investigations of precipitation in two different types of ... · in another precipitation hardening...

13year XXIV, no. 4/2015

KeywordsPrecipitation hardening stainless steel, G-phase, Carbide,

TEM, XRD

1. IntroductionPrecipitation hardening (PH) stainless steels are the steels

in which the final hardening mechanism is precipitation hardening. Precipitation hardening is the process of hardening or strengthening of an alloy by precipitating finely dispersed precipitates of the solute in a supersaturated matrix by nucleation and growth process. The ageing kinetics of any precipitation hardening stainless steel depends upon the alloying additions that are made in the alloy for the purpose of precipitation hardening. Generally copper, molybdenum, aluminum, titanium and niobium are added to increase the strength through precipitation hardening [1÷8]. It is possible to obtain an increase of hardness in the range of 150 to 200HV by precipitation hardening [9] and the yield strengths as high as 1500MPa and even more can be achieved. This increase in strength is due to the precipitation of very fine precipitates which can be copper, silicide or various inter-metallic phases depending upon the chemical composition. In case of Cu precipitation, an increment of up to 248 MPa per 1% Cu is obtainable [10].

Alloys of the family of precipitation hardening stainless steels include 17-4PH, 17-4 Mo, 17-7 PH, 15-5PH, 13-8 PH and 13-8Mo etc. 17-4PH, 18-3PH and PH13-8Mo are strengthened by the precipitation of copper in the martensite matrix [11]. 17-4PH alloy is more common than any other type of precipitation hardening stainless steel. This alloy contains more than 3 wt% Cu and the precipitation of copper rich phases takes place in the martensitic matrix through a solution treatment and aging practice [1]-[3],[12]. In another precipitation hardening stainless steels i.e. PH17-4Mo, Molybdenum containing inter-metallic precipitate Mo2C is formed with various carbides (M23C6 and M7C3) and Fe2Mo (Laves phase) is formed in the temperature range of 500-650°C after long ageing times [11]. In 14Cr-7Ni alloy the combined addition of Si-Ti causes a Nil6Ti6Si7 G-phase to be precipitated in grains [9]. The precipitation of NiAl and Ni3Al takes place in 17-7 PH stainless steel [13].

These precipitates are very small in the size, mostly in the range of few nano meters [14]. These precipitates hardly reach a size of 40-50 nm in the over-aged condition. This small size is beyond the resolution limit of optical microscope, therefore TEM is mainly used for study and identification of these precipitates. TEM is often used in combination with XRD for obtaining the complete picture, as TEM provides detailed

information of a small localized volume and XRD data is an average of a large sized sample [15]. This paper presents the results of a study carried out for the identification of nano-sized precipitates in two different precipitation hardening stainless steels by using XRD and TEM techniques. The TEM investigations have been performed by using thin foils and carbon extraction replica techniques.

2. ExperimentalTwo types of precipitation hardening stainless steels i.e.

X5CrNiCuNb16-4 (also known as 17-4PH stainless steel) and X4CrNiSiTi14-7 were studied in this work. Their exact chemical compositions are given in Table 1. The as-received samples were heated at 620°C for 8 hours in nitrogen environment followed by furnace cooling. This heat treatment produced samples in over-aged condition which were then used for investigations.

Table 1. Chemical composition of X5CrNiCuNb16-4 and X4CrNiSiTi14-7.

Elements

[wt%]

X5CrNiCuNb16-4 X4CrNiSiTi14-7

C 0.024 0.038

Si 0.706 1.475

Mn 0.78 0.35

P 0.0166 0.0258

S 0.0005 0.0006

Ni 5.04 7.03

Cr 15.17 13.61

Cu 3.3 0.6

Ti 0.016 0.33

Mo 0.115 0.79

Nb 0.26 0.006

Al 0.011 0.037

V 0.072 0.037

The investigations have been performed by using light optical metallography, X-ray diffraction and transmission electron microscope.

Initial investigations were done by using optical microscopy. Etching was done with Beraha I (24 g ammonium difluoride

Investigations of precipitation in two different types of precipitation hardening stainless steels

S. Sakhawat(1,a, M. N. Baig(1, b, A. Falahati(2, c, H.P. Degischer(2, M. Dománková(3, A. Mahmood(1

1) Pakistan Welding Institute, Islamabad, Pakistan 2) Vienna University of Technology, Vienna, Austria

3) Slovak University of Technology, Bratislava, Slovakia

E-mail: a)[email protected], b)[email protected] , c)[email protected]

14 year XXIV, no. 4/2015

+ 1000 ml distilled water + 200 ml of concentrated HCl) [16] and etching time of 5-10 seconds was employed. Light optical microscopy was performed by a Zeiss Axioplan microscope with a digital imaging system. The magnifications of 2.5x to 100x were available with the microscope.

X-ray diffraction measurements were performed on Philips PW 1710 diffractometer with cobalt fine focus X-ray tube. The angular 2θ range was from 40° to 140° with a recording time of 10 sec per step [17].

