Patent Application
20210079499
The present disclosure relates to an austenitic alloy having a high content of Ni, Mo and Cr which will, after solidification, have a low content of intermetallic phases (less than 0.3%). The present disclosure also relates to the use of the austenitic alloy in different products and to a method for manufacturing such an alloy.
Nickel-base alloys are used in many corrosive applications where the corrosion resistance and the stability of the microstructure of today's stainless steels are insufficient. However, there are problem associated with these alloys as they are prone to form microsegregations during the solidification process and thereby form unwanted intermetallic phases. These will, in turn, cause poor ductility and poor corrosion properties. The content of intermetallic phases may be reduced by using certain manufacturing methods, such as remelting and soaking, but these methods are very expensive.
Thus, there is a need of a nickel-base alloy having a low content of intermetallic phases, which can be manufactured by conventional metallurgical methods.
One aspect of the present disclosure is therefore to solve or to at least reduce the problems mentioned above. Thus, the present disclosure provides an austenitic alloy comprising in weight % (wt %):
E
Ni>1.864*ECr−19.92
wherein
E
Cr=[wt % Cr]+[wt % Mo]+1.5*[wt % Si] and
E
Ni=[wt % Ni]+30*[wt % C]+30*[wt % N]+0.5*[wt % Mn]+0.5*[wt % Cu].
The austenitic alloy as defined hereinabove or hereinafter will have a good corrosion resistance and a high ductility as the austenitic alloy will comprise less than 0.3% intermetallic phases after solidification which means that there will be less intermetallic phases present in the austenitic alloy. The intermetallic phases have a negative impact on any of the processes performed after solidification.
The present disclosure also relates to an object comprising the austenitic alloy as defined hereinabove or hereinafter. Examples of, but not limiting to, an object is a tube, a pipe, a bar, a rod, a hollow, a billet, a bloom, a strip, a wire, a plate and a sheet.
Furthermore, the present disclosure also provides a method for manufacturing an austenitic alloy comprising the following elements in weight %:
E
Ni>1.864*ECR−19.92
wherein
E
Cr=[wt % Cr]+[wt % Mo]+1.5*[wt % Si] and
E
Ni=[wt % Ni]+30*[wt % C]+30*[wt % N]+0.5*[wt % Mn]+0.5*[wt % Cu].
By integrating the above-mentioned steps into conventional metallurgical manufacturing processes, the obtained final object will have a low content of intermetallic phases, such as <0.3%.
The present disclosure relates to an austenitic alloy comprising the following elements in weight %:
E
Ni>1.864*ECr−19.92
wherein
E
Cr=[wt % Cr]+[wt % Mo]+1.5*[wt % Si] and
E
Ni=[wt % Ni]+30*[wt % C]+30*[wt % N]+0.5*[wt % Mn]+0.5*[wt % Cu].
The austenitic alloy of the present disclosure will have a low fraction (amount) of intermetallic phases (less than 0.3%) formed in the interdendritic areas during the solidification process. The fraction is calculated by dividing the volume of intermetallic phases in the interdendritic areas with the total volume of the material. Examples of intermetallic phases are sigma phase, laves phases and chi-phase.
Solidification is a phase transformation wherein an alloy will transform from a liquid phase to a solid crystalline structure phase. The solidification process starts with the formation of dendrites and during the solidification process microsegregation will occur. Microsegregation is an uneven distribution of alloying elements between the solidified dendrites which will promote the formation of unwanted intermetallic phases. The area between the dendrites is called interdendritic area. Typical solidification processes, but not limited to, are casting such as ingot casting, continuous casting and remelting.
The austenitic alloy as defined hereinabove or hereinafter will due to the low content of intermetallic phases in the intermetallic areas have a good corrosion resistance and a very good ductility. The austenitic alloy will therefore be very suitable for use in applications wherein high resistance to corrosion is necessary, such as in oil and gas industry, petrochemical industry and chemical industry. Furthermore, according to one embodiment of the present disclosure, in order for the alloy to have even better corrosion resistance, the austenitic alloy as defined hereinabove or hereinafter may also fulfill the condition of having a critical pitting temperature (CPT) greater than 88° C.
The present disclosure also relates an object comprising the austenitic alloy as defined hereinabove or hereinafter. Examples of an object, but not limited thereto, is a tube, a bar, a pipe, a rod, a hollow, a billet, a bloom, a strip, a wire, a plate and a sheet. Further examples include production tubing and heat exchanger tubing.
