Metal alloys for high impact applications

Abstract

A casting of a white cast iron alloy and a method of producing the casting are disclosed. A white cast alloy is also disclosed. The casting has a solution treated microstructure that comprises a ferrous matrix of retained austenite and chromium carbides dispersed in the matrix, with the carbides comprising 15 to 60% volume fraction of the alloy. The matrix composition comprises: manganese: 8 to 20 wt %; carbon: 0.8 to 1.5 wt %; chromium: 5 to 15 wt %; and iron: balance (including incidental impurities).

Claims

1. A white cast iron alloy comprising the following bulk chemistry: chromium: 7 to 36 wt %; carbon: 3 to 8.5 wt %; manganese: 5 to 18 wt %; silicon: 0 to 1.5 wt %; titanium: greater than 5 wt % to 13 wt %; and balance of iron and incidental impurities.

2. A white cast iron alloy comprising the following bulk chemistry: chromium: 7 to 36 wt %; carbon: 3 to 8.5 wt %; manganese: 5 to 18 wt %; silicon: 0 to 1.5 wt %; niobium: greater than 10 wt % to 33 wt %; and balance of iron and incidental impurities.

3. A white cast iron alloy comprising the following bulk chemistry: chromium: 7 to 36 wt %; carbon: 3 to 8.5 wt %; manganese: 5 to 18 wt %; silicon: 0 to 1.5 wt %; niobium and titanium: greater than 10 wt % to 25 wt %; and balance of iron and incidental impurities.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The white cast iron alloy and casting will now be described further by way of example only, and with reference to the accompanying drawings, in which:

(2) FIG. 1 is a micrograph of the microstructure of an as-cast iron alloy in accordance with an embodiment of the inventions.

(3) FIG. 2 is a micrograph of the microstructure of the as-cast iron alloy in FIG. 1 after heat treatment.

DETAILED DESCRIPTION

(4) Although a range of white cast iron alloy compositions are with the scope of the present invention, the following description is directed to one cast iron alloy in particular as an example.

(5) It is noted that the applicant has carried out extensive experimental work in relation to the white cast iron alloy of the present invention that has established the upper and lower limits of the ranges of the elements and the volume fractions of the carbides in the following as-cast microstructure of the present invention comprising: (a) a ferrous matrix comprising retained austenite, the matrix having a composition of: manganese: 8 to 20 wt % carbon: 0.8 to 1.5 wt %; chromium: 5 to 15 wt %; and iron: balance (including incidental impurities); and (b) chromium carbides comprising 5 to 60% volume fraction.

(6) The example white cast iron alloy had the following bulk composition: chromium: 20 wt %; carbon: 3 wt %; manganese: 12 wt %; silicon: 0.5 wt %; and a balance of iron and incidental impurities.

(7) A melt of this white cast iron alloy was prepared and cast into samples for metallurgical test work, including hardness testing, toughness testing and metallography.

(8) The test work was performed on as-cast samples that were allowed to cool in moulds to room temperature. Test work was also carried out on the as-cast samples that were then subjected to a solution heat treatment involving reheating the as-cast samples to a temperature of 1200 C. for a period of 2 hours followed by a water quench.

(9) A summary of the hardness and toughness test results is set out in Table 1 below.

(10) TABLE-US-00001 TABLE 1 Summary of Test Results Fracture Ferrite Hardness Hardness (HB - Toughness meter Alloy form (HV50) converted) (MPam) reading As cast 413 393 49.85 0% Solution 446 424 56.35 0% treated at 1200 Celsius

(11) The microstructure of the white cast iron alloy in the as-cast form (FIG. 1) shows large austenite dendrites in a matrix of eutectic austenite. By contrast, the solution heat treated form of the iron alloy (FIG. 2) shows austenite dendrites generally well dispersed in a retained austenite matrix. The ferrite meter readings for the as-cast and solution heat treated samples (that is, magnetism readings), show that the samples were non-magnetic. This, therefore, indicates that the castings did not include ferrite or martensite or pearlite in the ferrous matrix.

