US3042512A - Wear resistant cast iron - Google Patents

Description

W btates Patented July 3, 19-512.

The present invention relates to a unique cast iron alloy having improved and unusual combinations of properties, particularly in the direction of improved wear resistance.

In the field of abrasive wear, unusual hardness in a cast iron results when the excess carbon is in the form of carbides, as in a white iron, or when the matrix is in the form of martensite, as in heat-treated irons or certain alloyed irons.

Where excess carbon in the iron occurs as free carbides either wholly or in part, there is a marked reduction in the ability of the iron to resist the action of impact loading during service. There exists also a marked increase in the tendency of this cast iron to spall and flake during service, with a consequent reduction in its useful wearing life.

The majority of martensitic cast irons produced for wear resistance have their excess carbon in the form of hard, relatively brittle, carbides. For improved toughness it is usual to reduce the mount of carbides present by lowering the total carbon content of the iron. While this is an effective method, it limits the type of melting operation that can be used in commercial production and it introduces also the high shrinkage and poor castability characteristics of irons of lower total carbon content.

It is an object of this invention to provide a cast iron having a high combination of hardness and toughness.

It is another object of this invention to provide a tough cast iron of improved wear resistance and not subject to the difiiculties of shrinkage and castability associated with irons of lower carbon content.

Other objects and advantages of the present invention will be apparent to those skilled in the art from the following description:

The present invention provides a novel cast iron containing manganese in excess of 2.5% and molybdenum in excess of 0.4%. In addition to these elements, various other elements, particularly copper, may be present in the amounts found in the so called alloy cast irons where these alloys are present by chance or where they are added for special efiects.

The structure of the cast iron of this invention consists of austenite and martensite with the excess carbon in the nodular form. The preferred form of this product contains most of the excess carbon as graphite, with Very little carbon being present as free carbides.

As the product of this invention is free from the presence of embrittling carbides such as those found in cast irons produced for service in abrasive wear conditions, it has a much greater toughness than is usually associated with an iron of high hardness value. It has been found that the presence of the relatively soft nodular graphite in the smicture does not detract from the Wear resistance of the product.

The unique combination of properties exhibited by the alloy of this invention is achieved by the use of manganese and molybdenum in amount sufiicient to eliminate pearlite from the structure and replace it with austenite or martensite.

In the manufacture of conventional cast iron with graphite in the nodular form, manganese is not regarded as a useful element and, in fact, it is kept very low, i.e., below .5%, so as to provide ductility in the cast iron. Higher amounts are tolerated for special purposes such as for providing a pearlitic matrix, but the useful maximum has always been regarded as about 2.0%. It has not previously been disclosed that cast irons having nodular graphite and manganese in excess or" 2.5% have a Very unique and useful set of properties, or that irons of this manganese content may be produced with graphite in the nodular form.

To show the advantages of the alloy described in this specification, a test melt was made to the following analysis:

Percent Total carbon 3.37

Silicon 2.92

Manganese 3.24 Molybdenum 0.91 Copper 0.92

This melt was treated with a mixture of cerium and magnesium as a silicon alloy in an amount sufiicient to produce the nodular form of graphite. The melt so treated was cast into a number of test pieces, which were tested for mechanical properties and wear resistance.

The following test results were obtained:

Tensile strength 81,000 psi. Yield strength 65,000 psi. Elongation 1.0 percent. Transverse strength 18" centres, 1.2 bar 4350 pounds. Izod impact on 1.2" bar 83 foot/pounds. Brinell hardness 1.2 bar 528. Brinell hardness 3" section 472.

The structure of all test pieces was found to consist of the nodular form of graphite in a matrix of martensite.

To illustrate the unusually good combination of toughness and hardness exhibited by this alloy, the properties of a typical heat of the alloy of this invention is compared With that of a typical martensitic white iron in Table No.1:

The unique combination of properties of this product may be modified readily by heat-treatment. Thus, by annealing, the hardness may be reduced due to the formation of pearlite. By quenching, the hardness may be reduced by the formation of austenite and by deep freezing at temperatures as low as F. the hardness may be increased considerably, due to the formation of increased quantities of martensite. These treatments are all within the scope of those skilled in the art.

The chemical composition of this product may be varied to suit the cooling rate or casting section of the casting to be poured. Thus, for heavier, slower cooling sections, it is usual to raise the manganese content so as to ensure a martensitic structure or produce a partially austenitic structure. For quicker cooling, light casting sections, it is usual to reduce the manganese content, unless a larger quantity of austenite is desired.

