| Equipment of Ultra Super Critical (USC) under high temperature conditions are easily subjected to low-cycle fatigue (LCF), which is one of the important damage. Dynamic strain aging (DSA) appears in Heat-resistance steels under deformation. A lot of research shows that DSA reduces LCF life of heat-resistance steels. However, the temperature range and mechanism of DSA and DSA on the damage mechanisms of fatigue is unclear in heat-resistance steels under LCF loading. In this paper, by means of Inter Friction (IF) we study the mechanism of DSA in type heat-resistance steels such as P91 martensitic steel and TP347H austenitic steel. DSA effect on fatigue behaviors and fatigue damage models were studied. The deform mechanisms and damage mechanisms under LCF were investigated using TEM and SEM.Results show that DSA effect appears in different temperature ranges in P91 and TP347H steels. At strain rate of 3×10-2/s-3×10-5/s, the DSA region of P91 steel is from 250℃ to 500℃, while the DSA region of TP347H steel is from 300℃ to 700℃ at strain rate of 3×10-3/s-8×10-5/s.DSA effect reduces high temperature LCF life of P91 and TP347H steel used in USC boilers. The more effective DSA, the lower the LCF fatigue life in TP347H steel. A new parameter cyclic softening rate(R) is used to describe DSA effect. Relationship between cyclic softening rate(R) and the high temperature LCF life can be expressed that the lower R, the lower the LCF life in TP347H steel.DSA effect was not observed in P91 steel at USC working temperature regime of 550-650 ℃.However, DSA occurs when USC equipment starts up and shuts down, or works in the temperature range of 250-500℃. The investigation shows that pre-reformation of P91 steel at DSA region reduces the high temperature fatigue life due to remarkable Bauschinger effect.We study in detail LCF behavior of TP347H steel in the temperature range of 550-650 ℃, where DSA happens. It was found that LCF behavior depends on the strain amplitude and temperature. Cyclic softening increases with strain amplitude at RT, while cyclic softening weakens with strain amplitude at high temperature of 550-650℃. Planar dislocation structures formed at 550℃-650℃resulting from DSA, which reduced deformation coordination in grains and ductility. Deformation localization and reduction of ductility are responsible for reduction of LCF life. That is reflected on the fracture surface as multiple crack initiation, more second cracks and faster crack propagation rate.LCF life of TP347H steel is superior to P91 steel at RT and 550-650℃. LCF life of P91 steel under cyclic strain conditions at RT and 550-650℃,and TP347H steel at RT, were found to obey fatigue life prediction models, Manson-Coffin relationship. However failure life for TP347H steel at high temperature of 550-650℃ were deviated from the Coffin-Manson Law. It could be explained that DSA reduces ductility of TP347H at high temperature and influences plastic strain amplitude under cyclic deformation. Therefore, the Manson-Coffin relationship only including one parameter (plastic strain amplitude) could not predict the fatigue life of TP347H under high temperature condition.For the onset of serrated yielding in P91 steel, activation energy is about 73 kJ/mol, equal to activation energy of C atoms diffusion in a-Fe. The occurrence of DSA can be attributed to diffusion of C atoms in P91 steel. For the disappearance of serrated yielding in P91 steel, a rather higher activation energy of 202 kJ/mol was found, which is suggested that the high activation energy for the disappearance of serrations is the sum for the activation energy for diffusion of carbon solute and the binding energy of the solute to dislocation.The mechanism of DSA for TP347H is more complicated to that of P91. In the medium temperature range from 300℃ to 500 ℃, C atoms and (C, M) (M=Cr, and Nb) short-range-order (SRO) atmospheres are responsible for DSA, while at high temperature from 550℃ to 700℃, interaction between dislocation and substitutional atoms, such as Cr, Ni, and Nb, results in DSA effect. There are two factors for no occurrence of P91 steel at temperature higher 500 ℃. Firstly, the concentration of substitutional atoms in P91 is lower (about 8.44wt%Cr) than that in TP347H steel, about 20wt%(Cr+Nb), and the carbides easily precipitate and coarsen, which decreases the concentration of substitutional atoms in P91. Secondly, diffusion rate of substitutional atoms in body-centered cubic (bcc) structure for P91 steel is faster than that in face-centered cubic (fcc) structure for TP347H steel. Therefore, less substitutional atoms effectively lock dislocations in P91 at high temperature, at the same time, the interaction between more carbides and dislocation also restricts DSA effect. |
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