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83719. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2018

Examination of the Infl uence of Sinter Hardening Process on Prealloyed Metal Powders

Mehmet Günen, Adem Bakkaloğlu, Cihan Balaban

Yıldız Technical University, Department of Metallurgical and Materials Engineering, İstanbul, Turkey

Abstract

In this study, the influence of sinter-hardening process on prealloyed metals which are Astaloy Mo (Fe prealloyed with 1,5 wt.% Mo) based steels was examined. In line with this objective, diffusion alloyed Distaloy DH (Astaloy Mo-2 wt.% Cu) with 0,6 wt.% C, Astaloy Mo with 2 wt.% Cu and 0,6 wt.% C, and Astaloy Mo with 1 wt.% Cu and 0,6 wt.% C powder mixes were studied. The powder mixes were pressed under 600 MPa and were sintered at 1120°C under endogas atmosphere for 25 minutes in an industrial belt sintering furnaces. After the sintering process, 0,5°C/s, 1,5°C/s, 2°C/s and 3°C/s cooling rates were applied to the samples. Subsequent to production phase, sample preparation process was carried out to sinter-hardened samples. Thereafter, microstructures of the samples were analyzed under the optical and scanning electron microscopes, and then the samples were experimentalised by three-point bending, tensile and microhardness tests. Lastly, the findings obtained in consequence of experimental studies were discussed and concluded. Due to this study, the effects of cooling rates, copper amounts and prealloying methods on mentioned powders were examined. Also, sinter-hardening and traditional sintering processes were compared and the advantages of sinter-hardening process were shown. In regard to the experimental studies, it was observed that high cooling rate led to transformation of bainite to martensite and there was an increase in the hardness value of each sinter-hardened sample owing to increasing cooling rate. Furthermore, it was seen that materials having high hardness and strength can be produced by sinter-hardening. Additionally, it was determined that Distaloy DH specimens have less porous structure and have smaller pores in comparison to Astaloy Mo specimens. Therefore, it was concluded that Distaloy DH specimens have better mechanical properties.

1. Introduction

Since the transportation is a crucial necessity, the automotive industry has become one of the most prominent and largest industries in the world. According to 2017 data of Industrial Development Bank of Turkey [1], automotive industry corresponds to fourth biggest economy by 4 trillion dollars and 80 million employments directly and indirectly. This situation leads to more extensive researches and larger investments. At this point, product quality, production

cost and time have gained even more significance. Accordingly, high performance powder metallurgical parts and sinter-hardening process have started to draw attention. Extension of powder metallurgical parts’ usage can be possible by reducing the production cost of high performance parts.[2] Besides the quality, cost and time, oil consumption is another parameter by virtue of oil’s affordability and impact on the environment. At this point, lightness draws considerable attention, and material selection is the way of realizing this goal.[3] Sinter-hardening as one of the most used method in powder metallurgy is a cost-efficient production technique that incorporates sintering and hardening processes into one operation. Thus, it eliminates heat treating, provides saving on time and allows producing high-performance powder metallurgical parts.[4,5]

2. Experimental Procedure

In this study, determination of the effects of alloying methods, copper amounts and cooling rates on mechanical properties and microstructures of following powder mixes; Astaloy Mo-1Cu-0.6C, Astaloy Mo-2Cu-0.6C, Distaloy DH-0.6C is aimed. In line with this objective, transverse rupture strength, tensile strength and microhardness tests and also microscopical investigations were performed. Astaloy Mo is ferrous based powder prealloyed with 1,5 wt.% Mo. Distaloy DH is the diffusion-bonded form of Cu to Astaloy Mo. Prealloying with Mo, which has a low solid solution strengthening effect, and introducing Cu through diffusion bonding promotes compressibility of these powders.[6] The chemical compositions of investigated powder mixes are given in Table 1. Hereafter, Astaloy Mo-1Cu-0.6C, Astaloy Mo-2Cu-0.6C and Distaloy DH-0.6C will be referred to as AC1, AC2, and DH, respectively.

Table 1. Nominal chemical compositions (wt.%) of investigated powder mixes. 1Prealloyed with the base

powder, 2Diffusion-bonded to Astaloy Mo, 3Admixed.

Code Material Fe Mo Cu C

AC1 Astaloy Mo-1Cu-0.6C Base 11.5 31 30.6

AC2 Astaloy Mo-2Cu-0.6C Base 11.5 32 30.6

DH Distaloy DH-0.6C Base 11.5 22 30.6

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3. Results and Discussion

3.1. Microstructural Studies

Porosity measurements were performed by LOM. In consequence of this analysis, porosity rates of all specimens have decreased together with increasing cooling rate and the sample which has lowest pore volume is Distaloy DH.

Table 2. Porosity percentages of specimens according to cooling rates.

Porosity (%) Specimen 0,5°C/s 1,5°C/s 2°C/s 3°C/s

AC1 15,85 17,09 13,75 12,62 AC2 14,55 13,25 13,53 12,21 DH 11,00 9,87 10,52 7,12

Etched surfaces of specimens at 0,5°C/s, 1,5°C/s,2°C/s and 3°C/s cooling rates were observed by inverted microscope. In order to show the obvious difference between sintering and sinter-hardening, images belonging to specimens cooled at 0,5°C/s and 3°C/s cooling rates were given in Figure 1, Figure 2 and Figure 3. G, P, B and M letters state gap, pearlite, bainite and martensite, respectively.

