Solubility Product of VN in Austenite of High Cr Heat Resistant Steel

Abstract

Dissolved V in austenite of 7%Cr-0.4%V-0.09%N steel (hereinafter, % means mass%) was measured and the solubility product of VN was determined as follows:where T is the absolute temperature in K. The solubility product obtained is 3 to 5 times larger than those in the literature and about one order larger than that for iron. The decrease in the activity coefficient of nitrogen due to the presence of Cr and V in the experimental steel is responsible for the increase in the solubility product of VN.KEY WORDS: precipitation; MX; thermodynamics; activity coefficient. log([% ][% ])V N 4.38 ϭ Ϫ 8 436 T stituent elements. The detectable limit for V in this study is about 0.42 ppm for the experimental steel. The solubility product of VN is calculated using the following equation assuming that chemical composition of VN is stoichiometric.[%V] [%N] Symbols, [%V] and [%N], denote the dissolved V and N in austenite, and V t , N t denote the total V and N, respectively. Symbol, [%N 0 ], denotes the dissolved N in Fe-V-N alloy which is calculated from the observed values of the dissolved V. The amount of the dissolved vanadium in the matrix, [%V], is assumed to be equal to the amount of the dissolved vanadium observed. Experimental Results and Discussion Thermal EquilibriumCharacteristic features of the present analyzing method are that the duration of heat treatment is relatively short, and that the V content of a filtered solution is directly measured. Therefore, firstly, whether the equilibrium in the precipitation of VN is satisfactorily guaranteed by 1 h heating or not becomes a problem. Therefore, the effects of heating rate and heating duration should be discussed. Figure 1 shows the Arrhenius plot of the dissolved V. Open marks correspond to rapid heating where a small coupon was put into a pre-heated furnace and temperature of the coupon was reached to an aiming temperature within 5 min at least. Closed marks correspond to slow heating where a coupon was set in a cold furnace and gradually heated up to an aiming temperature taking 3 h. All coupons were held for 1 h at a temperature in both cases. The figure shows that a linear relationship is found in a wide range of temperature by the rapid heating and the regression line may probably correspond to the equilibrium solubility limit of V in austenite. On the other hand, the amount of the precipitates by the slow heating is insufficient at low temperatures as compared with those of the rapid heating. Figure 2 shows the time dependence of the dissolved V of the rapidly heated specimens at 1 100°C. This figure shows that the enough precipitation has been roughly accomplished by the 1 h rapid heating. Therefore, from Figs. 1 and 2 it is found that the equilibrium of the precipitation of VN in austenite is practically realized by a technique of the rapid heating of a small coupon followed by the short time holding at a temperature. The authors have studied the precipitation behavior of TaC and NbC and concluded that the precipitation of MX in austenite is accomplished wit...

Recovery and Precipitate Analysis of 9 Pct Cr-1 Pct MoVNb Steel during Creep

“…Similar dips were observed by simply tempering the high-chromium martensitic steels. [26,27,56,57] A similar dip was also reported accompanying the recrystallization process of cold-rolled lowcarbon ferritic steel. [58] Before and after the occurrence of a dip, individual compounds or a combination of compounds of M 23 C 6 , Cr 2 N, MX, and VX were dissolved temporarily followed by reprecipitation.…”

“…A dip in a hardness recovery curve took place at a longer time as the tempering temperature decreased. [26,27,56,57] The apparent activation energies obtained using the hardness dip data roughly agreed with those for the diffusion of the interstitial elements in bcc iron, although the continuous softening occurring during tempering was, of course, controlled by the diffusion of substitutional atoms. [56] This is an explanation shown in the previous work for the occurrence of a dip in the extracted-residue-vs-time curve.…”

“…During this time, drastic changes in the microstructure should happen, for example, the formation of subgrains referring to the previous work made on the similar martensitic steel. [26,56,57] Although in the acceleration creep region a considerable amount of fine particles of VX precipitated, the material continued to soften and proceed through to rupture. During this period, in the usual martensitic steels, [6,11,13,16,63,64] the width of the lath martensite or subgrain size increases and, consequently, the creep rate increases.…”

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

“…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.…”