PROCESS FOR MAKING THE SAME

DESCRIPTION

FIELD Embodiments of the subject matter disclosed herein relate in general to components for turbomachines and to turbomachines for "Oil & Gas" applications.

Some embodiments relate to (rotary) centrifugal compressors or pumps, as well as their components, operating in the field of production and treatment of oil and gas containing e.g . hydrocarbon plus hydrogen sulfide, carbon dioxide, with or without other contaminants. These materials are referred as "sour gas". Such apparatuses have at least one component made of a high corrosion resistant alloy, capable of resisting to corrosion better than state of art martensitic stainless steels and behaving similarly to premium nickel base superalloys.

Some embodiments relate to (rotary) gas turbines or steam turbines, as well as their components. Such apparatuses have at least one component made of a high mechanical resistant alloy, capable of resisting to fatigue and/or creep better than state of art materials. BACKGROUND ART

A compressor is a machine capable of raising the pressure of a compressible fluid (gas) through the use of mechanical energy. In centrifugal compressors, the compression of the fluid is carried out by one or more impellers assembled on a shaft with a rotating motion inside one or more stator parts (diaphragm) stacked together by bolts. The described assembly is normally called bundle. The fluid to be compressed is drawn into the bundle through one or more intake ducts, whereas the compressed fluid is expelled from the bundle towards one or more delivery ducts.

Commonly, the centrifugal compressors are actuated by electric motors or else by internal combustion engines, through a coupl ing for transmitting the motion .

Centrifugal compressors that operate in sour gas fields are subject to different type of interaction with the environment (corrosion) that can cause loss of performance and premature failure of compressor components. The sour service is characterized by hydrocarbons with wet hydrogen sulph ide (H ₂ S) where the pH ₂ S is higher than 0.0030 bar. This value is val id for carbon and low alloy steels. NACE MR0175/ISO 1 51 56-1 doesn't define a minimum pH ₂ S l imit for corrosion resistant alloys (CRAs), because this limit is a function also of acidity of the solution (pH) and the values can be lower than the one defined for carbon and low alloy steels.

There are several corrosion phenomena, where the following types are the most relevant:

General corrosion - an even attack of the surface of the material - Pitting corrosion - an uneven local ized attack

Stress corrosion cracking (SCC and CSCC)

It is pointed out that corrosion phenomena listed above can only occur if condensed water is present (wet gas), that acts as electrolyte for electrochemical process. Wet gas containing hydrocarbons, CO ₂ , H ₂ S, and chlorides (or other halides) eventually in presence of elemental sulphur, represents an environment where all the phenomena listed above can occur. Resistance of material to single or combination of damage mechan isms is therefore fundamental in order to guarantee rel iabil ity of products.

Among the corrosion mechan ism l isted above, the most critical is the stress corrosion cracking either by wet H ₂ S or chlorides (or in general hal ides), because it makes unavailable the un it for service.

In general , the mechan ism involves the d iffusion in the metal of hydrogen atoms generated by corrosion .

SSC can occur only if the following three cond itions are verified :

Tensile stress (residual and/or appl ied) · H ₂ S + condensed water

Material prone to SSC damage

Contam inants such as hal ides, arsen ic (As), antimony (Sb) and cyan ides (CN ^" ) act as catalyst, increasing the concentration of hydrogen atoms on surface and by preventing their recombination in hydrogen molecules making SSC more severe.

In general , centrifugal compressor components (impellers, shafts, d iaphragms and bolts) are exposed to tensile stress and wet gas cond itions.

Based upon experience, it has been found that the impellers and bolts constitute the most prone components to SSC and CSCC. Th is because the stress level is h igher than in the other components and because the stress remains appl ied during compressor stops (pressurized) where a wet gas at h igher partial pressure occurs. Therefore is mandatory, for sour service environments, to select materials that are able to withstand the severe environment cond itions. Material selection for such service is therefore based on a three dimensional space governed by partial pressure of H ₂ S (p(H ₂ S)), pH (mainly function of CO ₂ ), and chlorides (and/or other halides) content, as schematically represented in Figure 1 . Up to now different materials have been used with the aim of selecting the most cost effective solution for the specified environment.

