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