Abrasion Resistant Cast Iron

Abstract

A wear and abrasion resistant white iron having a composition as follows: And having a microstructure of predominantly chunky or needle carbides in a matrix predominantly austenitic.

Description

Our invention relates to a white cast iron and, more particularly, to a chromium-bearing white cast iron, having a combination of toughness and hardness, making it suitable to resist abrasion and wear when used for grinding or crushing ores, cements, rocks, and the like.

Many white cast irons are available for this purpose; most of them relying on the presence of hard iron carbides in their metallurgical structure, to provide a degree of wear resistance. Other white cast irons are alloyed with chromium, manganese, nickel, copper, vanadium, molybdenum, and the like, to provide harder carbides and, also, to improve the overall hardness of the matrix structure.

Some superior white cast irons, containing from 8 to 30 percent chromium are also in common use. Chromium provides particularly hard carbides and, when present in sufficient amount and under the right conditions, radically alters the structure of the carbides, so that they occur as cubic or needle-like particles quite different from massive carbides normally present in chromium-free cast irons.

Irons of this type have a ferritic matrix structure, unless alloys, such as nickel, are used to provide austenitic, bainitic or martensitic matrices. An object of this invention is to provide a wear resistant chromium-bearing cast iron containing orthorhombic or trigonal (chunky) carbides in an austenitic matrix.

A further object is to provide a white wear-resistant cast iron with an austenitic matrix, but not containing essential amounts of strategic and costly alloys, such as nickel and molybdenum.

A further object of this invention is to provide a wear-resistant white cast iron, wherein the improved wear characteristics of chromium are combined effectively with the wear characteristics of manganese, resulting in a white cast iron having a high degree of toughness for wear applications where some pounding impact is involved.

Still further objects may be apparent from the specification and the drawings in which;

FIG. 1 is a photomicrograph showing the structure of a prior art chromium and manganese-bearing cast iron in which the balance of these elements is such that the structure is similar to that of a white cast iron (magnification 400);

FIG. 2 is a photomicrograph showing the structure at 100 magnification of the alloy of this invention, wherein the chromium and manganese are correctly balanced and where the carbides exist as chunks having the trigonal and orthorhombic configuration;

FIG. 3 is an enlarged photomicrograph at 500 magnification of the alloy of this invention showing the typical austenitic matrix distinguished also by small quantities of low temperature pearlite, as indicated.

It has been established by those skilled in the art, that chromium, in sufficient amount, has a peculiar effect on the carbides that occur in a white cast iron. A sufficient amount of chromium, usually at least 8 percent by weight, produces first, a needle-like carbide known as an orthorhombic carbide supposedly having the composition (FeCr).sub.3 C. Increasing amounts of chromium, beyond this amount or a higher carbon equivalent value in the white cast iron, produce a chunky carbide known as a "trigonal" carbide, supposedly having the composition (FeCr).sub.7 C.sub.3.

The occurrence of substantial amounts of orthorhombic and, more particularly, trigonal carbides in the structure of the white cast iron, produces an increased impact resistance which largely contributes to the improved wear-resistant qualities exhibited by these cast irons.

It has been established, also, that increasing the carbon content and the silicon content of chromium-bearing cast iron, favors the increased production of orthorhombic and trigonal carbides. These carbides owe their presence to a hypereutectic composition, which enables them to be precipitated, first, from the melt during the process of solidification. While higher carbon and silicon contents do provide a more hypereutectic composition, it is still necessary to have at least about 8 percent chromium, before these preferred carbide types will be predominant in the structure.

It is also well known by those skilled in the art that manganese, as an alloying element, when used in sufficient amount, will produce an austenitic matrix structure. Austenite is particularly desirable as it can readily be decomposed by heat treatment by cold treatment or by mechanical working into martensite, which has a high intrinsic hardness, conferring considerably more wear resistance to any ferrous material. However, manganese, as an alloying element, alone, is incapable of producing carbides having the preferred orthorhombic or trigonal structure. Further than this, where manganese is present in the composition of the carbides of a white iron, it is not free to perform its function of producing an austenitic matrix. For this reason, relatively large amounts of manganese (at least about 7 percent), are necessary in a white iron containing carbides, to produce a predominantly austenitic structure.

