Technical field

The present disclosure relates to an austenitic nickel base alloy having a high content of Ni, Mo and Cr, which is suitable for use in welding applications as it will provide a surprisingly low amount of grain boundary precipitates in the heat affected zone. The present disclosure also relates to objects comprising the austenitic nickel base alloy and to the use thereof.

Background

Many nickel base alloys used today in wet corrosion applications comprises a high level of molybdenum (10 to 16 weight%). These alloys have a problem with precipitation of intermetallic phases in the grain boundaries during welding as these phases will decrease the microstructure stability and thereby decrease the corrosion, especially when objects made of these alloys are welded.

There is therefore a need for an austenitic nickel base alloy avoiding these problems.

Summary

The present disclosure therefore relates to an austenitic nickel base alloy comprising the following elements in weight% :

C < 0.03;

Si < 1.0;

Mn < 1.5;

S < 0.03;

P < 0.03;

Cr 25.0 to 35.0;

Ni 42.0 to 52.0;

Mo 6.0 to 9.0;

N 0.07 - 0.12; Cu < 0.4;

Balance Fe and unavoidable impurities;

and characterized in that the following conditions are fulfilled:

Cr _eq /Ni _eq is equal to or less than 0.80; and

PRE is equal or greater than 90;

wherein

Cr _eq is [wt%Cr] + [wt%Mo] + l.5*[wt%Si] + 0.5*[wt%Nb];

Ni _eq is [wt%Ni] + 30*[wt%C] + 30*[wt%N] + 0.5*[wt%Mn]; and

PRE is [wt%Cr] + l0*[wt%Mo] + 20*[wt%N].

The extent of grain boundary precipitation has a strong influence on primarily corrosion resistance but also on ductility, fracture toughness and/or formability. It has surprisingly been found that by fulfilling the conditions above, a low content of intermetallic phases is obtained in the grain boundaries in the heat affected zone. This means that the grain boundary rating will be low, which in turn means that the present austenitic nickel base alloy will have a stable microstructure stable in the heat affected zones. Furthermore, by fulfilling these conditions, the present austenitic nickel base alloy will have a good corrosion resistance, especially a good corrosion pitting resistance.

The present disclosure also relates an object comprising the austenitic nickel base alloy as defined hereinabove or hereinafter. Examples, but not limited thereto, of objects are a tube, a pipe, a bar, a rod, a hollow, a billet, a bloom, a strip, a wire, a plate and/or a sheet.

Brief description of the Figures

Figure 1 shows the heating and cooling cycles used in order to simulate the heat affected zone in multipass welding;

Figure 2 shows the result of the simulation wherein the decorated grain boundary in the form of grain bonding rating has been plotted against Cr _eq /Ni _eq . Detailed description

The present disclosure relates to a austenitic nickel base alloy comprising the following in weight% :

C < 0.03;

Si < 1.0;

Mn < 1.5;

S < 0.03;

P < 0.03;

Cr 25.0 to 35.0;

Ni 42.0 to 52.0;

Mo 6.0 to 9.0;

N 0.07 - 0.12;

Cu < 0.4;

Balance Fe and unavoidable impurities;

and fulfilling the following conditions:

Cr _eq /Ni _eq is equal or lower than 0.80; and

PRE is equal or greater than 90;

wherein Cr _eq is [wt%Cr] + [wt%Mo] + l.5*[wt%Si] + 0.5*[wt%Nb]

Ni _eq is [wt%Ni] + 30*[wt%C] + 30*[wt%N] + 0.5*[wt%Mn]

PRE is [wt%Cr] + l0*[wt%Mo] + 20*[wt%N].

As stated above, it has surprisingly been found that an austenitic alloy comprising the alloying elements as disclosed hereinabove or hereinafter in the ranges as disclosed hereinabove or hereinafter will have a low fraction of intermetallic phases in the grain boundaries in the heat affected zone, meaning that the grain boundary rating will be low. As the grain boundary rating will be low, the structural stability of the heat affected zone will be high. This is important for the weldability, particularly in multipass welding of large wall thicknesses where the heat affected zone is subjected to several temperature peaks. The austenitic nickel base alloy as defined hereinabove or hereinafter will also have good corrosion resistance due to a pitting resistance equivalence (PRE) above 90 and a low fraction of grain precipitation. Both these conditions are necessary for an alloy to be suitable for use in corrosive environments. The present austenitic nickel base alloy will therefore be very suitable for use in applications requiring high corrosion resistance and in corrosive environments, such as in the field of oil and gas industry, petrochemical industry and/or chemical industry.

The austenitic nickel base alloy as defined hereinabove or hereinafter may be comprised (found) in different products such as a bar, a seamless or welded tube, a sheet, a plate, a wire and/or a strip and a rod. Further examples include production tubing and heat exchanger tubing.

