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Eff ect of Deep Sub-Zero Treatment Time on Heat Treated Calmax® Cold Work Tool Steel

F. Koray Arslan², Aziz Hatman², İbrahim Altınsoy¹, Gözde Çelebi Efe¹, Tuba Yener¹, Mediha İpek¹, Cuma Bindal¹, Sakin Zeytin¹

¹Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering, Esentepe Campus, 54187 Serdivan-Sakarya, Turkey

²Uddeholm Turkey, TOSB (Taysad OSB), Cayirova/Kocaeli, Turkey

Abstract In this study, it was aimed to investigate the effect of subzero heat treatment duration as well as tempering on some microstructural and mechanical properties of commercial Calmax® cold work tool steels. Firstly, five steel samples were austenitized at 960oC following by quenching at 170°C and one of them was remained as just quenched for reference. After quenching, one of other four samples was tempered at 525°C for 30 min, while two of four samples were exposed to subzero heat treatment in liquid nitrogen medium having -197°C for 15 and 60 min, respectively and last sample was subjected to subzero heat treatment for 60 min. and then tempered at 525°C for 30 min. The hardness of quenched, quenched and tempered, quenched and subzero heat treated and quenched, subzero heat treated and tempered test materials were determined as 755, 527, 807, 829, 616 HV(0.1), respectively. The microstructure of test samples investigated by Scanning electron microscopy was mainly consisting of martensite and small amount of alloy carbides after heat treatments. On the other hand, martensitic zones in the microstructure increased by increasing deep cryogenic (sub-zero) duration and seconder alloy carbides became more visible by applying of tempering. XRD analysis revealed that remained austenite was considerably eliminated by only sub-zero heat treatment, but effectiveness of the process increased with additive tempering by observing of the ferritic and austenitic peaks. The presence of carbides (Cr23C6, Cr7C3) was also verified by XRD and SEM-dot EDS analysis. The retained amount of austenite in the microstructures of samples determined by quantitative analysis of austenitic and ferritic iron peaks was calculated as 9.8% for quenced sample, 2.5 % for quenced and tempered, 1.9% for only sub-zero heat treated for 15 min., 1.4% for only sub-zero heat treated for 60 min. and 0.6% for quenced and sub-zero heat treated for 60 min. following by tempering at 525 °C, in volume, respectively. The results of the study indicated that both sub-zero heat treatment and tempering decreases the amount of retained austenite. Sub-zero heat treatment increases the hardness of test materials, while tempering decreases the hardness of

samples. Additionaly, it was found that applying of sub-zero heat treatment for only 15 min. is enough for eliminating the amount of remained austenite to desirable level. 1. Introduction Tool steels represent an important segment of the total steel production [1]. Tools for metal forming (working) are essential for the production of metal parts in various industries. For example, the automotive industry in 2015 had a share of nearly 44% of metal parts on a global scale. Along with the increase in demand from the automotive industry, the global market for stamping / punching metals is growing at a steady pace [2]. They are used for the manufacture of tools, dies and components of mechanical devices that demand steels with special properties [1]. The manufacturing industry. Industry is always searching for means of increasing the durability of steel and appreciates enhanced tool life as an important economic factor for various operations such as forming, cutting, and molding. The enhanced life of cutting tool reduces cost of production, whereas productivity and quality of work done on the workpiece also matters [3,4]. Over the last decade, several researchers have reported that due to Deep Cryogenic Treatment (DCT) considerable reduction in wear rate (WR) and coefficient of friction ( ) of AISI D2 tool steels than those obtained either by Cold Treatment (CT) or by Conventional Heat Treatment (CHT). In addition, it has also been reported that DCT and multiple tempering after cryogenic treatment reduces the residual stresses and enhances the dimensional stability [5]. Deep cryogenic heat treatment is a complementary process performed on steels before tempering and after quenching at deep cryogenic temperatures and improvement in wear resistance is a result of the retained austenite elimination and a more homogenous carbide distribution which is achieved after the deep cryogenic heat treatment [6]. Compared to alternative methods to extend tool life, DCT is an inexpensive one-time process. In contrast to coatings, it affects the whole volume of the treated materials. However, the literature data on the wear resistance varies from a few

