FR2529910A1 - Corrosion-resistant cast iron - Google Patents

Abstract

The corrosion resistant cast iron is of the type containing elements such as carbon, silicon, manganese, nickel, chromium, copper, aluminium, magnesium, calcium, metals of the rare-earth series, and iron, and is characterised in that it additionally comprises barium, tantalum and niobium, the constituents being present in the following proportions in % by weight: carbon, 2.5 to 3.2, silicon, 0.95 to 2.4, manganese, 0.8 to 4.0, nickel, 12.0 to 18.0, chromium, 0.5 to 2.0, copper, 4.0 to 8.0, aluminium, 0.01 to 0.3, magnesium, 0.005 to 0.07, calcium, 0.01 to 0.10, rare-earth metals, 0.001 to 0.08, barium, 0.001 to 0.1, tantalum 0.003 to 0.02, niobium, 0.005 to 0.3, iron, the remainder. The invention applies to metallurgy and to foundry for the production of cast iron articles which are employed in mechanical constructions for the chemical and oil industries.

Description

 The present invention relates to corrosion resistant cast irons and can find application in metallurgy and foundry for the production of cast iron parts which are used in mechanical constructions for the chemical and petroleum industries.

 It is known that the segregation of the main alloying elements which takes place during the crystallization of the cast iron leads to an irregular distribution of the alloying components from the macro and microstructure point of view. The segregation is done. even feel at the level of the austenitic grain.

 When such a cast iron on cooling reaches a determined temperature limit, there occurs, depending on the distribution of the alloying elements from the macro and microstructure point of view, a partial decomposition of the metallic austenitic matrix with formation of martensite and bainite. The decomposition of the metallic austenitic matrix results in an increase in the volume of the cast iron and leads to an increase in the geometric dimensions of the casting. This undesirable phenomenon (the formation of the decomposition products of austenite, which are preferably arranged at the grain boundaries and in the areas adjoining the graphite inclusions) significantly affects the corrosion resistance.

 Therefore, corrosion-resistant cast irons must also have a high resistance to growth. The term “resistance to growth” is understood to mean the invariability of the austenitic metallic matrix of the cast iron in the case of a single or multiple cooling in a negative temperature range, as well as the absence of phase transformations which give rise to an irreversible variation in the dimensions of the casting and adversely affect the corrosion resistance of the cast iron.

Corrosion-resistant cast iron is known
(USSR author's certificate No. 451784), the composition of which is as follows (S by weight) carbon: 2.6 to 3.6 manganese: 0.3 to 1.5 copper: 0.5 to 9.0 magnesium: 0.02 to 0.12 yttrium: 0.01 to 0.10 tin: 0.01 to 0.10 silicon: 2.0 to 3.4 nickel: 14.0 to 17.0 chromium: O, G1 to 1 .8 calcium: 0.01 to 0.15 rare earth series metals: 0.01 to 0.10 aluminum: 0.005 to 0.3 iron: the rest
Endowed with high physical and mechanical properties, this cast iron has resistance to corrosion in certain aggressive media (ammonia solution, sodium hydroxide, trisodium phosphate, perhydrol, slaked lime, as well as methanol, benzene , carbon tetrachloride).

 However, known cast iron has a low corrosion resistance in petroleum saturated with hydrone sulfide as well as in water having a high content of iodine and bromine ions (iodine and bromine water). .

 Known cast iron resists growth only up to a temperature of -450C, which significantly reduces its field of application in mechanical constructions for the chemical and petroleum industries.

 The invention relates to a corrosion resistant cast iron whose composition, modified qualitatively and quantitatively, would stabilize the austenite and, therefore, would improve its resistance to corrosion and to growth, the field of application corrosion-resistant cast irons are then enlarged and the properties of the castings obtained are improved.

The problem thus posed is solved using a corrosion-resistant cast iron, of the type comprising elements such as carbon, silicon, manganese, nickel, chromium, copper, aluminum, magnesium, calcium, metals of the series. rare earths and iron, characterized, according to the invention, in that it further comprises barium, tantalum and niobium, the constituents being present in the following proportions (E by weight) carbon: 2.5 -2.5 to 3.2 silicon: 0.95 to 2.4 manganese: 0, -B to 4.0 nickel: 12.0 to 18.0 chromium: 0.5 to 2.0 copper: 4.0 to 8.0 aluminum: Q, O1 to 0.3 magnesium: 0.005 to 0.07 calcium: 0.01 to 0.10 rare earth serine metals: 0.001 to 0.08 barium: 0.001 to 0.1 tantalum : 0.003 to 0.02 niobium: 0.005 to 0.3 iron: the rest
The qualitative and quantitative composition of the cast iron according to the invention ensures high resistance to petroleum saturated with hydrogen sulphide as well as in iodine and bromine water. This cast iron resists growth up to a temperature of -600C '. All this makes it possible to widely use corrosion-resistant cast iron, according to the invention, in mechanical constructions for the chemical and petroleum industries and to improve the properties of use of the parts obtained.

