Fracture toughness of turbine-generator rotor forgings

En.+vrini:

Frwrrrru

Mechunics.

FRACTURE

1970. Vol.

I. pp.653-674.

Pergamon Press.

Printed in Great Britain

TOUGHNESS OF TURBINE-GENERATOR ROTOR FORGINGSt

H. D. GREENBERG, E. T. WESSEL and W. H. PRYLE Westinghouse Research Laboratories, Pittsburgh, Pa. 15235, U.S.A. Abstract-A comprehensive program is being conducted relative to applying fracture mechanics technology to large turbine-generator rotors. One facet of this program involves the determination of plane-strain fracture toughness (&,I over a range of temperatures for various types of rotor steels. Data have been obtained for ten large production forgings, representing three alloys, using various types of compact K,, and spin burst test specimens. These results demonstrate that valid I<[, data can be obtained in these types of intermediate-strength. high-toughness steels in the temperature range of practical interest. Data indicate that the plane-strain fracture toughness of these steels increases rapidly with increasing temperature and is rather high (K,,/a, > 1 in.“2), in the application range. As a result, the critical defect sizes for catastrophic failure upon a single cycle of loading are relatively large. The plane-strain fracture toughness measurements, as well as the application of these data, are presented and discussed.

1. INTRODUCTION As THE ratings of turbine generators continue to increase, there is a definite trend to the use of larger forgings- both in diameter and in length, and there is a desire-perhaps a necessity-to operate the rotors at higher stress levels. Fifteen years ago, a 200 MW turbine generator was considered a large unit. Turbine generators are now being manufactured with outputs exceeding 1000 MW and those of 1500 MW capacity are under consideration. The largest 3600 rev~min machines have rotor forgings which weigh over 80 tons, as rough machined, and those of the 1800 revfmin generators may weigh as much as 190 tons. Three basic compositions are used for practically all of the large turbine-generator rotor and disc forgings manufactured in the United States. ASTM specifications A469, A470, and A471 define their nominal compositions, as follows: (max. unless otherwise noted, Fe bal). c A469 A470 A471

0.27 0.25jO.35 0.28

Mn

P

s

0.70 1-O 060

0.015 O-015 0.015

0.018 0.018 0.018

Si

Ni

Cr

MO

v

0+15/0.30 3-Omin 0.50 0.20/,~ O-03 min 0. D/O*35 0.75 090/1~5 l.Oj1.5 0.201.30 0‘10 3.2514.0 1~25~2~00,301.~ O.OS/.IS

The 3% Ni alloy (hereafter referred to as Ni-MO-V) is used for practically all generator rotor forgings. The 1 Cr-1 MO alloy (Cr-MO-V) is used for turbine rotors which operate at temperatures above 850°F and the 3.5 Ni-1.5 Cr alloy (Ni-CrMO-V) has been used for some large generators, particularly those that operate in outdoor environments, and is used extensively and almost exclusively for low-pressure turbine rotors and discs. As rotor forgings, the alloys are used at yield strength levels between 60 and 120 ksi; turbine discs are heat treated to various yield strengths, as high as 160 ksi, min. *Presented at the National Symposium June 17-19.1968.

on Fracture 653

Mechanics.

