Creep Study

8/8/2019 Creep Study

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- - 9THE RELATIONSHIP BETWEEN RUPTURE LIFE AND CREEP PROPERTIES

OF 2 1/4 Cr-1 Mo STEEL*

R. L. KluehMetals and Ceramics DivisionOak Ridge National LaboratoryOak Ridge, Tennessee 37830

ABSTRACT

-NOTICE-This report was prepared as an account of worksponsored by the United States Government. Neitherthe United States nor the United States Atomic EnergyCommission, nor any of their employees, nor any oftheir contractors, subcontractors, or their employees,makes any warranty, express or implied, or assumes anylegal liability or responsibility for the accuracy, com-pleteness or usefulness of any information, apparatus,product or process disclosed, or represents that its usewould not infringe privately owned rights.

Several investigators have demonstrated empirical relationshipsbetween creep and rupture properties for various metals and alloys. Wehave examined empirical relationships between the rupture life and theminimum creep rate and the rupture life and the time to the end of steady-state creep (start of tertiary creep) for four heats of normalized-and-tempered 2 1/4 Cr-1 Mo steel with different carbon contents. The primaryobjective was the determination of the conditions that affect the correla-tion. The primary microstructural constituent (proeutectoid ferrite orbainite) of the matrix and the precipitates present in that matrix (beforetest or formed during test) played a significant role in the correlationof the data with the chosen empirical relationship.

INTRODUCTIONEmpirical relationships between creep and rupture properties have

been shown to exist for several metals and alloys at elevated tempera-tures [15].T Figure 1 shows a schematic creep-rupture diagram and

*Research sponsored by the U.S. Atomic Energy Commission undercontract with Union Carbide Corporation.

'"Numbers in brackets designate references at end of paper.

DISTRIBUTION OF THIS DOCUMENT IS


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ORNL-OWG 73-7669

TIME

Fig. 1. Schematic Diagram of a "Typic al " Creep -Rupt ure CurveShowing t h e Three S tages of Creep.


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defines the creep and rupture quantities of interest. Monkman andGrant II] shoved that several metals and alloys follow an empiricalrelationship between rupture time t and the steady-state creep rate esuch that

where C and a are constants. Several investigators 113] have founda to be near unity and assumed C to be independent of temperature andstress. For example, Monkman and Grant [1] tried to fit all ferriticsteel data to one curve and found considerable scatter. However, thedata included materials with up to 13% Cr and temperatures from 427 to704C (800 to 1300F).

Equation (1) was found by Garofalo et al. [4] to be obeyed bytype 316 stainless steel, but in this case C was slightly temperaturesensitive. They also found that empirical relationships exist betweenrupture time and the time to the end of secondary creep (start of tertiarycreep), tz, and. the time spent in secondary creep, t% t\~ namely,

T , (2)and

& , (3)

where t\ is the time to the end of transient creep and A, B, 8, and y a r econstants, with & and y near unity. However, at the higher test tempera-tures where deviations from Eq. (1) were noted for long-time tests,


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Gar ofal o et al . [4] found that Eqs . (2) and (3) w e r e s ti ll obeye d. Thedev ia ti on from Eq. (1) wa s attr ibuted to the nau tr e of th e gra in boun dar y

p r e c i p i t a t e .Following Garofalo et al . [4], Fig . 1 shows that

e 2 e =e tz - t\

Su bst it ut ion of Eq. (4) in to (2) and (3) gi ve s

(4)

e 2 - (5)and

B e 2 - (6)

t h u s i ndi c a t i n g a r e l a t ion s h i p be t w e e n e lon g a t ion a nd C of Eq. (1). Foran a , 3 and y o f uni ty , Eqs. (1), (5), and (6) indicate that the pr o-du c t of th e r u p t u r e t i m e a nd m i n i m u m c r e e p r a t e i s a con s t a n t a nd som efunc tion of the elongat ion durin g the s econdary sta ge of creep [2,4].Alt hough Gar ofalo et a l . 14] found e 2 ei to be a constan t w he n Eq. (1)ap p l i e d, oth e r i n v e s t i g a tor s [1] ha v e be e n u n abl e to e s t abl i s h a r e l a t ion-s h i p be t w e e n a m e a s u r e of cr e e p e lon g a t ion a nd C.