 The addition of tantalum to the cast iron according to the invention in the indicated ratio makes it possible to increase the degree of dispersion of the carbide inclusions and, therefore, makes it possible to reduce the microsegregation of the alloying elements which is harmful to the corrosion resistance of cast iron. The tantalum limit contents are determined taking into account the following factors: the upper limit being exceeded, the tantalum favors the crystallization of the cast iron according to a metastable system but beyond the lower limit, tantalum exerts no influence.

 The addition of niobium to the cast iron in accordance with the invention, in the indicated ratio, contributes to the reduction of the segregation of nickel and copper in the area adjacent to the carbide inclusions, promotes the improvement of the plastic properties of the cast iron and removing nitride inclusions from the grain boundaries.

The niobium content in the cast iron is a function of the rate of cooling and the degree of degassing of the cast iron due to the increase in its affinity for nitrogen. In the presence of high cooling rates and a niobium content beyond the lower limit, it is not possible to weaken the segregation of nickel and copper. The upper limit of the niobium content in the cast iron is determined by the degree of degassing of the modified cast iron and by its influence on the mechanical properties of the cast iron in the presence of low rates of cooling of the castings.
Barium is an effective modifier and, at the same time, an intense graphitizing element. Cast iron modified by barium taken in the indicated ratio is significantly less susceptible to quenching and subcooling.

It is advantageous that the corrosion-resistant cast iron further comprises cobalt, the constituents being present in the following proportions (% by weight): carbon: 2.5 to 3.2 silicon: 0.95 to 1.9 manganese: 0.8 to 4.0 nickel: 12.0 to 18.0 chromium: 0.5 to 2.0 copper: 4.0 to 8.0 cobalt: 0.05 to 0.3 aluminum: -0, 01 to 0.3 magnesium: 0.01 to 0.07 calcium: 0.01 to 0.10 rare earth series metals: 0.01 to 0.08 barium: 0.001 to 0.10 tantalum: 0.003 to 0 , 02 niobium: 0.005 to 0.2 iron: the rest
Cobalt, present in the cast iron in the proportion indicated weakens the segregation of manganese in the areas adjoining the grain boundaries while contributing to its more even distribution in the austenitic matrix of the cast iron and thereby reducing the risk of decomposition of austenite in localized areas of castings in the presence of low cooling rates. The resistance to growth of said corrosion-resistant cast iron goes down to -800C.

It is advantageous that the corrosion-resistant cast iron further comprises titanium, the components being present in the following proportions,% by weight: carbon: 2.5 to 3.2 silicon: 0.95 to 2.4 manganese : 0.8 to 4.0 nickel: 12.0 to 18.0 chromium: 0.5 to 2.0 copper: 4.0 to 8.0 aluminum: 0.01 to 0.3 magnesium: 0.005 to 0, 05 calcium: 0.01 to 0.05 rare earth series metals: 0.001 to 0.02 barium: 0.001 to 0.1 tantalum: 0.003 to 0.02 niobium: 0.01 to 0.3 cobalt: 0, 05 to 0.3 titanium: 0.05 to 0.5 iron: the rest
This cast iron has a high corrosion resistance in iodine and bromine water, in petroleum saturated with hydrogen sulfide, it has a resistance to growth down to -800C and is characterized by the properties of improved foundry.

 The corrosion-resistant cast iron, which is the subject of the invention, was obtained by making the alloy and the modification of the initial cast iron using different additives in the ladle.

 The initial cast iron is obtained by melting in an electric furnace. In particular, the cast iron considered was produced in an induction furnace. To this end, a charge made of conventional materials (nickel, cobalt, cementing agent) was charged and the charge was melted. A temperature of 15300C to 15800C having been reached, the molten material is poured into the ladle in which is already found an additive containing magnesium, metals of the rare earth series, calcium, barium and other elements. The quantity of additive is adjusted according to the quality of the initial materials, the thickness of the section of the castings to be obtained and according to the requirements imposed on these pieces.The casting of the cast iron was carried out at a temperature of 13500C at 1450 C.

 The chemical composition, mechanical properties, corrosion resistance and foundry properties were then determined.