Hereinafter, the alloying elements of the austenitic alloy as defined hereinabove or hereinafter are discussed in regard to their contribution to the properties of the alloy. It should be noted that the alloying elements could also contribute to other properties of the austenitic alloy even though it is not mentioned herein. The figures given herein are given in weight % (wt %):
Carbon (C): ≤0.03 wt %
C is an impurity contained in austenitic alloys. When the content of C exceeds 0.03 wt %, the corrosion resistance is reduced due to the precipitation of chromium carbide in the grain boundaries. Thus, the content of C is ≤0.03 wt %, such as ≤0.02 wt %.
Silicon (Si): ≤1.0 wt %
Si is an element which may be added for deoxidization. However, Si will promote the precipitation of the intermetallic phases, such as the sigma phase, therefore Si is contained in a content of ≤1.0 wt %, such as ≤0.5 wt %, such as ≤ to 0.3 wt %. According to one embodiment the lower limit of Si is 0.01 wt %.
Manganese (Mn): ≤1.5 wt %
Mn is often used to for binding sulphur by forming MnS and thereby increasing the hot ductility of the austenitic alloy. Mn will also improve deformation hardening of the austenitic alloy during cold working. However, a too high content of Mn will reduce the strength of the austenitic alloy. Accordingly, the content of Mn is set at ≤1.5 wt %, such as ≤1.2 wt %. According to one embodiment, the lower limit of Mn is lower 0.01 wt %.
Phosphorus (P): ≤0.03 wt %
P is an impurity contained in the austenitic alloy and is well known to have a negative effect on the hot workability and the resistance to hot cracking. Accordingly, the content of P is ≤0.03 wt %, such as ≤0.02 wt %.
Sulphur (S): ≤0.03 wt %
S is an impurity contained in the austenitic alloy, and it will deteriorate the hot workability. Accordingly, the allowable content of S is ≤0.03 wt %, such ≤0.02 wt %.
Copper (Cu): ≤0.4 wt %
Cu may reduce the corrosion rate in sulphuric acids. However, Cu will reduce the hot workability, therefore the maximum content of Cu is ≤0.4 wt %, such as ≤0.25 wt %. According to one embodiment, the lower limit of Cu is 0.01 wt %.
Nickel (Ni): 42.0 to 52.0 wt %
Ni is an austenite stabilizing element. Furthermore, Ni will also contribute to the resistance to stress corrosion cracking in both chlorides and hydrogen sulfide environments. Thus, a content of Ni of 42.0 wt % or more is required. However, an increased Ni content will decrease the solubility of N, therefore the maximum content of Ni is 52.0 wt %. According to one embodiment of the present austenitic alloy, the content of Ni is of from 43.0 to 51.0 wt %, such as of from 44.0 to 51.0 wt %.
Chromium (Cr): 25.0 to 33.0 wt %
Cr is an alloying element that will improve the pitting corrosion resistance. Furthermore, the addition of Cr will increase the solubility of N. When the content of Cr is less than 25.0 wt %, the effect of Cr is not sufficient for corrosion resistance, and when the content of Cr exceeds 33.0 wt %, secondary phases as nitrides and intermetallic phases will be formed, which will affect the corrosion resistance negatively. Accordingly, the content of Cr is of from 25.0 to 33.0 wt %, such as 25.5 to 32.0 wt %.
Molybdenum (Mo): 6.0 to 9.0 wt %
Mo is an alloying element which is effective in stabilizing the passive film formed on the surface of the austenitic alloy. Furthermore, Mo is effective in improving the pitting corrosion. When the content of Mo is less than 6.0 wt %, the resistance for pitting corrosion in harsh environments is not high enough and when the content of Mo is more than 9.0 wt %, the hot workability is deteriorated. Accordingly, the content of Mo is of from 6.0 to 9.0 wt %, such as of from 6.1 to 9.0 wt %, such as 6.4 to 9.0 wt %, such as of from 6.4 to 8.0 wt %.
Nitrogen (N): 0.07 to 0.11 wt %
N is an effective alloying element for increasing the strength of the austenitic alloy by using solution hardening and it is also beneficial for the improving the structure stability. The addition of N will also improve the deformation hardening during cold working. For having these effects in the present alloy, the content of N must be above 0.07 wt %. However, when the content of N is more than 0.11 wt %, then the flow stress will be too high for efficient hot working and the resistance against pitting corrosion will be reduced. Thus, the content of N is of from 0.07 to 0.11 wt %.
The austenitic alloy as defined hereinabove or herein after may optionally comprise one or more of the following elements Al, Mg, Ca, Ce, and B. These elements may be added during the manufacturing process in order to enhance e.g. deoxidation, corrosion resistance, hot ductility or machinability. However, as known in the art, the addition of these elements and the amount thereof will depend on which alloying elements are present in the alloy and which effects are desired. Thus, if added the total content of these elements is ≤1.0 wt %, such as ≤0.5 wt %.
