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Microstructure and hardness of spray-formed chromium-containing steel tooling
Scripta Materialia 55 (2006) 581–584 www.actamat-journals.com
Microstructure and hardness of spray-formed chromium-containing steel tooling Yaojun Lin,a,b,* Kevin M. McHugh,c Yizhang Zhoua and Enrique J. Laverniaa a
Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA b Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA c Industrial Technology Department, Idaho National Laboratory, Idaho Falls, ID 83415, USA Received 20 March 2006; revised 28 May 2006; accepted 23 June 2006 Available online 21 July 2006
The microstructure and hardness of a spray-formed chromium-containing tool steel H13 were studied. There is very little porosity in both as-spray-formed samples and spray-formed samples following heat treatment. The microstructure in the as-sprayformed material is characterized by proeutectoid carbides, lower bainite, martensite, and retained austenite. Spray-formed H13 aged at a given temperature exhibits higher hardness than commercial H13 conventionally heat treated and tempered at the same temperature. The microstructures and hardness are rationalized on the basis of numerical analysis. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Spray-formed tooling; H13 tool steel; Process development
Spray forming has been the topic of numerous studies published over the past few decades [1]. Recently, this technology has been successfully implemented for the fabrication of molds, dies and similar tooling [2,3]. During this approach, atomized droplets of a tool-forming alloy are deposited onto a tool pattern where they accurately capture features of the pattern during solidification to form the desired mold or die. By so doing, many of the machining, grinding, and polishing steps in conventional mold and die fabrication can be eliminated, thereby reducing their cost and lead time. Spray forming can also simplify the heat treatment practices that are required to tailor the properties of tooling due to the formation of a supersaturated solid solution and macrosegregation-free microstructure [3,4]. Rapid solidification during spray forming can suppress carbide precipitation and growth, allowing tool steels to be artificially aged directly from the as-spray-formed state to obtain the final mold/die products. In contrast, conventional annealing/austenitization/quench/temper heat treatments are required steps with conventionally processed die material. Spray-formed tooling has been shown to save energy, production time and costs, and avoid mold/die distortion if aging does not involve * Corresponding author. Address: Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA. E-mail: [email protected]
phase transformation of the matrix phase. This paper reports on an investigation of the microstructures (e.g., porosity and phases) and hardness of spray-formed H13 tooling. The microstructure and hardness obtained in experiments were rationalized on the basis of numerical simulation results. The rapid solidification process (RSP) tooling approach [5] was used to generate tooling samples. A schematic of the approach is shown in Figure 1. An alumina-base ceramic was slurry cast into a tool pattern using a silicone rubber mold. After setting up, ceramic patterns were demolded, fired in a kiln, and cooled to room temperature. AISI H13 tool steel (Fe–0.40C– 5.00Cr–1.10V–1.30Mo (wt.%)) with the mass of the feedstock of 9 kg was induction melted under a nitrogen atmosphere, superheated about 100 C, and atomized with nitrogen using a bench-scale atomizer operating at a pressure of 0.14 MPa, a gas flow rate of 5.8 g/s, and a metal discharge rate of 45.4 g/s. The atomizer was stationary and the spray was projected horizontally. The exit geometry of the atomizer is circular, leading to a conical spray pattern. An inert gas atmosphere within the spray apparatus minimized in-flight oxidation of the atomized droplets as they deposited onto the tool pattern (substrate) located 254 mm from the atomizer for an average deposit growth rate of 0.24 mm/s. The tool pattern, 100 mm wide, 100 mm long, and 50 mm thick, was preheated to a desired temperature prior to
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.06.020
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Y. Lin et al. / Scripta Materialia 55 (2006) 581–584
Figure 1. Schematic of approach used to spray form H13 tooling.