In order to simplify the complex rules behind material fit for purpose approach, the following principles cloud be considered:

For low p(H ₂ S) any pH, and high chlorides content duplex and superduplex alloys are the class of material of choice;

For low to moderate p(H ₂ S), any pH and low chlorides different classes of martensitic stainless steels are the class of material of choice;

For any p(H ₂ S), any pH and high chlorides nickel based alloys are the class of material of choice;

Representing these principles above in the 3D space, it is clear that there is a huge space between cost effective alloys (i.e. duplex, superduplex and martensitic stainless steels) and premium nickel base alloys, that could be covered by new alloys. Therefore, there is a need for components for centrifugal compressors in particular, but not exclusively on compressors operating in the field of production and treatment of oil and gas containing hydrocarbon plus hydrogen sulfide with or without other contaminants, capable of improving the reliability, increase the speed (given the higher specific strength of material) and provide a cost effective alloy by reducing expensive alloying elements, mainly nickel . Similar problems need to be addressed in pump design and service conditions or in some steam turbine application (i.e. geothermal fields).

A gas turbine is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between .

Atmospheric air flows through a compressor is brought to higher pressure in a combustion chamber where it is mixed and burnt with fuel (i .e l iquid or gas ) to increase its enthalpy. Th is high-temperature high- pressure flow enters in an expansion turbine, producing a shaft work output in the process. The turbine shaft work is used to drive the compressor and other devices such as an electric generator that may be coupled to the shaft.

This environment is characterized by a combination of high temperature, high stress in steady and cycl ing conditions. Materials for such application shall be designed to withstand creep, low and high cycle fatigue, oxidation and corrosion . This is normally accomplished by high strength steels or nickel base alloys.

Similar problems need to be addressed in steam turbine design and service conditions. The present Inventors have tried to achieve one or some or all of the above objects.

SUMMARY

According to first exemplary embodiments, there is a component of a turbomachine, the component being made of an alloy having a chemical composition consisting of:

C 0.005-0.03 wt%

Si 0.05-0.5 wt% Mn 0.1 -1 .0 wt%

Cr 1 9.5-22.5 wt%

Ni 35.0-37.0 wt%

Mo 3.0-5.0 wt%

Cu 1 .0-2.0 wt%

Co 0.0-1 .0 wt%

Al 0.01 -0.5 wt%

Ti 1 .8-2.5 wt%

Nb 0.2-1 .0 wt%

W 0.0-1 .0 wt% based on the alloy weight, the remaining being Fe and impurities, said impurities comprising S 0.0-0.01 wt% and P 0.0-0.025 wt%.

According to second exemplary embodiments, there is a process for making the above component, said process comprising at least one of the following steps of: a) melting the chemical composition of claim 1 through vacuum induction melting (VIM), or arc electric furnace; b) refining by Argon Oxygen Decarburization (A.O.D.), Vacuum Induction Degassing and Pouring (V. I .D.P), or Vacuum Oxygen Decarburization (V.O.D.); c) re-melting through electro-slag re-melting (E.S.R.), or vacuum arc re-melting (VAR).

According to third exemplary embodiments, there is a turbomachine comprising at least one component as defined in general above. BRIEF DESCRIPTION OF TH E DRAWINGS The present invention will become more apparent from the following description of exemplary embodiments to be considered in conjunction with accompanying drawings wherein :

Figure 1 shows a three dimensional space governed by partial pressure of H ₂ S (p(H ₂ S)), pH (mainly function of CO ₂ ), and chlorides (and/or other halides) content;

Figure 2 shows a typical cross section of centrifugal compressor;

Figure 3 shows a typical cross section of centrifugal pump;

Figure 4 shows a typical cross section of a steam turbine; Figure 5 shows a typical cross section of a gas turbine;

Figure 6A shows the phase equilibrium vs temperature of the alloy of Example 1 and Figure 6B shows the phase equilibrium vs temperature of the comparative UNS N0771 8; and

Figure 7A shows the Time Temperature Transformation curves for the alloy of Example 1 and Figure 7B shows the Time Temperature Transformation curves for the comparative UNS N0771 8.

