Patent Application
20130160905
Global requirements for energy are forecast to double by 2030 with a projected build of 40 nuclear power stations alone in the EU. In addition to this significant increase in fossil fuelled stations will be required to meet the predicted demand. In order to minimise emissions of greenhouse gases improved generating efficiency, through increased steam temperatures and pressures, and carbon capture technology will be required.
Currently martensitic steels used for turbine blades and casings, bolting and boiler tubes are limited to service temperatures of about 620° C. and the best commercially available alloy is Steel 92-9% Cr, 0.5% Mo, 2% W. Over the last 15 years significant effort has been invested by international consortia, such as COST 522 and 536, in Europe to raise the operating temperatures of these martensitic alloys. Attempts at alloy modifications have not thus far yielded any solutions backed up by long term creep data.
It is an object of this invention to provide a method for producing a tempered martensitic heat resistant steel for high temperature applications at an application temperature of up to 650° C.
A further object is to provide a tempered martensitic heat resistant steel for high temperature applications at an application temperature of up to 650° C.
The object is reached by a method as described in claim 1 and a steel as described in claim 9. Preferable embodiments are described in the dependent claims.
According to the invention, a method is provided for producing a tempered martensitic heat resistant steel for high temperature applications at an application temperature of up to 650° C., wherein the steel comprises, on the basis of percent by weight:
Tempered martensitic heat resistant steel derive their creep strength from four principal sources:
During exposure at elevated temperature the dislocation density is reduced and precipitate coarsening occurs both of which reduce the resistance to creep deformation. Coarsening of M23(C,B)6 is more rapid than M(C,N) and/or M2(C,N), but the rate can be reduced by boron additions. However the long term creep strength of these alloys is strongly dependent on the volume fraction and stability of the M(C,N) precipitate dispersion. The stability of the precipitates is enhanced by tempering above the application temperature and the volume fraction is controlled by the solution treatment temperature and cooling rate prior to tempering. The use of lower tempering temperatures according to this invention in combination with a tuned chemical composition will produce a high density of fine and stable precipitates. High solution treatment temperatures increase the alloy in solution prior to tempering, but in practice solution temperatures are restricted in order to ensure a fully martensitic microstructure and to prevent excessive grain growth which might lower creep ductility. Also, it is important to prevent formation of δ-ferrite during the solution treatment to ensure a substantially martensitic (α′) matrix on cooling because the occurrence of a dual phase (α′+δ) structure is detrimental for creep resistance. The presence of more or less austenite stabilising elements shift the transformation temperature to δ-ferrite up or down.
The solution treatment temperature should be high enough to dissolve completely the BN, M(2)(C,N) and M23(C,N)6 particles. The temperatures of complete dissolution of BN are usually higher than the transformation temperature to δ-ferrite. The same applies for some of the M(2)(C,N)-precipitates, depending on the composition and amounts. Therefore, not all particles will completely dissolve at the austenitization temperatures used. M23(C,N)6 particles dissolve at lower austenitization temperatures and are normally completely dissolved.
On tempering, alloying elements dissolved during austenitization are precipitated. Lowering the tempering temperature from the conventionally used 780° C. for tubes or 710° C. for blading (second step) to a temperature of e.g. 660° C. increases the volume fraction of M(2)(C,N) and M23(C,N)6. Lowering the carbon level significantly decreases the volume fraction of M23(C,N)6 in the ferrite region. In the case of M(2)(C,N) particles lowering the C:N ratio resulted in a further increase in their volume fraction.
The inventors found that control of the C: N ratio and the free boron in solid solution is essential in the control of the size and stability of the various precipitate types. The most important types with regard to high temperature creep properties are M23(C,B)6 wherein M is mainly Fe, Cr, W or Mo (and mixtures thereof), and carbo-nitrides of the type M(C,N) and/or M2(C,N) wherein M is mainly Nb, V, Ta or Cr (and mixtures thereof). The C:N ratio (as expressed in wt. %) must be below 1.3. Preferably the C:N is below 1.2.
