Metal Forming
The Influence of Thermomechanical Processing on Microstructure Development in Surgical Tools Made of X20Cr13 Steel Janusz Krawczyk, Bogdan Pawowski, Piotr Baa, Marek Pako Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krakow, Poland,
[email protected]; Abstract. This study analyzes the changes in the microstructure of surgical tool made of X20Cr13 steel at all stages of its forging. Moreover, an influence of forging parameters on development of microstructure of the investigated surgical tool is also discussed. The influence of the microstructure and chemical composition of as-received forging charge material, forging temperature, the symmetry of forging die set-up, and intermediate heat treatment parameters on the microstructure and properties of forgings was analyzed. An affect of forming stages on the microstructure and hardness of the investigated material was determined. Moreover, the character and reasons of the microstructure inhomogeneity connected with strain concentration was discussed. The role of the parameters of warm forming of the investigated steel in its microstructure development was also determined. Processing parameters as well as microstructural inhomogeneities leading to recrystallization of the material in the critical strain areas were evaluated. Precise control of the parameters of processing surgical tools should restrict the possibility of the formation of so called “microstructural notches”, decreasing their crack resistance. Proper parameters of the technology of forming of the investigated surgical tools were determined. Keywords: Metal forming, X20Cr13 steel, surgical tool, microstructure
1. INTRODUCTION
a)
Martensitic stainless steels containing 13% Cr due to their excellent mechanical properties and corrosion resistance are widely used to manufacture surgical instruments [1,2]. These steels are commonly used in quenched and tempered conditions [3,4]. Hot forging technology (including temperature of deformation, heating and cooling rate) and heat treatment parameters significantly affect the microstructure [5-7], mechanical properties and corrosion resistance [2-4,8,9]. The surgical tools surfaces are finished mainly by hand lapping to obtain desired surface roughness [10]. The influence of different ways of forging (including the symmetry of the forging die set-up) and heat treatment parameters as well as surface finishing procedures of surgical forceps made of X20Cr13 martensitic stainless steel on the microstructure and mechanical properties was analysed in this study.
b)
Figure 1. Microstructure of steel plates (cross-section perpendicular to the forging plane): a) material M, b) material P. Light microscope.
3. FORCEPS PRODUCTION PROCESS Both processes of the chirurgical forceps manufacturing consisted of: blanking - A, forging in first die - B, forging in second die - C, external flash cut off - D, cutting of a flash inside a ring - E, sand blasting - F. Successive stages of forming chirurgical forceps in two analyzed processes are presented in figures 2 and 3.
2. MATERIALS FOR TESTING Forgings of chirurgical forceps were made of X20Cr13 steel. Two processes of forging, made of two different primary materials M and P, were analysed in this study, together with further treatments of these forgings performed in two technological lines. The chemical compositions of initial materials are given in table 1. Steel plates characterized by a microstructure presented in figure 1 constituted initial materials. The material M hardness was 211 HV10 while the material P - was 215 HV10. As can be seen in figure 1, material M was characterized by more homogeneous microstructure and finer grain size as compared with material P. Table 1. Chemical compositions (wt. %) of the investigated steels C
Si Mn
P
S
Cu
Cr Ni Alc Mo material M 0.23 0.18 0.45 0.043 0.010 0.06 13.04 0.31 0.054 0.09 material P 0.19 0.22 0.40 0.027 0.003 0.05 13.33 0.33 0.035 0.10
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V
Fe
0.06
Bal.
0.05
Bal.
Figure 2. Successive stages of forming forceps made of material M. A soaking temperature prior to forging in the case of material M was 780 ºC, while of the material P was 830 ºC. The charge heating was also different. In the case
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Metal Forming
of material M blanks were passing through the pusher furnace and the time of their being inside the furnace was approximately 20 minutes. However, in the case of material P the whole charge of blanks was put in the furnace simultaneously, and therefore the first blanks were forged after 20 minutes soaking and the last blanks were forged after approximately 1 hour soaking.
P-Eb
P-Ea’
P-Gb
P-Gc
Figure 5. Sampling method for metallographic investigations in “a”, “b”, and “c” areas of the forceps.
5. RESULTS AND DISCUSSION
Figure 3. Successive stages of forming forceps made of material P.