Lehigh University,

Bethlehem,

Pa.,

654

H. D. GREENBERG.

E. ‘I‘. WESSEL

and W. H. PKY1.E

Inspection procedures on large rotor forgings normally entail visual inspection of the surface, magnetic particle inspection of the o.d. surface and bore, and ultrasonic inspections at high test sensitivity of the entire volume of the forging. If the forging does not contain any impe~ections, there obviously is no problem. However. in a few cases, forgings do reveal recordable indications of imperfections either during volumetric ultrasonic inspection or magnetic particle inspection of the bore. Over the years, many of these imperfections have been subjected to metallurgical investigations either by trepanning them from forgings which ultimately were used, or by rejecting the forging and evamating the defective condition by means of macroetch tests, tensite tests, and/or spin burst tests [ I]. As a result of these many investigations and a firm understanding of the distribution and magnitude of stresses present in the forgings, metallurgists and design engineers make individual judgements on the disposition of forgings containing ultrasonically detected imperfections. They necessarily are very conservative. If the defective condition is judged to be large enough to be of any concern whatsoever. it is removed. usually by overboring. Occasionally, forgings have been manufactured from which the questionable condition could not be removed and the forgings were rejected. For the past twenty years, Charpy impact test requirements have appeared in rotor specifications, and transition temperature concepts have been factored into design. We now feel that this approach can be improved upon because the rapidly developing technology of linear etastic fracture mechanics appears to have advanced sufficiently far as to offer a more exact method of establishing acceptance standards for these massive forgings. In early 1965, a high priority research program was organized at the Westinghouse Research Laboratories to exploit this development. At the beginning of this program, it appeared that the most practical test specimen for establishing the K,, of rotor forgings was the Wedge-Opening-Loading (WOL) specimen[Z] or a large spin burst test rotor[3]. Even though the WOL specimen requires the least volume of metal of any of the recognized fracture toughness specimens for low strength materials, the large size specimens required for measuring the fracture toughness of these alloys precluded taking them from excess metal from production rotor forgings. A decision was, therefore. made to test five forgings from each of the three alloys being used for rotor forgings and to confine the tests to forgings that had been rejected as a result of nondestructive testing. The steel companies cooperated in this effort by donating large discs cut from rejected rotor forgings. To date. ten large forgings have been cut up for WOL fracture toughness tests; spin burst test rotors containing fatigue precracks were also tested on three of them. This paper contains fracture toughness (K,,) data as a function of test temperature for ten turbine-generator forgings of three altoys, critical flaw size evaluations based on these data. and a general discussion of the applicability of the data. Although the entire program is not yet complete, sufficient KIe data have been accumulated to establish trends and thus to make a presentation worthwhile at this time. This paper is in the nature of a progress report. 2. MATERIAL To date, ten turbine-generator rotor and disc forgings have been cut up for this program. Their chemical compositions (ladle analyses) may be found in Table 1. Their heat treatments, relative sizes, and mechanical properties (averages of at least two. but usually four, specimens tested at the mill) are presented in Table 2.

Fracture toughness of turbine-gene~tor

rotor forgings

Table 1. Chemical compositions (ladle analyses) of the ten turbine-generator in the Westinghouse fracture toughness testing program Serial No. Ni-MO-V VI233 9-1638-l 1245357 2618 Cr-Mo-V HV6892 I24K406 HV6839 Ni-Cr-MO-V 3178 HV924 I H D9980

655 forgings included

Supplier

C

Mn

P

S

Si

Ni

Cr

MO

V

3 D A E

0.25 0.20 0.27 0.25

066 0.55 0.70 0.42

0.008 O*OlO 0.015 0.006

0.012 0+)09 0.020 0.014

0.23 0.19 0.03 0.02

2.92 2.96 3.20 3.46

0.25 0.48 060 0.38

0.43 0.29 0.35 0.39

0.09 0.08 0.09 0.08

c A e

0.32 0.31 0.30

0.81 0.78 0.77

0.009 OG)9 0.009

0+06 0.010 O-008

0.27 0.15 0.28 O-07 O-25 0.38

1.03 I.10 I.05

1.17 1.15 1.22

o-27 0.26 0.23

E

0.24 0.28 0.27

O-28 0.30 0.33

0.005 0.009 0.009

O-012 0.005 0.009

0.04 0.16 0.04

164 1.67 1.67

0.39 040 0.40

0.11 0.12 0.13

e C

3.51 3.40 3.48

The material furnished by the steel mills was in the form of 6-15 in. thick discs cut from the body (largest diameter section) of the rotor forgings or the hub of turbine disc forgings. The WOL specimens and spin burst test rotors were machined from the discs near the mid-radius position but occasionally were taken closer to the center of the discs. All of the notches (fatigue precracks) were in the radial direction of the forging. In addition to these tests, numerous tensile specimens, Charpy V-notch specimens, and 2 X 5 X %in. Drop Weight specimens were machined from material adjacent to the WOL specimens or directly from the broken spin burst test rotors. 3. EXPERIMENTAL