DESCRIPTION

The following description of exemplary embodiments refer to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed . Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

The term "room temperature" as used herein has its ordinary meaning as known to those skilled in the art and may include temperatures within the range of about 1 6°C (60°F) to about 32°C (90°F).

Regarding the alloy composition, the term "mandatory element" refers to an element that is present in the alloy and that, in combination with the other mandatory elements, allows to achieve the above objects. The mandatory elements in the alloy are Iron (Fe), Carbon (C), Silicon (Si), Manganese (Mn), Chromium (Cr), Nickel (Ni), Molybdenum (Mo), Copper (Cu), Aluminium (Al), Titanium (Ti), and Niobium (Nb). The term "optional element" refers to an element that is possibly present in addition to the mandatory elements defining the essential chemical composition of the alloy. The optional elements in the alloy are: Cobalt (Co), and Tungsten (W).

The term "impurity" or "impurity element", instead, refers to an element not provided in the design of the alloy composition in order to reach the aforesaid objects. However, said element may be present because, depending on the manufacturing process, its presence may be unavoidable. Impurities in the alloy comprise phosphorous (P), Sulphur (S), Boron (B), Bismuth (Bi), Calcium (Ca), Magnesium (Mg), Silver (Ag), Lead (Pb), Nitrogen (N), Tin (Sn), and Oxygen (O).

In one embodiment, at least one component of a turbomachine is made of a high corrosion high temperature resistant alloy, capable of resisting to corrosion and/or stress at high temperature better than state of art martensitic stainless steels and behaving similarly to premium nickel base superalloys like those complying the requirements of UNS N0771 8 e U NS N00625. Said alloy has a chemical composition consisting of:

C 0.005-0.03 wt%

Si 0.05-0.5 wt%

Mn 0.1 -1 .0 wt%

Cr 1 9.5-22.5 wt%

Ni 35.0-37.0 wt%

Mo 3.0-5.0 wt%

Cu 1 .0-2.0 wt%

Co 0.0-1 .0 wt%

Al 0.01 -0.5 wt%

Ti 1 .8-2.5 wt%

Nb 0.2-1 .0 wt%

W 0.0-1 .0 wt% based on the alloy weight, the remaining being Fe and impurities, said impurities comprising S 0.0-0.01 wt% and P 0.0-0.025 wt%.

The above alloy is advantageously a cost effective alloy, which at the same time surprisingly encompasses a reduced amount of expensive alloying elements, such as mainly nickel, but also chromium, molybdenum and titanium, without negatively affecting the mechanical and anticorrosion properties. Said alloy also shows a great resistance to high temperatures and pressures, so that the components made of the same result to be advantageously suitable for turbomachines, particularly centrifugal compressors.

Said impurities are P, S, B, Bi, Ca, Mg, Ag, Pb, N , Sn, O or a combination thereof.

Preferably, said impurities are less than 0.5 wt%; more preferably, less than 0.2 wt% . In preferred embodiments, said impurities are P up to 0.025 wt%, S up to 0.01 wt%, B, Bi, Ca, Mg, Ag, Pb, N, Sn, and O.

In some embodiments, the alloy has high resistance to corrosion at high temperature, in particular in the range of 200-250°C. In other embodiments, the alloy has high resistance to fatigue and/or creep at high temperature, in particular in the range of 400-700°C.

