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A study on the surface treatment of “Calmax” tool steel by a plasma transferred arc (PTA) process
Journal of Materials Processing Technology 128 (2002) 169–177
A study on the surface treatment of ‘‘Calmax’’ tool steel by a plasma transferred arc (PTA) process E. Bourithis, A. Tazedakis, G. Papadimitriou* Laboratory of Physical Metallurgy, National Technical University of Athens, 9 Heroon Polytechniou Street, 15780 Athens, Greece Received 29 June 2001; received in revised form 13 February 2002; accepted 26 June 2002
Abstract A tool steel (Calmax of Uddeholm) was subjected to surface melting using the plasma transferred arc (PTA) process. Single and multiple run melting was used under different working conditions and the geometric characteristics of melted and heat affected zones were studied as a function of working parameters, current, travel speed and gas flow rate. The parameters were adjusted in order to obtain hard layers with sufficient thickness, presenting at the same time low roughness and crack free surfaces. The same steel was subjected to a conventional heat treatment (quenching and tempering) for comparison. The PTA surface treated specimens showed fine martensitic microstructures in depth going up to 1.5E3 m. Their hardness was higher than the one of the conventionally heat treated steel. Pin on disk wear testing showed that PTA treated specimens have much better wear resistance, attributed mainly to the fine microstructure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PTA; Tool steel; Surfacing
1. Introduction Surface treatments of metals are commonly based on the use of high energy density sources, as they offer a means of rapid heating and subsequent quenching from the melt, leading to fine microstructures and consequently to possible improvement of mechanical, corrosion or tribological properties. Superficial layers of the appropriate thickness, free of cracks and with high hardness may be obtained by suitable control of the process variables. In this respect there has been considerable interest in the use of laser and electron beam sources for surface treating and melting of low carbon steels [1,2] and stainless and tool steels [3–5]. The impact of the process variables on temperature profiles [6–10], microstructure [11–16] and properties [17–21] have been examined and the results already obtained may serve as a reference for further investigations. However, only a limited number of investigations concerns the use of the plasma transferred arc (PTA) [22,23], although there are serious indications that its use may be quite attractive in industrial applications. It is, therefore, interesting to investigate the PTA process, which—despite its lower energy density—has
* Corresponding author. Tel.: þ30-10-772-2184; fax: þ30-10-772-2119. E-mail address: [email protected] (G. Papadimitriou).
the main advantage to require rather inexpensive equipment and the possibility to work with a higher heat input. In this context the present work was undertaken in order to study the possibility of superficial hardening of an air hardened tool steel (Calmax of Uddeholm) using the PTA process. This steel is widely utilized for moulds and dies in the forming industry of plastics and light alloys. Experiments were performed on steel plates by rapidly heating to peak temperatures over the melting point of steel, using both single and multiple track moving plasma beams. The geometrical characteristics (form and dimensions) of the heat affected layer and the corresponding microstructure were investigated in connection with the main process variables: heat input, scan speed and plasma gas flow. In order to assess the wear resistance, pin on disc tests have been done on plasma treated specimens and the results obtained were compared with those obtained with a conventionally heat treated (quenched and tempered) Calmax tool steel.
2. Procedures–experimental techniques The material under investigation is the commercial ‘‘Calmax’’ tool steel of Uddeholm-Sweden with chemical composition is given in Table 1. The steel is used in the soft-annealed condition, having a hardness of 220 HV.
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 4 4 7 - 8
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Table 1 Calmax steel chemical composition C Si Mn Cr Mo V
Table 3 Parameters utilized for scanning the surfaces with plasma arc 0.60% 0.35% 0.80% 4.50% 0.50% 0.20%
Specimens in the form of plates of 43E3 m 60E3 m area and 28E3 m thickness were treated superficially by the PTA process. Before their treatment the specimens were polished with abrasive paper down to 1200 grit fineness in order to remove oxides and obtain a smooth surface. The roughness was brought down to the level of Ra ¼ 0:04 mm, the same for all specimens, in order to keep stable arc conditions and avoid current fluctuations. The main equipment employed was a PW-200 Sabre Arc plasma console capable of controlling the plasma and shielding gas flow rates, a plasma torch (PWH-3A) for low/medium current applications, a KEMPI 2000 power source and an automatic machine for control of the torch movement in the horizontal level (x–y-axis). The fixed operating parameters of the PTA system are given in Table 2. In a first step of the investigation single runs have been performed along the central axis of a number of plates using different arc conditions. Their selection was made so that a qualitative approach on the influence of the three basic operating parameters, i.e. current (i), travel speed (u) and plasma gas flow rates (Q), could be achieved. All runs were visually inspected and Vickers hardness tests were performed, running vertically from the surface through the melted and the HAZ down to the bulk metal. Metallographic examination was carried out using both optical and scanning electron microscopy (SEM). In a second step based on the optimum conditions identified during the above experiments the whole surface area of specimens was scanned by running successive passes in the x-direction with a fixed overlapping in the y (perpendicular)direction. A 60% overlapping of the melting zone was used, in order to avoid important fluctuations of the hardened depth. For avoiding superheating the specimens were positioned on a copper plate and the latter placed inside a tank Table 2 Fixed values of plasma operating parameters Plasma gas Shielding gas Shielding gas flow Electrode Electrode diameter Electrode angle Electrode set back Stand off distance Tip diameter
Ar 99.9% Ar 99.9% 1.6E4 m3/s W-2% ThO2 2.4E3 m 458 1.8E3 m 2.5E3 m 2.35E3 m
i (A)
u (m/s)
Q (m3/s)
Surface 1 75
3.0E3
1.7E5
Surface 2 80
2.5E3
1.7E5
filled with water for cooling. Two series of operating parameters were chosen (Table 3). The above surfaces were subsequently examined using the same with the above experimental procedure. For comparison specimens of the Calmax steel were also conventionally heat treated by quenching and tempering, using conditions suggested by the steel producer and similar to those usually practiced in industry, i.e. austenitization for 30 min at 960 8C, followed by quenching in oil and double tempering at 500 and 470 8C for 2 h at each temperature. The results obtained were used as a basis for the assessment of the results achieved with the plasma treatment, namely the microstructural characteristics of the superficial layer, its uniformity, hardness and tribological behavior.
