EP1158067A1 - Martensitic hardenable heat treatable steel with improved thermal resistance and ductility - Google Patents

Abstract

Heat-treatable steel contains (in weight %): 9-13 Cr, 0.001-0.25 Mn, 2-7 Ni, 0.001-8 Co, 0.5-4 of at least one of W and Mo, 0.5-0.8 V, 0.001-0.1 of at least one of Nb, Ta, Zr and Hf, 0.001-0.05 Ti, 0.001-0.15 Si, 0.01-0.1 C, 0.12-0.18 N, maximum 0.025 P, maximum 0.015 S, maximum 0.01 Al, maximum 0.0012 Sb, maximum 0.007 Sn, maximum 0.012 As and a balance of iron. The weight ratio of vanadium to nitrogen is 3.5 to 4.2. An Independent claim is also included for a process heat treating the above heat-treatable steel comprising solution annealing at 1050-1250 degrees C; cooling to less than 300 degrees C; tempering and partially or completely re-austenizing at 600-900 degrees C; cooling to less than 300 degrees C; and annealing at 550-650 degrees C.

Description

Technical field

The invention relates to martensitic hardenable steels with elevated Nitrogen content. It affects both the choice and the quantitative proportion Tuning special alloy elements, which the setting of a exceptionally good combination of heat resistance and ductility enable, as well as a method for heat treatment of the alloy according to the invention.

State of the art

Martensitic hardenable steels based on 9-12% chromium are widespread Power plant engineering materials. It is known that the addition of chromium in the above range not only good resistance to atmospheric Corrosion, but also the complete hardenability of thick-walled Forgings made possible, such as as monoblock rotors or as Rotor disks are used in gas and steam turbines. Proven Alloys of this type usually contain about 0.08 to 0.2% carbon, which in solution is the setting of a hard martensitic structure enables. A good combination of heat resistance and ductility martensitic steels is made possible by a tempering treatment in which through the excretion of carbon in the form of carbides simultaneous recovery of the dislocation substructure a particle stabilized Sub-grain structure forms. The starting behavior and the resulting Properties can be effective through the choice and the proportion Matching special carbide formers such as Mo, W, V, Nb and Ta to be influenced.

Strengths over 850 MPa of 9-12% chrome steels can be set by lowering the tempering temperature, typically in the range 600 to 650 ° C, is held. However, the use of low tempering temperatures leads to high ones Transition temperatures from brittle to ductile (above 0 ° C), with which the material exhibits brittle fracture behavior at room temperature. Clear Improved ductility can be achieved if the tempered strength is lowered below 700 MPa. This is done by raising the Tempering temperature reached over 700 ° C. The application increased Tempering temperatures has the advantage that the set structural states are stable over long periods at elevated temperatures. A typical one Representative who is wide in steam power plants, especially as rotor steel German steel known as DIN has been used X20CrMoV12.1.

It is also known that the ductility is at a strength level of 850 MPa can be significantly improved by alloying with nickel. That's about it known that by alloying about 2 to 3% nickel even after a Tempering treatment at temperatures from 600 to 650 ° C Transition temperature from brittle to ductile state is still below 0 ° C, which results in a significantly improved combination of strength and Ductility. Such alloys are therefore widely used there Use where significantly higher demands on both strength and Ductility, typically used as disc materials for Gas turbine rotors. A typical representative of such alloys, which in the Gas turbine technology, especially as a material for rotor disks wide German steel known as DIN has been used X12CrNiMo12.

Various efforts have been made in the past to improve the special 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 6 ^th Liège Conference, ed. By J. Lecomte-Becker et. al., described the development of new rotor steels for steam turbine applications. In such alloys, the Cr, Mo, W contents were further optimized taking into account about 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 and creep rupture strengths for applications at 600 ° C to improve.

On the other hand, specifically for gas turbine applications Efforts have been made to either maintain the creep strength in the area from 450 to 500 ° C to improve the ductility or the Reduce embrittlement tendencies at temperatures between 425 and 500 ° C. For example, European patent application EP 0 931 845 A1 describes one in the Constitution similar to the German steel X12CrNiMo12, containing 12% nickel Chrome steel, in which the element molybdenum compared to the known steel X12 CrNiMo12 reduced, however an increased content of tungsten was added. DE 198 32 430 A1 describes a further optimization of the X12CrNiMo12 steel of the same type with the designation M152, in which by the Adding rare earth elements increases the tendency to become brittle Temperature range between 425 and 500 ° C is limited.

The disadvantage is that in none of the developments mentioned above the strength, especially the heat resistance at temperatures between 300 and 600 ° C improved the ductility level comparable to the steel X12CrNiMo12 could be.

A possible approach to improve the heat resistance at the same time as high Ductility has been associated with the development of steels with increased nitrogen levels suggested. EP 0 866 145 A2 describes a new class of martensitic Chromium steels with nitrogen contents in the range between 0.12 to 0.25% described. In this class of steel, the entire structure is determined by Formation of special nitrides, in particular controlled by vanadium nitrides, which through the forging treatment, through the austenitization, through a controlled cooling treatment or through a tempering treatment in various Way can be distributed. While the strength over the hardening effect The nitride is achieved by setting a high ductility in this Patent application through the distribution and morphology of nitrides, especially but by limiting the coarsening of the grain during forging and sought during solution heat treatment. This is mentioned in the Publication by an increased volume share as well as by a achieved high particle coarsening resistance of poorly soluble nitrides, so that a dense dispersion of nitrides contributes to the grain growth itself To effectively limit austenitizing temperatures from 1150 to 1200 ° C can The main benefits of those listed in EP 0 866 145 A2 Alloys lies in the possibility of combining strength and ductility solely through the formation of nitrides in terms of distribution and morphology to influence optimally by a suitable definition of the heat treatment.