The ferritic 2 1/4 Cr-1 Mo steel ha s wi de a ppl icat ion in the ch emic alprocess and petroleum industri es and *in power-generat ing equip ment . Atp r e s e n t m u c h of th e a v a i l abl e h i g h-t e m p e r a t u r e da t a i s c r e e p-r u p t u r edata that was collected at st ress es above those of direct inter est fordesign pu rp ose s. Anal ysi s of these data is i mportant, however , for theinformati on that can be derived rela tive to creep behav ior, espec ially


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any trends that may be detected with decreasing stress. Furthermore,much of the data obtained for use in setting ASME Boiler and PressureVessel Code Case allowable stresses are from rupture studies. Thus, anyinformation derived relative to the applicability of empirical relation-ships to 2 1/4 Cr-1 Ho steel should find practical application. In thepresent paper, we have analyzed data on selected heats of 2 1/4 Cr-1 Mo steel to determine the conditions under which the empiricalrelationships discussed above are applicable.

EXPERIMENTALWe have previously reported on the metallography and mechanical

properties of four heats of 2 1/4 Cr-1 Mo steel with varying carbon con-tents [6] Three of the heats were 300-lb heats melted, cast, andfabricated into 7/8-in. rods and contained 0.009, 0.030, and 0.120% C

(the last was a remelted commercial heat). The fourth steel was a1-in. plate of a commercial heat with 0.135% C. Since the details onthe materials and the creep-rupture test procedures were previouslygiven [6 ], .they will not be further discussed.

The tests were conducted on the steels in the normalized-and-tempered condition: 1 hr at 927C (1700F), air-cooled, 1 hr at 704C(1300F), air cooled (the steels were heat treated as 7/8-in. rods or1-in. plate). In this condition, tha carbon content determined theprimary microstructural constituents: the 0.009 and 0.030% C steelswere primarily proeutectoid ferrite, containing 12 and 10-15% baininte,respectively; the 0.120 and 0.135% C steels were entirely bainite.Creep-rupture tests were made at 454, 510, and 565C (850, 950, and1050F). Since In some instances the amount of prestrain on load


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appear ed to affect some of the creep pr ope rti es , the yield stre sses fort h e t e s t m a t e r i a l s a r e g i v e n i n Tabl e 1.

RESULTSConsi der first the data for the 0.009% C st eel in Table 2. In

g e n e r a l , i a nd c a p p e a r to de c r e a s e w i t h de c r e a s i n g s t r e s s . Th e p e r -vc e n t de c r e a s e i n Gj and , wi t h de c r e a s i n g s t r e s s s e e m s to i n c r e a s esign ifican tly at the lowest str esse s at 565 and 510C. No such effectis noted at 454C, wh er e a ll tests are a bove the yield stress and involvecons idera ble pr es tr ai n. At 510 and 565C, both 2 and 2 ~ i> t hee lon g a t ion du r i n g s e conda r y c r e e p , s how con s ide r abl e s c a t t e r, w h i l e a t454C, they a re somewhat more const ant .

For t h e da t a from t h e 0o030C s tee l in Table 3, similar obse rv a-tions foll ow. Howe ve r, for th is mat er ia l a ll te sts at 454 and 510Cwe r e abov e t h e y i e ld s t r e s s , a s w e r e t h e t e s t s a t 22.5 and 25 ksi a t565C. As obse rv ed for th e 0.009% C st ee l, 2 and 2 1 ar e re la ti ve lyconst ant , though wi th somewha t different va lu es above and bel ow t heyield s tr es s. The only except ion to this is th e 565C test at 13 ksi .In t h i s c a s e, w h e n t h e s t r e s s w a s c h a n g ed bu t 2 ksi , a n orde r of ma g n i-

tude decre ase w as observed in ,, 1 (similar to the lar ge decre asenoted for e. and ei for th e 0.009% C s t e e l ), 2, 2 1. The r ea sonfor t h i s c h a n g e i s obv iou s w h e n t h e c r e e p c u r v e for t h i s s p e c i m e n,F i g . 2, is exa m i n ed; t h i s s p e c i m e n did not di s p l a y a " t y p i c a l " c r e e pcurve of the type s hown i n Fig . 1. After about 300 hr , the ste ela p p a r e n t l y w e n t i n to t e r t i a r y c r e e p (t h e tz and 2 va lu es recorded inTabl e 2), in t h a t t h e c r e e p r a t e be g a n to i n c r e a s e w i t h t i m e . Aft e rabout 550 hr , howeve r, another period of "steady sta te" creep ensued


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Table 1. Yield Stress of 2 1/4 Cr-1 Mo Steels Tested8

CarbonContent(wt %)0.0090.0300.1200.135

Yield454C29.226.058.367.4

Stress,510C23.625.658.171.2

ksi565 C21.420.554.757.8

Yield454 C201179402465

Stress,510C163177401491

MPa565C148141377399

aTaken from ORNL-4922.