3. Results 3.1. ‘‘Single runs’’ The operating parameters utilized for the ‘‘single runs’’ were investigated within the following limits: the arc current was varied between 40 and 80 A and the travel speed between 2.5E3 and 4.0E3 m/s. The plasma gas flow rate was kept constant at two different levels, either 0.8E5 or 1.7E5 m3/s. Under these conditions the PTA treated metal of the single runs, observed both macroscopically and microscopically was free from cracks. The obtention of very hard layers with a thickness of the order of 1.5E3 m and hardness of the order of 800–900 HV was easily achieved using the upper current range and the 1.75E5 m3/s gas flow rate. The 0.8E5 m3/s gas flow rate gave also satisfactory layers having a similar hardness but a thickness lower by about 10–20%. It should be emphasized that the conventionally heat treated steel presents a hardness of 780 HV (about 63HRC) in the as quenched condition and 535 HV (about 51.5HRC) in the tempered condition. The microstructure, hardness and penetration characteristics of the process are examined in the following paragraphs. 3.1.1. Microstructure of single runs The original material in the soft-annealed condition consisted of spheroidal carbides up to 2 mm embedded in a ferritic matrix. A quantity of carbides was isolated by
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Fig. 1. (a) Single plasma pass cross-section. Plasma parameters i ¼ 60 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/m: (1) fully martensitic zone (melting zone and fully austenitized zone); (2) incomplete austenitized zone; (3) unaffected base metal. (b) Schematic diagram of a plasma pass, crosssection, illustrating (h, b, H): (1) fully martensitic zone; (2) incomplete austenitized zone. Value B represents the size of the totally affected material.
chemical dissolution of the ferritic matrix in a solution of hydrochloric and nitric acid in water, in order to be examined by X-ray diffraction (XRD). It was shown that the carbides present are of the M7C3 type (M: Fe, Cr, etc.). After the single pass treatment the microstructure under the impact of the plasma arc is completely modified and it takes the form of a circular segment with its center out of the specimen on the vertical axis of the arc. This is shown in Fig. 1a, which is a characteristic macrograph on a section perpendicular to the central axis of a single run on the y–z plane. Fig. 1b shows the corresponding drawing illustrating the characteristic dimensions of the affected zone: ‘‘H’’ and ‘‘B’’ are, respectively, the total depth and width of the entire zone affected by the heat of the arc and ‘‘h’’ and ‘‘b’’ are the depth and width of a hard—fully martensitic—zone directly under the arc. The dark zone of Fig. 1a enveloping the bright martensitic area corresponds to a variety of solid state transformation products coming from incomplete austenitization and terminating to the base metal. A detailed metallographic analysis allows to see that the hard fully martensitic zone, which is observed as a bright circular segment on the macrograph of Fig. 1a, comes both from: (a) transformations subsequent to the solidification of the melt directly under the arc (melting zone, MZ), and (b) from solid state martensitic transformation of the metal fully austenitized during the heating cycle (fully annealed zone, FAZ). As it is shown in the micrograph of Fig. 2a the melting zone—despite the martensitic transformation subsequent to the solidification—preserves its original dendritic microstructure. The fully austenitized zone begins with a narrow graingrowth zone (GGZ) coming from the overheated metal in the
Fig. 2. (a) Single plasma pass, cross-section (110). Plasma parameters i ¼ 60 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/s: (1) melting zone with dendritic structure; (2) grain-growth area; (3) quenching zone in the solid state condition; (4) incomplete austenitized zone. (b) Single plasma pass, cross-section (230). Plasma parameters i ¼ 60 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/s: (1) dendritic microstructure of the melting zone; (2) grain-growth area.
higher temperature austenitic range and continues with the quenched zone (QZ) coming from the martensitic transformation of the metal austenitized in the lower temperature austenitic range. The micrograph of Fig. 2b allows to see better the details of the transition from the melting to the solid state transformation zones. It is obvious that the complete austenitization of the above zones combined to the high hardenability of the Calmax steel has led to the formation of hard martensitic structures, irrespective of the original condition and form of the austenite grains.
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Fig. 3. Typical microhardness profile along the depth of a plasma pass: (1) fully martensitic zone; (2) incomplete austenitized zone; (3) unaffected base metal.
Fig. 4. Influence of the plasma operating parameters to the geometric dimensions of the affected area. (a) Influence of current (i) to the plasma pass geometry. Plasma torch speed (u): 2.5E3 m/s, plasma gas flow (Q): 1.7E5 m3/s. (b) Influence of the plasma torch speed (u) to the plasma pass geometry. Current (i): 75 A, plasma gas flow (Q): 1.7E5 m3/s. (c) Influence of current (i) to the measured dimension h, for two different PTA speeds. Plasma gas flow: 0.8E5 m3/s. (d) Influence of current (i) to the measured dimension h for two different plasma gas flow rates. Plasma torch speed (u): 2.5E3 m/s.
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The dark zone of Fig. 1a enveloping the martensitic area corresponds to a variety of solid state transformation products coming from incomplete austenitization and dissolution of the original M7C3 carbides as well as to lower cooling rates, too. This zone presents at its upper side high hardness values, which decrease progressively down to the hardness of the base metal (220 HV). Typical results of the microhardness survey are presented in Fig. 3. The graph reveals the hardness variation along the various distinct areas corresponding to the microstructures described above. A major increase in the hardness value can be identified in the layers with a fully martensitic microstructure where the hardness reaches values of around 850 HV. This value is slightly higher than the one of the as quenched Calmax steel (780 HV). However, from a practical point of view the hardness of the superficially treated steel should rather be compared to the value of 535 HV obtained for the same steel in the as quenched and tempered condition, which is used in industrial practice. Thereafter in the incomplete austenitized zone the hardness gradually falls down to its original value of the parent metal (220 HV).