However, an optimized state of formation of nitrides is only one factor Achieve maximum ductility. Another influencing factor is through the Effect of dissolved substitution elements such as nickel, cobalt and manganese expect. From manganese, it is known from carbon steels that this Element is more embrittling than it promotes ductility. In particular, it does embrittlement if the alloy undergoes long-term annealing Temperatures in the range of 350 to 500 ° C is exposed. It is also known that nickel in carbon steels tends to improve ductility, but also tends to do so lowers heat resistance at high temperatures. This will be reduced Carbide stability in nickel-containing steels related. In contrast is the effect of cobalt on the combination of heat resistance and ductility Very little is known, even in carbon-containing 9-12% chromium steels.

Presentation of the invention

The invention has for its object a martensitic curable To create tempered steel with high ductility, which is compared to the known prior art, in particular the steel X12CrNiMo12, by a characterized by increased heat resistance at temperatures from 300 to 600 ° C. It should on the one hand a suitable steel composition and on the other hand a Heat treatment process for materials of this composition be specified, which is the formation of a ductile and simultaneous heat-resistant martensitic tempering structure.

The core of the invention is a martensitic hardenable tempering steel with the following Composition (data in% by weight): 9 to 13% Cr, 0.001 to 0.25% Mn, 2 up to 7% Ni, 0.001 to 8% Co, at least one of W and Mo in total between 0.5 and 4%, 0.5 to 0.8% V, at least one of Nb, Ta, Zr and Hf in the sum between 0.001 to 0.1%, 0.001 to 0.05% Ti, 0.001 to 0.15% Si, 0.01 to 0.1% C, 0.12 to 0.18% N, maximum 0.025% P, maximum 0.015% S, maximum 0.01% Al, maximum 0.0012% Sb, maximum 0.007% Sn, maximum 0.012 % As, remainder iron and usual impurities, and the proviso that the Weight ratio of vanadium to nitrogen V / N in the range between 3.5 and 4.2 lies.

Preferred ranges for the individual alloy elements of the Composition according to the invention are contained in the subclaims.

• Lösungsglühen bei 1050 bis 1250 °C,
• danach Abkühlung auf eine Temperatur unter 300°C,
• Anlassbehandlung, partielle oder vollständige Reaustenitisierung bei 600 bis 900 °C,
• Abkühlung auf eine Temperatur unter 300°C,
• Glühung bei einer Temperatur von 550 bis 650 °C.

The heat treatment process for the alloy according to the invention is characterized by the following steps:

• Solution annealing at 1050 to 1250 ° C,
• then cooling to a temperature below 300 ° C,
• Tempering treatment, partial or complete reaustenitization at 600 to 900 ° C,
• Cooling to a temperature below 300 ° C,
• Annealing at a temperature of 550 to 650 ° C.

The advantage of the invention is that in the alloy mentioned Occasional structure is set, which is characterized by a tough basic matrix and the presence of nitrides that provide heat resistance. The toughness of the Basic matrix is preferred by the presence of substitution elements by nickel and secondarily by cobalt. The levels of this Substitution elements are designed to optimally unfold both martensite hardening and particle hardening using special nitrides, preferably vanadium nitrides, for setting the highest heat resistance enable.

One of the known martensitic-hardenable 9-12% chrome steels can be used good combination of heat resistance and ductility through a Stop compensation treatment, which is an austenitization treatment, a Quenching treatment and tempering treatment included. The attainable Strength is largely determined by the basic hardness of the quenched Martensite and the potential particle hardening effect of Elimination phases that are formed during the tempering treatment limited. An increase in strength over the basic hardness of the quenched Martensits is also through an occasion treatment in the so-called Secondary curing area possible. This is for those in power plant technology well-known 12% chrome steels in the temperature range between 450 and about 530 ° C.

Basically, both hardening mechanisms lower both the martensitic Hardening as well as precipitation hardening, ductility. More characteristic A ductility minimum is thus achieved in the area of secondary hardening observed. This ductility minimum does not need exclusively through the actual precipitation hardening mechanism. On certain contribution to embrittlement can also be obtained by segregation of Contamination to the grain boundaries or possibly through Proximity settings can be provided by dissolved alloy atoms.

An increase in the tempering temperature over the secondary hardening range leads to complete elimination with significant growth of carbides. Thereby the strength decreases and the ductility increases. It is essential that through the simultaneous recovery of dislocation substructure and particle coarsening the ductility increases so that the combination of strength and Overall ductility is improved. This improvement is the formation of a attributed to particle-stabilized subgrain structure. Form ductile structures through a tempering treatment in the 9-13% chromium steels with little nickel above 700 ° C. It can be assumed that both the ductility and the strength of particle-stabilized subgrain structures due to unevenness is reduced in the topology of the particle sub-grain structure. Excretions on sub-grain boundaries are subject to accelerated coarsening and tend to coagulate with neighboring excretions. Coarse and coagulated Phases generate breakage-causing voltage peaks, which increase ductility reduce. Above all, however, the uneven distribution of the Excretions also the most effective at high temperatures Hardening mechanism, namely particle hardening, severely limited.

