Re-dissolution of VN during Tempering in High Chromium Heat Resistant Martensitic Steel

Abstract

Precipitation behavior of VN during isothermal tempering at 740-800°C of 7%Cr-0.4%V-0.09%N steel (% denotes mass%, hereinafter) has been studied. Initially, rapid softening takes place accompanied by the precipitation of VN and, after that, the quasi-steady state in a hardness vs. tempering time diagram is continuing for a while. After the quasi-steady state, re-dissolution of VN particles rapidly occurs followed by final precipitation of VN. Just before the peak time of the re-dissolution of VN particles, both the temporal decrease in hardness and the temporal increase in the integral breadth of an X-ray diffraction peak take place. The similar precipitation phenomenon is confirmed in 0.14%C-9%Cr-1%Mo-0. 2%V-0.09%Nb steel. In both steels the re-dissolution of VN or NbC (hereinafter MX) accompanies the decomposition of martensite. The following reactions are suggested as a mechanism for the re-dissolution of MX type particles: local stresses induced by the recovery of martensite unlock the pinning of dislocations by the MX type particles and the consequent isolated particles, which become energetically unstable, are re-dissolving into the matrix.KEY WORDS: precipitation; hardness; X-ray integral breadth; Nb; VN.VN system as observed in the NbC system. And, therefore, the precipitation behavior of NbC during only tempering has been also re-investigated. Experimental ProcedureA 50 kg ingot was hot-rolled to 12 mm plate. The chemical composition of the experimental steel is 0.0032%C-0.15%Si-0.51%Mn-0.002%P-0.001%S-7.2%Cr-0.3%Mo -0.41%V-0.091%N and the balance is Fe. The Nb content is less than 0.0005 %. The hot rolled plate was normalized at 1 200°C followed by tempering at 740, 780 and 800°C for maximum 500 h. It was confirmed that neither any precipitate composed of V nor delta ferrite was observed after normalizing at 1 200°C. 13)A heat-treated specimen was dissolved in 6 N-HCl solution and the residues were separated using a filter with 0.2 mm pores. The dissolved V in the filtered solution is directly measured using an inductively coupled plasma atomic emission spectroscope (ICP). The detailed experimental procedure and the accuracy of the analyzing method are reported in the previous paper.13) Vickers hardness number was determined under 10 kg load and optical microstructure and scanning electron microscopy were observed after etching using the Vilella's reagent. Integral breadth of a ͕110͖ diffraction peak was also measured using Cu Ka 1 characteristic X-ray. An extracted carbon replica was observed using a transmission electron microscope of a field emission type (FE-TEM) operating at 200 kV and the chemical compositions of some particles were analyzed using an energy dispersive spectroscope (EDS). Experimental Results Dissolved VThe measured values of the dissolved V in the matrix of the tempered state are shown in Fig. 1. The dissolved V decreases with increasing tempering time up to about 10 h and the decreasing rate is fast when tempering temperature is high. This decrease corresponds to the precipitati...

Evolution Behavior of Carbides in 2.25Cr-1Mo-0.25V Steel

“…However, Tamura et al have found that MC can dissolve at first and then precipitate again during long-term aging. 14,15) In a word, dissolution of MC into matrix maybe occurred temporarily after 10 hours tempering.…”

Thermodynamic Modeling in Reduced Activation Steels

“…In this case, the authors presented evidences of a process of dissolution of the fine NbC particles during the early stages of tempering, which would be reprecipitated after a long tempering exposure. Moreover, in a very recent study 84) the same team proposed a mechanism to account for that redissolution in the case of Cr-V model alloys. On the other hand, our own EDX-STEM composition measurements on Ta-rich particles in the as-received state of the Eurofer97 alloy 85) turned out to be much more closer to the predicted composition values for the temperature of 1 050°C, suggesting that these precipitates are effectively far from equilibrium after 1 h tempering at 760°C.…”

Method of Estimating the Long-term Rupture Strength of 11Cr-2W-0.4Mo-1Cu-Nb-V Steel

“…Kubon et al [50] supported the idea proposed by Lundin et al [45,46] Such dynamical interactions of dislocations with constituent elements of MX are not a rare case; sequential events of precipitation on dislocations, energetic destabilization of the precipitated particles through the breaking away of dislocations, and the subsequent dissolution of precipitated particles are well recognized in the tempering of high-Cr heat-resistant steels. [51][52][53][54] These phenomena can be explained thermodynamically, and therefore, besides the MX particle, fine particles of not only ecarbide [55] and M 23 C 6 [56] but also even the stable oxide Y 2 Ti 2 O 7 [57] have been found to be easily dissolved in interactions with dislocations. [58] That is, it is clear that when we discuss the creep deformation of heat-resistant steel, we should take the local equilibrium around mobile dislocations into account, and microstructural considerations based on a simple thermal equilibrium and/or kinetic simulation are insufficient.…”