For economical reasons it is preferred to keep the molybdenum at close to 1.0%, although it may be varied '2 a over a broad range and still produce the structure of this product. The copper content is usually kept at a maximum of about 1%, because larger amounts may tend to afiect the nodularity of the graphite. Other alloys are used sparingly, unless desired for special effects. The silicon content is kept at a value related to the casting section and the amount of free carbides desired in the structure. The carbon content is usually above 3.0%, because this imparts better founding properties to the alloy.

The preferred composition for general purposes substantially falls in the range of Percent Total carbon 3040 Silicon 1.0-3 .5 Manganese 2.5-.10.0 Molybdenum 0.40-1.50 Copper -1.5

with other elements such as chromium, nickel or vanadium being present in various amounts, according to any special effects desired.

One of the outstanding qualities of the alloy of this invention is its ability to work harden or increase in hardness during service. is tied in with the amount of austenite in the matrix structure of the alloy and where larger quantities of austenite are present the initial hardness may be relatively low, say as low as 200 Brinell, but this hardness may increase to over 500 Brinell by the process of pounding impact. This is a useful property, because by providing a matrix high in austenite content, the toughness level of the alloy may be increased, so as to extend its useful life in service. It has not previously been disclosed that nodular cast irons may be given this ability to work harden through the combined use of manganese and molybdenum.

The abrasive qualities of this alloy may be illustrated by a simple test in which a test piece is rotated in a slurry of silicon carbide, along with a. standard medium carbon steel reference test piece and any other material being tested. Table II illustrates the relative wear rate of a number of materials where that of the reference test piece is given the arbitrary value of 100 as a wear index:

Table II Material: Wear index Reference medium carbon steel 100 Manganese molybdenum nodular iron Martensitic white cast iron Heat treated alloy cast iron 70 The degree of this work hardening This clearly shows that the cast iron of this invention, with a wear index of 30, is an outstanding material for abrasive wear conditions.

The alloy described has a much better castability than like materials, particularly alloy martensitic white cast irons. In a test run in a foundry, a series of mixer paddles were cast, using a martensitic white cast iron. The loss of castings due to common defects, such as shrinkage cracks, blowholes, misruns and breakage, amounted to about 19% of the total castings made. The same casting was then made for a period of time in the alloy of this invention. Loss of castings due to these defects was found to be only 1% of the total cast, thereby illustrating the improved foundry characteristics of this alloy.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from thespirit and scope of the invention, as those skilled in the art will readily understand. Such variations are considered to be within the purview and scope of the invention and the appended claims. 1

What is claimed is:

1. A cast iron containing manganese in the range of 2.5% to 10.0% and molybdenum. in the range of 0.4% to 1.5%, total carbon in the range of 3.0% to 4.0% and silicon in the range of 1.0% to 3.5%, said cast iron being further characterized by an as-cast matrix of martensite with substantially no free carbides and with excess carbon in the nodular graphite form.

2. A cast iron having substantially no free carbides and excess carbon in the nodular form and having an ascast matrix of martensite, said cast iron consisting essentially of manganese in the range of 2.5% to 10.0% and molybdenum in the range of 0.40% to 1.5%, the total carbon of said iron being in the range of 3.0% to 4.0% and silicon being in the range of 11.0% to 3.5

References Cited in the file of this patent UNITED STATES PATENTS 1,911,173 Colwell May 23, 1933 2,485,760 Millis et a1, Oct. 25', 1949, 2,842,437 Guenzi July 8, 1958 2,885,284 Moore May 5, 1959 FOREIGN PATENTS 455,531 Great Britain Oct. 22, 1936 545,102 Great Britain May 11, 1942 123,399 Australia Jan. 22, 1947

Claims (1)

1. A CAST IRON CONTAINING MANGANSESE IN THE RANGE OF 2.5% TO 10.0% AND MOLYBEDENUM IN THE RANGE OF 0.4% TO 1.5%, TOTAL CARBON IN THE RANGE OF 3.0% TO 4.0% AND SILICON IN THE RANGE OF 1.0% TO 3.5%, SAID CAST IRON BEING FURTHER CHARACTERIZED BY AN AS-CAST MATRIX OF MARTENSITE WITH SUBSTANTIALLY NO FREE CARBIDES AND WITH EXCESS CARBON IN THE NODULAR GRAPHITE FORM.