Good, long term creep resistance requires a high density of small precipitates which are resistant to coarsening during long term exposure under stress at elevated temperatures. The purpose is to stabilise the dislocation substructure and to inhibit dislocation mobility. The former is controlled mainly by M23(C,B)6 precipitates at lath boundaries and the latter by the nano-scale carbo-nitride precipitation at boundaries and within laths. It is surprising that the tempering just above the application temperature of the steel in service results in a better creep resistance, even after a subsequent post-weld heat treatment where improvements of 170% over conventionally heat treated samples are obtained.
In an embodiment of the invention, a method is provided for producing a tempered martensitic heat resistant steel for high temperature applications at an application temperature of at least 620° C., or preferably between 620 to 650° C.
According to the invention the tempering treatment is performed at a temperature in the range of 10 to 50° C. above the intended application temperature. For a service or application temperature of 650° C., the tempering treatment would be performed at a temperature of between 660 and 700° C. With intended application temperature the operating temperature is meant at which the heat resistant steel is used.
In an embodiment of the invention the solution treating is performed between 1150° C. and 1250° C. These temperatures allow for a complete dissolution of the M23(C,B)6, and the dissolution of the majority of the M(2)(C,N) particles.
The quenching after solution treatment should be as fast as possible to ensure that the dissolved elements remain in solid solution and to ensure formation of a fully martensitic microstructure. The quenching is preferably performed in oil. In an embodiment the oil is at ambient temperatures. However, the quenching could also be performed by other means such as forced air, (hot) mist or even (hot) water, as long as the martensitic microstructure is obtained, the dissolved elements remain in solid solution, and the stresses as a result of the quenching do not exceed critical levels so that no warping or cracking occurs.
In a preferable embodiment the tempering treatment is performed in at least two tempering treatments, and more preferably wherein the at least two separate heat treatments are performed at substantially the same tempering temperature for substantially the same period of time. Preferably the period of time at the tempering temperature is between 1 and 5 hours, preferably between 2 and 4 hours. It is clear that the reheating to the tempering treatment should be as fast as possible to prevent undesirable reactions taking place during the reheating. By choosing different tempering temperatures for the at least two tempering treatments within the range of between 10 and 50° C. above the application temperature or service temperature the composition and size distribution of the precipitates which govern the creep resistance can be very effectively and reliably controlled.
In an embodiment of the invention a method is provided wherein the first of the at least two separate heat treatments is at the temperature range from 500° C. up to 10° C. higher than the application temperature and second is or the following are at the temperature range from 10 to 50° C. higher than the application temperature. This will result in even finer dispersion of precipitates. The latter higher temperature temper is particularly relevant where a stress relieving treatment after welding is required. As an example for an application temperature of 650° C. the tempering can be done in the first step at 500-660° C. and the second step at 660-700° C.
In a preferable embodiment the steel comprises:
It should be noted that the term “and/or” when used in this description or the claims must be interpreted in the sense that one, more or all of the preferable ranges or process conditions may be applicable.
In a preferable embodiment the steel comprises:
In a second aspect of the invention a tempered martensitic heat resistant steel for high temperature applications is provided having a chemical composition as described hereinabove and produced in accordance with the method as described hereinabove wherein the microstructure of the steel after tempering comprises intragranular precipitates having a size of at most 70 nm of the M(C,N) and/or M2(C,N) type wherein M is one or more of Nb, V, Ta or Cr and wherein the microstructure of the steel after tempering comprises M23(C,B)6 precipitates wherein M is mainly composed of Cr and Fe on the lath, block, packets and/or prior austenite grain boundaries.
In an embodiment the M(C,N) and/or M2(C,N) precipitates have a size of at least 1 nm and at most 70 nm, preferably at most 50 nm and more preferably at most 30 nm. In an embodiment the M(C,N) and/or M2(C,N) precipitates have a size of between 10 and 70 nm, more preferably between 10 and 50 nm, even more preferably between 10 and 30 nm.
In a preferable embodiment both Ta and V are present as an alloying element.