4. EXPERIMENTAL Samples for the metallographic examinations were taken from the same place „a” on the material crosssection at various stages of the chirurgical forceps production (figure 4). In addition, for forceps made of material P metallographic specimens were prepared after cutting out the flash inside the ring and after grinding of the ring inner part (P-E), made after the sand blasting (P-G) in places marked in figure 5. Samples were etched with a reagent for corrosion resistant steels. Chemical composition of etching reagent was 30 g NH4F, 50 ml HNO3, 20 ml H2O. M-Aa
P-Aa
M-Ba
P-Ba
M-Ca
P-Ca
M-Da
P-Da
M-Ea
P-Ea
M-Fa
P-Fa
Figure 4. Sampling method for metallographic tests in place “a”. 1164
In the case of the first stage of forging material M, the top and the bottom die assembly was symmetrical, which caused the symmetrical distribution of strain concentration seen as changes in the microstructure (figure 6). The forging process of the material P was done with shifting of the top die axis versus the bottom one. This caused the asymmetrical distribution of the strain concentration zone (figure 7). In addition, in the case of the material P the brighter areas are seen in the near surface zone, which can indicate its decarburising. Such areas were also seen in the case of material M, but only after the final stage of forging (figure 8). This can be the result of a different degree of oxidation of charge materials related to a higher temperature of forging material P than material M and different time of a charge holding. However, the forging P microstructure is characterized by finer grains than forging M. This is most probably due to a higher strains occurring in the case of this forging.
Figure 6. Microstructure in the cross-section of M-Ba sample. Despite the forging die symmetry in the final forging stage, its asymmetry in the first stage of forming material P results in asymmetrical microstructure in the formed product (figure 9). The degree of decarburisation of the forging near surface zone is higher after the second stage of forging of material P than after the first stage and also higher than after the second stage of forging of material M. These observations confirm the investigations performed in place “a” of forgings of material M and P after removal of external flash (figures 10 and 11), the flash removal from the inner part of the forceps ring (figures 12 and 13) – which, of course, should not have any influence on the forging microstructure in place ‘a’ and after the sand blasting (figure 14). It can be only stated that in the
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Metal Forming
forging of material M the higher surface decarburisation can occasionally occur (figure 12), but it is fully removed in the sand blasting process (figure 14). After the flash cutting within the ring in material P the annealing was applied (temperature: 780 ºC, time: 4h) before the sand blasting, which caused that brighter areas are seen at the forging cross-section in place “a” (figure 15). They should be related to zones where the microstructure degradation resulting from a plastic strain causing recrystallization in the range close to the critical strain occurred. Similar areas are seen in polished sections made of annealed forgings in other than “a” places: P-Eb, P-Gb and P-Gc (figure 16). This was confirmed by the microhardness measurements by the Knoop method in brighter areas (173 HK) and outside them (207 HK), performed for sample P-Eb. Such areas are not seen in cases of not annealed forgings, e.g. PEa’ (figure 16).
Figure 10. Microstructure in the cross-section of M-Da sample.
Figure 11. Microstructure in the cross-section of P-Da sample. Figure 7. Microstructure in the cross-section of P-Ba sample.
Figure 12. Microstructure in the cross-section of M-Ea sample. Figure 8. Microstructure in the cross-section of M-Ca sample.
Figure 13. Microstructure in the cross-section of P-Ea sample. Figure 9. Microstructure in the cross-section of P-Ca sample. www.steelresearch-journal.com
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Figure 14. Microstructure in the cross-section of M-Fa sample.
Figure 17. Changes of hardness of the forged material (measured in place “a”) after each stage of the forging process.
6. SUMMARY
Figure 15. Microstructure in the cross-section of P-Fa sample. P-Eb
P-Ea’
P-Gb
P-Gc
Figure 16. Microstructure of the material P forging on different cross-section geometry and in different place than “a”. The forging die asymmetry (in a first stage of forging) as well as the higher forging temperature (more intense dissolution of alloy carbides, which improve a steel hardenability) and the smaller carbon content (probably smaller amount of retained austenite after quenching) in case of material P as compared to material M can be the reason of a significantly higher hardness of the material P forging than the material M forging after the first stage of forging and not much higher hardness after the final stage of forging and after the flash removal (figure 17). Annealing before the sand blasting lowers the hardness while the sand blasting alone causes its increase.
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The results obtained in this study allow formulating the following conclusions: 1. Die asymmetry causes the asymmetry of the strain concentration zones, which remains in the material, despite the successive stage of plastic deformation. 2. The annealing of forgings leads to origination of zones, in which the recrystallization processes occur causing a grain growth and structural notches in a form of an abrupt change of strength properties. 3. The application of a higher forging temperature influences the hardness increase of forgings.
7. REFERENCES [1]
J. Samolczyk, K. Jówiak, T. Kachlicki: Martensitic corrosion resistant steels subjected to warm forging, Obróbka Plastyczna Metali, XVIII(2) (2007), 23-32, (in Polish). [2] A. Nasery Isfahany, H. Saghafian, G. Borhani: The effect of heat treatment on mechanical properties and corrosion behavior of AISI420 martensitic stainless steel, Journal of Alloys and Compounds, 509(9) (2011), 3931-3936. [3] F.G. Caballero, L.F. Alvar3H />23D7:/ /@1T/ 23 23D7:/ :D/@3H /@1T/ 23