 The samples used to determine the mechanical properties were cut from the wedge-shaped test pieces with a length of 370 mm, a height of 140 mm, a width of 50 mm (top) and 30 mm (bottom).

The determination of the mechanical properties was carried out using techniques known per se.

 Samples, used for corrosion resistance tests in different aggressive media as well as for determining the microstructure of the cast iron, were cut from 10 mm thick cast plates, with a width of 50 mm and a length of 250 mm.

 The samples, used to determine the corrosion resistance in petroleum saturated with hydrogen sulfide, were immersed in petroleum after degreasing, drying and weighing. Hydrogen sulfide was continuously passed through the petroleum. The oil temperature was 100 ° C, the duration of the tests was 100 hours. When the tests were completed, the corrosion products were carefully removed from the samples and washing, drying and weighing were carried out. The rate of mass loss of the material was determined from the loss of mass of the samples per unit time compared to one unit of area.

 A similar technique was used for tests on the corrosion resistance of cast irons in other aggressive media, but at their ambient temperature. The duration of the tests in iodine and bromine water was 500 hours, including 120 hours at a temperature of the medium equal to 800C + 50C.

 The foundry properties are characterized by the fluidity and by the volume of shrinkage and pores.

 To determine the fluidity of the cast iron, quartz tubes of 3 + 0.1 mm in diameter were used which were filled with metal (the vacuum in the tubes is 210 + 5 mm Hg). The fluidity of the cast iron was determined by measurements of the length of the filled part of the quartz tube at different temperatures of casting of the cast iron.

 The volume of shrinkage and pores was determined using standardized techniques.

 For a better understanding of the invention, some examples of implementation are given below.

Example 1.

 The corrosion-resistant cast iron which is the subject of the invention was obtained in the following manner.

In an induction furnace, an initial cast iron was produced containing (% by weight) carbon: 2.90 silicon: 1.02 manganese: 1.40 nickel: 15.1 copper: 7.5 chromium: 1.30 sulfur: 0.039 phosnhore: 0.03 aluminum: 0.10 niobium: 0.11 tantalum: 0.01 iron: the rest
The treatment of the iron in the ladle was carried out at 15000C with an additive taken in an amount of 2.0% of the mass of the molten material.

The composition of the additive is as follows (by weight) calcium: 8.6 magnesium: 5.8 aluminum: 1.7 metals of the rare earth series: 5.3 silicon: 46.0 barium: 3.1
Casting was carried out at 13500C.

The composition of the corrosion-resistant cast iron, which was then obtained, is as follows (% by weight) carbon: 2.82 silicon: 1.59 manganese: 1.31 nickel: 15.06 copper: 7.5 chromium : 1.31 calcium: 0.06 sulfur: 0.016 phosphorus: 0.03 aluminum: 0.12 niobium: 0.11 tantalum: 0.01 barium: 0.04 magnesium: 0.04 rare earth series metals: 0 .05 iron: the rest
Corrosion-resistant cast iron has been tested to determine the chemical composition, mechanical properties and corrosion resistance.

 In order to make a comparison, a cast iron conforming to the USSR author's certificate no. 451784 was also tested.

The composition of the known cast iron is as follows (S by weight) carbon: 2.62 silicon: 3.37 nickel: 16.81 manganese: 0.3 chromium: 1.74 copper: 9.0 aluminum: 0.26 tin : 0.10 calcium: 0.12 magnesium: 0.06 rare earth series metals: 0.03 yttrium: 0.04 iron: the rest
The results of the microscopic analysis of the corrosion-resistant cast iron, which is the subject of the present invention, and of the corrosion-resistant cast iron conforming to the USSR author's certificate No. 451784, appear in the table 1.

TABLE 1
Characteristics Resist cast iron Resist cast iron
both to the so much to the
corrosion corrosion
conforms to conforms to
example 1 certificate
copyright USSR
No. 451784
Characteristics of graphite
spherical spherical shape
quantity,% 5 3
Characteristic of the metallic matrix
austenite,% 88 77
carbides, X 7 20
As can be seen from Table 1, the crystallization of cast iron, the chemical composition of which conforms to the URSS author's certificate No. 451784, gives rise to the formation of a large quantity of carbide inclusions.

 Table 2 combines the results of the tests aimed at determining the corrosion resistance of the cast iron in accordance with the invention and of the cast iron which is the subject of the USSR author's certificate No. 451784 in several aggressive liquids.

 Iodine and bromine water! appearing in the table and thereafter, contains 0.04 g / l of I and 0.31 g / l of Br, the total salt content is 176.5 g / l.

