Nb and V carbides in 12Cr-2.5Ni-2Mo-0.2V-0.08Nb-N creep resistant steel


Table 1: Chemical composition and heat treatment of conventional and developed disc forgings.


Table 2: Mechanical properties of conventional and developed disc forgings.


Figure 1: Embrittlement of conventional 12% Cr steel and developed 12CrNb steel.


Figure 2: TEM image of extraction replica from developed 12CrNb steel. (a) Nb and VC omplex carbide formation and (b) corresponding energy dispersion spectrum. Scale bar: 0.1 µm.

Carbide name: Nb and V carbides
Record No.: 1078
Carbide formula: No data
Carbide type: No data
Carbide composition in weight %: No data
Image type: EDS, TEM
Steel name: 12Cr-2.5Ni-2Mo-0.2V-0.08Nb-N
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Creep resistant steels
Steel composition in weight %: See the table 1.
Heat treatment/condition: Table 1 shows the chemical composition of these samples. The samples were prepared from 5 kg ingots by a high frequency induction furnace. Then, these ingots were forged into 30 mm square bar with initial temperature of 1150 C to final temperature of 850 C. These samples were quenched from 1050 C, and then, twice tempered for 5 h at 590 C and for 5 h at 605 C in turn, in order to obtain a completely martensitic structure. The samples for mechanical examination were machined for impact test (JIS Z 2202), tensile test (JIS Z 2201), and creep rupture test (JIS Z2271). Preparation of a real scale disc sample(about 1 t) was manufactured by different process because segregation of alloying elements causes serious defects for mechanical evaluation. The steel was prepared by electroslag remelting (ESR)as a 10 t ingot. The ingot was forged into a cylindrical shape, 935 mm dia. and 250 mm thickness, by a 6OOO t hydraulic press.
Tensile examination was carried out at room temperature by an Instron type testing machine of 5 t capacity. Impact examination was carried out at 20 C by 30 kgf-m Charpy impact equipment. Creep rupture examination was carried out by a lever type 3 t creep machine.
Note: Effect of adding elements on creep rupture strength and toughness for heat resistant 12% Cr steel of 1300 C class gas turbine disc was investigated. Creep rupture strength was increased by addition of Mo, W,V. Nb, and N. It was found that the addition of Nb causes a significant increase of creep rupture strength due to fine dispersion of NbC and of ductility due to grain refining. Chemical composition of 12Cr-2.5Ni- 2Mo-0.2V-0.08Nb-N steel is determined for gas turbine disc application. Preparation of real scale disc was carried out by electro-slag remelting process in order to avoid the segregation of adding elements in ingot. It is resulted that the mechanical properties of real scale disc specimen show the equivalent properties of small specimens. The steel discs for gas turbine were manufactured from the new steel.

Figure 1 shows fracture appearance transition temperature (FATT) and absorbed energy of the conventional and new 12Cr steel as a function of aging time with fractographs of studied samples. According to the fractographs, intergranular fracture is corresponding to the aging embrittlement of the samples. The conventional 12Cr steel sample aged for 13410 h at 450 C shows complete intergranular fracture surface. However, the 12CrNb steel shows transgranular fracture surface, which implies the fracture mode does not change even after aging for 10OOO h.The new 12CrNb steel discs have been operated in commercial power plants since 1988 without replacement.

As shown, addition of Nb increases both creep rupture strength and toughness for 12Cr steel. It is expected that an increase of toughness with Nb addition is caused by austenite grain refining prior to quenching. The samples were prepared by quenching after austenitization holding at 1050 C for 0.5 h. The grain size measurement of the samples is carried out by Hyne method. The effect of Nb on grain refining can be concluded that undissolved NbC prohibits grain growth of austenite during heat treatment. Figure 2 shows (a) bright field image of TEM of extraction replica of NbC and (b) its energy profile by EDX. It is revealed that Nb forms carbide which disperses homogeneously. It is reported that spherical NbC combines with rectangular V4C3. In consideration of interaction between dislocation and particle, this complex carbide can reduces the interspacing of neighbor particles by two times the side length of the longitudinal direction of V4C3. It is reported that Nb added 12Cr steel contains higher dislocation density. V forms either V4C3 or part of M in M23C6. It can be considered that complex carbide formation increases creep rupture strength becaus eof the decrease of inter particle distance for the mobile dislocations.
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