Erosion and corrosion resistant white cast irons

Abstract

A casting of a hypereutectic white iron that, in an as-cast form of the casting, has a microstructure that includes a ferrous matrix that contains 12-20 wt. % chromium in solution in the matrix, eutectic chromium carbides dispersed in the matrix, primary chromium carbides dispersed in the matrix, and optionally secondary carbides dispersed in the matrix. The eutectic carbides are 15-25 vol. % of the casting and the primary carbides are 25-35 vol. % of the casting. When present, the secondary carbides are up to 6 vol. % of the casting.

Claims

1. A casting of a hypereutectic white iron that, in an as-cast form of the casting, has a microstructure that includes a ferrous matrix that contains 12-20 wt. % chromium in solution in the matrix, eutectic carbides dispersed in the matrix, primary carbides dispersed in the matrix, and optionally secondary carbides dispersed in the matrix, where the eutectic carbides are 15-25 vol. % of the casting, the primary carbides are 25-35 vol. % of the casting, and when present, the secondary carbides are up to 6 vol. % of the casting, wherein the ferrous matrix also comprises C: 0.2-1.5 wt. % and Mn: 1.0-5.0 wt. %; wherein the eutectic carbides, the primary carbides, and the secondary carbides consist of M.sub.7C.sub.3 carbides, where M comprises Cr, Fe, and Mn; wherein the weight ratio of chromium and carbon in the casting is greater than 8:1 and less than 9.25:1; and wherein the bulk chemistry of the casting comprises: >37-40 wt. % Cr, 4-5 wt. % C, 1-3 wt. % Mn, <1.5 wt. % Si, and balance Fe and impurities.

2. The casting defined in claim 1 wherein the eutectic (Cr,Fe,Mn).sub.7C.sub.3 carbides and the primary (Cr,Fe,Mn).sub.7C.sub.3 carbides each comprise: Cr: 50-70 wt. %, C: 8.5-8.9 wt. %, and Mn: 0.5-5.0 wt. %.

3. The casting defined in claim 1 wherein the ferrous matrix comprises: Cr: 14-16 wt. %, C: 0.3-1.2 wt. %, and Mn: 1.0-5.0 wt. %.

4. The casting defined in claim 1 wherein the ferrous matrix comprises 13-17 wt. % Cr in solution in the matrix.

5. The casting defined in claim 1 comprising 25-30 vol. % primary carbides, 15-20 vol. % eutectic carbides, and up to 6 vol. % secondary carbides.

6. The casting defined in claim 1 wherein the combined amount of eutectic carbides and primary carbides in the as-cast casting is greater than 35 vol. %.

7. The casting defined in claim 1 wherein the combined amount of eutectic carbides and primary carbides in the as-cast casting is less than 55 vol. %.

8. The casting defined in claim 1 wherein the ferrous matrix is martensite.

9. The casting defined in claim 1 wherein the bulk chemistry of the casting comprises less than 4.9 wt. % C.

10. The casting defined in claim 1 wherein the bulk chemistry of the casting comprises less than 4.7 wt. % C.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) Embodiments of the invention are now described, by way of example only, with reference to the following Figures, of which:

(2) FIG. 1 which is a pie chart that illustrates the phases of one alloy casting in accordance with the invention produced and analysed during the above-mentioned experimental program carried out by the applicant;

(3) FIG. 2 is a representative SEM image of a sample as-cast and heat treated casting in accordance with the invention; and

(4) FIG. 3 is a representative SEM image of a test cast in the same heat as a field trial of an as-cast casting in accordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(5) As noted above, the experimental project carried out by the applicant found that HCWCI slurry pump wet-end components made from an experimental alloy having (a) a chromium carbide content of the order of 45 vol. % and (b) a ferrous matrix containing a chromium content of the order of 15 wt. % in solution in the matrix in the as-cast form of the casting, performed well in severe abrasive, impact, erosive and corrosive applications.

(6) On the basis of the experimental project, the applicant has realised that as-cast castings having a combination of the following features provide suitable performance as HCWCI slurry pump wet-end components exposed to environments where there is severe abrasive, impact and erosive wear and that is highly corrosive: (a) a high, at least 12 wt. %, chromium in solution in the matrix; (b) a combination of eutectic carbides and primary chromium carbides dispersed in the matrix; and (c) a high, typically at least 45 vol. %, combined amount of eutectic carbides and primary chromium carbides. In addition, the applicant has realised that as-cast castings having the following microstructure have an optimized combination of improved toughness, good corrosion resistance, and excellent wear resistance for a range of applications, including wet-end components in mill circuit slurry pumps, pipelines, mill liners, crushers, transfer chutes and ground-engaging tools: (a) a ferrous matrix that contains 12-20 wt. % chromium in solution, (b) 15-25 vol. % of the casting comprising eutectic chromium carbides dispersed in the matrix, (c) 25-35 vol. % of the casting comprising primary chromium carbides dispersed in the matrix, and (d) optionally, up to 6 vol. % of the casting comprising secondary carbides dispersed in the matrix.

