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The surface alloying behavior of martensitic stainless steel cut with wire electrical discharge machine
Applied Surface Science 252 (2006) 2915–2926 www.elsevier.com/locate/apsusc
The surface alloying behavior of martensitic stainless steel cut with wire electrical discharge machine Ching An Huang *, Chwen Lin Shih, Kung Cheng Li, Yau-Zen Chang Department of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan, ROC Received 27 January 2005; received in revised form 28 April 2005; accepted 29 April 2005 Available online 13 June 2005
Abstract The surface alloying behavior of tempered martensitic stainless steel multi-cut with wire electrical discharge machine (WEDM) is studied in this paper. Before machined with WEDM, the steel specimens were quenched at 1050 8C and then tempered at 200 8C, 400 8C, and 600 8C, respectively. The microstructure and surface morphology of the multi-cut surfaces were examined with scanning and transmission electron microscopes integrated with an energy-dispersive X-ray spectrometer for chemical composition analysis. Experimental results show that the cut surfaces of the steel specimens were alloyed with wireelectrode material in various extent. Especially the cut surface was much more alloyed when the steel was cut with the first rough cutting pass by using negatively biased potential and final fine cutting using positively biased potential. Alloying degree of cut surfaces can be distinguished with their anodic polarization curves in 0.5 M HClO4 + 0.2 wt% NaCl at 27 8C. Higher passive current density induces deeper alloyed surface. On the severely alloyed surface, a secondary anodic peak in the potential of 120 mV (versus Ag/AgClsat.) of its anodic polarization curve was observed. The presence of the secondary anodic peak was attributed to dissolution of copper, which was the major element of wire-electrode material from the alloyed surface. # 2005 Elsevier B.V. All rights reserved. Keywords: Wire electrical discharge machine (WEDM); Martensitic stainless steel; Microstructure; Electrochemical behavior
1. Introduction Due to its high hardenability, superior mechanical property and corrosion resistance, martensitic stainless steel is widely used for plastic molds, precision mechanical parts, and surgical tools [1]. In typical applications, the steel is quenched from 1050 8C to * Corresponding author. Tel.: +886 32118800x5346; fax: +886 32118740. E-mail address: [email protected] (C.A. Huang).
1150 8C to obtain hardened martensitic structure, and then tempered between 200 8C and 600 8C to achieve suitable strength and toughness. Because the hardened steel can be accurately shaped with wire electrical discharge machine through multi-cutting pass, WEDM is used in final surface extensively. The multi-cutting passes with WEDM can be briefly described as follows: First, the workpiece was roughly machined, or 1st-cut, with high flushing pressure of dielectric fluid, high power and low tension in wire-electrode to quickly separate and shape the
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workpiece. Then semi-finishing and finishing of 2nd to 5th cutting passes were carried out with laminar flow of dielectric fluid, relatively low power and high tension in wire-electrode. The cutting depth for the semi-finishing and finishing processes is only a few micrometers. The number of cutting passes required depends mainly on specified accuracy and surface roughness of the workpiece. Higher accuracy generally demands more cutting passes. From an application point of view, high accuracy can be achieved by a one-cut. However, smaller surface roughness requires more cutting passes in order to reduce overall machining time. The material removal mechanism of WEDM is the same as that of electrical discharge machine (EDM). It has been widely accepted that the metal removal mechanism in EDM is predominantly a thermal effect in nature [1]. The electrical discharging process generates a tremendous amount of heat causing melting or even evaporation in the local surface layers on both wire-electrode and workpiece sides. The heat also causes vaporization of the dielectric fluid and induces high pressure waves, which wash out the molten and/or vaporized metal into pieces from the workpiece. Continuously injected dielectric fluid then carries the droplets of metal away. Surface alloying between workpiece and wire-electrode materials was observed on the cut surface after cutting with WEDM [2,3]. Furthermore, phase transformation or heataffected zone (HAZ) could take place in the workpiece adjacent to the cut surface [4–7]. It has been reported in the literature [6–10] that surface modification through coating and phase transformation can be evaluated electrochemically. In this paper, we use electrochemical measurement to detect alloying degree and phase transformation of the surface of martensitic stainless steel, which was multicut with WEDM in 0.5 M HClO4 + 0.2 wt% NaCl at 27 8C.
Table 1 The chemical composition of martensitic stainless steel (AISI 440) used in this study Composition C Weight (%)
Cr
Mo
Mn
Si
S
P
0.39 15.89 1.02 0.87 0.46 <0.003 <0.003
for WEDM cutting as following. The steel billet was cut into several 15 mm thick plates by a sawing machine. The plate specimens were heated at 1050 8C for 1 h, and then quenched in oil. After quenching, some of the specimens were tempered at 200 8C, 400 8C and 600 8C, respectively, for one hour. Those plate specimens prepared, thus can be divided into four groups as-quenched, and tempered specimens in 200 8C, 400 8C and 600 8C, respectively. 2.2. Working condition A brass wire-electrode with a diameter of 0.25 mm was used in this study. The wire-electrode material was composed of 65 wt% copper and 35 wt% zinc. As
2. Experimental procedure 2.1. Materials treatment Martensitic stainless steel (AISI 440A2) was used in this study and its chemical composition is given in Table 1. The specimens for this study were prepared
Fig. 1. Schematic of the WEDM multi-cutting process.
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Table 2 Machining parameters used for multi-cutting passes with WEDM Cutting pass
Voltage (V)
CD (A)
TA (ms)
TB (ms)
WS (m/min)
WB (daN)
FP (bar)
1st-cut 2nd-cut 3rd-cut 4th-cut 5th-cut
80 120 80 80 +120
4 8 16 8 4
1.0 0.2 0.4 0.4 0.2
9.0 3.8 3.0 3.6 3.8
10 8.0 8.0 8.0 8.0
1.0 1.4 1.4 1.6 1.4
5.5 0.8 0.8 0.8 0.8
CD: discharging current; TA: duration of discharging time; TB: time between two pulses; WS: wire feeding speed; WB: wire tension; FP: flushing pressure.