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Fig. 5. Cross-section of a surface scanned with plasma arc (15). Plasma parameters i ¼ 70 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/s: (1) fully martensitic zone (melting zone and fully austenitized zone); (2) incompletely austenitized zone; (3) unaffected base metal.
Fig. 5 shows the macrograph of the heat treated area. The heat treatment zones were very similar to those obtained with a ‘‘single run’’ procedure with the difference that subsequent passes resulted in heat treatment of the previous passes. As a result successive layers of more or less tempered martensitic structures were obtained in the melting zone (Fig. 6a and b). XRD on the surface area of the specimens revealed a martensitic microstructure with very small amounts of retained austenite (Fig. 7).
3.1.2. Geometric characteristics of single runs Fig. 4 shows the influence of the operating parameters utilized for the single runs (current i, travel speed u and plasma gas flow rate Q) on the geometric dimensions (h, b, H) of the affected area under the arc. The size of each zone is directly related to the values of above parameters. It is observed that the size of all areas increases with increasing arc current and decreasing travel speed. This is consistent with the heat input definition. Increasing the plasma gas flow rate increases the dimensions of the affected area, as well. A multiple regression linear analysis allowed to find empirical equations relating the geometrical dimensions with the operational parameters h ¼ 0:476
i2 Q þ 2:800 u þ 98:133
(1)
i2 Q þ 780:506 (2) u þ 471:167 where h is measured in mm, b in mm, i in A, u in mm/s, and Q in lt/min. The fitting corresponds to R2 ¼ 0:87 for the Eq. (1) and 2 R ¼ 0:84 for the Eq. (2) for a level of confidence of 95%. b ¼ 1:674
3.2. Scanned surfaces Two sets of parameters were selected for scanning the whole surface area, as it is presented in Table 3. In both cases a 60% overlapping was selected in order to assure that the surface is most uniformly heat treated. Surface roughness (Ra-values) for the two scanned surfaces were 5 and 7 mm, respectively, thus minimizing or even eliminating the need for further finishing of the heat treated area, for many applications.
Fig. 6. (a) Martensitic structure layer of the scanned surface melting zone (1000). (b) Tempered martensitic structure layer of the melting zone (950).
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Fig. 7. XRD graph of a plasma arc scanned surface.
Martensitic and martensitic/bainitic microstructures were gradually obtained in depth in the fully austenized zone (Fig. 8a and b). A schematic drawing of two typical successive passes in a cross-section perpendicular to the x-axis of a scanned surface is presented in Fig. 9a. The same configuration is periodically repeated along the whole width of the surface consisting of 20 passes. The hardness profile of the heat treated area is presented in Fig. 9b at a depth of 0.4E3 m from the surface. As a result of the tempering of martensite the hardness in the melting zone drops to about 650–750 HV (from the original 850 HV hardness in the case of single runs). However, it remains by about 25% higher than the hardness achieved by conventional quenching and tempering (535 HV), mainly due to the fineness of the microstructure. Various areas with different microhardness can be identified in the scanned surfaces: (1) the unaffected melting zone with an almost untempered martensitic microstructure; (2) the heat treated melting zone from the next plasma pass; (3) the unaffected melting zone of the next plasma pass. The same results are periodically observed through the whole surface. 3.3. Pin on disc tests 3.3.1. Wear rates The two plasma treated surfaces and the conventionally heat treated specimen were tested under the same conditions for comparison. Tables 4 and 5 present, respectively, the
parameters utilized and the results obtained from the pin on disc tests. The wear rates were calculated in accordance with the ASTM standard test method G 99–95. The following equation has been used: Pin volume loss ¼
pðwear scan diameterÞ4 64ðsphere radiusÞ
Disk volume loss ¼
(3)
pðwear track radiusÞðtrack widthÞ 6ðsphere radiusÞ
(4)
A 340–440% increase in the wear resistance of the plasma treated surfaces in respect to the conventionally treated specimen is obtained. Indeed, very low wear rates, respectively, 219E15 m3/m for surface 1 and 170E15 m3/m for surface 2, have been achieved due to the increased hardness Table 4 Parameters utilized for the pin on disc setup Pin material Pin radius Friction distance Disc initial roughness, Ra Friction radius Load Temperature Rotation speed Number of rotations Humidity Sliding speed
Al2O3 3E3 m 380 m 0.04 mm 5E3 m 10 N 20 8C 191 rpm 12000 35% 0.1 m/s
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Fig. 9. Microhardness of plasma scanned surface at a depth of 0.4E3 m from the surface. (a) Schematic drawing of a scanned surface cross-section in x-axis: (1) melting zone; (2) incomplete austenitized zone of the next plasma pass; (3) melting zone of the next pass. (b) Hardness profile of a plasma arc scanned surface (y-axis). Measurements have been made 0.4E3 m below the scanned surface (melting zone): (1) the unaffected melting zone with an almost untempered martensitic microstructure; (2) the heat treated melting zone from the next plasma pass; (3) the unaffected melting zone of the next plasma pass.
Fig. 8. (a) Cross-section in x-axis of a plasma arc melted surface: (1) melting zone with a martensitic structure; (2) melting zone with tempered martensitic structure; (3) fully austenitized zone with martensitic/bainitic microstructure. (b) Martensitic/bainitic microstructure of the fully austenitized zone (1500).
and the fine microstructure obtained with the plasma treatment. For the conventionally heat treated specimen the wear rate is 746.25E15 m3/m. There is also a difference of the wear rates of the counterbodies, which is also better in the case of the plasma treated specimens. 3.3.2. Wear mechanisms Pin on disc tracks of the worn surfaces are presented in Figs. 10–12. The plasma treated surfaces present 2.5 times narrower tracks than the conventionally heat treated
Fig. 10. Pin on disc track of plasma heat treated surface.
