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A Study of Quantitative Metallography and X-Ray Diff raction Methods for Quantifying Carbide Amount in AISI D2 Tool Steel

Abdullah Sert¹, Selim Gürgen², Osman Nuri Çelik¹

¹Eskişehir Osmangazi University, ²Anadolu University - Türkiye

Abstract Carbide formation in tool steels can alter the mechanical properties. To accurately measure the amount of carbide is very important. The metallurgists in academia and industry often select different imaging techniques for quantifying carbide in tool steels. In this study, the amount of carbide by two groups of techniques was calculated. The first technique is quantitative metallography which includes OM with etching reagent of Vilella and backscattered electron - scanning electron microscope (BSE-SEM) imaging. Digital image analysis with Fiji software was used for calculating the amount of phases in the first technique. The second technique is chemical methods which include XRD analysis. The phase analysis of AISI D2 tool steel were acquired with elemental analysis (Fe and Cr) via EDS. The both of techniques (quantitative metallography and X-ray diffraction analysis) were applied to determine carbide in conventionally heat treated and cryogenically treated tool steels for a comparison differences. As a result of measurements, the cryogenically treated tool steel showed higher percentage and uniform distribution of carbide. 1. Introduction Conventional hardening and tempering methods are widely utilized in order to extend the wear properties of the tool steels. In recent studies, sub-zero treatments are suggested for improving the mechanical properties of the tool steels. Microstructures after conventional treatments include retained austenite which deteriorates the mechanical properties of the materials. However, the retained austenite in the microstructure is fully transformed to martensite by applying sub-zero treatments. Cryogenic treatment and then tempering form post precipitated carbides (secondary carbides) in the microstructure. Secondary carbides distributed through the matrix with the presence of primary carbides improve the

hardness, toughness and wear resistance of the tool steels [1-2]. Characterization of the materials plays an important role since the amount, shape and distribution of the carbides influence the mechanical properties of the material [3]. In this study, conventional hardening and cryogenic heat treatment of AISI D2 tool steel were investigated. In the evaluation of the results, quantitative metallography and X-ray diffraction methods were performed to compare the microstructural variations in the samples. 2. Experimental Procedure AISI D2 samples with following chemical composition 1.48 C, 0.39 Mn, 0.47 Si, 0.02 P, 0.02 S, 11.24 Cr, 0.81 Mo, 0.72 V and balance Fe (wt%) were used in this research. The samples were subjected to conventional heat treatment (HT - austenitization at 1030 °C for 1 hour and oil quenched, tempered at 200 °C for 1 hour) and cryogenic treatment (HCT - austenitization at 1030 °C for 1 hour and oil quenched, cryo-treatment at -196 °C for 36 hours, tempered at 200 °C for 1 hour). Microstructural examinations of polished and Vilella etched samples were conducted using JSM-5600LV (JEOL) scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and Eclipse L150 (Nikon) optical microscope. The size and quantity of carbides were calculated using the Fiji (ImageJ) image analysis software. The presence of phases in both heat treated samples was measured by (PANalytical) X-ray diffraction (XRD) using Cu-K radiation at 45 kV and 40 mA with 35-105° 2 . For macro hardness measurements, Rockwell C hardness tester was used with 150 kg load. 3. Results and Discussion Figure 1 shows the SEM images of HT and HCT samples including various sizes of carbides which are consisted of

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M7C3 and M23C6 (M=Fe, Cr, V, Mo). [4, 5]. Figure 2 shows the OM images of HT and HCT samples which were rendered for the amount and distribution of the carbides by using Fiji software. It is seen that carbide particles in HCT sample are smaller in size and well-dispersed in the matrix in comparison to HT sample. However, it should be noticed that OM analyses become difficult as the particle size of the carbides reduces in the microstructure. Regarding to this, Vimal et al. and Surberg et al. suggested that microstructural changes due to sub-zero heat treatment are troublesome in the OM analyses. [6, 7].

Figure 1. SEM images of HT and HCT.

Figure 3 shows the results of the analyses in HC and HCT samples. The results are given in terms of carbide fractions for corresponding carbide sizes obtained in both SEM and OM analyses. All image analysis were repeated 5 times in the different region of samples and the results

were averaged. Carbide sizes are divided into three groups which are small secondary carbides (SSCs) with the size of 1 μm, large secondary carbides (LSCs) between the size of 1 μm and 5 μm and primary carbides (PCs) with the size of 5 μm. Justification for the classification of carbides and selection of their size limits has been reported in a study [8]. It is seen that there is a wide difference between the results of OM and SEM for SSCs in HT sample. The fraction of SSCs in SEM analysis is approximately three times more than in OM analysis. This conflict stems from the ability to distinguish for high magnification rates in the SEM in comparison to the OM. However, SEM and OM results are in a good agreement for PCs which are large enough to be detected. Similar to the HT samples, there is a big difference between the fractions of SSCs in the results of SEM and OM for HCT samples. It is also important that SSCs are more likely to be formed in HCT than in HC since the SSCs in HCT sample is denser with a ratio of 49.2%. Besides, the SEM and OM analyses give closer results in HCT samples in comparison to HC sample.