PROCEDURE

AND TEST RESULTS

(a) Fr~cru~~ toughness tests Figure 1 shows the general configuration of the three types of compact-tension, fracture toughness specimens utilized in this investigation. Various sizes of specimens up to 4 in. thick (all other dimensions in direct proportion) were utilized. In order to maintain plane-strain conditions, the specimen size was successively increased as the test temperature was increased to the temperature range of practical interest to design engineers. Table 3 shows the overall dimensions of the various fracture toughness specimens as well as the measuring capacity of each in terms of the ratio of K,, to yieid strength. An extensive amount of plane-strain fracture toughness testing was conducted with compact K,, specimens over a range of temperatures for each forging. These data are summarized in Fig. 2 an+ 3 for the Ni-MO-V alloy, Figure 4 for the Ni-Cr-MO-V alloy, and Fig. 5 for the Cr-MO-V alloy. In several forgings, data were also obtained with other types of fracture toughness tests; namely, spin burst tests (discussed in detail later), single edge notch tension tests[4] and cock-line-loaded tests[S]. Within the scatter due to the somewhat inhomogeneous nature of these large forgings, all of the tests produced equivalent values for K,,. All of the data shown in Figs. 2-5 meet all the criteria that are currently advocated by ASTM Committee E24 for assessing the validity of plane-strain fracture toughness measurements [ 6, 7 I. All of the WOL and CT test specimens employed in this investigation were precracked by fatigue (zero to max in tension). The maximum stress intensity during precracking, hence the size of the fatigue plastic zone, was kept quite low (- 15-20 ksi (in.)“*) during fatiguing to prevent any influence on the subsequent measurement of KI,.

42

30

34

31

60

75 in. dia.

65 in. dia.

2618

Cr-MO-V XV6892

124K406

HV6839

Ni-Cr-MO-V 3178

HV9241

HD9980

1550

1550

1750

1775

1750

1460

1500

1.520

IS.50

Austenitizing Temp. (“F)

1090-40

1125-1.5

109040

1260-38

124040

126040

1110-38

1120-38

1180-40

1180-40

Tempering Temp. (“F-Hr)

;

E 0 g

A 0

0”

Data source

132 127 141 141 181 174

112 113 117 117 113 II2

of four tests.

117 113 124 126 165 154

93 88 92 94 89 89

80 86 86 85 83 80 90 93

101 105 101 102 103 100 113 110

Ultimate Yield strength strength tksi) (0.2% offsetjtksi)

20 19 19 22 15 18

19 18 18 18 19 19

22 22 22 21 25 16 20

24

63 63 62 65 45 58

53 46 53 50 54 56

63 58 60 52 60 53 56

62

Elong. Reduction % in 2 in. of area(%)

78 82 76 36 25

7.5 6 9 14 10 9

19 39 42 56 48 27

-20 -70 -SO -I:! 130

175 225 200 19.5 175 IS0

50 90 -13s

110

14s 92

-

Charpy V-Notch Data Ftlb at 75°F FATTPFI

of the ten turbine generator forgings included in the It fracture toughness testing program

i’suppliers’ data were obtained on radial core bars taken 3-5 in. below the surface-average SWestinghouse data were axial tests at mid-radius positions-average of two tests. min.

27 in.

40

1243357

x

31

9-1639-l

19 in.

43

Ni-MO-V V1233

x

Max. dia. (in.)

Serial No.

Table 2. Mechanical properties. impact properties. and heat treatments

Fracture toughness of turbine-generator

rotor forgings

657

oiv = B

w, = 1.356

a = .SB H = .5B

D = T =

,388 .5B

‘x’ Type WOL Specimen

W a H D II T

= = = = = =

2.558 I.06 1.246 ,708 3.28 .62SB

l------*1

----A 7’ Type WOL Specimen

W = a = Ii = D = Wl= H, =

?,OB i.Dr3 1.28 0.58 2.58 0.656

t___--

w

Corn&t

-I Tension Specimen

Fig. I. General configuration and dimensional proportions of the various types of compact fracture toughness specimens employed in this investigation.

