EP0424277A1 - Process for improving the corrosion resistance of a nickel based alloy and alloy thus produced - Google Patents

Abstract

Process for improving the pitting and crevice corrosion resistance of a nickel-based alloy and also the alloy thus improved. Such an alloy contains at least 50% Ni and approximately 20% Cr, 4% Nb, 1% Ti and Al. According to the invention, when the alloy is being produced, the proportions of carbon and silicon are regulated within narrow limits which make it possible, when the metal solidifies, to avoid the formation by segregation of carbides of M6C type, the proportion by weight of carbon being maintained at at least 0.03% without exceeding 0.05%, and the proportion by weight of silicon not exceeding 0.15%, the absence of carbides containing silicon being verified in the raw casting state. The invention applies especially to the alloys employed for the production of ingots or of cast pieces of large size.

Description

The subject of the invention is a process for improving the resistance to corrosion by pitting and crevices, in a chlorinated medium, of nickel-based alloys, and more particularly alloys containing at least 50% of nickel, around 20 % chromium, around 8% molybdenum, around 4% niobium and around 1% titanium and aluminum.

The invention also covers the alloys thus improved and their use for the production of wrought parts requiring ingots of large dimensions or else molded parts.

The corrosion resistance in chlorinated medium of alloys based on nickel-chromium, molybdenum, niobium is well known. Such alloys are commonly used for applications in sea water, brackish water, in industrial waste water or in acid oil wells.

In many applications, the parts produced from these alloys must have a high elastic limit, for example greater than 800 MPa which is generally obtained by cold working. However, this is difficult or impossible for solid parts and, more generally, parts molded or produced from powder.

To solve this problem, the Applicant Company has already proposed specific compositions of such alloys comprising in particular titanium and aluminum so as to allow structural hardening by precipitation by phase tempering of the Ni₃ (Ti, Al) and / or Ni₃Nb type. It is thus possible to obtain, without strain hardening, elastic limits greater than 800 MPa or even 1050 MPa depending on the quantity of elements participating in the structural hardening , that is to say essentially niobium, titanium, aluminum, and depending on the heat treatment used.

It has been noted, however, that such alloys could pose difficult manufacturing problems, in particular for the production of parts whose dimensions require ingots of large diameter, for example 400 nm or more. It has been observed, in fact, that even if the desired elastic limits were obtained, such large parts exhibited a particular sensitivity to corrosion, in particular in a chlorinated medium.

The object of the invention is to provide a solution to this problem, by perfecting alloys of this type so as to improve their resistance to corrosion by pitting and crevices, in particular in a chlorinated medium, while allowing obtaining a high yield strength, greater than 800 MPa.

The studies carried out for this purpose first of all made it possible to establish a link between the lower resistance to pitting corrosion and the segregation which occurs in particular during the solidification of large castings. , that it was not enough to minimize segregation by resorting, for example, to a method of manufacturing ingots by remelting a consumable electrode, under vacuum or under slag. he indeed, it appeared that, even in this case, it could occur in the interdendritic spaces of the ingots, enrichments in niobium, molybdenum and titanium capable of locally altering the corrosion resistance of the alloy.

It has thus been established that the corrosion resistance of such alloys is influenced at the same time by the formation of segregations, the composition of the alloy and its structure.

The subject of the invention is therefore, generally, a process for improving the resistance to pitting and crevice corrosion of a nickel-based alloy comprising:
at least 50% nickel, around 20% chromium,
around 8% molybdenum, around 4% niobium,
around 1% titanium and aluminum.

In accordance with the invention, during the development of the alloy, the proportions of carbon and silicon are adjusted within narrow limits making it possible, during solidification, to avoid the formation by segregation of carbide of M₆C type, the proportion by weight of carbon being maintained at 0.03% at least, without exceeding, however, 0.05% and the proportion by weight of silicon not exceeding 0.15%, the absence of carbides containing silicon being verified at 1 'gross state of casting.

In a particularly advantageous manner, an alloy according to the invention has the following composition, by weight:
50% to 55% nickel,
15 to 22% and preferably 19.5 to 21% of chromium,
8 to 10% and preferably 8 to 9% molybdenum
3 to 5% and preferably 3.5 to 4.3% of niobium,
1 to 2% and preferably 1 to 1.5% of titanium,
0.1 to 0.6% and preferably 0.1% to 0.35% aluminum,
1 to 3% and preferably 2 to 3% copper
not more than 2% cobalt
at least 0.03% and at most 0.05% carbon
not more than 0.15% silicon
the rest being iron.