In a preferred embodiment the steel according to the invention is used in the production of components for high temperature applications such as turbine blades or casings, bolting and boiler tubes, heat exchangers or other elements in power generation systems, for use at an application temperature of up to 650° C.
In a preferred embodiment of the invention the C:N ratio of the steel is below 1.2.
Now the preferable embodiments as to the chemical composition will be described:
Cr level should be selected according to the application temperature for steam oxidation and corrosion resistance. Recommended Cr level is 9.0-11.0%.
Co is optionally added only to avoid the formation of delta-ferrite on high solution treatment temperatures and is not necessary if there is no risk of formation of this phase at the temperatures that allow the dissolution of precipitates. For steels containing 9.0% Cr the addition of cobalt is not necessary. For 10.0-11.0% Cr the addition of Co is recommended and the recommended amount of Co is 1.5%. A suitable maximum content is 2%.
Cu is optionally added to influence the morphology of the Laves phase and to avoid formation of delta-ferrite in a similar way as Co. For 10.0-11.0% Cr the recommended amount of Cu is 1.5%, for 9.0% Cr this addition can be lower.
Mn and Ni are added to help to avoid formation of delta-ferrite similar to Co. Recommended levels are below 0.5% Mn and up to 0.6% Ni. In order to benefit from the effect of Ni and Mn a minimum amount of 0.1% for one or both elements is preferable. A suitable minimum Si level is 0.1%, preferably at least 0.15%.
W and Mo are added for solid solution strengthening. Tungsten additionally stabilizes M23(C,B)6. recommended combination is 1.5% W and 0.5% Mo. A suitable minimum W-content is 0.5%.
C:N ratio should be low in order to favour the formation of M(C,N) or M2(C,N) particles rich in nitrogen and reduce M23(C,B)6fraction. Examples of favourable C and N contents are: 0.073% C and 0.065% N, 0.02% C and 0.06% N.
V and Nb or V and Ta (or combination of V, Nb, Ta) is important for the nano-scale particles. The examples of favourable V and Nb or Ta contents are 0.18 to 0.25% V and 0.04 to 0.07% Nb or 0.07 to 0.12% Ta.
Addition of B is important for stabilization of M23 (C,B)6 precipitates. It is essential is to optimize the B:N ratio in order to maximise the boron in solid solution during solution treatment. It is also important to dissolve MX or M2X particles in the solution treatment. In these particles M=V, Nb, Ta, Cr or mixtures thereof and X═N and/or C. Therefore solution treatment temperature should be as high as possible but not resulting in formation of delta-ferrite. A recommended range of solution treatment temperatures is 1150 to 1250° C.
The use of low temperature tempering, up to 50° C. higher than application temperature, results in a very fine distribution of nano-scale M(C,N) or M2(C,N) particles. The tempering should be done in one or more, preferably one or two, steps at the same or different temperatures, up to 50° C. higher than the application temperature. Recommended temperatures are especially those in the range of 10 to 50° C. higher than application temperature as they are regarded as especially favourable for fine distribution of nano-scale MX(C,N) or M2(C,N) particles. It is also believed that low temperature tempering favours formation of M2(C,N) over M(C,N).
The invention will now be further explained by means of the following, non-limiting examples.
A 50kg air induction melt of Steel 92 material was produced (Steel A). A second 60kg cast with higher nitrogen and lower carbon contents was produced by vacuum induction melting (Steel B). Two additional casts (Steels C and D) of 50 kg each were also produced by vacuum induction melting. Cast C and D have the lowest C:N ratio to favour formation of M(C,N) and/or M2(C,N) particles. The chemical analyses are shown in Table 1. The ingots were forged to 50mm square bar then rolled to 19mm diameter round bar for the production of test specimens.
Samples from Steel A were given a standard heat treatment or a treatment involving a higher solution treatment temperature at 1150° C. and lower tempering temperature of 660° C. as outlined in Table 2. In order to study the effect of PWHT samples with the standard and inventive treatments were given a simulated PWHT of 1 hour at 740° C. followed by air cooling. For steel B samples were prepared with the low temperature tempering treatment and two solution treatments at 1150° C. and 1200° C. Steel C and D are given the treatment as it is shown in Table 3.