A ductility increase limited in its effect is due to a reduction the grain size possible. However, this is the case with those known in the art Alloys in large components are difficult to implement using forging technology leaves little effect. A somewhat more important measure for This is an increase in ductility in conventional, martensitic hardenable steels Alloying of nickel. However, the causes of this are not all Known points and are likely to depend heavily on the nickel content. So small ones Proportions of nickel can be very ductile, if that means about Formation of delta ferrite can be completely suppressed. With nickel contents In contrast, the Ac1 temperature (this is the one) is above 2% by weight Temperature at which ferrite converts to austenite during heating starts) to temperatures below 700 ° C. So should the strength by Lowering the tempering temperature below 700 ° C is then in Presence of increased nickel levels when tempering with a partial Calculation of transformation from ferrite to austenite. This is with a certain ductility-promoting new grain formation. However, is too note that the carbide excretion above the Ac1 temperature only is incomplete because the solubility of the austenite-stabilizing element Carbon is larger in austenite than in ferrite. The austenite that forms is further not sufficiently stabilized, so that a larger volume fraction of the regressed austenite of another martensitic transformation at Is subject to re-cooling after starting. In addition to the two The aforementioned contributions to effectiveness by nickel for increasing ductility can be a certain contribution of ductility of nickel in its effect as a substitution element come in solid solution. This can be explained electronically in such a way that the element nickel feeds additional free electrons into the iron lattice and thereby making the iron alloys even more "metallic".

Basically, conventional, martensitic hardenable steels have some Nickel alloys are not special compared to low-nickel alloys Heat resistance advantages. This is true at least for test temperatures above 500 ° C and could increase the nickel content with the above Reaustenitization related to tempering. It is also known that the alloying of nickel in such steels reduces the structural instability long-term aging conditions at elevated temperatures tightened. This long-term structural instability is with a accelerated coarsening of carbides.

Cobalt is an austenite-stabilizing element similar to nickel. In fixed Solution is therefore an effect similar to nickel in terms of ductility expect. From the chemical point of view, however, is an important one Make a difference than that cobalt supports ferromagnetism, the Curie temperature increases. Because the self-diffusion within the iron matrix increases suddenly when the Curie temperature is exceeded Exceeding the Curie temperature all diffusion-controlled recreational and Coarsening processes accelerated. By increasing the Curie temperature an improvement in tempering resistance can therefore be expected. Cobalt alloy structures should then be used during the tempering treatment delayed softening and thus the setting of increased strength enable. Also of importance is the fact that cobalt is the Ac1 temperature per weight percent alloy additive significantly less than Nickel.

Unlike nickel and cobalt, manganese is on the left next to that Element iron in the periodic system of the elements. It is a Electron-poorer element, which clearly shows its effect in solid solution should be different from nickel and cobalt. Nonetheless, it is a austenite-stabilizing element, which lowers the Ac1 temperature, however not a particularly positive, but rather an unfavorable effect on ductility leaves.

1) Die erfindungsgemässe Legierung soll ein wirksames Kornneubildungsverhalten aufweisen, so dass durch Schmieden und Normalisierung (Austenitisierung) verfeinerte Korn- und Blockstrukturen erzeugt werden können. Diese Kornfeinung soll durch eine Dispersion schwerlöslicher Nitride erzeugt werden, welche durch das Zulegieren von Stickstoff zu starken Nitridbildnern wie Vanadium, Niob, Tantal, Titan, Hafnium und Zirkon erzeugt wird. Die Kornfeinung selbst kann einen Beitrag zur Warmfestigkeit liefern, sofern die eingestellte Korn- und Blockstruktur durch die Nitride gegen Vergröberung gut stabilisiert ist. Entscheidend ist jedoch, dass die günstige Wirkung der Kornfeinung auf die Duktilität gegenüber der negativen Wirkung der gröberen Primärphasen auf die Duktilität überwiegt.

2) Eine verbesserte Warmfestigkeit wird durch eine thermisch stabile Ausscheidungsphase gewährleistet, welche in einem kleinen Volumenanteil, das heisst bei geringer Löslichkeit und damit hohem Widerstand gegen Vergröberung, eine maximale Teilchenhärtungswirkung pro Molvolumen bei erhöhten Prüftemperaturen ermöglicht. Die bevorzugte Ausscheidungsphase soll von demselben Typ sein wie diejenigen Nitride, welche die Korngrössenbegrenzung in Punkt 1 ermöglichen.

3) Die Ausscheidungsreaktionen sollen gleichmässig verlaufen, so dass die Kombination von Duktilität und Warmfestigkeit nicht durch grobe, filmartige Ausscheidungen und deren ungleichmässige Verteilung auf den Korn- und Subkorngrenzen beeinträchtigt wird.

4) Es soll Nickel zulegiert werden, um seine duktilitätsfördernde Wirkung als Substitutionselement zu nutzen und um die Ac1- und Ac3-Temperatur zu senken. Dieses Absenken der Umwandlungstemperatur ermöglicht die Ausscheidung von Nitriden bei tiefen Austenitisierungstemperaturen, nachdem das Material von Raumtemperatur auf die Aushärtungstemperatur aufgeheizt wird.