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Table 2. Creep a nd Creep-Rupt ure D at a for 2 1/4 Cr-1 Mo Steelwi th 0.009% C

Stress(ksi)

2522.5201512.5

42.540353027.525

504947.54542.54141

(MPa)

17215513810386m

293276241207189172

345338327310293283283

(

1.1.0.0.0.

4.2.1.0.0.0.

2.3.2.1.1.1.1.

%)

005165105

031433805

112953,7,5

Pri m a r y Cre e p

?0.450.500.100.290.02

1.81.20.900.350.230.02

1.41.51.31.251.00.951.0

(hr)

0.30.51.016.53.0

1.62.24.012.050.05.0

0.757.512.521287570

2

565C (10502.04.250.751.00.40

510C (9506.010.02.81.00.50.3

454C (8504.456.17.24.853.753.94.0

Secondary

(hr)F)

3.52.895357F ?

3.05.532.57375230F)

3.22280100240310410

Creep

(%/hr)

1.381.350.0650.0140.0065

1.20.5250.05750.00960.00300.00125

0.9350.2100.0750.0340.0110.00930.0073

Tertiary

23.038.219.527.031.0

33.635.630.838.032.038.0

28.031.032.530.430.430.432.0

Creep

( h ? )

5.8.33.429.797.

5.15.84.262.354.672.

4.32.120,181.560,655,817,

67627

544045

,1.6.0.5.5.3.1

oo


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Table 3. Creep an d Creep-Rupture Data for 2 1/4 Cr-1 Ho St ee lwith 0.030% C

Primary Creep Secondary Creep Tertiary Creepstress ^

25222020171513

484745404035

565656565655

.5

.5

.75.70.65.50.00.00

17215513813811710390

331327310276276241

391.390.390.389.386.379.

396612

4.24.41.52.91.31.7 o . i

4.03.52.42.11.52.8

5.87.76.55.24.04.7

1.62.00.551.00.500.450.062

3.02.751.91.381.251.0

3.66.94.93.32.73.4

1.7.1520275315

6.7.177030575

10040062041011501600

55

510'55

454"

10.112.56.58.53.35.00.5*C (950F)

7.59.04.47.54.85.7SC (850F)

8.78.07.47.56.15.7

534899395200160

3050836403501675

27553096092029002800

(ksi) (MPa) ( % ) ( % ) (hr) (% ) (hr) (%/hr) ( % ) (hr)

565C (1050oF)1.750.3080.0630.0750.0290.0230.0026

0.1500.1170.0290.00970.0100.0028

0.01650.00200.00260.00460.00120.0008

43.041.645.643.647.837.144.8

30.432.036.031.233.233.2

28.830.430.429.637.032.6

10.165.8214.5221.8536.3675.02062.9

37.759.1274.51212.71069.32807.9

332.0934.31560.01219.93776.83932.6

vO


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10

T I M E IN H O U R S

F i g . 2. The Creep-Rupture Curve for the 0.030% C Steel Tested at565C and 13,000 psi (90 MPa).


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11

(at a slightly increased rate) out to about 850 hr, after which the creeprate again began to increase and increased to rupture. When & z and2 "~ i a t 850 hr are compared with those for the specimens tested atthe higher stresses, the relative constancy of these quantities is main-tained. We previously pointed out the types of precipitation reactionsthat occur in 2 1/4 Cr-1 Mo steels [6 ]. As discussed below, we believethese reactions occurring during test must give rise to the kind of

mcreep curve shown in Fig. 2.In Figs. 3 and 4 the data for the 0.009 and 0.030% C steels at all

three temperatures are plotted in accordance with Eq s. (1) and (2) :log t is plotted against log e [Figs. 3(a) and 4(a)] and against2? slog *2 [Figs. 3(b) and 4(b)]. Although best-fit straight lines have beendrawn through the data, the lack of statistical experiments hinders theinterpretation.

In Fig. 3(a) for the 0.009% C steel, only one straight line wasdrawn for the log t vs log data. Although it was drawn to fit the454C d a t a , with three exceptions, the remainder of the data also fallrather nicely on the line with a very near unity. For the 0.030% Csteel, on the other hand, lines with a approaching unity were drawn forthe data at 454 and 565 C. The data at 510C, however, appear to haveconsiderable scatter: the short-time data fall on the 454C line,while the longer time data fall on the 565C line.