With the possible exception of some of the tests at very low temperatures (- - 320°F), the max K, (and plastic zone size) during fatiguing was well below that developed in the K,, test. The loading rate employed in the KIC tests was slow-representative of static loading conditions. The average rate for the various types and sizes of specimens was I( = 100 ksi (in.)~‘*/min. The effect of dynamic loading (fracture induced in a few milliseconds) is currently being investigated on specimens machined from several of these same forgings. In order to determine how reproducible the K,, vs. test temperature curves were for

EFM Vd

I. Na 4-F

6.58

H. D. GREENBERG

‘:. ‘I. WESSEL

and &‘. H. PRYLE

Table 3. Dimensions and measurement capacities of various compact fracture toughness specimens used in this investigation Estimated m~surement capacity* Type

K,,./ow

( K&Y.U 1L’

ix WGL 2x WOL 4XWOL 1TWOL 2TWOL 3T WOL 4T WOL 1TC.T. 2TC.T. 3T C.T. 4T CT.

0.45 0‘65 090 0.63 0.90 i.10 I.30 0.63 090 l-10 I.30

O.?O$ 0.423 0.82f O-40 0.80 1.20 1.60 0.40 0.80 1.10 1.60

Overall dimensions Thickness Height Width (in.) (in.) (in.) 1.0 2.0 4.0 1.0 2.0 3.0 4.0 I .o 2.0 3.0 4-o

1.0 2.0 4-o 2.48 4.% 7.46 9.92 2.4 4.8 7.2 9.6

I.44 288 5.75 3.20 6.20 9.20 12‘0 2.5 S-0 7.5 IO.0

tBased on currently suggested ASTM E24 minimum size criterion a & B ?z 2.5 ( K,&rr.s)2. $Currently limited by the crack length size requirement of the presently suggested criterion.

a given alloy, the curves for the four Ni-MO-V forgings were plotted on a single set of coordinates. These curves, as well as comparable ones for the Cr-MO-V and NiCr-MO-V forgings, may be found in Fig. 6. The three classes of steel forgings were compared to one another by replotting the data in Fig. 6 as scatter bands for each alloy, as shown in Fig. 7. The very high strength Ni-Cr-MO-V disc is excluded from this comparison because of its much greater yield strength. (b) Spin burst tests A portion of this overall investigation involved comparing K,, data determined by means of compact tension tests with those obtained from spin burst tests on the identical forgings. Since spin burst tests take advantage of inertia forces and specimens are accelerated in vacuum, large section sizes can be tested and a high degree of constraint maintained in the vicinity of the fatigue crack (test notch)[3]. East spin test rotor was about 13 in. dia. by about 9 in. long and had a test notch of about 4 in. length, one end of which terminated in a fatigue crack. The design and method of notching of the spin burst test specimen are shown schematically in Fig. 8. In brief, the fatigue crack is formed by hydrostatically loading the 0.25 in. dia. notched hole by means of an oil displacement technique[@ Fatigue crack growth is induced at relatively low stresses, 8~-40,~ psi (K, = 30 ksi (in.)1’2max>, at a frequency of one pressure cycle/set; it is monitored ultrasonically and cyclic loading is stopped after the fatigue crack grows to a length of Oal-0.2 in. Details of the precracking method and the calculation of KI, from spin burst test data have been presented elsewhere[9] and will not be repeated here. As shown in Figs. 2 and 5, the WOL test data correlate remarkably well with spin burst test data. (c) Intact tests Charpy V-notch

and Drop

Weight

tests (P-3 specimens-ASTM

specification

659

Fracture toughness of turbine-generatorrotor forgings I

I

/

I

124J 357 NiMoV

-EC! a

IXWOL

0 2x WOL

q 4XwoL .

-130

IT WOL

l 21 WM n 37 WOL

-110

S Spin Disc

._ :: --w a

160

/

ill

I

I X48

I

/

NiMoV

II t -300

-200

1%

T

-103

Temperature.

I ‘d

-70

\

0

:: $ v.

0

11:

100

am

4

Fig. 2. Temperature dependence of the plane-strain fracture toughness (K,,) of two NiMoV

alloy forgings.

E208) were conducted on each of the forgings. Both types of specimens were machined from the same locations as the WOL specimens; they were oriented axially with the notch (plane of fracture) in the radial direction. Impact data from the ten forgings are plotted in Fig. 9.