5) Die Wirkung von Kobalt soll als komplementäres Substitutionselement zu Nickel zur Duktilitätssteigerung genutzt werden. Ungleich des Nickels soll es ausschliesslich als ein gelöstes Element und nicht zur Phasenumwandlungsbeeinflussung (Ferrit/Austenit) genutzt werden. Als Substitutionselement soll es auch die Anlassbeständigkeit verbessern.

6) Die gewünschte Legierung, insbesondere das Einbringen von Stickstoff, soll unter der Rahmenbedingung hergestellt werden können, dass die Gehalte an versprödungsbringenden Elementen wie Silizium und Mangan den Anforderungen entsprechend gering gehalten werden können.

Based on these findings and hypotheses, the following alloy design is proposed for an improvement from the combination of heat resistance and ductility:

1) The alloy according to the invention should have an effective grain formation behavior, so that refined grain and block structures can be produced by forging and normalization (austenitization). This grain refinement is said to be produced by a dispersion of sparingly soluble nitrides, which is produced by alloying nitrogen with strong nitride formers such as vanadium, niobium, tantalum, titanium, hafnium and zircon. The grain refinement itself can contribute to the heat resistance, provided the grain and block structure is well stabilized by the nitrides against coarsening. It is crucial, however, that the favorable effect of grain refinement on ductility outweighs the negative effect of coarser primary phases on ductility.

2) Improved heat resistance is ensured by a thermally stable precipitation phase which, in a small volume fraction, i.e. with low solubility and thus high resistance to coarsening, enables maximum particle hardening effect per molar volume at elevated test temperatures. The preferred elimination phase should be of the same type as those nitrides that allow the grain size limitation in point 1.

3) The excretion reactions should run evenly so that the combination of ductility and heat resistance is not impaired by coarse, film-like precipitations and their uneven distribution on the grain and sub-grain boundaries.

4) Nickel should be alloyed in order to use its ductility-promoting effect as a substitution element and to lower the Ac1 and Ac3 temperature. This lowering of the transformation temperature enables the precipitation of nitrides at low austenitizing temperatures after the material is heated from room temperature to the hardening temperature.

5) The effect of cobalt should be used as a complementary substitute element for nickel to increase ductility. Unlike nickel, it should only be used as a dissolved element and not to influence the phase change (ferrite / austenite). As a substitution element, it is also said to improve temper resistance.

6) The desired alloy, in particular the introduction of nitrogen, should be able to be produced under the general condition that the contents of embrittlement elements such as silicon and manganese can be kept low according to the requirements.

From EP 0 866 145 A2 it is known that by controlled alloying of Nitrogen, vanadium and other special nitride formers such as niobium, titanium, tantalum, Zircon and hafnium the first three points are very well met and therefore for a steel alloy development with regard to improved mechanical Properties can be used. It plays a key role in this Vanadium nitride, which is used both for grain refinement and for Precipitation hardening can be used effectively. It is crucial that one Tempering of such steels at temperatures between 600 and 650 ° C the heat resistance compared to tempered but conventional Alloys can significantly increase. This is due to the onset Precipitation hardening by vanadium nitrides in this temperature range attributed, which at a temperature of 700 ° C for the first time by Göcmen, A. et al .: Precipitation Behavior and Stability of Nitrides in High Nitrogen Martensitic 9% and 12% Chromium Steels, ISIJ Int., 1996, 36, p. 769. was observed. It is important that this is fine and dense Precipitation states with high coherence of the vanadium nitrides to the iron lattice were found. That concludes that secondary curing is about Vanadium nitrides at 600 to 650 ° C compared to classic secondary hardening at 450 to 530 ° C offers no special ductility advantages.

Another important role of those listed in EP 0 866 145 A2 Alloys according to the invention are played from the element manganese. The Element manganese therefore comes in particularly in steels containing high levels of nitrogen significant role too because it like the solubility for nitrogen in the melt also increased in the austenite matrix. Manganese also has the property that it is the Austenite-ferrite transformation transition nose at longer times shifts. These properties of manganese result in favorable ones The vanadium nitrides are still required after a solution heat treatment before the martensitic transformation in the area of metastable austenite to leave again. On the side of carbon-containing 12% chrome steels however, manganese is understood as an impurity element which Promotes embrittlement significantly. Therefore, the manganese content, especially with regard to applications in the temperature range between 350 and 500 ° C, usually limited to very small quantities.

A substitution of manganese with nickel in high nitrogen containing 9-13% Chrome steels create new advantages and opportunities. It is assumed, that nickel as a substitute element in solid solution, the ductility of the Crystal matrix improved. By alloying with nickel, the Ac1 and Ac3 temperature lowered. In N and V alloy systems, this creates the advantage the vanadium nitride at low austenitization temperatures, i.e. in one austenitic matrix can be excreted. The decisive advantage However, the vanadium nitride is difficult to germinate can be easily germinated in the martensitic, dislocation-rich matrix, before they are transferred to the austenitic matrix. So there is intention then finely separate the vanadium nitrides in the austenitic matrix it is no longer necessary to outsource the connection directly to the Solution heat treatment in "metastable" austenite, as in EP 0 866 145 A2 has been described. Because the germs for the vanadium nitrides now can be easily formed in the martensitic matrix The aging time for the maturation of nitrides in austenite can be significantly reduced. Alloying with nickel thus offers a new possibility: Vanadium nitrides quickly and effectively in the transformable austenitic Eliminate matrix. Because the austenite hardening is now effective without manganese The stability of the martensitic matrix can also be carried out against embrittlement due to a strong limitation of manganese be improved.