shown in Fig. 1, the feeds for the 2nd to 5th passes were 40 mm, 15 mm, 4 mm and 3 mm, respectively. The operating parameters for multi-cutting process are shown in Table 2, and these are based on the expert system in WEDM (Charmilles Technologies Robofil 300). For the 1st (rough) cutting pass, which was used to effectively separate the workpiece, the highest cutting power in the wire-electrode and the largest flushing pressure in dielectric fluid were applied. The purpose of the 2nd and the 3rd cutting passes was to enhance the geometrical and dimensional accuracy of the workpiece, while the last 4th and 5th passes were applied for final surface shaping. Unlike the preceding four cutting passes, the electrode was reverse biased in the 5th cutting pass. The conductivity of dielectric fluid for each cutting was precisely controlled within 5.0 0.5 mS/cm and the working temperature was kept at 20 0.5 8C. 2.3. Preparation of the cut surfaces The multi-cut surfaces described above were examined using a scanning electron microscope (SEM) integrated with an energy-dispersive X-ray spectrometer (EDS) for chemical composition analysis. The specimens, quenched- and tempered-steel plates, were cut by WEDM with different number of cutting passes. The machined surfaces were carefully preserved in wax and sliced into 8 mm 15 mm 15 mm pieces with a diamond saw. Then the specimens were ultrasonically cleaned in an acetone bath for 10 min to remove wax on the surfaces. After flushed with alcohol, the specimens were dried using a cool air blaster for SEM/EDS study, and electrochemical measurements. With the same preparing procedure the surface roughness in arithmetic mean value, Ra, of the specimens was measured with a surface profiler (Hommel Werk T 4000).
Microstructures of finished surfaces were studied with analytical transmission electron microscope (ATEM, JEOL 2010 and Links-EDX system). Cross-sectioned specimens for ATEM examination were taken from 600 8C-tempered steel specimen, which was prepared after rough, 4th and 5th cutting passes. The same processes to prepare SEM specimen of WEDM-cut surface were applied on the ATEM specimens. The cut surfaces were sectioned in small plate with WEDM in a dimension of 1 mm thick, 1.5 mm long and 1 mm wide, stuck the cut faces of two plates together with M-Bond 610, and then mounted vertically with G1 epoxy (Gatan company, USA) in a 3 mm diameter copper ring. The specimens were then mechanically ground to 100 mm in thickness, and both sides of the specimen were further dimpled with Dimpler (VCR, Dimpler) to a thickness of 10 mm in the center. Finally, a low angle (88) Ar+ion milling (VCR, XLA 2000) machine under 5 kV voltage was used to sputter the specimen until a tiny hole was produced in the adhered cut interface, around which the cut surface was so thin that electron diffraction and image could be observed and examined with the TEM/EDS. During the ion milling, the specimen holder was cooled with liquid nitrogen to avoid thermal effects on the specimen. Details of specimen preparation were presented in [11,12]. 2.4. Electrochemical analysis The electrochemical measurements were carried out in a three-electrode electrochemical cell. The Ag/AgCl electrode in the saturated KCl solution was used as the reference electrode, and a platinized Ti-mesh as the counter electrode. The exposed area of the multi-cut specimen was 1 cm2 and testing temperature was kept at 27 1 8C in a circulated water bath. The electrolyte was composed of 0.5 M HClO4 with 0.2 wt% NaCl.
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Anodic polarization measurement was conducted with a potentiostat/galvanostat (EG&G Model 273A) in a flat cell (EG&G Model K0235). De-aeration was carried out by purging N2 gas into the electrolyte during electrochemical measurement. Before electrochemical measurement, the specimen was immersed in the electrolyte for 10 min to ensure dynamic stability. The potentiodynamic polarization behavior of the multi-cut specimen was studied by scanning the specimens with a potential difference range from 250 (versus opencircuit potential) to 1200 mV (versus Ag/AgClsat.) in a scan rate of 0.3 mV/s.
3. Results and discussions 3.1. Surface morphology after WEDM multi-cutting The surface morphologies of 600 8C-tempered specimens machined with WEDM after different cutting passes are shown in Fig. 2(a)–(e). As shown in Fig. 2(a), a matt and porous morphology can be observed on the rough-cut, or 1st-cut surface. According to the rough cutting condition presented in Fig. 1, the wire-electrode with the highest power
Fig. 2. The surface morphologies of martensitic stainless steel after the 1st to 5th WEDM cutting passes [denoted (a)–(e)].
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was used to separate the workpiece; besides, dielectric fluid was strongly injected into the gap with the highest flushing pressure between workpiece and wire-electrode. Hence, we can expect rougher surfaces after 1st cutting than those after the fine cutting passes. On the surfaces after the 2nd, 3rd and 4th cutting passes, there are many craters and gas holes, a typical surface appearance machined with WEDM, which are derived from molten and recast processes and are presented in Fig. 2(b)–(d). On the other hand, many fine nodules can be found on the fine cut surface after the 5th cutting pass (see Fig. 2(e)). The surface roughness values, Ra, of 200 8C-tempered specimens after different cutting passes are presented in Table 3. As can be clearly seen in the table, the surface roughness decreased with an increasing of cutting passes. In spite of nodular morphology, the lowest surface roughness Ra of 0.8 mm can be found after the 5th cutting pass. To detect the chemical composition of multi-cut surfaces, a large electron beam was used to incident SEM-image area of cut surface. Fig. 3 shows the EDS
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Table 3 The surface roughness of 200 8C-annealed specimens after different cutting passes with WEDM Cutting pass
Ra (mm)
1st-cut
2nd-cut
3rd-cut
4th-cut
5th-cut
3.9
2.1
1.7
1.1
0.8
analysis results of the cut surfaces, which are 600 8Ctempered specimen after 1st to 5th cutting passes. Components of wire-electrode material (copper and zinc), as well as the substrate (iron and chromium) were detected with EDS. Obviously, the elements of the steel, iron and chromium, were found only on the surfaces after 2nd to 4th cutting passes. On the contrary, the wire-electrode materials were detected only on the surfaces after 1st and 5th cutting passes. It implies that the steel surfaces cut after 1st and 5th passes were much more alloyed with wire-electrode material than those cut after other cutting passes. The alloyed region on the surface after WEDM cutting should exist in the recast layer, in which
Fig. 3. The EDS analysis of the 600 8C-tempered specimens after different WEDM cutting passes. (No. 1–5 indicate the 1st, 2nd, 3rd, 4th, and 5th cutting passes.)