660

H. D. GREENBERG,

E. T. WESSEL

and W. H. PRYLE I

I

P 1638- I NiMoV

7

1

150

0

21 CT

* 2T WOL /

r4TWoL

V 1233 NiMoV

Temperature.

OF

Fig. 3. Temperature dependence of the plane-strain fracture toughness (K,,.) of two NiMoV

alloy forgings.

(d) Tensile tests Tensile specimen blanks were also taken from the same locations as the impact specimens; they were oriented in the axial direction ~longitudinal tests) and were machined to the configuration of the 0.350 in. dia. specimen shown in ASTM Specification A370. Figs. 3 and 4 (#4). Specimens were tested over a range of temperatures:

661

Fracture toughness of turbine-generator rotor forgings

220

l&l .3 140

5 f aA 0

‘LQ

z D

60

20

I

I

I

I

I HV9241

NiCrMoV

-

220

-

180

l 21 WOL l 31 WOL

.

41 WOL

- 60

24D

I

I

/

/ HD9980

I

0

Fig.

4. Temperature

dependence

I

NiCrMoV

of the plane-strain fracture forgings.

toughness

(K,,)

220

21 CT

of three

NiCrMoV

alloy

_.”

I

\

d-._._..L

-T-------1

i

. .._ __.._._

160

I

’

I

I

i

I

/ i+vf&H

I

i

i

/

-I

_..._.AU

crhtov

IM .27kw

T--

ii

.

WI--

Fig.

5. Temperature

dependence

of the plane-strain fracture CrMoV alloy forgings. 662

toughness

-

(A’,,1 of three

1% -*----

21118 124J3Y

130

110

‘5

m

:: E Y.

70

j! 0

50

30

10

ml -----

3118 W924I HOPWO

220

i@l ,s

-----.N____

g I40 ; g D

Mt

70

im

L'i,

110 ? (am 2 VI 2 10 z D

M

N

IO -m

-m

-1m

0

xa

Isn*ltraturr.7 Fig. 6. Summary

of the fracture

toughness

behavior types. 663

of various

forgings

for the three

alloy

664

H. D. GREENBERG.

E. T. WESSEL

and W. H. PRYLE

Fracture toughness of turbine-generator rotor forgings

665

Specimen Blank

SpirrBurst Toughness Specimen Fig. 8. Details of specimen preparation of fatigue precracked spin-burst test specimens.

the data are too voluminous to be included in this paper but are available authors upon request. Yield strength is plotted as a function of tem~rature forging in Figs. 2-5. Room temperature data may be found in Table 2. With exceptions, the data are comparable to suppliers’ data on radial tests taken the o.d.

from the for each very few close to

4. APPLICATION OF DATA It is by now generally agreed in the engineering community that linear elastic fracture mechanics provides a quantitative means for estimating critical flaw sizes in massive structures provided that valid KI, data can be obtained at the temperature of interest. The data presented in this paper are not yet sufficient to permit a rigorous calculation of true critical flaw sizes in turbine generator rotors for reasons which are discussed later: however. the general approach can be described, as follows: First, let us consider the case of a disc-shaped crack located at the mid-wall position of a turbine generator; its critical depth (2~). that is its diameter, in the radial direction. can be established from the equation[ lo]: (K,,)” = ?rtr%llQ f

