Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• It is an object of th is i nvention to provide a method for prod uci ng a tempered martensitic heat resistant steel for high temperature applications at an application temperature of up to 650°C.
• a fu rther object is to provide a tempered martensitic heat resistant steel for high temperature applications at an application temperature of up to 650°C.
• Tempered martensitic heat resistant steel derive their creep strength from four principal sources:
• the solution treatment temperature should be high enough to dissolve completely the BN, M(2)(C,N) and M 23 (C,N) 6 particles.
• the temperatures of complete dissolution of BN are usually higher than the transformation temperature to ⁇ -ferrite.
• M 2 3(C,N) 6 particles dissolve at lower austenitization temperatures and are normally completely dissolved.
• the most important types with regard to high temperature creep properties are M 23 (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 M 2 (C, N) wherein M is mainly Nb, V, Ta or Cr (and m ixtures thereof).
• the C: N ratio (as expressed in wt.%) must be below 1.3.
• the C: N is below 1.2.
• the tempering treatment is performed at a temperature in the range of 10 to 50°C above the intended application temperature.
• the tempering treatment would be performed at a temperature of between 660 and 700°C.
• intended application temperature the operating temperature is meant at which the heat resistant steel is used.
• the solution treating is performed between 1150°C and 1250°C. These temperatures allow for a complete dissolution of the M 2 3(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.
• the oil is at ambient temperatures.
• 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.
• 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 tem perature for substantially the same period of time.
• the period of time at the tempering temperature is between 1 and 5 hours, preferably between 2 and 4 hours.
• a method 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.
• a tempered martensitic heat resistant steel for high temperature applications having a chemical composition as described hereinabove and produced in accordance with the method as described hereinabove wherein the microstructure of the steel after tem pering comprises intragranular preci pitates having a size of at most 70 nm of the M (C,N) and/or M 2 (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 M 2 3(C,B) 6 precipitates wherein M is mainly composed of Cr and Fe on the lath, block, packets and/or prior austenite grain boundaries.
• the M(C,N) and/or M 2 (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 M 2 (C,N) preci pitates have a size of between 10 a nd 70 n m, more preferably between 10 and 50 nm, even more preferably between 10 and 30 nm.
• both Ta and V are present as an alloying element.
• the steel accord ing to the i nvention 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.
• the C: N ratio of the steel is below 1.2.
• Cr level should be selected accordi ng to the application tem perature 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 tem peratu res that al low the dissolution of preci pitates.
• a suitable maximum content is 2%.
• Cu is optionally added to influence the morphology of the Laves phase and to avoid formatio n of delta-ferrite in a similar way as Co.
• the recommended amount of Cu is 1.5%, for 9.0% Cr this addition can be lower.
• M n a nd Ni are added to hel p to avoid formation of delta-ferrite similar to Co.
• Recommended levels are below 0.5% Mn and up to 0.6% Ni.
• Ni and M n a minim um 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 M 2 3(C,B) 6 .
• recommended combination is 1.5% W and 0.5% Mo.
• a suitable minimum W-content is 0.5%.
• N ratio should be low in order to favour the formation of M(C,N) or M 2 (C,N) particles rich in nitrogen and reduce M 23 (C,B) 6 fraction.
• 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.
• the tempering should be done in one or more, preferably one or two, steps at the same or different temperatu res, 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 M 2 (C,N) particles. It is also believed that low temperature tempering favours formation of M 2 (C,N) over M(C,N).
• Step A A 50kg air induction melt of Steel 92 material was produced (Steel A).
• a second 60kg cast with h ig her n itrogen and lower carbon contents was prod uced 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 M 2 (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 outl ined in Table 2.
• a sim u lated PWHT of 1 hour at 740°C followed by air cooling.
• steel B samples were prepared with the low tem peratu re tem peri ng treatment and two sol ution treatments at 1 150°C a nd 1200°C.
• Steel C and D are given the treatment as it is shown in Table 3.
• Table 2 Heat Treatment Schedule for Steel A and B.
• 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.
• microstructure of the samples in as tempered condition has been characterised and the results thereof are presented in the figures 1 to 4.
• Figure 1 and 2 show examples of the microstructure comprising very fine M 2 (C,N) and M(C,N) precipitates formed i n Steel C A4-T2 sample (solutionised at 1200°C, quenched in oil and tempered at 660°C/3h AC + 660°C/3h AC).
• Figure la TEM Bright Field (BF) micrograph of Steel C A4-T2 sample foil showing martensite lath interior precipitates.
• Figure lb TEM Dark Field (DF) micrograph of Steel C A4-T2 with M2X precipitates taken in diffraction spot "dfl" of Figure lc.

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• 2011
  • 2011-06-10 CA CA2801637A patent/CA2801637A1/en not_active Abandoned
  • 2011-06-10 WO PCT/EP2011/059657 patent/WO2011154515A1/en not_active Ceased
  • 2011-06-10 US US13/701,017 patent/US20130160905A1/en not_active Abandoned
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