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The sensitization of ferritic steels containing less than 12% Cr
Corrosion Science, 1978, Vol. 18, pp. 371 to 395. Pergamon Press. Printed in Great Britain
THE SENSITIZATION OF FERRITIC STEELS CONTAINING LESS THAN 12% Cr* BRYAN POULSON Materials Research, A.T.D., Clarke Chapman Limited, Gateshead, England Abstract--The susceptibility of a range of ferritic Cr Mo steels to sensitization during welding has been investigated. No localized attack occurred in any of the standard test solutions used to assess susceptibility to intergranular corrosion. However potentiostatic etching in 16 wtYoH2SO4 at 18°C produced narrow trenching close to the heat affected zone-parent metal interface in the 12~.Cr 1%Mo and 9 ~oCr 1 ~oMo steels. The thermal history and the metallurgical structure of the attacked region have been examined. The effect of post weld heat treatment on the occurrence of localized attack has been studied and correlated with hardness changes. Analytical and electrochemical measurements have been used to test the various theories proposed to explain intergranular corrosion in ferritic materials. Finally the implications of those findings are considered. INTRODUCTION INTERGRANULAR CORROSION(IGC) is a well known problem with a wide range of Fe and Ni base alloys. 1'2 These include the austenitic and ferritic stainless steels and the Ni based alloys such as Incoloy 800 and lnconel 600. Recently it has been found that materials susceptible to IGC, i.e. sensitized materials, can suffer intergranular stress corrosion cracking (s.c.c.)in high temperature high purity water; 3 in this environment the general corrosion rates are low and in the absence of stress I G C does not occur. A recent summary of s.c.c, failures in the nuclear industry 4 indicated that at least 25 ~o of failures were associated with sensitized materials. Another class of materials used in the power generating industry are the C r - M o ferritic steels. These steels were developed when the desire for increased efficiency required steels with better creep and oxidation behaviour than the carbon steels then in use. The 5 ~oCr 1 ~oMo and the 9 ~oCr 1 ~oMo steels are also used in the petrochemical industries to meet specific corrosion requirements, particularly resistance to sulphur attack. However their resistance to localized corrosion, such as I G C , has not been extensively examined. This probably stems from the view that at least 1 2 ~ C r is needed to produce passivity. 5 Alloys containing less than 12~oCr will not be passive and will behave like iron. In particular they will not develop an active-passive situation, as a result of chromium depletion at the grain boundaries, with the resultant rapid attack there. However, in some environments steels containing less than 12 ~ C r are passive. For example the successful operation of boilers depends on a thin layer of magnetite protecting the steel. 6 There are, however, more than theoretical reasons for suspecting I G C could be a problem with steels containing less than 12~oCr. For example, intergranular oxide penetration has been observed on a 9 ~oCr I ~oMo steel, after exposure to what was probably a caustic environment. 7 Standard immersion tests have been developed to assess a materials degree of sensitization, 1,2 i.e. its susceptibility to I G C in an environment which produces this *Manuscript received 25 August 1976; received for publication 28 July 1977. 371
372
B. POULSON
f o r m o f attack. These tests are time consuming, destructive an d n o t simply quantifiable in all cases. Th e effect o f heat t r e a t m e n t on susceptibility to I G C is usually presented in the f o r m o f t i m e - t e m p e r a t u r e - s e n s i t i z a t i o n diagrams. While these are u n d o u b t e d l y useful they are i n a d e q u a t e for m a n y purposes. T h e y cannot, for example, be used to predict the possibility o f sensitization during welding, or its relief during post weld heat treatment. F u r t h e r m o r e they are unsatisfactory in assessing the possibility o f sensitization during solution annealing treatments. Bearing these points in mind this investigation was intended to: 1. E x a m i n e the susceptibility o f steels c o n t a i n i n g i 2 % C r or less to I G C , particularly in relation to welding. 2. I f possible develop a rapid electrochemical test for assessing the possibility o f I G C . 3. E x a m i n e the effects o f post weld heat treatments on any tendency towards localized attack. EXPERIMENTAL METHOD A series of Cr Mo ferritic steels was obtained (Table l) and welds were prepared according to the procedures outlined in Table 2. The standard half hour post weld heat treatment at various temperatures was carried out in an air furnace. The specimens were thus slowly heated and slowly cooled. Other shorter post weld heat treatments were performed in a salt bath and the specimens waterquenched, The non-welded specimens were quenched into iced brine after half hour at 950°C in an argon atmosphere, and tempered in a salt bath. Jominy end quench tests were performed in the specified way? TABLE 1. Designation M.S. Tube 1Cr Tube 2¼Cr Tube 5Cr Tube 9Cr Tube 9Cr Rod 9Cr Spacers 12Cr Tube
MANUFACTURERS'ANALYSISOF STEELSUSED Composition P Ni
C
Ma
Si
S
0.08 0.1 0.12 0.1 0.1 0.1 0.1 0.19
0.58 0.57 0.56 0.46 0.47 0.43 0.45 0.52
0.15 0.26 0.27 0.27 0.44 0.59 0.53 0.45
0.03 0.024 0.024 0.004 0.017 0.008 0.011 0.01
TABLE2. Material M.S. Tube 1Cr Tube 2~42r Tube 5Cr Tube 9Cr Tube 9Cr Rod 9Cr Spacers 12Cr Tube
0.04 0.021 0.016 0.013 0.017 0.014 0.009 0.01
. 0.04 ----0.1 0.51
.
Cr
Mo
others
. 0.85 2.24 4.97 9.07 8.55 8.9 11.7
. 0.49 0.98 0.56 1 0.99 0.97 1.02
------0.33V
OUTLINEOF WELDINGPROCEDURES Process Production orbital TIG Production orbital TIG Production orbital TIG Experimental manual autogenous TIG Production orbital TIG Experimental autogenous orbital TIG Experimental autogenous linear TIG Experimental autogenous orbital plasma
Specimens approx. 1.5 in. long and 0.25 in. wide (containing a section through the weld) were cut from the welded tubes. The bore surface was then polished to grade 600 emery or finer. Unwelded samples approximately 4 cms in area were cut from the tube or rod and polished to grade 600 emery or better. All specimens were drilled and tapped to fit a Stern-Makrides Electrode Holder) Solutions (summarized in Table 3) were prepared from laboratory grade chemicals and deionized water.
The sensitization of ferritic steels containing less than 12 yoCr TABLE 3.
Solution
SUMMARYOF SOLUTIONSUSED
Composition
Acid ferric sulphate
50 wt ~H~SO4 + 25 g/l Fe~(SO~)a 16 wt ~oHsSO~ + 100 g/l CuSO, + Cu 65 wt ~.HNOa 10 wt ~oH2CsO4.2H~O
Acid copper, copper sulphate Nitric acid Oxalic acid
373
Temperature Boiling Boiling Boiling 18°C
Standard immersion tests, all constant potential tests and electrochemical measurements were performed in a multi-necked 1 1. flat bottomed flask. All potentials were measured using a saturated calomel electrode (SCE) and are reported on this scale. Polarization curves were obtained using a linear sweep generator (Chemical Electronics) to drive a potentiostat (Wenking Model ST72), the current being recorded on a chart recorder (Smiths Servoscribe) by manual switching between ranges. After a number of the constant potential tests the solution was analysed for Fe and Cr by atomic adsorption spectroscopy, a Hilger Watts instrument being used. All specimens for examination in the SEM were ultrasonically cleaned in methanol directly the test finished and stored until required. E X P E R I M E N T A L RESULTS
Development of the test method There was no indication of localized attack after exposing as welded 9Cr tube samples to any of the standard test solutions used to detect susceptibility to IGC. In acid ferric sulphate, rapid hydrogen evolution occurred, the solution quickly changed colour from yellow to green, and the specimen dissolved rapidly. This is shown in Fig. l(a) where it c a n also be seen that the weld metal and the heat affected zone (HAZ) dissolved marginally more quickly than the parent metal. In the acid copper sulphate test, copper deposition occurred and irregular general dissolution took place beneath this layer (Fig. Ib). In nitric acid, little general and no localized dissolution appeared to have taken place (Fig. lc). In the oxalic acid etch, the weld metal remained almost unattacked, the H A Z was lightly etched and the parent metal over etched (Fig. 2). Another attempt to detect any tendency to suffer localized dissolution was by potantiostatic etching in 16 wt ~H2SOs at 18°C. When specimens were held at certain potentials localized hydrogen evolution was noticeable in the region of the H A Z parent metal interface and after 20 h deep narrow trenching had occurred there (Fig. ld).
The effect of potential and Cr content on the susceptibility to localized dissolution As welded samples of all the steels were exposed to the 16 Wt~oH~SO5 over a range of potentials. Localized trenching occurred only on the 12Cr and 9Cr steels. The effect of potential on the form of attack is summarized in Table 4, typical specimens are shown in Fig. 3. Figure 4 shows the very localized attack that occurs, in the region of the H A Z - p a r e n t metal interface, at potentials above the range promoting trenching, as indicated in Table 4. Although the 5Cr steel showed a tendency towards trenching--as evidenced by localized hydrogen evolution--this situation tended to be unstable. The specimen became active and dissolved rapidly.
