Key material characteristics for optimized heat treatment of PM steels

Transcription

1 Key material characteristics for optimized heat treatment of PM steels Dimitris Chasoglou, Höganäs AB, SE Höganäs, Sweden, Ola Bergman, Höganäs AB, SE Höganäs, Sweden, Abstract Heat treatment operations such as case and through hardening are commonly used for obtaining high performance PM components. Such operations require a number of specific material data in order to be successful. In the PM industry today, however, the material data are often inadequate and heat treatments are generally performed based on previous experience on solid steels. The latter may lead to poor heat treatment results and PM parts with deteriorated mechanical properties. This paper presents results from quench dilatometry experiments and, in connection with metallography, the respective CCT diagrams for pre-alloyed PM steel grades suitable for heat treatment. The hardenability of the same materials was also investigated by means of Jominy end quench test. The obtained results are a valuable tool for the design and control of a successful heat treatment of PM steels. Introduction The main challenge today for the PM industry is its expansion by identifying potential applications and replacing existing technologies for the production of metallic components i.e transmission gears for automotive applications [1]. Therefore, the demands on performance of PM parts are high and need to be met in a consistent and reliable manner. One of the ways to achieve that is through the optimization of the final microstructure in connection with the targeted application [2]. One important tool for controlling the final microstructure is the incorporation of a relevant heat treatment process in the production of a PM part. Of course heat treating is not a new practice in PM but the increasing demands in mechanical performance as well as the development of new powder grades open up new possibilities for using available heat treatment processes i.e. case and through hardening [3][4]. One of the main problems however that sometimes is not allowing the correct implementation of a heat treatment stage which in turn can take full advantage of the material potential, is the lack of specific material data that are necessary for performing successful heat treatments. Therefore, heat treatments are generally performed based on previous experience on solid steels without considering the material as well as some special characteristics of the component i.e. geometry, density etc. The above can lead to poor heat treatment results and/or unreliable processes and in turn in poor mechanical performance. Some of the most basic material data, necessary for performing a successful heat treatment, include the Continuous Cooling Transformations (CCT) as well as the Time Temperature Transformation (TTT) diagrams of the desired material which describe the phase transformations occurring in a material system with regard to the applied cooling rate [2]. Additional useful information are hardenability data for the material system of interest which provide the hardening response of the system during quenching. The purpose of this study is to present the ongoing efforts of acquiring the basic material data described above. Due to the large amount of collected data it is possible to present only a small part of the work in this article with the ambition that in the future it will be possible to present these results in an organized and coherent collection which will be a useful groundwork when designing the heat treatment of PM steels. Materials and experimental procedure In this study a number of commercially available pre-alloyed steel powder grades produced by Höganäs AB-Sweden were admixed with different C-contents in order to study their hardenability in a complete manner and acquire the specific material data (Table 1). Presented at WordPM 2016 in Hamburg on October 11, 2016 Page 1