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to some hundred percent of improvement. One of the most discussed parameters is the holding time at the cryogenic temperature. Numerous investigations have shown that increasing the soaking time increases the wear resistance [7]. The aim of this study is investigate how the DCT process time as well as tempering affects the evaluation of microstructural and mechanical properties of the Calmax cold work tool steel. 2. Experimental Procedure The chemical composition of the samples exposed to heat treatment processes was presented in Table 1. Table 1. Chemical composition of Calmax® cold work tool steel.

wt.% Sample C Mn Ni Cr Mo VCalmax® 0,6 0,8 0,35 4,5 0,5 0,2

Five steel samples having dimension of 10x10x10 mm3 were austenitized at 960oC following by quenching at 170°C and one of them was remained as just quenched for reference. After quenching, one of other four samples was tempered at 525°C for 30 min, while two of four samples were exposed to subzero heat treatment in liquid nitrogen medium having -197°C for 15 and 60 min, respectively and last sample was subjected to subzero heat treatment for 60 min. and then tempered at 525°C for 30 min. After heat treatments, the microstructural evaluations of the samples were examined by Jeol 6060 LV SEM. The presence of the carbides were investigated by EDS and XRD analyses. Amount of retained austenite in the microstructures according to DCT process time and tempering was also determined via Rigaku XRD instrument. The variation of microhardness in the samples was measured by Leica VM-HT Mod Vickers indenter. The notations of the samples were demonstrated as C1 for just quenched sample, C2 for quenched and tempered at 525 °C for 30 min., C3 for DCT processed sample for 15 min., C4 for DCT processed sample for 60 min. and C5 for sample exposed to DCT for 60 min following by tempering at 525 °C for 30 min. 3. Results and Discussion SEM microstructures of the heat treated steel samples were given in Figure 1a-d. From the Figure 1a-d, it can be seen that microstructures consisted of martensite as well as some retained austenite (C1), tempered martensite with some small carbides (C2), increasing amount of refined martensites by DCT process time and more visible small carbides (C3, C4) and tempered martensite with higher

amount of small carbides (C5), respectively. As a result of sub-zero heat treatment, it was observed that the area of white zones formed after quenching decreased with the increase of the duration of the deep sub-zero process. It was foreseen that these open white areas belongs to residual austenite (Fig. 1a-b). Furthermore, it was clear that martensite plates was further tapered by increasing time of DCT process (Fig. 1c-d). The interface of the carbide-matrix was left behind, that is, the distinct layer around the carbides was scattered as result of tempering after DCT process. Also, it appeared that smaller secondary carbides (<1 m) was precipitated due to the effect of the deep sub-zero heat treatment process (Fig. 1e).

(a) (b)

(c) (d)

(e) Figure 1. SEM images of steel samples exposed to heat treatment (a) C1 (b) C2 (c) C3 (d) C4 (e) C5.

The EDS dot analysis results with SEM (in BES mode) images of the heat treated steel samples were demonstrated in Figure 2a-e. It can be seen from the Figure 2a-e that microstructure of the samples consist of Fe dominant and some Cr-complex carbides having spherical morphology in a white zones become more visible by tempering with DCT process. Also, It was seen that the weight percent of the carbide-forming alloy elements in the matrix were reduced due to tempering as well as DCT. It was highly resulted from precipitation of secondary carbides.

UCTEA Chamber of Metallurgical & Materials Engineers’s Training Center Proceedings Book

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SEM image of C1 Mark 1

Mark 2 Mark 3

SEM image of C2 Mark 1

Mark 2 Mark 3

Mark 4

SEM image of C3 Mark 1

Mark 2 Mark 3

Mark 4 is similar to Mark 2

Figure 2. SEM-dot EDS analyses of heat treated steel samples a) C1 b) C2 c) C3

SEM image of C4 Mark 1

Mark 2 Mark 3 Mark 4 is similar to Mark 3

SEM image of C5 Mark 1

Mark 3 Mark 4 Mark 2 similar to Mark 1

Figure 2 (cont’d). SEM-dot EDS analyses of heat treated steel samples d) C4 e) C5 c) C3

The results of XRD analysis realizing for detecting remained austenite in the samples according to heat treatment processes were showed in Figure 3.