374
B. POULSON
(o)
(b)
(c)
(d)
FIG. 1. Morphology of attack on 9Cr weld after exposure to solutions used to detect susceptibility to intergranular attack. (a) 15 rnin in acid ferric sulphate. (b) 3 h in acid, copper, copper sulphate. (c) 24 h in nitric acid. (d) 20 h in 16 wt~oH2SO4 at ÷ I00 mV SCE.
Effects of post weld heat treatment on the susceptibility o f 9Cr welds towards localized attack The manufacturers recommended post weld heat treatment ( P W H T ) varies between 730 and 800°C for times as short as 15 rain to as long as 60 min. 1°-1"- In some codes a m a x i m u m hardness is specified, e.g. 250 Hv. 1~ Clarke C h a p m a n have adopted a standard P W H T of 30 min at 710-760°C. After this treatment any tendency towards localized corrosion, in the present test, was removed and hardness values were below 265 Hv. To examine how critical heat treatment conditions are a series of P W H T were carried out. These covered times between 1 and 30 rain at temperatures from 400 to 750°C. The specimens were then potentiostatically etched at + 100 mV SCE in H~SO4 as previously described. The general trend o f results was that with increasing heat treatment time and temperature the region of the weld susceptible to attack altered in the following way:
FIG. 2.
Morphology of attack on 9Cr weld after oxalic acid etch.
FIG, 4.
Fine cracks and resultant surface (after fracturing) of 12Cr weld exposed at + 200 mV SCE for 100 h.
The sensitization of ferritic steels containing less than 12~,Cr
377
'a
(b)
(c)
(d}
(e)
FXG. 3. Effect of potential (SCE) on morphology of attack of 12Cr steel after 20h in 16 wt Y.H~S04. (a) + 200 mY; (b) 0; (c) -- 200 mY; (d) - 300 mY; (e) - 600 mY. Trenching at H A Z - p a r e n t metal region -> trenching + attack on H A Z --->attack on HAZ + some attack on weld metal ~ immunity to attack. Perhaps more important than the site of the attack is the severity of attack. In a simple way the results of the tests are summarized in Fig. 5 which shows the changing site of attack and Fig. 6 which illustrates the severity of the attack. This severity of attack is classified in a generalized way as follows: 1. Gross attack--when trenching or other visible attack has occurred. 2. Slight a t t a c k - - w h e n attack has occurred but is visible only metallographieally. 3. No attack--when examined metallographically at 400 ×. The important points to note are: (a) Inadequate P W H T can sensitize the entire HAZ. (b) Specimens heat treated for 30 min should be immune to localized attack as long as the temperature is above 700°C.
378
B. POULSON
TABLE 4. TVlE EFFECT OF POTENTIAL ON DISSOLUTION MORPHOLOGY OF WELDS IN 9 Yo AND 12 ~oCr STEEL
Potential (SEE), mV 300 250 200 150 100 50 0 -- 50 -- 100
- -
12y,Cr
9 ~Cr
t No attack
t No attack
I
I
1- very fine L attack
velocity of trenching - trenching plus 0.03 mm/h general corrosion 0.05 mm/h which increases
200
with decreasing potential
1-very fine L attack velocity of trenching I" trenching plus 0.13 mm/h | general corrosion [ which increases 0.09mm/h r with decreasing 0.04mm/h L potential
0.06 mm/h
-- 300
General corrosion
0.02 mm/h
--400
General corrosion
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FIG. 6. Effect of PWHT on the severity of attack on 9Cr weld potentiostatically etched in HsSO4. (c) Susceptibility to trenching disappears quickly at 750°C and specimens H.T. for 8 min showed no signs of localized attack. The morphology of the attack in these tests was both intergranular, producing grain fall out, and transgranular along the martensite laths, giving rise to a platelike structure. Examples of these different types of attack are shown in Fig. 7 which also includes an example of the type of non-localized attack after a P W H T of 30 min at 750ac. Although a detailed study of the factors affecting dissolution morphology was not made, the proportion of intergranular attack appeared to be greatest in the high temperature H A Z close to the weld metal.
FIG. 5.
Effect of PWHT on the pattern of attack on 9Cr weld potentiostatically etched in HsSO4. (a) 650°C for l rain; (b) 700°C for I rain; (c) 650°C for 30 rain.
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The sensitization of ferritic steels containing less than 12Y.Cr
381
Characterization of 9 ~oCr welds On as-welded specimens the region susceptible to trenching etched up, in Vilellas reagent, 14 slightly faster than the rest of the specimen. With specimens which had been given a P W H T in such a way as to sensitize the entire HAZ, this could not be ascertained from their etching characteristics. There were no mierosctructural features visible in the optical microscope which enabled sensitized materials to be recognized. Hardness profiles were obtained for each of the welds which were susceptible to trenching. The results are shown in Fig. 8 from which it appears that sensitization is associated with that region of the HAZ which has undergone partial transformation and resulted in some, but not maximum hardness increases.
600 _
12 Cr Tube
o/o/O
SO0
o~
4 0 0 -300
'''--°--'~
7-
07
oi
4 0 0 _ 9 Cr
o~
o ~
o--------o ~ ° - °
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300 200 400 _
9Cr
Rod
~ o ~ O
300 200 ~ 400 9
0
Cr Spacer J ° ~ o - ~ - - - _
o_
o
300 200
0-'
- a ~ I
Imm
]
FIG. 8. Hardness profiles across welds susceptible to trenching. Hatched region denotes area susceptible to trenching. Hardness profiles were also obtained on many of the 9Cr welds after different PWHT. Typical results, shown in Fig. 9, were chosen to include specimens grossly sensitized, slightly sensitized and immune. As a rough guide it seems that if the maximum hardness is greater than ca. 280 Hv some localized attack appears likely in the present test. However gross attack was usually associated with hardnesses > 325 Hv but it must be emphasized that the attack was not necessarily associated with the hardest region. Furthermore this hardness level, below which localized attack will not occur, will probably depend on the composition of the steel and needs to be examined for steels other than the 9Cr steel.
Reproduction of sensitization on non-welded samples A simple way of obtaining a large range of cooling rates on a single specimen is the Jominy end quench test. Such a test was performed on a 9Cr bar which had been austenitized for 1 h at 950°C. Hardness measurements along the bar indicated a
382
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FIG. 9. Effectof PWHT on hardness profiles across 9Cr tube welds. maximum value of 460 Hv and a minimum value of 420 Hv. After potentiostatic etching for 20 h at q- 100 mV(SCE) in 16 wt %HzSO4 no localized attack attributable to sensitization had occurred. It thus appears that rather slow cooling rates are require to induce sensitization in these hardenable ferritic steels. Specimens of 9Cr rod were austenitized for 1 h at 950°C quenched into iced brine and then tempered for times up to 16 min. These specimens were then potentiostatically etched in H2SO4 for 15 min and examined metallographically. Quenched samples remained bright and unattacked. Samples tempered for 1 min were heavily attacked, specimens tempered for longer times gradually regained their corrosion resistance. This is summarized in Fig. I0 which includes hardness measurements on each of the samples. Thus it appears that quenched samples are not sensitized but this rapidly occurs after only 1 min at 750°C and then gradually is removed till at 8 and 16 rain no localized attack could be observed.
Electrochemical measurements
Potentiodynamic anodic polarization curves (at 10 mV/min) in 16 wt%H2SO4 were obtained for each of the steels in the quenched condition. Figure 11 shows the portion of the polarization curves in the vicinity of the passivation potential, which is defined in the present investigation as the potential at which the current density decreases below 0.1 mA/cm 2. The effect of Cr additions on this passivation potential is shown in Fig. 12 which also includes previous work. z~,16 Reasonable agreement is evident and it is apparent that up to 5 %Cr there is only a small effect whereas at higher Cr levels large shifts in the passivation potential occur. Attempts were made to see if electrochemical measurements could detect sensitized welds. Potentiodynamic anodic polarization curves (at 100 mV/min) and current transients--at + 100 mV (SCE)--were obtained. Tests in which the entire HAZ and the weld metal had been sensitized (i.e. by a P W H T of 1 min at 750°C) did give indications that sensitized welds could be detected by rapid electrochemical measurements. However on as welded samples, where the attack is much more localized,
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The sensitization of ferritic steels containing less than 127oCr
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Potentiodynamic anodic polarization curves for quenched steels in 16 wt ~o HsSO~. Sweep rate 10 mV min starting from + 500 mV SCE.
