Patent Application
20170342533
See Application Data Sheet.
Not applicable.
Not applicable.
Not applicable.
Not applicable.
The present invention relates to a steel alloy made in foundry, the composition of which includes inter alia iron (Fe), carbon (C), nickel (Ni) and cobalt (Co).
The present invention will find its application mainly in the field of the manufacture of tools in foundry from a steel alloy, said tools being afterwards used for forming various parts, namely obtained, in turn, from an alloy including, for example, titanium.
From European patent document EP 0 343 292 is known a steel alloy including the following elements in the indicated proportions: C between 0.4 and 0.8%; Si less than or equal to 1%; Mn less than or equal to 1%; Ni between 30 and 40%; Co between 2 and 8%; S less than or equal to 0.2%; P less than or equal to 0.2%; Mg and/or Ca less than or equal to 0.3%; with Ni+Co*0.75 between 32 and 40%, the remainder being iron and unavoidable impurities. The alloy is heated at a temperature between 600 and 1000° C. and is then tempered.
This alloy is of interest in the manufacture of mechanical precision machines, for example machine-tools, measuring instruments, semiconductor manufacturing machines and optical machines, due to a low expansion coefficient at temperatures between 20 and 100° C.
As regards JP 2003 138336, which describes a steel alloy including between 26 and 32% Ni, between 5 and 12% Co, between 0.5 and 1.7% C, less than 1% Si, less than 0.5% Mn, the remainder being iron, said alloy including a proportion of rare earths (lanthanum, cerium, praseodymium and neodymium) between 0.01 and 0.009% and having a low thermal expansion coefficient up to a temperature of 200° C.
However, in the field covered by the alloy of the present invention, the manufacture of foundry tools made of a steel alloy, the alloy in question must have a low thermal expansion coefficient at temperatures much higher than 100° C., and even than 200° C.
Also known in the state of the art are tools specifically made in foundry for forming parts made of titanium alloy at temperatures ranging from 400 to 1000° C. The expansion coefficients of these tools are relatively high (higher than 17·10−6 K−1).
Now, novel methods for producing parts require tools with a thermal expansion coefficient close to that of the alloys to be formed, namely for titanium alloys, in the range of 10.3×10−6 K−1 at 830° C., within the temperature range used for the forming.
The tools must also have good creep rupture properties. Indeed, such a feature permits to avoid the tools from deforming at high temperature.
For forming temperatures above 400° C., tools with a low expansion coefficient exist, manufactured only by machine-welding. However, the latter are likely to deform at high temperature.
For forming temperatures up to 400° C. are known in the state of the art tools made in foundry from a number of alloys having a thermal expansion coefficient and mechanical strength, which is not the case for higher temperatures.
Thus is known the patent application FR 3 025 807, also filed by the applicant, which describes an alloy of spheroidal graphite or lamellar graphite cast iron, known as Ferrynox N29K, and comprising the following elements: carbon (C) between 1.2% and 3.5%—silicon (Si) between 1.0% or 1.2% and 3%—nickel (Ni) between 26% and 31%—cobalt (Co) between 15% and 20% and optionally: magnesium (Mg) between 0.02% and 0.10%—manganese (Mn) less than or equal to 1.5%—chromium (Cr) less than or equal to 0.5% and/or phosphorus (P) less than or equal to 0.12 or 0.04% and/or sulfur (S) less than or equal to 0.11 or 0.03%, and/or molybdenum (Mo) less than or equal to 0.5%, and/or copper (Cu) less than or equal to 0.5%, the remainder being iron and unavoidable impurities.
This alloy permits to obtain tools having an interesting, i.e. low and stable, thermal expansion coefficient for temperatures up to 400° C. The so manufactured tools have an interesting application in the manufacture of parts made of composite or thermoplastic materials.
However, the expansion coefficient at temperatures above 400° C. is too high.
For temperatures above 400° C., there exist however alloys with a low thermal expansion coefficient for tools made in foundry, but their creep rupture properties is low.
Now, some materials, such as titanium alloys for example, can only be formed at temperatures above 400° C.
Therefore, the tools used for forming them must have not only a low thermal expansion coefficient, but also good creep rupture properties, in order to avoid a deformation of said tools at temperatures above 400° C. during the forming, for example, of parts made of titanium alloy.
The aim of the invention is to permit the manufacture by molding of a part made of steel the thermal expansion of which is low at high temperatures and namely up to 1000° C., and which exhibits good creep rupture properties.
In particular, the aim of the invention is to design an alloy that permits to manufacture tools having a low expansion coefficient and good creep rupture properties, including at very high temperatures.
To this end, the subject matter of the invention is a steel alloy characterized in that it is comprised, in % by weight with respect to the total weight of the alloy, of the following elements:
carbon (C) between 0.08% and 0.4%,
silicon (Si) between 0.15% and 2%,
nickel (Ni) between 24% and 31%,
cobalt (Co) between 15% and 30%,
niobium (Nb) between 0.01% and 2.5%,
the remainder being iron and unavoidable impurities.
Optionally, the alloy according to the invention comprises molybdenum (Mo) at a content of less than or equal to 3% by weight and/or manganese (Mn) at a content of less than or equal to 1.5% by weight and/or chromium (Cr) at a content of less than or equal to 1.5% by weight and/or phosphorus (P) at a content of less than or equal to 0.04% by weight and/or sulfur (S) at a content of less than or equal to 0.03% by weight and/or copper (Cu) at a content of less than or equal to 0.5% by weight.
According to particular embodiments, the alloy according to the invention may include one or more of the following features:
The subject matter of the invention is also a part made at least in part from a steel alloy, said alloy having a composition as defined above.
This part may namely consist of a tool, the purpose of which is to permit the forming of other parts made of alloys, for example titanium-based alloys. In other words, said tool made of the alloy according to the invention is likely to receive an alloy at a high temperature between 400 and 1000° C., and it must have a low expansion coefficient and good creep rupture properties within this temperature range, and namely up to a forming temperature of 1000° C.
The invention also relates to a method for manufacturing a part as defined above, characterized in that it includes at least the following steps:
preparing an alloy in foundry that has a composition as defined above;
said alloy is poured into a mold in order to obtain said part.
The FIGURE is a graph illustration, showing the thermal expansion behavior of four alloys.
The invention will be better understood when reading the following detailed description, given only by way of an example and with reference to the single FIGURE in which is shown the thermal expansion behavior of four alloys (GX40CrNiSi25-20, GXX40NiCrSi38-19, Ferrynox N29K and Example 1), among which an example of an alloy according to the invention (Example 1), by means of the evolution of their average thermal expansion coefficient (in 10−6 m/m° C. or 10−6 K−1) depending on the temperature (in ° C.) and also with reference to the table below, which gives the creep rupture properties of these different alloys.
The subject matter of the invention is a particular alloy composition. This alloy permits to obtain parts the thermal expansion coefficient of which is low and the creep rupture properties of which are high up to a temperature of 1000° C.
The part is for example a tool, namely a tool for forming metal parts.
All the compositional indications are given below in % by weight of the total weight of the alloy.
A first aspect of the invention is the chemical composition of the alloy.
The alloy is a molded steel alloy made in foundry. Its basic component is iron (Fe). It also includes unavoidable impurities resulting from the preparation of said alloy.
The alloy according to the invention has a composition comprised, besides iron (Fe) and said impurities, of carbon (C) between 0.08% and 0.4%, silicon (Si) between 0.15% and 2%, nickel (Ni) between 24% and 31%, cobalt (Co) between 15% and 30% and niobium (Nb) between 0.01% and 2.5%.
The nickel (Ni) content of the alloy may preferably vary between at least 25% or 26% and at most 30% or 30.5% by weight.
The cobalt (Co) content of the alloy may preferably vary between at least 16% and at most 19% or 27% by weight.
The niobium (Nb) content of the alloy may preferably vary between at least 0.05% and at most 1.5% or 2% by weight.
The carbon (C) content of the alloy may preferably vary between at least 0.1% or 0.12% and at most 0.2% or 0.3% by weight.
The silicon (Si) content of the alloy may preferably vary between at least 0.2% and at most 1% or 1.5% by weight.
In addition, the alloy may comprise manganese (Mn) at a content between trace amounts and 1.5%.
The manganese (Mn) content of the alloy may preferably vary between at least 0.05% or at least 0.1% and at most 1.0% by weight.
In addition, the alloy may comprise copper (Cu) at a content between trace amounts and 0.5% by weight.
The copper (Cu) content of the alloy may preferably be less than 0.4%, yet more preferably less than 0.3% by weight.
In addition, the alloy may comprise molybdenum (Mo) at a content between trace amounts and 3%.
The molybdenum (Mo) content of the alloy may preferably vary between 0.05% or 0.5% and at most 1.2% or 1.5% by weight.
In addition, the alloy may comprise chromium (Cr) at a content between trace amounts and 1.5%.
The chromium (Cr) content of the alloy may preferably be less than or equal to 1% by weight, more preferably less than or equal to 0.4% and, most preferably, this chromium content is less than or equal to 0, 3%.
In addition, the alloy may comprise phosphorus (P) at a content between trace amounts and 0.04% by weight.
In addition, the alloy may comprise sulfur (S) at a content between trace amounts and 0.03% by weight.
Since the end of the 19th century has been known an iron-based alloy with a low expansion coefficient, which is Fe—Ni36. This alloy is known under the trade name INVAR® (TRADEMARK).
In its composition, this alloy includes 36% by weight of nickel, the remainder being iron.
The curve representing the expansion coefficient of the iron/nickel alloys shows an anomaly around 36% nickel: the expansion coefficient is then much lower than for the other compositions. However, this only applies for low temperatures up to 130° C., above which the expansion coefficient of the alloy is no longer stable. Thus, the implementation of this alloy at higher temperatures is not advantageous.
Various alloys were then developed starting from this base, namely with cobalt as an addition element. For example, iron/nickel/cobalt steel with 32% nickel and 5.5% cobalt has a lower expansion coefficient than INVAR®, and especially retains this property at higher temperatures. However, its creep rupture properties at high temperature are low.
Known are molded refractory steels (GX40CrNiSi25-20, GX40NiCrSi38-19), which now permit to obtain good creep rupture properties at high temperature (rupture stress in 1000 h equal to 28 and 30Mpa, respectively, at 900° C.), but with a high expansion coefficient (18.6 and 17.3·10−6 K−1, respectively, at 900° C.).
According to examples, the alloy comprises, besides iron (Fe) and the unavoidable impurities, only the following elements, within the indicated limits:
This alloy is a molded steel.
The thermal expansion coefficient of Example 1, as compared with thermal expansion coefficients of the three alloys known from the prior art (GX40CrNiSi25-20, GX40NiCrSi38-19, Ferrynox N29K) is shown in the single FIGURE.
The thermal expansion coefficient of the alloy having the composition of Example 1 is less than 12.6*10−6 K1 at a temperature below 980° C.
More generally speaking, one observes that the thermal expansion coefficient of the alloy having the composition of Example 1 is significantly lower than the expansion coefficient of the standardized alloys GX40CrNiSi25-20 and GX40NiCrSi38-19, and in nearly the entire temperature range that has been tested, in particular for high temperatures.
Therefore, the alloy according to the invention has a low expansion coefficient at temperatures between 400 and 1000° C., applied during the forming of parts made of titanium alloy, which is of particular interesting.
The creep rupture properties of the alloy according to the invention and having a composition according to Example 1 was also tested and compared with those of the alloys GX40CrNiSi25-20 and GX40NiCrCrSi38-19 and the alloy known as Ferrynox N29K.
More specifically, and as a reminder, the Ferrynox N29K consists of a spheroidal graphite, or lamellar graphite, cast-iron alloy, with low expansion and subject matter of patent application FR 3 025 807.
The creep rupture properties are as follows:
The creep rupture properties of the steel alloy according to the invention having the contents according to Example 1 are much higher than those of the alloy known as Ferrynox N29K.
These improved creep rupture properties are observed for the alloy of Example 1 at all the temperatures that have been tested and that are likely to be used during the manufacture of parts made of titanium, and this as well after 100 hours or after 1000 hours of exposure.
The improvement of the creep rupture properties is due to the presence, in the composition of the alloy according to the invention, of niobium, which causes the formation at the grain boundaries of niobium carbides. Niobium carbides permit a blocking of creep by preventing the grains from sliding relative to each other.
Furthermore, the breaking stress value at 1000 h of a part obtained from the alloy of Example 1, namely 25 MPa at 900° C., is comparable to the breaking stresses of the alloys GX40CrNiSi25-20 and GX40NiCrSi38-19, 28 and 30 MPa, respectively, at 900° C., as indicated above. Now, these alloys GX40CrNiSi25-20 and GX40NiCrSi38-19 are considered to have good creep rupture properties at high temperature.
Another aspect of the alloy according to the invention is that it is weldable.
A second aspect of the invention is a part made of an alloy as defined above. The part is naming a tool. The tool may comprise only portions made of the alloy according to the invention or may entirely be made of this alloy.
A third aspect of the invention is the method for manufacturing a part made of an alloy according to the invention.
First of all, the alloy is prepared in foundry with the compositions that have been described above. In other words, the various constituents are mixed according to the contents that have been indicated.
Said alloy is then poured into a mold having a shape and dimensions configured to permit the manufacture of the desired part, the part consisting, for example, of a tool.
After the cooling step, the part can be heat-treated.
The alloy is used namely for manufacturing tools implemented thereafter for forming metal parts (for example made of titanium alloy).
The technical field can be aeronautics.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1654914 | May 2016 | FR | national |