(12) Compositional analysis of the retained austenite matrix is revealed a chromium content in the matrix solid solution of about 12 wt % and a carbon content in the matrix of about 1.1 wt %. The retained austenite matrix therefore can be regarded as a manganese steel with relatively high chromium content in solid solution for improved hardness and improved corrosion resistance, which are not features of conventional austenitic manganese steel.

(13) Additionally, the volume percentage of chromium carbides contributed to hardness and overall wear resistance. Although the hardness results in Table 1 are below typical hardness measurements of wear resistant cast iron alloys, it was found that hardness of the iron alloy increased after work hardening treatments to a level that is comparable to hardness of known wear resistant cast iron alloys.

(14) Further samples of the same white cast iron alloy were cast and then subjected to heat treatment at 1200 C. for a period of 2 hours.

(15) The samples had a microstructure comprising primary austenite dendrites plus eutectic carbides and eutectic austenite.

(16) Microanalysis of the samples revealed the following: Both the elements chromium and carbon partition heavily to the carbide phase which was identified as (Fe, Cr, Mn).sub.7C.sub.3 by Electron Back Scattered Diffraction. To a first approximation, the element manganese is evenly distributed between the carbides and austenite phases. 11.3% by volume of the microstructure consisted of primary austenite dendrites. 22.3% by volume of the microstructure consisted of eutectic carbides. 66.4% by volume of the microstructure consisted of eutectic austenite. The carbon content of the austenite phase was 0.98 wt %. The manganese content of the austenite phases was 11.8 wt % and 11.6 wt %. The ferrous matrix of the alloy consisted of 11.3% by volume primary austenite dendrites and 66.4% by volume eutectic austenite. The chemistry of the ferrous matrix was Fe-12Cr-12Mn-1.0C-0.4Si, which is essentially a basic manganese steel containing 12% chromium in solid solution.

(17) Fracture toughness testing was carried out on two samples according to the procedure described in Double Torsion Technique as a Universal Fracture Toughness Method, Outwater, J. O. et al., Fracture Toughness and Slow-Stable Cracking, ASTM STP 559, American Society for Testing and Materials, 1974, pp 127-138.

(18) The applicant found that the presence of manganese in the alloy allowed the ferrous matrix to become surface work hardened by the action of compressive loading during service to provide a material with moderate wear resistance and excellent toughness, attributable to the presence of a metastable austenitic structure formed by water quenching of the casting from a temperature of about 1200 C. to room temperature. The wholly austenitic structure could be retained during cooling to room temperature due to the presence of both a high manganese content and a specific carbon content.

(19) Because of the synergistic combination of the presence of the manganese, a casting that was made out of a white cast iron alloy of the invention offers significantly improved fracture toughness compared to regular high chromium white cast iron, in combination with the advantages of white cast iron of (a) high abrasion and erosion wear resistance, (b) relatively high yield strength, and (c) moderate corrosion resistance in acidic environments.

(20) The white cast iron alloy of the above-mentioned example had an average fracture toughness of 56.3 MPam. This result compares favourably with toughness values of 25-30 MPa.Math.m. for high chromium white cast irons. It is anticipated that this fracture toughness makes the alloys suitable for use in high impact applications, such as pumps, including gravel pumps and slurry pumps. The alloys are also suitable for machinery for crushing rock, minerals or ore, such as primary crushers.

(21) One advantage of the white cast iron alloy of the present invention is that hot working of the as formed alloy breaks up the carbide into discrete carbides, thereby improving the ductility of the alloy.

(22) Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other country.

(23) Many modifications may be made to the preferred embodiment of the present invention as described above without departing from the spirit and scope of the present invention.

(24) It will be understood that the term comprises or its grammatical variants as used in this specification and claims is equivalent to the term includes and is not to be taken as excluding the presence of other features or elements.