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Defining the pores in PM components
technical trends
Defining the pores in PM components Porosity levels are important for powder metallurgists. Liquids or gases can penetrate pores during the manufacturing process and have a negative influence on properties. An Austro-Swedish team looked at the transition from open to closed porosity and pore connectivity at different densities and temperatures...
T
he most significant difference between cast and wrought materials and powder metallurgy precision parts is the much higher level of porosity in PM. It is a very important factor and has to be seen as a real microstructural component with potentially great influence on properties [1, 2, 3, 4, 5]. For that reason it is necessary to have methods to characterise the porosity thoroughly. Porosity can be defined in different ways. The most frequently used definition is the total porosity, which is the total volume fraction of empty space in the specimen [2]. This parameter is usually measured using water displacement methods. Of equal importance is the pore connectivity. The pores may be interconnected or isolated. In the case of fully interconnected pores there exists only one big, very complex-shaped and branched, pore, while isolated pores are distributed in the matrix [2]. In practice the terms “open” and “closed porosity” are used more frequently than “interconnected” and “isolated” pores, although this is not necessarily the same. Open porosity is that part of porosity which is directly connected to the surface of the samples. Therefore it can be measured easily via penetrating methods like oil impregnation or pycnometric analyses such as mercury
30
MPR February 2010
pycnometry. The open porosity is very important in the matter of thermochemical treatment [6]. The open pores act as transport channels that enable the gas components to penetrate the whole specimen. This may result in over-carburising or over-nitriding the parts. Further, specimens collect a lot of oil in the open pores when oil quenching is used as a cooling step. In this study, an advanced method was used to characterise the transition between open and closed porosity, namely helium pycnometry, since it is of high importance to know the intersection open / closed porosity, especially when the investigated materials are of interest for thermochemical heat treatment. Three groups of materials, each differently compacted and sintered, were characterised to study the influence on the transition between open and closed porosity. Another way of describing the porosity of a material is the plane porosity, which was introduced by Slesar [1]. It is a quantitative parameter which includes the free particle surfaces and also the pores
in the sintering contacts. Using the plane porosity allows a more exact description of the relationship between strength and microstructure than the total porosity. A fractographic picture is used for the analysis of this parameter and therefore permits the specification of the three-dimensional pore network at the real load bearing cross section [6]. The team used a special computer programme called Pixcal to characterise the load bearing cross section, comparing it to the total porosity of the three different materials sintered at different temperatures. 0. The three materials were Astaloy CrL, Astaloy Mo and ASC 100.29 all admixed with 0.6 %C. Their green state was determined after pressing at 400, 600 and 800 MPa using die wall lubrication. The sintered state was investigated at five desired density levels listed in Table 1. In order to find the transition between open and closed porosity, two different ways of characterising the density were needed. The first one was the total porosity obtained via water displacement – the
Table 1: Desired density levels for all the materials Desired Density (g/cm3)
Base powder; code Astaloy CrL “C” Astaloy Mo “M”
7.1
7.25
7.4
7.5
7.6
ASC 100.29 “A”
0026-0657/10 ©2010 Elsevier Ltd. All rights reserved.
Archimedes equation. The second was a pycnometric method using helium as a displacement agent. To calculate the total porosity the density was measured using the Archimedes principle (Equation 1). To avoid penetration of the open pores, the samples were impregnated with a water stop agent (commercial water stop spray) - the weight of which is much too low to affect the measurement - and then the weight of the sample in air and the weight in water were determined. The calculation of the theoretical (or pore-free) density was necessary to enable calculation of the total porosity (Equation 2). In particular at low porosity levels, the definition of the theoretical density is very critical for the results. In this case the theoretical density was calculated from the density of the phases through the rule of mixture. Carbon was estimated to be present as cementite. mA ȡ= mA – mW t Equation 1. Archimedes equation for the density (neglecting air displacement). ȡ ... sintered density [g/cm3] mA ... weight in air [g] mW ... weight in water [g] 0,998 ... density of water at 23°C ȡ P=1– ȡ t Theoretical Equation 2. Calculation of the total porosity. P ... porosity [%] ȡTheoretisch ... theoretical density of the material [g/cm3]
an added volume called Va. Last, the gas pressure is reduced to p3. p2(VC - VS) = p3(VC - VS + Va) The above mentioned equation can be changed to VS = VC +
p2 (VC - VS) = nRT Next, a valve was opened, and the sample chamber Vc was connected to
metal-powder.net
1 - (p2 / p3)
.