H. ID. GREENBERG,

666

E. T. WESSEL

and W. Ii. PRYLE

where K,, is the plane strain fracture toughness in ksi (in.)lj2, (T is the applied stress normal to the flaw in ksi. a is the radius of the crack and Q is the flaw shape paremeter. The critical sizes of cracks intersecting a surface may be estimated in a similar fashion with the only difference between the expressions being that the Q parameter is changed. the flaw depth for surface flaws is ‘a’, and a surface factor term. 1-ZI. is incorporated in the expression. Calculations of critical flaw sizes are actually quite simple since curves showing how Q changes at various a/r,, ratios and flaw geometries, and giving KI as a function of applied stress and a/Q. have been developed and may be found elsewhere in this publication. Turbine rotors and discs are very complex pieces of machinery which are subjected to large changes in temperature and stress during start-up, making it difhcult to present typical stress, distributions in a paper. Generator rotors, on the other hand. are much simpler to analyze; Fig. 10 shows a typical distribution of tangential and radial stresses in a generator rotor[l 11. The absolute value of stress varies considerably depending on the diameter, speed. and rating of the rotor. For illustration, critical flaw sizes have been calculated for the ten forgings from which KI, data are available at four applied stress levels (100. 75, 50 and 25 per cent of yield strength) and at two temperature levels (0°F and 150°F). These data may be found in Table 4. The writers wish to emphasize that the critical flaw sizes tabulated in Table 4 are not yet in a form in which they can be applied directly to design and qua&y control problems, for the following reasons: (1) in numerous cases, the sizes of the critical flaws listed in Table 4 are so large that the stresses acting on the remaining section would become critical in a tensile overload situation or serious imbalance and vibrations would occur well before the flaws grew to the critical size. Furthermore, the large sizes of these flaws would sometimes involve their being located in severe stress gradients; the analysis used to determine the critical flaw sizes is not directly applicable to this condition. (2) All of the K,, determinations were made in clean metal. It is reasonable to assume that the fracture toughness of dirty metal, i.e. areas cont~ning heavy concentrations of nonmetallic inclusions. would be significantly less. Tests are currently being conducted to evaluate this factor. (3) We do not know how valid it is to extrapolate K,, data to higher temperatures of Table 4. Estimated critical flaw sizes of ten forgings for brittle fracture upon a single application of bad at 0°F and 15O”F

Serial No. Ni-Mc+V VI233 9-1638-l 124J357 2618 Cr-Me-V HV6892 124K406 HV6839 Ni-Cr.MwV 3178 HV9241 H D9980

Supplier

Yield strength (0.2% offset). ksi @ 150°F Q 0°F

@O-F

6 D A E

89 89 82 95

80 XI -.

44 70 60 53

c A c

88 95 92

x2 88 x5

e c C

II? 128 I60

_. -_

‘I’

@ IWF

I IO 130t

Critical flaw sizer @ CF Internal disc-shapedcrack (c = 10 dia. tin.1 ra Y.S. @ 25% Y.S. @ 5o$r, Y.S. e 75% Y.S. --_-

b-l 15%

1.h

0.7 I.6 1.5 0.8

tP4 Il.9 a.7 0.4

..

13-2 8.0

3.8 3.1 1.x

40 45 46

62 x4 9%

5.2 ?+l h.4

I,3 I.4 1.6

o-5 0% 0.6

O.? 0.3 0.4

172 160 7@

._

4&X 39.6 4-7

14.X 9.x I.2

h3 4.3 0.5

32 2,.x 0.2

‘rExtrapolated. &, data not available. Note: Rotor forgings with smaller flaws than these ‘critical sizes’ may become inoperative because of imbalance. redistribution of stresses, slow crack growth under cyclic loading, or tensile overload failures.

Fracture toughness of turbine-generator

rotor forgings

667

interest to turbine designers. We are currently machining 6T, 8T and 10T WOL specimens from forging 3 178 in order to get valid data at higher test temperatures. These tests should be completed by early fall. (4) Turbine-generator rotors are expected to operate for very long times and are generally required to undergo some degree of load cycle duty. This involves reversals of stress numbering in the thousands. We have obtained fatigue crack growth data on one Ni-MO-V forging, but have not yet tested the other alloys. From the one set of data, it appears as though the critical flaw sizes in Table 4 may be reduced as much as 50 per cent, if one takes fatigue crack growth into consideration, but we do not have sufficient data to make a positive statement in this regard, as yet. Tests are currently underway to get fatigue crack growth data on clean and dirty areas of the three alloys. 5. DISCUSSION (a) General observations There are some significant general observations relative to the data shown in Fig. 2-5. First, we are now certain that it is possible to obtain the K,, parameter in these intermediate-strength, high-toughness materials within the temperature range of practical interest (approx. O-300°F depending upon the particular application).? Secondly, it is now well established that K,, increases relatively rapidly in the temperature range of practical interest. While there is some variance in the rate of increase in K,, between the different alloys, and to a lesser degree within any given alloy, the tendency for a relatively rapid increase is general. Finally, the fracture toughness of most of these materials is quite high for the application temperature range. From Fig. 6(a), it is apparent that the KI, fracture toughness varies appreciably between forgings in the temperature range of practical interest, for example at 0°F. This variance in toughness is not directly associated with a variation in yield strength; therefore, it is apparent that other metallurgical factors, e.g. composition, impurities, heat treatment, ingot size and/or microstructure have a collective influence on the K,, toughness. However, the variance in K,, between forgings in a given class does not appear to be so large as to preclude the use of an appropriate lower limit for each class of forgings in conservative design considerations. The KI, fracture toughness appears th outdoor installations. the minimum temperature of interest for startup conditions of generators is 0°F. In turbine rotors, the major interest is centered at higher temperatures, SOOF-300°F. where high stresses can occur during start-up due to thermal gradients.