2 Table 1. Nominal composition of commercial powder grades and reference materials used in this study Commercial names Nominal Chemical Composition of powder grades (wt.%) Cr Mo Fe Astaloy 85Mo - 0,85 Base Astaloy Mo - 1,5 Base Astaloy CrA 1,8 - Base Commercial name Admixed C (wt.%) 0.2, 0.4, 0.6, 0.8 Nominal Chemical Composition reference material (wt.%) *Jominy C Si max. Mn P max. S Cr Mo Fe 20MnCr5S Base Quench dilatometry is an accurate and consistent method used for the production of CCT/TTT curves which provide basic information for the phase transformations of the material systems of interest. The CCT diagrams presented in this article were created using the dilatometry curves produced using a Linseis R.I.T.A Quench Dilatometer. Cylindrical samples (Ø: 6mm and height: 10mm) were pressed to a density of 6,8-6,9 g cm -3. In turn the samples were sintered at 1120 C for 30 in a N 2 /H 2 90/10% atmosphere. Due to the size of the samples it was necessary to have methane additions in the atmosphere in order to avoid any C-loss. The sintered samples were then induction heated (10 C/s) up to the austenization temperature (960 C) where they were held for 3 minutes and then were cooled down at different cooling rates (0,1 C/s, 0,5 C/s, 1 C/s, 2 C/s, 5 C/s, 10 C/s, 20 C/s, 50 C/s and 100 C/s) using He gas which was also the atmosphere used throughout heating and isothermal holding as well. Metallographic investigation followed for all the samples. The CCT curves were produced using the equipment software WinZTU after the phase transformation points were identified from every dilatometry curve. The hardenability of steels is determined, according to ASTM [5], by measuring the depth below a surface of a sample which after austenization it has been quenched, where the steel is hardened. The Jominy test is one of the two methods used for the evaluation of the hardenability of steels, the other one being the Grossman method. The Jominy test, also known as end-quench test, involves the water quenching of one end of a cylindrical bar of a standard diameter and measuring the hardening response of the steel as a function of the distance from the quenched end. Studies have shown that the Jominy test is suitable for determining the hardenability of PM steels [6]. For performing such Jominy tests, at first the material blends mentioned in Table 1 also including 0,6wt.% Intralube E as a lubricant were pressed into rectangular bars of the following dimensions L:120mm, W:30mm, H:30mm at a density of 7,2 g cm -3. The bars were then sintered at 1120 C for 30 in a N 2 /H 2 90/10% atmosphere with some relevant methane additions in order to avoid any C-loss. The above was confirmed by C-analysis after sintering were the material was showing the nominal C-content. From the sintered rectangular bars cylindrical samples were machined in the geometry and dimensions shown in (Figure 1). As a reference, solid bars of 20CrMn5, which is a typical material used in transmission gears, were also used for comparison and process verification. The produced bars were then heated up to 900 C where they were held for 30 and in turn were inserted to the Jominy test apparatus where the bottom end was subjected to water quenching. The quenched bars were ground in order to avoid the potentially decarburized surface and hardness measurements (Rockwell C) as well as microhardness measurements HV 0,1 were performed along the length of the bar from the quenched end. Figure 1.Geometry and dimensions of Jominy test bar Presented at WordPM 2016 in Hamburg on October 11, 2016 Page 2

3 Results and discussion In Figure 2-4, as an example, one can see the CCT diagrams of Mo pre-alloyed powder grades which were designed using the phase transformation points from the dilation curves from the quench dilatometry experiments. In Figure 2 and 3 the only difference between the powder grades is the Mo content which increases the hardenability of the steel. This is also evident on the CCT curves where in the case of Ast85Mo+0.6%C it is possible to acquire a fully martensitic structure only using high cooling rates (above 20K/sec) whereas for AstMo+0.6%C a fully martensitic structure appears with a cooling rate of 5K/sec. These diagrams, in connection with the heat treatment parameters, can give insight on different matters. For example, diagrams like Figure 2 and 3 can provide the expected surface microstructure for a component that is going to be case hardened aiming at a specific surface C-content (i.e 0.6%) or the expected microstructure for an alloy containing i.e. 0.6%C that is going to be through hardened. Figure 4 shows the CCT diagram of AstMo with low C-content which could very well be the core C-content of a case hardened component. In combination with hardness profiles and microscopy images one can use this information for controlling the heat treatment stage and have the suitable cooling rate (i.e. in the core of the component) in order to i.e. avoid the formation of ferrite. Figure 2. CCT diagram of Ast85Mo containing 0.6%C Figure 3. CCT diagram of AstMo containing 0.6%C Figure 4. CCT diagram of AstMo containing 0.2%C From the hardness measurements (Rockwell C) the Jominy hardenability curves were obtained and are presented below in Figure 5. The expected hardenability difference between Ast85Mo and AstMo is clearly seen in the hardenability curves in the higher hardening depth of the latter, e.g for 0.8%C Ast85Mo is showing hardening depth of ~6mm whereas AstMo is showing 7.5mm. Hardening depth is defined as the depth where the hardness value of the curve reaches 50HRC [7]. It is interesting that Cr-alloyed steel (AstCrA) is showing essentially a threshold above or below which the difference in C-content do not result in significant changes in the hardenability curves. This threshold lies between %C. Presented at WordPM 2016 in Hamburg on October 11, 2016 Page 3