With the mass of the samples determined, the density was calculated very easily. The measurement yielded results in which the density is lowered compared to the theoretical by the closed porosity, while the open pores are filled with He and therefore are not detected.
Plane Porosity Currently used software systems for image analysis are classified as semi-automatic and fully automatic, respectively. Both system types have some drawbacks when applied to fracture surfaces. For the semiautomatic systems this is mainly the timeconsuming manual work, and the results depend on the skill of the operators. For the fully automatically systems there exist primarily two problems. One is the contrast problem, eg that cleavage areas cannot be measured correctly, and the results are strongly influenced by the adjustment of the measuring parameters [8]. Pixcal is software to analyse the load bearing cross section Ac [9]. But it can also be used to calculate the amount
Open porosity
Transition from open to closed Comparing the three different materials it is easily recognised that for both sintering temperatures the transition from open to closed porosity - defined as the density at which Popen = Pclosed - occurs at lower density levels for the chromium containing material, whereas molybdenum in the materials seems to cause higher transition values. This is due to the fact that chromium is an alloying element that increases sintering activity [10]. When examining transition at a higher sintering temperature compared to the transition at a lower one (Graph 1 and Graph 2) it can be clearly seen that at 1250°C the values for the transition from open to closed porosity are shifted to lower density values. This means that after sintering at higher temperatures, there are fewer interconnected pores open to the surface even at lower density levels. This is a beneficial factor when heat treating PM parts. As stated
10.00 ASC 1120°C open porosity ASC 1120°C closed porosity CrL 1120°C open porosity CrL 1120°C closed porosity Mo 1120°C open porosity Mo 1120°C closed porosity
9.00 8.00
Porosity [%]
In order to find the correct amount of open porosity, the closed porosity was measured and then subtracted from the total porosity. The closed porosity was measured with a helium pycnometer (Ultra Pycnometer 1000T) [7]. This method uses the Boyle-Mariotte law. A solid sample with a specific volume Vs was placed in the sample chamber Vc, and the cell was flooded with a measuring gas. R was the gas constant, n the amount of gas and T the gas temperature.
Va
of plane porosity Px of the material [1] since Px = 1 - Ac. It is based on counting pixels of marked areas. The pores must be marked manually, and the programme then calculates the amount of marked pixels in comparison to the number of pixels of the entire photo. To make the measurement a fracture surface obtained through low-deformation fracture had to be analysed [6]. To get such a surface, the samples were cooled in liquid nitrogen just before impact fracturing.
7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 7.00
7.10
7.20
7.30
7.40
7.50
7.60
Density [g/cm] Graph 1. Open and closed porosity as a function of the density for ASC 100.29, Astaloy CrL and Astaloy Mo sintered at 1120°C.
February 2010 MPR
31
10.00
7.7
Porosity [%]
8.00 7.00
Transition density [g/cm]
ASC 1250°C open porosity ASC 1250°C closed porosity CrL 1250°C open porosity CrL 1250°C closed porosity Mo 1250°C open porosity Mo 1250°C closed porosity
9.00
6.00 5.00 4.00 3.00 2.00
ASC CrL Mo
7.6 7.5 7.4 7.3 7.2 7.1
1.00 0.00 7.00
7.10
7.20
7.30
7.40
7.50
7.60
7 1100
1150
Density [g/cm]
Porosity [%]
MPR February 2010
1300
1350
1400
Graph 3. Influencing parameters on the transition density from mainly closed to mainly open porosity (defined as the density for which Popen = Pclosed).
