Selection of optimum quenching parameters for large rotors of steel 25KhN3MFABorisov, I.
doi: 10.1007/BF00779390pmid: N/A
1.
To obtain a fine-grained structure during final heat treatment, the following are recommended:
a)
accelerated heating of forgings to austenitization temperature with 2-h holding after through heating;
b)
minimizing the austenitization temperature and choosing it with regard for the temperature—time dependence of the increase in austenite grain size across the forging, as well as in relation to the critical radius of the forging and the cooling medium;
c)
austenizing at the chosen temperature with the aim of obtaining stable properties;
2.
In selecting the quenching parameters, it is necessary to consider the future service conditions of the parts and the property values required of them:
Fracture of steel at different stages of hydrogen embrittlementSavchenkov, É.; Svetlichkin, A.
doi: 10.1007/BF00779394pmid: N/A
1.
In places of segregation in a polycrystalline body hydrogen facilitates microplastic deformation under the influence of torsional stresses in the first stage and the formation of microcracks under the influence of normal stresses, which reduces the capacity of steel for even deformation and strengthening.
2.
The susceptibility of steels to hydrogen embrittlement varies with the distribution of hydrogen, since local hydrogen weakening increases the sensitivity of steel to overloading and premature brittle failure.
3.
In the second stage of reversible embrittlement the capacity of the steel for even strengthening is reestablished, although hydrogen bubbles begin to form, reducing the ductility, and the characteristic signs of the stage of irreversible embrittlement appear.
4.
In conformity with the rate at which brittleness develops, the morphology of the fracture also changes. The original ductile fracture changes to typical brittle fracture during reversible embrittlement. With the development of irreversible embrittlement the fracture becomes mixed—brittle sections along with ductile sections.
5.
The most even structure, chemical composition, fields of internal stress, distribution of structural components, and dispersity of structural components increase the resistance of steel to hydrogen embrittlement.
Defects and long-term strength of materials with low deformabilityPetrenya, Yu.; Chizhik, A.
doi: 10.1007/BF00779395pmid: N/A
1.
To calculate the long-term strength of parts used in power plants that are subject to brittle fracture under creep conditions it is necessary to use the proposed system of kinetic equations that reflect the effect of invariants of the stress tensor on the micromechanism of failure and the orientation of defectiveness.
2.
This system of equations can be most effectively used in the case of mixed fracture under conditions of creep where it is necessary to extrapolate the test results to operating times of more than 100,000 h and also for fluctuating force and temperature fields leading to a change in the mechanism of microfracture.
Microfracture of metals during high-temperature creepBetekhtin, V.; Kadomtsev, A.; Petrov, A.
doi: 10.1007/BF00779396pmid: N/A
1.
A change from creep at moderate temperatures to high-temperature creep is accompanied by equalization of the defect density Δρ/ρ in the volume and in surface layers of polycrystalline metals resulting from microdiscontinuities. In this case Δρ/ρ varies with the applied stress, while the character of accumulation of defects in the process of creep changes.
2.
The characteristics of high-temperature creep of polycrystalline materials are evidently due to a considerable extent to the activation of healing of microcracks in presurface layers at elevated temperatures.
Structure, defectiveness, and service life of heat-resistant steel during prolonged operationKumanin, V.
doi: 10.1007/BF00779397pmid: N/A
1.
After prolonged operation of various steam pipes (85\2-105,000 h) the structural condition of the steel becomes uniform.
2.
With uniform structure of steel after prolonged operation at high temperatures the main factor determining the service life is the level of defectiveness.
A correlation was found between the creep rate and the level of defectiveness of steel with a uniform structure: in
$$\dot \varepsilon = \kappa \omega ^n .$$
3.
One of the main reasons for the onset of the third stage of creep is the sudden intensification of diffusion processes in the presence of a branched system of pores.
4.
To reduce the level of defectiveness it is expedient to conduct restoration cyclic heat treatment (RCHT) consisting in repeated phase recrystallization.
After 8–10 cycles of RCHT the volume of defects decreases 50–60% in greatly damaged pipe, as the result of which the creep rate decreases considerably.
Long-term strength of heat-resistant steels under creep conditionsZlepko, V.; Melamed, M.; Shvetsova, T.
doi: 10.1007/BF00779399pmid: N/A
1.
Changes in the structure that occur in the process of creep are similar in steels 12Kh1MF, 15Kh1M1F, 12Kh2M1, and 14Kh1GMF at 540–565°.
2.
The service life and consistency of the properties in the course of operation depend mainly on the structure in the original condition. With the optimal bainitic-ferritic structure in the condition as-received the steam pipes of these steels are fairly reliable in operation up to 100,000 h and longer.
3.
The possibility of steam pipes of Cr-Mo-V and Cr-Mo steels operating for more than 100,000 h depends on the rate of softening processes, which depends on the consistency of the composition of the solid solution, carbide reactions, and the processes of pore formation and rebuilding of dislocation arrays. Each of these factors may have the decisive effect on the accumulation of damage in different stages of creep.
4.
The condition of the metal before failure is characterized by the mean size of carbides, the distance between them, and the lattice constant of the α solid solution.