Table 5 Wear rates of the plasma arc scanned surfaces and conventionally heat treated surface Pin
Surface 1 Surface 2 Surface 3
Disc
Volume loss (m3)
Volume loss/sliding distance (m3/m)
Volume loss (m3)
Volume loss/sliding distance ( m3/m)
4.19E13 1.33–13 21.21E13
1.10E15 0.35E15 5.58E15
83.37E12 64.65E12 283.57E12
219.19E15 170.14E15 746.25E15
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Fig. 11. Conventionally heat treated surface, pin on disc track.
Fig. 12. Oxide layer formation on the conventionally heat treated surface.
specimens. Furthermore they also present small amounts of oxide blisters, while quenched and tempered specimens have an oxidized blister surface which masks more than 35% of the track.
4. Discussion The above results indicate that plasma heat treatment can provide a reliable tool for the surface treatment of Calmax tool steels producing very fine microstructure, responsible for the high hardness values achieved and the excellent tribological behavior. The original ferritic microstructure with uniformly dispersed spherical carbides (M7C3) has been transformed to a depth of the order of 1.5E3 m to a partially tempered martensitic microstructure containing a small amount of retained austenite. The higher hardness values achieved compared to those obtained with conventional quenching and tempering should be attributed to several different mechanisms, mainly the refinement of the structure during rapid solidification of the melt and the secondary hardening of martensite. Cracks are not present in any of the examined specimens, despite the very high cooling rates achieved. It was observed that a non-uniform hardness profile was obtained perpendicular to the axis of the passes, depending
on the size of each zone, which in turn depends on the operating parameters utilized. As expected, the martensitic melting zone produced as a result of the high solidification rates obtained by heat removal into the material provides very high hardness values. In addition, the use of a water tank for cooling the specimen helps to attain a quasi stationary thermal regime during the scanning of the surface, resulting in even higher cooling rates and avoiding excessive tempering of the martensitic structures already obtained. The quenching in the solid state condition in the austenitized areas results also in higher hardness values as a consequence of grain refinement in these areas. The latter may also be attributed to the fact that small amounts of retained austenite detected in the melted area are absent in these zones. The subsequent gradual reduction of the hardness within the heat affected (partially annealed) zone is attributed to the phases present in these areas including all those phases in between the martensitic structure of the molten metal and the ferrite with a dispersion of spherical carbides in the parent metal. However, the macrohardness results obtained in the multiple pass treatments reveal lower hardness values at the surface of the examined specimen, as a result of the cyclic heat treatment of the previously melted zones during successive runs. This treatment effectively refines the grain and leads to the tempering of the martensite. Furthermore, subsequent runs also tend to anneal out residual stresses caused by previous runs which also result in lower hardness values, but eliminate the risk of cracks. Effectively, the multiple track procedure can be considered as a multiple stage process consisting of alternate quenching and subsequent tempering stages. The larger the number of runs the greater is the volume fraction of reheated weld metal that is produced which also gives lower hardness values. Nevertheless, it must be mentioned that higher than expected hardness values observed in small regions of the multiple run surface treatment are attributed to secondary hardening effects of the steel. The use of the water tank is restricting the influence of the previous runs which provide a certain preheat and which also tend to extend cooling time, thus making the size of the produced zones more predictable. As already mentioned above, the size of each zone is controlled to a large extent by the operating parameters utilized along with the overlapping which in turn depends on the y-axis movement of the plasma torch. In general, the size of each zone is proportional to the arc current, the plasma gas flow rate and inversely proportional to the PTA speed for the parameter range utilized. Furthermore, the high energy concentration produced with plasma along with the high arc pressures resulted in important melted zone depths. The depth of the melted zone may reach 1.5E3 m. However, it must be emphasized that hardness values achieved depend primarily on the metal structure obtained and also that parameter variation in order to control plasma macromelting is restricted by the stability of the plasma arc. The latter is particularly affected by plasma gas flow rate and PTA current variations. Therefore, parameters are selected with
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the view of ensuring both, sufficient thermal power and thermal stability. Surface roughness measurement indicated that from a practical standpoint, plasma surface treatment induces roughness which can easily be removed by additional machining (it might even be permissible for the majority of tool types). However, from a practical standpoint plasma melting under the experimental conditions used does not induce a large surface roughness, considering the depth of the melted zone achieved. Lower wear rates obtained with the plasma surface treatment is one more indication of the higher hardness phases obtained with the treatment. The wear resistance of the plasma processed material is superior to that of the conventionally heat treated (quenched and tempered) material. High hardnesses and refined microstructures achieved are considered to be responsible for the enhanced wear resistance of the plasma processed material. Furthermore, subsequent metallographic examination did not reveal any brittle fracture or chipping, indicating a positive cracking resistance of the hardened surfaces.
5. Conclusions On the basis of the results of this investigation the following conclusions can be drawn. Deep hardened zones can be produced with the plasma surface treatment. Their geometric characteristics are accurately determined through selection of PTA parameters. Non-uniformities in local microhardness were observed associated to the local microstructure. Plasma transformation hardening improves the wear resistance for the Calmax tool steel. A 340–440% increase in the wear resistance of the plasma treated surfaces in respect to the conventionally treated specimen is obtained. Multiple run procedures result in local decrease of hardness values achieved due to the tempering effect produced by each plasma pass on the martensite of the earlier formed melt and austenitization zones, nevertheless the surfaces treated always present better hardness characteristics than the ones obtained by conventional quenching and tempering. Crystallization cracks were not detected, although high cooling rates were used.
