Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-05-04
2002-10-15
Lee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S663000, C420S038000
Reexamination Certificate
active
06464804
ABSTRACT:
The invention relates to martensitic-hardenable steels with high nitrogen contents. It relates to both the selection and the adaptation in terms of quantitative ratios of specific alloying elements which allow an extremely good combination of resistance to heat and ductility to be established, and to a process for the heat treatment of the alloy according to the invention.
Martensitic-hardenable steels based on 9-12% chromium are materials which are in widespread use in power plant engineering. It is known that the addition of chromium in the abovementioned range not only allows good resistance to atmospheric corrosion but also allows thick-walled forgings, as are used, for example, as monobloc rotors or as rotor disks in gas and steam turbines, to be hardened all the way through. Proven alloys of this type usually contain approximately 0.08 to 0.2% carbon, which in solution allows a hard martensitic structure to be established. A good combination of resistance to heat and ductility in martensitic steels is made possible by a tempering treatment, in which, as a result of the precipitation of carbon in the form of carbides with simultaneous recovery of the dislocation substructure, a particle-stabilized subgrain structure is formed. The tempering performance and the resultant properties can be actively influenced by the selection and the quantitative adaptation of specific carbide-forming elements, such as for example Mo, W, V, Nb and Ta.
Strengths of over 850 MPa in 9-12% chromium steels can be established by maintaining a low tempering temperature, typically in the range between 600 and 650° C. However, the use of low tempering temperatures leads to high transition temperatures from the brittle state to the ductile state (over 0° C.), with the result that the material exhibits a brittle fracture behavior at room temperature. Significantly improved ductilities can be achieved if the heat-treated strength is reduced to below 700 MPa. This is achieved by raising the tempering temperature to over 700° C. The use of higher tempering temperatures has the advantage that the microstructure states which are established are stable for longer periods at elevated temperatures. A typical representative which has found widespread use in steam power plants, in particular as rotor steel, is the German steel which is known under DIN as X20CrMoV12.1.
Furthermore, it is known that the ductility can be considerably improved at a strength level of 850 MPa by the addition of nickel to the alloy. For example, it is known that the addition of approximately 2 to 3% nickel to the alloy, even after tempering at temperatures of from 600 to 650° C., leads to a transition temperature from the brittle to the ductile state which lies below 0° C., so that overall it is possible to establish a significantly improved combination of strength and ductility. Alloys of this type are used wherever significantly higher demands are imposed both in terms of strength and in terms of ductility, typically as disk materials for gas turbine rotors. A typical representative of alloys of this type which has found widespread use in gas turbine technology, in particular as a material for rotor disks, is the German steel which is known under DIN as X12CrNiMo12.
In the past, various efforts have been made to improve specific properties of these steels. For example, the publication by Kern et al.: High Temperature Forged Components for Advanced Steam Power Plants, in Materials for Advanced Power Engineering 1998, Proceedings of the 6th Liège Conference, ed. by J. Lecomte-Becker et. al., has described the development of new types of rotor steels for steam turbine applications. In alloys of this type, the levels of Cr, Mo and W have been further optimized, taking account of approximately 0.03 to 0.07% N, 0.03 to 0.07% Nb and/or 50 to 100 ppm B, in order to improve the creep strength and creep rupture strength for applications at 600° C.
On the other hand, specifically for gas turbine applications, efforts have been made either to improve the creep rupture strengths in the range from 450 to 500° C. at a high ductility level or to reduce the tendency to become brittle at temperatures of between 425 and 500° C. For example, European patent application EP 0 931 845 A1 describes a nickel-containing 12% chromium steel, the constitution of which is similar to the German steel X12CrNiMo12, in which the element molybdenum is reduced compared to the known steel X12CrNiMo12 but a higher tungsten content is added to the alloy. DE 198 32 430 A1 has disclosed a further optimization of a steel which is of the same type as X12CrNiMo12 and is known as M152, in which, as a result of the addition of rare earth elements, the tendency to become brittle in the temperature range between 425 and 500° C. is restricted.
A drawback is that in none of the abovementioned developments was it possible to improve the strength, in particular the resistance to heat, at temperatures of between 300 and 600° C. to a similarly high ductility level to that of the steel X12CrNiMo12.
One possible approach with a view to improving the resistance to heat combined, at the same time, with a high ductility was proposed with the development of steels with high nitrogen contents. EP 0 866 145 A2 describes a new class of martensitic chromium steels with nitrogen contents in the range between 0.12 and 0.25%. In this class of steels, the overall microstructure formation is controlled by the formation of special nitrides, in particular of vanadium nitrides, which can be distributed in numerous ways by means of the forging treatment, by means of the austenitization, by means of a controlled cooling treatment or by means of a tempering treatment. While the strength is achieved by means of the hardening action of the nitrides, in this patent application it is desired to establish a high ductility through the distribution and morphology of the nitrides, but primarily by restricting the grain coarsening during the forging and during the solution annealing treatment. In the abovementioned document, this is achieved by both an elevated volumetric proportion and a high particle coarsening resistance of relatively insoluble nitrides, so that a close dispersion of nitrides was able to effectively limit the grain growth even at austenitization temperatures of from 1150 to 1200° C. The significant benefit of the alloys listed in EP 0 866 145 A2 lies in the possibility of optimally influencing the combination of strength and ductility solely through the formation of nitrides, with regard to distribution and morphology, by means of a suitable definition of the heat treatment.
However, an optimized formation of nitrides is only one factor involved in achieving a maximum ductility. A further factor of influence is to be expected from the action of dissolved substitution elements, such as nickel, cobalt and manganese. It is known that manganese in carbon steels tends to have an embrittling effect rather than promoting ductility. In particular, it causes embrittlement if the alloy is exposed to prolonged annealing at temperatures in the range from 350 to 500° C. Furthermore, it is known that in carbon steels nickel improves the ductility but tends to reduce the resistance to heat at elevated temperatures. This is related to a reduced carbide stability in nickel-containing steels. By contrast, the effect of cobalt on the combination of resistance to heat and ductility is relatively unknown even in carbon-containing 9-12% chromium steels.
The invention is based on the object of providing a martensitic-hardenable heat-treated steel with high ductility which compared to the known prior art, in particular the steel X12CrNiMo12, is distinguished by a high resistance to heat at temperatures of from 300 to 600° C. It is intended firstly to specify a suitable steel composition and secondly a heat treatment process for materials of this composition which allows a ductile and, at the same time, heat-resistant martensitic tempered microstructure to be formed.
The essence of the invention is a martensitic-harden
Ernst Peter
Goecmen Alkan
Alstom (Switzerland Ltd
Burns Doane Swecker & Mathis L.L.P.
Lee Deborah
LandOfFree
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