International Journal of Minerals, Metallurgy and Materials V olume 20 , Number 12 , December 2013 , P age 1164 DOI: 10.1007/s12613-013-0850-0
Metallographic etching and microstructure characterization of NiCrMoV rotor steels for nuclear power Peng Liu1) , Feng-gui Lu2) , Xia Liu3) , and Yu-lai Gao1) 1) School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China 2) School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 3) Shanghai Turbine Plant, Shanghai 200240, China (Received: 12 April 2013; revised: 21 May 2013; accepted: 22 May 2013)
Abstract: The grain size of prior austenite has a distinct influence on the microstructure and final mechanical properties of steels. Thus, it is significant to clearly reveal the grain boundaries and therefore to precisely characterize the grain size of prior austenite. For NiCrMoV rotor steels quenched and tempered at high temperature, it is really difficult to display the grain boundaries of prior austenite clearly, which limits a further study on the correlation between the properties and the corresponding microstructure. In this paper, an effective etchant was put forward and further optimized. Experimental results indicated that this agent was effective to show the details of grain boundaries, which help analyze fatigue crack details along the propagation path. The optimized corrosion agent is successful to observe the microstructure characteristics and expected to help analyze the effect of microstructure for a further study on the mechanical properties of NiCrMoV rotor steels used in the field of nuclear power. Keywords: metallography; etching; austenite; grain size and shape; etchants; nuclear power plants
1. Introduction NiCrMoV rotor steels, combined deep hardenability with high ductility, high strength, high fatigue strength, and creep resistance, are widely applied in aircraft, pressure vessels, and automotive industry [1]. Moreover, NiCrMoV steels are extensively used for low-pressure steam turbine rotor shafts in power generation plants, attributing to their excellent mechanical properties [2-3]. It is well known that the microstructure will produce distinct influence on the mechanical, electrical, and magnetic properties, so it is really significant to clearly reveal the grain boundaries and then determine the grain size for this rotor steel [4-5]. To determine the grain size of prior austenite, in principle, two fundamental stages are involved: namely, it clearly reveals the grain boundaries of prior austenite first and then determines their size accurately. In particular, to obtain excellent and comprehensive mechanical properties, NiCrMoV steels are generally quenched and tempered for dozens of hours at high-temperature atmosphere before they are made into the rotor of steam turbines. UnfortuCorresponding authors: Feng-gui Lu
E-mail:
[email protected]
nately, the difference between the microstructure of prior austenite in the grain boundaries and that in the intragranular zone becomes much inconspicuous after long time high-temperature temper. Therefore, it is difficult to display the prior austenite grain clearly and further calculate the grain size accurately. Due to the unavoidable differences in chemical composition, heat treatment, and other undeterminable factors, the etch of metallographic sample is generally a complex electrochemical process, and the validity and the efficiency of etching methods to reveal austenite grain boundaries in steels are still uncertain [6]. Therefore, the revealing of prior austenite grain boundaries could be a difficult task, especially in medium carbon micro-alloyed steels, which showed low sensitivity to the chemical etching [7-8]. NiCrMoV rotor steels belong to medium carbon micro-alloyed steels, and it is a challenge to clearly reveal the grain boundaries after high-temperature temper. Some investigations [9] involving medium carbon micro-alloyed steels demonstrated that the procedures Yu-lai Gao
E-mail:
[email protected]
c University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2013
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P. Liu et al., Metallographic etching and microstructure characterization of NiCrMoV ...
based on the special combination of heat treatment and chemical etching are not effective to reveal the austenite grain boundaries for certain austenitization conditions in a given steel, even if these steels have similar chemical composition. Different etchants to reveal the prior austenite grain boundaries have been reported [7-10]. Picric acid, ammonium persulfate, and nital are the three etchants that were often used to reveal the prior austenite grain boundaries. Zhu et al. [11] adopted 4% nital in alcohol to show the prior austenite grain boundaries of 25Cr2Ni1MoV rotor steel, but the prior austenite grain boundaries were not clearly enough. In the process of revealing prior austenite grain boundary, even minor variations in the composition of the etchant or the etching temperature and time might influence the results [12]. In view of this condition, the variety of agent, corrosion temperature, and other related parameters need to be tested to reveal the grain boundaries for NiCrMoV rotor steels. As a suitable chemical etchant, picric acid is often employed in an appropriate solvent in combination with a wetting agent, which was a system originally developed by Cho et al. [13]. Subsequently, researchers [10,14-15] have further improved the effects of wetting agents or additives on the etching of prior austenite grain boundaries in low-alloy steels. However, the best
Table 1. C 0.30
Si 0.12
Chemical etchant Etchant 1 Etchant 2
2. Experimental In this work, 25Cr2Ni2MoV was selected as the example of NiCrMoV rotor steels due to the wide application in the low pressure turbine rotor. The chemical composition of 25Cr2Ni2MoV rotor steel is listed in Table 1. The material was oil quenched and tempered at 580◦ C for 27 h. All the samples to be examined were ground and polished using standard metallographic techniques. Reagents listed in Table 2 were used to reveal the prior austenite grain boundaries. Etchant 1 is the 4% nitric acid in alcohol that is the conventional etchant, while etchant 2 is the presently developed agent consisting of picric acid, distilled water, dodecyl benzenesulfonic acid sodium salt (C18 H29 NaO3 S), and HCl.
