EP3077558B1

EP3077558B1 - Nickel-based alloy, method and use

Info

Publication number: EP3077558B1
Application number: EP14830602.0A
Authority: EP
Inventors: Luca FORONI; Raimondo MONTANI
Original assignee: Foroni SpA
Current assignee: Foroni SpA
Priority date: 2013-12-05
Filing date: 2014-12-05
Publication date: 2017-08-23
Anticipated expiration: 2034-12-05
Also published as: ITVA20130061A1; JP2017503085A; US20180171456A1; JP6571103B2; AU2014358718A1; AU2014358718B2; ES2644391T3; EP3077558A1; WO2015083133A1

Description

Scope of the invention

. In recent years, as a result of the significant increase in the energy demand, for the oil extraction industry the problem has arisen of finding oil at increasingly greater depths, both on land and on the seabed.
. At the same time the size of the equipment has also increased, reaching up to 18 inches (460 mm) in the wall diameter or thickness. This increase has forced manufacturers to more than double the size of the starting ingots which, given the chemical compositions at play, have presented significant problems regarding the chemical homogeneity of the products even after long and costly homogenisation heat treatments.
. Commercially alloys known with the signs N07718, N07716, N07725, currently available on the market for use in the most critical ambients have the following limitations:

the N07718 alloy, with which good mechanical characteristics can be achieved without compromising the grain boundary with elevated precipitations of detrimental phases, can be used at moderate temperatures and not in the presence of elemental sulphur;
the N07716 alloy, aged so as to obtain high mechanical characteristics, has micro-structures with grain boundaries decorated by significant phase precipitations which affect the behaviour of this alloy in laboratory tests of intergranular corrosion and which, therefore, hamper its use in the HPHT (High Pressure High Temperature) sphere and in those environments in which the presence of nascent hydrogen is possible, making products in this alloy incredibly fragile.

. The installation requirements of the oil extraction industry have urged the manufacturers of traditionally used alloys to significantly increase (10-15%) the mechanical properties of the standard alloys (N07718, N07716, N07725) acting mainly on ageing heat treatments which, unfortunately, are not devoid of effects on the corrosion properties of said alloys. US5000914 discloses a precipitation-hardening type Ni-based alloy exhibiting improved resistance to stress corrosion cracking, having a chemical composition comprising Ni 58.5%, Cr 23%, Mo6.1%, Fe 7.3%, Nb 4.52%, Mn 0.01%, P 0.002%, S 0.002%, Ti<0.01%, C 0.002%, Si 0.31%, Al 0.20%, N 0.14%, which has been hot worked, solution annealed, water quenched, aged and heat treated, and having a yield strength of >900 MPa and sulfide stress cracking resistance.
. The Applicant, after having worked for some years on the V.A.R. (Vacuum Arch Remelting) recasting method so as to minimise the chemical irregularities and on the heat transformation process to optimise it and make it repetitive using sophisticated machinery, has concluded that to meet the current needs of the oil and natural gas extraction industry it is necessary to influence the chemical composition of the alloy to obtain elevated mechanical characteristics without invalidating the micro structure and resistance to corrosion, even for large pieces.
. The present invention lies thus in the above context, proposing to provide a method of manufacture and nickel-based alloys able to overcome the drawbacks spoken of in relation to the prior art.
. More specifically, the alloys which the present invention relates to are able to combine a number of desirable features, discussed below, which to date have been deemed to be substantially mutually irreconcilable.

Description of the invention

. Such objectives are achieved by a manufacturing method of a nickel-based alloy comprising the steps of:

i) forging and solution treating a metal mass;
ii) subjecting the product of step i) to a first ageing step at a higher temperature;
iii) cooling the product of step ii) in air;
iv) subjecting the product of step iii) to a second ageing step at a lower temperature;
v) cooling, preferably in air, the product of step iv) to obtain the nickel-based alloy.