Fe

Fe

CrFe

Fe

Fe

Cr Fe

Fe

Cr

Fe

Fe

Cr

Fe

Fe

Fe

Cr

Fe

Fe

Fe Cr

Fe

Fe

Fe

Cr

Fe

Fe

Fe

Cr

Fe

Fe

Fe

Cr

Fe

Fe

Fe

Cr

Fe

FeFe

Cr

Fe

Fe

Fe

Cr

Fe

Fe

Fe

Cr

Fe

Fe

Fe

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Figure 3. XRD patterns of heat treated Calmax steel samples.

By effect of DCT process after quenching, it was detected that the peak intensity of the remained austenite in 50.788° a little decreased, while the martensite peaks in 64.028 and 82.34° increased. On the other hand, the intensity of the M7C3-M23C6 peaks increased due to precipitation of that carbides by additive tempering (Figure 3).

Volume ratio of residual austenite in heat treated cold work steel samples were calculated and the results given in Table 2. In order to determine the residual austenite volume ratio, the peaks in the planes of martensite (200) and (211) and those of planes (111) and (200) for austenite were used. It was measured that the highest residual austenite volume fraction was obtained as a result of the quenching process. By applying conventional heat treatment (quenching following by tempering), the residual austenite volume ratio in the samples dropped below 5%. DCT process applied to the cold work tool steel samples after quenching residual austenite values further reduced compared to conventional heat treatment. It has been seen that the DCT process applied to Calmax cold tool steels for 15 and 60 minutes reduces the residual austenite value slightly more than that of obtained only by tempering. It was observed that the residual austenite in steels samples subjected to tempering after DCT decreased to negligible levels. The amount of residual austenite further decreased with the increase in the duration of the DCT (Table 2).

Table 1. Variation of remained austenite in the samples according to various heat treatments

Sample Volume Ratio of Retained Austenite, %

C1 9.8

C2 2.5

C3 1.9C4 1.4C5 0.6

The variation of microhardness values of the steel samples depending on the different heat treatments was given in Table 3. From Table 3, it can be claimed that the microhardness of the DCT processed samples (C3, C4) according to only quenched sample (C1) increased due to the transformation of retained austenite into martensite. On the other hand, for tempered samples (C2, C5) the microhardness values decreased according to C1 sample. However, the microhardness of C5 sample was higher than C2 sample because of the precipitation of some seconder Cr-complex carbides induced by prior DCT process and time. It was found that the hardness-reducing effect of the tempered martensite structure was higher than hardness gain obtained by residual austenite martensite transformation and carbide precipitation (Table 3).

Table 3. Microhardness values of the samples applied to different heat treatments

Sample Microhardness (HV0.1)

ref 210

C1 755

C2 527

C3 807C4 829C5 616

4. Conclusion

Amount of remained austenite in quenched samples was significantly reduced by DCT and efficiency of DCT process was slightly increased by time progress. The remained austenite was further decreased by tempering process as tempering also leads some remained austenite to martensite transformation. This results were verified by SEM, EDS and XRD analysis. It was found that the DCT process resulted in increasing in microhardness of the samples exposed to quenching. The least microhardness values was measured in C2 sample that only tempered after quenching and microhardness of the C5 sample tempered after DCT process was higher than C2 sample due to some precipitation of seconder carbides induced by the DCT process. It can be claimed that applying of DCT process just for 15 min. reduced the amount of retained austenite to desirable level and samples exposed to tempering following by DCT process performs higher hardness than only tempering samples after quenching. So, the optimum hardness-toughness values was obtained in C5 sample.

References

[1] P. Psyllakia, G. Kefalonikasb, G. Pantazopoulosc, S. Antonioub, J. Sideris, Surface and Coatings Technology, 162 (2002) 67–78. [2] D. Tobola, W. Brostow, K. Czechowski, P. Rusek, Wear, 29-39 (2017) 382-383. [3] M. Arun, N. Arunkumar, R. Vijarayaj, B. Ramesh, Measurement (2018) In press, Accepted Manuscript. [4] D. A. College, Y. Zhu, Materials Science and Engineering A, 722 (2018) 167-172. [5] D.V. Korade, K. V. Ramana, K. R. Jagtap, N. B. Dhokey, Materials Today:Proceedings, 4 (2017) 7665-7673.[6] A. Akhbarizadeh, S. Javadpour, K. Amini, A. H. Yagthin, Vacuum, 90 (2013) 70-74. [7] A. Oppenkowski, S. Weber, W. Theisen, Journal of Materials Processing Technology, 210 (2010) 1949-1955.