Chemical composition of 25Cr2Ni2MoV rotor steel wt%
Mn 0.10-0.30 Table 2.
compositions and etching conditions vary a lot as for different steels. Therefore, a specific etchant and associated etching method have to be developed for each particular case. The aim of this work was to develop an effective etchant to clearly reveal the prior austenite grain boundaries of NiCrMoV rotor steels and then evaluate the grain size, which is one of the significant parameters to determine the inherent properties of the rotor steel. Besides, the propagation of the fatigue crack was also clearly observed attributing to the clear appearance of the grain boundaries.
P 0.015
S 0.015
Cr 2.15-2.45
Mo 0.60-0.85
V 0.12
Ni 2.00-2.50
Cu 0.17
Chemical etchants to reveal austenite grain boundaries
Description 4% nital (4 mL nital (HNO3 ) + 96 mL ethanol (C2 H5 OH)) 5 g picric acid + 200 mL distilled H2 O + 4 g dodecyl benzenesulfonic acid sodium salt (C18 H29 NaO3 S) + 2 mL 5at% HCl
After the prior austenite grain boundaries were revealed, the grain sizes were measured by the optical microscope (OM) and Image Pro-Plus based on the planimetric procedure in ASTM E 112. The grain size of prior austenite was represented by the area (A), mean diameter (D), round rate (R), length (L) and width (W), all of which were calculated by the planimetric procedure at least 50 intercepts in total. Among these parameters, the mean diameter was defined as characteristic dimension to characterize the grain size.
3. Results and discussion 3.1. Revealing of prior austenite grain The microstructure of 25Cr2Ni2MoV etched by etchant 1 (E1) was shown in Fig. 1. Fig. 1(a) showed the micrographs in low magnification, and Fig. 1(b) showed the high magnification of the white frame position marked
Etching condition 25◦ C and 10 s 75◦ C and 150 s
with arrow 1. It is found that only some indistinct and discontinuous grain boundaries of prior austenite could be observed. The unclear boundaries were indicated by the white arrows, and it is no doubt that the results are unsatisfactory to reveal the prior austenite grain boundaries and unacceptable to evaluate the grain size. In order to compare the effects of different etching results with these two etchants, the results of microstructure etched by etchant 2 (E2) were taken from the same position as that in Fig. 1, and the corresponding zone shown in Fig. 2. In Fig. 2(a), the grain boundaries of prior austenite could be observed in the entire vision field. Fig. 2(b) was the high magnification micrograph of the white frame position marked with arrow 2. The clear boundaries of prior austenite grains could be observed and pointed by the white arrows. Compared with the corrosive results in Figs. 1 and 2, it is clear that the corrosive effects etched by etchant 2 are much more satisfied.
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Fig. 1.
Int. J. Miner. Metall. Mater., V ol. 20 , No. 12 , Dec. 2013
Optical micrographs of 25Cr2Ni2MoV etched with etchant 1: (a) austenite grain boundaries in low mag-
nification; (b) austenite grain boundaries in high magnification.
Fig. 2.
Optical micrographs of 25Cr2Ni2MoV etched with etchant 2: (a) austenite grain boundaries in low mag-
nification; (b) austenite grain boundaries in high magnification.
It is worth illustrating the mechanism for the different results etched by these two etchants. Generally speaking, the chemical etchants are able to reveal the grain boundaries of the prior-austenite because the chemical potential of grain boundaries is slightly higher than that of the crystal grains. An appropriately chosen chemical etchant can reflect that small difference in potential. However, the difference between grain boundaries and the grain inside are remarkably reduced for 25Cr2Ni2MoV rotor steel due to its high temperature (580◦ C) and long time (27 h) temper. As for the conventional etchant consisting of mineral acid, such as nital solution and hydrochloric solution, the corrosive speed is very fast, and as a result, the slight difference between grain boundaries and the grain cannot be revealed. In contrast, for the etchant 2 consisting of picric acid, distilled H2 O, dodecyl benzene sulfonic acid sodium salt, and a little HCl, the situation is really different. Two possible reasons may result in this phenomenon. On one hand, as an anionic organic inhibitor, dodecyl benzenesulfonic acid sodium salt can be ionized into anion in the supersaturated picric acid solution. The anion is easy to
be adsorbed on the anode substrate, forming the corrosive liquid layer on the surface of the metal. Thus, the tempered matrix is protected from being etched. Nevertheless, the effect of adsorption is comparatively weak in the grain boundaries because of its too many defects and higher energy in this zone. On the other hand, the Fe3+ from the chemical reaction of Fe and HCl can restrain the etching reaction in the interior of the grains. These coupled effects produced by etchant 2 strengthened the difference between the grain boundaries and the grain inside. Therefore, clear grain boundaries were successfully revealed based on this newly developed chemical etchant.