. As a result of the steps i)-v), the hardening metal phases of the nickel-based alloy are precipitated in a uniform manner in the grains of the latter.
. Indeed, the hardening metal phases (γ' and γ") precipitate eminently in steps ii) and iv), and the ageing steps have been selected in such a way as to have the maximum precipitation rate of said phases.
. During the cooling steps the hardening metal phases may continue to precipitate but no longer at an optimal rate while the carbide phases (also referred to as "carbides" in this description), and optionally the undesired intermetallic phases, may have high precipitation rates during this process.
. For this reason the cooling steps indicated above which follow each step of ageing are much faster than traditionally used treatments.
. According to a particularly advantageous embodiment, as a result of steps i) -v), the nickel-based alloy comprises hardening phases γ' and γ" precipitated in an essentially non-intergranular position, and carbides precipitated in a discontinuous manner at least along the boundary of the aforesaid grains.
. It follows that, innovatively, since the aforesaid method combined with appropriate analytical balancing minimises the precipitation of carbides and unwanted phases at the grain boundary and promotes a uniform distribution of the metal hardening phases in the metal grains of the alloy (specifically: outside the intergranular zones), the alloy produced using the aforesaid method is subject to the phenomenon of intergranular corrosion in an extremely limited, if not non-existent manner, at least compared to the alloys currently used.
. More specifically, the alternation of the two ageing treatments both followed by cooling makes it possible to modulate the generation rate of the precipitates in each step iii) and v). Furthermore, for each step of the method, only the precipitation of the metal phases advantageous to the properties of the nickel-based alloy (i.e. γ' and γ") is stimulated.
. The metal mass comprises, expressed in percentages by weight: C = 0.030 max, Si = 0.50 max, Mn = 0.50 max, Cr = 20.0-24.0, Ni = 55.0- 60.0, Mo = 5.5 - 7.0, S = 0.005 max, P = 0.015 max, Cu = 1.0 max, Co = 1.0 max, Al = 0.80 max, Ti = 0.50 - 1.50, Nb = 4.0 - 5.5 and Fe for the remaining percentage. Preferably, the Fe may be present in a percentage of about 5-15_or about 5-12.
. Preferably, the nickel-based alloy forged and solution treated in step i) comprises, expressed as percentages by weight: C = 0.022 max, Si = 0.20 max, Mn = 0.20 max, Cr = 21.0-23, Ni = 57.0-59.0, Mo = 5.5-6.0, Al = 0.30-0.60, Ti = 0.70-1.0, Nb = 4.5-5.0, Fe = 5 as a minimum percentage.
. Merely by way of example, the nickel-based alloy forged and solution treated in step i) could comprise, expressed as percentages by weight: Ni = 58, Cr = 21.5, Mo = 5.8, Nb = 4.8, Ti = 0.9, Al = 0.4, Fe = 8%.
. nickel-based alloy.
. According to a variant, step i) comprises substeps of forging the nickel-based alloy at a temperature of approximately 1000-1160°C and then solution treating said mass at a temperature of approximately 1030-1080°C.
. The sub-step of solution treating is followed by a cooling step in water before step ii), or by a rapid cooling of an equivalent type.
. It is important to note that the product of step iii) by itself already constitutes a product of industrial interest and with its own market.
. It follows that, according to a possible embodiment of the invention, this method could comprise a step of separating the product of step iii), and a step of transforming a first part of the separated product into a first finished product, e.g. with lower performances, and/or a step of storing said separated product.
. In other words, not all the forged and solution treated metal mass starting the process needs necessarily to lead to the product of step v), but a part thereof could be withdrawn at the end of step iii), and be transformed as indicated above, or even simply stored.
. The production and logistic advantage compared to conventional alloys is thus evident.
. According to a preferred variant, the step iii) product could be characterised by a yield strength, measured at ambient temperature, equal to or greater than approximately 827 MPa.
. According to a further advantageous embodiment, the method may comprise a step of sending to step iv) (and subsequently to step v)) a second part of the aforesaid separated product at step iii), to obtain a second product, e.g. of higher performances, made of the nickel-based alloy.
. It follows that the separated and/or stored product may be subjected to step iv) at a different time from step iii), for example as a result of an order for the nickel-based alloy.
. According to a preferred embodiment, following step v), the nickel-based alloy is characterised by a yield strength, measured at ambient temperature, equal to or greater than about 950-970 MPa, preferably greater than or equal to 970 MPa.
. It should be noted that within this description the terms "higher" and "lower" will be construed as relative terms within the method or alloys themselves, and not as absolute terms.
. It follows that the alloy having lower performances will be considered such only in relation to the higher performance alloy, and preferably limited to the yield strength parameter only. This does not mean that the "lower" alloy from this point of view, might not be better if compared relative to other factors, for example in relation to the anti-corrosion properties.
. Similarly, the terms "higher temperature" and "lower temperature" mentioned in relation to the described ageing steps will have a relative meaning only.
. As regards step ii), said step is specifically used in order to minimise the precipitation of carbides and other unwanted phases at the grain boundaries.
. Step ii) is conducted at a temperature (defined as "higher") of about 720-780°C for about 3-8 hours, or for about 3-6 hours. Step iv) is conducted at a temperature (defined as "lower") of about 600-640°C for about 4-10 hours.
. Preferably one or both the cooling steps iii) and/or v) could be performed in air at room temperature, preferably up to about an ambient temperature of the respective products.
. Within the present invention the term "ambient temperature" - unless otherwise specified - is understood to mean a temperature external to the strongly heated ambient in which the ageing steps ii) and iv) are conducted. Specifically, "ambient temperature" could refer to the temperature outside the furnace used to perform the aforesaid ageing heat treatments, more precisely at the cooling planes situated inside the production plant.
. More precisely, the ambient temperature could be the temperature of the production plant, changing greatly depending on the season of the year in which the production takes place and/or on the latitude of the production site in which the aforesaid method takes place.
. The aforesaid objective is also resolved by a nickel-based alloy obtained through the steps of:

i) forging and solution treating the aforesaid metal mass;
ii) subjecting the product of step i) to a first ageing step at a higher temperature;
iii) cooling the product of step ii) in air;
iv) subjecting the product of step iii) to a second ageing step at a lower temperature;
v) cooling, preferably in air, the product of step iv) to obtain a nickel-based alloy.

. As a result of the steps i)-v), the nickel-based alloy comprises metal hardening phases precipitated uniformly throughout its grains.
. As regards preferred or advantageous variants for the manufacture of said alloy, refer to the description above.
. According to a particularly advantageous embodiment, at the end of steps i) -v), the nickel-based alloy comprises hardening phases γ' and γ" precipitated in an essentially non-intergranular position, and carbides precipitated in a discontinuous manner at least along the boundary of said grains.
. More precisely, the steps ii) and iv) interspersed by step iii), promote the precipitation of the hardening phases γ' and γ" in a uniform and preferably fine manner, minimising the precipitation carbides and unwanted intermetallic phases at the grain boundary.
. The aforesaid objective is also resolved by a nickel-based alloy consisting of, expressed in percentages of weight: C = 0.030 max, Si = 0.50 max, Mn = 0.50 max, Cr = 20.0-24.0, Ni = 55.0- 60.0, Mo = 5.5 - 7.0, S = 0.005 max, P = 0.015 max, Cu = 1.0 max, Co = 1.0 max, Al = 0.80 max, Ti = 0.50 - 1.50, Nb = 4.0 - 5.5 and Fe for the remaining percentage. Said alloy is characterised in that it comprises hardening phases γ' and γ" precipitated essentially in a non-intergranular position, advantageously evenly and preferably finely, and carbides precipitated discontinuously at least at the boundary of said grains.
. For example, the latter alloy could be obtained using the method according to any of the embodiments illustrated above. For this reason, even where not specifically indicated, preferred or advantageous variants of said alloy could comprise any manufacturing step deductible from the aforesaid description.
. Advantageously, the alloy of the present invention is preferably usable for making equipment and pipes for the chemical or petrol industries.
. The purpose of the present invention will now be illustrated on the basis of non-limiting examples. Example 1: Means for implementing the method.
. The nickel-based alloy which the invention relates to is preferably melted in an electric arc furnace, refined in A.O.D. (Argon Oxygen Decarburization) so as to obtain an intense desulphurisation, thorough deoxidisation and a very restricted analytical range of compositions to ensure repeatability of the mechanical and corrosion properties.
. The refining process could be completed by at least one of the following operations:

further elaboration of liquid steel at V.I.D.P. (Induction Vacuum Degassing and Pouring);
source casting in moulds suitable for subsequent forging;
source casting of ingots intended for subsequent remelting V.A.R. / E.S.R. (Vacuum Arch Remelting / Electro Slag Remelting).

. According to a variant, the ingots obtained after V.A.R. or E.S.R. remelting may be subjected to appropriate homogenisation heat treatment and then transformed into blooms through use of a forging press, for example having two integrated, fully automated manipulators programmable both for the entity and deformation rate for each cycle.
. The blooms, after intermediate grinding, could be transformed into billets/bars through the use of a hydraulic press with four synchronised hammers, for example with dual manipulator and/or new concept RCD (Round Continuous Deforming) rolling mill. These last two systems could also be automated and programmable.
. The heat-processing plants, designed ad hoc for long products (in particular having a extension length substantially greater than the width or thickness, such as pipes or bars), make it possible to work within narrow temperature ranges so as to have a good control of the grain uniformity and avoid the precipitation of deleterious phases especially for resistance corrosion in the environments in which the products, made from the alloy according to the invention, are intended for use.
. According to a preferred variant, the metal mass - and thus the corresponding alloy according to the invention - could contain on average the following percentages by weight of the basic elements: Ni = 58, Cr = 21.5, Mo = 5.8, Nb = 4.8, Ti = 0.9, Al = 0.4, Fe = 8%. Example 2: Comparison of the nickel-based alloy of the invention with traditional alloys currently used.
. The nickel-based alloy of the present invention, after heat transformation in the temperature range 1000-1160°C and solution treatment in the range 1030-1080°C, typically has the mechanical features shown in Figure 1.
. The nickel-based alloy of the present invention (called "AF.955"), after solution treatment as in the above paragraph, if aged in the temperature range 720-780°C for 3-8 hours and air cooled (or equivalent cooling, or in case of faster cooling) typically has the mechanical features specified in Figure 2 and the resistance data to intergranular corrosion and pitting referred to in Figures 6-7-8.
. The alloy, after a second ageing at 600°-640°C for a time ranging from 4-10 hours, followed by air cooling, presents the mechanical characteristics specified in Figure 3, and the resistance to intergranular corrosion and pitting referred to Figures 6-7-8.
. The results of the SCC corrosion tests (Stress Corrosion Cracking) as per Figure 9, complemented with additional tests, permit the inclusion of the alloy in the Nace Standard MR 0175/ISO 15156-3 (2009).
. The SSRT (Slow Strain Rate testing) test results as in Figure 10 shows the reduced sensitivity of the alloy to the phenomenon of hydrogen embrittlement.
. Figures 4-5 shows the chemical composition and characteristic mechanical properties of the alloys most commonly used in the numerous environments encountered in the oil and natural gas extraction industry.
. For all the alloys, the 3 grades (Gr. 3) differ from the 3HS grades (Gr.3HS) only in the methods (temperatures/times) of the thermal ageing treatment.
. More specifically, for the alloy according to the invention the 3 grades (Gr.3) relate to a nickel-based alloy subjected to a single ageing step and subsequent cooling as per steps ii) and iii) mentioned above. Conversely, the 3HS grades (Gr.3HS) relate to an alloy which has also undergone the second steps of ageing and cooling - namely also steps iv) and v).
. Figures 6-7-8-9-10 provide information on the resistance capacity of the materials in the laboratory corrosion tests compared to the alloy of the invention.
. Further comparing the characteristic micro structures (Figures 11A-11F for the alloy AF.955; Figures 12A-12F for the alloy N07718 of the prior art; Figures 13A-13F for the alloy N07716 of the prior art), the results of the intergranular corrosion, pitting, SCC, SSRT tests compared to those obtained for traditional alloys with a nickel comparable content (N07718, N07716), clearly show the improvements achieved by the introduction of this innovative analytical balance of alloy elements, combined with thorough and innovative heat treatment methods.
. With reference to the aforementioned Figures, Figures 11A-11B show the metallography of the alloy AF.955 (at 100X and 500X magnifications) before step ii), namely at the end of forging and solution treating of the metal mass only. Figures 11C-11D show the metallography - again at the aforesaid magnifications - of the aforesaid step iii) product, i.e. following the first ageing and subsequent air cooling step. Figures 11E-11F lastly show metallographies corresponding to the previous ones, relative to the alloy AF.955 at the end of step v).
. Figures 12A-12F and Figures 13A-13F show a metallography respectively of the alloy N07718 and of the alloy N07716 corresponding to the aforementioned Figures (after solution treating, 12A-12B for the alloy N07718 and 13A-13B for the alloy N07716; after a first type of ageing for each alloy Gr. 3 (12C-12D and 13C-13D) and after a second ageing for Gr.3HS for each alloy (12E-12F and 13E-13F).
. These improvements to the structural and corrosion resistance, combined with the elevated mechanical properties achieved without exceeding in the ageing heat treatment time, advantageously allow the use of the alloy in all "sour" ambients, even at great depths (HPHT applications), which previously forced users to a targeted and not always optimal choice between the alloys N07718 and N07716 and the homologous N07725.
. Innovatively, the method and nickel alloys of the present invention make it possible to brilliantly resolve the drawbacks spoken of in relation to the prior art.
. More specifically, the method and the nickel alloys of the present invention are substantially free of intergranular metal phase precipitates, in particular of carbides, so that the corrosion phenomenon at the grain boundary is dramatically reduced, if not substantially absent compared to the prior art.
. In addition, the alloy of the present invention has greater mechanical and traction resistance properties, and in particular lengthening and pinch point data, than the metal alloys it was compared to, a considerable resistance to corrosion under stress and very low hydrogen embrittlement with elongation at rupture characteristics still high enough to guarantee safe use of the alloy in environments in which nascent hydrogen may develop.
. As discussed at the beginning, no alloy of the prior art was so far able to achieve said technical results, especially for a product forged and produced on an industrial scale.
. Advantageously, the components of the nickel-based alloy, the heat and/or thermo-mechanical treatments which the present invention relates to make the precipitation phenomena of the metal hardening phases unique and characteristic.
. Advantageously, in the method of the present invention, a clear temporal separation between the precipitation phases makes it possible to obtain different alloys of different types, markedly different in performance.
. Nonetheless, this method makes it possible to achieve important production economies, not only by virtue of the common process which characterises the manufacture of the various obtainable alloys.
. In fact, on the market there are currently two types of products, i.e. products with Ys > 120 KSi (MPa 827) and products with Ys > 140 KSi (MPa ca 966). With the method and with the alloy of the present invention it is thus possible to guarantee the aforesaid minimum levels with the step iii) product and the step v) product respectively, which are part of the same production chain.
. Lastly, advantageously, even the length of the cooling positively influences the precipitation of the most useful phases for the mechanical properties, of resistance to corrosion and the embrittlement of the alloy described.
. Without wanting in any way to provide a scientific explanation of the phenomenon, the unwanted phases and carbides tend to precipitate at the grain boundary. Technically it is therefore important to minimise such precipitation and ensure that these precipitates are not continuous at the grain boundary. A grain boundary with greater, lesser or no precipitates affects the resistance to intergranular corrosion and hydrogen embrittlement, but the phenomenon of stress corrosion cracking in a more limited manner.
. Advantageously, even the step iii) product proves to be optimal for certain industrial applications.