We have discovered that manganese and chromium can be used together in a white cast iron in such a way that both orthorhombic and trigonal carbides can be produced simultaneously with an austenitic matrix, thereby providing a cast iron of unusual wear resisting characteristics. This is quite an unusual effect, when it is considered that manganese and chromium are quite opposite in nature and would tend to counteract each other, when used together.

Chromium is a ferrite former, tending to prevent the retension of austenite, whereas manganese tends to promote the formation of austenite.

We do not know why manganese is able to prevent the formation of ferrite, which is commonly present in high chromium cast irons, particularly because a large percentage of the manganese is tied up in the formation of carbides.

As a matter of fact, electro-probe analysis shows that, in the carbide phase the manganese to chromium ratio is about three to one with manganese at 36 percent and chromium at 12 percent; whereas in the matrix the manganese to chromium ratio is one to two, with manganese at 5 percent and chromium at about 10 percent.

We presume that there must be sufficient manganese present in the matrix to promote austenite and that, when manganese and chromium are used together, in sufficient amount, the austenite to ferrite change is rendered so sluggish that austenite is retained.

We have found that, up to 40 percent low temperature pearlite may also occur in the matrix, but that it is still predominantly austenitic. We have found that a manganese content of at least 7 percent and a chromium content of at least 8 percent and a combined manganese and chromium content of at least 15 percent, is necessary to produce the preferred structure of the alloy of our invention. The combination we actually prefer to use, under most conditions, is equal parts of manganese and chromium at about 10 to 12 percent of each element. We find that this particular combination is less sensitive to sectional variations in castings.

There is no upper limit to the amounts of manganese and chromium we can use, but in the interests of economy and to maintain good casting and foundry characteristics, we prefer to limit the combined manganese and chromium content to about 30 percent, with manganese up to 15 percent and chromium up to 15%.

It is well known that other alloys, particularly nickel, may be used in conjunction with chromium, to provide cast irons having an austenitic matrix and white cast irons containing orthorhombic and trigonal carbides. Nickel, however, is a strategic alloy, not always readily available in unlimited quantity to the producers of wear resistant castings and nickel, also, is a relatively expensive alloy, being at least five times as expensive and often eighty times as manganese. The application of manganese, as an essential alloying ingredient in the alloy of our invention, therefore, has solved an industrial problem of some magnitude. The use of nickel, as a tramp element or in small quantities, by deliberate intent, is not precluded from our alloy composition, providing the manganese and the chromium present are sufficient to provide the preferred structure, if the nickel were entirely absent.

By the same token, alloys like molybdenum, are often used in high chromium cast irons or in manganese cast irons, to provide an increased tendency to form a martensitic or bainitic matrix structure. Molybdenum also is a highly strategic and expensive alloy and would, therefore, not normally be present in any appreciable amount in the alloy composition of our invention. It may be used for special effects, where desired, providing that the manganese and chromium contents present are, by themselves, present in sufficient amount to produce the preferred structure of orthorhombic and trigonal carbides in an austenitic matrix.

Alloys, such as tungsten (acting similarly to molybdenum) and vanadium (acting similarly to chromium) may also be present, but they too are relatively expensive and we have also found that vanadium, present in large amounts (i.e. above about 2 percent) tend to produce carbides which are not as desirable as the preferred orthorhombic and trigonal types.

Copper may also be used as an alloying element, but it too is expensive and only appears to exert a relatively slight effect on our alloy composition. In general, we prefer to limit all of these special alloying elements commonly found in white cast irons to a total value of about 2 percent each and certainly to a combined value of no more than 5 percent.