TEM observations were performed in a microscope JEOL 200CX operating at 200 kV. Thin foils suitable for TEM observation were prepared. Small discs of 3 mm in diameter and about 0.08 mm thickness were jet-electro polished in electrolyte HNO3 : CH3OH = 3 : 7, at –20°C and 15V to obtain, electron transparent areas near the central hole. The jet-electro polishing was done in TENUPOL 2.

Carbon extraction replicas were prepared for examination by TEM and for analysis by EDX to identify the precipitates. The samples were etched in etchant consisting of 1-2 g of picric acid, 10 ml of ethanol, and 5 ml of water. The surface was coated by a C-layer and the replicas were removed from the sample surfaces electrochemically at 20V using 3% nital as electrolyte.

3. Results and discussionsPrecipitations in X5CrNiCuNb16-4 alloy

Figure 1 shows the microstructure of X5CrNiCuNb16-4 in as-received condition. The microstructure is typical martensitic. No retained austenite, delta ferrite or any precipitate is viewable in this micrograph. The over-ageing of this sample resulted in tempered martensitic matrix as shown in Figure 2. Reverse austenite was not identifiable in the microstructure, also delta ferrite was not found in the material. The optical microscopy was not able to resolve the precipitations that were expected to be formed in the samples because being in the range of nano meters they were beyond the resolution limit of optical microscopy.

with some amount of austenite. Two types of carbides were observed, M7C3 (~0.2%) and M23C6 (~0.7%), where M can be Cr. No indication of precipitation other than carbides was obtained from XRD; this may be due to the fact that the total

fraction of the precipitates was below the detection limit of diffractometry and hence they could not be detected. However the TEM investigations revealed more interesting results.

Figure 3 is the bright field image of X5CrNiCuNb16-4 alloy in over-aged condition, showing the presence of a number of precipitates in the matrix of tempered martensite. Figure 4 is the

Figure 1. Light optical micrograph of X5CrNiCuNb16-4 in as-received condition.

Figure 2. Light optical micrograph of X9CrNiCuTi15-7 in over-aged condition.

Figure 3. TEM bright field image of X5CrNiCuNb16-4 showing precipitates.

The XRD diffraction pattern of this alloy in as-received condition revealed only martensite phase thus showing that structure of this alloy in as-received condition was martensitic without the presence of any retained austenite. The XRD results in over-aged condition showed the presence of martensite phase

dark field image of the precipitates. The selected area diffraction pattern corresponding to the substructure illustrated in Figure 4 is shown in Figure 5. The reflection G-phase was used to obtain dark field image and these precipitates were identified as G-phase precipitates which were found uniformly dispersed within the grains. These precipitates were spherical in shape with size varying in the range of 10-45nm. The inter-metallic G-phase is a ternary silicide having formula of the form A16B6C7. In this formula A and B are the transition elements and C is a Group IV element. The ideal stichiometry is Ti6Ni16Si7, where Fe, Mo can substitute to Ni, and Cr, Mn or Nb can substitute to Ti. G-phase has a face-centred cubic unit cell containing 116 atoms with lattice parameter of 1.12 nm. The name of G-phase was assigned due to its prominent appearance at grain boundaries


[18÷25]. However it is shown in many studies that besides precipitating at grain boundaries the G-phase also precipitate out as a dispersed phase in the interior of the grains in various Fe-Cr-Ni alloys [25] as it precipitated in present study. Murayama et al. reported the formation of G-phase within the matrix in 17-4PH alloy. They suggested that martensite/Cu interface is a suitable site for G-phase nucleation. They observed that the small Cu precipitates are always adjacent to the G-phase in this alloy by

the three-dimensional atom probe. Thus copper precipitates provide heterogeneous nucleation sites for G-phase formation [25]. In our case Cu precipitates could not be observed. This suggests that Cu precipitates were still coherent and did not cause large strain contrast due to the small strain field around the precipitates [1]. Wang et al. [26] reported the formation of G-phase in the same alloy i.e.17-4PH and suggested that the precipitation of G-phase requires an incubation time. The temperatures employed by Murayama et al. and Wang et al. were 400°C and 350°C respectively where the diffusion rate is low, so the G-phase formation took a very long time. In present study the exposure temperature was 620°C and the incubation time was greatly reduced and G-phase precipitates were observed after 8 h. As the ideal composition of G-phase is proposed as Ti6Ni16Si7, but Since Ti is present in an extremely small amount it is expected that the possible composition of this G-phase can be Mn6Ni16Si7 or Nb6Ni16Si7. Although the alloy under investigation contained more than 3 wt% Cu but Cu precipitates could not be

observed by TEM foils. The attempt to search Cu precipitate through extraction replica technique could not succeed due to the two reasons: firstly the Cu was expected to dissolve during replica producing procedure and secondly the specimen holder

for replica sample was made up of Cu itself, therefore it could generate the wrong signals and misleading results.

Besides G-phase another type of precipitates was also found in this alloy. Figure 6 shows the dark field image of the same sample by using reflection M23C6 showing the second type of precipitate. These precipitates were identified as M23C6 carbides. Figure 7 is the selected are diffraction pattern of the Figure 6.