droplet deposition. It was manipulated in the spray manually. The operator visually monitored the exposed surface of the spray during deposition and attempted to maintain a uniform temperature profile. The thickness of the final deposit was 50 mm. Samples investigated in the present study were sectioned using a wire electron discharge machine (EDM) from three locations of the spray-formed deposits: near the deposit/substrate interface, central region and near the exposed surface. Some samples were artificially aged under various conditions (Table 2) by placing sections in a preheated furnace that was purged with nitrogen. Each sample was coated with boron nitride and placed in a sealed metal foil packet as a precautionary measure to prevent decarburization. Artificially aged samples were soaked at 500 C for 2 h and air cooled. Others were soaked at 630 C for 2 h, air cooled to room temperature, heated again at 630 C for 2 h, and air cooled. Spray-formed H13 that was conventionally heat treated was austenitized at 1052 C for 30 min, air quenched, and double tempered (2 h plus 2 h) at 593 C. Analysis was performed on the as-deposited samples and the aged samples. Optical microscopy (OM) in conjunction with AnalysisTM software was used to determine porosity on the sample surfaces that were polished and not etched. OM, scanning electron microscopy (SEM, Philips XL30), and transmission electron microscopy (TEM, Philips CM20) was used to examine microstructure. The samples for OM and SEM analysis were prepared using standard metallographic techniques. The etchant used was a solution consisting of 10% nitric acid and 90% ethanol. The preparation of TEM samples is described as follows. The samples were mechanically thinned to 40–50 lm in thickness and then were twin jet polished using a solution consisting of 62 ml perchloric acid (70%), 700 ml ethanol, 100 ml butanol, and 137 ml distilled water. A Series 500 Wilson Rockwell C Hardness Tester was used to carry out hardness testing by averaging eight indentations. The processing of tooling via spray forming can be divided into two distinct but closely related stages: droplet flight and droplet deposition. During flight, the thermal energy of the atomized droplets is extracted via convection heat transfer between the droplets and the atomization gas and via radiation heat transfer. The temperature gradient inside a droplet can safely be neglected as the Biot number is below 0.1 for the typical droplet size during gas atomization [6]. The temperature and the solid fraction of individual droplets during flight can be calculated using an energy conservation equation [7]. At the deposition distance, assuming a droplet-size distribution predicted using the Lubanska correlation
[8], the average temperature and solid fraction of incoming droplets at impact can be evaluated using the enthalpy method [9], and used as input data for the calculation of the cooling process in the deposition stage. During deposition, heat conduction can be assumed to be 1-D along the thickness of the spray-formed material since the thickness of molds/dies is usually much smaller than their width/length. The buildup of the deposit occurs via discrete deposition of individual droplets [10], i.e., there exists a time interval between two groups of droplets that successively arrive at the previously deposited material’s surface. Accordingly, a 1-D Fourier’s equation is used to deal with heat conduction within the deposit during a time interval. At the end of this time interval, a new group of droplets is incorporated into the previously deposited material to generate new deposited material. Fourier’s equation is then implemented to compute heat conduction within the new deposit. In the present study, the alternating-direction explicit (ADE) method is used to discretize Fourier’s equation as described in Ref. [10]. The assumptions of 1-D heat transfer along the thickness and use of the ADE method are also applicable to the pattern (substrate) due to the much smaller thickness than the width and length. These assumptions are used to calculate the pattern’s temperature and to calculate cooling within the deposit. The objective of the numerical analysis described herein is to identify relative trends of temperature as a function of time. The simulation results, based on the above relatively simple droplet and deposition models help understand the thermal history and the cooling and solidification behavior of the deposited materials. In the calculations that follow, the relevant thermal and physical properties of H13 tool steel and alumina tool pattern are summarized in Table 1. The values of thermal conductivity, density and specific heat and those of latent heat of fusion and equilibrium distribution coefficient correspond to room temperature and the melting temperature range of H13 (1315– 1454 C), respectively. Since the properties of H13 and alumina change insignificantly with temperature, it is assumed that the values of these properties remain unchanged with temperature. This assumption may satisfy the objective of numerical analysis herein to investigate the temperature dependency on time rather than absolute values of temperature. Table 1. Thermal and physical properties of H13 tool steel and ceramic pattern Material H13
Ceramic pattern a b
Property Thermal conductivity Density Specific heat Latent heat of fusion Liquidus temperature Solidus temperature Equilibrium distribution coefficient Thermal conductivity Density Specific heat
Value
Reference a
28 W/mK 7000 kg/m3a 447 J/kg Ka 2.8 · 105J/kgb 1454 C 1315 C 0.35b
[11]
1 W/mKa 1700 kg/m3a 900 J/kg Ka
[11]
[12]
At room temperature. At the melting temperature range of H13 (1315–1454 C).