Figures 3(b) and 4(b) show the data plotted in accordance withEq. (2). Only one best-fit straight line has been drawn, and althoughthere is some scatter, the data fit this curve quite well with 3 verynear unity. These results indicate that the percent of the test timespent in the primary and secondary stages of creep is relatively constant


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12

to 3c

2

1O 2

5

2

10 '

5

2

10

: i" ] - -

#H. . . . i

\

r

\V

D

. ]

1

s

s\

- ... _

Z H ~ S

9 454C 510CO 565C "

o

P V\

5 10" 5 10'1 2es

5 10'

10 3

5

2to 2

5

210'

5

2/

/

,{ T i

* /

if //

d - -j

i y/W \ r

-

- +F f fia 4' 50

0R NttII5 4 < > C0C>5C

L-D ATG 73-12681

10 2 5 10' 2 S 10s 2 5 10s 2 5 104 2 5 105/, (hr)

Fig. 3. Relationships Between Rupture Life and (a) Minimum CreepRate and (b) End ofSecondary Creep Stage for 2 1/4 Cr-1 MoSteelWith 0.009%C.


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13

10"

s2

t o '5.

2

10*5

2ml

V\

\s 5 ~

5" ^vs

._. AA

S - -5s s

5IOC565C

v

\

* S ss sN

ORWL-OIVG i j " |2|6y|9|

5 10"3 2 5 to"2 2 5 10"' 2 5 10 2 5 101


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14

with little dependence on temperature and stress. The only real excep-tions to this are the longer time tests at 56 5C, and examination of therespective creep curves indicates that they are of the type shown inFig. 2. Also, a superposition of the t, t z curves for the 0,009 and0.030% C steels indicates that the data for both follow a similar rela-tionship (i.e., similar B and $ ) .

We now turn to the data for the two completely bainitic steels with

commercial carbon contents (Tables 4 and 5 ) . At 56 5C, where all testswere at stresses below the yield stress, i, ., 2 , and 2 ei showlittle change with stress, despite the fact that there was a change infracture mode from intergranular to transgranular that resulted ina slope change in the creep-rupture curve and a change in the ruptureelongation, e [6 ]. At 510C, there is considerable data scatter, and,as pointed out below, this may result from precipitation reactions thatare occurring at this temperature (all tests for the 0.135% C steeland all but one for the 0.120% C steel) are at stresses below the yieldstress). At 454C the results of Table 4 for the 0.120% C steel (alltests above the yield) show a slightly decreasing 1 and E. when the

vstress decreases. Somewhat similar conclusions follow for the 0.135% Csteel of Table 5 (the 75 and 79 ksi tests are above the yield, though70 ksi is very near the yield). The three low-stress tests show a

*relatively constant e l t ., 2, and 2 Ei, while the 79 and 75 ksivtests show higher values, and the results of the 70 ksi test fall inbetween.

In Fig. 5(a) the data for all three temperatures for the 0.120and 0.135% C steels are plotted according to Eq. (1). Figure 5(b) showsthe combined data represented according to Eq . (2) in a log t , log t % plot.


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Table 4. Creep and Creep-Rupture Data for 2 1/4 Cr-1 Mo Stee lwith 0.120% C

Stress(ksi)

45353530302520

6055504540

757067.565

(MPa)

310241241207207172138 *

414379345310276

517483465448

i

0200000

10000

0000

c % >

.05

.15

.63

.75

.75.44

.73

.80.43

.15

.08

.03

.6.5

.39

.33

Prima'ry Creep

f* 1.6

0.380.350.300.300.45

1.350.250.10.060.01

0.450.300.210.23

t x(hr)

0.4.1.142019175

0.0.2.0.2,

1.52083

565C107

5108C25,7505,5454C

.3

E2(%)

(10502.03.82.82.82.81.92.0(9503.101.530.950.230.50(8500.851.450.800.89

Secondary

F)

F)

F)

*2(hr)

0.5181288120160850

1.363416100

3.35962345

Creepes(%/hr)

3.750.140.190.0260.0210.0070.0016

1.550.2100.0250.0120.005

0.1350.01950.00890.0020

Tertiary

o S24.125.128.125.332.824.214.9

24.026.828.031.628.4

23.922.0 -19.219.3

Creep*r(hr)

1.32.25.194.

