cracking. As a consequence of this, thin oxide layers (=scale) become detached from the surface of the cast part. Ideally a thin oxide layer, which adheres well in the long-term and blocks oxygen diffusion, should be formed on the surfaces of the turbocharger housing and/or exhaust manifold which are exposed to the exhaust gas. 
         [0003]    EP 076 701 B1 discloses a heat-resistant ferritic cast iron with spheroidal graphite. The alloy contains up to 3.4 wt % C, from 3.5 to 5.5 wt % Si, up to 0.6 wt % Mn, from 0.1 to 0.7 wt % Cr, from 0.3 to 0.9 wt % Mo and up to 0.1 wt % of a component forming spheroidal graphite. The alloy is used for the production of turbocharger housings in motor vehicle manufacture. 
         [0004]    EP 1 386 976 B1 discloses an alloy for cast iron products with high thermal stability. The alloy consists of from 2.5 to 2.8 wt % C, from 4.7 to 5.2 wt % Si, from 0.5 to 0.9 wt % Mo, from 0.5 to 0.9 wt % Al, up to 0.04 wt % Mg, up to 0.02 wt % S, from 0.1 to 1.0 wt % Ni, from 0.1 to 0.4 wt % Zr, remainder Fe and usual impurities. The alloy is used for exhaust manifolds and turbocharger housings in motor vehicle manufacture. 
         [0005]    It is an object of the invention to provide a cast iron alloy that can be used at temperatures which are as high as possible, is as economical as possible to produce and ensures as long as possible a service life under frequent temperature changes. 
       SUMMARY OF THE INVENTION 
       [0006]    The foregoing object is achieved by providing a cast iron alloy for cast iron products with a high oxidation resistance at surface temperatures of from 800 to 950° C. having the chemical constituents from 2.8 to 3.6 wt % C, from 2.0 to 3.0 wt % Si, from 2.5 to 4.3 wt % Al, up to 1.0 wt % Ni, up to 0.8 wt % Mo, up to 0.3 wt % Mn, from 0.002 to 0.1 wt % Ce, from 0.023 to 0.06 wt % Mg, up to 0.01 wt % S, remainder Fe and usual impurities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  represents the transition of the present alloy from the ferritic phase to the austenitic phase as a function of temperature. 
           [0008]      FIG. 2  represents the thermal expansion coefficient of the new alloy with the designation SiMo1000plus, measured as a function of temperature, compared with other cast iron alloys. 
           [0009]      FIG. 3  represents the thermal conductivity of the alloy SiMo1000plus compared with other cast iron alloys as a function of temperature. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    It is advantageous for the cast parts to expand elastically as regularly as possible at the operating temperature. This is achieved by the temperature of the transition from the ferritic phase to the austenitic phase of the alloy lying above 880° C. It is also achieved by the thermal expansion of the alloy specimens as measured by a dilatometer varying uniformly and constantly up to a temperature of 880° C. It is also achieved by the alloy having a thermal expansion coefficient of from 8 to 12 10 −6 /K at 25° C. and from 13.5 to 15.5 10 −6 /K at 900° C. These are values which, plotted against the temperature, are consistently about 30% lower than the values of so-called Ni resist alloys with the standard designations D5S or GJSA XNiSiCr35-5-2. 
         [0011]    It is furthermore advantageous for the cast parts not to be brittle at room temperature. This is achieved by the alloy having strength values of from 500 to 650 MPa for the tensile strength R m , from 470 to 620 MPa for the yield point R p0.2  and from 2.0 to 4.0 for the elongation at break A 5 . These are strengths values which are about 1.3 to 1.5 times as great as those of so-called Ni resist alloys. The ductility of the cast iron alloys proposed here corresponds to the average value of standard commercial ferritic materials which, however, cannot be exposed to temperatures of more than 860° C. 
         [0012]    It is also advantageous for the cast parts to be readily processable. This is achieved by the alloy having a Brinell hardness of from 220 to 250. 
         [0013]    It is also advantageous for the alloy to be composed of elements which are as economical as possible. This is achieved by the alloy containing less than 0.8 wt % Mo, less than 1 wt % Cr and less than 1 wt % Ni. Ni resist alloys typically contain about 30 to 35 wt % Ni and about 2 to 5 wt % Cr. Spherocast alloys alloyed with molybdenum normally contain about 0.8 wt % molybdenum. 
         [0014]    Furthermore, it is also advantageous for the cast parts to be as insensitive as possible to heat. This is achieved by the alloy specimens having a thermal conductivity of 25 W/mK at 25° C. and a thermal conductivity of 26 W/mK at 900° C. Ni resist alloys have a thermal conductivity which is 20 to 50% lower at 400° C. 