 The concentration of hydrogen sulfide a in petroleum, indicated in the table and thereafter, is 0.3 g / l.

TABLE 2
Medium Concen- Speed of loss of
tration, mass g / m2.h
% vo
lume Resis cast iron Resis cast iron
both to the so much to the
corrosion corrosion
conform to conform. at
example I certificate
copyright
USSR n0451784
Sulfuric acid 75 0.069 0.081
Sodium hydroxide 40 0.0028 0.004
Hydrated lime 20 Q, 0049 0.005
Ammonia 10 0.010 0.012
Trisodium phosphate 3 0.0101 0 + 020
Perhydrol - 0.0143 0.023
Methanol 0.0108 0.015
Benzene - 0.0098 0.011
Carbon tetrachloride - 0 * 0095 o, oi5
Iodine and bromine water 0.0650 0.116
Oil saturated with hydrogen sulfide 0.0059 0.0645
As can be seen from Table 2, the corrosion-resistant cast iron which is the subject of the invention has a higher corrosion resistance in corrosive environments and especially in petroleum saturated with hydrogen sulfide and in iodine and bromine water.

 Table 3 shows the results of the tests aimed at determining the physical and mechanical properties of the corrosion-resistant cast iron according to the invention and of the corrosion-resistant cast iron which is the subject of the author's certificate. 'USSR No. 451784.

TABLE 3
Characteristics Resist cast iron Resist cast iron
both to the so much to the
corrosion corrosion
conforms to conforms to
example 1 certificate
from the USSR No. 451784
Tear resistant,
MPa 425.3 402
Relative elongation 16 13
Resilience, J / cm2 50 27.5
Hardness, HB 133 167
As can be seen from table 3, the corrosion-resistant cast iron, which is the subject of the invention, presents resistant properties and improved plastics.

Example 2.

Corrosion-resistant cast iron according to the invention contains (% by weight): carbon: 3.19 silicon: 1.9 nickel: 18.0 manganese: 2.01 chromium: 2.0 copper: 8.0 barium: s 0.10 niobium: 0.20 tantalum: 0.02 aluminum: 0.3 calcium: 0.1 magnesium: 0.07 rare earth series metals: 0.08 iron: the rest
The results of the tests for this font are shown below. The results of the microscopic analysis are as follows
Characteristics of graphite
shape: spherical
quantity,%: 5
Characteristics of the metal matrix
austenite,%: 87
fuel,%: 8
Corrosion resistance
a) petroleum saturated with hydrogen sulfide
duration of the tests, h: 100
mass loss rate, g / m2.h: 0.0125
depth of attack, mm / year: 0.0132
b) iodine and bromine water
duration of tests, h: 500
mass loss rate, g / m2.h 0.0619
depth of attack, mm / year: 0.0724
Physical and mechanical properties
breaking strength, MPa o 441.5
relative elongation,%: 10.0
resilience, J / cm2: 34.3
hardness, HB: 165.0
Example 3.

The corrosion-resistant cast iron which is the subject of the invention contains (S by weight) carbon: 2.50 silicon: 0.95 nickel: 15.0 manganese: 0.8 chromium: 0.5 copper: 4 0.0 barium: 0.01 niobium: 0.005 tantalum: 0.003 aluminum: 0.01 calcium: 0.01 magnesium -: 0.01
The results of the indicated cast iron tests are given below
Results of the microscopic analysis Characteristics of graphite
shape: spherical
quantity,%: 7
Characteristics of the metal matrix
austenite, X: 88
carbides, X: 5
Corrosion resistance
a) petroleum saturated with hydrogen sulfide
duration of the tests, h: 100
mass loss rate, g / m2.h: 0.0069
depth of attack, mm / year: 0.0081
b) diode and bromine water
duration of the tests, h: soo
mass loss rate, g / m2.h: 0.0746
depth of attack, mm / year: 0.0872
Physical and mechanical properties:
breaking strength, MPa: 372.8
relative elongation,%: 20.0
resilience, J / cm2: 54.9
hardness, HB: 127
Example 4.