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A study on the surface treatment of ‘‘Calmax’’ tool steel by a plasma transferred arc (PTA) process E. Bourithis, A. Tazedakis, G. Papadimitriou* Laboratory of Physical Metallurgy, National Technical University of Athens, 9 Heroon Polytechniou Street, 15780 Athens, Greece Received 29 June 2001; received in revised form 13 February 2002; accepted 26 June 2002
Abstract A tool steel (Calmax of Uddeholm) was subjected to surface melting using the plasma transferred arc (PTA) process. Single and multiple run melting was used under different working conditions and the geometric characteristics of melted and heat affected zones were studied as a function of working parameters, current, travel speed and gas flow rate. The parameters were adjusted in order to obtain hard layers with sufficient thickness, presenting at the same time low roughness and crack free surfaces. The same steel was subjected to a conventional heat treatment (quenching and tempering) for comparison. The PTA surface treated specimens showed fine martensitic microstructures in depth going up to 1.5E3 m. Their hardness was higher than the one of the conventionally heat treated steel. Pin on disk wear testing showed that PTA treated specimens have much better wear resistance, attributed mainly to the fine microstructure. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PTA; Tool steel; Surfacing
1. Introduction Surface treatments of metals are commonly based on the use of high energy density sources, as they offer a means of rapid heating and subsequent quenching from the melt, leading to fine microstructures and consequently to possible improvement of mechanical, corrosion or tribological properties. Superficial layers of the appropriate thickness, free of cracks and with high hardness may be obtained by suitable control of the process variables. In this respect there has been considerable interest in the use of laser and electron beam sources for surface treating and melting of low carbon steels [1,2] and stainless and tool steels [3–5]. The impact of the process variables on temperature profiles [6–10], microstructure [11–16] and properties [17–21] have been examined and the results already obtained may serve as a reference for further investigations. However, only a limited number of investigations concerns the use of the plasma transferred arc (PTA) [22,23], although there are serious indications that its use may be quite attractive in industrial applications. It is, therefore, interesting to investigate the PTA process, which—despite its lower energy density—has
* Corresponding author. Tel.: þ30-10-772-2184; fax: þ30-10-772-2119. E-mail address: [email protected] (G. Papadimitriou).
the main advantage to require rather inexpensive equipment and the possibility to work with a higher heat input. In this context the present work was undertaken in order to study the possibility of superficial hardening of an air hardened tool steel (Calmax of Uddeholm) using the PTA process. This steel is widely utilized for moulds and dies in the forming industry of plastics and light alloys. Experiments were performed on steel plates by rapidly heating to peak temperatures over the melting point of steel, using both single and multiple track moving plasma beams. The geometrical characteristics (form and dimensions) of the heat affected layer and the corresponding microstructure were investigated in connection with the main process variables: heat input, scan speed and plasma gas flow. In order to assess the wear resistance, pin on disc tests have been done on plasma treated specimens and the results obtained were compared with those obtained with a conventionally heat treated (quenched and tempered) Calmax tool steel.
2. Procedures–experimental techniques The material under investigation is the commercial ‘‘Calmax’’ tool steel of Uddeholm-Sweden with chemical composition is given in Table 1. The steel is used in the soft-annealed condition, having a hardness of 220 HV.
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Table 1 Calmax steel chemical composition C Si Mn Cr Mo V
Table 3 Parameters utilized for scanning the surfaces with plasma arc 0.60% 0.35% 0.80% 4.50% 0.50% 0.20%
Specimens in the form of plates of 43E3 m 60E3 m area and 28E3 m thickness were treated superficially by the PTA process. Before their treatment the specimens were polished with abrasive paper down to 1200 grit fineness in order to remove oxides and obtain a smooth surface. The roughness was brought down to the level of Ra ¼ 0:04 mm, the same for all specimens, in order to keep stable arc conditions and avoid current fluctuations. The main equipment employed was a PW-200 Sabre Arc plasma console capable of controlling the plasma and shielding gas flow rates, a plasma torch (PWH-3A) for low/medium current applications, a KEMPI 2000 power source and an automatic machine for control of the torch movement in the horizontal level (x–y-axis). The fixed operating parameters of the PTA system are given in Table 2. In a first step of the investigation single runs have been performed along the central axis of a number of plates using different arc conditions. Their selection was made so that a qualitative approach on the influence of the three basic operating parameters, i.e. current (i), travel speed (u) and plasma gas flow rates (Q), could be achieved. All runs were visually inspected and Vickers hardness tests were performed, running vertically from the surface through the melted and the HAZ down to the bulk metal. Metallographic examination was carried out using both optical and scanning electron microscopy (SEM). In a second step based on the optimum conditions identified during the above experiments the whole surface area of specimens was scanned by running successive passes in the x-direction with a fixed overlapping in the y (perpendicular)direction. A 60% overlapping of the melting zone was used, in order to avoid important fluctuations of the hardened depth. For avoiding superheating the specimens were positioned on a copper plate and the latter placed inside a tank Table 2 Fixed values of plasma operating parameters Plasma gas Shielding gas Shielding gas flow Electrode Electrode diameter Electrode angle Electrode set back Stand off distance Tip diameter
Ar 99.9% Ar 99.9% 1.6E4 m3/s W-2% ThO2 2.4E3 m 458 1.8E3 m 2.5E3 m 2.35E3 m
i (A)
u (m/s)
Q (m3/s)
Surface 1 75
3.0E3
1.7E5
Surface 2 80
2.5E3
1.7E5
filled with water for cooling. Two series of operating parameters were chosen (Table 3). The above surfaces were subsequently examined using the same with the above experimental procedure. For comparison specimens of the Calmax steel were also conventionally heat treated by quenching and tempering, using conditions suggested by the steel producer and similar to those usually practiced in industry, i.e. austenitization for 30 min at 960 8C, followed by quenching in oil and double tempering at 500 and 470 8C for 2 h at each temperature. The results obtained were used as a basis for the assessment of the results achieved with the plasma treatment, namely the microstructural characteristics of the superficial layer, its uniformity, hardness and tribological behavior.
