Chemical and Petroleum Engineering, VoL 33, No. 5, 1997

MATERIALS SCIENCE AND CORROSION PROTECTION

C U R R E N T S T A T E O F T H E W E L D I N G O F T W O - P H A S E

C O R R O S I O N - R E S I S T A N T S T E E L S

K. A. Yushehenko, A. K. Avdeeva, and Yu. N. Kakhovskii UDC 621.791:669.15-194

Austenitic-ferritic steels have already been studied fairly extensively [1] as part of an effort to conserve nickel and use these steels in place of anstenitic steels with an 18% chromium content and 10-12% nickel content. Interest in these steels has risen again in recent years in connection with their use as structural materials that not only have high strength, but most importantly are resistant to corrosion and corrosion cracking - including in media with a high concentration of hydrogen sulfide. It was noted in [2] and several other studies that austenitic-ferritic steels are widely used in the chemical, petrochemical, and gas industries, power generation, and the construction of offshore oil platforms. Their range of application could be broadened even further, since efficient methods exist for the production of steels with high contents of alloying elements. This applies in particular to chromium.

Although a large number of grades of austenitic-ferritic steels with different degrees of alloying (Table 1) and different physicomechanical properties (Table 2) have been developed in different countries, a common approach can be taken to solving problems related to their welding: ensure that the metal of welds and heat-affected zones (HAZs) is resistant to the formation of embrittling phases (a-phase, carbonitride phase, etc.), that the proportions of ferrite and austenite in the HAZ vary as little

as possible from the analogous proportions in the base metal, and that the weld is stable against the formation of cracks and voids.

The steels being examined here can be divided into four groups (with respect to the alloying system): 20-22 % C r - 5 % Ni, with stabilization by titanium and niobium; 20-22% C r - 6 % N i - 2 % Mo, with stabilization by titanium; 23-25% Cr-6-8%

N i -3% Mo, with alloying by nitrogen, copper, and tungsten; 28% C r - < 10% N i -2 % Mo, with alloying by nitrogen and copper.

As a rule, two-phase steels have a low content of carbon (roughly 0.02-0.03%). When carbon content is higher, the

steels must be stabilized by titanium and niobium. The phase diagram of these steels permks only an approximate evaluation of their phase composition as a function of

the concentration of ferrite-forming (Creqv) and austenite-forming (Nieqv) elements. Ref'med phase diagrams of the weld metal can be used for relatively thin welds. When weld thickness increases to more than 10-12 mm and when welding is done in multiple passes, secondary heating begins to have a significant effect on phase redistribution both in the weld and in the HAZ.

These changes in phase composition are due to the extremely low structural stability of austenite and ferrite. Martensitic transformations can occur in austenite, while the a-phase (FeCr), the a-phase (Fe36Cr12Mo), or secondary austenite can form in ferrite [3]. Thus, special attention should be given to problems connected with attaining a relatively stable phase composition. Figure 1 shows the microstructure of the metal of an HAZ and a weld of several two-phase steels differing in the contents of

the main alloying elements and, thus, in phase composkion. Studies of steels of the system 20-22% Cr-3-10% Ni conducted by the E. O. Paton Institute of Electric Welding over

several years [4-7] have made it possible to establish several principles regarding welds. The most important of these principles are as follows.

Even a small change in the proportions of Creq v and Nieq v leads to qualitative and quantitative changes in grain size, the relative amounts of the 3'- and a-phases, redistribution of the alloying elements, an improvement in the stability of the austenite and ferrite, and a corresponding improvement in the properties of the metal. Fi~mare 2 shows the effect of nickel content and the phase ratio on these characteristics.

Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 5, pp. 81-83, September-October, 1997.

0009-2355/97/3305-0595518.00 �9 Plenum Publishing Corporation 595

1 t 3 �9

a)

I 2 3" q // mll b)

Fig. 1. Microstructure of the metal of welds of austenitic-

ferritic steels with different contents of the a-phase (x 200): a) HAZ: 1) 45%; 2) 67%; 3) 80%; 4) 98%; b) weld: 1)

10%; 2) 30%; 3) 60%; 4) 90%.

~ , / m l

. Ferrite

xcv,,Ilcm 2

.b " / \ < 6t ~ :

~' . , / .

80

50

~.0

ZO

0 , 6 a ,o .i.~,.

4t 5 ~r$ 27 i ~ i t ~ ~ I I a ) 05 7~ ~I ~ ~r6 27 t5 3,5 at,%

b)

Fig. 2. Effect of nickel content (change in the content of the a-phase): a) on the size d of the grains of austenite and ferrite; b) on the mechanical properties of weld metal

containing 21-22% chromium.