3.2. Calculation of grain size The standards to determine grain size are set in ASTM E 112. There are three distinct methods to determine the grain size: the comparison procedure, the intercept procedure, and the planimetric procedure. Here, the planimetric procedure was applied due to its less deviation than the other two methods. Based on this method, the grains are counted inside a
P. Liu et al., Metallographic etching and microstructure characterization of NiCrMoV ...
circle with known area. Grains that are cut from the circle are counted as one half. After conversion to grains per mm2 , the grain size is read from a table. Using the standardized procedures allows the determination of a grain size number or the mean grain size. The entire polished and etched cross-section (10 mm × 20 mm) was captured by a digital image analysis system in the form of a mosaic of many single micrographs. According to the relevant requirement of the ASTM E112, an appropriate and continuous field containing at least 50 grains is necessary for the calculation of prior austenite grain size number. Fig. 3 is the overall micrograph of the selected area for calculating the grain size number of prior austenite. The grain size is hence evaluated from areas where a network of coherent grains is clearly visible. The clear prior austenite grain boundaries can be observed in Figs. 4(a) and 4(b), which are the two parts of the overall micrograph marked with arrows 1 and 2. It is noticed that the counting of single noncoherent grains may cause the possibility of neglecting small grains. To avoid this error, the prior austenite grains are then manually marked, and the
Fig. 4.
parts in grain were filled with black, as displayed in Fig. 5.
Fig. 3.
Overall micrograph of the selected area for cal-
culating the grain size number of prior austenite.
Micrographs of two parts of the selected area indicated by arrows 1 (a) and 2 (b) in Fig. 3.
According to ASTM E 112, the grain size number (G) can be calculated by G = 3.321928 lgNA − 2.954
(1)
NIntercepted )/A (2) 2 where NA is the number of prior austenite grains per square millimeter, A (mm2 ) is the area of the test circle, NInside is the number of grains completely locating inside the test circle, and NIntercepted is the number of grains that intercept the test circle. According to the calculation based on formulas (1) and (2), the grain size number (G) is approximately equal to 5.3. In addition, the parameters of grain size are calculated with the software named Image Pro-Plus, and the results are shown in Table 3. To increase the creditabilNA = (NInside +
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ity and accuracy of the calculation, parameters in Table 3 were calculated by grains that were completely inside the test circle. As the most important characteristic parameter, the mean diameter of prior austenite grains is about 72.0 µm.
3.3. Observation of fatigue crack propagation Attributing to the clear appearance of the grain boundaries, it is convenient to observe the structurerelated measurement information, e.g., the observation of fatigue crack propagation. This will be helpful to analyze the regularity and then probe their failure mechanism. Fig. 6 shows the microstructure of 25Cr2Ni2MoV rotor steel along the fatigue crack, and the most typical morphology of the transgranular mode is marked with broken line and white arrows. Therefore, the successful appearance of the grain boundaries is the precondition to
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further investigate the influence of microstructure on the related properties of rotor steel.
Fig. 5.
Overall micrograph after grain boundaries
were marked and the parts in the grain were filled with
Fig. 6.
Microstructure of 25Cr2Ni2MoV rotor steel
along the fatigue crack.
black. Table 3. Item Minimum Maximum Mean
µm2
Area / 143.7 18690.1 2788.4
Parameters of grain size in the test circle Diameter / µm 18.4 216.5 72.0
4. Conclusions The 4% nital etchant, to some extent, can meet the requirement to observe the microstructure of NiCrMoV rotor steels. However, this etchant is not good enough to clearly reveal the grain boundaries of prior austenite. In contrast, the newly developed etchant, consisting of picric acid, dodecyl benzenesulfonic acid sodium salt, HCl, and distilled H2 O, can successfully reveal the grain boundaries of prior austenite. Moreover, the clearly etched metallographic results provide the basis to determine the grain size. The prior austenite grain size number of 25Cr2Ni2MoV rotor steel in this experiment is 5.3. Moreover, the mean diameter of prior austenite grains is 72.0 µm. At ambient temperature, the fatigue crack growth is by transgranular mode, which can be obviously observed after the sample etched using this newly developed etchant.
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