Solidification and solid state transformations during cooling of chromium-molybdenum white cast irons
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I.
INTRODUCTION
Table I.
Composition of Alloys in Wt Pct (Fe Soide)
FoR massive
castings in industrial applications, high chromium white irons have been extensively used. Since 1930, these alloys are currently chosen when good abrasive wear resistance is required. The use of these steels has been enlarged in recent years: cold work tools, mill liners, pulverizer rolls, slurry pumps, impellers, and rolling mill rolls. Optimization of the alloy compositions with additional elements (in particular, Mo) and heat treatments to improve mechanical properties have been extensively studied. In 1973 a whole colloquium was devoted to this subject. These white irons are used either in the as-cast state with an austenitic metastable matrix or after different heat treatments. For economical reasons, considerable experience has been accumulated in the last decade. Particularly we should mention the studies from Gundlach, ~ Cias, 2 Parks, 3 and more recently Gahr and Doane 4 concerning the microstructure in correlation with the mechanical properties. The microstructure of the alloys we investigated has been previously studied by Maratray. 5'6 Our purpose is to investigate more particularly the nature and the distribution of the phases and the solidification paths. Besides the usual methods for characterization we have used the method of unidirectional solidification interrupted by quenching 7 to emphasize the structure and facilitate quantitative analysis of the phases.
II.
EXPERIMENTAL
PROCEDURE
The compositions of the alloys are reported in Table I and on the Jackson's representation of the Fe-Cr-C system (Figure 1). Rod alloys (8 mm diameter) are directionally solidified then quenched during cooling to delineate the J. D.B. DeMELLO, M. DURAND-CHARRE, and S. HAMARTHIBAULT are all with lnstitut National Polytechnique de GrenobleLaboratoire de Thermodynamique et Physico-Chimie M6tallurgiques (L. A. 29)--E. N. S. E. E. G. DomaineUniversitaireB. R44, 38401 Saint Martin d'H~res, France. Manuscript submitted October 19, 1982.
METALLURGICALTRANSACTIONSA
Element Alloy 21 3 4 5 6 7 8 9 10
C
Cr
Mo
Mn
Si
1.33 1.92 2.74 3.26 3.91 0.92 1.65 2.36 2.74 3.38
6.80 9.65 12.45 16.10 18.75 11.35 16.05 21.40 26.60 32.10
2.38 3.25 3.84 3.14 3.00 2.95 3.20 2.98 2.94 3.01
0.74 1.04 0.92 1.04 1.10 0.74 1.03 1.04 0.98 1.07
0.23 0.45 0.22 0.45 0.61 0.33 0.56 0.54 0.54 0.55
3
4
5%C
%Cr 50
40
30
20
10
0
1
2
Fig. 1--Compositions of alloys are reported on the liquidus surfaceof the Fe-Cr-C system according to Jackson) I: Cr/C = 5, II: Cr/C = 10. Crystallization paths of some alloys are indicated by the arrows.
VOLUME14A,SEPTEMBER1983--1793
solid-liquid interface. A rather slow freezing rate was chosen (2.35 cm per hour) so as to obtain large structures which are easier to observe and analyze. Thermal differential analysis (DTA) was performed for each alloy on 2 g specimens. The DTA apparatus was also employed to program cooling at rates ranging from 60 to 1000 ~ per hour. All the specimens were optically observed after poli
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