108 Prakt. Met. Sonderband 38 (2006) 
For better illustration of the effect of porosity on the material properties of pressed and TIE 
sintered steels, impact toughness and macrohardness (HV30) of the tested materials were The 
determined. cor 
—— _ — _ tots 
Compacting | Theoretical Green Sintered Impact Macro- 
Pressure density Density Density | Porosity [%] Toughness hardness 
[MPa g/cm g/cm g/cm J/cm HV-30 
300 17.5 9,1 BY 5 
500 7.05 6,87 6,93 Aa 17,8 105 
800 7,85 7,20 7,28 1,3 30,6 138 Wit 
Table 1: Theoretical density, green and sintered densities, calculated porosity, impact toughness and pix 
macrohardness of powder metallurgical Fe-0,78m%C steels compacted at different pressures of 
pre 
2.2 METALLOGRAPHIC PREPARATION Dot 
Samples were cold mounted in epoxy resin (Epofix, 30mm moulds) and prepared after a We 
procedure slightly modified from the recommendations presented in [3] in four steps. filte 
Between the preparation steps samples and sample-holder were submersed in an val 
ultrasonic bath of distilled water for at least 3 min to avoid contamination of the following alte 
preparation step. Critical for correct porosity preparation was the polishing step with 3 um use 
diamond. As already discussed in [3], sufficient long polishing times are necessary for the ce 
complete removal of debris accumulated in pores during previous preparation steps. After 
an initial polishing step of 8 min with 3 um diamond samples were examined 
microscopically to detect inhomogeneities in preparation (i.e. areas with smaller and or 
less visible pores) and at increasing magnifications to evaluate the opening of pores. wit 
Afterwards an additional polishing step of 2 min with 3 um diamond was carried out and val 
microscopic examination was repeated. Only after further preparation steps did not the 
significantly change the visible porosity, the final 1 um diamond polishing step was carried 
out, afterwards samples were rinsed with methanol, submersed for 2 min in a methanol Us: 
ultrasonic bath and dried in warm air. be 
Step plane grinding | fine grinding diamond polishing 1 diamond polishing 2 | 
Abrasive diamond grinding disc | diamond suspension ! diamond suspension * diamond suspension Le 
(ca, 120#) 2 9M ao Sum po pn uma) 
Rounds 300 150 150 150 At 
per min_ _ = i pol 
Time | 10min 4 min on enn 1 min i 
Force/ 
Sample | SON 25N 20N 15N pre 
Table 2: Preparation steps for correct porosity preparation on pressed and sintered Fe-0,78m%C pearlitic - im: 
steels (porosity from 17,5 to 7,3%) 
2.3 MICROSCOPY AND DIGITAL IMAGE PREPARATION 
wit 
To develop a reliable method for porosity quantification of powder metallurgical steels by sig 
image analysis two influencing factors, namely magnification and digital preparation were sta 
examined. Development of the image processing routine was carried out on the material wit 
with a calculated porosity of 11,7%. From two samples prepared after the procedure qu 
described in 2.2 at 100x, 200x, and 500x magnification 10 images each were taken in bre 
bright field. The resulting 20 images per magnification were transformed from 24Bit RGB