It should also be considered that the alloy is sufficiently high Apply solubility for preferred levels of nitrogen. It is known, that manganese increases solubility for nitrogen and nickel increases solubility for Nitrogen lowers. The particular advantage of the desired alloy design is that the required solubility for nitrogen is already due to the Element Vanadium is offered, which is necessary for the formation of an optimal structure in an almost stoichiometric ratio to nitrogen is added. The dominant effect of vanadium on the solubility of nitrogen makes it possible that the increased and preferred levels of nitrogen due to Vanadium and almost stoichiometric to vanadium without the use of Overpressure can be introduced and therefore only subordinate to the Presence of nickel and cobalt are impaired.

The element cobalt also offers the possibility of aging Without delaying nitrides and the recovery of dislocations when starting that an increased austenite reversion is provoked when starting.

Below are the preferred amounts in weight percent for each Element and the reasons for the chosen according to the invention Alloy ranges in connection with the resulting Possibilities of heat treatments shown.

Chrome:

A chromium content of 9-13% enables good hardenability thick-walled components and provides sufficient resistance to oxidation up to a temperature of 550 ° C safely. A weight percentage below 9% is impaired through remuneration. Levels above 13% lead to accelerated education of hexagonal chrome nitrides during the tempering process, which besides Nitrogen also binds vanadium, and thus the effectiveness of curing with vanadium nitrides. The optimal chromium content is 10.5 to 11.5%.

Manganese and silicon:

Together with silicon, these elements promote temper embrittlement and must therefore be limited to the lowest levels. The one to be specified The area should take into account the ladle metallurgical possibilities in the range between 0.001 and 0.25% for manganese and between 0.001 and 0.15 % for silicon.

Nickel:

Nickel is used as an austenite stabilizing element to suppress delta ferrite used. In addition, it is said to be a resolved element in the ferritic matrix improve ductility. Nickel contents up to about 3.5% by weight remain homogeneously dissolved in the matrix if the tempering temperature, or the stress relieving temperature at the end of the whole Remuneration treatment does not exceed 600 ° C. For alloys which low temperatures, i.e. at 600 to 640 ° C preferred nickel content at 3 to 4 wt .-% before. Nickel contents above 4% by weight strengthen the austenite stability in such a way that after solution annealing and Annealing an increased proportion of residual austenite or tempering austenite in tempered martensite. In the presence of stoichiometric However, nitrogen and vanadium are suitable for those with high nickel content Steel on a special heat treatment. Will such an alloy Solution annealed at high temperatures, e.g. at 1150 to 1200 ° C, is a increased residual austenite content after tempering on the effect of the high Nitrogen and vanadium concentration in solution to the resulting Increase in the martensite start temperature. Another However, reaustenitization at temperatures between 700 and 850 ° C allows a re-excretion of vanadium nitrides, which the martensite start temperature can be raised again so that a complete Reconversion to martensite becomes possible again by quenching. Such low reaustenitization temperatures prevent premature Aging of the vanadium nitrides so that they are still an essential one Particle hardening contribution to deliver. Through this process, one can Vanadium nitrides form well-stabilized martensite, which is characterized by the preceding grain formation process the setting of a particularly high Ductility enables. Another tempering treatment at about 600 ° C leads to Formation of small austenite islands, which are opposed to a reverse transformation in the martensite is sufficiently stabilized. The volume fraction of this austenite is below 5%, provided the nickel content does not exceed 7%. Even higher Volume fractions increase the risk of embrittlement in one Long-term aging at elevated temperatures. That kind of Heat treatment is suitable for alloys with 2 to 7% nickel. A particularly good combination of heat resistance and ductility is under Consideration of this special heat treatment technology with nickel contents achieved in the range between 4.5 and 6.5%.

Cobalt:

This element is used as a substitute element for iron in solid solution for the final fine-tuning of ductility and heat resistance used. On A weight percentage of up to 10% cobalt can be added without any Austenite transformation at tempering temperatures in the range from 600 to 650 ° C is expected. The optimal cobalt content depends on the proportion of molybdenum and tungsten. This proves to be above about 8% by weight Alloying cobalt as uneconomical. A preferred range of alloys which takes into account the high alloying costs of cobalt, is 3.5 to 4.5% by weight.

Molybdenum and tungsten:

Both elements improve the creep resistance through mixed crystal hardening partially loosened elements and by precipitation hardening during one Long-term use. An excessive proportion of these elements leads however, to embrittlement during long-term exposure, which is caused by the Elimination and coarsening of the Laves phase (W, Mo) and Sigma phase (Mo) is given. For this reason, the total Mo + W share must be 4% be limited. An ideal range for W + Mo is in the range 1 to 4%.