700 4 0 0 &L--~ 0 ~ o • 300--
600
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Effect of Cr content on passivation potential.
sensitization could not be detected. Thus in general it appears unlikely that electrochemical measurements will be useful in detecting sensitization in welds where the actual region attacked is a small fraction of the total area. However electrochemical measurements on quenched, and quenched and tempered samples of 9Cr steel were more fruitful. Potentiodynamic anodic polarization curves are shown in Fig. 13. It can be seen that there are distinct differences between the samples. In particular sensitization causes the appearance of a secondary active peak and the current density of the primary active peak is higher than the values obtained from the quenched samples. Subsequent tempering reduces the magnitude of both primary and secondary peaks. Previous work on steels with 12% and higher Cr contents found very similar effects, which were attributed to Cr depleted regions. 17'1s Current transients (Fig. 14) at + I00 mV (SCE) also give indications of sensitized material in that the current increased for sensitized samples as opposed to nonsensitized samples for which there was a continuous decay.
386
B. POUI.~ON
200
QUENCHEDAND TEMPERED
Imin /
750°C
U
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o -200
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UENCHED. ' - ' ~ - - - ' ~ " ~ ~
-400 I
-000
I I IIIIll
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CURRENT DENSITY mA/cm~ Fla. 13.
Effect of heat treatment on potentiodynamic anodic polarization curves in 16 %HISO~.
IO
E
Quenched and Tempered 16m/750°C
c
d
II
o
FIG. 14.
I
Quenched
I I I I I I I I I I I I I
2
3 4
5
6 7 8 9 Time minutes
IO II
12 B
14
Effect of heat treatment on current transients (at + 100 mV SCE) on 9Cr in 16 wt ~,HISO4.
Analytical measurements After many of the constant potential exposures the solution was analysed for Cr and Fe. If it is assumed that the other elements dissolve stoichiometrically then since the composition of each alloy is known, an apparent Cr content of the material which has dissolved can be calculated from: apparent Cr ~o =
ppm Cr × 100 ppm Cr + ppm Fe + ppm other elements 1
The sensitization of ferritic steels containing less than 12%Cr
387
The results of such calculations are shown in Table 5. Since in a number of tests, grain fallout occurred and the entire grains subsequently dissolved these values will tend to be higher than the Cr content of depleted regions. Nevertheless the apparent Cr content of the dissolved material varies systematically with potential, being at a minimum at potentials causing trenching. What is not at present understood is why there are differences between the 9Cr and 12Cr steels in the correlation between actual Cr content of the alloy and maximum apparent Cr content. One possible suggestion is that some of the Cr is tied up as carbide and this does not dissolves in the 9Cr case but does in the 12Cr, the V additions possibly influencing its behaviour. I f it is assumed that all the carbon forms a carbide of the type Cr~ C5 this could account for 1.6 ~oCr in the 9Cr steel being tied up. TABLE5. SUMMARYOF ANALYTICALRESULTS Potential mV SCE in 16 wt %H~SO4 (mV)
-
-
-
-
-----
200 100 50 0 50 100 200 300 400 600
10~o Oxalic acid at 1A cm~
Apparent ~Cr of dissolved metal 12Cr steel 9Cr steel
10.5 9.4 8.5 9.2 10.4 11.9 --
6.6 6.4 4.9 5.3 4.8 5.8
15.8
It is also significant that the apparent Cr content after an oxalic acid etch is much higher than the Cr content of the alloy suggesting that carbides dissolve in this test. DISCUSSION
Mechanical hnplications Various theories have been proposed to explain the occurrence of intergranular attack on the higher Cr, non-hardenable, ferritic steels. These are basically one of two types: the first group suggests precipitates either dissolve readily 19 or cause adjacent material to corrode rapidly by either acting as local efficient cathodes z° or straining the matrix. 21 The second group of theories postulates that localized attack is due to regions near the grain boundary being depleted in Cr and dissolving preferentially. ~2 The general objections to mechanisms of the first kind are the difficulty in explaining how dissolution occurs between the carbide particles and why only steels containing more than 5 ~ C r are susceptible to sensitization. Specific objections to the noble efficient cathode carbide theory are: (a) That is it unlikely that a carbide can function as an efficient local cathode at anodic potentials in potentiostatic tests. (b) The unexplained limited range of potentials promoting attack.
388
B.
POULSON
(c) The increased resistance to attack after long tempering times with little apparent change in the carbide. The idea that the cause of attack is carbide dissolution would appear most improbable since the evidence of this and other investigations~ is that it does not at potentials promoting attack. Similarly models based on precipitates inducing strainenhanced dissolution would seem unlikely to be a first order effect. Reasons include the fact that maximum sensitization occurs before maximum carbide precipitation, that additions of Ti prevent sensitization of the higher Cr steels but there is no evidence that TiC strains the lattice less than carbides of the type M~ Ce2~ and finally there is scant evidence suggesting strain can increase the dissolution rate sufficiently to produce this attack. However the Cr depletion model makes several important predictions which c a n be tested: 1. Preferential attack should be limited to the potential range between the passivation potential of the matrix and the passivation potential of the Cr depleted zone. ss 2. Since with decreasing Cr contents, the difference between the passivation potential of the matrix and the depleted zone will get less, the potential range causing intergranular attack should decrease with decreasing Cr content. The minimum Cr content at which sensitization can occur will be that which causes a certain minimum difference between the passivation potentials of the matrix and the depleted zones. 3. If material depleted in Cr is dissolving this should be apparent from an analysis of the solution after a test. 24 Figure 15 shows the results of electrochemical and analytical measurements designed to test these predictions for the 12Cr steel. It can be seen that trenching occurs at potentials just into the passive range and there is a close correlation between trenching and the apparent Cr content of the dissolved material. It has previously been shown (see Table 4) that the 9Cr steel suffered localized attack over a narrower
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Correlation between anodJc polarization curve apparent Cr content and trenching behavJour for ]2Cr welds.
The sensitization of ferritic steels containing less than 12~Cr
389
range of potentials than did the 12Cr steel. All this data is exactly what is predicted by the Cr depletion model. Analysis of intergranular fracture surfaces of hydrogen embrittled sensitized and non-sensitized steels should provide direct evidence of Cr depletion. There are details of the mode of Cr depletion in the 9Cr and 12Cr steels which are different in a number of interesting ways to the sensitization of the austenitic or the ferritic stainless steels. During continuous cooling from the annealing temperature to room temperature no allotropic phase changes occur in either the ferritic or the austenitic stainless steels. Thus in the ferritic stainless steels carbide precipitation occurs from ferrite and in the austenitic stainless steels from austenite. Since the diffusion rates of Cr are much quicker in ferrite than in austenite this results in markedly different kinetics of sensitization. 1 Specifically, ferritie stainless steels can sensitize during rapid cooling while austenitic steels require much slower cooling rates to cause sensitization. The ferritic 9Cr and 12Cr steels investigated in this study are hardenable, this means that during cooling from the normalizing temperature the structure is austenitic until either the Ms temperature is reached or the austenite transforms to ferri~e and carbide. The transformation kinetics of the 9Cr steel have been investigated .5 and are summarized in Fig. 16. During continuous cooling from the normalizing temperature relatively slow rates of cooling will be required for carbide formation and thus sensitization. This is evidenced by the lack of any sensitization in the jominy specimen at cooling rates which would induce sensitization in the ferritic stainless steels. Thus the 9Cr and 12Cr are more austenitic like in the sensitization behaviour during continuous cooling,
Austtnitised
IOOC m
! hoUr at 9.50°C i 2
800
./
.---'~~ hour at IUOUt.
m
u o 60C
E
4OC
Ms Mf
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II
I
i
I0
I00 Time
FIG.
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I
I000
minutes
Continuous cooling transformation diagram for a 9~Cr I~Mo steel {Ref. 25).
Below the Ms temperature the structure o f the 9Cr and 12Cr steels will be martensitic and on subsequent tempering the possibility of sensitization will occur. In this case the kinetics of sensitization will be more like the ferritic stainless steels than the austenitic stainless steels, and relatively short times (i.e. 1 min at 750°C) are required to induce sensitization which is subsequently removed after c a . 8 min.
390
B. Pout.soN A u s t e n i t i c or f e r r i t i c nonhordenoble stainless s~reels
Ferrific
Cr Mo hordenoble steels ^
I
I
H
% Cr
Before weldir~
B f t e r welding
GB I Before weldin,
At pec~ temperature
After weldin
Fio. 17. Differencesin mode of sensitization during welding between hardenable and non hardenable Cr steels. There are similar important differences in the mechanism of sensitization during welding (Fig. 17). The initial structure of the 9Cr and 12Cr steels is ferrite plus carbide. During welding some carbide dissolution and reprecipitation must occur to induce sensitization which occurs in the H A Z - - a s it does in the austenitic steels. However during subsequent P W H T the ferritic-like behaviour ensues. Tiffs means that while all the HAZ and weld metal can be sensitized by very short P W H T times (i.e. 1 min at 750°C) the standard P W H T of 30 min at 750°C completely removes sensitization. This treatment would induce sensitization of an austenitic steel which would require very long times at 750°C before it was removed.
