Prakt. Met. Sonderband 38 (2006) 107 
during cutting and grinding and the removal debris accumulated in pores during the 
polishing steps. Since true porosity of pressed and sintered steels becomes visible only 
and pores after sufficiently long polishing with small grain size diamond, the relevant preparation 
s exhibits a steps are repeated until no significant changes in visual porosity can be observed 
y darker in microscopically [3]. This procedure allows evaluating image preparation by supplying a 
e between sample which is defined as correctly prepared by the operator. However, this method 
amount of introduces misinterpretation by the operator as a new source of systematic error. 
enting the Statistical errors are caused by the statistical distribution of porosity in the material. 
3m dark to Several samples of the same material should be examined to avoid incorrect results since 
antification porosity visible on the sample surface could deviate from the average porosity of the 
method for material. Since pores are also statistically distributed over the sample surface, several 
pores and pictures of the same sample are necessary to reliably quantify porosity. From standard 
ent of the deviation and the number of pictures taken, intervals of confidence for the results can be 
r porosity calculated. Vice versa the number of images necessary for a certain size of the interval of 
d sintered confidence can be estimated from a reasonably well known standard deviation. 
Magnification is a source of both statistical and systematic error. At large magnifications 
fewer pores are visible on each image, the standard deviation of the images increases. At 
small magnifications standard deviation is comparatively small, but the systematic error in 
quantification increases. The boundary area between matrix and pores is larger compared 
to the overall pore area, since the mean pore area is the same as at larger magnifications. 
but the overall pore perimeter increases 
2.1 EXAMINED MATERIALS 
A sintered steel was prepared at three different density levels from a powder mixture of a 
water atomised plain iron powder (ASC100.29) with 0,80 mass% graphite (Kropfmuhl UF4) 
0,5 mass% of pressing lubricant (HWC) were added. The samples were pressed at 300, 
500 and 800 MPa to obtain different density levels (dimensions approx. 100 x 12 x 9,2 to 
10,4 mm). Green densities were determined geometrically from sample volume and mass. 
After a separate dewaxing operation to remove the lubricant, the samples were sintered at 
1180°C for 40 min in high purity N2 (99.999%) and slowly cooled to room temperature to 
ensure a pearlitic microstructure. The carbon content after sintering was determined by 
combustion analysis as 0,78 mass%. The sintered density p, was measured after 
Archimedes by weighing samples in air and in water. The samples were impregnated prior 
; to density measurements with a commercial impregnating spray for leather, to avoid water 
steel with gs . . . . 
x and matrix- infiltration of surface pores. Theoretical density p, was calculated (assuming that all 
70:128 pixel carbon is present as cementite) after 
ti 1 mg We (1) 
| statistical Dr Pr De 
ation [2]. A 
moans With p,, density of plain iron at room temperature (7.87 g.cm™), p, density of cementite at 
checken room temperature (7.69 g.cm®), m, mass fraction of iron and m, mass fraction of 
ial with the cementite. Porosity ¢ was calculated after 
ainst each 
sources of el%] = (1-25). 100 
‘he aim of Pm, 
r deformed 
(2}