Post on 26-Feb-2018
7/25/2019 UNS S 32760_7
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Celso Antonio Barbosa e Alexandre Sokolowsk
REM: R. Esc. Minas, Ouro Preto, 66(2), 201-208, abr. jun. | 2013
Steel Cr 2N GBP (%)
Conventional 0.84 0.14
Rebalanced 0.01
SteelCPT Critical Pitting Temperature (C)
Individual Values AverageConventional 53 55 60 56
Rebalanced 73 85 - 76
Table 6Critical pitting temperatures values
determined using potentiostatic methodof the conventional and rebalanced steels.
Table 7Amount of Cr
2N precipitated at ferrite
grain boundaries in the conventional andrebalanced steels.
Figure 13
Ferrite and Austenite PREN number
for the Conventional and Rebalanced
steel calculated by Thermo-Calc.
Figure 12Effect of the amount of Cr
2N inter-
granular precipitation on the CriticalPitting Temperature-CPT determined bypotentiostatic method (average values).
precipitation in the conventional steel,we evaluated the amount of precipitatesin both steels. Table 7 shows the Cr
2N
percentages found. The rebalanced steelshowed no evidence of Cr
2N precipitation
in ferrite grain boundaries both in LOMas well as in SEM.
The relationship between Cr2N pre-
cipitation in the ferrite grain boundaries
and CPT can be seen in Figure 12. Thereduction of CPT temperature near to50C explains why the G-48 test devel-oped a high pitting level in the corrosionspecimens, as showed in Figure 4 and aninacceptable mass loss, Table 3.
The PREN can be calculated us-ing the Thermo-Calc simulation thatgives us the chemical compositions ofthe equilibrium phases present in the
steel at each temperature. The chemicalcomposition of austenite and ferrite werecalculated in three different temperaturesincluding the recommended solutionannealing temperature for this steelgrade of 1120C. Figure 13 shows thecalculation results for the temperaturesof 1050, 1120 and 1200C. The firstobservation is that the austenite PREN
is lower than ferrite PREN and tendsto reach the ferrite number only at hightemperatures, near 1200C. Second, therebalanced steel shows a PREN numberboth in austenite as in ferrite higher thanthe conventional steel.
As Thermo-Calc gives also thephase molar fraction, we have alsoestimated the PREN total of each steelcomposition, considering only the aus-
tenite and ferrite phases. The calculatedtotal PREN value is higher than 40 a1120C for the rebalanced steel, Figur14, showing that the compositionachanges introduced led to a good combination of stronger corrosion resistanaustenite and ferrite in a proper balanceincreasing the corrosion resistance of threbalanced steel.
In the Table 8 we can observe thimprovement in toughness as evaluated bythe absorbed impact energy. It is interesting to notice that the minimum requiredvalue was obtained even in the conventional steel that failed in corrosion, showingthat the improper austenite/ferrite balancand chromium nitride precipitation didnot deteriorate the toughness to a leveunder the specified values.
45
55
65
75
85
0,00 0,01 0,10 1,00 10,00
IP (%)
CPT
(C)
1040
33
34
35
36
37
38
39
40
41
42
43
44
1060
Temperature (C)
P
RE
N
1080 1100 1120 1140 1160 1180 1200 1220
Rebalanced Ferrite
Conventional Ferrite
Conventional Austenite
Rebalanced Austenite