Molybdenum becomes due to its higher solubility compared to tungsten prefers. A preferred range is by a molybdenum content in the range of 1 to 2% and a tungsten content of less than 1%. Better is one Molybdenum content from 1 to 2.5% and a tungsten content of less than 0.5%. A particularly preferred range is due to a negligibly small one Tungsten content, but given molybdenum contents of 1 to 3%.

Vanadium and nitrogen:

These two elements together significantly control the Grain size formation and precipitation hardening. The Microstructure formation forms are optimal if the elements vanadium and Nitrogen are alloyed in an almost stoichiometric ratio to each other. The ideal weight ratio V / N is 3.6. Because the nitrogen solubility Vanadium is a slightly overstoichiometric V / N To strive for relationship. A slightly over-stoichiometric ratio increased sometimes the stability of the vanadium nitride with respect to the chromium nitride. Overall, a V / N ratio in the range between 3.5 to 4.2 is preferred. On a particularly preferred range is 3.8 to 4.2. The concrete content of Nitrogen and vanadium nitrides are based on the optimal volume fraction the vanadium nitrides, which are insoluble during solution treatment Primary nitrides should remain. The greater the total proportion of vanadium and nitrogen, the greater the proportion of vanadium nitrides which no longer goes into solution and the greater the grain refining effect. The positive influence of grain refinement on ductility is limited, however, because with increasing volume fraction of primary nitrides the primary nitrides themselves Limit ductility. The preferred nitrogen content is in the range 0.13 to 0.18% by weight and that of vanadium is in the range between 0.5 and 0.8 % By weight.

Titanium:

Titanium nitride is a sparingly soluble nitride that supports grain refinement. In contrast to vanadium nitride, however, it can already be found in the melting and form in particular in the solidification phase, with which the solidification as a whole calmer and finer. Too high proportions of weight lead to very large ones Primary nitrides that worsen ductility. Therefore the top one Titanium content can be limited to 0.05%.

Niobium, tantalum, zircon and hafnium:

These are all powerful nitride formers that support the grain refining effect. In order to keep the volume share of the primary nitrides small, their total share must limited to 0.1%. A particularly preferred nitride former is niobium, because Niobium dissolves in small amounts in the vanadium nitride and thus the stability of the Vanadium nitride can improve. Niobium is preferred in the range between 0.01 and 0.07%.

Phosphorus, sulfur, arsenic, antimony and tin:

Together with silicon and manganese, these elements reinforce the Embrittlement due to long-term aging in the range between 350 and 500 ° C. These elements should therefore be limited to minimally tolerable proportions become.

Aluminum:

This element is a strong nitride generator, which nitrogen is already in the Melt sets and thus the effectiveness of the added nitrogen is strong impaired. The aluminum nitrides formed in the melt are very coarse and lower ductility. Aluminum must therefore have a weight fraction of Be limited to 0.01%.

Carbon:

Carbon forms chromium carbides when tempered, which for an improved Creep resistance are beneficial. If the carbon content is too high, this leads to this resulting increased volume fraction of carbides, however, to a Ductility reduction, which is particularly due to the carbide coarsening comes into play during long-term outsourcing. The carbon content should therefore be capped at 0.1%. Another disadvantage is that The fact that carbon increases the hardening during welding. The particularly preferred carbon content is in the range between 0.02 and 0.07 % By weight.

Brief description of the drawing

Several exemplary embodiments of the invention are shown in the drawing.

Fig. 1

eine graphische Darstellung, bei der die Streckgrenze ausgewählter Legierungen bei Raumtemperatur in Abhängigkeit der Übergangstemperatur des Bruchaussehens (FATT) aufgetragen ist und die Wirkung des Nickels und der Wärmebehandlungstemperaturen auf die Streckgrenze und auf FATT entnehmbar ist;

Fig. 2

eine graphische Darstellung, bei der die Streckgrenze ausgewählter Legierungen bei Raumtemperatur in Abhängigkeit der Übergangstemperatur des Bruchaussehens (FATT) aufgetragen ist und die Wirkung des Nickels und des Kobalts auf die Streckgrenze und auf FATT entnehmbar ist;

Fig. 3

eine graphische Darstellung, bei der die Streckgrenze ausgewählter Legierungen bei einer Prüftemperatur von 550 °C gegenüber der Übergangstemperatur des Bruchaussehens (FATT) aufgetragen ist und die Wirkung des Nickels und des Kobalts auf die Streckgrenze bei 550 °C und auf FATT entnehmbar ist;

Fig. 4

eine graphische Darstellung, bei der die Streckgrenze der erfindungsgemässen Legierung "alloy D" aus verschiedenen Wärmebehandlungen resultierend zusammen mit den Vergleichslegierungen X12CrNiMo12 (martensitisch-härtbarer Stahl) und IN706 (ausscheidungshärtbare Ni-Fe Legierung) und den dazugehörigen Kerbschlagarbeiten Av gegenüber der Prüftemperatur aufgetragen ist

Fig. 5

eine graphische Darstellung, bei der die Streckgrenze der erfindungsgemässen Legierung "alloy E" aus verschiedenen Wärmebehandlungen resultierend zusammen mit den Vergleichslegierungen X12CrNiMo12 (martensitisch-härtbarer Stahl) und IN706 (ausscheidungshärtbare Ni-Fe Legierung) und den dazugehörigen Kerbschlagarbeiten Av gegenüber der Prüftemperatur aufgetragen ist

Show it:

