DE102020007257A1 - Copper-nickel cast alloy - Google Patents

Abstract

The invention relates to a copper-nickel cast alloy with the following composition in % by weight: Ni: 24 to 36% Mg: 1.5 to 6.0% Al: 1.5 to 6.0% optionally Ti: 0, 05 to 0.5% optional B: 0.01 to 0.15% optional Ca: 0.02 to 0.1% optional Fe: 0.05 to 1.0% balance copper, oxygen bound in magnesium oxide and unavoidable impurities.

Description

The invention relates to a copper-nickel cast alloy which also contains magnesium and aluminum.

Many copper materials are characterized by good corrosion resistance. Alloys with a specified property profile can be created by alloying in certain elements. Not only do the properties in the final state play an important role, but the alloys must also be suitable for the manufacture of semi-finished products such as rods, tubes or strips by hot and/or cold forming. This restriction does not apply to materials that are used in the cast condition. This opens up possibilities to optimize the properties of the alloy for special applications. In particular, applications at high temperatures are increasingly coming to the fore in various technical fields.

From the pamphlet EP 3 252 179 A1 a high-temperature copper alloy with the composition 10.6 to 18.0% Al, 10.5 to 14.5% Ni, the remainder copper is known. Fe, Co, Ti, Mn, B, Ca and C can be added to the alloy as optional elements. In the cast condition, the tensile strength at room temperature is at least 700 MPa.

Furthermore, from the pamphlet CN 103 469 006 A a copper alloy known, which is used in the cast state as a valve body. In addition to copper, the alloy includes 15-17% by weight nickel, 2-3% by weight aluminum, 8-9% by weight magnesium and small amounts of other elements. In order to produce a valve body of sufficient quality, a precisely specified procedure must be followed. No information can be found in the publication about the properties of the alloy at high temperatures.

The invention is based on the object of providing an alloy which, in the as-cast state, has a high high-temperature strength. It should also be resistant to corrosion and oxidation at high temperatures. In particular, the alloy should have high strength and good wear resistance at temperatures above 400°C.

The invention is represented by the features of claim 1. The further dependent claims relate to advantageous developments and refinements of the invention.

The invention includes copper-nickel cast alloy with the following composition in % by weight: Ni: 24 to 36% mg: 1.5 to 6.0% Al: 1.5 to 6.0% optional Ti: 0.05 to 0.5% optionally B: 0.01 to 0.15% optional Ca: 0.02 to 0.1% optionally Fe: 0.05 to 1.0%

Rest copper, oxygen bound in magnesium oxide and unavoidable impurities.

The alloy is a cast alloy with a dendritic structure after chill casting. The grain size is approx. 0.6 to 1 mm in the as-cast state. The dendritic phase mainly consists of a mixed crystal containing copper and nickel. A nickel content of at least 24% by weight is required in order to achieve sufficiently large solid solution hardening even at high temperatures. Nickel contents above 36% by weight would make the alloy expensive with no benefit to compensate for the increase in price.

The alloy is characterized by an accumulation of magnesium oxide in the interdendritic areas. Magnesium oxide has refractory properties, the melting point of magnesium oxide is 2800 °C. These interdendritic areas with magnesium oxide lead to a very good high temperature temperature resistance. The proportion of these interdendritic areas is at least 6% by volume. The magnesium content in the interdendritic areas is 9 to 14% by weight, depending on the magnesium content in the alloy.

A further strength-increasing effect in the dendrite arms themselves is provided by finely dispersed, approximately spherical, nickel- and aluminum-containing precipitations whose average equivalent diameter is 20 to 100 nm. Equivalent diameter here means the diameter of a sphere that has the same volume as the precipitate under consideration.

To achieve this structure and the resulting high-temperature properties, Mg and Al contents of at least 1.5% by weight are required. Contents of Mg and Al above 6.0 wt% would degrade the castability and mechanical properties of the alloy. A Mg content of more than 6% by weight would lead to embrittlement of the alloy, for example. Compositions with 3.0 to 5.0% by weight Mg and 3.0 to 5.0% by weight Al have particularly excellent high-temperature properties.

The ratio of the Mg component to the Al component can preferably be at least 0.8 and at most 1.2. The alloy has particularly favorable properties within this preferred range.