Y. Lin et al. / Scripta Materialia 55 (2006) 581–584
Table 2 shows the measured porosity of five sprayformed H13 samples sectioned from different locations in the deposit and heat treated under different conditions. In all of the samples, porosity is less than 1%. In Figure 2a, the OM image of as-spray-formed H13 (sample no. 4) exhibits dark acicular features separated by the light-colored regions. In the SEM image of this sample (Fig. 2b), proeutectoid carbides can be observed (white particles indicated by arrows). The SEM image in Figure 2b also provides the details of the dark acicular features (labeled ‘‘A’’) and the light-colored regions (labeled ‘‘B’’) that are lower bainite and martensite respectively. The above microstructures are confirmed by TEM analysis. The TEM bright field image in Figure 3 exhibits typical lower bainite morphology: bainitic ferrite (labeled ‘‘A’’), and (Fe, M)3C carbides (where M represents Cr, V, and Mo substituting for Fe) between bainitic ferrite and inside bainitic ferrite. Figure 4a is the TEM bright field image of martensite and Figure 4b is the corresponding diffraction pattern. Figure 5 shows TEM morphology of retained austenite. Tables 3 and 4 compare the hardness of spray-formed and aged H13 with commercial forged H13 that was conventionally heat treated. Conventional heat treatment was conducted by austenitizing at 1015 C, air quenching, and tempering at the temperatures shown in Table 4. For a given aging (or tempering) tempera-
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Figure 4. TEM micrograph of martensite: (a) bright field image and (b) diffraction pattern.
Figure 5. TEM micrographs showing retained austenite: (a) bright field image and diffraction pattern (inset) and (b) dark field image from the diffraction spot 111.
Table 3. Hardness of spray-formed and aged H13 tool steel Table 2. Porosity in various spray-formed H13 steel samples Sample location
Sample no.
Heat treatment
Porosity (vol.%)
Deposit/substrate interface
1 2 3 4 5
As-deposited 500 C · 2 h 630 C · (2 + 2) h As-deposited 1052 C (air quenching) and 593 C · (2 + 2) h
0.10 0.12 0.71 0.05 0.55
Exposed surface Middle
Figure 2. Microstructure of as-deposited H13 (sample no. 4): (a) OM image and (b) SEM image.
Figure 3. TEM micrograph of lower bainite: bright field image and diffraction pattern of (Fe, M)3C (inset).
Sample location
Sample no.
Aging treatment
Hardness (HRC)
Deposit/substrate interface
1 2 3 4
As-deposited 500 C · 2 h 630 C · (2 + 2) h As-deposited
55.2 56.6 40.3 54.3
Exposed surface
Table 4. Hardness of conventionally processed H13 tool steel [13] Tempering temperature
Hardness (HRC)
As-quenched 500 C 630 C
50.0 53.3 39.6
ture, the hardness value of spray-formed H13 was found to be higher than that of conventional H13. Figure 6 shows the calculated cooling curve of an H13 tool steel deposit. Temperature is plotted as a function of time for three regions of the deposit: the deposit/ substrate interface, half thickness, and the exposed surface of the deposited material. Results indicate that during spray forming, liquid phase is present throughout the entire thickness of the deposited material. By assuming equilibrium conditions for the simulation in Figure 6, an estimation of liquid fraction is obtained. In Figure 7, it is shown that throughout the entire thickness of the deposited material, the maximum liquid fraction is above 20%, which is required for complete removal of porosity [14]. The liquid fraction exists for the time interval from 30–480 s. These results support the observed very low porosity values in Table 2. In order to predict the phases in as-spray-formed H13, the calculated cooling curve for temperatures less
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Y. Lin et al. / Scripta Materialia 55 (2006) 581–584 1600 Liquidus Line Solidus Line
1200 1000 800 600 400 200
Interface Half Thickness Exposed Surface
Deposition Ends
Temperature (°C)
1400
0
500
1000 Time (s)
1500
2000
Figure 6. Calculated temperature as a function of time of sprayformed H13 tool steel at the deposit/substrate interface, half thickness, and exposed surface.