Corrosion-resistant cast iron according to the invention contains (X by weight): carbon: 2.8 silicon: 1.5 nickel: 16.2 manganese: 1.4 chromium: 1.2 copper: 5.9 barium: 0.05 niobium: 0.1 tantalum: 0.01 aluminum: 0.15 calcium: 0.04 magnesium: 0.03 rare earth series metals: 0.03 cobalt: 0.14 iron: the rest
The results of the tests of this font are given below
Results of microscopic analysis
Characteristics of graphite
shape: spherical
quantity,%: 5
Characteristics of the metal matrix
austenite,%: 91
carbides,%: 4
Corrosion resistance
a) petroleum saturated with hydrogen sulfide
duration of the tests, h: 100
mass loss rate, g / m2.h-: 0.0046
depth of attack, mm / year: 0.0054
b) iodine and bromine water
duration of tests, h: 500
mass loss rate, gXm2.h: 0.0280
depth of attack, mm / year: 0.0336
Physical and mechanical properties
breaking strength, MPa: 487.4
relative elongation,%: 19.2
resilience, J / cm2: 55.0
hardness, HB: 127.0
Example 5.

The corrosion-resistant cast iron which is the subject of the invention contains (% by weight) carbon: 2.8 silicon: 1.5 nickel: 16.2 manganese: 1.4 chromium: 1.2 copper: 5Z9 barium: 0.02 niobium: 0.1 tantalum: 0.01 aluminum: 0.15 calcium: 0.02 m. magnesium-: o, 02 metals of the earth series. rare: 0.005 cobalt: 0.12 titanium: 0.17 iron: the rest
The results of the tests for this font are shown below.

Results of microscopic analysis
Characteristics of graphite
shape: vermicular
quantity, z: 8
Characteristics of the metal matrix:
austenite, Z: 89
carbides, z: 3
Corrosion resistance :
a) petroleum saturated with hydrogen sulfide:
duration of tests, h; 100
club losing speed, g / mh: 0.0054
depth of attack, mm / year: 0.0063
b) iodine and bromine water
duration of tests, h: 500
mass loss rate, g / m2.h: 0.0374
depth of attack, mm / year: 0.0437
Physical and mechanical properties
breaking strength, MPa: 265.4
relative elongation, Z: 6.0
resilience, J / cmZ: 21.5
hardness; H9: 127
Foundry properties
at a casting temperature of the cast iron
equal to 13500C
fluidity, mm: 320
at a casting temperature of the cast iron
equal to 13000C:
fluidity, mm: 300
at a casting temperature of the cast iron
equal to l20O0C
fluidity, mm: 270
Total volume of shrinkage and pores,%: 4.0 including the volume of pores due to shrinkage, S: 1.6
It goes without saying that the invention is not limited to the modes of application and embodiments indicated, it also includes all the variants.

Claims (3)

 1. Corrosion-resistant cast iron, of the type containing elements such as carbon, silicon, manganase, nickel, chromium, copper, aluminum, magnesium, calcium, metals of the rare earth series and iron, characterized in that it comprises, in addition, barium, tantalum and niobium, the constituents being present in the following proportions, in% by weight carbon: 2.5 to 3.2 silicon: 0.95 to 2.4 manganese: 0.8 to 4 , 0 nickel: 12.0 to 18.0 chromium: 0.5 to 2.0 copper: 4.0 to 8.0 aluminum: 0.01 to 0.3 magnesium: 0.005 to 0.07 calcium: 0.01 to 0.10 rare earth series metals: 0.001 to 0.08 barium: 0.001 to 0.1 tantalum: 0.003 to 0.02 niobium: 0.005 to 0.3 iron: the rest  2. Corrosion-resistant melt according to claim 1, characterized in that it additionally comprises cobalt, the constituents being present in the following proportions, in% by weight of carbon: 2.5 to 3.2 silicon: 0.95 to 1.9 manganese: 0.8 to 4.0 nickel: 12.0 to..18, O chromium: 0.5 to 2.0 copper: 4.0 to 8.0 aluminum: 0.01 to 0.3 magnesium: o, oi to 0.07 calcium: 0.01 to 0.10 rare earth series metals: 0.01 to 0.08 barium: 0.001 to 0.10 tantalum: 0.003 to 0, 02 niobium: 0.005 to 0.2 cobalt: 0.05 to 0.3 iron: the rest  3. Corrosion-resistant cast iron according to claim 1, characterized in that it additionally comprises titanium, the constituents being present in the following proportions, in% by weight of carbon: 2.5 to 3.2 silicon: 0.95 to 2.4 manganese: 0.8 to 4.0 nickel: 12.0 to 18.0 chromium: 0.5 to '2.0 copper: 4.0 to 8.0 aluminum: 0.01 to 0.3 magnesium: 0.005 to 0.05 calcium: 0.01 to 0.05 rare earth series metals: 0.001 to 0.02 barium: 0.001 to 0.1 tantalum: 0.003 to 0.02 niobium: 0, 01 to 0.3 cobalt: 0.05 to 0.3 titanium: 0.05 to 0.5 iron: the rest.