[0001] The present invention relates to a ferrochromium alloy and more particularly to an
erosion and corrosion resistant ferrochromium alloy.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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
up to 1% of each of one or more micro-alloying elements selected from the group
consisting of titanium, zirconium, niobium, boron, vanadium and tungsten, and
balance, iron and incidental impurities, with a microstructure comprising eutectic
chromium carbides in a matrix comprising one or more of ferrite, retained austenite
and martensite.
[0011] 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.
[0012] The term "austenite" is herein understood to mean face-centred cubic iron containing
solid solutions of carbon and chromium.
[0013] The term "martensite" is herein understood to mean a transformation product of austenite.
[0014] It is preferred that the matrix contains a 25-35 wt. % solid solution of chromium.
[0015] It is preferred that the microstructure further comprises one of primary chromium
carbides, primary ferrite or primary austenite in the matrix.
[0016] The preferred amount in wt % of the elements chromium is 36 to 40 and % carbon is
1.9 to 2.1
[0017] With the foregoing preferred composition it is preferred that the matrix contains
a 29-32 wt. % solid solution of chromium.
[0018] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
1. An erosion and corrosion resistant ferrochromium alloy comprising the following composition,
in wt. %.
34 - 50 chromium
1.5 - 2.5 carbon
1 - 2 manganese
0.5 - 1.5 silicon
1 - 2 molybdenum
1 - 5 nickel
1 - 2 copper
up to 1% of each of one or more micro-alloying elements selected from the group consisting
of titanium, zirconium, niobium, boron, vanadium and tungsten, and balance, iron and
incidental impurities, with a microstructure comprising eutectic chromium carbides
in a matrix comprising one or more of ferrite, retained austenite and martensite.
2. An alloy as claimed in Claim 1, characterised in that the microstructure further comprises
one of primary chromium carbides, primary ferrite or primary austenite in the matrix.
3. An alloy as claimed in Claim 1 or Claim 2, characterised in that the matrix contains
a 25-35 wt. % solid solution of chromium.
4. An alloy as claimed in any preceding Claim, characterised by an chromium content of
36 - 40 wt % and a carbon content of 1.9 - 2.1 wt %.
5. A method of producing an alloy as claimed in any preceding Claim, characterised by
heat treating the alloy at a temperature in the range of 600 - 1000°C and air cooling
the alloy.
1. Erosions- und korrosionsbeständige Ferrochromlegierung, umfassend die folgende Zusammensetzung
in Gew. %:
34-50 Chrom
1,5-2,5 Kohlenstoff
1-2 Mangan
0,5-1,5 Silizium
1-2 Molybdän
1-5 Nickel
1-2 Kupfer
und bis zu jeweils 1 % von einem oder mehreren der mikrolegierenden Elemente, ausgewählt
aus der aus Titan , Zirkon, Niob, Bor, Vanadin und Wolfram bestehenden Gruppe, Rest
Eisen und beiläufige Verunreinigungen, mit einer Mikrostruktur, welche eutektische
Chromcarbide in einer Matrix aus einem oder mehreren Bestandteilen aus der Reihe Ferrite,
verbliebenem Austenit und Martensit enthält.
2. Legierung gemäß Anspruch 1, dadurch gekennzeichnet, daß die Mikrostruktur ferner einen
Bestandteil aus der Reihe primärer Chromcarbide, primärer Ferrit oder primärer Austenit
in der Matrix enthält.
3. Legierung gemäß Anspruch 1 oder 2, dadurch gekennzeichnet, daß die Matrix eine 25-35
gew.%ige feste Lösung von Chrom enthält.
4. Legierung nach einem der voranstehenden Ansprüche, gekennzeichnet durch einen Chromgehalt
von 36-40 Gew. % und einen Kohlenstoffgehalt von 1,9-2,1 Gew. %.
5. Verfahren zur Herstellung einer Legierung gemäß einem der voranstehenden Ansprüche,
gekennzeichnet durch eine Wärmebehandlung der Legierung bei einer Temperatur im Bereich
von 600-1000° C und die Luftkühlung der Legierung.
1. Alliage de ferro-chrome résistant à l'érosion et à la corrosion, comprenant la composition
suivante :
34 à 50% en poids de chrome
1,5 à 2,5% en poids de carbone
1 à 2% en poids de manganèse
0,5 à 1,5% en poids de silicium
1 à 2% en poids de molybdène
1 à 5% en poids de nickel
1 à 2% en poids de cuivre
jusqu'à 1% en poids de chacun de l'un ou de plusieurs des éléments pour microalliage
choisis dans le groupe consistant en titane, zirconium, niobium, bore, vanadium et
tungstène, et
le restant étant constitué par du fer et des impuretés accidentelles, avec une microstructure
comprenant des carbures de chrome eutectiques dans une matrice comprenant un ou plusieurs
constituants parmi une ferrite, une austénite conservée et une martensite.
2. Alliage suivant la revendication 1, caractérisé en ce que la microstructure comprend
de plus l'un parmi des carbures de chrome primaires, une ferrite primaire ou une austénite
primaire dans la matrice.
3. Alliage suivant les revendications 1 ou 2, caractérisé en ce que la matrice contient
une solution solide de 25 à 35% en poids de chrome.
4. Alliage suivant l'une quelconque des revendications précédentes, caractérisé par une
teneur en chrome de 36 à 40% en poids et une teneur en carbone de 1,9 à 2,1% en poids.
5. Procédé pour la production suivant l'une quelconque des revendications précédentes,
caractérisé par un traitement à chaud de l'alliage à une température comprise dans
une gamme de 600°C à 1000°C et un refroidissement à l'air de l'alliage.