The stability of austenite against martensitic transformation and, thus, the attainment of an acceptable level of cold- shortness in the metal (in the absence of embrittling phases) are realized when the nickel content of the T-phase is greater than

6.5%. For the given composition, this level of concentration corresponds to a phase ratio close to unity. With the T- and a- phases present in such a proportion, the weld metal is more stable against the formation of hot cracks (see Fig. 3). The optimum relationship between the strength and ductility of the welds also depends on the concentration of the T- and c~-phases; the best combination is attained when they are present in roughly equal amounts (see Fig. 2b).

The stability of ferrite requires special study, especially for steels alloyed with molybdenum, vanadium, silicon, niobium, and titanium. These elements significantly reduce the phase stability of ,~- and 8-ferrite.

The a-phase may be formed as a result of redistribution of the alloying elements - particularly chromium and molybdenum - in austenitic-ferritic steel in connection with the development of interphase chemical heterogeneity.

The second generation of austenitic-ferritic steels, characterized by high contents of chromium ( > 24%), molybdenum (> 3 %), and nitrogen ( > 0.2%), is distinguished by a high degree of chemical heterogeneity. Thus, the probability of formation of a a-phase increases especially in welds alloyed with nitrogen, where the phase ratio deviates from unity. In this case, the

phase imbalance is due to the fact that nitrogen, being an austenite-former, increases the concentration of T-phase in the structure, increases the amount of chromium in the ferrite, and thus, increases the solubility of nitrogen in the ferrite and slows the formation of nitrides of chromium. The process of formation of the a-phase is activated under such conditions. The a-phase

596

TABLE 1

Grade of steel <C Cr Ni

12Kh21N5T 0,12 22 5,5

08Kh22N6T 0,08 22 6,0

03Kh23N6 0,03 22 6,0

08Kh21N6M2T 0,08 22 5.5

03Kh22N6 0,03 22 5,5

UR-45N

1.4~2/RREZ Falc 223 I SM 22C

Chemical compositio n, %

Mo N Ti Cu W

- - - - 0 , 4 - - - -

- - - - 0 , 4 - - - -

2,0 -- 0.3 -- --

2.0 . . . .

K 2205 0,03 22 5,5 3,0 0,14

] Remaint 4462

'US 22 SAF 2304 0,03 23 4,0 -- 0~40 --

U R 35 0r03 23 4,0 -- -- 0,12

AL 2205 0,03 23 5,0 3.0 0,14 0,15

Zerron 25 0,03 25 4r0 2,5 0,15

AISI 329 0,08 25 5,0 1,0 0,15

Zerron 25 0,03 25 4,0 2,5 0,15 0,16

UR 47 N 0,03 25 7,0 3,0 0,16 0,16

UR 52 N 0,03 25 7,0 3,0 0,16 0,20

DRZ 0,03 25 6r5 3,0 0r2 0r28

SAF 5207 0,03 25 7,0 4r0 0,28 0~25

Zerron 100 0~03 25 6,5 3~7 0,25 0,25

Falc I00 0,03 25 7,0 3,5 0,25 0r20

SUS 329 0,03 25 6,0 3,0 0,20 0,20

FMN 0,04 25 5,0 8,0 0,20 0,37

A 905 0,03 25,5 3,7 2,3 0,37 0,26

UR 52N 0,03 25 6,0 3,8 0,26

Uranus 50 0,05 26 5,0 2,0 0,25

9.4452S 0,03 26 7,0 3,5 0,25 0,17

Fertalium 255 0,04 26 5,5 3,3 0,17 0,20

Remaint 4460 0,08 26 4,0 2,0 0,20

7-Mo Plius 0,03 27,5 4,5 1,5 0,25

Ferralium 288 0,08 27,5 7,0 _ 2,0 0,20

04Kh33N7 0,04 33,0 7,5

- Manufacturing Mn Si country

-- -- [CIS (Commonweal th - - of - - Independent States)

France Germany Japan

Switzerland

Japan

Germany

Switzerland

France

I,O U~S. U.S.

2,0 England

France

England

1,o I,O Switzerland

England Germany

Iapan

England

I,O 1.o Austr ia

5,8 France

Germany

U.S.

Germany

U.S.

0,2

1,5

0,5 0,3

0fl 0,7

0,7 0,7

1,5

3,0

0,6

2,0

I

1,0

Its.s.

