Tough and corrosion resistant white cast irons

Abstract

An end-use casting of a high chromium white iron, i.e. a casting that has been heat-treated, includes a ferrous matrix and at least two different chromium carbides dispersed in the matrix, with at least one of the chromium carbides including a transformation product of an as-cast chromium carbide.

Claims

1. A heat-treated casting of a high chromium white iron for improving toughness and corrosion resistance, includes a ferrous matrix and primary carbides including at least two different chromium carbides dispersed in the ferrous matrix, the primary carbides having a core including an as-cast chromium carbide and a shell surrounding the core including a transformation product of the as-cast chromium carbide wherein the as-cast chromium carbide is a M.sub.7C.sub.3 chromium carbide and the transformation product is a M.sub.23C.sub.6 chromium carbide, wherein the M.sub.23C.sub.6 carbide is formed during heat treatment of the casting, and wherein the composition of the casting comprises the following formula: Cr: 30-40 wt. % C: 1.5-3 wt. % Cr/C ratios (wt. %): 9:1-15:1 Up to 3 wt. % each of any one or more than one of Mn, Si, Ni, Mo, or Cu, Incidental impurities Balance: Fe.

2. The heat-treated casting defined in claim 1 wherein M comprises Cr, Fe, and Mn.

3. The heat-treated casting defined in claim 1 wherein the chromium carbides dispersed in the ferrous matrix are 40-50 vol. % of the casting.

4. The heat-treated casting defined in claim 1 wherein the M.sub.23C.sub.6 chromium carbides are 25-30 vol. % of the casting.

5. The heat-treated casting defined in claim 1 wherein the matrix is 40-70 vol. % of the casting.

6. Equipment used in the mining and mineral processing industries that includes the heat-treated casting defined in claim 1.

7. The heat-treated casting defined in claim 1, wherein the M.sub.7C.sub.3 chromium carbides are 40-50 vol. % of the casting.

8. The heat-treated casting defined in claim 1, wherein the M.sub.7C.sub.3 chromium carbides are 10-20 vol. % of the casting.

9. The heat-treated casting defined in claim 1, wherein the M.sub.7C.sub.3 chromium carbides are 15-20 vol. % of the casting.

10. A heat-treated casting of a high chromium white iron for improving toughness and corrosion resistance, includes a ferrous matrix and primary carbides including at least two different chromium carbides dispersed in the matrix, the primary carbides having a core including an as-cast chromium carbide and a shell surrounding the core including a transformation product of the as-cast chromium carbide wherein the as-cast chromium carbide is a M.sub.7C.sub.3 chromium carbide and the transformation product is a M.sub.23C.sub.6 chromium carbide and wherein the composition of the casting is consisting of the following formula: Cr: 30-40 wt. % C: 1.5-<3 wt. % Cr/C ratios (wt. %): 9:1-15:1 Up to 3 wt. % each of any one or more than one of Mn, Si, Ni, Mo, or Cu, Incidental impurities Balance: Fe; and wherein the transformation product is formed during heat treatment of the as-cast form of the casting when heated to between 800 and 1,000 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a Cr/C diagram that shows two embodiments of ranges (i.e. regions) of Cr and C concentrations in high chromium white cast irons in accordance with the invention;

(3) FIG. 2A is a representative SEM image of a sample end-use casting, i.e. as-cast and heat-treated casting, in accordance with an embodiment of the invention;

(4) FIG. 2B is a pie chart of the constituents of the microstructure of the end-use casting shown in FIG. 2A;

(5) FIG. 3 is a graph of relative corrosion resistance versus C concentration of compositions of samples of end-use castings, i.e. as-cast and heat-treated casting, in accordance with an embodiment of the invention and samples of embodiments of end-use castings of known HCWCIs exposed to solutions having different pHs;

(6) FIGS. 4A, 4B, and 4C are graphs of relative mass loss versus C concentration of compositions of samples of embodiments of end-use castings, i.e. as-cast and heat-treated casting, in accordance with the invention and samples of embodiments of end-use castings of known HCWCIs exposed to solutions having different pHs;

(7) FIG. 5 is graph of relative erosion resistance and relative impact resistance of a sample embodiment of an end-use casting, i.e. as-cast and heat-treated casting, in accordance with the invention and end-use castings of Cr27 and Cr35 HCWCIs; and

(8) FIG. 6 is a representative SEM image of a sample end-use casting, i.e. as-cast and heat-treated casting, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) As noted above, the applicant has identified a combination of composition and microstructure of castings of high chromium white irons that exhibit corrosion resistance and toughness that are very useful in a number of end-use applications of the castings.