The optional elements Ti, B, Ca and Fe are used for grain refinement.

The alloy offers excellent properties especially for high temperature applications and applications where good wear and corrosion resistance is required. Depending on the exact composition of the alloy, excellent high-temperature strength, characterized by a 0.2% yield point of 400 MPa at 600 °C and a 0.2% yield point of up to 780 MPa at 400 °C, or alternatively a very good combination of strength, toughness and wear resistance at room temperature with tensile strengths of 700 MPa and impact energy of up to 60 J in the as-cast condition with excellent wear resistance. Due to the high nickel content, the alloy has better corrosion and oxidation resistance than hot-work tool steels. This advantage occurs in particular in a maritime environment and in a salty atmosphere.

Within the scope of a preferred embodiment of the invention, the Ni content can be at least 29% by weight. Very good resistance to corrosion, oxidation and cavitation is achieved thanks to a high proportion of Ni. The resistance to cavitation is advantageous when the alloy is used as a material for pump rotors.

In a special configuration of this embodiment, the Ni content can be at least 34% by weight, the Mg content at least 4.5% by weight and at most 5.5% by weight and the Al content at least 4.5% by weight % and not more than 5.5% by weight. With such a composition, an alloy with excellent high-temperature strength is achieved. This is characterized by a 0.2% compression limit of 400 MPa at 600 °C and a 0.2% compression limit of up to 780 MPa at 400 °C.

Within the scope of a further special refinement of this embodiment, the Ni content can be at most 31% by weight, the Mg content at most 3.5% by weight and the Al content at most 3.5% by weight. With an alloy composition within this specification, cast materials can be formed that exhibit a very advantageous combination of strength, ductility, toughness and wear resistance. In particular, alloys according to the invention with an Mg content of 2.5 to 3.5% by weight and an Al content of 2.5 to 3.5% by weight at room temperature achieve a notched bar impact work of 15 J and an elongation at break of almost 4% and good strength at temperatures above 400 °C. In an alternative special configuration of alloys according to the invention with an Mg content of at least 1.5% by weight and less than 2.5% by weight and an Al content of at least 1.5% by weight and less than 2.5% by weight, a notched bar impact work of up to 60 J and a tensile strength of 700 MPa are achieved.

The invention is explained in more detail using exemplary embodiments and comparative examples.

Rm:

Zugfestigkeit (in MPa)

Rp0.2:

0,2%-Stauchgrenze (in MPa)

A:

Bruchdehnung (in %)

HV 0.1:

Vickers-Härte

W:

Kerbschlagarbeit (in J)

Samples No. 1 to No. 4 according to the invention and comparative samples No. 5 to No. 7 and their mechanical properties at room temperature are compiled in Table 1. Table 1: Composition of the samples and their properties in the as-cast state at room temperature. Samples marked with * are comparison samples. sample Composition in % by weight _Rm to MPa R _p0.2 in MPa % Hardness HV 0.1 W to J number 1 CuNi30Mg2Al2 700 510 8th 264 55 No. 2 CuNi30Mg3Al3 500 440 3.8 343 15 #3 CuNi35Mg5Al5 350 320 0.8 351 2.5 #4 CuNi30Mg5Al5 - - - 337 - No. 5* CuNi25Mg10 260 - 0.1 422 2 No. 6* CuNi30Mg8 200 - 0 419 0.7 No. 7* CuNi30Mn1 490 300 12 190 100

Rm:

Tensile strength (in MPa)

Rp0.2:

0.2% compression limit (in MPa)

A:

Elongation at break (in %)

HV 0.1:

Vickers hardness

W:

Impact work (in J)

Samples No. 1 (CuNi30Mg2AI2) and No. 2 (CuNi30Mg3Al3) are characterized by a very advantageous combination of strength, ductility, toughness (expressed by the notched bar impact work) and hardness. These two samples show the highest tensile strength and 0.2% yield point values at room temperature. The ductility, which is quantified by the elongation at break, is also at a relatively high level. Only Comparative Sample No. 7 has a higher elongation at break, but with lower hardness. With the exception of hardness, sample #1 exhibits better properties than sample #2 at room temperature. Elevated temperature testing reveals that above 400°C sample #2 has higher tensile strength and 0.2% yield point than sample No. 1 has. The higher proportions of Mg and Al therefore favor high-temperature strength. Depending on the requirements, the composition of the alloy can be varied in order to achieve properties that are optimal for the respective application.