(7) The microstructure of the experimental alloy in the as-cast form, i.e. prior to any downstream after-casting treatment, is illustrated diagrammatically in the pie chart of FIG. 1.

(8) With reference to FIG. 1, the microstructure comprises: A ferrous matrix consisting of martensite and some retained austenite and 15 wt. % chromium in solution in the matrix, with the ferrous matrix making up 55 vol. % of the casting. Fine, continuous 3-D network of eutectic chromium carbides similar to the chromium carbides in Cr27 (20 vol. %) casting making up 20 vol. % of the casting. The presence of the continuous, 3-D network of eutectic chromium carbides in the microstructure of Cr27 casting substantially reduces the fracture toughness. The eutectic carbides are M.sub.7C.sub.3 carbides (where M comprises Cr, Fe, and Mn). Coarse, discrete primary chromium carbides making up 25 vol. % of the casting that formed during solidification and also adversely influence the fracture toughness of the casting by decreasing the amount of the tougher ferrous matrix in the microstructure. The primary carbides are M.sub.7C.sub.3 carbides (where M comprises Cr, Fe, and Mn). Optionally, fine, discrete secondary carbides making up to 6 vol. % of the casting that formed after solidification and also adversely influence the fracture toughness of the casting by (a) decreasing the amount of the tougher ferrous matrix in the microstructure and (b) destabilizing the austenite phase which decomposes to martensite. The secondary carbides are M.sub.7C.sub.3 carbides (where M comprises Cr, Fe, and Mn).

(9) FIG. 2 is a representative SEM image of a sample as-cast and heat treated casting in accordance with the invention. The image has been marked-up to show the distribution of primary and eutectic carbides in the ferrous matrix.

(10) In nominal FeCrC alloys, the microstructural and micro-analytical features of stoichiometry of the (Cr,Fe,Mn).sub.7C.sub.3 carbides, the vol. % of primary carbides, the vol. % of eutectic carbides, the carbide distribution, and the amounts of elemental chromium, iron, and carbon in (a) the carbides and (b) the ferrous matrix of castings of the alloys are greatly dependent on the partitioning behaviour of each individual element in the alloy during the solidification and cooling processes to form the castings.

(11) The factors determining the partitioning coefficients for each element are complex and not accurately known and must be established by trial and error.

(12) In the experimental project the applicant produced a number of FeCrC2 Mn0.5 Si alloys in the laboratory and the resultant microstructures and microanalyses of the various phases were determined by a detailed examination using Scanning Electron Microscopy, Energy-Dispersive Spectroscopy, Wavelength-Dispersive Spectroscopy and X-Ray Diffraction.

(13) From this experimental data, the applicant was able to establish alloys with microstructural features similar to (or close to) the selected requirements for the three phases in the casting as shown in FIG. 1, with particular focus on working towards the requirements for 15 wt. % chromium in solution in the matrix, the ferrous matrix making up 55 vol. % of the casting, and the eutectic and primary carbides each making up 20 and 25 vol. %, respectively, of the casting.

(14) The nominal bulk chemistry of the casting having the microstructural features described in the preceding paragraph was determined by summing the microanalyses and proportions of each phase. A typical nominal bulk chemistry for an example casting with selected microstructural features in accordance with the invention is shown in Table 2 below.

(15) TABLE-US-00002 TABLE 2 Nominal chemistry of an example casting with selected microstructural features. Composition (wt. %) Phases Vol. % Fe Cr C Mn Si sum Primary 28.00 29.20 60.00 8.80 2.00 0.00 100 Carbide Eutectic 20.00 33.20 55.00 8.80 3.00 0.00 100 Carbide Ferrous 52.00 80.40 15.00 0.600 3.00 1.00 100 Matrix Total 100 56.62 35.60 4.54 2.72 0.52 100

(16) The applicant followed the following steps in the selection process:

(17) Selecting 25 vol. % primary carbides for the casting fixed the eutectic carbides at approximately 20 vol. % and the ferrous matrix at approximately 55 vol. % of the casting.

(18) Selecting the chemistry of the ferrous matrix in the casting to be Fe-15 Cr0.8 C2 Mn0.7 Si fixed the bulk carbon content of the alloy.

(19) The bulk carbon content of the alloy established the solidification parameters (liquidus and solidus temperatures for the alloy. The liquidus temperature, in turn, determined the final amount of primary carbides in the microstructure.