3. Results 3.1. ‘‘Single runs’’ The operating parameters utilized for the ‘‘single runs’’ were investigated within the following limits: the arc current was varied between 40 and 80 A and the travel speed between 2.5E3 and 4.0E3 m/s. The plasma gas flow rate was kept constant at two different levels, either 0.8E5 or 1.7E5 m3/s. Under these conditions the PTA treated metal of the single runs, observed both macroscopically and microscopically was free from cracks. The obtention of very hard layers with a thickness of the order of 1.5E3 m and hardness of the order of 800–900 HV was easily achieved using the upper current range and the 1.75E5 m3/s gas flow rate. The 0.8E5 m3/s gas flow rate gave also satisfactory layers having a similar hardness but a thickness lower by about 10–20%. It should be emphasized that the conventionally heat treated steel presents a hardness of 780 HV (about 63HRC) in the as quenched condition and 535 HV (about 51.5HRC) in the tempered condition. The microstructure, hardness and penetration characteristics of the process are examined in the following paragraphs. 3.1.1. Microstructure of single runs The original material in the soft-annealed condition consisted of spheroidal carbides up to 2 mm embedded in a ferritic matrix. A quantity of carbides was isolated by
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Fig. 1. (a) Single plasma pass cross-section. Plasma parameters i ¼ 60 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/m: (1) fully martensitic zone (melting zone and fully austenitized zone); (2) incomplete austenitized zone; (3) unaffected base metal. (b) Schematic diagram of a plasma pass, crosssection, illustrating (h, b, H): (1) fully martensitic zone; (2) incomplete austenitized zone. Value B represents the size of the totally affected material.
chemical dissolution of the ferritic matrix in a solution of hydrochloric and nitric acid in water, in order to be examined by X-ray diffraction (XRD). It was shown that the carbides present are of the M7C3 type (M: Fe, Cr, etc.). After the single pass treatment the microstructure under the impact of the plasma arc is completely modified and it takes the form of a circular segment with its center out of the specimen on the vertical axis of the arc. This is shown in Fig. 1a, which is a characteristic macrograph on a section perpendicular to the central axis of a single run on the y–z plane. Fig. 1b shows the corresponding drawing illustrating the characteristic dimensions of the affected zone: ‘‘H’’ and ‘‘B’’ are, respectively, the total depth and width of the entire zone affected by the heat of the arc and ‘‘h’’ and ‘‘b’’ are the depth and width of a hard—fully martensitic—zone directly under the arc. The dark zone of Fig. 1a enveloping the bright martensitic area corresponds to a variety of solid state transformation products coming from incomplete austenitization and terminating to the base metal. A detailed metallographic analysis allows to see that the hard fully martensitic zone, which is observed as a bright circular segment on the macrograph of Fig. 1a, comes both from: (a) transformations subsequent to the solidification of the melt directly under the arc (melting zone, MZ), and (b) from solid state martensitic transformation of the metal fully austenitized during the heating cycle (fully annealed zone, FAZ). As it is shown in the micrograph of Fig. 2a the melting zone—despite the martensitic transformation subsequent to the solidification—preserves its original dendritic microstructure. The fully austenitized zone begins with a narrow graingrowth zone (GGZ) coming from the overheated metal in the
Fig. 2. (a) Single plasma pass, cross-section (110). Plasma parameters i ¼ 60 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/s: (1) melting zone with dendritic structure; (2) grain-growth area; (3) quenching zone in the solid state condition; (4) incomplete austenitized zone. (b) Single plasma pass, cross-section (230). Plasma parameters i ¼ 60 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/s: (1) dendritic microstructure of the melting zone; (2) grain-growth area.
higher temperature austenitic range and continues with the quenched zone (QZ) coming from the martensitic transformation of the metal austenitized in the lower temperature austenitic range. The micrograph of Fig. 2b allows to see better the details of the transition from the melting to the solid state transformation zones. It is obvious that the complete austenitization of the above zones combined to the high hardenability of the Calmax steel has led to the formation of hard martensitic structures, irrespective of the original condition and form of the austenite grains.
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Fig. 3. Typical microhardness profile along the depth of a plasma pass: (1) fully martensitic zone; (2) incomplete austenitized zone; (3) unaffected base metal.
Fig. 4. Influence of the plasma operating parameters to the geometric dimensions of the affected area. (a) Influence of current (i) to the plasma pass geometry. Plasma torch speed (u): 2.5E3 m/s, plasma gas flow (Q): 1.7E5 m3/s. (b) Influence of the plasma torch speed (u) to the plasma pass geometry. Current (i): 75 A, plasma gas flow (Q): 1.7E5 m3/s. (c) Influence of current (i) to the measured dimension h, for two different PTA speeds. Plasma gas flow: 0.8E5 m3/s. (d) Influence of current (i) to the measured dimension h for two different plasma gas flow rates. Plasma torch speed (u): 2.5E3 m/s.
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The dark zone of Fig. 1a enveloping the martensitic area corresponds to a variety of solid state transformation products coming from incomplete austenitization and dissolution of the original M7C3 carbides as well as to lower cooling rates, too. This zone presents at its upper side high hardness values, which decrease progressively down to the hardness of the base metal (220 HV). Typical results of the microhardness survey are presented in Fig. 3. The graph reveals the hardness variation along the various distinct areas corresponding to the microstructures described above. A major increase in the hardness value can be identified in the layers with a fully martensitic microstructure where the hardness reaches values of around 850 HV. This value is slightly higher than the one of the as quenched Calmax steel (780 HV). However, from a practical point of view the hardness of the superficially treated steel should rather be compared to the value of 535 HV obtained for the same steel in the as quenched and tempered condition, which is used in industrial practice. Thereafter in the incomplete austenitized zone the hardness gradually falls down to its original value of the parent metal (220 HV).