Figure 1. AC1 – 200x magnification; (a) 0,5°C/s,(b) 3°C/s.

Figure 2. AC2 – 200x magnification; (a) 0,5°C/s,(b) 3°C/s.

Figure 3. DH – 200x magnification; (a) 0,5°C/s,(b) 3°C/s.

B

G

p

G

B M

M

G

G P

B

M

G

P

B

G

(a)

(b)

(a)

(b)

(a)

(b)

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ı Eğ i t im MerkeziBildir i ler Kitab ı

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As it seen in the figures above, acicular structures demonstrate that all specimens cooled at 3°C/s cooling rate has almost completely martensitic microstructure. This statement is supported by microhardness test results. EDS analyses were performed in order to determine the distribution of alloying elements. For each sample, results were obtained from 3 different points. Elemental compositions of individual points were determined and distribution of detected elements mapped out. One of the SEM and EDS results is shown in Figure 4, and average elemental compositions and percentages of elements are given in Table 2.

Figure 4. An image obtained from EDS analysis.

Table 2. Elemental compositions and percentages of elements.

Specimen Point Fe % Mo % Cu %

AC11 98,303 1,697 02 96,191 1,644 2,165 3 98,590 1,410 0

AC21 93,769 1,614 4,617 2 98,374 1,626 03 93,845 1,358 4,797

DH1 94,423 1,526 4,052 2 94,931 1,760 3,309 3 95,622 1,217 3,161

In the light of EDS analysis results, it is seen that Mo exists in each point. Because it is added to Fe by prealloying. In this method, alloying elements are alloyed with the base material, the molten iron, and atomized. Thus, Mo is distributed homogeneously in structure. Cu was detected at each point of DH specimens whereas it was absent at some points of AC1 and AC2. This situation shows that diffusion alloying provides better homogeneity than admixing.

3.2. Mechanical Studies

The results of identification of microstructures were proved by Vickers hardness test. Average HV0.1 microhardness values for each specimen are shown in Figure 5. The microstructures for all specimens cooled at a rate of 0,5°C/s (5 Hz) consist of fine-pearlite and bainite. Microstructures are mostly bainitic and contain martensite in small quantities at 1,5°C/s (15 Hz) cooling rate. The specimens cooled at a rate of 2°C/s (20 Hz) contain bainite and martensite together.

Microstructures are almost completely martensitic at 3°C/s (30 Hz) cooling rate.

Figure 5. Average microhardness values of specimens according to cooling rates.

The test results relating to transverse rupture strengths are given as graphic in Figure 6. For each specimen, decrease in TRS was observed in parallel with increasing cooling rate. This decrease for AC1 specimens is 12,56%, for AC2 specimens is 19,71% and for DH specimens is 12,68%.

Figure 6. Average TRS values of specimens according to cooling rates.

The test results relating to tensile strengths are given in Figure 7.

Figure 7. Average tensile strength values of specimens according to cooling rates.

In common with transverse rupture strengths, tensile strengths of all specimens have decreased in conjunction with increasing cooling rate. The main reason of these decreases in strength values is related to dislocations. Applied force originates crack initiation by creating stresses at pores. In company

1 2

3 +

+ +

Cu

Fe

Cu Mo

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with increasing cooling rate, structure turns into martensitic form and this situation ends up with restricting the dislocation movements. Therefore, having cornered, big and/or a large number of pores occasions quicker crack propagation in brittle materials because of the limited dislocation movements. So, higher cooling rates cause lower strengths.

4. Conclusion

Depending on porosity measurements, Distaloy DH has the lowest porosity and smaller pores. From the comparison of AC1 and AC2, higher copper amount has provided lower porosity. The result that diffusion alloying method provides better homogeneity than admixing method has been deduced from EDS analyses results. All specimen types at same cooling rates include same forms but different amounts. DH provides slightly higher microhardness than others at lower cooling rates while 2% copper amount ensures better microhardness than 1% copper amount. Boosting the cooling rate from 0,5°C/s to 3°C/s has caused decrease in TRS and TS values of all specimens. Distaloy DH has offered higher tensile and transverse rupture strengths among three type powders. Besides, the material showing the lowest variance in strength values, in other saying the most stable material has been Distaloy DH.

References

[1] S. Pi kin, Türkiye Otomotiv Sanayi Rekabet Gücü ve Talep Dinamikleri Perspektifinde 2020 ç Pazar Beklentileri, Türkiye S nai Kalk nma Bankas , 2017, Istanbul, Turkey. [2] P. Johanson and E. Y. Yüksekda , Sinter Hardening-A Time/Cost Efficient Production Line, 6thInternational Powder Metallurgy Conference, 05-09 October 2011, Middle East Technical University, Ankara, Turkey. [3] P. Lindskog, The History of Distaloy, Powder Metallurgy, 56/5 (2013) 351-361. [4] W. B. James, What Is Sintering, International Conference on Powder Metallurgy and Particulate Materials, 31 May – 4 June 1998, Las Vegas, USA. [5] L. A. Dobrzanski, J. Otreba, Z. Brytan and M. Rosso, Utilisation Of Sinter-Hardening Treatment For Various Sintered Steels, Journal Of Achievements in Materials and Manufacturing Engineering, 24/2 (2007) 187-190. [6] G. F. Bocchini, New Iron Base Powders for Advanced Automotive PM Applications, International Journal of Materials and Product Technology, 15(3/4/5) (2000) 172–192.