Practical implications It has been shown that ferri.tic steels containing less than 12~oCr, and possibly as little as 5 ~oCr, can be sensitized by certain treatments. As discussed previously, this sensitization is thought to be due to localized regions being depleted in Cr. This is analogous to the sensitization, weld decay, i.e. susceptibility to intergranular attack, which is a well-known problem with the austenitic stainless steels. In these hardenable ferritic steels it appears that material could be sensitized in a .number of ways: 1. During welding, a zone close to the parent metal-HAZ interface becomes sensitized. 2. Inadequate PWHT, the entire HAZ and weld metal can become sensitized. 3. Insufficient tempering after normalizing will result in sensitization. It is possible that material inadvertently tempered at too high a temperature (i.e. above the lower critical temperature) could be sensitized. 4. During cooling after annealing it appears possible that sensitization could result, if certain as yet undefined critical cooling rates are employed. There is also the chance of correctly heat treated material being sensitized in service: the first possibility is that a rapid thermal transient to above 805°C would induce sensitization. The second is that since the carbide solubility at the operating temperature is lower than at the tempering temperature, carbide precipitation will occur. This could possibly tend to reduce the corrosion/oxidation resistance and could cause mild sensitization. Sensitization is not invariably detrimental and sensitized austenitic stainless steels can be used in a number of environments. Figure 18 shows the region on an as-welded
Region susceptible to trenching on 9Cr weld, after 100 h exposure to inhibited citric acid containing dissolved oxygen. FiG. 19. Effect of weld on surface pitting of plain carbon steel after exposure to inhibited citric acid containing dissolved oxygen. F{G. 18.
The sensitization of ferritic steels containing less than 12~,Cr
393
sample of 9 ~oCr which is sensitized. In this case it had been exposed to inhibitedcitric acid for 100 h and very little localized attack has occurred. What is probably more important is how the degree of sensitization will effect the materials s.c.c, resistance. Thus Antil126 reported that as-welded 9Cr 1Mo steel capsules failed after approximately 4000 h with 3 ~oNaCl at 350°C and failed in less than 170 h with 20~oNaOH at 360°C. In both environments correctly heat treated capsules did not show any indications of cracking. Similarly Bignold z7 has tested solution annealed (at 1050°C) 9Cr 1Mo material and found it susceptible to transgranular s.c.c, in a chloride environment (0.5M MgCl2 + 0.5M NaCI) and intergranular s.c.c, in a complex hydroxide environment (0.5 NaOH + 0.5M NaCI + sat. Na,2SO4 + sat. Na~SiOa) both at elevated temperature. Bignold 27also indicated that it was difficult to reproduce the sensitization solution annealing treatment. This confirms earlier indications ~a that annealed 9Cr 1Mo material could be susceptible to s.c.c. As indicated previously there is a possibility that annealing 9Cr 1Mo material could sensitize it if certain rather critical cooling rates are employed. An alternative possibility is the quoted failures of 9Cr were caused by hydrogen embrittlement of the hard HAZ or solution annealed material. Recently it has been shown 29 that quenched 9Cr (Hv ,,, 460) can undergo hydrogen embrittlement in media as diverse as inhibited citric acid and 15 ~ N a O H at 250°C. The standard test solutions, used to detect IGC in stainless steels are not suitable for testing steels containing 12~oCr or less. A standard potentiostatic etching technique in 16 wt~H2SO4 at 18°C has been developed which satisfactorily detects sensitization in these steels. Some indication of the degree of sensitization of homogeneous samples can be obtained from electrochemical measurements, but with welds, due to the localized nature of attack, this does not seem possible. Clearly it is desirable that steels containing 12~oCr and 9~oCr should be heat treated after welding to remove any sensitization. For the 9Cr steel the manufacturers recommended P W H T appears quite adequate. From the results of varying PWHT for heat treatment times of 30 rain the temperature should be 700°C or above while at 750°C times as short as 8 min should remove sensitization. As an indication, values of hardness in excess of 280 Hv suggest that a weld in 9Cr material might be sensitized; the exact value will probably depend on the detailed composition. Finally it must be emphasized that it is unlikely that sensitization is the only cause of localized weld corrosion problem in the lower Cr ferritic hardenable steels. For example localized pitting in the HAZ of weld in plain carbon steels has occurred after exposure to inhibited HCI a° and welds have been shown to influence the occurrence of surface pitting in inhibited citric acids containing oxygen ~1 (Fig. 19). CONCLUSIONS I. As-welded 1 2 ~ C r l~oMo, 9~oCr 1 ~oMo and possibly 5~oCr ½~oMo steels are sensitized in the region of the parent metal-HAZ interface, during welding. This sensitization can be removed by suitable Post Weld Heat treatments, i.e. 30 m at temperatures over 700°C or times over 8 min at 750°C. Inadequate PWHT can result in the entire HAZ and weld metal becoming sensitized. 2. Standard test methods do not detect sensitization in steels containing less than 12~oCr. A suitable test is potentiostatic etching in 16 wt~oH~SO4 at potentials
394
3.
4.
5. 6.
B. POUI.,SON
j u s t in the passive range. Electrochemical m e a s u r e m e n t s c a n n o t detect local sensitization near welds b u t are m o s t useful in m e a s u r i n g the sensitization o f h o m o g e n e o u s samples. H a r d n e s s profiles across welds give a g o o d i n d i c a t i o n o f the presence o f sensitized material. F o r the 9Cr Z½Mo steel hardness levels a b o v e a b o u t 280 Hv are to be avoided. This will also minimize the risk o f h y d r o g e n embrittlement. T h e available evidence suggests t h a t the degree o f sensitization m u g h t be i m p o r t a n t in d e t e r m i n i n g the susceptibility to stress c o r r o s i o n cracking in certain environments. A n a l y t i c a l a n d electrochemical m e a s u r e m e n t s p r o v i d e s u p p o r t for the cause o f sensitization to be related to local regions which are d e p l e t e d in Cr. I m p o r t a n t differences between the sensitization process in the ferritic h a r d e n a b l e steels e x a m i n e d in this investigation a n d the ferritic a n d austenitic stainless steels a r e described. F e r r i t i e h a r d e n a b l e steels are "austenitic like" d u r i n g cooling f r o m the n o r m a l i z i n g t e m p e r a t u r e b u t "ferritic like" d u r i n g subsequent tempering. D u r i n g welding s o m e c a r b i d e dissolution m u s t occur p r i o r to r e p r e e i p i t a t i o n in h a r d e n a b l e steels for sensitization to occur.
Acknowledgements--The author thanks the management of Clarke Chapman Limited and the
C.E.G.B. who have been associated with this work for permission to publish the outcome of the development and the results. REFERENCES I. R. L. COWANand C. S. TEDMOND,Jr., lntergranular Corrosion.oflron-Nickel-Chromium Alloys, Advances in Corrosion Science, Vol. 3. Plenum Press, New York (1973). 2. M. 1-~NTltO~, Localized Corrosion--Cause of Metal Failure. ASTM STP 516 ASTM (1972). 3. J. S. Amv~o, Corrosion 10, 319 (1968). 4. S. H. BUSHand R. L. DILLON,Stress Corrosion Crackb~g and Hydrogen Embrittlement of lron Base Alloys. Firminy. To be published by NACE, Houston, Texas. 5. K. Btr~GA~T, Nichtrostende Stable. Werkstoff-Handbuch Stahl und Eisen, Dusseldorf (1965). 6. P. H. EFFERTZ,Proc. 5th Int. Cong. on Metal Corrosion, p. 926. NACE, Houston, Texas (1974). • 7. D. ROBINSON,Clarke Chapman Report No. ML/73/68, December (1973). 8. B.S. 4437 (1969). 9. M. STERNand A. C. MAKRIDES,J. electrochem. Soc. 107, 782 (1960). 10. Sandvik, Data Sheet for HT7 Steel. 11. T.I. Data Sheet for 9YoCr 1 ~ M o Steel (1968). 12. United Steel Co. Data Sheet for Esshette CR M9 (1967). 13. A.S.A. Code for Pressure Piping Section B31, 3. Petroleum Refinery Piping. 14. ASTM Standard E 407-70 (1970). 15. R. OtaVlER, Thesis, University of Leiden (1955). 16. R. P. FRANKENTHALand H. W. PICKERING,J. electrochem. Soc. 120, 23 (1973). 17. M. B. ROCKEL, Corrosion 27, 95 (1971). 18. P. SLmRYand T. GEIGER, Werkstoffe und Korros. 20, 665 (1968). 19. E. HOUDREMONTand W. TOFAUTE,Stahl undEisen 72, 539 (1957). 20. R. STICKLERand A. VINKIER, Trans. A S M 54, 362 (1961). 21. R. A. LULA,A. J. LENAand G. C. KIEFER, Trans. A S M 4 4 , 197 (1954). 22. A. P. BOND, Trans. Met. Soc. A I M E 2 4 5 , 2127 (1969). 23. V. CIHALand M. PRAZAK,Corrosion 16, 5306 (1960). 24. C. ED~.LEANU,J. Iron Steel Inst. 185, 282 (1957). 25. R. V. DAY and J. BARFORD,CERL Lab. Report RD/L/R1379, June (1966). 26. J. E. A~rrlLL, private communication. 27. G. J. BIGNOLD,private communication. 28. J. S. ARMIJO, LMFBR Heat Exchanger Materials Development Program. General Electric Co. Report GEAP-1319-2.