According to one embodiment, the austenitic alloy consists of all the alloying elements mentioned hereinabove or hereinafter in the ranges mentioned hereinabove or hereinafter.
The term “impurities” as referred to herein means substances that will contaminate the austenitic alloy when it is industrially produced, due to the raw materials, such as ores and scraps, and due to various other factors in the production process and are allowed to contaminate within the ranges not adversely affecting the properties of the austenitic alloy as defined hereinabove or hereinafter. Examples of allying elements which are considered to be impurities are Co and Sn. Carbide formers, such as Nb and W, are considered in the preset disclosure to be impurities and/or trace elements and if present they are only present in very low levels, meaning they will not form any carbides, and thus will not have an impact on the final properties of the austenitic alloy.
The present disclosure also provides a method for manufacturing an austenitic alloy having the composition of the following elements in weight % (wt %):
wherein the austenitic alloy will have an intermetallic content of less than 0.3% after solidification, wherein said method comprises the steps of:
E
Ni>1.864*ECr−19.92
wherein
E
Cr=[wt % Cr]+[wt % Mo]+1.5*[wt % Si] and
E
Ni=[wt % Ni]+30*[wt % C]+30*[wt % N]+0.5*[wt % Mn]+0.5*[wt % [Cu].
The inventors have by thorough investigations surprisingly found that by integrating this method into conventional metallurgical manufacturing processes, the object obtained thereof will have a low content of intermetallic phases after solidification which will have a positive impact on the outcome of other metallurgically processes used.
Per one embodiment of the present method as defined hereinabove or hereinafter, the equation of may also be used when designing the austenitic alloy, i.e. before the austenitic alloy is melted.
The analyzing of the melt may be performed using e.g. X-ray fluorescence spectrometry, Spark discharge optical emission spectrometry, Combustion analysis, Extraction analysis and Inductively coupled plasma optical emission spectrometry. The obtained element content from the analyze is then inserted into the equation. If the condition (equation) is not fulfilled, then alloying elements are added until the equation is fulfilled. When the additional alloying elements have been added, the melt may be analyzed again, and these steps may be repeated several times until the equation (condition) is fulfilled.
According to yet another embodiment of the present method, optionally samples may be taken from the austenitic alloy after solidifying for measuring and verifying the intermetallic phases.
According to one embodiment of the method as defined hereinabove or hereinafter, the solidifying method is casting.
After the solidifying step, the method may comprise conventional metal manufacturing steps such as hot working and/or cold working. The method may optionally comprise heat treatment steps and/or aging steps. Examples of hot working processes are hot rolling, forging and extrusion. Examples of cold working processes are pilgering, drawing and cold rolling. Examples of heat treatment processes are soaking and annealing, such as solution annealing or quench annealing. Examples of, but not limited to, objects, which may be obtained by the method as defined hereinabove or hereinafter is a tube, a pipe, a bar, a, rod, a hollow, a billet, a bloom, a strip, a wire, a plate and a sheet.
The present disclosure is further illustrated by the following non-limiting examples.
The alloys of Table 1 were made by melting in a HF (High Frequency) induction furnace of 270 kg and thereafter they were made into ingots by casting into 9″mould. After casting and solidification, the moulds were removed and the ingots were quenched in water. The compositions of the experimental heats, Cr- and Ni-equivalents and fraction of intermetallic phases in interdendric areas are given in Tables 1 and 2.
Samples were cut from the upper part of the ingots and were metallographically prepared and etched in Beraha etchant 9b. This etching showed the dendrite structure and intermetallic phases. Light optical microstructure (LOM) studies (light optical microscope Nikon) were performed in order to investigate the intermetallic phase. The percentage (%) of intermetallic phases in interdendritic areas were measured by using an inserted grid of 10×10 lines in magnification 200 times and then by counting the number of intersections in the grid which hit the intermetallic phases in the interdendritic area and divided by the total number intersections. A total of 10 randomly positioned fields over the metallographical sample were measured to decide the fraction of intermetallic phases in interdendritic area.
Two typical examples of the microstructure, one with intermetallic phases in interdendritic areas, see
The Cr- and Ni-equivalents of the heats are plotted in
Three specimens were obtained from the cold rolled and solution annealed material and were tested for pitting corrosion according to ASTM G150 with 3 M MgCl2 as electrolyte. The average corrosion pitting temperature (CPT) values for each heat are shown in the table below:
| Number | Date | Country | Kind |
|---|---|---|---|
| 18173865.9 | May 2018 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/063297 | 5/23/2019 | WO | 00 |