0.3
Interface Half Thickness Exposed Surface
0.2 0.15
Deposition Ends
Liquid Fraction
0.25
0.1 0.05 0 0
80
160 240 Time (s)
320
400
Figure 7. Calculated liquid fraction as a function of time in sprayformed H13 tool steel.
1200 (1)
Temperature (°C)
1000
(2)
(2): Average cooling curve during conventional air quenching [15]
A→ C A→P
600 20%
A: austenite C: carbides P: pearlite B: bainite M: martensite
Ms A→B
40%
200
80% A→M
0 1
The authors gratefully acknowledge the contributions of Bruce Wickham and Lisa Moore-McAteer during the processing of H13 tool steel and financial support from the United States Department of Energy under FWP Number 42C5-05.
(1): Calculated cooling curve
800
400
of H13 in Figure 8. During spray forming of H13 tooling, austenite decomposition starts from the austenite single-phase region, while in conventional quenching of H13 it starts from the phase region containing austenite and carbides [15]. As a result, more alloying elements and carbon are dissolved in the matrix of spray-formed H13 tooling. However, the cooling rate in spray-formed H13 is less than that in conventional quenching of H13 (Fig. 8), leading to more precipitation of alloying elements and carbon in the form of carbides. By combining the two factors, the former factor overwhelms the latter factor, leading to a higher aging hardness in sprayformed H13 tooling as shown in Tables 3 and 4. In conclusion, H13 steel with porosity less than 1% has been spray-formed for tooling purposes. A liquid fraction above 20% and sufficiently long period of time (e.g., 30–480 s) where the liquid phase is present in the deposited H13 are responsible for the low porosity. The microstructure in the as-spray-formed H13 consists of proeutectoid carbides, lower bainite, martensite and retained austenite. Spray-formed H13 tool steel exhibits higher hardness than conventionally processed material. The higher starting temperature of austenite decomposition during spray forming than is found during conventional processing results in the higher hardness of spray-formed H13 tool steel.
10
100 1000 Time (s)
10
4
10
5
Figure 8. The prediction of phases in as-deposited H13 tool steel.
than 1100 C was mapped to the transformation diagram of H13 steel [15], as shown in Figure 8. It is noted that austenite decomposition occurs only when the temperature deceases to a value that is lower than the temperature corresponding to the austenite single-phase region in the equilibrium phase diagram of H13. The lowest temperature corresponding to the austenite single-phase region is estimated to be 1100 C [15]. On this basis, Figure 8 predicts as-spray-formed H13 consists of proeutectoid carbides, bainite, martensite, and retained austenite, consistent with the aforementioned SEM (Fig. 2b) and TEM (Figs. 3–5) observations. The higher hardness values of spray-formed H13, compared to conventionally heat-treated H13, can also be rationalized on the basis of Figure 8. In order to do this, the average cooling curve during air quenching [16] is shown overlapping the transformation diagram
[1] E.J. Lavernia, Y. Wu, Spray Atomization and Deposition, John Wiley & Sons, New York, NY, 1996. [2] K.M. McHugh, J.E. Folkestad, in: Proceedings of SDMA 2003 and ICSF V, Bremen, Germany, 2003, pp. 5–123. [3] K.M. McHugh, in: S.P. Marsh, J.A. Dantzig, R. Trivedi, W. Hofmeister, M.G. Chu, E.J. Lavernia, J.H. Chun (Eds.), Solidification 1998, TMS, Warrendale, PA, 1998, pp. 427–438. [4] E.S. Lee, W.J. Park, K.H. Baik, S. Ahn, Scripta Mater. 39 (1998) 1133. [5] J.R. Knirsch, in: Proceedings of 2001 International Die Casting Congress, Cincinnati, OH, 2001. [6] C.G. Levi, R. Mehrabian, Metall. Trans. A 13 (1982) 221. [7] Q. Xu, E.J. Lavernia, Acta Mater. 49 (2001) 3849. [8] K. Lubanska, J. Met. 22 (February) (1970) 45. [9] P.S. Grant, B. Cantor, L. Katgerman, Acta Metall. Mater. 41 (1993) 3109. [10] Q. Xu, V.V. Gupta, E.J. Lavernia, Metall. Mater. Trans. B 30 (1999) 527. [11] B.C. Moon, Z.H. Lee, D.R. White, E.J. Lavernia, J. Mater. Res. 15 (2000) 1669. [12] Crucible Tool Co., private communication, 2004. [13] Source Book on Industrial Alloy and Engineering Data, American Society for Metals, Materials Park, OH, 1978, p. 267. [14] S. Annavarapu, R. Doherty, Int. J. Powder Metall. 29 (1993) 331. [15] G.A. Roberts, R.A. Cary, Tool Steels, American Society for Metals, Materials Park, OH, 1980, pp. 563–625. [16] Metal Handbook Vol. 4: Heat Treating (ninth ed.), American Society for Metals, Materials Park, OH, 1981, p. 59.