254.435.2519.

2.13.

104.173.829.

18.136.242.707.

3405395

9,4,3,5,9

.0

.9

.9

.5

in


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Table 5. Creep and Creep-Rupture Data for 2 1/4 Cr-1 Mo St ee lwith 0.135% C

(ksi)

454040353025

5955505047454035

797570666564

S t r e s s(MPa)

310276276241207172

407379345345324310276241

545517483455448441

(

0.0.0.1.0.0.

0.0.0.0.0.0.0,0,

0,00000

ei% )

3777800575

16751028,30,10.65.40

.52

.88

.30

.10

.18

.11

P r i m a r y C r e e p

0.150.40.320.540.220.36

0.10.530.040.150.250.060.300.20

0.380.500.220.080.0970.09

(hr)565C0.21.252.58.827125510C1.23.041164

100200

454C0.888155025

E2(%)

(1050F)2.252.752.252.402.002.00

(950F)0.481.050.260.720.950.961.101.90

(850F)1.701.630.720.350.450.29

Secondaryti(hr)

1.89

10.530150370

77.525491041063001400

82545210210295

Cr e e p

(%/hr)

1.100.300.190.0520.01020.0030

0.0640.0700.0130.0110.00750.00800.00240.001

0.1640.0440.01150.00130.00160.00086

Ter t i a r yer(%)

18.521.023.122.115.715.2

22.423.227.224.125.126.420.819.4

18.018.416.818.420.020.1

Creeptr(hr)

3.15.22.86.362.1020.

28.36.

130.181.428.416.1558.

4133.

1453125542,513831

840129

5807,1.0,9,5

.4

.5

.0

.3

.3

.1

O\


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17

Two s t r a i g h t l i n e s h a v e be e n s how n on F i g . 5(a). Al l t h e 565C dat aand th e long-time tests at 510C fall on one lin e, wh il e all the 454Cdat a a nd t h e s hor t-t i m e t e s t s a t 510C fal l on t h e ot h e r l i n e . Ag a i n t h es lop e s of th e t wo c u r v e s a r e s u c h t h a t a i s v e r y n e a r u n i t y . Th e l a r g e stdev ia ti on of a from uni ty is obser ved for the 0.135% C ste el at 454C,pr i m a r i l y be c a u s e of th e t h r e e l on g-t i m e da t a poi n t s . In F i g . 5(a) t h e s epoi n t s w e r e i g nor ed. Not e t h a t de s p i t e a c h a n g e i n fr a c t u r e mode (fromt r a n s g r a n u l a r to i n t e r g r a n u l a r) a t 565C, th e r e i s n o a p p a r e n t effe c t ont h e fi t of th e da t a i n F i g . 5.

In F i g . 5(b) th e da t a for both s t e e l s a r e g i v e n i n on e log t ,log * 2 p l ot . Wi t h t h e exc e p t ion of se v e r a l of th e 510C te s t s, w h i c hfa l l to t he l eft of th e c u r v e , t h e da t a fol low on e s t r a i g h t l i n e w i t h3 n e a r u n i t y . Wh e n t h e c r e e p c u r v e s for t h e t e s t s t h a t de v i a t e fromt h e c u r v e s w e r e exa m i n ed, i t w a s qu i t e obv iou s t h a t t h e y exh ibi t ed" a t y p i c a l " c r e e p c u r v e s s i m i l a r to t h at of F i g . 2 . If t h e t i m e to t h eend of the se cond "s te ady-st at e" pe riod is taken for tz, t h e a g r e e m e n tw i t h t h e r e m a i n de r of t h e d a t a i s e xc e l l e n t .

Tabl e s 6 a nd 7 g i v e t h e coeffi c i e n t s for t h e l e a s t squ a r e s l i n e sfor the data shown in F ig s. 3 through 5, as determ ine d from Eqs, (1)and (2). For the t , t rel ati onsh ip s of Table 6, the insu fficie ncy ofda t a h i n de r s t h e u s e fu l n e s s of t h e r e l a t i o n s h i p s . Ne v e r t h e l e s s , t h ea p p r oa c h of a to u n i t y for mos t of th e da t a i s a p p a r e n t . Wh e r e t wo s e t s

4

of coeffi c i e n t s a r e g i v e n, t h e s e cond w a s de t e r m i n ed aft e r a poi n t t h a tobviously devia ted wa s dropped. The combined c urv e [shown in Fig . 5(a)]for th e 0.120 and 0.135% C stee l w as deter mine d with out the thr ee long-time tes ts for the 0.135% C st ee l.