Plane Porosity
Table 1 shows the amount of plane porosity obtained through Pixcal. The CrL material contains less porosity compared to the two other materials. Comparing the plane porosity measured with Pixcal and the total porosity measured through Archimedes, it is clearly seen that the values for the porosity are much higher for the Pixcal calculation. This is simply due to the fact that a fracture surface always includes more pores than a section, in particular in the case of interconnected pores, ie isolated sintering contacts [6], in which case the “sponge” model rather than the “swiss cheese” model applies. Correlating the Plane / Total Porosity ratio and the Open / Closed Porosity ratio it is seen that the higher amount of open porosity results in higher amounts of plane porosity. This means that with increasing closed porosity, also the load bearing cross section increases [3]. ASC open porosity The higher 16 ASC closed porosity CrL open porosity sintering temCrL closed porosity 14 Mo open porosity perature results Mo closed porosity 12 in lower amounts 10 of open poros8 400 MPa ity, and the ratio 6 of Plane/Total 600 MPa 4 porosity decreases 800 MPa 2 (Graph 5). This 0.00 is also seen when 6.8 6.4 6.6 7 7.2 7.4 7.6 examining the Density [g/cm] fracture surface with Pixcal. For Graph 4. Transition from open to closed porosity of the green state a given density, at (compacted with die wall lubrication).
32
1250
Temperature [°C]
Graph 2. Open and closed porosity as a function of the density for ASC 100.29, Astaloy CrL and Astaloy Mo sintered at 1250°C.
above, open pores here act like transporting channels, and so the reactive agents, such as carburising gases, and also the hardening medium, especially oil, can penetrate the pores and thus the entire part. It is remarkable that at higher sintering temperatures the differences in the density values at which open porosity is transformed to closed are more pronounced. At 1120°C the transition density is not too different for all three materials while at 1250°C this does not hold anymore. Graph 3 sums up the parameters influencing the transition density from mainly open to mainly closed porosity, which are the sintering temperature and the alloying element. Both have a considerable effect. The investigations in the green state, pressed with die wall lubrication, pointed out that no significant closed porosity exists after cold pressing (Graph 4). This means that the entire amount of this porosity type is formed during sintering.
1200
1120°C (Figure 1) there is a significantly higher level of plane porosity (blue marked areas) than in the samples sintered at 1250°C or 1380°C. The team concluded that transition at lower density levels is very positive in
References [1] M Slesar, E Dudrova, E Rudnayova; Powder Metallurgy International Vol. 24, 4, 1992 pp 232-236. [2] H Danninger, G Jangg, B Weiss; Powder Metallurgy International Vol 25, 3 1993 pp 111-117 [3] H Danninger, G Jangg, B Weiss; Powder Metallurgy International Vol 25, 4 1993 pp 170-173 [4] A Salak, V Miuskovic, E Dudrova, E Rudnayova; Powder Metallurgy International Vol. 6, 3 1974 pp 128-132 [5] K Widanka; Powder Metallurgy Vol 51, 3, 2008 pp 268-271 [6] H Danninger et al.; Praktische Metallographie Vol 31, 2, 1994 pp 56-69 [7] Quantachrome Instruments; User Manuals, 2006 [8] H Danninger U. Sonntag, B.Kuhnert, R. Ratzi; Praktische Metallographie Vol. 39, 8 2002 [9] C Xu; Ph D Thesis, TU Wien (2005) [10] S Kremel, H. Danninger, Y. Yu; Powder Metallurgy Progress Vol. 2, 4, 2002 pp 211-221
metal-powder.net
Table 1. Results of the measurements of the plane porosity with Pixcal. 7,1 g/cm3
7,4 g/cm3
Table 2. Total porosity (calculation from the Archimedes density). 7,1 g/cm3
7,6 g/cm3
1120 ˚C
7,4 g/cm3
7,6 g/cm3
1120 ˚C
ASC 100, 29
30±4
16±3
7±1
ASC 100, 29
9,8
6,2
3,8
Astaloy CrL
28±1
13±2
8±2
Astaloy CrL
9,5
5,6
3,7
Astaloy Mo
35±8
22±3
13±3
Astaloy Mo
9,5
5,6
3,7
1250 ˚C ASC 100, 29
23±1
Astaloy CrL Astaloy Mo
1250 ˚C
11±1
6±2
ASC 100, 29
17±3
8±2
6±2
Astaloy CrL
8,8
4,7
3,3
18±5
10±2
6±1
Astaloy Mo
9,5
6,0
4,0
terms of heat treatment. Open porosity usually acts as transporting channels for carburising/nitriding agents and for
quenching oil. Closed pores eliminate these problems; furthermore, isolated pores (which are closely related to closed
4.0
35 1120°C PP/TP 1250°C PP/TP 1120°C OP/CP 1250°C OP/CP
3.5 3.0
30 25 20
2.0 15
OP/CP
PP/TP
2.5
1.5 10
1.0
5
0.5 0 7.00
7.10
7.20
7.30
7.40
7.50
0 7.60
Density [g/cm] Graph 5. Correlation of the ratio of Plane / Total and Open / Closed Porosity for the ASC 100,29 + 0,6 %C.