4 Figure 5. Hardenability curves for Ast85Mo, AstMo, AstCrA and reference steel 20MnCr5 Due to the nature of PM steels with their porosity microhardness profiles can be of more use for the interpretation of the hardening response of the material. Figure 6 shows the hardening response of the powder grades of the study with C-content 0.2% and the reference material. It is important to point out that in comparison with a typical cast (fully dense) material suitable for heat treatment, PM grades (AstMo and AstCrA) with comparable C-content are performing relatively well when considering the lower total alloying element content and despite the porosity levels (up to ~10%). The case depths are well defined especially for AstMo and AstCrA and the distance from the surface end where the microhardness is 400HV0.1 is approx. 6mm for the reference material and 4.8 and 5.5 for the PM materials respectively. Of course there is still a distance between the heat treatment response of the cast and the PM materials, which however is smaller than what is commonly considered thus showing the potential for applications requiring specific heat treatments i.e. case hardening for transmission gears. Presented at WordPM 2016 in Hamburg on October 11, 2016 Page 4

5 Figure 6. Microhardness profiles for the powder grades of the study containing 0.2%C and the reference material Figure 7 shows the microhardness profiles of the powder grades of the study containing 0.6%C. Also here the difference in hardenability between the grades with different Mo content is evident on the hardening depth. Figure 7. Microhardness profiles for Ast85Mo, AstMo and AstCrA containing 0.6%C The microstructures observed in the Jominy samples at positions with increasing distance from the quench end are typical of the materials under investigation. As an example one can see in that close to the quench end the microstructure of Ast85Mo-0.6C is fully martensitic whereas approx. 8.5mm from the quench end the microstructure is a mixture of martensite and bainite (dark). Further inside the samples the Mo-prealloyed grades, apart from martensite and bainite, show also signs of pearlite whereas the Cralloyed material is predominantly bainitic with some traces of martensite (Figure 9). Presented at WordPM 2016 in Hamburg on October 11, 2016 Page 5 Figure 8. Microstructure of Ast85Mo-0.6C at i) ~1mm and ii) ~8.5mm from the quench end

6 Figure 9. Microstructures at the interior of the Jominy samples ~25mm from the quench end i) AstMo-0.6C showing martensite, bainite and pearlite and ii) AstCrA-0.6C showing bainite and some traces of martensite Conclusions Some of the above can be considered already known and fully understood throughout many years of practical experience, however the fact remains that there are more than a few cases where the heat treatment result has been far from the desired one. This study focuses on showing how such basic material information would be of great help when it comes to performing a successful heat treatment. The combination of CCT (and TTT) diagrams which can provide information regarding the resulting microstructural changes during the cooling stage of a heat treatment and consequently assist in optimizing different parameters of the treatment itself or changing the material composition if possible. The above can be complemented by hardenability curves which provide information regarding the hardening response of the material in connection with metallographic investigation. The utilization of key material data can reveal the full potential of PM steels in terms of hardenability which as shown is in the levels of conventional steels. This in turn can give the opportunity of performing successful material processing through heat treatment which will enable the further use of PM steels in demanding applications. Acknowledgements The authors would like to acknowledge and thank the University Carlos III of Madrid, Spain and especially Prof. Monica Campos for performing the Jominy testing. References Presented at WordPM 2016 in Hamburg on October 11, 2016 Page 6

7 [1] J. Capus, Höganäs PM transmission gear initiative takes another step forward, Met. Powder Rep., vol. 71, no. 1, pp , Jan [2] Höganäs Handbook for Sintered Components. Höganäs AB. [3] M. Dahlström, M. Larsson, and Y. Giraud, High Performance PM Components Heat Treated by Low Pressure Carburizing and Gas Quenching, in Euro PM2013 Congress and Exhibition, 2013, pp. Vol.2, [4] T. Holm, E. Olsson, and E. Troell, Eds., Steel and its Heat treatment - a handbook. Swerea IVF, [5] Standard Test Methods for Determining Hardenability of Steel A255-02, 2002nd ed. ASTM International, [6] G. F. Bocchini, B. Rivolta, A. Baggioli, M. G. Ienco, and M. R. Pinasco, Jominy test applied to PM steels for heat treatment, Powder Metall., vol. 49, no. 2, pp , Jul [7] Introduction to Surface Hardening of steels, in ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes, ASM International, 2013, pp Presented at WordPM 2016 in Hamburg on October 11, 2016 Page 7