Microstructure and hardness of spray-formed chromium-containing steel tooling Yaojun Lin,a,b,* Kevin M. McHugh,c Yizhang Zhoua and Enrique J. Laverniaa a
Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616, USA b Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA c Industrial Technology Department, Idaho National Laboratory, Idaho Falls, ID 83415, USA Received 20 March 2006; revised 28 May 2006; accepted 23 June 2006 Available online 21 July 2006
The microstructure and hardness of a spray-formed chromium-containing tool steel H13 were studied. There is very little porosity in both as-spray-formed samples and spray-formed samples following heat treatment. The microstructure in the as-sprayformed material is characterized by proeutectoid carbides, lower bainite, martensite, and retained austenite. Spray-formed H13 aged at a given temperature exhibits higher hardness than commercial H13 conventionally heat treated and tempered at the same temperature. The microstructures and hardness are rationalized on the basis of numerical analysis. 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Spray-formed tooling; H13 tool steel; Process development
Spray forming has been the topic of numerous studies published over the past few decades [1]. Recently, this technology has been successfully implemented for the fabrication of molds, dies and similar tooling [2,3]. During this approach, atomized droplets of a tool-forming alloy are deposited onto a tool pattern where they accurately capture features of the pattern during solidification to form the desired mold or die. By so doing, many of the machining, grinding, and polishing steps in conventional mold and die fabrication can be eliminated, thereby reducing their cost and lead time. Spray forming can also simplify the heat treatment practices that are required to tailor the properties of tooling due to the formation of a supersaturated solid solution and macrosegregation-free microstructure [3,4]. Rapid solidification during spray forming can suppress carbide precipitation and growth, allowing tool steels to be artificially aged directly from the as-spray-formed state to obtain the final mold/die products. In contrast, conventional annealing/austenitization/quench/temper heat treatments are required steps with conventionally processed die material. Spray-formed tooling has been shown to save energy, production time and costs, and avoid mold/die distortion if aging does not involve * Corresponding author. Address: Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA. E-mail: [email protected]
phase transformation of the matrix phase. This paper reports on an investigation of the microstructures (e.g., porosity and phases) and hardness of spray-formed H13 tooling. The microstructure and hardness obtained in experiments were rationalized on the basis of numerical simulation results. The rapid solidification process (RSP) tooling approach [5] was used to generate tooling samples. A schematic of the approach is shown in Figure 1. An alumina-base ceramic was slurry cast into a tool pattern using a silicone rubber mold. After setting up, ceramic patterns were demolded, fired in a kiln, and cooled to room temperature. AISI H13 tool steel (Fe–0.40C– 5.00Cr–1.10V–1.30Mo (wt.%)) with the mass of the feedstock of 9 kg was induction melted under a nitrogen atmosphere, superheated about 100 C, and atomized with nitrogen using a bench-scale atomizer operating at a pressure of 0.14 MPa, a gas flow rate of 5.8 g/s, and a metal discharge rate of 45.4 g/s. The atomizer was stationary and the spray was projected horizontally. The exit geometry of the atomizer is circular, leading to a conical spray pattern. An inert gas atmosphere within the spray apparatus minimized in-flight oxidation of the atomized droplets as they deposited onto the tool pattern (substrate) located 254 mm from the atomizer for an average deposit growth rate of 0.24 mm/s. The tool pattern, 100 mm wide, 100 mm long, and 50 mm thick, was preheated to a desired temperature prior to
1359-6462/$ - see front matter 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2006.06.020
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Y. Lin et al. / Scripta Materialia 55 (2006) 581–584
Figure 1. Schematic of approach used to spray form H13 tooling.