(10) The combination identified by the applicant is high chromium white irons that have compositions that are characterised by (a) ranges of Cr and C concentrations and (b) Cr:C ratios within these ranges that can be cast and then heat-treated so that at least part of the chromium carbides in as-cast forms of the castings transform to another chromium carbide, whereby the end-use forms of the castings have mixtures of chromium carbides with at least one of the chromium carbides including a transformation product of an as-cast chromium carbide.

(11) The amount of the transformation product may be selected based on a range of factors, including the requirements for the end-use form of the casting and the composition of the casting.

(12) The microstructures of the end-use forms of the castings are quite different to the microstructures of other end-use forms of castings of HCWCIs such as Cr27 and Cr35.

(13) It is evident form the above that (a) selected Cr and C concentrations, (b) selected Cr:C ratios and (c) heat-treated microstructures of end-use castings of these Cr and C concentrations and Cr:C ratios are important to produce end-use castings of the invention.

(14) FIG. 1 is a Cr/C diagram that shows two embodiments of Cr and C concentration ranges in high chromium white cast irons in accordance with the invention. The two embodiments are identified as Regions I and II in the Figure.

(15) FIG. 1 also shows the Cr and C concentration regions of the known Cr27 and Cr35 high chromium white cast irons. The Cr27 regions are described as ASTM A532 IIIA and IIA, IIB and IID in the Figure.

(16) With reference to FIG. 1, the composition ranges of Region I are: Cr: 30-40 wt. % C: 1.5-3 wt. %

(17) As a result of direct experimental work and casting and metallurgy experience of the applicant and computer modelling work carried out by the applicant, the applicant has also identified that the Cr/C ratios (wt. %) in Region I should be in a range of 9:1-15:1, typically, 10:1-15:1. The applicant has also identified in this work that it is 30 preferable that the total carbides in an end-use form of the casting be 30-60 vol. %, and the composition have up to 3 wt. % each of any one or more than one of Mn, Si, Ni, Mo, and Cu, with incidental impurities, and the balance Fe.

(18) With further reference to FIG. 1, the composition ranges of Region II are: Cr: 10-23 wt. % C: 3.3-5.5 wt. %

(19) As a result of direct experimental work and casting and metallurgy experience of the applicant and computer modelling work carried out by the applicant, the applicant has also identified that the Cr/C ratios (wt. %) in Region II should be in a range of 2:1-4:1. The applicant has also identified in this work that it is preferable that the total carbides in an end-use form of the casting 30-70 vol. %, and the composition have up to 3 wt. % each of any one or more than one of Mn, Si, Ni, Mo, and Cu, incidental impurities, and balance Fe.

(20) FIG. 2A is a representative SEM image of a sample end-use casting, i.e. an as-cast and heat-treated casting, in Region I of FIG. 1.

(21) FIG. 2B is a pie chart of the constituents of the microstructure of the casting shown in FIG. 2A.

(22) The sample comprised 35 wt. % Cr.

(23) With reference to FIG. 2A, the microstructure of the end-use casting comprises a ferrous matrix (shown by way of example as points 3 and 6 in FIG. 2A) and chromium carbides dispersed in the matrix. The chromium carbides comprise M.sub.7C.sub.3 carbides (see points 1 and 4 in FIG. 2A) and M.sub.23C.sub.6 carbides (see points 2 and 5 in FIG. 2A), where M comprises Cr, Fe, and Mn. At least a part of the M.sub.23C.sub.6 carbides form as a transformation product of M.sub.7C.sub.3 carbides in the as-cast form of the casting. At least some of the chromium carbides have a hard core of M.sub.7C.sub.3 that is surrounded by a softer shell of M.sub.23C.sub.6. This feature is discussed further below in relation to FIG. 6.

(24) FIG. 2B shows that the matrix in the end-use casting in FIG. 2A is 56 vol. % and the chromium carbides in the end-use casting in FIG. 2A are 44 vol. % of the total volume of the end-use casting, with the matrix comprising 14 wt. % Cr and 0.25 wt. % C, and the chromium carbides comprising 16 vol. % M.sub.7C.sub.3 carbides and 28 vol. % M.sub.23C.sub.6 carbides of the total volume of the casting.

(25) The as-cast form of the casting comprised an austenite matrix with M.sub.7C.sub.3 carbides dispersed in the matrix. Heat treatment of the as-cast form of the casting to produce the sample shown in FIG. 2A transformed the austenite matrix to martensite and transformed part of the M.sub.7C.sub.3 carbides to M.sub.23C.sub.6 carbides.

