306 Prakt. Met. Sonderband 38 (2006) 
tempering at 650 °C for 5 hours [2]. After each treatment, a sample was extracted for its Fic 
metallographic inspection and subsequent micrographic study, Fig. 8. qu 
All the samples for metallographic inspection were mechanically ground, and thereafter ex 
polished with 6, 1 and 0.25 um diamond paste and etched with nital-2 % reagent [3]. The the 
samples were observed in a Nikon Epiphot metallographic bench and the micrographs pre 
recorded with a Sony digital viewing and recording system. in | 
Hardness tests were performed on the upper and lower faces of poleshoes “5”, “9”, and 
“10” after oil quenching and after tempering. Likewise, hardness tests were conducted on Tal 
samples extracted from poleshoes “A” and “B” after each thermal treatment. an 
est 
3. RESULTS AND DISCUSSION Fig 
col 
Table | shows the values obtained in the chemical analysis. Note that poleshoe “5” does sol 
not comply with the specification in terms of %C, which may favor crack formation during sal 
quenching and influence the final microstructure, favoring the presence of retained ma 
austenite [4]. Poleshoe “9” presents a high content in S, though within the specifications of [6]. 
the standard, which is indicative of deficient desulphurization. As regards Al, it should be giv 
noted that it presents the lowest value in all the poleshoes, an indication of weak tim 
deoxidation. It should also be added that almost half the total aluminum in the sample is agl 
present in the inclusionary mode (= 42.6 %). The compositions of poleshoes “10”, “A” and glo 
“B” comply relatively well with the aforementioned specifications. x 
Table Ill shows the values of the critical transformation temperatures A; and A. calculated tou 
according to the formulation proposed by Andrews with regard to the austenization and SIZ 
maximum tempering temperature treatments respectively. The martensite start and finish in \ 
temperatures Ms and M; were calculated according to the corrected version of Steven's be 
formulation made by Irving, which served for the determination of the temperature interval qu 
of martensite formation in the quenching, as well as to a rough estimate of the crack . 
susceptibility in quenching. Additionally, the ideal critical diameters, De, were determined Fig 
for a 99% martensitic transformation according to the expressions of Hollomon and Jaffe tre 
[5]. It seems reasonable to consider a common austenization temperature of 880 °C, thus MI 
ensuring that tempering at 650 °C supposes a margin of some 120 °C below the eutectoid Sp: 
temperature. Poleshoe “5” presents a low Ms temperature; lower than those of the other bot 
poleshoes, which favors quench cracking due to the increase in the thickness of the piece inte 
[6]. This condition, combined with an M; value that is also low, and bearing in mind that the So 
quenching operation is carried out in a low-severity medium (Hoi = 0.5), allows us to PIC 
guarantee that the stability of retained austenite will be favored during cooling, with the hid 
possible appearance of appreciable proportions of retained austenite at room temperature ho! 
[4]. The transformation of the martensite and the retained austenite during tempering gives an) 
different constituents: Martensite—o+Cem, and from Yret — Upper plus lower bainite. The etc 
above combination of microstructures may be responsible for poor toughness behavior. 
The high value of the pearlitic quenchability found in the 5 studied components guarantees 
the absence of this constituent. Poleshoes “10” and “B” present a lower value of 4. 
Deisoe (bainitic), indicating a lesser tendency to reach high quenching depths. Th 
4 
Figs. 1 and 2 show the transformation curves corresponding to poleshoes “A” and “B”. lie 
Figs. 3 and 4 enable us to ascertain the final structure in both materials as a function of the uni 
cooling rate to which material is subjected [7]. PO 
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