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molten materials of steel substrate and wire-electrode were alloyed through thermal effect of electrical discharging. It can be expected that the thicker the recast layer was developed from the specimen, the deeper the alloying depth could be. Cross-sectioned SEM micrographs of 600 8C-tempered specimen after multi-cutting passes are presented in Fig. 4. As shown in the figure, the thickness of recast layer decreased rapidly with the number of cutting passes. The thickness of recast layer was ca. 10 mm for 1st-cut specimen, 3 mm for 2nd-cut specimen, 1 mm for 3rdcut specimen, and almost undetectable with SEM for 4th- and 5th-cut specimens, i.e. the alloying depth from outermost cut surface decreases with the increasing cutting pass from 1st to 4th cutting when the wire-electrode was negatively biased. However, from the results of EDS-spectra shown in Fig. 3, the 5th-cut surface was severely alloyed with wireelectrode material. This alloyed effect was attributed to positively biased wire-electrode during the 5th cutting pass. Although, either 4th or 5th WEDM cutting pass was recommended for final surface shaping, significant difference in the cut surface morphologies were observed. An interpretation of the difference could be the existence of two cutting passes with opposite biased voltage. In the case of 4th cutting pass, the voltage of the wire-electrode was negatively biased,
where most heat was generated in the workpiece [4]. Thus, the surface of the workpiece around the wireelectrode was molten or even vaporized preferentially. On the other hand, most heat was concentrated in the wire-electrode in the 5th cutting pass, where the wireelectrode was positively biased during the cutting. Comparing to the workpiece, the exposed area of the wire-electrode is relatively small; hence, the current density is much higher in the wire-electrode. Furthermore, dielectric fluid could not flush out all of the vaporized and molten material, leaving the wireelectrode components to condense on the surface or even alloy with the workpiece. These arguments can be deduced from EDS analysis of Fig. 3, where the wire-electrode elements, Cu and Zn were detected on the surface machined with the 1st- and 5th-cut, but not found on other cut surfaces. 3.2. Microstructure examined with TEM The TEM-micrographs of cross-sectioned 1st-, 4thand 5th-cut surfaces are presented in Figs. 5–7, where all specimens were tempered at 600 8C. On the 1st-cut surface shown in Fig. 5, several spherical deposits, mainly composed of wire-electrode material, were found which are evenly distributed within the recast layer. The thickness of recast layer was estimated to be a few micrometers. Surface alloying occurs in rough
Fig. 4. Cross-sectioned SEM micrographs of 600oC-tempered specimen after WEDM multi-cutting passes. The thickness of recast layers were indicated in the above micrographs except (d) and (e).
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Fig. 5. The TEM-micrograph of cross-sectioned rough-cut surface of the 600 8C-tempered specimen.
cutting by sputtering the molten wire-electrode material on the cut surface. The sputtered deposits were then co-deposited with the recast. Fig. 6 shows the TEM-micrograph of crosssectional surface after 4th-cut. A layer composed of equiaxed martensite can be obviously found on the
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outermost surface. The martensitic structure was transformed through heating effect of discharging process during WEDM cutting. When compared with the result of cut surface analyzed with EDS shown in Fig. 3, we can conclude that almost no wire-electrode material was detected on the cut surface. Contrary to the surface after 4th-cut, a dense deposited layer of wire-electrode material was detected on the outermost surface after the 5th-cut (see Fig. 7). However, the deposited layer was so thin that wire-electrode elements, copper and zinc were not found in the depth of 100 nm measured from the outermost cut surface. The EDS analysis was conducted by using 4 nm electron beams to analyze the chemical composition on a specific position in the specimen. The EDS-spectra are shown in Fig. 7, in which elements, Cu and Zn, were not detected in the position B, 100 nm from outermost cut surface, while a very thin alloyed layer was detected in the position A, located directly on the outermost cut surface. Wire-electrode components were found on the 1stcut surface, although the cutting pass has the same potential-biased direction as those of the 2nd, 3rd and 4th cutting passes. This alloyed phenomenon could be attributed to the splitting process where the wireelectrode was surrounded with workpiece during the 1st cutting pass, in spite of the high flushing pressure of dielectric fluid. There was a great chance that the
Fig. 6. (a) The TEM-micrograph of cross-sectioned surface after the WEDMed 4th cutting pass of the 600 8C-tempered specimen. (b) EDS analysis at the position A indicated in (a).
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Fig. 7. (a) The TEM-micrograph of cross-sectioned 5th-cut surfaces of the 600 8C-tempered specimen. (b) EDS analysis at positions A and B shown in (a).
materials of wire-electrode would sputter on the surface of workpiece. In this study we confirm that 1stcut surface can reach an alloyed depth of a few tens of micrometer and the wire-electrode components were evenly distributed in the recast layer. On the other hand, high density of wire-electrode deposition was detected on the 5th-cut surface, but the depth of the alloyed surface was only below 0.1 mm. From the above results we confirm that the alloying degree of the cut surface strongly depends on the applied cutting pass. As cut surfaces with different degree of alloying could have dissimilar electrochemical behavior in a suitable electrolyte, the anodic polarization experiments were conducted. Results of the electrochemical measurement are presented in the next section.
behavior. A higher anodic current density in 600 8Ctempered specimen was measured, when all of the specimens were polarized at the same overpotential. It suggests that 600 8C-tempered specimen has a higher anodic dissolution rate than the other specimens. Several studies [13] reported that multiple phases and various precipitates were developed in the substrate of martensitic stainless steel when the steel was quenched and then tempered at temperatures higher than 500 8C. Due to galvanic effect among the multi
3.3. Electrochemical measurement Fig. 8 shows the anodic polarization curves of asquenched, 200 8C, 400 8C and 600 8C-tempered specimens in 0.5 M HClO4 with 0.2 wt% NaCl solution. With the increase of the anodic overpotential, a typical active, passive and transpassive behavior of all specimens was detected. Except 600 8C-tempered specimen, as-quenched, 200 8C, and 400 8C-tempered specimens have almost the same anodic polarization
Fig. 8. The anodic polarization curves of as-quenched, 200 8C, 400 8C and 600 8C-tempered specimens in 0.5 M HClO4 with 0.2 wt% NaCl solution.