18/28

18

s

s2

5

2

5

2

" I N\E L

i .

a #454'C0

'So

r i !\

65*C

\

0RNL-DW6 7S-KCTS

* * \^ NS S

10 " * 2 5 0 "s 2 5 1 0 ~2 2 S 10" ' 2 5 10 2 5 101

w n r n . u w tg-icouc

2I0 1

5 sr m

/-

m

|

tl-7i V "

7*. K/p../0/

at 454*10* DO 568*

y

cc -

/ ]

j

y .

8 1 0 * 2 5 * 2 5 K ) 4

Fig. 5. Relati onships Betwee n Rupture Life a n d (a) Min im um Cree pi

Rate a n d (b) End of Sec ondary Creep St age for Both t h e 0.120 a n d 0.135% CS t e e l s .


19/28

19

Table 6. Coefficients for Empirical Relationship BetweenRupture Life and Minimum Creep Rate

Carbon(wt %)

0.0090.030

0.030

0.1200.1350.1,20 and

0.1350.1200.1350.120 and0.135

Temperature(C)454454565454454454565565565

Numberof Data

765bh4687613

a1.050.800.840.830.950.870.740.92

0.970.950.96

C0.7191.111.051.351.260.560.670.46

0.700.670.67

SEa

0.110.170.070.130.050.090.060.09

0.080.040.07

aStandard error of estimate.One obviously deviating point excluded.


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20

Table 7. Coeffici ent for Empi ric al Relat ionship Be twe enRu pt ur e Life an d Tim e to the Start of Tertiary- Creep

Carbon(wt 500.0090.0300.009 a nd0.030

0.120 an d0.135

Numberof Dat a17183536

e1.040.930.971.01

A0.290.450.380.46

S Ea

0.270.190.230.19

aS t a n d a r d e r r or of e s t i m a t e .


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The t , *2 relationships of Table 7, where the data from all tempera-tures are rather closely accommodated by one curve, indicate that $approaches unity in all cases. All data were included in fitting thecurves of Table 7, and no attempt was made to improve the fit for thepoints that obviously deviate from the majority of the data and can beaccounted for by "atypical" creep curves. As stated before, super-position of the curves for the 0.009 and 0.030% C steels indicated goodagreement. To verify that a common equation could be used to representthese data, we plotted the 95% joint confidence ellipses about theestimated equation parameters. Such a plot showed a large region commonto both data sets and implies that the data may be represented by acommon equation. (If the two ellipses did not overlap, the data shouldnot be combined.) On the other hand, a superposition of the curve forthe 0.120 and 0.135% C steels on the curve for the 0.009 and 0.030% Csteels shows that it is shifted slightly to the right of the curves forthe other two steels. When the joint confidence ellipses for bothcombined data sets are plotted, only a small region of overlap was found.Although this smaller region indicates a shift in the data, the overlapimplies that a single equation may be used to represent the data, anindication that A may be insensitive to a difference in the primarymicrostructural constituent (proeutectoid ferrite and bainite). Withoutfurther data, however, we have not chosen to write an equation for alldata.


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DISCUSSION

On prolonged exposure of 2 1/4 Cr-1 Mo steel at elevated tempera-tures, precipitation reactions occur; the type of reaction dependsupon the microstructural constituents [6,73 In proeutectoid ferrite,the primary constituent of the 0.009 and 0.030% C steels, M02C, formsfirst, then eventually gives way to MeC. In bainite, the microstructuralconstituent of the 0.120 and 0.135% C steels, a more complicated reactionsequence occurs, which Baker and Nutting [7] have written as

Ce m e nt i t e ^C e m e n t i t e+Mo2C

In either proeutectiod ferrite or bainite, a finely dispersed M02Cprecipitate enhances the creep strength (all other precipitates form withmorphologies that do not enhance creep strength). Although the kineticsof M02C formation and eventual disappearance differ in proeutectoidferrite and bainite, we believe that it is the formation (strengtheningeffect) and eventual agglomeration or overaging (loss of strengtheningeffect) that gives rise to the (observations on the) effect of tempera-ture on C of Eq. (1).