To study the corrosion resistance of such alloys, two experimental methods were used, test G 48 and test G 28, method A of ASTM.

ASTM test G 48 uses a solution of 100g of ferric chloride in 900 ml of distilled water. The test is carried out at 25 and 50 ° C. To make the test more selective, it can be modified compared to the ASTM method by lowering the pH of the solution to 0.5 by addition of hydrochloric acid and by raising the bath temperature to 100 ° C. This test therefore corresponds to an acidified chlorinated medium. It detects sensitivity to pitting and crevice corrosion.
ASTM test G 28, method A uses a solution of 25g of ferric sulfate in 600 ml of 50% sulfuric acid brought to the boil. We measure the weight loss after a 120h immersion.

In addition, for each casting, we checked the structure of the alloy in the laboratory and observed the formation of several kinds of carbides:

1 / Type MC carbides where M is a mixture of metallic elements which we will specify.

In the solid state of solidification, we observed carbides of composition:
Nb _{0.86 / 0.90} ; Ti _{0.05 / 0.08} ; Mo _{0.02. / 0.06} ; Cr _{0 / 0.02} ; Ni _{0 / 0.04} ) C where the contents are expressed in atomic percentages. These primary carbides resulting from solidification, remain in the finished product, even if it is strongly wrought.

There is also a small amount of intergranular secondary carbides of little different composition:
Nb _0.62 ; Mo _0.26 ; Ti _0.06 ; Cr _0.06 ) C.

It can be seen that, in both cases, these carbides do not contain silicon.

2 / M₆C type carbides.

In the raw solidification state (primary carbides), the composition was found: Ni _0.33 ; Mo _0.25 ; Cr _0.19 ; Nb _0.18 ; If _0.03 ; Fe _0.02 ) ₆ C. We also observe secondary carbides precipitated at the grain boundaries having the composition:
(Ni _0.11 ; Mo _0.27 ; Cr _0.56 ; Nb _0.02 ; Si _0.04 ) ₆C. It appears that, in both cases, these carbides contain silicon.

3 / M₂₃C₆ type carbides

These carbides are not observed in the solid state of solidification. They appear at the grain boundaries of processed products. In these alloys, we were able to determine their composition: (Cr _0.80 , Mo _0.13 , Ni _0.05 , Nb _0.01 , Fe _0.01 ) ₂₃C₆.

Various alloy compositions have thus been developed for which the structure obtained for solidification and the corrosion resistance have been studied in parallel, and it has been possible to determine that M₆C type carbides have a very unfavorable influence on the resistance. corrosion, the two types of tests mentioned above having made it possible to detect that such carbides constituted preferential attack sites.

Furthermore, it has been found that the appearance of such M₆C type carbides was linked to the contents of the silicon and carbon alloy.

Until now, in fact, it seemed normal to limit the silicon content, but silicon contents of up to 1% were admitted, the patent Fr 2,154,871 cited above, lowering this limit to 0.5 %. However, this limitation of the silicon content was always intended to preserve the weldability properties, the influence of silicon, in this regard, being well known.

However, it has been established that, to avoid the appearance of carbides of the MuresC type, the silicon content should be much lower, in particular when the carbon content is very low and that it should not exceed a content of 0 , 15% silicon, preferring a content of 0.10% maximum.

As regards the carbon content, it was also recognized that this should be reduced as much as possible during the production of the metal and should not exceed 0.1 to 0.2%.

But, it was found that a content too low in carbon could lead to an increase in the importance of segregation in interdentric spaces. However, it appears that, during the solidification of the ingots, the greater the segregation in niobium is important in the interdendritic spaces the more their specific mass rises, this having the consequence of increasing, during solidification, the probability of hydrodynamic instabilities which can lead to the formation of strongly segregated macro-defects of solidification and rich in M₆C carbides, which, as we have seen, contain silicon and constitute privileged points for attack by corrosive media, this risk increasing with size ingots and becoming important when their dimensions exceed 400 mm.

It was thus possible to determine that the alloy should contain at least 0.03% carbon.

However, if a minimum quantity of carbides of the MC or M₂₃ C₆ type is required, it has been observed, beyond a certain limit, the formation in the vicinity of these carbides, of chromated and demolybdenized zones which constitute sites attacked in particular by reagent G 28, method A of ASTM. It therefore appears that the carbon content must be kept below a certain limit, much lower than the commonly accepted limits and, in practice, it has been found that the carbon content should not exceed 0.05%.