(20) Using the data in Table 2 as a starting point to produce a trial casting, the microstructural features of the trial casting were quantified and compared to the desired features in Table 2.

(21) By a process of iteration, fine adjustments to the bulk chemistry of successive castings were made to establish the final bulk chemistry exhibiting the desired microstructural features of FIG. 1, and as exemplified in FIG. 2.

(22) With regard to the last dot point, determining the required bulk chemistry to produce samples with a ferrous matrix containing a chromium content of the order of 15 wt. % in solution in the matrix at ambient temperature required an assessment to be made of the chromium content prior to cooling to ambient temperature. Noting that direct measurement at temperature is not possible, the measurements were made by solution treating the samples at 1200 C. followed by water quenching to ambient temperature. This treatment retained the chromium in solution, and the maximum elemental chromium content achievable in the ferrous matrix in the as-cast condition could then be determined.

(23) In addition to the above mentioned experimental project, the applicant has produced a number of castings in accordance with the invention and tested these casting in field trials, some of which have been completed and assessed.

(24) The castings were produced in accordance with standard procedure of the applicant for high chromium white cast irons. The procedure is an inoculation process described in a patent family that includes U.S. Pat. No. 5,803,152. The disclosure in the US patent is incorporated herein by cross-reference. The castings were produced from 1-3 tonne heats of selected bulk chemistries. Pouring temperatures were in the range of 1350 to 1450 C. The castings were allowed to cool naturally in their moulds. The castings were heat treated depending on the specific field trial application.

(25) One of the series of field trials was carried out on impeller and throatbush components of a 150 MCU pump of the applicant in a mill circuit of a mining company operation. The trial ran for 1766 hours and wear rate was assessed and compared to wear rates for a high chromium white cast iron currently used in the same type of pump in the same mill circuit.

(26) Another of the series of trials was carried out on impeller, throatbush, frame plate liners, and volute components of a 350 MCU pump of the applicant in a mill circuit of another mining company operation. The trial ran for 4100 hours and wear rate was assessed and compared to wear rates for a high chromium white cast iron currently used in the same type of pump in the same mill circuit.

(27) The wet chemical analysis of the bulk chemistry used to form the castings in one of the field trials is set out below in Table 3.

(28) TABLE-US-00003 TABLE 3 Wet Chemical analysis Element Cr C Mn Ni Si Fe Wt. % 37.5 4.4 2.0 1.7 0.43 Bal.

(29) The analysis was carried out on inoculated samples and, therefore, there would have been approximately 1 wt. % less chromium and approximately 0.1 wt. % less carbon in the original casting.

(30) FIG. 3 is a representative SEM image of a test cast in the same heat as the field trial produced from the bulk chemistry in Table 3. The image shows the distribution of primary and eutectic carbides in the ferrous matrix. By estimate, the test cast (and therefore the field trial castings) contained 18 vol. % of eutectic chromium carbides, 28 vol. % of primary carbides, 2-3 vol. % of secondary carbides, and 12-16 wt. % Cr in solution in the matrix.

(31) It was found that the wear rate in the field trial was 0.3-0.4 mm/day. This is a 40% improvement over the high chromium white cast iron currently used in the same type of pump in the same mill circuit.

(32) From a practical perspective, when casting actual products in moulds in a foundry, it will be necessary to take into account the impact of cooling conditions on the microstructure of castings and the extent to which chromium and other elements will precipitate from solution. In the context of chromium concentration, different amounts of chromium will precipitate out of solution as a casting in a mould cools to ambient temperature depending on the thermal profile of the mould and the size of the casting. It will be necessary to take this into account when determining a bulk chemistry required to result in a ferrous matrix containing a target chromium content of the order of 15 wt. % (or another target concentration) in solution in the matrix at ambient temperature.

(33) In addition, it is noted that in standard foundry practice, castings of an alloy may be subjected to a further heat treatment procedure, for example, heating to 950-1050 C., holding at temperature for 4-6 hours, and air-cooling to ambient temperature. This heat treatment procedure hardens the ferrous matrix by 100-200 Brinell points due to:

(34) (a) secondary hardening by the precipitation of secondary chromium carbides in the ferrous matrix, destabilisation of the retained austenite in the ferrous matrix; and

(35) (b) subsequent transformation of any Cr-depleted austenite to martensite in the ferrous matrix on cooling to room temperature.

(36) It is estimated that the formation of secondary chromium carbide precipitates during such heat treatment at 950-1050 C. will reduce the elemental chromium content of the ferrous matrix in solution by up to 3 wt. %.

(37) Many modifications may be made to the embodiments of the invention described in relation to the Figures without departing from the spirit and scope of the invention.

(38) In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word comprise or variations such as comprises or comprising is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.