The sensitization of ferritic steels containing less than 12~oCr
395
29. B. POULSON,unpublished results. 30. I. N. PUTILOVA,S. A. BALEZINand V. P. BARANIK,Metallic Corrosion Inhibitors (Translator G. RYn^cK, Ed. E. BISHOP),p. 85. Pergamon Press, Oxford (1960). 31. B. POULSON,Clarke Chapman Report RD/P/772, July (1975).
THE SENSITIZATION OF FERRITIC STEELS CONTAINING LESS THAN 12% Cr* BRYAN POULSON Materials Research, A.T.D., Clarke Chapman Limited, Gateshead, England Abstract--The susceptibility of a range of ferritic Cr Mo steels to sensitization during welding has been investigated. No localized attack occurred in any of the standard test solutions used to assess susceptibility to intergranular corrosion. However potentiostatic etching in 16 wtYoH2SO4 at 18°C produced narrow trenching close to the heat affected zone-parent metal interface in the 12~.Cr 1%Mo and 9 ~oCr 1 ~oMo steels. The thermal history and the metallurgical structure of the attacked region have been examined. The effect of post weld heat treatment on the occurrence of localized attack has been studied and correlated with hardness changes. Analytical and electrochemical measurements have been used to test the various theories proposed to explain intergranular corrosion in ferritic materials. Finally the implications of those findings are considered. INTRODUCTION INTERGRANULAR CORROSION(IGC) is a well known problem with a wide range of Fe and Ni base alloys. 1'2 These include the austenitic and ferritic stainless steels and the Ni based alloys such as Incoloy 800 and lnconel 600. Recently it has been found that materials susceptible to IGC, i.e. sensitized materials, can suffer intergranular stress corrosion cracking (s.c.c.)in high temperature high purity water; 3 in this environment the general corrosion rates are low and in the absence of stress I G C does not occur. A recent summary of s.c.c, failures in the nuclear industry 4 indicated that at least 25 ~o of failures were associated with sensitized materials. Another class of materials used in the power generating industry are the C r - M o ferritic steels. These steels were developed when the desire for increased efficiency required steels with better creep and oxidation behaviour than the carbon steels then in use. The 5 ~oCr 1 ~oMo and the 9 ~oCr 1 ~oMo steels are also used in the petrochemical industries to meet specific corrosion requirements, particularly resistance to sulphur attack. However their resistance to localized corrosion, such as I G C , has not been extensively examined. This probably stems from the view that at least 1 2 ~ C r is needed to produce passivity. 5 Alloys containing less than 12~oCr will not be passive and will behave like iron. In particular they will not develop an active-passive situation, as a result of chromium depletion at the grain boundaries, with the resultant rapid attack there. However, in some environments steels containing less than 12 ~ C r are passive. For example the successful operation of boilers depends on a thin layer of magnetite protecting the steel. 6 There are, however, more than theoretical reasons for suspecting I G C could be a problem with steels containing less than 12~oCr. For example, intergranular oxide penetration has been observed on a 9 ~oCr I ~oMo steel, after exposure to what was probably a caustic environment. 7 Standard immersion tests have been developed to assess a materials degree of sensitization, 1,2 i.e. its susceptibility to I G C in an environment which produces this *Manuscript received 25 August 1976; received for publication 28 July 1977. 371
372
B. POULSON
f o r m o f attack. These tests are time consuming, destructive an d n o t simply quantifiable in all cases. Th e effect o f heat t r e a t m e n t on susceptibility to I G C is usually presented in the f o r m o f t i m e - t e m p e r a t u r e - s e n s i t i z a t i o n diagrams. While these are u n d o u b t e d l y useful they are i n a d e q u a t e for m a n y purposes. T h e y cannot, for example, be used to predict the possibility o f sensitization during welding, or its relief during post weld heat treatment. F u r t h e r m o r e they are unsatisfactory in assessing the possibility o f sensitization during solution annealing treatments. Bearing these points in mind this investigation was intended to: 1. E x a m i n e the susceptibility o f steels c o n t a i n i n g i 2 % C r or less to I G C , particularly in relation to welding. 2. I f possible develop a rapid electrochemical test for assessing the possibility o f I G C . 3. E x a m i n e the effects o f post weld heat treatments on any tendency towards localized attack. EXPERIMENTAL METHOD A series of Cr Mo ferritic steels was obtained (Table l) and welds were prepared according to the procedures outlined in Table 2. The standard half hour post weld heat treatment at various temperatures was carried out in an air furnace. The specimens were thus slowly heated and slowly cooled. Other shorter post weld heat treatments were performed in a salt bath and the specimens waterquenched, The non-welded specimens were quenched into iced brine after half hour at 950°C in an argon atmosphere, and tempered in a salt bath. Jominy end quench tests were performed in the specified way? TABLE 1. Designation M.S. Tube 1Cr Tube 2¼Cr Tube 5Cr Tube 9Cr Tube 9Cr Rod 9Cr Spacers 12Cr Tube
MANUFACTURERS'ANALYSISOF STEELSUSED Composition P Ni
C
Ma
Si
S
0.08 0.1 0.12 0.1 0.1 0.1 0.1 0.19
0.58 0.57 0.56 0.46 0.47 0.43 0.45 0.52
0.15 0.26 0.27 0.27 0.44 0.59 0.53 0.45
0.03 0.024 0.024 0.004 0.017 0.008 0.011 0.01
TABLE2. Material M.S. Tube 1Cr Tube 2~42r Tube 5Cr Tube 9Cr Tube 9Cr Rod 9Cr Spacers 12Cr Tube
0.04 0.021 0.016 0.013 0.017 0.014 0.009 0.01
. 0.04 ----0.1 0.51
.
Cr
Mo
others
. 0.85 2.24 4.97 9.07 8.55 8.9 11.7
. 0.49 0.98 0.56 1 0.99 0.97 1.02
------0.33V
OUTLINEOF WELDINGPROCEDURES Process Production orbital TIG Production orbital TIG Production orbital TIG Experimental manual autogenous TIG Production orbital TIG Experimental autogenous orbital TIG Experimental autogenous linear TIG Experimental autogenous orbital plasma
Specimens approx. 1.5 in. long and 0.25 in. wide (containing a section through the weld) were cut from the welded tubes. The bore surface was then polished to grade 600 emery or finer. Unwelded samples approximately 4 cms in area were cut from the tube or rod and polished to grade 600 emery or better. All specimens were drilled and tapped to fit a Stern-Makrides Electrode Holder) Solutions (summarized in Table 3) were prepared from laboratory grade chemicals and deionized water.
The sensitization of ferritic steels containing less than 12 yoCr TABLE 3.
Solution
SUMMARYOF SOLUTIONSUSED
Composition
Acid ferric sulphate
50 wt ~H~SO4 + 25 g/l Fe~(SO~)a 16 wt ~oHsSO~ + 100 g/l CuSO, + Cu 65 wt ~.HNOa 10 wt ~oH2CsO4.2H~O
Acid copper, copper sulphate Nitric acid Oxalic acid
373
Temperature Boiling Boiling Boiling 18°C
Standard immersion tests, all constant potential tests and electrochemical measurements were performed in a multi-necked 1 1. flat bottomed flask. All potentials were measured using a saturated calomel electrode (SCE) and are reported on this scale. Polarization curves were obtained using a linear sweep generator (Chemical Electronics) to drive a potentiostat (Wenking Model ST72), the current being recorded on a chart recorder (Smiths Servoscribe) by manual switching between ranges. After a number of the constant potential tests the solution was analysed for Fe and Cr by atomic adsorption spectroscopy, a Hilger Watts instrument being used. All specimens for examination in the SEM were ultrasonically cleaned in methanol directly the test finished and stored until required. E X P E R I M E N T A L RESULTS
Development of the test method There was no indication of localized attack after exposing as welded 9Cr tube samples to any of the standard test solutions used to detect susceptibility to IGC. In acid ferric sulphate, rapid hydrogen evolution occurred, the solution quickly changed colour from yellow to green, and the specimen dissolved rapidly. This is shown in Fig. l(a) where it c a n also be seen that the weld metal and the heat affected zone (HAZ) dissolved marginally more quickly than the parent metal. In the acid copper sulphate test, copper deposition occurred and irregular general dissolution took place beneath this layer (Fig. Ib). In nitric acid, little general and no localized dissolution appeared to have taken place (Fig. lc). In the oxalic acid etch, the weld metal remained almost unattacked, the H A Z was lightly etched and the parent metal over etched (Fig. 2). Another attempt to detect any tendency to suffer localized dissolution was by potantiostatic etching in 16 wt ~H2SOs at 18°C. When specimens were held at certain potentials localized hydrogen evolution was noticeable in the region of the H A Z parent metal interface and after 20 h deep narrow trenching had occurred there (Fig. ld).