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phases, we can expect a higher dissolution rate in 600 8C-tempered specimen than that in the specimens tempered at relatively low temperatures. The anodic polarization curves of the specimens tempered at different temperatures after WEDM multi-cutting in 0.5 M HClO4 with 0.2 wt% NaCl electrolyte are presented in Fig. 9. It can be seen that the variation of anodic current density in each specimen was closely related to the cutting passes of WEDM. The lowest anodic current density was detected on the specimen without WEDM cutting. In the same anodic polarized potential, the anodic current density of each specimen subsequently decreased as the specimen was machined after 1st to 4th cutting passes. Severe alloyed cut surfaces, the 1st- and 5th-cut surfaces, have the highest anodic current densities in their anodic polarization curves
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than the surfaces cut after the other passes. Compared to EDS-spectra shown in Fig. 3 and the thickness variation of recast layer presented in Fig. 4, it is evident that the extent of alloying in cut surface can be revealed from its anodic polarization curve tested at 27 8C in 0.5 M HClO4 with 0.2 wt% NaCl, and the higher passive current density, the higher degree alloyed surface. As shown in Fig. 9, there is an obvious secondary anodic current peak in the anodic polarization curves of 1st- and 5th-cut surfaces, where the corresponding potential of the peak was at ca. 120 mV. Since the surfaces were severely alloyed, the secondary anodic current peak must be related to alloying of wireelectrode material. In order to clarify the existence of the secondary anodic current peak, potentiostatic etching at potentials of 90 mV and 200 mV, which
Fig. 9. The anodic polarization curves of (a) as-quenched, (b) 200 8C-, (c) 400 8C-, and (d) 600 8C-tempered specimens after the WEDMed 1st to 5th cutting passes.
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were in the neighborhood of the potential of secondary anodic current peak, for 200 s was conducted. After potentiostatic etching, the etched surfaces were EDS-analyzed and the EDS-spectra are presented in Fig. 10. As shown in Fig. 10, the Cu element was detected on the cut surface after potentiostatic etching at 90 mV, but was not found if etched at 120 mV. We thus conclude that the secondary anodic current peak was attributed to dissolution of copper, the main element of wireelectrode material in the cut surface. To further study the degree of surface alloying machined with WEDM, two definite alloyed conditions, the 4th- and 5th-cut surfaces, of 600 8Ctempered specimen were potentiostatically set at 200 mV for 1500 s in 0.5 M HClO4 with 0.2 wt%
Fig. 10. The EDS-spectra of the cut surfaces after potentiostatic etching at (a) 90 mV, (b) 200 mV for 200 s.
Fig. 11. The current density variation of the WEDMed 4th- and 5thcut surfaces of the 200 8C-tempered specimen during potentiostatic etching at 200 mV for 3000 s.
NaCl, and the results are shown in Fig. 11. The anodic current density of 5th-cut surface decreased with the duration increase of potentiostatic etching. Moreover, the current density of the 5th-cut surface was almost the same as that of the 4th-cut surface after potentiostatic etching for 120 s. From the TEMmicrograph of the cross-sectioned 5th-cut surface, the alloyed depth was only within 0.1 mm from the outermost surface. Adjacent to the alloyed layer, a martensite structure was detected. In the beginning of etching, high anodic current density of the 5th-cut surface was due to the alloying effect of wire-electrode material. However, as the thin alloyed layer was fully dissolved, the anodic current density of 5th-cut surface reduced to the same value as that of 4th-cut surface. This can be also validated from the EDS-spectra presented in Fig. 10, in which the alloy element, Cu was not be detected after potentiostatic etching at 200 mV for 200 s. The electrochemical behavior of an electrode is the response of the interface between electrode surface and electrolyte. Thus, anodic dissolution behavior depends strongly on the condition of electrode surface and testing electrolyte. Detecting the surface alloying condition with electrochemical method was shown in the literature [14–16]. We also studied the anodic polarization behavior of the surfaces multi-cut with WEDM in 1 M H2SO4 + 0.2 wt% NaCl. The anodic polarization curves of the multi-cut surfaces of
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4. Conclusion
Fig. 12. The anodic polarization curves of the 200 8C-tempered specimen in 1 M H2SO4 with 0.2 wt% NaCl solution after the WEDMed 1st to 5th cutting passes.
200 8C-tempered specimens are presented in Fig. 12, in which higher anodic current densities can be found in the surfaces machined with the 1st and 5th cutting passes than those machined with other cutting passes. It implies that more surface alloy was developed after 1st and 5th cutting passes, but no obvious difference in the anodic polarization curves of 2nd-, 3rd- and 4thcut surfaces. However, we propose the electrolyte comprising of 0.5 M HClO4 + 0.2 wt% NaCl, which was also proposed to detect the degree of sensitization of austenitic stainless steel [17,18] for detecting the degree of surface alloying. The anodic current densities of these cut surfaces can be successfully differentiated and the anodic current density of the cut surface decreased with increasing the cutting pass from 1st to 4th-cut. Difference in anodic current density of the specimens could be attributed to recast layer thickness and surface roughness. Alloying between wire-electrode and workpiece materials might take place in the recast layer, and the thickness decreased with the cutting pass (see Fig. 4). The surface roughness of specimens decreased with the cutting pass (see Table 3), and the rougher surface of the specimen has higher anodic current density. Although the surface after the 5th cutting pass has the lowest surface roughness, higher anodic current density was detected than those after 2nd, 3rd and 4th cutting passes. It means that relatively high degree of surface alloying takes place on the 5th-cut surface. This is fully in agreement with the result of TEMexamination shown in Fig. 7.
In this paper, we detected the various alloying degree of WEDM-cut surfaces of martensitic stainless steel specimens with electrochemical tests in 0.5 M HClO4 + 0.2 wt% NaCl. Before cutting with WEDM, the steel specimens were quenched at 1050 8C and then tempered at 200 8C, 400 8C and 600 8C, respectively. The results show that the surfaces of the steel specimens were alloyed with wire-electrode material in various degrees according to different cutting passes with WEDM. We conclude that alloying degree of cut surfaces can be evaluated with their anodic polarization curves in 0.5 M HClO4 + 0.2 wt% NaCl at 27 8C. The significance of surface alloying is proportional to the passive current density. On the severely alloyed surfaces, the 1st- and 5th-cut surfaces, a secondary anodic current peak in 120 mV (versus Ag/AgClsat.) of its anodic polarization curve was observed. The presence of the secondary anodic peak was attributed to dissolution of copper, the main element of wire-electrode material, from the alloyed surface. Acknowledgment The authors would like to thank National Science Council, ROC for the financial support to this research under contract number: NSC 88-2216-E-182-006.