9,2
5,3
3,6
ones, though not identical) are beneficial for the load bearing cross section and thus for the mechanical properties. A fairly new method for characterising the plane porosity from fracture surfaces with the Pixcal software was accomplished. Comparing it with the more common total porosity it is evident that the plane porosity is much higher than Ptot. It is a more realistic way for describing the porosity in parts than metallographic methods, since pores are usually three dimensional [1]. The Pixcal program is a comparatively easy tool for the investigations of plane porosity and avoids most of the drawbacks of conventionally used programs, except the expenditure of time, which is still rather high.
Acknowledgement This work was carried out in the Höganäs Chair IV programme which is funded by Höganäs AB Sweden.
The Authors This feature was derived from an updated version of Pycnometric investigation of the transition between open and closed porosity for different iron based PM materials, a paper by Magdalena Dlapka1, Herbert Danninger1, Chistian Gierl1 and Björn Lindqvist2. The original was given at EuroPM 2009 in Copenhagen.
Figure 1. Astaloy CrL and Astaloy Mo admixed with 0,6 % C measured with Pixcal after sintering at 1120 °C, 1250 °C and 1380 °C and breaking after cooling in liquid nitrogen.
metal-powder.net
1Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9//164-CT 1060 Wien, Austria. 2Höganäs AB, SE-263 83 Höganäs, Sweden.
February 2010 MPR
33
Defining the pores in PM components Porosity levels are important for powder metallurgists. Liquids or gases can penetrate pores during the manufacturing process and have a negative influence on properties. An Austro-Swedish team looked at the transition from open to closed porosity and pore connectivity at different densities and temperatures...
T
he most significant difference between cast and wrought materials and powder metallurgy precision parts is the much higher level of porosity in PM. It is a very important factor and has to be seen as a real microstructural component with potentially great influence on properties [1, 2, 3, 4, 5]. For that reason it is necessary to have methods to characterise the porosity thoroughly. Porosity can be defined in different ways. The most frequently used definition is the total porosity, which is the total volume fraction of empty space in the specimen [2]. This parameter is usually measured using water displacement methods. Of equal importance is the pore connectivity. The pores may be interconnected or isolated. In the case of fully interconnected pores there exists only one big, very complex-shaped and branched, pore, while isolated pores are distributed in the matrix [2]. In practice the terms “open” and “closed porosity” are used more frequently than “interconnected” and “isolated” pores, although this is not necessarily the same. Open porosity is that part of porosity which is directly connected to the surface of the samples. Therefore it can be measured easily via penetrating methods like oil impregnation or pycnometric analyses such as mercury
30
MPR February 2010
pycnometry. The open porosity is very important in the matter of thermochemical treatment [6]. The open pores act as transport channels that enable the gas components to penetrate the whole specimen. This may result in over-carburising or over-nitriding the parts. Further, specimens collect a lot of oil in the open pores when oil quenching is used as a cooling step. In this study, an advanced method was used to characterise the transition between open and closed porosity, namely helium pycnometry, since it is of high importance to know the intersection open / closed porosity, especially when the investigated materials are of interest for thermochemical heat treatment. Three groups of materials, each differently compacted and sintered, were characterised to study the influence on the transition between open and closed porosity. Another way of describing the porosity of a material is the plane porosity, which was introduced by Slesar [1]. It is a quantitative parameter which includes the free particle surfaces and also the pores
in the sintering contacts. Using the plane porosity allows a more exact description of the relationship between strength and microstructure than the total porosity. A fractographic picture is used for the analysis of this parameter and therefore permits the specification of the three-dimensional pore network at the real load bearing cross section [6]. The team used a special computer programme called Pixcal to characterise the load bearing cross section, comparing it to the total porosity of the three different materials sintered at different temperatures. 0. The three materials were Astaloy CrL, Astaloy Mo and ASC 100.29 all admixed with 0.6 %C. Their green state was determined after pressing at 400, 600 and 800 MPa using die wall lubrication. The sintered state was investigated at five desired density levels listed in Table 1. In order to find the transition between open and closed porosity, two different ways of characterising the density were needed. The first one was the total porosity obtained via water displacement – the
Table 1: Desired density levels for all the materials Desired Density (g/cm3)
Base powder; code Astaloy CrL “C” Astaloy Mo “M”
7.1
7.25
7.4
7.5
7.6
ASC 100.29 “A”
0026-0657/10 ©2010 Elsevier Ltd. All rights reserved.
