Research Trends of Heat Treatment of Austenitic Heat Resistant Steel
In order to reduce CO2 emissions, extensive research, and development of A-USC equipment. 23Cr-45Ni-7W alloy (ASTM UNS N06674) is a material that improves creep strength by strengthening Laves phase precipitation. It is a substitute material for high-temperature steam piping and boiler tubes in A-USC equipment. Boiler tubes sometimes need to be cold-bent during construction, so it is necessary to understand the changes in characteristics during processing. Research reports have evaluated the effect of cold working on the creep strength characteristics of the alloy.
The application of Ni-based superalloys with better heat resistance strength than conventional heat-resistant steels in 700℃-class A-USC steam turbine impellers was studied. As a Ni-based alloy for A-USC turbine impellers, Ni-0.05C-18Cr-13Co-9Mo-1.3Al-1.4Ti-0.1Ta-0.3Nb alloy was developed. The alloy is made of Inconel 617 (ASTM UNS N06617) is the basic material, and the alloy composition has been adjusted and improved. There are research reports on the effect of heat treatment conditions on the mechanical properties of the alloy.
As a Ni-based alloy material that can be applied to casting parts of the impeller box and nozzle box of the ultra-supercritical pressure (A-USC) generator set with a steam temperature of 700 ℃, there are research reports on the effect of Al and Ti addition on Ni- The influence of 0.07C-13Cr-9Mo-19Cr-0.1Ta-0.3Nb-Al-Ti alloy on mechanical properties was evaluated. This alloy is compounded with Ta and Nb added to the material based on ASTM UNS N06617 (equivalent to alloy 617).
There are research reports on the application of precipitation-strengthened forged Ni-based superalloy Ni-19Cr-12Co-6Mo-2Al-3Ti-1W-0.05C-0.005B in 700℃-class A-USC steam turbine rotating parts. As a result, it can be seen that the phase as an intermetallic compound precipitates as needles at a nozzle temperature of 850°C. Therefore, the low-cycle fatigue test and tensile test were carried out on the aged material, and the influence of the relative mechanical properties was studied.
In order to enable Fe-18.4Cr-31.0Ni-3.3Nb alloy to be used in A-USC units with a steam temperature of 700℃, a new Fe-18.4Cr-31.0Ni-3.3Nb alloy (mass%) is proposed. The intermetallic compound containing Nb serves as a strengthening phase. The grain boundaries of this alloy are almost covered by the Laves phase, which is the TCP phase, and the Ni3Nb phase of the GCP phase is precipitated in the crystal grains, so it has good high-temperature strength.
In order to improve the corrosion resistance of heat-resistant materials, Cr must be added, but the relationship between the amount of Cr added in this alloy and the structure is not clear. Therefore, based on the 18Cr additive, some researchers introduced the relationship between the structure change and the creep characteristics when the amount of Cr addition increased or decreased at 1623mass%.
In order to improve the power generation efficiency of thermal power generation equipment, we have actively promoted the development of 700℃ super ultra-supercritical pressure (A-USC) power generation equipment. In these development projects (thermie700 in Europe and DOE-Vision21 in the United States, etc.), one of the standard alternative materials for boiler equipment that can be used in harsh environments is alloy 617 (JIS-NW6617).
Therefore, a research report evaluated the high-temperature tensile properties of NW6617 and the low-cycle fatigue properties of various waveforms, combined with the cross-sectional observation, clarified the detailed high-temperature strength characteristics, and evaluated the damage of the creep-fatigue life. . In particular, the characteristics of the high-temperature characteristics of NW6617 have been clarified by comparing austenitic heat-resistant steel and high-chromium ferritic heat-resistant steel.
Fe-Ni alloy HR6W (Fe-23Cr-45Ni-5/7W-Ti, Nb) is an alternative material for A-USC boiler piping. As we all know, boiler piping is generally a large-diameter pipe, and residual stress will occur in the welding heat-affected zone during welding. As a method to reduce this residual stress, there is SR (stress relief) treatment, but its treatment conditions for HR6W are not yet clear.
Therefore, in order to confirm the SR treatment conditions, a stress relaxation test was performed on the microstructure changes after the SR treatment, the material, and the base material after welding. Later, a research report investigated the creep rupture strength of HR6W after SR treatment.
In order to develop a new generation of high-efficiency 700℃-class A-USC equipment, various alloys have been developed so far. In particular, research on a new type of Fe-18.4Cr-31.0Ni-3.3Nb alloy (mass%) that uses intermetallic compounds containing Nb as a strengthening phase is also being actively carried out.
Some researchers have investigated the structure change and creep characteristics of this alloy when the Cr addition amount is changed and found that when the Cr addition amount reaches 23 mass%, the creep strength will be greatly reduced. Therefore, in order to make the intragranular strengthening phase more refined and stable, Zr was selected as an additive element, and the influence of the addition of Zr on the microstructure and creep characteristics of the Cr alloy was studied.
In order to develop 700°C-level A-USC power generation equipment, a new type of austenitic heat-resistant steel Fe-20Cr-30Ni-2Nb (at.%) with two intermetallic compounds (TCP phase and GCP phase) as the strengthening phase Designed. The steel can meet the required characteristics of A-USC boiler tubes (700°C, 105h creep rupture strength of 100MPa or more) because the Fe2Nb-Laves phase (TCP phase) can strengthen the intergranular precipitation.
Therefore, in order to clarify the application of this strengthening mechanism in 800°C-class power generation equipment (FA-USC), a research report investigated the 1073K creep characteristics where only the Laves phase was precipitated, focusing on the Laves phase at the grain boundary to study it. Relationship with the organization.
The creep strength of Fe-20Cr-30Ni-2Nb (at.%) strengthened by the grain boundary TCP-Fe2NbLaves phase and the intragranular GCP-Ni3Nb phase is much higher than the existing steel grades. The steel basically meets the target value of 700°C A-USC power generation equipment (700°C, 100MPa, 105h). In addition, heat-resistant steel that can be used for a higher steam temperature of 800°C is designed.
In order to maintain the creep strength of the material under the water vapor environment of 800℃ in the boiler tube, the oxidation resistance of the material is also very important. Therefore, a research report investigated the creep behavior of the steel in a water vapor atmosphere simulating the actual power generation equipment boiler tube and compared it with the test results in the atmosphere.
Fe-20Cr-30Ni-2Nb steel is proposed as a new type of heat-resistant steel that can be used for 700℃-class A-USC generator set boiler tubes. The actual use shows that this steel can precipitate the TCP phase (Fe2Nb-) to the grain boundary and make GCP The phase (Ni3Nb-) precipitates into the crystal and has good creep characteristics. It provides help for the development of super heat-resistant steel that can withstand steam temperatures up to 800°C.
If the transition metal elements M constituting the two compounds are different, the strength can be further improved by independently controlling the structure of the grain boundary (Fe2M1) and the intragranular (Ni3M2). Therefore, from the point of view of the phase balance between -Fe/TCP phases, some research reports point out that the transition metal element M can be selected as the Fe-Ni-Nb-M4 elementary metal after Nb is fixed, and the phase balance and the distribution of Nb and M in each phase were studied.