In preferred embodiments, the alloy has a chemical composition consisting of:

C 0.005-0.03 wt%

Si 0.05-0.2 wt%

Mn 0.1 -0.6 wt%

Cr 20.0-21 .5 wt%

Ni 35.0-37.0 wt%

Mo 3.5-4.0 wt%

Cu 1 .2-2.0 wt%

Co 0.0-0.2 wt%

Al 0.05-0.4 wt%

Ti 1 .9-2.3 wt%

Nb 0.2-0.5 wt%

W 0.0-0.6 wt% based on the alloy weight, the remaining being Fe, with Fe at least 30 wt%, and impurities, said impurities comprising S 0.0-0.001 wt% and P 0.0-0.02 wt%.

In more preferred embodiments, the alloy has a chemical composition consisting of:

C 0.005-0.02 wt%

Si 0.05-0.2 wt%

Mn 0.1 -0.6 wt% Cr 20.0-21 .5 wt%

Ni 35.0-37.0 wt%

Mo 3.5-4.0 wt%

Cu 1 .2-2.0 wt%

Co 0.0-0.2 wt%

Al 0.05-0.4 wt%

Ti 1 .9-2.3 wt%

Nb 0.2-0.5 wt%

W 0.0-0.6 wt% based on the alloy weight, the remaining being Fe, with Fe at least 30 wt%, and impurities, said impurities comprising S 0.0-0.001 wt% and P 0.0-0.02 wt%.

In even more preferred embodiments, the alloy has a chemical composition consisting of:

C 0.005-0.02 wt%

Si 0.06-0.1 5 wt%

Mn 0.2-0.4 wt%

Cr 20.2-21 .0 wt%

Ni 36.0-36.5 wt%

Mo 3.6-3.8 wt%

Cu 1 .3-1 .7 wt%

Co 0.0-0.1 wt%

Al 0.1 -0.3 wt%

Ti 2.0-2.2 wt%

Nb 0.25-0.4 wt%

W 0.0-0.4 wt% based on the alloy weight, the remaining being Fe, with Fe at least 30 wt%, and impurities, said impurities comprising S 0.0-0.001 wt% and P 0.0-0.015 wt%. In the most preferred embodiments, the alloy has a chemical composition consisting of: c 0.01 5 wt%

Si 0.09 wt%

Mn 0.3 wt%

Cr 20.4 wt%

Ni 36.2 wt%

Mo 3.7 wt%

Cu 1 .41 wt%

Co 0.03 wt%

Al 0.25 wt%

Ti 2.04 wt%

Nb 0.27 wt%

W 0.1 wt%

Fe balance having the following impurities

P up to 0.01 3 wt%

S up to 0.0002 wt%

B up to 0.003 wt%

Bi up to 0.3 ppm

Ca up to 50 ppm

Mg up to 30 ppm

Ag up to 5 ppm

Pb up to 5 ppm

N up to 1 00 ppm

Sn up to 50 ppm

O up to 50 ppm

In some embodiments, the alloy has a grain size finer than plate 3 as per ASTM E1 1 2. The above alloy can be obtained by any casting process. However, it is preferred to obtain said alloy by a process comprising at least one of the following steps of: a) melting the above chemical composition through vacuum induction melting (VIM), or arc electric furnace; b) refining by Argon Oxygen Decarburization (A.O.D.), Vacuum Induction Degassing and Pouring (V. I .D.P), or Vacuum Oxygen Decarburization (V.O.D.); c) re-melting through electro-slag re-melting (E.S.R.), or vacuum arc re-melting (VAR).

In this way, the presence of impurities, segregation thereof and in-homogeneities is significantly reduced and at the same time improved mechanical characteristics and corrosion resistance of the alloy are achieved . In some embodiments, the alloy resulting from the above described casting processes is subjected to a step d) of homogenization at a high temperature, preferably above 1 1 00° C for at least 6 hours.