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The surface alloying behavior of martensitic stainless steel cut with wire electrical discharge machine Ching An Huang *, Chwen Lin Shih, Kung Cheng Li, Yau-Zen Chang Department of Mechanical Engineering, Chang Gung University, Taoyuan 333, Taiwan, ROC Received 27 January 2005; received in revised form 28 April 2005; accepted 29 April 2005 Available online 13 June 2005
Abstract The surface alloying behavior of tempered martensitic stainless steel multi-cut with wire electrical discharge machine (WEDM) is studied in this paper. Before machined with WEDM, the steel specimens were quenched at 1050 8C and then tempered at 200 8C, 400 8C, and 600 8C, respectively. The microstructure and surface morphology of the multi-cut surfaces were examined with scanning and transmission electron microscopes integrated with an energy-dispersive X-ray spectrometer for chemical composition analysis. Experimental results show that the cut surfaces of the steel specimens were alloyed with wireelectrode material in various extent. Especially the cut surface was much more alloyed when the steel was cut with the first rough cutting pass by using negatively biased potential and final fine cutting using positively biased potential. Alloying degree of cut surfaces can be distinguished with their anodic polarization curves in 0.5 M HClO4 + 0.2 wt% NaCl at 27 8C. Higher passive current density induces deeper alloyed surface. On the severely alloyed surface, a secondary anodic peak in the potential of 120 mV (versus Ag/AgClsat.) of its anodic polarization curve was observed. The presence of the secondary anodic peak was attributed to dissolution of copper, which was the major element of wire-electrode material from the alloyed surface. # 2005 Elsevier B.V. All rights reserved. Keywords: Wire electrical discharge machine (WEDM); Martensitic stainless steel; Microstructure; Electrochemical behavior
1. Introduction Due to its high hardenability, superior mechanical property and corrosion resistance, martensitic stainless steel is widely used for plastic molds, precision mechanical parts, and surgical tools [1]. In typical applications, the steel is quenched from 1050 8C to * Corresponding author. Tel.: +886 32118800x5346; fax: +886 32118740. E-mail address: [email protected] (C.A. Huang).
1150 8C to obtain hardened martensitic structure, and then tempered between 200 8C and 600 8C to achieve suitable strength and toughness. Because the hardened steel can be accurately shaped with wire electrical discharge machine through multi-cutting pass, WEDM is used in final surface extensively. The multi-cutting passes with WEDM can be briefly described as follows: First, the workpiece was roughly machined, or 1st-cut, with high flushing pressure of dielectric fluid, high power and low tension in wire-electrode to quickly separate and shape the
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.04.035
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workpiece. Then semi-finishing and finishing of 2nd to 5th cutting passes were carried out with laminar flow of dielectric fluid, relatively low power and high tension in wire-electrode. The cutting depth for the semi-finishing and finishing processes is only a few micrometers. The number of cutting passes required depends mainly on specified accuracy and surface roughness of the workpiece. Higher accuracy generally demands more cutting passes. From an application point of view, high accuracy can be achieved by a one-cut. However, smaller surface roughness requires more cutting passes in order to reduce overall machining time. The material removal mechanism of WEDM is the same as that of electrical discharge machine (EDM). It has been widely accepted that the metal removal mechanism in EDM is predominantly a thermal effect in nature [1]. The electrical discharging process generates a tremendous amount of heat causing melting or even evaporation in the local surface layers on both wire-electrode and workpiece sides. The heat also causes vaporization of the dielectric fluid and induces high pressure waves, which wash out the molten and/or vaporized metal into pieces from the workpiece. Continuously injected dielectric fluid then carries the droplets of metal away. Surface alloying between workpiece and wire-electrode materials was observed on the cut surface after cutting with WEDM [2,3]. Furthermore, phase transformation or heataffected zone (HAZ) could take place in the workpiece adjacent to the cut surface [4–7]. It has been reported in the literature [6–10] that surface modification through coating and phase transformation can be evaluated electrochemically. In this paper, we use electrochemical measurement to detect alloying degree and phase transformation of the surface of martensitic stainless steel, which was multicut with WEDM in 0.5 M HClO4 + 0.2 wt% NaCl at 27 8C.
Table 1 The chemical composition of martensitic stainless steel (AISI 440) used in this study Composition C Weight (%)
Cr
Mo
Mn
Si
S
P
0.39 15.89 1.02 0.87 0.46 <0.003 <0.003
for WEDM cutting as following. The steel billet was cut into several 15 mm thick plates by a sawing machine. The plate specimens were heated at 1050 8C for 1 h, and then quenched in oil. After quenching, some of the specimens were tempered at 200 8C, 400 8C and 600 8C, respectively, for one hour. Those plate specimens prepared, thus can be divided into four groups as-quenched, and tempered specimens in 200 8C, 400 8C and 600 8C, respectively. 2.2. Working condition A brass wire-electrode with a diameter of 0.25 mm was used in this study. The wire-electrode material was composed of 65 wt% copper and 35 wt% zinc. As
2. Experimental procedure 2.1. Materials treatment Martensitic stainless steel (AISI 440A2) was used in this study and its chemical composition is given in Table 1. The specimens for this study were prepared
Fig. 1. Schematic of the WEDM multi-cutting process.
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Table 2 Machining parameters used for multi-cutting passes with WEDM Cutting pass
Voltage (V)
CD (A)
TA (ms)
TB (ms)
WS (m/min)
WB (daN)
FP (bar)
1st-cut 2nd-cut 3rd-cut 4th-cut 5th-cut
80 120 80 80 +120
4 8 16 8 4
1.0 0.2 0.4 0.4 0.2
9.0 3.8 3.0 3.6 3.8
10 8.0 8.0 8.0 8.0
1.0 1.4 1.4 1.6 1.4
5.5 0.8 0.8 0.8 0.8
CD: discharging current; TA: duration of discharging time; TB: time between two pulses; WS: wire feeding speed; WB: wire tension; FP: flushing pressure.