Archimedes equation. The second was a pycnometric method using helium as a displacement agent. To calculate the total porosity the density was measured using the Archimedes principle (Equation 1). To avoid penetration of the open pores, the samples were impregnated with a water stop agent (commercial water stop spray) - the weight of which is much too low to affect the measurement - and then the weight of the sample in air and the weight in water were determined. The calculation of the theoretical (or pore-free) density was necessary to enable calculation of the total porosity (Equation 2). In particular at low porosity levels, the definition of the theoretical density is very critical for the results. In this case the theoretical density was calculated from the density of the phases through the rule of mixture. Carbon was estimated to be present as cementite. mA ȡ= mA – mW t Equation 1. Archimedes equation for the density (neglecting air displacement). ȡ ... sintered density [g/cm3] mA ... weight in air [g] mW ... weight in water [g] 0,998 ... density of water at 23°C ȡ P=1– ȡ t Theoretical Equation 2. Calculation of the total porosity. P ... porosity [%] ȡTheoretisch ... theoretical density of the material [g/cm3]
an added volume called Va. Last, the gas pressure is reduced to p3. p2(VC - VS) = p3(VC - VS + Va) The above mentioned equation can be changed to VS = VC +
p2 (VC - VS) = nRT Next, a valve was opened, and the sample chamber Vc was connected to
metal-powder.net
1 - (p2 / p3)
.
With the mass of the samples determined, the density was calculated very easily. The measurement yielded results in which the density is lowered compared to the theoretical by the closed porosity, while the open pores are filled with He and therefore are not detected.
Plane Porosity Currently used software systems for image analysis are classified as semi-automatic and fully automatic, respectively. Both system types have some drawbacks when applied to fracture surfaces. For the semiautomatic systems this is mainly the timeconsuming manual work, and the results depend on the skill of the operators. For the fully automatically systems there exist primarily two problems. One is the contrast problem, eg that cleavage areas cannot be measured correctly, and the results are strongly influenced by the adjustment of the measuring parameters [8]. Pixcal is software to analyse the load bearing cross section Ac [9]. But it can also be used to calculate the amount
Open porosity
Transition from open to closed Comparing the three different materials it is easily recognised that for both sintering temperatures the transition from open to closed porosity - defined as the density at which Popen = Pclosed - occurs at lower density levels for the chromium containing material, whereas molybdenum in the materials seems to cause higher transition values. This is due to the fact that chromium is an alloying element that increases sintering activity [10]. When examining transition at a higher sintering temperature compared to the transition at a lower one (Graph 1 and Graph 2) it can be clearly seen that at 1250°C the values for the transition from open to closed porosity are shifted to lower density values. This means that after sintering at higher temperatures, there are fewer interconnected pores open to the surface even at lower density levels. This is a beneficial factor when heat treating PM parts. As stated
10.00 ASC 1120°C open porosity ASC 1120°C closed porosity CrL 1120°C open porosity CrL 1120°C closed porosity Mo 1120°C open porosity Mo 1120°C closed porosity
9.00 8.00
Porosity [%]
In order to find the correct amount of open porosity, the closed porosity was measured and then subtracted from the total porosity. The closed porosity was measured with a helium pycnometer (Ultra Pycnometer 1000T) [7]. This method uses the Boyle-Mariotte law. A solid sample with a specific volume Vs was placed in the sample chamber Vc, and the cell was flooded with a measuring gas. R was the gas constant, n the amount of gas and T the gas temperature.