Consider first the 0.009 and 0.030% C steels, whose properties aredetermined by what occurs in the proeutectoid ferrite. Before the M02Cprecipitates, the creep strength is primarily determined by the ferritematrix. Likewise, after agglomeration (overaging) the ferrite matrixdetermines the creep strength. We have previously shown [6] that inthese two steels M02C precipitation begins during the 1-hr temper at


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704C, but optimum strengthening (as measured by hardness) requiresbetween 1 and 10 hr at 704C and longer at lower temperatures. Wealso know that overaging occurs first in the low-carbon steel.

All indications are that the 0.009% C steel quickly overages at565C, and overaging was complete before the test was begun [6 ]. Thestrength at 565 C is therefore characteristic of the ferrite matrix.With the exception of three tests at 510C, the remainder of the testsshown in Fig. 3(a) fall on the straight line with the 565C data. Hence,the small amount of precipitate formed in this low-carbon steel at704C must not affect the strength (i.e., at all temperatures, thestrength is characteristic of the ferrite).

The data for the 0.030% C steel in Fig. 4(a) appear somewhat morecomplicated. The same precipitation reaction occurs, but because ofthe increased carbon content, the time period over which it occurs ata given temperature will increase over that for the 0.009% C steel. Wehave previously used the precipitation reactions to explain thesimilarity of the creep-rupture properties of the 0.030% C steel andthose of the 0.120 and 0.135% C steels at 454 and 510C 16 ]. We willhere rely on the same explanation, although verification in both instancesawaits electron microscopy studies.

At the highest te&t temperature, 565C, the M02C has little effecton the strength, probably because the M02C precipitate particles havegrown to such an extent that the interparticle spacing is too large toappreciably enhance the strength much over that of the ferrite matrix.At 454 C, on the other hand, the creep rate and its relationship torupture life is affected by the M02C that formed during the temper.


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The two parallel curves, therefore, represent these two microstructuralextremes: dispersion-strengthened ferrite at 454C, unstrengthenedferrite at 565 C. Finally, at 510C, where a line through the datawould approach the 454C line for short rupture times and approach the565C line for long rupture times, the temperature is such that theshort-time rupture properties are determined by the Mo2C-strengthenedferrite, and the data fall on the 454C curve, while the long-timerupture properties are those determined by the unstrengthened (overaged)matrix, and the data fall on the 565C curve.

In Fig. 3(a) for the 0.009% C steel, three of the long-time testsat 510C deviate from the indicated curves. Likewise, in Fig. 4(a) forthe 0.030% C steel, two long-time tests deviate one at 510C, one at565C. Although we feel that these deviations, which are of the typefound by Garofalo et al. [4], involve precipitation, perhaps MeCformation, electron microscopy is required for verification.

The explanation for the two straight lines for the log t , log representation for the 0.120 and 0.135% C steels is again sought in thecarbide precipitates. In bainite, the M02C again enhances the creep-rupture strength, but it is much more quickly replaced by other carbidesthat have little or no effect on creep strength. Hardness measurementsindicated that the maximum strengthening effect for M02C in these steelsis exceeded during the 704C temper, although considerable M02C is stillpresent in the microstructure after the temper [6 ]. Therefore, the 454and 565C curves in Fig. 5(a) must represent different stages in the overagingprocess. That is, since the overaging proceeds much more quickly, thehigher the temperature, the 565C curve must represent material that islittle affected by M02C, while the 454C curve represents material still


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strengthened by the M02C formed during tempering. The transition fromone curve to the other occurs at 510 C, where, given enough time, over-aging proceeds to such an extent that the properties are similar tothose at 565C, while for shorter rupture times, they correspond to thoseat the lower temperature.

With three exceptions, both materials follow the same t , erelationships [Fig. 5(a)]. The exceptions are the three long-time testsfor the 0.135% C steel at 454C. At present, no logical explanation forthis deviation is available. All three of these tests were below theyield stress, but all 510C tests, which follow the relationship, werealso below the yield. Since these tests appear to fit the t , tzrelationship [Fig. 5(b)], whatever causes the deviation affects tz andt similarly. It must again be the result of precipitate reactions, bu tfurther electron microscopy is required. These points were ignored inthe curve drawn in Fig. 5( a) .

Grant [5] has defined 2, the total elongation during t h e primaryand secondary stages of creep, as the "true elongation" because: "theuseful life of a metal ends when the first cracks a p p e a r or w h e ndistinct diminution of the cross section occurs ..." As w e h a v e s e eni n this study, however, the end of secondary c r e e p , a s nor m a l l y defi n ed(increasing creep rate), does not always coincide with neck or crackformation. Although we have not verified i t , we feel t h e t z that fitsEq . (2) is the one defined by Grant [5 ], while the premature tz of"atypical" creep curves (end of first "steady-state" period) is notindicative of the end of the useful life.