In some embodiments, the alloy resulting from the above described casting processes and subsequent step d) of homogenization heat treatment, is further subjected to a step e) of hot or cold plastic deformation through at least one plastic deformation cycle, in order to attain a minimum total reduction ratio of 2: 1 . Such plastic deformation cycles include forging (open or close die), rolling, extrusion, cold expansion, to produce a raw component shape or more generally a raw shape to be further machined to produce centrifugal compressor, pump, gas and steam turbine, as well as components thereof.

In other embodiments, the alloy resulting from step e) is then subjected to a step f) of heat treatment to induce solubil ization through at least one heat cycle, preferably at a temperature of 1 020-1 1 50°C, that can be carried out inside furnaces, under air, controlled atmosphere or vacuum, and followed by fast cool ing in l iquid or gas med ia, in order to put and keep in solution the alloying elements (i .e copper, titanium, aluminium, niobium, etc .. ) for optional subsequent heat treatment steps.

In further embodiments, said step f) of heat treatment is followed by a step g) of an ageing treatment.

Preferably, said step g) of an ageing treatment comprises the following sub-steps: g-1 ) heating the alloy to a temperature of 71 0-780°C for 4-8 h; g-2) cooling at a cooling rate of 40-60°C/h down to a temperature of 61 0-670°C; g-3) keeping the alloy to a temperature of 61 0-670°C for at least 6 h, and g-4) letting the alloy to cool at room temperature in air.

Alternatively, said step g) of an ageing treatment comprises the following sub-steps: g-1 ') heating the alloy to a temperature of 780-820°C for 2-8 h; and g-2') letting the alloy to cool at room temperature in air.

Owing to the above described chemical composition, level of impurities, grain size resulting from controlled plastic deformation process and heat treatment conditions, the alloy advantageously shows the following properties: - superior anticorrosion characteristics in terms of general and local ized corrosion, threshold stress in solution A method A as per NACE MR0175, higher Stress Corrosion Cracking (SCC) resistance, higher Chloride Stress Corrosion Cracking (CSCC), Sulphide Stress Cracking (SSC), Galvanically-induced Hydrogen Stress Cracking (GHSC);

- higher tensile properties at room and high temperature;

- suitable toughness properties;

- higher high and low cycle fatigue properties; - higher creep strength;

- higher oxidation and hot corrosion resistance; with respect to stainless steels (martensitic, ferritic, austenitic and austenitic-ferritic) and comparable to premium nickel base superalloys.

In some embodiments, the alloy is further atomized to produce powder and then treated by powder metallurgy. Preferably, with the term "powder metallurgy" it is meant that said powder is consolidated by Cold Isostatic Pressing (CIP), by Metal Injection Moulding (MIM), Sintering, Hot Isostatic Pressing (H IP), or fabricated by MIM and exposed to a H IP process. Basically, powders are fed into a die, compacted to a desired shape. The pressed powder is then sintered or hipped in a controlled atmosphere furnace at room or high pressure to produce metallurgical bonds among powder particles. Optional post- sintering operations, such as isothermal forging, infiltration, finish machining or surface treatment, may then be applied to complete the component.

Figures 2, 3, 4 and 5 show different turbomachines where one or more components as set out above may be used . Figure 2 shows a typical cross section of centrifugal compressor, Figure 3 shows a typical cross section of centrifugal pump, Figure 4 shows a typical cross section of a steam turbine, and Figure 5 shows a typical cross section of a gas turbine. Thanks to its high resistance to corrosion (even at high temperature) and/or to its high resistance to fatigue and/or creep, the component is very useful, in particular it is very useful for components that get in touch with the working fluid of the turbomachine.

EXAMPLES Example 1 .