Claims

Manufacturing method of a nickel-based alloy comprising the steps of:
i) forging and solution treating a nickel-based alloy consisting of, expressed as percentages by weight: C = 0.030 max, Si = 0.50 max, Mn = 0.50 max, Cr = 20.0-24.0, Ni = 55.0-60.0, Mo = 5.5 - 7.0, S = 0.005 max, P = 0.015 max, Cu = 1.0 max, Co = 1.0 max, Al = 0.80 max, Ti = 0.50 - 1.50, Nb = 4.0 - 5.5 and Fe = 5,0 - 15,0, wherein forging is carried on at a temperature of 1000-1160°C and the solution treating takes place at a temperature of 1030-1080°C, followed by a cooling step in water;

ii) subjecting the product of step i) to a first ageing step at a higher temperature of about 720-780°C for about 3-8 hours;

iii) cooling the product of step ii) in air at ambient temperature

iv) subjecting the product of step iii) to a second ageing step at a lower temperature of 600-640°C for about 4-10 hours;

v) cooling in air the product of step iv) to obtain the nickel-based alloy;
wherein, following said steps i)-v), the hardening metal phases of the nickel-based alloy are precipitated in a uniform manner in the grains of the latter.
Method according to the previous claim, wherein following the steps i) - v), the nickel-based alloy comprises metal hardening phases γ' and γ" precipitated in an essentially non-intergranular position, and carbide phases precipitated in a discontinuous manner at least along the boundary of said grains.
Method according to any of the previous claims, further comprising a step of separating the product of step iii), and a step of transforming a first part of the separated product into a first finished product
Method according to the previous claim 3, comprising a step of further processing the second part of said separated product to step iv) and subsequently to step v) to obtain a second product, of higher performance, made of said nickel-based alloy.
Method according to any of the previous claims, wherein the product of step iii) is characterised by a yield strength, measured at ambient temperature, of approximately 827 MPa or more and wherein, following step v), the nickel- based alloy is characterized by a yield strength, measured at ambient temperature, of approximately 950-970 MPa.
Method according to any of the previous claims, wherein the nickel-based alloy forged and solution treated in step i) consists of, expressed as percentages by weight: C = 0.022 max, Si = 0.20 max, Mn = 0.20 max, Cr = 21.0-23, Ni = 57.0-59.0, Mo = 5.5-6.0, Al = 0.30-0.60, Ti = 0.70-1.0, Nb = 4.5-5.0, Fe = 5,0 - 12,0.
Method according to any of the previous claims, wherein the nickel-based alloy forged and solution treated in step i) consists of, expressed as percentages by weight: Ni = 58, Cr = 21.5, Mo = 5.8, Nb = 4.8, Ti = 0.9, Al = 0.4, Fe = 8%.
Method according to any of the previous claims, wherein step ii) is performed for about 3-6 hours.
Method according to any of the previous claims, wherein steps of cooling iii) and v) are carried out in air at ambient temperature, namely at a temperature outside the heated environment in which the ageing steps ii) and iv) are performed, to about an ambient temperature of the respective products.
Nickel-based alloy, for example made by the method according to claims 1-11, comprising a metal mass consisting of, expressed in percentages by weight: C = 0.030 max, Si = 0.50 max, Mn = 0.50 max, Cr = 20.0-24.0, Ni = 55.0- 60.0, Mo = 5.5 - 7.0, S = 0.005 max, P = 0.015 max, Cu = 1.0 max, Co = 1.0 max, Al = 0.80 max, Ti = 0.50 - 1.50, Nb = 4.0 - 5.5 and Fe = 5,0 - 15,0;
said alloy being characterised in that it comprises metal hardening phases γ' and γ" precipitated in an essentially non-intergranular position, and carbide phases precipitated in a discontinuous manner at least along the boundary of said grains.
Use of the alloy according to claims 12 or obtained through the method of claims 1-11 for making equipment and pipes for the chemical or petrol industries.