Total carbon, which is present in all cast irons, is an important element in the composition of our alloy. We generally prefer a total carbon content of about 3.30 percent or more, in order to favor the production of the preferred carbides. In any case, we find that with carbon contents of less than 3 percent, it is quite difficult to obtain a reasonable proportion of trigonal carbides in the structure. Carbon contents above 4 percent tend to produce massive carbide segregation and the formation of very large carbides, which detract from the homogeneity of an alloy.

While silicon may be entirely absent from an alloy, we prefer to have it present in an amount ranging from 0.5 to 2.5 percent. At silicon levels of between 1.0 and 2.0 percent we find that the production of the preferred carbides is somewhat easier and that silicon appears to work together with carbon, so that the amount of silicon used is usually lower at higher carbon contents. In any case, the presence or absence of silicon is not critical to the production of the cast iron of our invention.

The mechanical properties of our alloy are particularly suited to wear and abrasion resistance. The overall Brinell hardness may be as low as 300, because of the relatively soft austenitic matrix, but on the other hand, particularly when thermal treatments are used, may be as high as 700 Brinell. We find that the as-cast hardness normally ranges between 320 and 550 Brinell and we prefer to have an as-cast hardness of about 400 Brinell, where a higher impact resistance is important.

The tensile strength of our alloy is usually more than 50,000 p.s.i., but may be as high as 100,000 p.s.i. As tensile strength is not particularly important in white cast irons, we do not regard it as an index of quality.

The impact strength of our alloy is higher than that associated with white cast irons and is very similar to that normally associated with other austenitic, ferritic or martensitic white irons containing chunky carbides; thus, on a 2 inch diameter bar we would normally expect an impact strength of 100 to 150 foot pounds in the product of our invention. This is approximately twice the value normally obtained in white irons not having the preferred carbide structure. This high impact strength or toughness provides good wear resistance under conditions where some pounding is present and where ordinary white irons would tend to abrade very rapidly by attrition or flaking of the metal surface.

Because of the relatively stable nature of the structure of our alloy and presumably because of the oxidation resistance of chromium, we have found that our wear resisting alloy is surprisingly resistant to the effects of heat or certain corrosive media. This enhances the value of this alloy, where the combined effects of heat and wear or the combined effects of corrosion and wear must be contended with in industrial applications.

While heat and corrosion resistance are found to be relatively good, the prime purpose of our alloy is to provide improved wear and abrasion resistance in castings such as mill liners, grinding balls, drop balls, grizzly discs, rolls, dipper teeth, pulverizing hammers and etc.

Many examples of the product of our invention may be given, but one example clearly indicates the unusual nature and the structural combination which is normal to this alloy. A heat was melted to the following composition:

Total Carbon 3.63% Silicon 1.65% Manganese 10.50% Chromium 9.62% Sulphur 0.08% Phosphorus 0.09%

This heat was cast into a 2 inch diameter test bar and into a casting used for grinding rock. The structure of the casting was examined and was found to consist of carbides in a matrix of about 70 percent austenite and 30 percent low temperature pearlite (non-resolvable pearlite). The carbides were approximately 60 percent of the trigonal (chunky) type and about 40 percent of the orthorhombic (needle) type.

The test bar was tested for impact at 12 inch centers under a drop test machine. It broke at 125 foot pounds. The hardness of the casting was found to be 430 Brinell on an overall basis.

Microhardness measurements on the carbides in the structure indicated hardness ranging from 800 to 1050 Brinell.

The casting was placed in service and performed for 152 hours, compared to ordinary white irons usually lasting about 60 hours and to alloyed white irons containing strategic alloys usually lasting about 130 hours.

Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details may be resorted to, without departing from the spirit and the scope of the invention hereinafter claimed.

Claims

What is claimed is:

1. A wear-resistant white iron having a substantially austenitic structure containing predominantly trigonal and orthorhombic chromium-manganese carbides consisting essentially of from 7.0 to 15.0 percent manganese, 8.0 to 15.0 percent chromium, 3.0 to 4.0 percent total carbon, 0 to 2.5 percent silicon, and with the total of manganese and chromium being at least 15.0 percent.

2. White iron in the composition range of: