Bainitic welding and component

Abstract

The invention relates to a method for welding a component made of steel, in which a built-up welding takes place, wherein the weld site is then allowed to cool and maintained at a holding temperature above a martensite-forming temperature for two to ten hours, or until a bainitic join has completely formed, and then it is reduced to an ambient temperature in a controlled manner, in particular, thereby concluding the heat treatment.

Description

[0001] Description

[0002] Bainitic welding and components

[0003] The invention relates to the welding of steels in which a bainitic structure is established and a component.

[0004] The conventional way to increase the hardness or strength of steels is hardening and tempering.

[0005] A second, rarer method is bainiticizing, formerly called intermediate hardening and tempering.

[0006] During this heat treatment, the component is austenitized in the same way as during hardening, i.e., depending on the material, heat treatments are carried out at temperatures of 1073K - 1323K. Quenching then takes place in a warm salt bath.

[0007] The temperature of the salt bath depends on the material and ranges between 533 K and 663 K. The component remains in the salt bath at a constant temperature (isothermal) until the microstructural transformation from austenite to bainite (= intermediate stage) is complete. No martensite forms during this process!

[0008] Depending on the material, the conversion can be completed in a few minutes. Sometimes it takes several hours.

[0009] The component is then cooled in air.

[0010] This process has the advantage of the best possible toughness while maintaining high hardness.

[0011] Bainite structures have very special properties such as high impact toughness (even at lower temperatures), improved fatigue strength, greater elongation, less hardening distortion, increased necking, better bending behavior.

[0012] It is therefore an object of the invention to simplify and shorten heat conduction after welding while maintaining good mechanical properties.

[0013] The object is achieved by a method according to claim 1 and a component according to claim 6. The transfer of bainitizing to welding is the subject of this invention with the intention of simplifying and shortening the heat conduction after welding while maintaining the good mechanical properties.

[0014] The subclaims list further advantageous measures which can be combined with each other as desired to achieve further advantages.

[0015] The figure shows three curves, the temperature curve for bainitizing, the temperature curve of a conventional post-heat treatment (tempering at T>673K), and the temperature curve according to the invention.

[0016] This invention can be used to solve three different problems: a) For large turbine rotors, particularly gas turbines with an output of 200 MW or more, post-heat treatment (tempering or stress relieving) at 873 K is not possible because structural conditions, such as different thermal expansion rates of the materials and components, could deform or destroy the rotor. Furthermore, safety reasons speak against the conventional process. b) For rotors for which post-heat treatment is possible, such as when welding rotor bearing parts, post-heat treatment is often very complicated, time-consuming and expensive, but is nevertheless the industry standard because the residual stresses and material properties in the heat-affected zone cannot otherwise be achieved. c) The quality of the targeted heat treatment used in the conventional process (tempering orStress relieving) depends on the environmental parameters and represents an increased challenge in order to ensure the respective, sometimes local, temperatures in construction site conditions.

[0017] Where post-heat treatment was possible, welding repairs could only be performed with reduced mechanical material properties. Furthermore, the cost and time required are significantly higher. If post-heat treatment is not possible, welding repairs with the required material properties were not feasible.

[0018] Another solution is to machine a repair site, although this measure can usually only be carried out once on one and the same rotor for safety reasons and the resulting impact on other components in the system.

[0019] The above-mentioned problems can be avoided by special heat management after welding of steels, especially those with a ferritic and bainitic structure.

[0020] The service life of a damaged rotor can be extended. Alternative methods include:

[0021] 1. ) Turning the bearing surface and adjusting the bearings:

[0022] The bearing adjustment can only be performed after the machining process has been completed, which means it takes longer than the described solution. The time required depends on the country-specific logistics / production capabilities of the bearing shells. The time difference to restore operational readiness is estimated at at least one to two weeks. Known downtime penalties range between 80,000 and 500,000 g per day. The assembly costs of both processes (conventional and new) are assumed to be equal, as the new process requires additional welding. This process can usually only be performed once on the same rotor for safety reasons and the resulting impact on other components in the system.

[0023] 2. ) Replacing the damaged bearings requires the following activities:

[0024] The destacking of a gas turbine rotor, the provision and adjustment of the components to be replaced, and the restacking. This is followed by a final turning operation to ensure the required bearing clearance tolerances in the stacked state. During this period, technicians and machinery must be deployed on-site. The overall effort represents a total economic loss.

[0025] 3. ) Installation and provision of a new rotor:

[0026] This option would not affect the timing, but would be more costly than any repair procedure.

[0027] The invention uses steels that can form a bainitic structure within technically reasonable times.

[0028] These are, in particular, steels (i.e., iron with steel) containing nickel (Ni), chromium (Cr), molybdenum (Mo), and vanadium (V). Particularly preferred are the material class of CrMoV steels containing 2.5-4% nickel, in particular 26NiCrMoV14-5, 26NiCrMoV14-5 mod., 26NiCrMoVll-5, or 26NiCrMoVll-5 mod., especially according to DIN / EN, the steels 1.6957 / 26NiCrMoV14-5, 1.6963 / 26NiCrMoV14-5 mod., 1.6948 / 26NiCrMoVll-5, and 1.6962 / 26NiCrMoVll-5 mod.

[0029] As a rule, a build-up welding is carried out.

[0030] The filler material used for the build-up welding is different from the material of the component on which the build-up welding is carried out.

[0031] Different here means that at least one alloying element is present in greater or lesser quantities, or that the proportion of at least one alloying element differs by at least 10%, especially by 20%. Less important is whether the material of the overlay weld can form a bainitic structure.

[0032] In particular, the material of the overlay weld is a MnNiCrMo steel with silicon.

[0033] In particular, a wire is used for build-up welding, which is applied under argon shielding gas.

[0034] According to applicable standards, a welding procedure specification (WPS) and an annealing protocol for heat treatment are mandatory for every welding process. This allows verification of whether the process to be patented was used.

[0035] The figure shows several curves of temperature time courses.

[0036] Curve 1 shows the temperature profile for achieving a bainitic structure, where the material is heated to an austenitizing temperature of 1073K to 1323K, then cooled, and held at a temperature of at least 573K for one to two hours, followed by cooling, as discussed in the prior art.

[0037] In conventional welding (curve 4), temperatures of up to 1973K are reached, whereby the local temperature of the weld area then cools down quickly and is kept at a temperature well below 573K for one to two hours, then slowly cooled over several hours, before being heated again for several hours to a temperature greater than 773K, followed by further cooling.

[0038] This is a time-consuming process (>40 hours).

[0039] In the method according to the invention according to curve 7, temperatures of up to 1973K are also reached during welding, whereby the local temperature of the weld also cools down quickly, but then at a temperature greater than 573K, or generally speaking above the martensite formation temperature, ie at least 10K above, in particular at least 20K above it, so that the weld is kept in particular between 623K and 673K, in particular until a bainitic structure has been established.

[0040] The holding time at this temperature may be longer than at the first plateau of the commercial post-heat treatment (curve 4), but as a rule the total post-heat treatment time is reduced by at least 50%, since the post-heat treatment is completed after the plateau, particularly after the controlled cooling according to the invention.

[0041] The new heat management system eliminates the financial expense and time required for conventional post-heat treatment of steels that can form a bainitic structure, while maintaining or improving material properties.

[0042] The additional effort for conventional on-site heat treatment is estimated at two to three days for two to three employees per component.

Claims

Patent claims 1. Method for welding a component made of steel, in particular with a ferritic microstructure, in which a surfacing weld is carried out, wherein the weld area is then allowed to cool and is held at a holding temperature above, in particular at least 10K above, and most particularly at least 20K above, a martensite formation temperature for two to ten hours or until the complete formation of a bainitic microstructure, and is then in particular cooled down in a controlled manner, in particular to room temperature, and in particular the heat treatment is thereby completed.

2. The method according to claim 1, wherein components made of steel containing nickel (Ni), chromium (Gr), molybdenum (Mo) and vanadium (V) are welded, in particular the material class of 2.5-4% NiCrMoV steels, and most especially the steels 26NiCrMoV14-5, 26NiCrMoV14-5 mod., 26NiCrMoV11-5 or 26NiCrMoV11-5 mod.

3. Method according to one or both of claims 1 or 2, wherein the holding temperature is at least 573K, in particular at least 623K to 673K, but at most 773K. A method according to one or more of claims 1, 2, or 3, wherein the material used for the weld overlay is a material different from the material of the component to be welded, in particular a MnNiCrMo steel with silicon. A method according to one or more of claims 1, 2, 3, or 4, wherein a wire is used for the weld overlay, which is applied in particular under argon shielding gas. A steel component with a steel weld overlay, wherein the component has a bainitic microstructure at least in the area of ​​the weld overlay. A component according to claim 6, wherein the component comprises a steel containing nickel (Ni), chromium (Cr), molybdenum (Mo), and vanadium (V), in particular a steel of the material class of 2.5–4% NiCrMoV steels, and most in particular the steels 26NiCrMoV14-5, 26NiCrMoV14-5 mod. , 26NiCrMoVll-5 or 26NiCrMoVl 1-5 mod..Component according to one or both of claims 6 or 7, wherein the material for the weld overlay comprises a material that is different from the material of the component to be welded, in particular a MnNiCrMo steel with silicon.

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