173
Fig. 5. Cross-section of a surface scanned with plasma arc (15). Plasma parameters i ¼ 70 A, u ¼ 2:7E3 m/s, Q ¼ 1:7E5 m3/s: (1) fully martensitic zone (melting zone and fully austenitized zone); (2) incompletely austenitized zone; (3) unaffected base metal.
Fig. 5 shows the macrograph of the heat treated area. The heat treatment zones were very similar to those obtained with a ‘‘single run’’ procedure with the difference that subsequent passes resulted in heat treatment of the previous passes. As a result successive layers of more or less tempered martensitic structures were obtained in the melting zone (Fig. 6a and b). XRD on the surface area of the specimens revealed a martensitic microstructure with very small amounts of retained austenite (Fig. 7).
3.1.2. Geometric characteristics of single runs Fig. 4 shows the influence of the operating parameters utilized for the single runs (current i, travel speed u and plasma gas flow rate Q) on the geometric dimensions (h, b, H) of the affected area under the arc. The size of each zone is directly related to the values of above parameters. It is observed that the size of all areas increases with increasing arc current and decreasing travel speed. This is consistent with the heat input definition. Increasing the plasma gas flow rate increases the dimensions of the affected area, as well. A multiple regression linear analysis allowed to find empirical equations relating the geometrical dimensions with the operational parameters h ¼ 0:476
i2 Q þ 2:800 u þ 98:133
(1)
i2 Q þ 780:506 (2) u þ 471:167 where h is measured in mm, b in mm, i in A, u in mm/s, and Q in lt/min. The fitting corresponds to R2 ¼ 0:87 for the Eq. (1) and 2 R ¼ 0:84 for the Eq. (2) for a level of confidence of 95%. b ¼ 1:674
3.2. Scanned surfaces Two sets of parameters were selected for scanning the whole surface area, as it is presented in Table 3. In both cases a 60% overlapping was selected in order to assure that the surface is most uniformly heat treated. Surface roughness (Ra-values) for the two scanned surfaces were 5 and 7 mm, respectively, thus minimizing or even eliminating the need for further finishing of the heat treated area, for many applications.
Fig. 6. (a) Martensitic structure layer of the scanned surface melting zone (1000). (b) Tempered martensitic structure layer of the melting zone (950).
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Fig. 7. XRD graph of a plasma arc scanned surface.
Martensitic and martensitic/bainitic microstructures were gradually obtained in depth in the fully austenized zone (Fig. 8a and b). A schematic drawing of two typical successive passes in a cross-section perpendicular to the x-axis of a scanned surface is presented in Fig. 9a. The same configuration is periodically repeated along the whole width of the surface consisting of 20 passes. The hardness profile of the heat treated area is presented in Fig. 9b at a depth of 0.4E3 m from the surface. As a result of the tempering of martensite the hardness in the melting zone drops to about 650–750 HV (from the original 850 HV hardness in the case of single runs). However, it remains by about 25% higher than the hardness achieved by conventional quenching and tempering (535 HV), mainly due to the fineness of the microstructure. Various areas with different microhardness can be identified in the scanned surfaces: (1) the unaffected melting zone with an almost untempered martensitic microstructure; (2) the heat treated melting zone from the next plasma pass; (3) the unaffected melting zone of the next plasma pass. The same results are periodically observed through the whole surface. 3.3. Pin on disc tests 3.3.1. Wear rates The two plasma treated surfaces and the conventionally heat treated specimen were tested under the same conditions for comparison. Tables 4 and 5 present, respectively, the
parameters utilized and the results obtained from the pin on disc tests. The wear rates were calculated in accordance with the ASTM standard test method G 99–95. The following equation has been used: Pin volume loss ¼
pðwear scan diameterÞ4 64ðsphere radiusÞ
Disk volume loss ¼
(3)
pðwear track radiusÞðtrack widthÞ 6ðsphere radiusÞ
(4)
A 340–440% increase in the wear resistance of the plasma treated surfaces in respect to the conventionally treated specimen is obtained. Indeed, very low wear rates, respectively, 219E15 m3/m for surface 1 and 170E15 m3/m for surface 2, have been achieved due to the increased hardness Table 4 Parameters utilized for the pin on disc setup Pin material Pin radius Friction distance Disc initial roughness, Ra Friction radius Load Temperature Rotation speed Number of rotations Humidity Sliding speed
Al2O3 3E3 m 380 m 0.04 mm 5E3 m 10 N 20 8C 191 rpm 12000 35% 0.1 m/s
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Fig. 9. Microhardness of plasma scanned surface at a depth of 0.4E3 m from the surface. (a) Schematic drawing of a scanned surface cross-section in x-axis: (1) melting zone; (2) incomplete austenitized zone of the next plasma pass; (3) melting zone of the next pass. (b) Hardness profile of a plasma arc scanned surface (y-axis). Measurements have been made 0.4E3 m below the scanned surface (melting zone): (1) the unaffected melting zone with an almost untempered martensitic microstructure; (2) the heat treated melting zone from the next plasma pass; (3) the unaffected melting zone of the next plasma pass.
Fig. 8. (a) Cross-section in x-axis of a plasma arc melted surface: (1) melting zone with a martensitic structure; (2) melting zone with tempered martensitic structure; (3) fully austenitized zone with martensitic/bainitic microstructure. (b) Martensitic/bainitic microstructure of the fully austenitized zone (1500).
and the fine microstructure obtained with the plasma treatment. For the conventionally heat treated specimen the wear rate is 746.25E15 m3/m. There is also a difference of the wear rates of the counterbodies, which is also better in the case of the plasma treated specimens. 3.3.2. Wear mechanisms Pin on disc tracks of the worn surfaces are presented in Figs. 10–12. The plasma treated surfaces present 2.5 times narrower tracks than the conventionally heat treated
Fig. 10. Pin on disc track of plasma heat treated surface.