shown in Fig. 1, the feeds for the 2nd to 5th passes were 40 mm, 15 mm, 4 mm and 3 mm, respectively. The operating parameters for multi-cutting process are shown in Table 2, and these are based on the expert system in WEDM (Charmilles Technologies Robofil 300). For the 1st (rough) cutting pass, which was used to effectively separate the workpiece, the highest cutting power in the wire-electrode and the largest flushing pressure in dielectric fluid were applied. The purpose of the 2nd and the 3rd cutting passes was to enhance the geometrical and dimensional accuracy of the workpiece, while the last 4th and 5th passes were applied for final surface shaping. Unlike the preceding four cutting passes, the electrode was reverse biased in the 5th cutting pass. The conductivity of dielectric fluid for each cutting was precisely controlled within 5.0 0.5 mS/cm and the working temperature was kept at 20 0.5 8C. 2.3. Preparation of the cut surfaces The multi-cut surfaces described above were examined using a scanning electron microscope (SEM) integrated with an energy-dispersive X-ray spectrometer (EDS) for chemical composition analysis. The specimens, quenched- and tempered-steel plates, were cut by WEDM with different number of cutting passes. The machined surfaces were carefully preserved in wax and sliced into 8 mm 15 mm 15 mm pieces with a diamond saw. Then the specimens were ultrasonically cleaned in an acetone bath for 10 min to remove wax on the surfaces. After flushed with alcohol, the specimens were dried using a cool air blaster for SEM/EDS study, and electrochemical measurements. With the same preparing procedure the surface roughness in arithmetic mean value, Ra, of the specimens was measured with a surface profiler (Hommel Werk T 4000).
Microstructures of finished surfaces were studied with analytical transmission electron microscope (ATEM, JEOL 2010 and Links-EDX system). Cross-sectioned specimens for ATEM examination were taken from 600 8C-tempered steel specimen, which was prepared after rough, 4th and 5th cutting passes. The same processes to prepare SEM specimen of WEDM-cut surface were applied on the ATEM specimens. The cut surfaces were sectioned in small plate with WEDM in a dimension of 1 mm thick, 1.5 mm long and 1 mm wide, stuck the cut faces of two plates together with M-Bond 610, and then mounted vertically with G1 epoxy (Gatan company, USA) in a 3 mm diameter copper ring. The specimens were then mechanically ground to 100 mm in thickness, and both sides of the specimen were further dimpled with Dimpler (VCR, Dimpler) to a thickness of 10 mm in the center. Finally, a low angle (88) Ar+ion milling (VCR, XLA 2000) machine under 5 kV voltage was used to sputter the specimen until a tiny hole was produced in the adhered cut interface, around which the cut surface was so thin that electron diffraction and image could be observed and examined with the TEM/EDS. During the ion milling, the specimen holder was cooled with liquid nitrogen to avoid thermal effects on the specimen. Details of specimen preparation were presented in [11,12]. 2.4. Electrochemical analysis The electrochemical measurements were carried out in a three-electrode electrochemical cell. The Ag/AgCl electrode in the saturated KCl solution was used as the reference electrode, and a platinized Ti-mesh as the counter electrode. The exposed area of the multi-cut specimen was 1 cm2 and testing temperature was kept at 27 1 8C in a circulated water bath. The electrolyte was composed of 0.5 M HClO4 with 0.2 wt% NaCl.
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Anodic polarization measurement was conducted with a potentiostat/galvanostat (EG&G Model 273A) in a flat cell (EG&G Model K0235). De-aeration was carried out by purging N2 gas into the electrolyte during electrochemical measurement. Before electrochemical measurement, the specimen was immersed in the electrolyte for 10 min to ensure dynamic stability. The potentiodynamic polarization behavior of the multi-cut specimen was studied by scanning the specimens with a potential difference range from 250 (versus opencircuit potential) to 1200 mV (versus Ag/AgClsat.) in a scan rate of 0.3 mV/s.
3. Results and discussions 3.1. Surface morphology after WEDM multi-cutting The surface morphologies of 600 8C-tempered specimens machined with WEDM after different cutting passes are shown in Fig. 2(a)–(e). As shown in Fig. 2(a), a matt and porous morphology can be observed on the rough-cut, or 1st-cut surface. According to the rough cutting condition presented in Fig. 1, the wire-electrode with the highest power
Fig. 2. The surface morphologies of martensitic stainless steel after the 1st to 5th WEDM cutting passes [denoted (a)–(e)].
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was used to separate the workpiece; besides, dielectric fluid was strongly injected into the gap with the highest flushing pressure between workpiece and wire-electrode. Hence, we can expect rougher surfaces after 1st cutting than those after the fine cutting passes. On the surfaces after the 2nd, 3rd and 4th cutting passes, there are many craters and gas holes, a typical surface appearance machined with WEDM, which are derived from molten and recast processes and are presented in Fig. 2(b)–(d). On the other hand, many fine nodules can be found on the fine cut surface after the 5th cutting pass (see Fig. 2(e)). The surface roughness values, Ra, of 200 8C-tempered specimens after different cutting passes are presented in Table 3. As can be clearly seen in the table, the surface roughness decreased with an increasing of cutting passes. In spite of nodular morphology, the lowest surface roughness Ra of 0.8 mm can be found after the 5th cutting pass. To detect the chemical composition of multi-cut surfaces, a large electron beam was used to incident SEM-image area of cut surface. Fig. 3 shows the EDS
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Table 3 The surface roughness of 200 8C-annealed specimens after different cutting passes with WEDM Cutting pass
Ra (mm)
1st-cut
2nd-cut
3rd-cut
4th-cut
5th-cut
3.9
2.1
1.7
1.1
0.8
analysis results of the cut surfaces, which are 600 8Ctempered specimen after 1st to 5th cutting passes. Components of wire-electrode material (copper and zinc), as well as the substrate (iron and chromium) were detected with EDS. Obviously, the elements of the steel, iron and chromium, were found only on the surfaces after 2nd to 4th cutting passes. On the contrary, the wire-electrode materials were detected only on the surfaces after 1st and 5th cutting passes. It implies that the steel surfaces cut after 1st and 5th passes were much more alloyed with wire-electrode material than those cut after other cutting passes. The alloyed region on the surface after WEDM cutting should exist in the recast layer, in which
Fig. 3. The EDS analysis of the 600 8C-tempered specimens after different WEDM cutting passes. (No. 1–5 indicate the 1st, 2nd, 3rd, 4th, and 5th cutting passes.)