Fig. 1

a graphic representation in which the yield strength of selected alloys is plotted at room temperature as a function of the transition temperature of the fracture appearance (FATT) and the effect of the nickel and the heat treatment temperatures on the yield strength and on FATT can be seen;

Fig. 2

a graphic representation in which the yield strength of selected alloys is plotted at room temperature as a function of the transition temperature of the fracture appearance (FATT) and the effect of nickel and cobalt on the yield strength and on FATT can be seen;

Fig. 3

a graph showing the yield strength of selected alloys at a test temperature of 550 ° C compared to the transition temperature of the fracture appearance (FATT) and the effect of nickel and cobalt on the yield strength at 550 ° C and on FATT can be seen;

Fig. 4

a graphic representation in which the yield strength of the alloy "alloy D" according to the invention from various heat treatments resulting from the comparison alloys X12CrNiMo12 (martensitic-hardenable steel) and IN706 (precipitation-hardenable Ni-Fe alloy) and the associated impact energy Av is plotted against the test temperature

Fig. 5

a graphic representation in which the yield strength of the alloy "alloy E" according to the invention from various heat treatments resulting from the comparison alloys X12CrNiMo12 (martensitic-hardenable steel) and IN706 (precipitation-hardenable Ni-Fe alloy) and the associated impact energy Av is plotted against the test temperature

Ways of Carrying Out the Invention

The invention is explained below using exemplary embodiments and FIG. 1 to 5 explained in more detail.

Table 1 shows a number of alloys according to the invention.

With the exception of the alloys AP35 and AP38, which melt as 10 kg in were melted in an induction furnace, all other alloys were Form of 60-80kg electrodes according to the electro-slag remelting process manufactured. Furthermore, with the exception of the AP28M alloys Adjustment of the specified nitrogen content during melting, or during the remelting process, no excess pressure applied. These alloys were therefore at 0.9 bar (atmospheric pressure) melted, or remelted. The resulting Nitrogen analyzes (Table 1) show that the preferred nitrogen levels even with high nickel contents (up to 5.5%) without excess pressure during the Production can be brought.

W2:
Lösungsglühen bei 1080 °C / 2h / Luft-Abkühlung auf Raumtemperatur
Anlassbehandlung bei 640 °C / 2h / Luft-Abkühlung auf Raumtemperatur
Spannungsarmglühung bei 600 °C / 1h

W4:
Lösungsglühen bei 1180 °C / 2h / Luft-Abkühlung auf Raumtemperatur
Anlassbehandlung bei 640 °C / 2h / Luft-Abkühlung auf Raumtemperatur
Spannungsarmglühung bei 600 °C / 1h

T2C:
Lösungsglühen bei 1180 °C / 2h / Luft-Abkühlung auf Raumtemperatur
Reaustenitisierung bei 750 °C / 2h / Luft-Abkühlung auf Raumtemperatur
Spannungsarmglühung bei 600 °C / 1h

The following heat treatments provide a framework for solution heat treatment and tempering (reaustenitization) temperatures within which the heat treatments were performed:

W2:
Solution annealing at 1080 ° C / 2h / air cooling to room temperature
Tempering treatment at 640 ° C / 2h / air cooling to room temperature
Stress relieving at 600 ° C / 1h

W4:
Solution annealing at 1180 ° C / 2h / air cooling to room temperature
Tempering treatment at 640 ° C / 2h / air cooling to room temperature
Stress relieving at 600 ° C / 1h

T2C:
Solution annealing at 1180 ° C / 2h / air cooling to room temperature
Reaustenitization at 750 ° C / 2h / air cooling to room temperature
Stress relieving at 600 ° C / 1h

In all other heat treatments, the solution annealing and tempering temperatures were changed so that they were still between those of W2, W4 and T2C. Forged blocks with cross-sectional dimensions of 7x7 cm ² were used for the heat treatment.

FIG. 1 shows AP28M for 3 different alloys according to the invention, "alloy D "and" alloy E ", which are all alloyed with 4% by weight cobalt and are otherwise mainly differ in nickel content, the adjustable Combination of yield strength at room temperature and transition temperature appearance (FATT). The results will be one with those commercial alloy of the type X12 CrNiMo12 compared, which at 1060 ° C solution annealed, tempered at 640 ° C and stress relieved at 600 ° C has been. Basically, it can be seen that the alloys selected improved yield strength values and / or ductility can be achieved. It is crucial to determine that the achievable combination of Yield strength and ductility are sensitive both by the content of Nickel can also be influenced by the solution annealing temperature. Out Figure 1 clearly shows that by raising the solution annealing temperature the yield point is effective, but at the same time at the same time reducing ductility, can be improved. Furthermore, it is clear from FIG. 1 that a Raising the nickel content effectively, but at the expense of ductility Strength, can be improved. From the combination of both positions new opportunities arise by optimizing the nickel content on the one hand and by optimizing the heat treatment on the one hand other side alloys with improved strength and To set ductility properties. A simple heat treatment related (W2: 1080 ° C / 640 ° C / 600 ° C) an optimal combination of Heat resistance and ductility with nickel contents in the range between 3 and 3.5% achieved. An exceptionally good combination of yield strength and ductility can be used in particular with high - nickel alloys ("alloy E") set a two-stage austenitizing treatment at 1180 and 750 ° C. This favorable combination of properties is due to the low Ac3 temperature of the Allows high-nickel alloy. For example, it can be assumed that in Alloys with nickel contents of 5.5% by weight almost complete the matrix at 750 ° C is completely austenitic. This means that a large volume share of Vanadium nitrides, which was dissolved at 1180 ° C, in the following Annealing treatment at 750 ° C in austenite. The Alloying with nickel can obviously be used well after a Solution heat treatment and quenching to room temperature To introduce aging annealing at lower austenitizing temperatures, before the heat treatment with the classic quenching and Tempering treatment is continued.