Sample No. 3 (CuNi35Mg5Al5) is mostly inferior to samples No. 1 and 2 at room temperature. This sample is only at the level of sample 2 in terms of hardness. Tests at elevated temperatures show that the higher proportions of Mg and Al lead to better properties at temperatures above 300 °C. This is discussed in more detail in connection with Table 2.

Sample no. 4 (CuNi30Mg5Al5) has a reduced nickel content compared to sample 3. This leads to a small drop in hardness. It is assumed that the other mechanical properties change in a similar way.

Samples No. 5 (CuNi25Mg10) and No. 6 (CuNi30Mg8) differ from Samples No. 1 to No. 4 essentially by a higher proportion of Mg and the absence of Al. Although samples No. 5 and No. 6 achieve a very high level of hardness, their strength, elongation at break and notched bar impact work are at a low level. These two samples are very brittle and cannot be processed mechanically.

Sample No. 7 (CuNi30Mn1) is a copper-nickel alloy that has been known for a long time and is characterized in particular by high ductility at room temperature and good corrosion resistance. However, it does not reach the hardness of Sample Nos. 1 to 4 and is therefore inferior in wear resistance. Table 2 documents the temperature dependency of the 0.2% compression limit for selected samples. sample designation 0.2% compression limit in MPa at temperature 300ºC 400ºC 500ºC 600ºC 700°C No. 2 CuNi30Mg3Al3 660 600 450 260 #3 CuNi35Mg5Al5 730 780 600 400 200 No. 7* CuNi30Mn1 500 430 360 200 No. 8* CuAI14Ni12 650 500 270 160 100 No. 9* X37CrMoV5-1 800 620 310

Table 2: Temperature dependence of the compression limit for selected samples

Table 2 documents the temperature dependency of the compression limit for samples no. 2, 3 and 7 from table 1. In addition, two other comparison samples No. 8 (CuAl14Ni12) and No. 9 (hot work steel X37CrMoV5-1) were added. The measurement data document the excellent high-temperature strength of sample no. 3: at temperatures of up to 500 °C, the compression limit of this alloy is at a level just below that of the hot-work tool steel X37CrMoV5-1 (sample no. 9). At temperatures above 500 °C, an advantage in favor of sample no. 3 can even be seen. Furthermore, at temperatures above 300°C, the measurement data of sample #3 is superior to that of sample #2. The relative advantage of sample #3 over sample #2 increases with increasing temperature.

Sample #2 exhibits approximately the same yield point as Sample #8 at 300°C. At temperatures above 300°C, however, there are clear advantages for sample no. 2. Sample no. 7 does not reach the level of samples no. 2 and no. 3 at any temperature.

The measurement data document the advantages of the alloy according to the invention, both at room temperature and, in particular, at temperatures above 300°C. With an alloy composition according to Sample No. 3, a compression limit at the level of a hot-work tool steel can be achieved. The alloy according to the invention has better corrosion resistance than hot-work steel.

The alloy described above can be used in particular for sliding elements that are used at high temperatures, for turbine materials and in apparatus construction, for example in seawater desalination plants.

QUOTES INCLUDED IN DESCRIPTION

This list of the documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 3252179 A1 [0003]
• CN 103469006A [0004]

Claims (4)

Copper-nickel cast alloy with the following composition in % by weight: Ni: 24 to 36% mg: 1.5 to 6.0% AI: 1.5 to 6.0% optional Ti: 0.05 to 0.5% optionally B: 0.01 to 0.15% optional Ca: 0.02 to 0.1% optionally Fe: 0.05 to 1.0% Rest copper, oxygen bound in magnesium oxide and unavoidable impurities. Copper-nickel cast alloy according to claim 1 , characterized in that the Ni content is at least 29% by weight. Copper-nickel cast alloy according to claim 2 , characterized in that the Ni content is at least 34% by weight, the Mg content is at least 4.5% by weight and at most 5.5% by weight and the Al content is at least 4.5% by weight and is at most 5.5% by weight. Copper-nickel cast alloy according to claim 2 , characterized in that the Ni content is at most 31% by weight, the Mg content is at most 3.5% by weight and the Al content is at most 3.5% by weight.