Hereinafter, the alloying elements of the austenitic nickel base alloy as defined hereinabove or hereinafter are discussed, weight% is 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 < to 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 the content of Si is < 1.0 wt%, such as< 0.5 wt%, such as < 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 nickel base alloy. Mn will also improve deformation hardening of the austenitic nickel base alloy during cold working. However, too high content of Mn will reduce the strength of the austenitic nickel base alloy. Accordingly, the content of Mn is set at < 1.5 wt%, such as < to 1.2 wt%. According to one embodiment, the lower limit of Mn is 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 nickel base alloy, and it will deteriorate the hot workability. Accordingly, the allowable content of S is < 0.03 wt%, such as < 0.02 wt%.

Copper (Cu): < 0.4 wt%

Cu may reduce the corrosion rate in sulphuric acids. However, Cu together with Mn 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 is 0.01 wt%.

Nickel (Ni): 42.0 to 52.0 wt%

Ni is an austenite stabilizing element as it will stable the austenitic microstructure in combination with Cr and Mo. 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 42.0 to 51.0 wt%.

Chromium (Cr): 25.0 to 35.0 wt%

Cr is an alloying element that will improve the stress corrosion cracking 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 35.0 wt%, secondary phases as nitrides and sigma phase will be formed, which will affect the corrosion resistance negatively. Accordingly, the content of Cr is of from 25.0 to 35.0 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 nickel base alloy. Furthermore, Mo is effective in improving the stress corrosion cracking resistance, especially in H ₂ S -environments. When the content of Mo is less than 6.1 wt%, the resistance for stress corrosion cracking resistance in H ₂ S- environments is not 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 6.1 to 9.0 wt%, such as of from 6.4 to 9.0 wt%.

Nitrogen (N): 0.07 to 0.12 wt%

N is an effective alloying element for increasing the strength of the austenitic nickel base 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. However, when the content of N is more than 0.12 wt%, the flow stress will be too high for efficient hot working and the stress corrosion cracking resistance will also be reduced. Thus, the content of N is of from 0.07 to 0.12 wt%.

The austenitic nickel base 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 less than or equal to 1.0 wt%, such as 0.5 wt%. According to one embodiment, the austenitic nickel base 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 nickel base 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 nickel base alloy as defined hereinabove or hereinafter.

The alloy as defined hereinabove or hereinafter may be manufactured by using conventional metallurgical manufacturing methods, for example by manufacturing methods comprising steps such as hot working and/or cold working. The manufacturing 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.

The present disclosure is further illustrated by the following non- limiting examples.

EXAMPLES

The present disclosure is further illustrated by the following non- limiting examples. Example 1

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 ingot by casting into 9"mould. After casting, the molds were removed and the ingots were quenched in water. The

compositions of the experimental heats are given in Table 1. The ingots were forged to flat bars and hot rolled to plates 10 mm in thickness. After quench annealing at l200°C for 15 minutes followed by water quenching, and pickling, the plates were cold rolled to 2 mm thickness. The cold rolled material was quench annealed at l200°C for 10 minutes and subsequently machined to test coupons 2 x 20 x lOOm.

The PRE for each sample (alloy) was calculated using the formula:

[wt%Cr] + lO*[wt%Mo] + 20*[wt%N].

Samples were taken from each alloy.

Table 1 - Composition and PRE for each sample. The balance is iron (Fe) and unavoidable impurities. Samples within the present disclosure is marked with a

The nickel base alloys of table 1 were exposed to several heating and cooling cycles using a resistance heated thermal simulator (Gleeble) in order to simulate the temperature history created by multipass welding for an alloy in the heat affected zone (see figure 1). For each alloy one sample was used for evaluation of the microstructure.

The microstructure was evaluated using light optical microscope (Leica) after etching in oxalic acid. Evaluation of portion decorated grain boundaries has shown to be a good measure of the structure stability in austenitic alloy. The evaluation was made by estimating the portion of decorated grain boundaries in five levels. See table 2, right column. This is a simplified method of evaluating the portion of grain boundaries decorated with intermetallic phases. Previous experience has shown that the results of this fast method is in good agreement with the results of the more time-consuming intercept method. Each heat was evaluated by ten fields of view. The results were subsequently converted to grain boundary rating according to table 2. Mean values of the ten fields were calculated for each heat, see table 3. The average grain boundary rating (GBR) was plotted versus Cr _eq /Ni _eq (see figure 2). As can be seen from figure 2, there is a drastic reduction of GBR when the Cr _eq /Ni _eq is less than or equal to 0.80 which means an improvement of the microstructure compared to that of an alloy with Cr _eq /Ni _eq greater than 0.80.

Table 2 Conversion table for grain boundary ratio ( GBR) and the number of decorated grain boundary ( GBD) obtained by experience

Table 3 Cr _eq and Ni _eq values together with average grain boundary rating ( GBR).