Figure 2 shows the effect of cobalt on the Combination of yield strength at room temperature and the FATT transition temperature. In this example various heat treatments with solution annealing temperatures between 1080 and 1200 ° C, tempering temperatures between 640 and 750 ° C and one final stress relieving treatment at 600 ° C. Clearly too can be seen that all alloy with low cobalt contents in the lower Quadrants with low yield strength values and high FATT values are therefore similar alloys with increased cobalt contents in terms of the adjustable Combination of yield strength and ductility are clearly inferior.

In Figure 3 are analogous to the results shown in Figure 2 at The test results on the same alloys (same Heat treatment) but taking into account the hot yield point at 550 ° C shown. Provide alloys with cobalt, e.g. B. AP28M, a much better one Combination of the hot stretch at 550 ° C and the transition temperature the fracture appearance as alloys without cobalt, e.g. B. "alloy A". best Properties are achieved with a two-stage solution heat treatment (1180 ° C, 750 ° C) reached.

Figures 4 and 5 show graphical representations in which the yield point the alloys "alloy D" and "alloy E" depending on the performed heat treatment is plotted against the test temperature. Furthermore, the yield strengths of the comparative alloys are X12CrNiMo12 (matensitically hardenable steel) and IN706 (precipitation hardenable alloy), and their notch impact work Av compared to the inventive Alloys drawn. It can be seen that for the inventive Alloys "alloy D" and "alloy E" improve the heat resistance up to high test temperatures regardless of the heat treatment and independent of the nickel content is retained. The new alloys according to the invention show in comparison with austenitic high temperature alloys (IN706), which are designed on a nickel-iron basis, an extraordinarily good one Combination of impact energy at room temperature and heat resistance at 550 ° C.

Of course, the invention is not limited to that described Embodiments limited.

Claims (17)

Martensitic hardenable tempering steel characterized by the following composition (data in% by weight): 9 to 13% Cr, 0.001 to 0.25% Mn, 2 to 7% Ni, 0.001 to 8% Co, at least one of W and Mo in the sum between 0.5 and 4%, 0.5 to 0.8% V, at least one of Nb, Ta, Zr and Hf in the sum between 0.001 to 0.1%, 0.001 to 0.05% Ti, 0.001 to 0.15% Si, 0.01 to 0.1% C, 0.12 to 0.18% N, a maximum of 0.025% P, a maximum of 0.015% S, a maximum of 0.01% Al, a maximum of 0.0012% Sb, a maximum of 0.007% Sn, a maximum of 0.012% As, balance iron and usual impurities, and the proviso that the weight ratio of vanadium to Nitrogen V / N is in the range between 3.5 and 4.2. Martensitic hardenable tempering steel according to claim 1, characterized by 2 to 4.5% Ni. Martensitic-hardenable tempering steel according to claim 1, characterized by 2.7 to 3.7% Ni. Martensitic-hardenable tempering steel according to claim 1, characterized by 4 to 7% Ni. Martensitic-hardenable tempering steel according to claim 1, characterized by 4.5 to 6.5% Ni. Martensitic-hardenable tempering steel according to one of claims 1 to 5, characterized by 0.5 to 6% Co. Martensitic-hardenable tempering steel according to claim 6, characterized by 2 to 6% Co. Martensitic-hardenable tempering steel according to claim 6, characterized by 3.5 to 4.5% Co. Martensitic-hardenable tempering steel according to one of claims 1 to 8, characterized by 10 to 12% Cr. Martensitic-hardenable tempering steel according to claim 9, characterized by 10.5 to 11.5% Cr. Martensitic-hardenable tempering steel according to one of claims 1 to 10, characterized by 0.02 to 0.07% C. Martensitic-hardenable tempering steel according to one of Claims 1 to 11, characterized by 0.5 to 0.7% V and 0.14 to 0.17% N. Martensitic-hardenable tempering steel according to one of claims 1 to 12, characterized by 0.01 to 0.07% Nb. Martensitic-hardenable tempering steel according to one of claims 1 to 13, characterized by a sum of Mo and W in the range between 1 and 4%. Martensitic-hardenable tempering steel according to claim 14, characterized by less than 1% W and a sum of Mo and W in the range between 1 and 2.5%. Martensitic-hardenable tempering steel according to claim 15, characterized by less than 0.5% W and a sum of Mo and W in the range between 1 and 2.5%. Process for the heat treatment of a steel with a composition according to one of Claims 1 to 16, characterized by the following temporally successive process steps: Solution annealing at 1050 to 1250 ° C, Cooling to a temperature below 300 ° C, Tempering treatment, partial or complete reaustenitization at 600 to 900 ° C, Cooling to a temperature below 300 ° C, Annealing at a temperature of 550 to 650 ° C.

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