An alloy has been prepared having the following composition :

C 0.01 5 wt%

Si 0.09 wt%

Mn 0.3 wt%

Cr 20.4 wt%

Ni 36.2 wt%

Mo 3.7 wt%

Cu 1 .41 wt%

Co 0.03 wt%

Al 0.25 wt%

Ti 2.04 wt%

Nb 0.27 wt%

W 0.1 wt%

Fe balance having the following impurities:

P up to 0.01 3 wt%

S up to 0.0002 wt%

B up to 0.003 wt% Bi up to 0.3 ppm

Ca up to 50 ppm

Mg up to 30 ppm

Ag up to 5 ppm

Pb up to 5 ppm

N up to 1 00 ppm

Sn up to 50 ppm

O up to 50 ppm

The above chemical composition was melted through vacuum induction melting (VIM), refined by Argon Oxygen Decarburization (A.O.D.), and re-melted re-melting through electro-slag re-melting (E.S.R.).

The resulting alloy was homogenized at a temperature above 1 1 00° C for at least 6 hours.

The alloy was then subjected to two cycles of hot plastic deformation . Subsequently, the alloy was subjected to a heat treatment to induce solubilization at a temperature of 1 020-1 1 50°C, followed by fast cooling in liquid or gas media.

The resulting alloy has been tested to assess mechanical and anticorrosion properties. The results have been compared to a known Martensitic Stainless Steel (shortly 'Martensitic SS') in the following Table 1 .

Martensitic Stainless steels is a category of stainless steels having a microstructure mainly composed by tempered Martensite. Matertensite is formed by a rapid cooling of austenite phase which is achieved by a quenching heat treatment. Traditional martensitic steels have a high carbon content, in the 0.08-1 .% range, a Chromium in the 1 2-1 7% range. Their main characteristic compared other stainless steel classes is the high strength and fair corrosion resistance. Table 1 .

Additional verified SSC properties are reported in Table 2 and Table 3. Table 2.

Table 3.

The alloying elements' weight percent is tailored to avoid or minimizing topologically closed packed phases (TCP). Excessive quantities of Cr, Mo, W would promote the precipitation of intermetall ic phases which are rich in these elements. Generally speaking, the TCP phases have chemical formulae A _x B _y . For example, the μ phase is based on the ideal stoichiometry A ₆ B ₇ and has a rhombohedral cell containing 1 3 atoms, such as W ₆ Co ₇ and Mo6Co ₇ .

The σ phase is based upon the stoich iometry A ₂ B and has a tetragonal cell containing 30 atoms, such as Cr ₂ Ru, Cr ₆ i C039 and Re67Mo33- The P phase, for example, Cri ₈ Mo ₄ 2N i ₀ is primitive orthorhombic, containing 56 atoms per cell .

As it is shown in Figures 6A (thermodynamic equilibrium) and 7A (kinetics estimation), only σ phase is thermodynamically possible and precipitation kinetics is so slow that neither during solution annealing, nor during ageing can happen .

The chemical composition of this alloy is optimized to enlarge the hot workability window. This is accomplished by a low nickel content and reducing the temperature of precipitation of hardening secondary phases (gamma prime). As it can be seen in Figure 6, the theoretic workability range at equilibrium is quite large and is between 1 020°C and 1 280°C. This interval is larger than those provided by UNS N0771 8 (Figures 6B and 7B).

Equilibrium intervals do not take into account kinetics and visco-plastic phenomena, but can give an idea of how much better this alloy behaves in comparison with other well known commercial premium nickel base alloys.

Practically, this alloy has a hot forming range between 900°-1 200°C, thus reducing the risk of failure during production and cycle time.

The alloy has a combination of chemical elements so as to provide secondary phases hardening such as to provide a minimum yield strength of 750 Mpa with a max hardness of 34 HRC thus enhancing stress corrosion properties. The reduced hardness level results in a better machining if compared with premium nickel based alloys like UNS N0771 8. This level of hardness allows the turbomachinery components to be machined in aged conditions resulting in an optimization of manufacturing cycle if compared with premium nickel based alloys like UNS N0771 8.

This alloy is designed to be easy welded by common arc welding processes (SMAW and GTAW) with homologous or different nickel base filler materials like UNS N06625, U NS N07725, or UNS N09925.