The present invention relates to a ferrochromium alloy and more particularly to an erosion and corrosion resistant ferrochromium alloy.

The present invention is designed for use in the formation of parts for lining pumps, pipes, nozzles, mixers and similar devices which, in service, can be subjected to mixtures containing a corrosive fluid and abrasive particles.

Typical applications for such parts include flue gas desulphurization, in which the parts are exposed to sulphuric acid and limestone, and fertiliser production, in which the parts are exposed to phosphoric acid, nitric acid and gypsum.

U.S. patents, 4,536,232 and 4,080,198, assigned to Abex Corporation (the "Abex U.S. patents"), disclose ferrochromium alloys containing approximately 1.6 wt. % carbon and 28 wt. % chromium which are characterized by primary chromium carbide and ferrite islands in a martensite or austenite matrix containing a solid solution of chromium. The level of chromium in the alloys suggests that the alloys should exhibit good corrosion resistance characteristics. However, the performance of such alloys from the corrosion resistance viewpoint is not entirely satisfactory. Moreover, the Australian patent nos. AU-B-43163/72 and (2) AU-B-14453/70 as well as Britsh patent GB-A-401644 are concerned with erosion and corrosion resistant iron chromium alloys.

An object of the present invention is to provide a ferrochromium alloy which has improved erosion and corrosion resistance compared with the alloys disclosed in the Abex U.S. patents.

The mechanism for erosion and corrosion of alloys of the type disclosed in the Abex U.S. patents in acidic environments is by accelerated corrosion due to the continuous removal of the passive corrosion-resistant layer by erosive particles in the fluid stream.

In order to replenish the passive layer it is necessary to have the chromium concentration at as high a level as possible in the matrix.

However, simply increasing the chromium content to improve corrosion resistance tends to cause the formation of the sigma phase which is undesirable in view of the embrittlement problems associated with the sigma phase.

The present invention is based on the realization that by increasing both the chromium and carbon concentrations of alloys of the type disclosed in the Abex U.S. patents it is possible to increase the volume fraction of the chromium carbide phase, and thereby improve the wear resistance characteristics of the ferrochromium alloys, while maintaining the matrix at a chromium concentration which is at a level that will not lead to the formation of significant amounts of sigma phase. It can be appreciated that by improving the wear resistance of the ferrochromium alloys, in view of the mechanism by which erosion and corrosion occurs, as noted above, it is possible to realize an improvement in the erosion and corrosion resistance of the ferrochromium alloys.

According to the present invention there is provided an erosion and corrosion resistant ferrochromium alloy comprising the following composition, in wt. %.

34 - 50

chromium

1.5 - 2.5

carbon

1 to 2

manganese

0.5 to 1.5

silicon

1 to 2

molybdenum

1 to 5

nickel

1 to 2

copper

The term "ferrite" is herein understood to mean body-centred cubic iron (in the alpha and/or delta forms) containing a solid solution of chromium.

The term "austenite" is herein understood to mean face-centred cubic iron containing solid solutions of carbon and chromium.

The term "martensite" is herein understood to mean a transformation product of austenite.

It is preferred that the matrix contains a 25-35 wt. % solid solution of chromium.

It is preferred that the microstructure further comprises one of primary chromium carbides, primary ferrite or primary austenite in the matrix.

The preferred amount in wt % of the elements chromium is 36 to 40 and % carbon is 1.9 to 2.1

With the foregoing preferred composition it is preferred that the matrix contains a 29-32 wt. % solid solution of chromium.

In accordance with the invention, increasing both the chromium and carbon contents of the ferrochromium alloy above the levels disclosed in the Abex U.S. patents permits the formation of a greater volume fraction of hard carbides to enhance wear resistance. More specifically, and preferably, a stoichiometric balance in the increase in chromium and carbon contents permits the formation of a greater volume fraction of chromium carbides without increasing the chromium content of the matrix to a critical level above which sigma phase embrittlement occurs.

It has been found that preferred alloys of the present invention exhibit superior corrosion and erosion resistance to the alloys disclosed in the Abex U.S. patents. This is illustrated in Table 1 below which lists the results of laboratory scale potentiodynamic corrosion and disc wear tests on alloys disclosed in the Abex U.S. patents and preferred alloys of the present invention. The compositions of the alloys are listed in Table 2 below.

It will be noted from Table 1 that the corrosion and erosion resistance of the preferred alloys of the present invention is significantly better than that of the Abex alloys.

The alloy of the present invention has a different microstructure to that of the alloys disclosed in the Abex U.S. patents. The difference is illustrated in the accompanying figures which comprise photocopies of photomicrographs of an alloy disclosed in the Abex U.S. patents and preferred alloys of the present invention.

Figure 1 shows the microstructure of an Abex alloy which comprises 28.4% chromium, 1.94% carbon, 0.97% manganese, 1.48% silicon, 2.10% molybdenum, 2.01% nickel and 1.49% copper, the balance substantially iron. The microstructure consists of primary austenite dendrites (50% volume) and a eutectic structure comprising eutectic carbides in a matrix of eutectic ferrite, retained austenite and martensite.

Figure 2 shows the microstructure of one preferred alloy of the present invention which comprises 35.8% chromium, 1.94% carbon, 0.96% manganese, 1.48% silicon, 1.94% carbon, 0.96% manganese, 1.48% silicon, 2.06% molybdenum, 2.04% nickel, 1.48% copper, the balance substantially iron. The microstructure is hypereutectic with primary ferrite dendrites (20% volume) and a eutectic structure comprising finely dispersed eutectic carbides in a matrix of eutectic ferrite. It is noted that when compared with the microstructure of the Abex U.S. patent shown in Figure 1 the microstructure of Figure 2 reflects that there is a reduced volume of primary dendrites and an increased volume of the eutectic matrix and since the eutectic matrix has a relatively high proportion of carbides there is an overall increase in the volume fraction of hard carbides in the alloy when compared with the Abex alloy. It is noted that the foregoing phenomenon is also apparent to a greater extent from a comparison of the microstructures shown in Figs. 3 to 5 and Fig. 1.

Figure 3 shows the microstructure of another preferred alloy of the present invention which comprises 40.0% chromium, 1.92% carbon, 0.96% manganese, 1.59% silicon, 1.95% molybdenum, 1.95% nickel, 1.48% copper, the balance substantially iron. The microstructure consists of eutectic carbides in a matrix of eutectic ferrite.

Figure 4 shows the microstructure of another preferred alloy of the present invention which comprises 40.0% chromium, 2.30% carbon, 2.77% manganese, 1.51% silicon, 2.04% molybdenum, 1.88% nickel, 1.43% copper, the balance substantially iron. The microstructure is hypereutectic with primary M₇C₃ carbides and a eutectic structure comprising eutectic carbides in a matrix of eutectic ferrite.

Figure 5 shows the microstructure of another preferred alloy of the present invention which comprises 43% chromium, 2.02% carbon, 0.92 manganese, 1.44% silicon, 1.88% molybdenum, 1.92% nickel, 1.2% copper, the balance substantially iron. The microstructure in this case is hypereutectic with trace amounts of primary M₇C₃carbides and a eutectic structure comprising eutectic carbides in a matrix of eutectic ferrite.

Any suitable conventional casting and heat treatment technology may be used to produce the alloys of the present invention. However, it is preferred that the alloys are formed by casting and then heat treating at a temperature in the range of 600 to 1000°C followed by air cooling.