Several investigators have pointed out that the constant C ofEq. (1) is related to some measure of elongation, but with the exception


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of Garofalo et al. [4], have not determined a relationship [1]. Althoughour data show some indication of a constancy of Zz ~ i> the scatted istoo great to verify any relationship to C.

SUMMARY AND CONCLUSIONWe have collated and plotted data from four heats of nonnalized-and-

tempered 2 1/4 Cr-1 Mb steel according to empirical relationships betweenrupture life and minimum creep rate and rupture life and the time totertiary creep. The primary objective of this paper was not the determi-nation of a curve or curves for 2 1/4 Cr-1 Mo st ee l tha t could be usedunder all conditions. Rath er, we were interested in determinin g what thelimitations were on the use of these equati ons and wha t refinem entsmigh t be ma de to improve thei r usefulnes s. Toward that e nd, w e showedthat preci pita tion process es occurr ing durin g test i nflue nce t he fit ofthe empirical equations. When these precipita tion react ions are takeninto ac count, the deg re e of corre lat ion is considera bly imp roved. Asa re sult , w e suggest that when ever such correlations ar e of interest,it is most us eful to hav e a detail ed underst anding of the me tal lur gyof the mat eria ls under st udy. By understanding the mat eri al , it ma yals o prove p oss ible to expla in the l arg e amount of sca tte r ofte n foundwh en a lar ge data set obtained from var ious sourc es i s re prese ntedaccording to these em pirical equat ions.

ACKNOWLEDGMENTSThe aut hor g rat efull y acknowle dges T. E. Hebble for making the

statistical calculations; W. R. Martin, R. W. Swindeman, and E. E, Bloom


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for helpful discussions on the manuscript; Sigfred Peterson for a reviewof the manuscript; and Rhonda Castleberry of the Reports Office forpreparing the manuscript.

REFERENCES1. Honkman, F. C , and Grant, N. J., "An Empirical Relationship Between

Rupture Life and Minimum Creep Rate," Proceed ings of t he ASTM3 Vol. 56,1 9 5 6 , pp. 593-605.

2 . Davies, P. W., and Wilshire, B., "An Interpretation of the Relation-ship Between Creep and Fracture," Structural Processes in Cre ep 3Iron and Steel Institute, London, 1961, pp. 34-43.

3. Servi, I. S., and Grant, N. J., "Creep and Stress-Rupture Behavior ofAluminum as a Function of Purity," Tra nsactions of the AIME, Vol. 191,N o . 10 , Oct. 1951, pp. 909-916.

4 . Garofalo, F., et al., "Creep and Creep-Rupture Relationships in anAustenitic Stainless Steel," Tra nsactions of the AIMEt Vol. 310,N o . 4, Apr. 196 1, pp. 310-319.

5. Grant, J. M., "Stress Rupture Testing," High Temperat ure Propertiesof Met alsj American Society for Metals, Cleveland, 1951, pp. 4172.

6 . K l u e h , R . L . , E f f e c t of C a r b o n on t h e M e c h a n i c a l P r o p e r t i e s of2 1/4 Cr -1 Mo Ste el, ORNL-4922, Nov. 1973.7 . Baker, R. S., and Nutting, J., "The Tempering of 2 1/4% Cr-1% Mo

Steel After Quenching and Normalizing," Journal of the Iron a nd SteelInstitute, Vol. 192 , July 1959, pp. 257-268.


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LIST OF FIGURESFig. 1. Schematic Diagram of a "Typical" Creep-Rupture Curve

Showing the Three Stages of Creep.Fig. 2. The Creep-Rupture Curve for the 0.030% C Steel Tested at

565C and 13,000 psi (90 MP a).Fig. 3. Relationships Between Rupture Life and (a) Minimum Creep

Rate and (b) End of Secondary Creep Stage for 2 1/4 Cr-1 Mo SteelWith 0.009% C.

Fig. 4. Relationship Between Rupture Life and (a) Minimum CreepRate and (b) End of Secondary Creep Stage for 2 1/4 Cr-1 Mo SteelWith 0.030% C.

Fig. 5. Relationships Between Rupture Life and (a) Minimum CreepRate and (b) End of Secondary Creep Stage for Both the 0.120 and 0.135% C

Steels.

Creep Study

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