Precipitations in X4CrNiSiTi14-7 alloy

Figure 8 shows the microstructure of X4CrNiSiTi14-7 sample in as-received condition. The microstructure is martensitic with very fine grains and unclear grain boundaries. The over-ageing of this sample resulted in tempering of martensitic matrix (Figure 9). The microstructure consists of finer grains which are not separately distinguishable. Retained austenite, delta ferrite or any type of precipitation could not be identified.

The XRD of the as-received sample indicated the presence of single phase i.e. martensite, however the XRD of this alloy in over-aged condition showed the presence of martensite, reverse austenite, and carbides, M7C3 (~1%) and M23C6 (~2%). No indication of precipitation other than carbide was obtained from XRD.

Figure 4. TEM dark field image of X5CrNiCuNb16-4 showing precipitates.

Figure 5. Selcted area diffraction pattern corresponding to Figure 4.

Figure 6. TEM dark field image of X5CrNiCuNb16-4 showing carbides precipitates.



Figure 10 shows the substructure of the sample as obtained from TEM foil showing elongated laths. Figure 11 shows the TEM dark field image of X4CrNiSiTi14-7 alloy in over-aged

next approach was to use extraction replica technique for the identification of precipitates. In extraction replica technique the precipitates are deposited in carbon film after dissolving

Figure 8. Light optical micrograph of X4CrNiSiTi14-7 in as-received condition.

Figure 9. Light optical micrograph of X4CrNiSiTi14-7 in over-aged condition.

Figure 10. TEM bright field image of X4CrNiSiTi14-7 showing lath like substructure.

Figure 11. TEM dark field image of X4CrNiSiTi14-7 showing carbides precipitates.


condition. The only type of precipitate that could be identified through TEM foil was carbides. Since no precipitate other than carbides could be found through TEM foil method, so the

the matrix. The advantage of this technique is that since the particles are present in the replica or carbon film, they can be analyzed by diffraction or analytical techniques without interference from the surrounding matrix; therefore the visibility of precipitates is improved, especially in high dislocation density matrices. In present case the matrix was martensite phase with high dislocation density so the replica was successful in resolving the nano sized precipitates without interference from the martensitic matrix.

Figure 13 is the carbon extraction replica showing the substructure of the sample containing irregularly shaped grains elongated few µm and less than 1µm wide. A number of precipitations were observed in the sample which are viewed clearly in Figure 14 with higher magnification. These precipitates were separated from each other on the basis of their size and their chemical composition as determined by EDX. Three different types of precipitates were identified namely G-phase, M23C6 and Ti(C). The G-phase precipitates were smallest amongst the three types of precipitation with d ~ 20nm and were of regular shape. EDX analysis of G-phase precipitates showed that they are mainly constituted of Si, Ti,


and Ni. Fe and Cr were also present in these precipitates. The chemical compositions of three precipitates and their average are presented in Table 2.

Figure 13. Carbon extraction replica of X4CrNiSiTi14-7 showing the substructure and precipitates.

Figure 14. Carbon extraction replica of X4CrNiSiTi14-7 showing the precipitates.

Table 2. Chemical composition of G-phase precipitates.

S. No Chemical composition [wt %]

Si Ti Cr Fe Ni

1 24.1 14.9 5 8.5 47.4

2 20.8 15.5 5.9 9 48.8

3 20.1 14.9 3.5 8.4 53.1

Average 21.7 15.1 4.8 8.7 49.8

The table shows that the chemical composition of these particles is not much different from each other. G-phase particles are very fine and well dispersed in the matrix, indicating that they are the main phase responsible for increasing strength and hardness. This finding is also in agreement with literature [9], [27]-[28].

The second precipitate that was found through replica was M23C6 carbide. They were of irregular shape with a size of ~ 50 nm. It is found that the diffraction pattern of G-phase and the M23C6 carbide were the same so they could not separately identified from each other on the basis of diffraction pattern alone therefore the EDX analysis was necessary for

their separate identification. The chemical compositions of G-phase is very different from the chemical composition of M23C6, hence by knowing the chemical composition they can be separately distinguished. It is also suggested that Cr/Ni ratio can be used to distinguish between the two types of precipitates [29].


The third type of precipitate that was identified in this alloy was a big precipitate of ~ 200 nm size and quasi spherical shape. This precipitate was identified as Ti(C) as shown in Figure 14.

4. ConclusionsAn investigation was performed for the identification of

precipitations in X5CrNiCuNb16-4 and X4CrNiSiTi14-7 alloys. The optical metallography was not able to resolve the nano-sized precipitates. No precipitation was found in the as-received condition. In over-aged condition the XRD was successful in observing the presence of carbides (M23C6 and M7C3) in both alloys. Carbides were also resolvable by TEM foil method. In addition to carbides, G-phase precipitation was found in both alloys. In X5CrNiCuNb16-4 alloy G-phase was detected by TEM foil method while in the second alloy i.e. X4CrNiSiTi14-7 the G-phase could not the detected by TEM foil due to the high dislocation density matrix. In this case the carbon extraction replica technique was used to find the precipitates without the interference from the matrix and fine size G-phase precipitates were observed within the matrix. Ti(C) precipitates were also identified through replica technique.

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