WO2003045626A2 - Graphitic cast iron of high mechanical strength - Google Patents

Abstract

Graphitic cast iron of high mechanical strength, presenting a microstructure with adequate proportions of nodular graphite and adequate proportions of vermicular graphite to define a total amount of graphite in the range of 12-15 % in volume of the microstructure.

Description

^*GRAPHITIC CAST IRON OF HIGH MECHANICAL STRENGHT"

The present report is about a graphitic cast iron of high mechanical strength, and more specifically, a graphitic cast iron of high tensile strength with high capacity to damp vibrations, good heat conductivity, good machinability and free from solidification porosities.

This graphitic cast iron of high tensile strength being preferentially used in the production of internal combustion engine blocks and internal combustion engine heads.

As known cast iron is usually classified, according to its graphite form, in gray cast iron (graphite in the form of vein) , vermicular cast iron (graphite in the form of worms) and nodular cast iron (graphite in the form of nodules) . Each group of cast iron presents typical properties with specific uses and applications.

In the case of internal combustion blocks and heads, it is usual to apply the gray cast iron group due to its reasonable tensile strength (200 to 350 Mega Pascal of ultimate tensile strength) , its good heat conductivity, machinability and absence of solidification porosities even in complex geometry parts.

The vermicular cast iron has been increasingly applied to engine blocks and heads and to brake disks and drums, providing an ultimate tensile strength higher than that of the gray cast iron (300 to 500 Mega Pascal of ultimate tensile strength) , with intermediate values of heat conductivity.

The biggest difficulties observed in the use of the latter are the presence of solidification porosity in certain geometries and low machinability, which reflects in the reduction of a cutting tool life if compared to gray cast iron.

Another unfavorable aspect observed is its capacity to damp vibrations which is smaller than the gray cast iron. This aspect shall be considered in the project of automotive components as vibration causes noise problems that shall be avoided.

The nodular cast iron group, due to its high tensile strength (450 to 900 Mega Pascal) and to good elongation values (20 to 2%), has been used in several automotive and machine components. On the other hand, its use in engine blocks and heads has been restricted to a few cases mainly due to the presence of solidification porosities and machinability much lower than the gray cast iron and the vermicular cast iron, and its low capacity to damp vibrations and low heat conductivity.

One of the goals of the present invention is to provide a graphitic cast iron of high tensile strength that presents an ultimate tensile strength up to 600 Mega Pascal, and an yield strength up to 380 Mega Pascal. These and other goals and advantages of the present invention are met with a graphitic cast iron of high tensile strength that presents a microstructure with adequate proportions of nodular graphite (forms I and II as per Standard ASTM A247) and adequate proportions of vermicular graphite (form IV as per Standard ASTM A247) ; the percentage of nodular graphite comprised in the range of 20- 40% and the percentage of vermicular graphite comprised in the range of 60-80% so that the total amount of graphite be 12-15% in volume, being the matrix a pearlitic one, and such microstructure of metallic matrix is comprised of refined pearlite.

The graphitic cast iron of high tensile strength, thus, presents a content of Carbon from 3,5 to 3,9%, a content of Si from 2,0 to 2,6%, a content of Cr lower than 0,05%, a content of Mn lower than 0,40% and a content of Ti lower than 0,015 %; controlled additions of Cu, Sn, Ni and Mo are foreseen according to the thickness of the part.

With this microstructure provided to the graphitic cast iron of high tensile strength, it is possible to counterbalance the contraction of surface solidification, thus, producing parts free from solidification porosities. At the same time, this microstructure presents good machinability due to the lubricant effect the graphite exerts on the cutting tool.

With a high amount of vermicular graphite in the structure of the graphitic cast iron of high tensile strength, it is also possible to obtain other properties such as good heat conductivity and the damping of vibrations .

The tensile strength advantages previously mentioned are, thus, increased.

Despite the fact that a preferred constructive concept has been described and illustrated, it is important to mention that changes are feasible and possible to be carried out without deviating from the scope of the present invention.

Claims

Claims

1- 'GRAPHITIC CAST IRON OF HIGH MECHANICAL STRENGHT" , characterized by presenting a microstructure with adequate proportions of nodular graphite and adequate proportions of vermicular graphite, being the percentage of nodular graphite in the range of 20-40% and the percentage of vermicular graphite in the range of 60-80%.

2- 'GRAPHITIC CAST IRON OF HIGH MECHANICAL STRENGHT", as per claim 1, characterized by presenting a total amount of graphite in the range of 12-15% in volume of the microstructure.

3- 'GRAPHITIC CAST IRON OF HIGH MECHANICAL STRENGHT", as per claim 1 or 2, characterized by presenting a microstructure of metallic matrix comprised of refined pearlite.

4- 'GRAPHITIC CAST IRON OF HIGH MECHANICAL STRENGHT", as per claim 1 or 2 or 3, characterized by presenting a content of Carbon from 3,5 to 3,9%, a content of Si from 2,0 to 2,6%, a content of Cr lower than 0,05%, a content of Mn lower than 0,40% and a content of Ti lower than 0,015 %; controlled additions of Cu, Sn, Ni and Mo are foreseen according to the thickness of the part.

5- 'GRAPHITIC CAST IRON OF HIGH MECHANICAL STRENGHT", as per claim 1 or 2 or 3 or 4, characterized by presenting an ultimate tensile strength up to 600 Mega Pascal and an yield strength up to 380 Mega Pascal.