TABLE 2

Steel alloying system*

Cr 22; Ni (5--6)

Cr 22; Ni (5--6); Ti 0.4

Cr 22; Ni (5--6)

Mo 2; Ti 0 3

Cr 22; Ni (5--6);

Mo (2--3); N 0,14

Cr 23; Ni 4; N 0,14

Cr 24; Ni 6; Mo 2

Cr 25; Ni (4---7)

Mo (2--4); N (0,1--0,25)

Cr 26; Ni (5--7)

Mo (2--3,5); N 0,2

Cr 33; Ni 7,5

Mechanical properties

ac , MPa a,~, MPa 650--720 450--500

650--760 450--550 680--780

680--840 -450---570

600 400

760--840 500--700

730--850 500--680

700--830 490--600

Not

25--30

20--25

25--30

25

30--40

25--35

25--35

*The content of the elements is shown in %.

597

8

6

4

2

%r, ram/rain

0 fO ZO JO ~0 50 GO 70 80 a,%

Fig. 3. Effect of the ferritic o~-phase on the critical rate ~cr of deformation of the austenitic-ferritic metal of a weld with a content of 20-22% Cr: I, II) presence and absence of

cracks, respectively; the dashed region represents the

unstable state.

,k, g/(m2n)

j, rr~crn ~

3O I

r

t / t ~ / /

/ ' \ /

J . / A

k . . . .

0 2 ~ 6 8 10 o~;~.,

Fig. 4. Effect of the relative surface areas of ferrite and austenite electrodes, working in 56 % boiling HNO 3, on the

density of the corrosion current ( - - - ) and corrosion rate

~'e (~): I) juVk = f(~'); 2) juvk = riot).

is seen immediately after the argon-arc welding of nitrogen-alloyed (0.3 %) chromium-nickel-molybdenum welds made in

one pass with a nonconsumable electrode and filler metal. The formation of the a-phase in the weld metal and the HAZ is facilitated by the addition of heat to the metal during

welding (linear energy) and a steady cooling rate. Optimizing these factors can help reduce the deviation of the phase ratio from unity and keep less of the a-phase from being formed.

Phase composition has a particularly strong effect on the corrosion resistance of the metal. While being resistant to corrosion cracking, steels of the given class have a tendency to undergo structure-sensitive corrosion. In this case, austenite

or ferrite undergo selective dissolution, depending on the composition of the corrosive medium. The appearance of such corro- sion is influenced by the proportions of the -y- and c~-phases. Exigeriments performed by the method described in [6] have

conf~med that the sizes of these phases (the corresponding areas of the surfaces in contact with the medium) affect the electrochemical processes that take place at the microscopic level (between grains) and the selective dissolution of the phases

(Fig. 4). These principles are probably observed by all austenitic-ferritic steels, including conventional low-carbon steels and

low-carbon steels alloyed with nitrogen. Thus, it becomes possible not orily to establish a general approach to developing new welding materials and new steels that can be easily welded, but also to develop new welding processes. Each type (in relation

to Creq v and Nieqv) of steel will undoubtedly require materials developed especially for it and special welding methods and techniques. Here, it is important to maintain control over the welding heat and cooling rate and the conditions under which

the specified composition of the metal is obtained.

598

REFERENCES

1.

2.

3.

4.

5.

6.

7.

A. A. Babakov and M. V. Pridantsev, Corrosion-Resistant Steels and Alloys [in Russian], MetaUurgiya, Moscow (1971). Conf. "Duplex failless steels-91," Oct. 28-30, 1991, Beaune, Bourgogne. Editions de Physique (1991). L. Nassau, H. Meelker, and I. Hilres, "Welding duplex and super-duplex stainless steel - a gaaide for industry-ll,"

Dos. 11W 11-C-891-91, IX-H-244-91. N. I. Kakhovskii and K. A. Yushchenko, "Effect of welding heat on the microstructure and properties of steels of the

types 21-3 and 21-5," Avtom. Svarka, No. 10, 15-25 (1963). N. I. Kakhovskii and K. A. Yushchenko, "Effect of ferrite-forming elements on the properties of chromium-nickel

welds of the types 20-5 and 20-7," Avtom. Svarka, No. 10, 35-40 (1964). N. I. Kakhovskii, N. A. Langer, K. A. Yushchenko, and G. I. Chalyuk, "Electrochemical properties of welds of austenitic-ferritic chromium-nickel steels of the type 21-5,* Avtom. Svarka, No. 12, 30-37 (1964). N. I. Kakhovskii and K. A. Yushchenko, "Effect of nickel on the microstructure and properties of welds with 20-22 %

chromium," Avtom. Svarka, No. 5, 26-30 (1966).

599