Plain and notched stress rupture tests in the temperature range 600-675° C. were carried out according to BS EN 10291:2000. Stress rupture results for Steel 92 (A) are shown in Table 4 and 5 (b stands for broken and ub for unbroken samples). Tests were carried out at 600-675° C. at stresses designed to give aim lives between 1,000 and 30,000 hours. In most cases for the standard heat treatment the alloy failed close to the aim lives. However at longer aim durations and higher temperatures the actual lives were shorter than intended.
The inventive heat treatment gave dramatic improvements in creep life compared with material given the standard treatment. Aim lives were generally exceeded by significant margins, although the improvement decreased with increasing temperature.
The stress rupture data for the high nitrogen Steel B at 600 and 650° C. are shown in
Table 6 for material given the low temperature temper and at two solution treatment temperatures of 1150 and 1200° C. The performance is compared with the conventional composition given the low temperature temper and the standard treatment.
The high nitrogen-low carbon Steel B is outperforming the conventional composition of Steel 92 (Steel A). The best properties were obtained using the higher solution treatment temperature of 1200° C. At 650° C. and 92 MPa Steel A with the inventive heat treatment failed after 12,089 hours for a 10,000 hour aim. The high nitrogen Steel B given a 1200° C. solution treatment failed after 27,908 hours, meaning that Steel B outperforms Steel A by a factor 2.
For Steel 92 (Steel A) with conventional heat treatment the tests at 675° C. failed far below the aim life. Better performance was achieved for the same steel but with higher solution treatment and lower tempering temperatures. However, the best creep lives at this temperature and stress level were achieved for Steels C and D (Steel B was not tested) again with higher solution treatment and lower tempering temperatures than conventional heat treatment. The aim life was exceeded by up to a factor of seven. The aim life at 650° C./122 MPa was also exceeded by a factor of seven-eigth in the case of Steel B.
The stress rupture tests at 650° C. for novel steels with inventive heat treatments demonstrate excellent results, especially for Steel B and C.
The results of a test programme for Steel A to assess notched properties reveal that the notched properties have been similar to, or in excess of, the plain properties. The effect of PWHT is that PWHT (Table 7) has little effect on the conventionally heat treated material as the tempering and PWHT temperatures are similar. For both steels the inventive heat treated material has exceeded the base values for normally heat treated materials. The results show that the 740° C. PWHT reduced the creep life to about between 63% and 76% of the non-PWHT value but still showed an improvement of 170% compared with the standard heat treatment in the case of Steel A.
The microstructure of the samples in as tempered condition has been characterised and the results thereof are presented in the
a: TEM Bright Field (BF) micrograph of Steel C A4-T2 sample foil showing martensite lath interior precipitates.
b: TEM Dark Field (DF) micrograph of Steel C A4-T2 with M2X precipitates taken in diffraction spot “df1” of
c: SAD pattern from wide sample area of
d: Indexation of SAD as M2X crystal type (ε-Fe2N type, S.G. P-31m), a=0.492 nm, c=0.447 nm; zone axis [236]. Lattice spacing indicated in Angstroms.
e: Indexation of SAD of Figure is as BCC lattice, a=0.286 nm; zone axis [101]. Lattice spacing indicated in Angstroms.
a: TEM BF micrograph of Steel C A4-T2 with MX precipitates
b: SAD pattern from wide sample area of
c: Indexation of SAD
d: Indexation of SAD
The example of EDX spectra from M(C,N) precipitate in Steel C A4-T2 is shown in
The example of EDS spectra from M2(C,N) precipitate in Steel C A4-T3 (solutionised at 1200° C., quenched in oil and tempered at 600° C./3 h AC+660° C./3 h AC) is presented in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10165598.3 | Jun 2010 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/059657 | 6/10/2011 | WO | 00 | 2/12/2013 |