         [0015]    The key concept of the invention is to provide a cast iron alloy which allows as high as possible a working temperature with a high scaling resistance in turbocharger housings and exhaust manifolds, and which can be produced as economically as possible and as simply as possible in a casting process. Previous standard solutions for higher working temperatures reside in the use of expensive cast steel and austenitic cast iron or in the use of elaborately produced sheet metal designs. 
       Example 
       [0016]    An exhaust manifold made of spherocast for a combustion engine of an automobile with the following chemical composition in percentages by weight: 3.02 C, 2.96 Si, 2.53 Al, 0.79 Ni, 0.65 Mo, 0.23 Mn, 0.04 Cu, 0.031 P, 0.026 Cr, 0.023 Mg, 0.017 Ti, less than 0.01 S and 0.002 Ce, has a ferritic lattice. The exhaust manifold is cast directly into the molds from a melt, which was pretreated with magnesium in a GF converter. Subsequent time-consuming heat treatment, such as solution annealing or austempering, is not necessary. 
         [0017]    The treatment with magnesium has a favorable effect on the sulfur content of the alloy and ensures the formation of graphite in the spheroidal or vermicular form. Magnesium has a desulphurizing effect, although sufficient Mg must remain in solution in order to promote growth of the spheroliths (=spheroidal graphite particles). An Mg content of about 0.025 wt % is ideal for the present Al content of about 2.5 wt %. The alloy specimens have a density which is at least 5% less than the density of comparable conventional cast iron alloys. 
         [0018]    The carbon content of from 2.8 to 3.6 wt % ensures a composition which lies close to the eutectic. Less than 2.8% C is unfavorable for the feedstock of the cast parts. More than 3.6% C is unfavorable for the high-temperature properties of the alloy. 
         [0019]    Cerium is added in amounts of from 0.002 to 0.1 wt % as a nucleation promoter. More than 0.1% Ce is unfavorable and leads to the formation of so-called chunky graphite. 
         [0020]    The silicon content of from 2 to 3 wt % in the present alloy has a positive effect on formation of the ferritic phase, improves the fluidity of the melt, raises the yield point and improves the heat resistance of the cast parts. Less than 2% Si is unfavorable for the chill depth. More than 3% Si increases the brittleness of the cast parts. 
         [0021]    The aluminum content of from 2.5 to 4.3 wt % likewise has a positive effect on formation of the ferritic phase and neutralizes the nitrogen. Less than 2.5% Al is unfavorable for the graphite stabilization. More than 4.3% Al is unfavorable for the formation of spheroidal graphite. 
         [0022]    The nickel content of from 0.1 to 1 wt % raises the yield point without substantially increasing the brittleness and improves the corrosion resistance. Less than 0.1% Ni is unfavorable for the graphite stabilization. More than 1% Ni is unfavorable for the formation of bainite and martensite in thinner regions of the cast parts. Nickel is a comparatively expensive alloy element. 
         [0023]    The molybdenum content of from 0.4 to 0.8 wt % has a positive effect on increasing the yield point, the thermal stability, the creep strength and therefore the thermal cycling stability. Less than 0.4% Mo is unfavorable for the graphite stabilization. More than 0.8% Mo is unfavorable for the formation of carbides and gas bubbles. Molybdenum is a very expensive alloy element. 
         [0024]    The manganese content of up to 0.3 wt % has a positive effect on the binding of sulfur. More than 0.3% Mn is unfavorable for the formation of grain boundary carbides and impairs of the nucleation state. Too much Mn promotes the formation of perlite in the crystal lattice. The bainitic lattice becomes increasingly brittle. 
         [0025]    The chromium content of up to 1 wt % has a positive effect on the creep strength and the thermal stability of the castings. 
         [0026]    In general, lower contents of the alloy additives are favorable for reducing the formation of grain boundary carbides and the brittleness at room temperature. This is the case for example with the copper and titanium contents. 
         [0027]    Compared with cast steel, the melting temperatures for spherocast are about 100 to 200° C. lower. This means that less energy is consumed and less alloy elements are released to the environment by evaporation. 
         [0028]      FIG. 1  represents the transition of the present alloy from the ferritic phase to the austenitic phase as a function of temperature. It may be seen here that an equilibrium phase transition takes place at about 900° C. The way in which the alloy changes aggregate state at a melting temperature of from 1240 to 1280° C. may also be seen here. 
         [0029]      FIG. 2  represents the thermal expansion coefficient of the new alloy with the designation SiMo1000plus, measured as a function of temperature, compared with other cast iron alloys. 
         [0030]      FIG. 3  represents the thermal conductivity of the alloy SiMo1000plus compared with other cast iron alloys as a function of temperature. Here, D5S stands for the so-called Ni resist alloys, and GJV SiMo and SiMoNi stand for the previously known spherocast alloys alloyed with about 1% Mo.