(26) It can readily be appreciated that the relative proportions of the matrix and the chromium carbides in the casting, the Cr and C concentrations in the matrix, and the relative proportions of the M.sub.7C.sub.3 carbides and the M.sub.23C.sub.6 carbides in the chromium carbides may be varied as required having regard to the requirements of end-use applications of the castings. In this regard, as noted above, the important variables include the Cr and C concentrations within Region I, the selection of the Cr:C ratios within Region I to be within the range of 9:1-15:1, and the heat treatment conditions to achieve a required transformation of as-cast M.sub.7C.sub.3 carbides to M.sub.23C.sub.6 carbides.

(27) FIG. 6 is a representative SEM image of another sample end-use casting, i.e. as-cast and heat-treated casting, in Region I of FIG. 1.

(28) The purpose of FIG. 6 is to provide more detail on the chromium carbides that have a hard core of M.sub.7C.sub.3 that is surrounded by a softer shell of M.sub.23C.sub.6 that is described above in relation to FIG. 2A. With reference to FIG. 6, a representative chromium carbide particle generally identified by the numeral 11 comprises a core 13 of M.sub.7C.sub.3 and an outer shell 15 of M.sub.23C.sub.6 in a matrix 17. The M.sub.23C.sub.6 in the particle forms as a transformation product of as-cast M.sub.7C.sub.3. The M.sub.23C.sub.6 acts as a transition zone between the much softer metal matrix 17 and the extremely hard M.sub.7C.sub.3 carbide core 15, allowing dissipation of impact energy leading to a reduced propensity for the primary carbides to crack during large particle impingement and impact.

(29) FIG. 3 is a graph of relative corrosion resistance versus C concentration of compositions of samples of end-use castings, i.e. as-cast and heat-treated castings, in accordance with the invention and samples of end-use castings of known HCWCIs exposed to solutions having different pHs.

(30) FIG. 3 shows relative corrosion resistance results for (a) samples of end-use castings having a nominal C concentration of 3 wt. % in Region I of FIG. 1 in accordance with the invention and (b) samples of end-use castings of known HCWCIs have respective nominal C concentrations of 1 wt. %, 2 wt. %, 4 wt. %, 5 wt. % and 6 wt. %. The samples were exposed to solutions of pH3, pH5, and pH7.

(31) It is clear from FIG. 3 that, in relative terms, the corrosion resistance of the sample in accordance with the invention performed considerably better than the samples of known HCWCIs.

(32) FIGS. 4A, 4B, and 4C are graphs of relative mass loss versus C concentration of compositions of samples of end-use castings, i.e. as-cast and heat-treated casting, in accordance with the invention and samples of end-use castings of known HCWCIs exposed to solutions having different pHs.

(33) The experimental work reported in FIGS. 4A, 4B, and 4C was carried out in accordance with ASTM 1095. The experimental work assessed slurry pot erosion at impingement angles of 45 and 90. FIGS. 4A, 4B, and 4C show the results for (a) samples of end-use castings having a nominal C concentration of 3 wt. % in Region I of FIG. 1 in accordance with the invention and (b) samples of end-use castings of known HCWCIs have respective nominal C concentrations of 1 wt. %, 2 wt. %, 4 wt. %, 5 wt. % and 6 wt. %. The samples were exposed to solutions of pH3, pH5, and pH7.

(34) It is clear from FIGS. 4A, 4B, and 4C that, the erosion resistance of the sample in accordance with the invention performed considerably better than the samples of known HCWCIs.

(35) FIG. 5 is graph of relative erosion resistance and relative impact resistance of a sample end-use casting, i.e. as-cast and heat-treated casting, in accordance with the invention and end-use castings of Cr27 and Cr35 HCWCIs.

(36) The relative erosion resistance tests were carried out in accordance with a standard Coriolis Scouring Erosion Testing procedure of the National Research Council of Canada. The relative impact resistance tests were carried out in accordance with a procedure and on a test rig developed by the applicant. In accordance with the procedure, impact particles were allowed to free-fall and hit a sample casting at a velocity of 9 m/s.

(37) FIG. 5 shows that the erosion resistance of the sample end-use casting in accordance with the invention was better than that of the Cr27 sample.

(38) FIG. 5 also shows that the impact resistance of the sample end-use casting in accordance with the invention was better than that of the Cr35 sample.

(39) The applicant has found in a series of field trials that the combination of corrosion resistance, erosion resistance, and impact resistance, i.e. toughness, of the end-use casting in accordance with the invention is well-suited to a range of applications.

(40) One field trial was carried out to evaluate the performance of castings of 150MCU impellers of pumps on SAG duty at a gold mining operation. The current 150MCU impellers are made from Cr35 HCWCI and these were found to shatter at about 2500 hours in service. The impellers all wore thin and shattered. In a field trial with an impeller comprising an end-use casting in accordance with the invention, the impeller remained intact for 3000 hours and was changed on the basis of a scheduled maintenance at this service life.

(41) 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.

(42) 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.