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molten materials of steel substrate and wire-electrode were alloyed through thermal effect of electrical discharging. It can be expected that the thicker the recast layer was developed from the specimen, the deeper the alloying depth could be. Cross-sectioned SEM micrographs of 600 8C-tempered specimen after multi-cutting passes are presented in Fig. 4. As shown in the figure, the thickness of recast layer decreased rapidly with the number of cutting passes. The thickness of recast layer was ca. 10 mm for 1st-cut specimen, 3 mm for 2nd-cut specimen, 1 mm for 3rdcut specimen, and almost undetectable with SEM for 4th- and 5th-cut specimens, i.e. the alloying depth from outermost cut surface decreases with the increasing cutting pass from 1st to 4th cutting when the wire-electrode was negatively biased. However, from the results of EDS-spectra shown in Fig. 3, the 5th-cut surface was severely alloyed with wireelectrode material. This alloyed effect was attributed to positively biased wire-electrode during the 5th cutting pass. Although, either 4th or 5th WEDM cutting pass was recommended for final surface shaping, significant difference in the cut surface morphologies were observed. An interpretation of the difference could be the existence of two cutting passes with opposite biased voltage. In the case of 4th cutting pass, the voltage of the wire-electrode was negatively biased,
where most heat was generated in the workpiece [4]. Thus, the surface of the workpiece around the wireelectrode was molten or even vaporized preferentially. On the other hand, most heat was concentrated in the wire-electrode in the 5th cutting pass, where the wireelectrode was positively biased during the cutting. Comparing to the workpiece, the exposed area of the wire-electrode is relatively small; hence, the current density is much higher in the wire-electrode. Furthermore, dielectric fluid could not flush out all of the vaporized and molten material, leaving the wireelectrode components to condense on the surface or even alloy with the workpiece. These arguments can be deduced from EDS analysis of Fig. 3, where the wire-electrode elements, Cu and Zn were detected on the surface machined with the 1st- and 5th-cut, but not found on other cut surfaces. 3.2. Microstructure examined with TEM The TEM-micrographs of cross-sectioned 1st-, 4thand 5th-cut surfaces are presented in Figs. 5–7, where all specimens were tempered at 600 8C. On the 1st-cut surface shown in Fig. 5, several spherical deposits, mainly composed of wire-electrode material, were found which are evenly distributed within the recast layer. The thickness of recast layer was estimated to be a few micrometers. Surface alloying occurs in rough
Fig. 4. Cross-sectioned SEM micrographs of 600oC-tempered specimen after WEDM multi-cutting passes. The thickness of recast layers were indicated in the above micrographs except (d) and (e).
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Fig. 5. The TEM-micrograph of cross-sectioned rough-cut surface of the 600 8C-tempered specimen.
cutting by sputtering the molten wire-electrode material on the cut surface. The sputtered deposits were then co-deposited with the recast. Fig. 6 shows the TEM-micrograph of crosssectional surface after 4th-cut. A layer composed of equiaxed martensite can be obviously found on the
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outermost surface. The martensitic structure was transformed through heating effect of discharging process during WEDM cutting. When compared with the result of cut surface analyzed with EDS shown in Fig. 3, we can conclude that almost no wire-electrode material was detected on the cut surface. Contrary to the surface after 4th-cut, a dense deposited layer of wire-electrode material was detected on the outermost surface after the 5th-cut (see Fig. 7). However, the deposited layer was so thin that wire-electrode elements, copper and zinc were not found in the depth of 100 nm measured from the outermost cut surface. The EDS analysis was conducted by using 4 nm electron beams to analyze the chemical composition on a specific position in the specimen. The EDS-spectra are shown in Fig. 7, in which elements, Cu and Zn, were not detected in the position B, 100 nm from outermost cut surface, while a very thin alloyed layer was detected in the position A, located directly on the outermost cut surface. Wire-electrode components were found on the 1stcut surface, although the cutting pass has the same potential-biased direction as those of the 2nd, 3rd and 4th cutting passes. This alloyed phenomenon could be attributed to the splitting process where the wireelectrode was surrounded with workpiece during the 1st cutting pass, in spite of the high flushing pressure of dielectric fluid. There was a great chance that the
Fig. 6. (a) The TEM-micrograph of cross-sectioned surface after the WEDMed 4th cutting pass of the 600 8C-tempered specimen. (b) EDS analysis at the position A indicated in (a).
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Fig. 7. (a) The TEM-micrograph of cross-sectioned 5th-cut surfaces of the 600 8C-tempered specimen. (b) EDS analysis at positions A and B shown in (a).
materials of wire-electrode would sputter on the surface of workpiece. In this study we confirm that 1stcut surface can reach an alloyed depth of a few tens of micrometer and the wire-electrode components were evenly distributed in the recast layer. On the other hand, high density of wire-electrode deposition was detected on the 5th-cut surface, but the depth of the alloyed surface was only below 0.1 mm. From the above results we confirm that the alloying degree of the cut surface strongly depends on the applied cutting pass. As cut surfaces with different degree of alloying could have dissimilar electrochemical behavior in a suitable electrolyte, the anodic polarization experiments were conducted. Results of the electrochemical measurement are presented in the next section.
behavior. A higher anodic current density in 600 8Ctempered specimen was measured, when all of the specimens were polarized at the same overpotential. It suggests that 600 8C-tempered specimen has a higher anodic dissolution rate than the other specimens. Several studies [13] reported that multiple phases and various precipitates were developed in the substrate of martensitic stainless steel when the steel was quenched and then tempered at temperatures higher than 500 8C. Due to galvanic effect among the multi
3.3. Electrochemical measurement Fig. 8 shows the anodic polarization curves of asquenched, 200 8C, 400 8C and 600 8C-tempered specimens in 0.5 M HClO4 with 0.2 wt% NaCl solution. With the increase of the anodic overpotential, a typical active, passive and transpassive behavior of all specimens was detected. Except 600 8C-tempered specimen, as-quenched, 200 8C, and 400 8C-tempered specimens have almost the same anodic polarization
Fig. 8. The anodic polarization curves of as-quenched, 200 8C, 400 8C and 600 8C-tempered specimens in 0.5 M HClO4 with 0.2 wt% NaCl solution.