It therefore appeared that a very precise definition of the carbon content range, between 0.03% and 0.05% and a strict limitation of the silicon content which should not exceed 0.15% made it possible on the one hand to avoid the formation of M₆C carbides and on the other hand, to limit segregation and solidification defects which constitute weak points for resistance to pitting corrosion.

It has also been necessary to define ranges of precise contents for the main elements of the alloy.

Alloys of this type are based on nickel and, generally, it is preferred that the nickel content is at least 55%. This avoids the precipitation of topologically compact phases such as the LAVES phase during the operations of using the alloy (solidification, welding or transformation). These phases have a negative influence on the forgeability and / or corrosion resistance. The LAVES phase, for example, results in an inability of the metal to be processed by spinning and a poor result with the ASTM G 28 test. This solution is effective but expensive, the price of nickel being high.

However, it has been found that the nickel content of the alloy can be reduced to a minimum of 50%, on the one hand thanks to the actions relating to segregation which are mentioned above, on the other hand by compensating for this lowering of the nickel content by adding 1.0 to 3% of copper (or preferably 2 to 3%) which increases the structural stability of the alloy, i.e. decreases its propensity to precipitation of topologically compact phases.

Furthermore, it is known that such an addition of copper is in itself favorable to the resistance corrosion resistance in particular in chlorinated and sulfuric environments.

Ultimately, to obtain the desired result, the following composition of an alloy according to the invention could be defined:
Ni: 50 to 55%
Cr: 15 to 22% and preferably 19.5 to 21%
Mo: 8 to 10% and preferably 8 to 9%
Nb: 8 to 5% and preferably 3.5 to 4.3%
Ti: 1 to 2% and preferably 1 to 1.5%
Al: 0.1 to 0.6% and preferably 0.1 to 0.35%
Cu: 1 to 3% and preferably 2 to 3%
Co: at most 2% of Co
the rest being iron.

To illustrate the influence of the composition of the alloy on the extent of segregation, on the structure and on the corrosion resistance, we will now give several examples of castings with the results of the examinations carried out on these castings.

A first example highlights the influence of carbon content on niobium segregation.

A la coulée elle a été divisée en deux lingots A et B de teneurs en carbone égales,respectivement,à 0,016% pour le lingot A et 0,038% pour le lingot BA laboratory casting produced in the vacuum induction furnace was carried out with the following basic composition 5:

At the casting it was divided into two ingots A and B of carbon contents equal, respectively, to 0.016% for ingot A and 0.038% for ingot B

For each ingot, the niobium segregation rates were measured in two places for example, the interdendritic spaces and the dendrites and it was found that the ratios of the niobium contents between these two points were, respectively, 3.59 for ingot A and 2.30 for ingot B. However, we know that a ratio of 2.3 can be corrected using thermomechanical homogenization treatments but that a ratio of 3.59 cannot be tolerated. This example therefore shows that there is a precise limit for the carbon content of the alloy below which the niobium segregations become too large.

A second example corresponding to an industrial casting illustrates the effect of too low carbon contents on the structure. The following analysis flow was transformed into forged products:

In the products produced from this casting, primary carbides (that is to say formed during solidification) of M₆C type have been observed, the metal M indicated above being a mixture of the metals molybdenum, niobium, chrome, silicon and nickel.

Samples of this metal were subjected to ASTM G 48 test. Pitting and crevices were observed after 24 h immersion at 60 ° C, which is considered a bad result for this type of alloy.

A third example illustrates the influence of the composition on the presence or absence of the topologically compact Laves phase, and on the forgeability. Two industrial castings A and B of the following compositions were transformed into billets intended for the extrusion of tubes. Elements VS Mn Yes Or Cr Mo Ti Nb Fe Al Flows AT 0.022 0.04 0.30 60.12 20.72 8.86 1.37 3.53 4.64 0.22 B 0.038 0.01 0.08 60.62 20.30 8.20 1.29 3.75 4.60 0.15

In the metal from casting A, significant quantities of Laves phase and M₆C carbides have been detected. On the contrary, these phases have not been detected in casting B, the carbon and silicon contents of which are in accordance with the invention. .