Va
of plane porosity Px of the material [1] since Px = 1 - Ac. It is based on counting pixels of marked areas. The pores must be marked manually, and the programme then calculates the amount of marked pixels in comparison to the number of pixels of the entire photo. To make the measurement a fracture surface obtained through low-deformation fracture had to be analysed [6]. To get such a surface, the samples were cooled in liquid nitrogen just before impact fracturing.
7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 7.00
7.10
7.20
7.30
7.40
7.50
7.60
Density [g/cm] Graph 1. Open and closed porosity as a function of the density for ASC 100.29, Astaloy CrL and Astaloy Mo sintered at 1120°C.
February 2010 MPR
31
10.00
7.7
Porosity [%]
8.00 7.00
Transition density [g/cm]
ASC 1250°C open porosity ASC 1250°C closed porosity CrL 1250°C open porosity CrL 1250°C closed porosity Mo 1250°C open porosity Mo 1250°C closed porosity
9.00
6.00 5.00 4.00 3.00 2.00
ASC CrL Mo
7.6 7.5 7.4 7.3 7.2 7.1
1.00 0.00 7.00
7.10
7.20
7.30
7.40
7.50
7.60
7 1100
1150
Density [g/cm]
Porosity [%]
MPR February 2010
1300
1350
1400
Graph 3. Influencing parameters on the transition density from mainly closed to mainly open porosity (defined as the density for which Popen = Pclosed).
Plane Porosity
Table 1 shows the amount of plane porosity obtained through Pixcal. The CrL material contains less porosity compared to the two other materials. Comparing the plane porosity measured with Pixcal and the total porosity measured through Archimedes, it is clearly seen that the values for the porosity are much higher for the Pixcal calculation. This is simply due to the fact that a fracture surface always includes more pores than a section, in particular in the case of interconnected pores, ie isolated sintering contacts [6], in which case the “sponge” model rather than the “swiss cheese” model applies. Correlating the Plane / Total Porosity ratio and the Open / Closed Porosity ratio it is seen that the higher amount of open porosity results in higher amounts of plane porosity. This means that with increasing closed porosity, also the load bearing cross section increases [3]. ASC open porosity The higher 16 ASC closed porosity CrL open porosity sintering temCrL closed porosity 14 Mo open porosity perature results Mo closed porosity 12 in lower amounts 10 of open poros8 400 MPa ity, and the ratio 6 of Plane/Total 600 MPa 4 porosity decreases 800 MPa 2 (Graph 5). This 0.00 is also seen when 6.8 6.4 6.6 7 7.2 7.4 7.6 examining the Density [g/cm] fracture surface with Pixcal. For Graph 4. Transition from open to closed porosity of the green state a given density, at (compacted with die wall lubrication).
32
1250
Temperature [°C]
Graph 2. Open and closed porosity as a function of the density for ASC 100.29, Astaloy CrL and Astaloy Mo sintered at 1250°C.
above, open pores here act like transporting channels, and so the reactive agents, such as carburising gases, and also the hardening medium, especially oil, can penetrate the pores and thus the entire part. It is remarkable that at higher sintering temperatures the differences in the density values at which open porosity is transformed to closed are more pronounced. At 1120°C the transition density is not too different for all three materials while at 1250°C this does not hold anymore. Graph 3 sums up the parameters influencing the transition density from mainly open to mainly closed porosity, which are the sintering temperature and the alloying element. Both have a considerable effect. The investigations in the green state, pressed with die wall lubrication, pointed out that no significant closed porosity exists after cold pressing (Graph 4). This means that the entire amount of this porosity type is formed during sintering.