droplet deposition. It was manipulated in the spray manually. The operator visually monitored the exposed surface of the spray during deposition and attempted to maintain a uniform temperature profile. The thickness of the final deposit was 50 mm. Samples investigated in the present study were sectioned using a wire electron discharge machine (EDM) from three locations of the spray-formed deposits: near the deposit/substrate interface, central region and near the exposed surface. Some samples were artificially aged under various conditions (Table 2) by placing sections in a preheated furnace that was purged with nitrogen. Each sample was coated with boron nitride and placed in a sealed metal foil packet as a precautionary measure to prevent decarburization. Artificially aged samples were soaked at 500 C for 2 h and air cooled. Others were soaked at 630 C for 2 h, air cooled to room temperature, heated again at 630 C for 2 h, and air cooled. Spray-formed H13 that was conventionally heat treated was austenitized at 1052 C for 30 min, air quenched, and double tempered (2 h plus 2 h) at 593 C. Analysis was performed on the as-deposited samples and the aged samples. Optical microscopy (OM) in conjunction with AnalysisTM software was used to determine porosity on the sample surfaces that were polished and not etched. OM, scanning electron microscopy (SEM, Philips XL30), and transmission electron microscopy (TEM, Philips CM20) was used to examine microstructure. The samples for OM and SEM analysis were prepared using standard metallographic techniques. The etchant used was a solution consisting of 10% nitric acid and 90% ethanol. The preparation of TEM samples is described as follows. The samples were mechanically thinned to 40–50 lm in thickness and then were twin jet polished using a solution consisting of 62 ml perchloric acid (70%), 700 ml ethanol, 100 ml butanol, and 137 ml distilled water. A Series 500 Wilson Rockwell C Hardness Tester was used to carry out hardness testing by averaging eight indentations. The processing of tooling via spray forming can be divided into two distinct but closely related stages: droplet flight and droplet deposition. During flight, the thermal energy of the atomized droplets is extracted via convection heat transfer between the droplets and the atomization gas and via radiation heat transfer. The temperature gradient inside a droplet can safely be neglected as the Biot number is below 0.1 for the typical droplet size during gas atomization [6]. The temperature and the solid fraction of individual droplets during flight can be calculated using an energy conservation equation [7]. At the deposition distance, assuming a droplet-size distribution predicted using the Lubanska correlation
[8], the average temperature and solid fraction of incoming droplets at impact can be evaluated using the enthalpy method [9], and used as input data for the calculation of the cooling process in the deposition stage. During deposition, heat conduction can be assumed to be 1-D along the thickness of the spray-formed material since the thickness of molds/dies is usually much smaller than their width/length. The buildup of the deposit occurs via discrete deposition of individual droplets [10], i.e., there exists a time interval between two groups of droplets that successively arrive at the previously deposited material’s surface. Accordingly, a 1-D Fourier’s equation is used to deal with heat conduction within the deposit during a time interval. At the end of this time interval, a new group of droplets is incorporated into the previously deposited material to generate new deposited material. Fourier’s equation is then implemented to compute heat conduction within the new deposit. In the present study, the alternating-direction explicit (ADE) method is used to discretize Fourier’s equation as described in Ref. [10]. The assumptions of 1-D heat transfer along the thickness and use of the ADE method are also applicable to the pattern (substrate) due to the much smaller thickness than the width and