The effect of potential and Cr content on the susceptibility to localized dissolution As welded samples of all the steels were exposed to the 16 Wt~oH~SO5 over a range of potentials. Localized trenching occurred only on the 12Cr and 9Cr steels. The effect of potential on the form of attack is summarized in Table 4, typical specimens are shown in Fig. 3. Figure 4 shows the very localized attack that occurs, in the region of the H A Z - p a r e n t metal interface, at potentials above the range promoting trenching, as indicated in Table 4. Although the 5Cr steel showed a tendency towards trenching--as evidenced by localized hydrogen evolution--this situation tended to be unstable. The specimen became active and dissolved rapidly.
374
B. POULSON
(o)
(b)
(c)
(d)
FIG. 1. Morphology of attack on 9Cr weld after exposure to solutions used to detect susceptibility to intergranular attack. (a) 15 rnin in acid ferric sulphate. (b) 3 h in acid, copper, copper sulphate. (c) 24 h in nitric acid. (d) 20 h in 16 wt~oH2SO4 at ÷ I00 mV SCE.
Effects of post weld heat treatment on the susceptibility o f 9Cr welds towards localized attack The manufacturers recommended post weld heat treatment ( P W H T ) varies between 730 and 800°C for times as short as 15 rain to as long as 60 min. 1°-1"- In some codes a m a x i m u m hardness is specified, e.g. 250 Hv. 1~ Clarke C h a p m a n have adopted a standard P W H T of 30 min at 710-760°C. After this treatment any tendency towards localized corrosion, in the present test, was removed and hardness values were below 265 Hv. To examine how critical heat treatment conditions are a series of P W H T were carried out. These covered times between 1 and 30 rain at temperatures from 400 to 750°C. The specimens were then potentiostatically etched at + 100 mV SCE in H~SO4 as previously described. The general trend o f results was that with increasing heat treatment time and temperature the region of the weld susceptible to attack altered in the following way:
FIG. 2.
Morphology of attack on 9Cr weld after oxalic acid etch.
FIG, 4.
Fine cracks and resultant surface (after fracturing) of 12Cr weld exposed at + 200 mV SCE for 100 h.
The sensitization of ferritic steels containing less than 12~,Cr
377
'a
(b)
(c)
(d}
(e)
FXG. 3. Effect of potential (SCE) on morphology of attack of 12Cr steel after 20h in 16 wt Y.H~S04. (a) + 200 mY; (b) 0; (c) -- 200 mY; (d) - 300 mY; (e) - 600 mY. Trenching at H A Z - p a r e n t metal region -> trenching + attack on H A Z --->attack on HAZ + some attack on weld metal ~ immunity to attack. Perhaps more important than the site of the attack is the severity of attack. In a simple way the results of the tests are summarized in Fig. 5 which shows the changing site of attack and Fig. 6 which illustrates the severity of the attack. This severity of attack is classified in a generalized way as follows: 1. Gross attack--when trenching or other visible attack has occurred. 2. Slight a t t a c k - - w h e n attack has occurred but is visible only metallographieally. 3. No attack--when examined metallographically at 400 ×. The important points to note are: (a) Inadequate P W H T can sensitize the entire HAZ. (b) Specimens heat treated for 30 min should be immune to localized attack as long as the temperature is above 700°C.
378
B. POULSON
TABLE 4. TVlE EFFECT OF POTENTIAL ON DISSOLUTION MORPHOLOGY OF WELDS IN 9 Yo AND 12 ~oCr STEEL
Potential (SEE), mV 300 250 200 150 100 50 0 -- 50 -- 100
- -
12y,Cr
9 ~Cr
t No attack
t No attack
I
I
1- very fine L attack
velocity of trenching - trenching plus 0.03 mm/h general corrosion 0.05 mm/h which increases
200
with decreasing potential
1-very fine L attack velocity of trenching I" trenching plus 0.13 mm/h | general corrosion [ which increases 0.09mm/h r with decreasing 0.04mm/h L potential
0.06 mm/h
-- 300
General corrosion
0.02 mm/h
--400
General corrosion
75C 7OC - .
.
65C U
o
i
60(
--O
•
•
•
•
55( - - O
•
•
•
•
~ s~
• No a t t a c k
•
4so - -
O Slight attack
•
4OO __
• Gross attack
I
I
I
2
I
3
Time
I
4
I II III
5
6
7 8 9 I0
I
20
I
30
minutes
FIG. 6. Effect of PWHT on the severity of attack on 9Cr weld potentiostatically etched in HsSO4. (c) Susceptibility to trenching disappears quickly at 750°C and specimens H.T. for 8 min showed no signs of localized attack. The morphology of the attack in these tests was both intergranular, producing grain fall out, and transgranular along the martensite laths, giving rise to a platelike structure. Examples of these different types of attack are shown in Fig. 7 which also includes an example of the type of non-localized attack after a P W H T of 30 min at 750ac. Although a detailed study of the factors affecting dissolution morphology was not made, the proportion of intergranular attack appeared to be greatest in the high temperature H A Z close to the weld metal.
FIG. 5.
Effect of PWHT on the pattern of attack on 9Cr weld potentiostatically etched in HsSO4. (a) 650°C for l rain; (b) 700°C for I rain; (c) 650°C for 30 rain.
"
~i
~i~
"~
~ i N . ' : i o
~o~
0
0
.~.o
~ ÷.~
~2
The sensitization of ferritic steels containing less than 12Y.Cr
381
Characterization of 9 ~oCr welds On as-welded specimens the region susceptible to trenching etched up, in Vilellas reagent, 14 slightly faster than the rest of the specimen. With specimens which had been given a P W H T in such a way as to sensitize the entire HAZ, this could not be ascertained from their etching characteristics. There were no mierosctructural features visible in the optical microscope which enabled sensitized materials to be recognized. Hardness profiles were obtained for each of the welds which were susceptible to trenching. The results are shown in Fig. 8 from which it appears that sensitization is associated with that region of the HAZ which has undergone partial transformation and resulted in some, but not maximum hardness increases.
600 _
12 Cr Tube
o/o/O
SO0
o~
4 0 0 -300
'''--°--'~
7-
07
oi
4 0 0 _ 9 Cr
o~
o ~
o--------o ~ ° - °
Tubt
300 200 400 _
9Cr
Rod
~ o ~ O
300 200 ~ 400 9
0
Cr Spacer J ° ~ o - ~ - - - _
o_
o
300 200
0-'
- a ~ I
Imm
]
FIG. 8. Hardness profiles across welds susceptible to trenching. Hatched region denotes area susceptible to trenching. Hardness profiles were also obtained on many of the 9Cr welds after different PWHT. Typical results, shown in Fig. 9, were chosen to include specimens grossly sensitized, slightly sensitized and immune. As a rough guide it seems that if the maximum hardness is greater than ca. 280 Hv some localized attack appears likely in the present test. However gross attack was usually associated with hardnesses > 325 Hv but it must be emphasized that the attack was not necessarily associated with the hardest region. Furthermore this hardness level, below which localized attack will not occur, will probably depend on the composition of the steel and needs to be examined for steels other than the 9Cr steel.
Reproduction of sensitization on non-welded samples A simple way of obtaining a large range of cooling rates on a single specimen is the Jominy end quench test. Such a test was performed on a 9Cr bar which had been austenitized for 1 h at 950°C. Hardness measurements along the bar indicated a
382
B. Potn_sorq
400
TtnT~roturel °C /minutes Time
-
/
-
/
o
. . . . . .
~o
/
--o--o--o~--4-o/--o--o
o °
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650/4'7 600130
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-
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/o
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o o~o----~ ° -
75o)30
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Imm [
FIG. 9. Effectof PWHT on hardness profiles across 9Cr tube welds. maximum value of 460 Hv and a minimum value of 420 Hv. After potentiostatic etching for 20 h at q- 100 mV(SCE) in 16 wt %HzSO4 no localized attack attributable to sensitization had occurred. It thus appears that rather slow cooling rates are require to induce sensitization in these hardenable ferritic steels. Specimens of 9Cr rod were austenitized for 1 h at 950°C quenched into iced brine and then tempered for times up to 16 min. These specimens were then potentiostatically etched in H2SO4 for 15 min and examined metallographically. Quenched samples remained bright and unattacked. Samples tempered for 1 min were heavily attacked, specimens tempered for longer times gradually regained their corrosion resistance. This is summarized in Fig. I0 which includes hardness measurements on each of the samples. Thus it appears that quenched samples are not sensitized but this rapidly occurs after only 1 min at 750°C and then gradually is removed till at 8 and 16 rain no localized attack could be observed.