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phases, we can expect a higher dissolution rate in 600 8C-tempered specimen than that in the specimens tempered at relatively low temperatures. The anodic polarization curves of the specimens tempered at different temperatures after WEDM multi-cutting in 0.5 M HClO4 with 0.2 wt% NaCl electrolyte are presented in Fig. 9. It can be seen that the variation of anodic current density in each specimen was closely related to the cutting passes of WEDM. The lowest anodic current density was detected on the specimen without WEDM cutting. In the same anodic polarized potential, the anodic current density of each specimen subsequently decreased as the specimen was machined after 1st to 4th cutting passes. Severe alloyed cut surfaces, the 1st- and 5th-cut surfaces, have the highest anodic current densities in their anodic polarization curves
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than the surfaces cut after the other passes. Compared to EDS-spectra shown in Fig. 3 and the thickness variation of recast layer presented in Fig. 4, it is evident that the extent of alloying in cut surface can be revealed from its anodic polarization curve tested at 27 8C in 0.5 M HClO4 with 0.2 wt% NaCl, and the higher passive current density, the higher degree alloyed surface. As shown in Fig. 9, there is an obvious secondary anodic current peak in the anodic polarization curves of 1st- and 5th-cut surfaces, where the corresponding potential of the peak was at ca. 120 mV. Since the surfaces were severely alloyed, the secondary anodic current peak must be related to alloying of wireelectrode material. In order to clarify the existence of the secondary anodic current peak, potentiostatic etching at potentials of 90 mV and 200 mV, which
Fig. 9. The anodic polarization curves of (a) as-quenched, (b) 200 8C-, (c) 400 8C-, and (d) 600 8C-tempered specimens after the WEDMed 1st to 5th cutting passes.
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were in the neighborhood of the potential of secondary anodic current peak, for 200 s was conducted. After potentiostatic etching, the etched surfaces were EDS-analyzed and the EDS-spectra are presented in Fig. 10. As shown in Fig. 10, the Cu element was detected on the cut surface after potentiostatic etching at 90 mV, but was not found if etched at 120 mV. We thus conclude that the secondary anodic current peak was attributed to dissolution of copper, the main element of wireelectrode material in the cut surface. To further study the degree of surface alloying machined with WEDM, two definite alloyed conditions, the 4th- and 5th-cut surfaces, of 600 8Ctempered specimen were potentiostatically set at 200 mV for 1500 s in 0.5 M HClO4 with 0.2 wt%
Fig. 10. The EDS-spectra of the cut surfaces after potentiostatic etching at (a) 90 mV, (b) 200 mV for 200 s.
Fig. 11. The current density variation of the WEDMed 4th- and 5thcut surfaces of the 200 8C-tempered specimen during potentiostatic etching at 200 mV for 3000 s.
NaCl, and the results are shown in Fig. 11. The anodic current density of 5th-cut surface decreased with the duration increase of potentiostatic etching. Moreover, the current density of the 5th-cut surface was almost the same as that of the 4th-cut surface after potentiostatic etching for 120 s. From the TEMmicrograph of the cross-sectioned 5th-cut surface, the alloyed depth was only within 0.1 mm from the outermost surface. Adjacent to the alloyed layer, a martensite structure was detected. In the beginning of etching, high anodic current density of the 5th-cut surface was due to the alloying effect of wire-electrode material. However, as the thin alloyed layer was fully dissolved, the anodic current density of 5th-cut surface reduced to the same value as that of 4th-cut surface. This can be also validated from the EDS-spectra presented in Fig. 10, in which the alloy element, Cu was not be detected after potentiostatic etching at 200 mV for 200 s. The electrochemical behavior of an electrode is the response of the interface between electrode surface and electrolyte. Thus, anodic dissolution behavior depends strongly on the condition of electrode surface and testing electrolyte. Detecting the surface alloying condition with electrochemical method was shown in the literature [14–16]. We also studied the anodic polarization behavior of the surfaces multi-cut with WEDM in 1 M H2SO4 + 0.2 wt% NaCl. The anodic polarization curves of the multi-cut surfaces of
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4. Conclusion
Fig. 12. The anodic polarization curves of the 200 8C-tempered specimen in 1 M H2SO4 with 0.2 wt% NaCl solution after the WEDMed 1st to 5th cutting passes.
200 8C-tempered specimens are presented in Fig. 12, in which higher anodic current densities can be found in the surfaces machined with the 1st and 5th cutting passes than those machined with other cutting passes. It implies that more surface alloy was developed after 1st and 5th cutting passes, but no obvious difference in the anodic polarization curves of 2nd-, 3rd- and 4thcut surfaces. However, we propose the electrolyte comprising of 0.5 M HClO4 + 0.2 wt% NaCl, which was also proposed to detect the degree of sensitization of austenitic stainless steel [17,18] for detecting the degree of surface alloying. The anodic current densities of these cut surfaces can be successfully differentiated and the anodic current density of the cut surface decreased with increasing the cutting pass from 1st to 4th-cut. Difference in anodic current density of the specimens could be attributed to recast layer thickness and surface roughness. Alloying between wire-electrode and workpiece materials might take place in the recast layer, and the thickness decreased with the cutting pass (see Fig. 4). The surface roughness of specimens decreased with the cutting pass (see Table 3), and the rougher surface of the specimen has higher anodic current density. Although the surface after the 5th cutting pass has the lowest surface roughness, higher anodic current density was detected than those after 2nd, 3rd and 4th cutting passes. It means that relatively high degree of surface alloying takes place on the 5th-cut surface. This is fully in agreement with the result of TEMexamination shown in Fig. 7.
In this paper, we detected the various alloying degree of WEDM-cut surfaces of martensitic stainless steel specimens with electrochemical tests in 0.5 M HClO4 + 0.2 wt% NaCl. Before cutting with WEDM, the steel specimens were quenched at 1050 8C and then tempered at 200 8C, 400 8C and 600 8C, respectively. The results show that the surfaces of the steel specimens were alloyed with wire-electrode material in various degrees according to different cutting passes with WEDM. We conclude that alloying degree of cut surfaces can be evaluated with their anodic polarization curves in 0.5 M HClO4 + 0.2 wt% NaCl at 27 8C. The significance of surface alloying is proportional to the passive current density. On the severely alloyed surfaces, the 1st- and 5th-cut surfaces, a secondary anodic current peak in 120 mV (versus Ag/AgClsat.) of its anodic polarization curve was observed. The presence of the secondary anodic peak was attributed to dissolution of copper, the main element of wire-electrode material, from the alloyed surface. Acknowledgment The authors would like to thank National Science Council, ROC for the financial support to this research under contract number: NSC 88-2216-E-182-006.
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