The forgeability of these castings was evaluated by rapid tensile test at temperatures 1150, 1180 and 1200 ° C as follows: For each temperature, the test piece is first preheated to a temperature 50 ° C higher than the temperature test, then it is brought back to this test temperature before breaking it. The para meter measured is the necking at break, which must be at least 60% at the three test temperatures for the metal to be suitable for processing by extrusion.
The results obtained are as follows: Breaking strength% Temperature 1150 ° C 1180 ° C 1200 ° C Flows AT 63 13 10 B 80 67.5 63.0

It can be seen that metal A is unsuitable for transformation by extrusion while metal B has good aptitude for this transformation.

Finally, a fourth example illustrates the influence of the composition of the alloy on its resistance to corrosion and shows the particular resistance of the composition which is the subject of the invention.

The two flows A and B of the previous example and a third flow C were compared with regard to their resistance to corrosion.
The analysis of flow C was as follows:

• 1°) Mise en solution : 1 h à 1040°C - refroi­dissement air
• 2°) Revenu 8 h à 750°C - refroidissement à 50°C/h jusqu'à 620°C - maintien 8 h - refroi­dissement à l'air.

Metal from these three castings was rolled into 20 mm diameter bars and subjected to the following heat treatment:

• 1 °) Dissolution: 1 h at 1040 ° C - air cooling
• 2 °) Tempering 8 h at 750 ° C - cooling at 50 ° C / h to 620 ° C - holding 8 h - air cooling.

The tensile characteristics measured on the three castings were little different, as can be seen in the table below which indicates, for each cast, the tensile strength R _m , the conventional elastic limit at 0.2% and the elongation in%. Casting R _m (MPa) R _0.002 (MPa) AT% AT 1330 920 32 B 1370 952 31 VS 1355 935 35

These three flows were subjected to the ASTM G 48 test modified by acidification of the reagent as expressed above.

Flow A shows pitting and crevices after 10 hours of immersion in the ferric chloride solution acidified to pH 0.5 and brought to 50 ° C.

Flow B resists up to 95 ° C and shows some cracks after 24 hours at this temperature. Flow C does not show any pitting or crevices after 24 h at 97 ° C. It shows only a few cracks from 72 h at this temperature.

Claims (5)

1. Process for improving the resistance to corrosion by pitting and crevices of a nickel-based alloy comprising:
at least 50% nickel
around 20% chromium
around 8% molybdenum
around 4% niobium
around 1% titanium and aluminum
characterized in that during the development of the alloy, the proportions of carbon and silicon are adjusted within narrow limits allowing, during solidification to avoid the formation by segregation of carbides of type M₆C, the proportion by weight of carbon being maintained at least 0.03% without exceeding 0.05% and the proportion by weight of silicon not exceeding 0.15%, the absence of carbides containing silicon being verified in the raw state of casting. 2. Method according to claim 1, characterized in that, during the development of the alloy, the proportions of the various constituents are adjusted within the following limits:
C from 0.03 to 0.05%
Mn less than 0.35%
If less than 0.15% and preferably not more than 0.10%
Cr from 15 to 22% and preferably from 19.5 to 21%
Mo from 8 to 10% and preferably from 8 to 9%
Nb from 3 to 5% and preferably from 3.5 to 4.3%
Ti from 1 to 2% and preferably from 1 to 1.5
Al from 0.1 to 0.6% and preferably from 0.1 to 0.35%
Ni from 50 to 55%
Cu from 1 to 3% and preferably from 2 to 3%
Co less than 2%
Fe complement 3.A nickel-based alloy resistant to pitting and crevices in chlorinated media and with an elastic limit greater than 830 MPa and mainly comprising:
around 20% chromium
around 8% molybdenum
around 4% niobium
around 1% titanium and aluminum
at least 50% nickel
as well as impurities and accidental elements characterized in that the carbon and silicon contents are adjusted during the development of the alloy, within narrow limits, respectively:
at least 0.03% and at most 0.05% carbon
and at most 0.15% silicon
said alloy not containing carbides of type M₆C in the raw casting state. 4. A nickel-based alloy according to claim 1, characterized in that it comprises:
Ni: 50 to 55% Cr: 15 to 22% and preferably 19.5 to 21%
Mo: 8 to 10% and preferably 8 to 9%
Nb 3 to 5% and preferably 3.5 to 4.3%
Ti: 1 to 2% and preferably 1 to 1.5%
Al 0.1 to 0.6% and preferably 0.1 to 0.35%
Cu: 1 to 3% and preferably 2 to 3%
Co: at most 2% of Co
the rest being iron. 5.Use of an alloy according to one of claims 4 and 5 for the production of large ingots and molded parts of which at least one dimension exceeds 300 mm.