Table 5 Wear rates of the plasma arc scanned surfaces and conventionally heat treated surface Pin
Surface 1 Surface 2 Surface 3
Disc
Volume loss (m3)
Volume loss/sliding distance (m3/m)
Volume loss (m3)
Volume loss/sliding distance ( m3/m)
4.19E13 1.33–13 21.21E13
1.10E15 0.35E15 5.58E15
83.37E12 64.65E12 283.57E12
219.19E15 170.14E15 746.25E15
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Fig. 11. Conventionally heat treated surface, pin on disc track.
Fig. 12. Oxide layer formation on the conventionally heat treated surface.
specimens. Furthermore they also present small amounts of oxide blisters, while quenched and tempered specimens have an oxidized blister surface which masks more than 35% of the track.
4. Discussion The above results indicate that plasma heat treatment can provide a reliable tool for the surface treatment of Calmax tool steels producing very fine microstructure, responsible for the high hardness values achieved and the excellent tribological behavior. The original ferritic microstructure with uniformly dispersed spherical carbides (M7C3) has been transformed to a depth of the order of 1.5E3 m to a partially tempered martensitic microstructure containing a small amount of retained austenite. The higher hardness values achieved compared to those obtained with conventional quenching and tempering should be attributed to several different mechanisms, mainly the refinement of the structure during rapid solidification of the melt and the secondary hardening of martensite. Cracks are not present in any of the examined specimens, despite the very high cooling rates achieved. It was observed that a non-uniform hardness profile was obtained perpendicular to the axis of the passes, depending
on the size of each zone, which in turn depends on the operating parameters utilized. As expected, the martensitic melting zone produced as a result of the high solidification rates obtained by heat removal into the material provides very high hardness values. In addition, the use of a water tank for cooling the specimen helps to attain a quasi stationary thermal regime during the scanning of the surface, resulting in even higher cooling rates and avoiding excessive tempering of the martensitic structures already obtained. The quenching in the solid state condition in the austenitized areas results also in higher hardness values as a consequence of grain refinement in these areas. The latter may also be attributed to the fact that small amounts of retained austenite detected in the melted area are absent in these zones. The subsequent gradual reduction of the hardness within the heat affected (partially annealed) zone is attributed to the phases present in these areas including all those phases in between the martensitic structure of the molten metal and the ferrite with a dispersion of spherical carbides in the parent metal. However, the macrohardness results obtained in the multiple pass treatments reveal lower hardness values at the surface of the examined specimen, as a result of the cyclic heat treatment of the previously melted zones during successive runs. This treatment effectively refines the grain and leads to the tempering of the martensite. Furthermore, subsequent runs also tend to anneal out residual stresses caused by previous runs which also result in lower hardness values, but eliminate the risk of cracks. Effectively, the multiple track procedure can be considered as a multiple stage process consisting of alternate quenching and subsequent tempering stages. The larger the number of runs the greater is the volume fraction of reheated weld metal that is produced which also gives lower hardness values. Nevertheless, it must be mentioned that higher than expected hardness values observed in small regions of the multiple run surface treatment are attributed to secondary hardening effects of the steel. The use of the water tank is restricting the influence of the previous runs which provide a certain preheat and which also tend to extend cooling time, thus making the size of the produced zones more predictable. As already mentioned above, the size of each zone is controlled to a large extent by the operating parameters utilized along with the overlapping which in turn depends on the y-axis movement of the plasma torch. In general, the size of each zone is proportional to the arc current, the plasma gas flow rate and inversely proportional to the PTA speed for the parameter range utilized. Furthermore, the high energy concentration produced with plasma along with the high arc pressures resulted in important melted zone depths. The depth of the melted zone may reach 1.5E3 m. However, it must be emphasized that hardness values achieved depend primarily on the metal structure obtained and also that parameter variation in order to control plasma macromelting is restricted by the stability of the plasma arc. The latter is particularly affected by plasma gas flow rate and PTA current variations. Therefore, parameters are selected with
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the view of ensuring both, sufficient thermal power and thermal stability. Surface roughness measurement indicated that from a practical standpoint, plasma surface treatment induces roughness which can easily be removed by additional machining (it might even be permissible for the majority of tool types). However, from a practical standpoint plasma melting under the experimental conditions used does not induce a large surface roughness, considering the depth of the melted zone achieved. Lower wear rates obtained with the plasma surface treatment is one more indication of the higher hardness phases obtained with the treatment. The wear resistance of the plasma processed material is superior to that of the conventionally heat treated (quenched and tempered) material. High hardnesses and refined microstructures achieved are considered to be responsible for the enhanced wear resistance of the plasma processed material. Furthermore, subsequent metallographic examination did not reveal any brittle fracture or chipping, indicating a positive cracking resistance of the hardened surfaces.
5. Conclusions On the basis of the results of this investigation the following conclusions can be drawn. Deep hardened zones can be produced with the plasma surface treatment. Their geometric characteristics are accurately determined through selection of PTA parameters. Non-uniformities in local microhardness were observed associated to the local microstructure. Plasma transformation hardening improves the wear resistance for the Calmax tool steel. A 340–440% increase in the wear resistance of the plasma treated surfaces in respect to the conventionally treated specimen is obtained. Multiple run procedures result in local decrease of hardness values achieved due to the tempering effect produced by each plasma pass on the martensite of the earlier formed melt and austenitization zones, nevertheless the surfaces treated always present better hardness characteristics than the ones obtained by conventional quenching and tempering. Crystallization cracks were not detected, although high cooling rates were used.
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