Electrochemical measurements
Potentiodynamic anodic polarization curves (at 10 mV/min) in 16 wt%H2SO4 were obtained for each of the steels in the quenched condition. Figure 11 shows the portion of the polarization curves in the vicinity of the passivation potential, which is defined in the present investigation as the potential at which the current density decreases below 0.1 mA/cm 2. The effect of Cr additions on this passivation potential is shown in Fig. 12 which also includes previous work. z~,16 Reasonable agreement is evident and it is apparent that up to 5 %Cr there is only a small effect whereas at higher Cr levels large shifts in the passivation potential occur. Attempts were made to see if electrochemical measurements could detect sensitized welds. Potentiodynamic anodic polarization curves (at 100 mV/min) and current transients--at + 100 mV (SCE)--were obtained. Tests in which the entire HAZ and the weld metal had been sensitized (i.e. by a P W H T of 1 min at 750°C) did give indications that sensitized welds could be detected by rapid electrochemical measurements. However on as welded samples, where the attack is much more localized,
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sensitization could not be detected. Thus in general it appears unlikely that electrochemical measurements will be useful in detecting sensitization in welds where the actual region attacked is a small fraction of the total area. However electrochemical measurements on quenched, and quenched and tempered samples of 9Cr steel were more fruitful. Potentiodynamic anodic polarization curves are shown in Fig. 13. It can be seen that there are distinct differences between the samples. In particular sensitization causes the appearance of a secondary active peak and the current density of the primary active peak is higher than the values obtained from the quenched samples. Subsequent tempering reduces the magnitude of both primary and secondary peaks. Previous work on steels with 12% and higher Cr contents found very similar effects, which were attributed to Cr depleted regions. 17'1s Current transients (Fig. 14) at + I00 mV (SCE) also give indications of sensitized material in that the current increased for sensitized samples as opposed to nonsensitized samples for which there was a continuous decay.
386
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QUENCHEDAND TEMPERED
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Analytical measurements After many of the constant potential exposures the solution was analysed for Cr and Fe. If it is assumed that the other elements dissolve stoichiometrically then since the composition of each alloy is known, an apparent Cr content of the material which has dissolved can be calculated from: apparent Cr ~o =
ppm Cr × 100 ppm Cr + ppm Fe + ppm other elements 1
The sensitization of ferritic steels containing less than 12%Cr
387
The results of such calculations are shown in Table 5. Since in a number of tests, grain fallout occurred and the entire grains subsequently dissolved these values will tend to be higher than the Cr content of depleted regions. Nevertheless the apparent Cr content of the dissolved material varies systematically with potential, being at a minimum at potentials causing trenching. What is not at present understood is why there are differences between the 9Cr and 12Cr steels in the correlation between actual Cr content of the alloy and maximum apparent Cr content. One possible suggestion is that some of the Cr is tied up as carbide and this does not dissolves in the 9Cr case but does in the 12Cr, the V additions possibly influencing its behaviour. I f it is assumed that all the carbon forms a carbide of the type Cr~ C5 this could account for 1.6 ~oCr in the 9Cr steel being tied up. TABLE5. SUMMARYOF ANALYTICALRESULTS Potential mV SCE in 16 wt %H~SO4 (mV)
-
-
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10~o Oxalic acid at 1A cm~
Apparent ~Cr of dissolved metal 12Cr steel 9Cr steel
10.5 9.4 8.5 9.2 10.4 11.9 --
6.6 6.4 4.9 5.3 4.8 5.8
15.8
It is also significant that the apparent Cr content after an oxalic acid etch is much higher than the Cr content of the alloy suggesting that carbides dissolve in this test. DISCUSSION
Mechanical hnplications Various theories have been proposed to explain the occurrence of intergranular attack on the higher Cr, non-hardenable, ferritic steels. These are basically one of two types: the first group suggests precipitates either dissolve readily 19 or cause adjacent material to corrode rapidly by either acting as local efficient cathodes z° or straining the matrix. 21 The second group of theories postulates that localized attack is due to regions near the grain boundary being depleted in Cr and dissolving preferentially. ~2 The general objections to mechanisms of the first kind are the difficulty in explaining how dissolution occurs between the carbide particles and why only steels containing more than 5 ~ C r are susceptible to sensitization. Specific objections to the noble efficient cathode carbide theory are: (a) That is it unlikely that a carbide can function as an efficient local cathode at anodic potentials in potentiostatic tests. (b) The unexplained limited range of potentials promoting attack.
388
B.
POULSON
(c) The increased resistance to attack after long tempering times with little apparent change in the carbide. The idea that the cause of attack is carbide dissolution would appear most improbable since the evidence of this and other investigations~ is that it does not at potentials promoting attack. Similarly models based on precipitates inducing strainenhanced dissolution would seem unlikely to be a first order effect. Reasons include the fact that maximum sensitization occurs before maximum carbide precipitation, that additions of Ti prevent sensitization of the higher Cr steels but there is no evidence that TiC strains the lattice less than carbides of the type M~ Ce2~ and finally there is scant evidence suggesting strain can increase the dissolution rate sufficiently to produce this attack. However the Cr depletion model makes several important predictions which c a n be tested: 1. Preferential attack should be limited to the potential range between the passivation potential of the matrix and the passivation potential of the Cr depleted zone. ss 2. Since with decreasing Cr contents, the difference between the passivation potential of the matrix and the depleted zone will get less, the potential range causing intergranular attack should decrease with decreasing Cr content. The minimum Cr content at which sensitization can occur will be that which causes a certain minimum difference between the passivation potentials of the matrix and the depleted zones. 3. If material depleted in Cr is dissolving this should be apparent from an analysis of the solution after a test. 24 Figure 15 shows the results of electrochemical and analytical measurements designed to test these predictions for the 12Cr steel. It can be seen that trenching occurs at potentials just into the passive range and there is a close correlation between trenching and the apparent Cr content of the dissolved material. It has previously been shown (see Table 4) that the 9Cr steel suffered localized attack over a narrower
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The sensitization of ferritic steels containing less than 12~Cr
389
range of potentials than did the 12Cr steel. All this data is exactly what is predicted by the Cr depletion model. Analysis of intergranular fracture surfaces of hydrogen embrittled sensitized and non-sensitized steels should provide direct evidence of Cr depletion. There are details of the mode of Cr depletion in the 9Cr and 12Cr steels which are different in a number of interesting ways to the sensitization of the austenitic or the ferritic stainless steels. During continuous cooling from the annealing temperature to room temperature no allotropic phase changes occur in either the ferritic or the austenitic stainless steels. Thus in the ferritic stainless steels carbide precipitation occurs from ferrite and in the austenitic stainless steels from austenite. Since the diffusion rates of Cr are much quicker in ferrite than in austenite this results in markedly different kinetics of sensitization. 1 Specifically, ferritie stainless steels can sensitize during rapid cooling while austenitic steels require much slower cooling rates to cause sensitization. The ferritic 9Cr and 12Cr steels investigated in this study are hardenable, this means that during cooling from the normalizing temperature the structure is austenitic until either the Ms temperature is reached or the austenite transforms to ferri~e and carbide. The transformation kinetics of the 9Cr steel have been investigated .5 and are summarized in Fig. 16. During continuous cooling from the normalizing temperature relatively slow rates of cooling will be required for carbide formation and thus sensitization. This is evidenced by the lack of any sensitization in the jominy specimen at cooling rates which would induce sensitization in the ferritic stainless steels. Thus the 9Cr and 12Cr are more austenitic like in the sensitization behaviour during continuous cooling,
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Below the Ms temperature the structure o f the 9Cr and 12Cr steels will be martensitic and on subsequent tempering the possibility of sensitization will occur. In this case the kinetics of sensitization will be more like the ferritic stainless steels than the austenitic stainless steels, and relatively short times (i.e. 1 min at 750°C) are required to induce sensitization which is subsequently removed after c a . 8 min.
390
B. Pout.soN A u s t e n i t i c or f e r r i t i c nonhordenoble stainless s~reels
Ferrific
Cr Mo hordenoble steels ^
I
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% Cr
Before weldir~
B f t e r welding
GB I Before weldin,
At pec~ temperature
After weldin
Fio. 17. Differencesin mode of sensitization during welding between hardenable and non hardenable Cr steels. There are similar important differences in the mechanism of sensitization during welding (Fig. 17). The initial structure of the 9Cr and 12Cr steels is ferrite plus carbide. During welding some carbide dissolution and reprecipitation must occur to induce sensitization which occurs in the H A Z - - a s it does in the austenitic steels. However during subsequent P W H T the ferritic-like behaviour ensues. Tiffs means that while all the HAZ and weld metal can be sensitized by very short P W H T times (i.e. 1 min at 750°C) the standard P W H T of 30 min at 750°C completely removes sensitization. This treatment would induce sensitization of an austenitic steel which would require very long times at 750°C before it was removed.
Practical implications It has been shown that ferri.tic steels containing less than 12~oCr, and possibly as little as 5 ~oCr, can be sensitized by certain treatments. As discussed previously, this sensitization is thought to be due to localized regions being depleted in Cr. This is analogous to the sensitization, weld decay, i.e. susceptibility to intergranular attack, which is a well-known problem with the austenitic stainless steels. In these hardenable ferritic steels it appears that material could be sensitized in a .number of ways: 1. During welding, a zone close to the parent metal-HAZ interface becomes sensitized. 2. Inadequate PWHT, the entire HAZ and weld metal can become sensitized. 3. Insufficient tempering after normalizing will result in sensitization. It is possible that material inadvertently tempered at too high a temperature (i.e. above the lower critical temperature) could be sensitized. 4. During cooling after annealing it appears possible that sensitization could result, if certain as yet undefined critical cooling rates are employed. There is also the chance of correctly heat treated material being sensitized in service: the first possibility is that a rapid thermal transient to above 805°C would induce sensitization. The second is that since the carbide solubility at the operating temperature is lower than at the tempering temperature, carbide precipitation will occur. This could possibly tend to reduce the corrosion/oxidation resistance and could cause mild sensitization. Sensitization is not invariably detrimental and sensitized austenitic stainless steels can be used in a number of environments. Figure 18 shows the region on an as-welded
Region susceptible to trenching on 9Cr weld, after 100 h exposure to inhibited citric acid containing dissolved oxygen. FiG. 19. Effect of weld on surface pitting of plain carbon steel after exposure to inhibited citric acid containing dissolved oxygen. F{G. 18.