Figure 2. Rendered OM image of HT and HCT.

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Figure 3. Image analyses results of amount fraction

of carbides.

Figure 4. EDS analysis of AISI D2 tool steel with HCT

treatment.

Figure 4 shows the EDS analysis result of the HCT sample which was taken inside of the red frame (area 1) on a carbide particle in the SEM image. The chart shows the presence of Cr, Fe, V, Mo and C elements in the composition of the carbide particle. In tool steels including chromium, two types of Cr carbides (Cr7C3 and Cr23C6) are often observed. Depending on the chemical composition of the steel, alloying elements such as Fe and Mo enrich these carbides. As the temperature increases, the precipitation sequence in tool steels is noted as follows; M3C M7C3 M23C6 where M refers to metal atoms. The transformation of M3C (Fe3C) to M7C3 (Cr7C3) is formed by the nucleation at the Fe3C (M3C)/ferrite interface [9]. In the light of this result, it is possible to mention that the chemical structure of the carbide is M7C3 (M=Fe, Cr, V, Mo). Also it can be seen that the chemical composition of the area 2 is different than area 1. Area 2 is taken from the matrix and the weight percent of Cr, V, Mo and C elements are lower than area 1. In order to detect the phase transformations after the treatments, samples were investigated with XRD analysis. In the XRD results, both treatments result in the identical peak positions while the intensity of the peaks varies depending on the treatment as shown in Figure 5. The peak characteristics were determined with X’Pert HighScore software and previous studies in the literature [4, 10]. The XRD chart gives that carbides of M7C3 and M23C6 are formed in the matrices after the treatments. In fact, detection of the carbides is quite problematic since lower amounts of carbides are formed in the matrix and their peaks coincide with the peaks of martensite. In the evaluation of the outcomes, coincident peaks are discussed considering the martensitic phases. Formation of the secondary carbides arises from the transformation of the retained austenite during the sub-zero treatment. Cold treatment after conventional heat treatments may cause retained austenite in the matrix. For example, martensite finish temperature of AISI D2 steel is -125 C which means that fully transformation to martensite is not possible in the conventional treatments for this steel. However, cryogenic heat treatment ensures the martensitic transformation for the retained austenite. During the transformation, crystallographic defects and dislocations with high densities may be seen in the matrix. Carbon atoms form clusters in the vicinity of the defects and act as the sources for carbides during the treatment [3]. It is known that XRD peaks become wider as the structural defects and stacking faults increases in the matrix [11]. Furthermore, full-width at half-maximum (FWHM) of XRD peaks increases due to the enhanced point defects and material hardness [12].

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Figure 5. XRD analyses of AISI D2 tool steels with HT and

HCT treatments. Table 1 gives the FWHM of the XRD peaks shown in Figure 5. Peak of M7C3 carbides are located between the angles of 42.5 and 43.1 . The difference between the FWHMs after HT and HCT is obtained as 16%. The variations for the other martensite and carbide peaks are 5.99%, 2.14%, 6.29% and 6.02% respectively where the values are higher in HCT samples. It is also important that intensity and width of the peaks increases in HCT sample due to the increased amount of the secondary carbides in the matrix.

Table 1. FWHM analyses of HT and HCT treated AISI D2

tool steels.

Peak 2theta HT-FWHM

HCT-FWHM

42.5 – 43.1 0.21754 0.25905 43.5 – 45.5 0.33984 0.36151 63.5 – 65.9 0.73836 0.75452 80.7 – 83.5 0.67572 0.72114 97.8 – 99.1 0.90154 0.95932

Table 2. Comparison of HT and HCT.

Hardness HRC

SSCs -SEM

SSCs -OM

FWHM -XRD (M7C3)

Relative Intensity

-XRD (M7C3)

HT 60 19.31% 6.00% 0.21754 3.43% HCT 61 38.06% 27.89% 0.25905 7.12% Table 2 gives the comparison of HT and HCT in terms of mechanical, microstructural and XRD results. The hardness measurement showed +1 HRC enhancement. The macro hardness enhancements +0.5 HRC [13] and +1 HRC [10] have been reported in the literature. The increase of the SSCs is consistent with both SEM and OM investigations. FWHM and relative intensity of M7C3 peaks are given for the scanning angle range of 42.5 and 43.1 . FWHM of the peaks in HCT sample is 16% better than that of in HT sample.

4. Conclusion In this study, AISI D2 tool steel was treated with various heat treatments to investigate the carbide formation in the matrix. Metallographic and XRD investigations were performed for the samples in the analyses. HT and HCT methods accelerate the formation of the carbides in the matrix. However, carbides are more prone to be formed in HCT. On the other hand, visualization techniques of SEM and OM were compared in the microstructural investigations. It is seen that SEM is more precise than OM especially for detection of the SSCs. Last, the change in the amount of M7C3 in the SEM results is found to be consistent with the XRD results. Acknowledgments The authors A.Sert and S.Gürgen acknowledge the support of the Scientific and Technological Research Council of Turkey (TÜB TAK) under Program 2211. References [1] D. Das, R. Sarkar, A. K. Dutta, and K. K. Ray,

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