US20100178192A1

US20100178192A1 - Cast Iron Comprising Cobalt and Component

Info

Publication number: US20100178192A1
Application number: US12/087,797
Authority: US
Inventors: Stefan Janssen; Shilun Sheng
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 2006-01-16
Filing date: 2007-01-03
Publication date: 2010-07-15
Also published as: CN101400812A; EP1974068B1; WO2007082788A1; EP1808504A1; EP1974068A1

Abstract

Cast iron alloys have application limits with regard to temperature. By means of the use of cobalt an optimal ferritic structure can be achieved such that with an alloy containing silicon 2.0-4.5 wt. %, cobalt 0.5-5 wt. %, carbon 2.5-4 wt. %, molybdenum≦1 wt. %, manganese≦0.25 wt. %, nickel≦0.3 wt. %, the remainder iron where the proportion of silicon cobalt and molybdenum is less than 7.5 wt. % the application limits are shifted to higher temperatures.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2007/050057, filed Jan. 3, 2007 and claims the benefit thereof. The International Application claims the benefits of European Patent Office application No. 06000851.3 EP filed Jan. 16, 2006, both of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to an alloy, a cast iron comprising cobalt and a component thereof.

BACKGROUND OF INVENTION

The known cast iron alloys now employed (so-called GJS spherocast alloys) primarily use silicon and molybdenum to increase the creep strength, scaling resistance and endurance strength. Over time, however, these elements lead to a significant decrease in the ductility.
Molybdenum furthermore exhibits a very high susceptibility to segregation.

SUMMARY OF INVENTION

It is therefore an object of the invention to provide an alloy and a component, which overcome the aforementioned disadvantages and have better mechanical strengths over the service life.
The object is achieved by an alloy as claimed in an independent claim and e.g. by a component as claimed in a further independent claim.
Further advantageous measures are listed in the dependent claims, and these may advantageously be combined with one another in any desired way.
The invention consists in cobalt partially or fully replacing molybdenum. The working limitations presented by the previous GJS alloy can therefore be overcome. The alloy according to the invention has high elongations for the application field in the temperature range of 450° C.-550° C., and has the following composition (in wt %):


	silicon	2.0%-4.5%
	cobalt	0.5%-5%
	carbon	2.0%-4.5%, in particular 2.5%-4%,
	molybdenum	<1.5%, in particular ≦1.0%,
	manganese	<0.5%, in particular ≦0.25%,
	nickel	<0.5%, in particular ≦0.3%,
	remainder iron.

Advantageously, the proportion of silicon, cobalt and molybdenum is less than 7.5 wt %.
Preferably, the proportion of cobalt in the alloy lies between 0.5 and 1.5 wt % cobalt.
Advantageous mechanical values are achieved for the alloy respectively when the cobalt content is 0.5 wt %, with 1 wt % cobalt, with 1.5 wt % cobalt and 2.0 wt % cobalt.
The alloy may contain further elements. Preferably, however, the alloy consists of iron, silicon, cobalt and carbon.
Particular advantages are also achieved when the alloy consists of iron, silicon, cobalt, carbon and manganese.
Further advantages are obtained with an alloy which consists of iron, silicon, cobalt, carbon and optionally admixtures of molybdenum, manganese and/or nickel.
The alloy may optionally contain undesired impurities of at most


	phosphorus	0.007 wt %
	sulfur	0.008 wt %
	magnesium	0.049 wt %.

Furthermore, there is preferably no chromium (Cr) in the alloy except for the usual impurities.
Likewise, there is preferably no magnesium (Mg) in the alloy except for the usual impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be explained in more detail with the aid of the following figures, in which:


	FIG. 1	shows a micrograph,
	FIG. 2	shows mechanical characteristics,
	FIG. 3	shows a steam turbine,
	FIG. 4	shows a gas turbine.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows an almost optimal ferritic structure (etched) with spherical graphite made of an alloy with about 2 wt % cobalt:


	carbon	3.67 wt %,
	molybdenum	2.41 wt %,
	manganese	0.029 wt %,
	nickel	1.94 wt %,
	iron	remainder.

FIG. 2 shows the influence of cobalt on the mechanical properties of the alloys, which are listed in the following table (data in wt %).


cobalt	0	0.54	1.04	1.94
carbon	3.63	3.61	3.68	3.67
silicon	2.45	2.44	2.47	2.41
manganese	0.067	0.036	0.03	0.029
phosphorus	0.007	0.006	0.007	0.007
Sulfur	0.009	0.006	0.008	0.008
Magnesium	0.044	0.04	0.05	0.049

The elongation at break R_p02increases from 271 N/mm²to 284 N/mm².
The tensile strength Rm increases from 403 N/mm²to 412 N/mm².
The elongation at break A5 increases from 15.5% to 21.9%.
Likewise, the necking at fracture Z increases from 13.8% to 29.5%.
Even small proportions of cobalt (0.5 wt % to 1.0 wt % or 1.0 wt % to 1.5 wt %) improve the mechanical characteristics.
FIG. 3 shows a steam turbine 300, 303 having a turbine shaft 309 extending along a rotation axis 306.
The steam turbine comprises a high-pressure turbine part 300 and a medium-pressure turbine part 303, each with an inner housing 312 and an outer housing 315 enclosing the latter. The high-pressure turbine part 300 is, for example, configured in pot design. The medium-pressure turbine part 303 is, for example, configured to be twin-streamed. It is likewise possible for the medium-pressure turbine part 303 to be configured to be single-streamed.
A bearing 318 is arranged along the rotation axis 306 between the high-pressure turbine part 300 and the medium-pressure turbine part 303, the turbine shaft 309 comprising a bearing region 321 in the bearing 318. The turbine shaft 309 is mounted on a further bearing 324 beside the high-pressure turbine part 300. In the region of this bearing 324, the high-pressure turbine part 300 comprises a shaft seal 345. The turbine shaft 309 is sealed relative to the outer housing 315 of the medium-pressure turbine part 303 by two further shaft seals 345. Between a high-pressure steam intake region 348 and a steam outlet region 351, the turbine shaft 309 in the high-pressure turbine part 300 comprises the high-pressure rotor blading 357. With the associated rotor blades (not represented in detail), this high-pressure rotor blading 357 constitutes a first blading region 360.
The medium-pressure turbine part 303 comprises a central steam intake region 333. Associated with the steam intake region 333, the turbine shaft 309 comprises a radially symmetric shaft shield 363, a cover plate, on the one hand to divide the steam flow into the two streams of the medium-pressure turbine part 303 and also to prevent direct contact of the hot steam with the turbine shaft 309. In the medium-pressure turbine part 303, the turbine shaft 309 comprises a second blading region 366 with the medium-pressure rotor blades 354. The hot steam flowing through the second blading region 366 flows from the medium-pressure turbine part 303 out of a discharge port 369 to a low-pressure turbine part (not shown) connected downstream.
The turbine shaft 309 is composed for example of two turbine shaft parts 309 a and 309 b, which are connected firmly to one another in the region of the bearing 318. Each turbine shaft part 309 a and 309 b comprises a cooling line 372 formed as a central bore 372 a along the rotation axis 306. The cooling line 372 is connected to the steam outlet region 351 via a feed line 375 comprising a radial bore 375 a. In the medium-pressure turbine part 303, the coolant line 372 is connected to a cavity (not shown) below the shaft shield. The feed lines 375 are configured as a radial bore 375 a, so that “cold” steam from the high-pressure turbine part 300 can flow into the central bore 372 a. Via the discharge line 372 also formed in particular as a radially directed bore 375 a, the steam passes through the bearing region 321 into the medium-pressure turbine part 333 and there onto the lateral surface 330 of the turbine shaft 309 in the steam intake region 333. The steam flowing through the cooling line is at a much lower temperature than the temporarily superheated steam flowing into the steam intake region 333, so as to ensure effective cooling of the first rotor blade row 342 of the medium-pressure turbine part 303 and the lateral surface 330 in the region of this rotor blade row 342.
FIG. 4 shows a gas turbine 100 by way of example in a partial longitudinal section.
The gas turbine 100 internally comprises a rotor 103, which will also be referred to as the turbine rotor, mounted so as to rotate about a rotation axis 102 and having a shaft 101.
Successively along the rotor 103, there are an intake manifold 104, a compressor 105, an e.g. toroidal combustion chamber 110, in particular a ring combustion chamber, having a plurality of burners 107 arranged coaxially, a turbine 108 and the exhaust manifold 109.
The ring combustion chamber 110 communicates with an e.g. annular hot gas channel 111. There, for example, four successively connected turbine stages 112 form the turbine 108.
Each turbine stage 112 is formed for example by two blade rings. As seen in the flow direction of a working medium 113, a guide vane row 115 is followed in the hot gas channel 111 by a row 125 formed by rotor blades 120.
The guide vanes 130 are fastened on an inner housing 138 of a stator 143 while the rotor blades 120 of a row 125 are fastened on the rotor 103, for example by means of a turbine disk 133.
Coupled to the rotor 103, there is a generator or a work engine (not shown).
During operation of the gas turbine 100, air 135 is taken in and compressed by the compressor 105 through the intake manifold 104. The compressed air provided at the end of the compressor 105 on the turbine side is delivered to the burners 107 and mixed there with a fuel. The mixture is then burnt to form the working medium 113 in the combustion chamber 110. From there, the working medium 113 flows along the hot gas channel 111 past the guide vanes 130 and the rotor blades 120. At the rotor blades 120, the working medium 113 expands by imparting momentum, so that the rotor blades 120 drive the rotor 103 and the work engine coupled to it.
During operation of the gas turbine 100, the components exposed to the hot working medium 113 experience thermal loads. Apart from the heat shield elements lining the ring combustion chamber 110, the guide vanes 130 and rotor blades 120 of the first turbine stage 112, as seen in the flow direction of the working medium 113, are heated the most.
In order to withstand the temperatures prevailing there, they may be cooled by means of a coolant.
Substrates of the components may likewise comprise a directional structure, i.e. they are monocrystalline (SX structure) or comprise only longitudinally directed grains (DS structure).
Iron-, nickel- or cobalt-based superalloys are for example used as material for the components, in particular for the turbine blades 120, 130 and components of the combustion chamber 110.
Such superalloys are known for example from EP 1 204 776 B1, EP 1 306 454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; with respect to the chemical composition of the alloys, these documents are part of the disclosure.
The blades 120, 130 may likewise have coatings against corrosion (MCrAlX; M is at least one element from the group iron (Fe), cobalt (Co), nickel (Ni), X is an active element and stands for yttrium (Y) and/or silicon, scandium (Sc) and/or at least one rare earth element, or hafnium). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1, EP 0 412 397 B1 or EP 1 306 454 A1 which, with respect to the chemical composition, are intended to be part of this disclosure.
On the MCrAlX, there may furthermore be a thermal barrier layer which consists for example of ZrO₂, Y₂O₃—ZrO₂, i.e. it is not stabilized or is partially or fully stabilized by yttrium oxide and/or calcium oxide and/or magnesium oxide.
Rod-shaped grains are produced in the thermal barrier layer by suitable coating methods, for example electron beam deposition (EB-PVD).
The guide vane 130 comprises a guide vane root (not shown here) facing the inner housing 138 of the turbine 108, and a guide vane head lying opposite the guide vane root. The guide vane head faces the rotor 103 and is fixed on a fastening ring 140 of the stator 143.

Claims

1-35. (canceled)

36. An alloy comprising in wt %:

silicon 2.0%-4.5%; cobalt 0.5%-5%; carbon 2.0%-4.5%; molybdenum ≦1.5%; manganese ≦0.25%; nickel ≦0.5%; and iron.

37. The alloy as claimed in claim 36, wherein the proportion of silicon, cobalt and molybdenum is less than 7.5 wt %.

38. The alloy as claimed in claim 36, comprising from 1.0 wt % to 2.0 wt % cobalt.

39. The alloy as claimed in claim 36, further comprising molybdenum.

40. The alloy as claimed in claim 36, free of molybdenum.

41. The alloy as claimed in claim 36, further comprising manganese.

42. The alloy as claimed in claim 41, wherein the manganese content is ≦0.07 wt %.

43. The alloy as claimed in claim 36, free of manganese.

44. The alloy as claimed in claim 36, further comprising nickel.

45. The alloy as claimed in claim 36, free of nickel.

46. The alloy as claimed in claim 36, further comprising 2.0 wt %-3.0 wt % silicon.

47. The alloy as claimed in claim 36, further comprising from 3.5 wt % to 4.0 wt % carbon.

48. The alloy as claimed in claim 36, further comprising at most 0.07 wt % phosphorus.

49. The alloy as claimed in claim 36, further comprising at most 0.008 wt % sulfur.

50. The alloy as claimed in claim 36, further comprising at most 0.05 wt % magnesium.

51. The alloy as claimed in claim 36, free of chromium.

52. The alloy as claimed in claim 36, free of magnesium.

53. The alloy as claimed in claim 36, consisting of iron, silicon, cobalt and carbon.

54. The alloy as claimed in claim 36, consisting of iron, silicon, cobalt, carbon and manganese.

55. A housing part, comprising an alloy having iron and in wt %:

silicon 2.0%-4.5%; cobalt 0.5%-5%; carbon 2.0%-4.5%; molybdenum ≦1.5%; manganese ≦0.25%; nickel ≦0.5%.

56. A component, comprising an alloy having iron and in wt %:

silicon 2.0%-4.5%; cobalt 0.5%-5%; carbon 2.0%-4.5%; molybdenum ≦1.5%; manganese ≦0.25%; nickel ≦0.5%,

wherein the component is a steam turbine or a gas turbine.

57. The component as claimed in claim 56, a substrate which is iron-based or steel-based.