The sensitization of ferritic steels containing less than 12~,Cr
393
sample of 9 ~oCr which is sensitized. In this case it had been exposed to inhibitedcitric acid for 100 h and very little localized attack has occurred. What is probably more important is how the degree of sensitization will effect the materials s.c.c, resistance. Thus Antil126 reported that as-welded 9Cr 1Mo steel capsules failed after approximately 4000 h with 3 ~oNaCl at 350°C and failed in less than 170 h with 20~oNaOH at 360°C. In both environments correctly heat treated capsules did not show any indications of cracking. Similarly Bignold z7 has tested solution annealed (at 1050°C) 9Cr 1Mo material and found it susceptible to transgranular s.c.c, in a chloride environment (0.5M MgCl2 + 0.5M NaCI) and intergranular s.c.c, in a complex hydroxide environment (0.5 NaOH + 0.5M NaCI + sat. Na,2SO4 + sat. Na~SiOa) both at elevated temperature. Bignold 27also indicated that it was difficult to reproduce the sensitization solution annealing treatment. This confirms earlier indications ~a that annealed 9Cr 1Mo material could be susceptible to s.c.c. As indicated previously there is a possibility that annealing 9Cr 1Mo material could sensitize it if certain rather critical cooling rates are employed. An alternative possibility is the quoted failures of 9Cr were caused by hydrogen embrittlement of the hard HAZ or solution annealed material. Recently it has been shown 29 that quenched 9Cr (Hv ,,, 460) can undergo hydrogen embrittlement in media as diverse as inhibited citric acid and 15 ~ N a O H at 250°C. The standard test solutions, used to detect IGC in stainless steels are not suitable for testing steels containing 12~oCr or less. A standard potentiostatic etching technique in 16 wt~H2SO4 at 18°C has been developed which satisfactorily detects sensitization in these steels. Some indication of the degree of sensitization of homogeneous samples can be obtained from electrochemical measurements, but with welds, due to the localized nature of attack, this does not seem possible. Clearly it is desirable that steels containing 12~oCr and 9~oCr should be heat treated after welding to remove any sensitization. For the 9Cr steel the manufacturers recommended P W H T appears quite adequate. From the results of varying PWHT for heat treatment times of 30 rain the temperature should be 700°C or above while at 750°C times as short as 8 min should remove sensitization. As an indication, values of hardness in excess of 280 Hv suggest that a weld in 9Cr material might be sensitized; the exact value will probably depend on the detailed composition. Finally it must be emphasized that it is unlikely that sensitization is the only cause of localized weld corrosion problem in the lower Cr ferritic hardenable steels. For example localized pitting in the HAZ of weld in plain carbon steels has occurred after exposure to inhibited HCI a° and welds have been shown to influence the occurrence of surface pitting in inhibited citric acids containing oxygen ~1 (Fig. 19). CONCLUSIONS I. As-welded 1 2 ~ C r l~oMo, 9~oCr 1 ~oMo and possibly 5~oCr ½~oMo steels are sensitized in the region of the parent metal-HAZ interface, during welding. This sensitization can be removed by suitable Post Weld Heat treatments, i.e. 30 m at temperatures over 700°C or times over 8 min at 750°C. Inadequate PWHT can result in the entire HAZ and weld metal becoming sensitized. 2. Standard test methods do not detect sensitization in steels containing less than 12~oCr. A suitable test is potentiostatic etching in 16 wt~oH~SO4 at potentials
394
3.
4.
5. 6.
B. POUI.,SON
j u s t in the passive range. Electrochemical m e a s u r e m e n t s c a n n o t detect local sensitization near welds b u t are m o s t useful in m e a s u r i n g the sensitization o f h o m o g e n e o u s samples. H a r d n e s s profiles across welds give a g o o d i n d i c a t i o n o f the presence o f sensitized material. F o r the 9Cr Z½Mo steel hardness levels a b o v e a b o u t 280 Hv are to be avoided. This will also minimize the risk o f h y d r o g e n embrittlement. T h e available evidence suggests t h a t the degree o f sensitization m u g h t be i m p o r t a n t in d e t e r m i n i n g the susceptibility to stress c o r r o s i o n cracking in certain environments. A n a l y t i c a l a n d electrochemical m e a s u r e m e n t s p r o v i d e s u p p o r t for the cause o f sensitization to be related to local regions which are d e p l e t e d in Cr. I m p o r t a n t differences between the sensitization process in the ferritic h a r d e n a b l e steels e x a m i n e d in this investigation a n d the ferritic a n d austenitic stainless steels a r e described. F e r r i t i e h a r d e n a b l e steels are "austenitic like" d u r i n g cooling f r o m the n o r m a l i z i n g t e m p e r a t u r e b u t "ferritic like" d u r i n g subsequent tempering. D u r i n g welding s o m e c a r b i d e dissolution m u s t occur p r i o r to r e p r e e i p i t a t i o n in h a r d e n a b l e steels for sensitization to occur.
Acknowledgements--The author thanks the management of Clarke Chapman Limited and the
C.E.G.B. who have been associated with this work for permission to publish the outcome of the development and the results. REFERENCES I. R. L. COWANand C. S. TEDMOND,Jr., lntergranular Corrosion.oflron-Nickel-Chromium Alloys, Advances in Corrosion Science, Vol. 3. Plenum Press, New York (1973). 2. M. 1-~NTltO~, Localized Corrosion--Cause of Metal Failure. ASTM STP 516 ASTM (1972). 3. J. S. Amv~o, Corrosion 10, 319 (1968). 4. S. H. BUSHand R. L. DILLON,Stress Corrosion Crackb~g and Hydrogen Embrittlement of lron Base Alloys. Firminy. To be published by NACE, Houston, Texas. 5. K. Btr~GA~T, Nichtrostende Stable. Werkstoff-Handbuch Stahl und Eisen, Dusseldorf (1965). 6. P. H. EFFERTZ,Proc. 5th Int. Cong. on Metal Corrosion, p. 926. NACE, Houston, Texas (1974). • 7. D. ROBINSON,Clarke Chapman Report No. ML/73/68, December (1973). 8. B.S. 4437 (1969). 9. M. STERNand A. C. MAKRIDES,J. electrochem. Soc. 107, 782 (1960). 10. Sandvik, Data Sheet for HT7 Steel. 11. T.I. Data Sheet for 9YoCr 1 ~ M o Steel (1968). 12. United Steel Co. Data Sheet for Esshette CR M9 (1967). 13. A.S.A. Code for Pressure Piping Section B31, 3. Petroleum Refinery Piping. 14. ASTM Standard E 407-70 (1970). 15. R. OtaVlER, Thesis, University of Leiden (1955). 16. R. P. FRANKENTHALand H. W. PICKERING,J. electrochem. Soc. 120, 23 (1973). 17. M. B. ROCKEL, Corrosion 27, 95 (1971). 18. P. SLmRYand T. GEIGER, Werkstoffe und Korros. 20, 665 (1968). 19. E. HOUDREMONTand W. TOFAUTE,Stahl undEisen 72, 539 (1957). 20. R. STICKLERand A. VINKIER, Trans. A S M 54, 362 (1961). 21. R. A. LULA,A. J. LENAand G. C. KIEFER, Trans. A S M 4 4 , 197 (1954). 22. A. P. BOND, Trans. Met. Soc. A I M E 2 4 5 , 2127 (1969). 23. V. CIHALand M. PRAZAK,Corrosion 16, 5306 (1960). 24. C. ED~.LEANU,J. Iron Steel Inst. 185, 282 (1957). 25. R. V. DAY and J. BARFORD,CERL Lab. Report RD/L/R1379, June (1966). 26. J. E. A~rrlLL, private communication. 27. G. J. BIGNOLD,private communication. 28. J. S. ARMIJO, LMFBR Heat Exchanger Materials Development Program. General Electric Co. Report GEAP-1319-2.
The sensitization of ferritic steels containing less than 12~oCr
395
29. B. POULSON,unpublished results. 30. I. N. PUTILOVA,S. A. BALEZINand V. P. BARANIK,Metallic Corrosion Inhibitors (Translator G. RYn^cK, Ed. E. BISHOP),p. 85. Pergamon Press, Oxford (1960). 31. B. POULSON,Clarke Chapman Report RD/P/772, July (1975).