length. These assumptions are used to calculate the pattern’s temperature and to calculate cooling within the deposit. The objective of the numerical analysis described herein is to identify relative trends of temperature as a function of time. The simulation results, based on the above relatively simple droplet and deposition models help understand the thermal history and the cooling and solidification behavior of the deposited materials. In the calculations that follow, the relevant thermal and physical properties of H13 tool steel and alumina tool pattern are summarized in Table 1. The values of thermal conductivity, density and specific heat and those of latent heat of fusion and equilibrium distribution coefficient correspond to room temperature and the melting temperature range of H13 (1315– 1454 C), respectively. Since the properties of H13 and alumina change insignificantly with temperature, it is assumed that the values of these properties remain unchanged with temperature. This assumption may satisfy the objective of numerical analysis herein to investigate the temperature dependency on time rather than absolute values of temperature. Table 1. Thermal and physical properties of H13 tool steel and ceramic pattern Material H13
Ceramic pattern a b
Property Thermal conductivity Density Specific heat Latent heat of fusion Liquidus temperature Solidus temperature Equilibrium distribution coefficient Thermal conductivity Density Specific heat
Value
Reference a
28 W/mK 7000 kg/m3a 447 J/kg Ka 2.8 · 105J/kgb 1454 C 1315 C 0.35b
[11]
1 W/mKa 1700 kg/m3a 900 J/kg Ka
[11]
[12]
At room temperature. At the melting temperature range of H13 (1315–1454 C).
Y. Lin et al. / Scripta Materialia 55 (2006) 581–584
Table 2 shows the measured porosity of five sprayformed H13 samples sectioned from different locations in the deposit and heat treated under different conditions. In all of the samples, porosity is less than 1%. In Figure 2a, the OM image of as-spray-formed H13 (sample no. 4) exhibits dark acicular features separated by the light-colored regions. In the SEM image of this sample (Fig. 2b), proeutectoid carbides can be observed (white particles indicated by arrows). The SEM image in Figure 2b also provides the details of the dark acicular features (labeled ‘‘A’’) and the light-colored regions (labeled ‘‘B’’) that are lower bainite and martensite respectively. The above microstructures are confirmed by TEM analysis. The TEM bright field image in Figure 3 exhibits typical lower bainite morphology: bainitic ferrite (labeled ‘‘A’’), and (Fe, M)3C carbides (where M represents Cr, V, and Mo substituting for Fe) between bainitic ferrite and inside bainitic ferrite. Figure 4a is the TEM bright field image of martensite and Figure 4b is the corresponding diffraction pattern. Figure 5 shows TEM morphology of retained austenite. Tables 3 and 4 compare the hardness of spray-formed and aged H13 with commercial forged H13 that was conventionally heat treated. Conventional heat treatment was conducted by austenitizing at 1015 C, air quenching, and tempering at the temperatures shown in Table 4. For a given aging (or tempering) tempera-
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Figure 4. TEM micrograph of martensite: (a) bright field image and (b) diffraction pattern.
Figure 5. TEM micrographs showing retained austenite: (a) bright field image and diffraction pattern (inset) and (b) dark field image from the diffraction spot 111.
Table 3. Hardness of spray-formed and aged H13 tool steel Table 2. Porosity in various spray-formed H13 steel samples Sample location
Sample no.
Heat treatment
Porosity (vol.%)
Deposit/substrate interface
1 2 3 4 5
As-deposited 500 C · 2 h 630 C · (2 + 2) h As-deposited 1052 C (air quenching) and 593 C · (2 + 2) h
0.10 0.12 0.71 0.05 0.55
Exposed surface Middle
Figure 2. Microstructure of as-deposited H13 (sample no. 4): (a) OM image and (b) SEM image.
Figure 3. TEM micrograph of lower bainite: bright field image and diffraction pattern of (Fe, M)3C (inset).
Sample location
Sample no.