1200
1120°C (Figure 1) there is a significantly higher level of plane porosity (blue marked areas) than in the samples sintered at 1250°C or 1380°C. The team concluded that transition at lower density levels is very positive in
References [1] M Slesar, E Dudrova, E Rudnayova; Powder Metallurgy International Vol. 24, 4, 1992 pp 232-236. [2] H Danninger, G Jangg, B Weiss; Powder Metallurgy International Vol 25, 3 1993 pp 111-117 [3] H Danninger, G Jangg, B Weiss; Powder Metallurgy International Vol 25, 4 1993 pp 170-173 [4] A Salak, V Miuskovic, E Dudrova, E Rudnayova; Powder Metallurgy International Vol. 6, 3 1974 pp 128-132 [5] K Widanka; Powder Metallurgy Vol 51, 3, 2008 pp 268-271 [6] H Danninger et al.; Praktische Metallographie Vol 31, 2, 1994 pp 56-69 [7] Quantachrome Instruments; User Manuals, 2006 [8] H Danninger U. Sonntag, B.Kuhnert, R. Ratzi; Praktische Metallographie Vol. 39, 8 2002 [9] C Xu; Ph D Thesis, TU Wien (2005) [10] S Kremel, H. Danninger, Y. Yu; Powder Metallurgy Progress Vol. 2, 4, 2002 pp 211-221
metal-powder.net
Table 1. Results of the measurements of the plane porosity with Pixcal. 7,1 g/cm3
7,4 g/cm3
Table 2. Total porosity (calculation from the Archimedes density). 7,1 g/cm3
7,6 g/cm3
1120 ˚C
7,4 g/cm3
7,6 g/cm3
1120 ˚C
ASC 100, 29
30±4
16±3
7±1
ASC 100, 29
9,8
6,2
3,8
Astaloy CrL
28±1
13±2
8±2
Astaloy CrL
9,5
5,6
3,7
Astaloy Mo
35±8
22±3
13±3
Astaloy Mo
9,5
5,6
3,7
1250 ˚C ASC 100, 29
23±1
Astaloy CrL Astaloy Mo
1250 ˚C
11±1
6±2
ASC 100, 29
17±3
8±2
6±2
Astaloy CrL
8,8
4,7
3,3
18±5
10±2
6±1
Astaloy Mo
9,5
6,0
4,0
terms of heat treatment. Open porosity usually acts as transporting channels for carburising/nitriding agents and for
quenching oil. Closed pores eliminate these problems; furthermore, isolated pores (which are closely related to closed
4.0
35 1120°C PP/TP 1250°C PP/TP 1120°C OP/CP 1250°C OP/CP
3.5 3.0
30 25 20
2.0 15
OP/CP
PP/TP
2.5
1.5 10
1.0
5
0.5 0 7.00
7.10
7.20
7.30
7.40
7.50
0 7.60
Density [g/cm] Graph 5. Correlation of the ratio of Plane / Total and Open / Closed Porosity for the ASC 100,29 + 0,6 %C.
9,2
5,3
3,6
ones, though not identical) are beneficial for the load bearing cross section and thus for the mechanical properties. A fairly new method for characterising the plane porosity from fracture surfaces with the Pixcal software was accomplished. Comparing it with the more common total porosity it is evident that the plane porosity is much higher than Ptot. It is a more realistic way for describing the porosity in parts than metallographic methods, since pores are usually three dimensional [1]. The Pixcal program is a comparatively easy tool for the investigations of plane porosity and avoids most of the drawbacks of conventionally used programs, except the expenditure of time, which is still rather high.
Acknowledgement This work was carried out in the Höganäs Chair IV programme which is funded by Höganäs AB Sweden.
The Authors This feature was derived from an updated version of Pycnometric investigation of the transition between open and closed porosity for different iron based PM materials, a paper by Magdalena Dlapka1, Herbert Danninger1, Chistian Gierl1 and Björn Lindqvist2. The original was given at EuroPM 2009 in Copenhagen.
Figure 1. Astaloy CrL and Astaloy Mo admixed with 0,6 % C measured with Pixcal after sintering at 1120 °C, 1250 °C and 1380 °C and breaking after cooling in liquid nitrogen.
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1Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9//164-CT 1060 Wien, Austria. 2Höganäs AB, SE-263 83 Höganäs, Sweden.
February 2010 MPR
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