Aging treatment
Hardness (HRC)
Deposit/substrate interface
1 2 3 4
As-deposited 500 C · 2 h 630 C · (2 + 2) h As-deposited
55.2 56.6 40.3 54.3
Exposed surface
Table 4. Hardness of conventionally processed H13 tool steel [13] Tempering temperature
Hardness (HRC)
As-quenched 500 C 630 C
50.0 53.3 39.6
ture, the hardness value of spray-formed H13 was found to be higher than that of conventional H13. Figure 6 shows the calculated cooling curve of an H13 tool steel deposit. Temperature is plotted as a function of time for three regions of the deposit: the deposit/ substrate interface, half thickness, and the exposed surface of the deposited material. Results indicate that during spray forming, liquid phase is present throughout the entire thickness of the deposited material. By assuming equilibrium conditions for the simulation in Figure 6, an estimation of liquid fraction is obtained. In Figure 7, it is shown that throughout the entire thickness of the deposited material, the maximum liquid fraction is above 20%, which is required for complete removal of porosity [14]. The liquid fraction exists for the time interval from 30–480 s. These results support the observed very low porosity values in Table 2. In order to predict the phases in as-spray-formed H13, the calculated cooling curve for temperatures less
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Y. Lin et al. / Scripta Materialia 55 (2006) 581–584 1600 Liquidus Line Solidus Line
1200 1000 800 600 400 200
Interface Half Thickness Exposed Surface
Deposition Ends
Temperature (°C)
1400
0
500
1000 Time (s)
1500
2000
Figure 6. Calculated temperature as a function of time of sprayformed H13 tool steel at the deposit/substrate interface, half thickness, and exposed surface.
0.3
Interface Half Thickness Exposed Surface
0.2 0.15
Deposition Ends
Liquid Fraction
0.25
0.1 0.05 0 0
80
160 240 Time (s)
320
400
Figure 7. Calculated liquid fraction as a function of time in sprayformed H13 tool steel.
1200 (1)
Temperature (°C)
1000
(2)
(2): Average cooling curve during conventional air quenching [15]
A→ C A→P
600 20%
A: austenite C: carbides P: pearlite B: bainite M: martensite
Ms A→B
40%
200
80% A→M
0 1
The authors gratefully acknowledge the contributions of Bruce Wickham and Lisa Moore-McAteer during the processing of H13 tool steel and financial support from the United States Department of Energy under FWP Number 42C5-05.
(1): Calculated cooling curve
800
400
of H13 in Figure 8. During spray forming of H13 tooling, austenite decomposition starts from the austenite single-phase region, while in conventional quenching of H13 it starts from the phase region containing austenite and carbides [15]. As a result, more alloying elements and carbon are dissolved in the matrix of spray-formed H13 tooling. However, the cooling rate in spray-formed H13 is less than that in conventional quenching of H13 (Fig. 8), leading to more precipitation of alloying elements and carbon in the form of carbides. By combining the two factors, the former factor overwhelms the latter factor, leading to a higher aging hardness in sprayformed H13 tooling as shown in Tables 3 and 4. In conclusion, H13 steel with porosity less than 1% has been spray-formed for tooling purposes. A liquid fraction above 20% and sufficiently long period of time (e.g., 30–480 s) where the liquid phase is present in the deposited H13 are responsible for the low porosity. The microstructure in the as-spray-formed H13 consists of proeutectoid carbides, lower bainite, martensite and retained austenite. Spray-formed H13 tool steel exhibits higher hardness than conventionally processed material. The higher starting temperature of austenite decomposition during spray forming than is found during conventional processing results in the higher hardness of spray-formed H13 tool steel.
10
100 1000 Time (s)
10
4
10
5
Figure 8. The prediction of phases in as-deposited H13 tool steel.
than 1100 C was mapped to the transformation diagram of H13 steel [15], as shown in Figure 8. It is noted that austenite decomposition occurs only when the temperature deceases to a value that is lower than the temperature corresponding to the austenite single-phase region in the equilibrium phase diagram of H13. The lowest temperature corresponding to the austenite single-phase region is estimated to be 1100 C [15]. On this basis, Figure 8 predicts as-spray-formed H13 consists of proeutectoid carbides, bainite, martensite, and retained austenite, consistent with the aforementioned SEM (Fig. 2b) and TEM (Figs. 3–5) observations. The higher hardness values of spray-formed H13, compared to conventionally heat-treated H13, can also be rationalized on the basis of Figure 8. In order to do this, the average cooling curve during air quenching [16] is shown overlapping the transformation diagram
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