Table 4. (contd.) Critical flaw sizes o 0°F Elliptical crack intersecting the bore depth x length (c = 5~) (in.) e 50% Y.S. @ 75% Y.S. @ Y.S.

0.3 0.7 0.6 0.4

x x x x

3 7 6.3 3.9

WI 0.3 0.3 0.1

xt X2.8 x 2.7 x I.4

0.06 0.1 0.1 0.07

Critical Raw sizes .@ 150°F Internal disc-shaped crack (c = ~1) @25%Yy.S. @5ORY.S.@75%Y.S. @Y.S.

X 0.6 XI.4 XI.3 x 0.7

44.4 a a

0.2 x 2.2 0.3 x 2.5 0.3 * 2.7

0.09 x 0.9 0.1 Xl 0.1 x I.1

0.05 Y 0.5 0.05 x 0.5 0.06 x 0.6

13.4 23.6 30.0

2.9 X 29 1.9x 19 0.2 Y 2.2

1.1 x II 0.8 x7.7 0.09 x 0.9

0.6 X6 0.4 X4.2 0.05 x 0.5

a a a

Critical flaw sizes @ 150°F Elliptical crack intersecting the bore depth x length (c = 50) (in.) e25rOY.S. e5O%Y.S. e75%Y.S. B Y.S.

I I.6 15.6 a a

5.0 6.3 a a

2.6 3.4 a a

7.7 x 71 9.9x99 a a

2 x20 2.9 x29 a a

0.8x 8 1.1 x II a a

0.4 x 4 0.6 X 6 a a

3.2 5.6 7.9

I.6 2.4 3.1

0.8 1.3 I.7

2.4 x 24 4.3 x 43 4.9 x 49

0.6 * 6.4 1.1 x I1 ,.4x I4

0.3 x 2.5 0.4 X 4.2 0.5 x 5.3

0.1 x I.3 0.2 x 2.3 0.3 Y 3.0

a a a

a a a

a a a

a a a

a a a

a a a

a a a

H. D. GREENBERG,

668 100

E. T. WESSEL

and W. H. PRYLE

P 60

60 0 : 0’ 60 0’ 8 240

60; i8 40

2 E c

20

;

f f

20

4 .A

0

o-

0

80

80

c

i

60

1 p P L

40

J

20

i

I

T

I

I

I

I

-

I

loo

-

HD*III NiCrMoV w6wcrhw

e

3

60

b 3

40

t 5 :

20

* % b p” 4

60

60

40

J40

20

0

0

-100

0

loo

T#m#rature,

Fig. 9. Charpy

200

400-200

-100

OF

V notch impact

0

loo

szao

Tomprrature,

properties

of the various

OF

forgings.

300

f lk . E ai

400

i#

Fracture toughness of turbine-generator rotor forgings

Fig. IO. Distribution

669

of tangential and radial stresses in a generator rotor at 4320 revlmin.

to be more consistent between forgings in the Cr-MO-V class of steels as seen in Fig. 6(c). In the Ni-Cr-MO-V steels, Fig. 6(b), there is a very marked difference in fracture toughness for the one forging heat treated to a high yield strength. It is probable that the much poorer fracture toughness of this forging is predominately due to its higher yield strength rather than other metallurgical factors; however, this observation requires further investigation before a definite conclusion can be drawn.

670

H. D. GREENBERG.

E. T. WESSEL

and W. H. PRYLE

Within any given forging there is also a variation in KfC,,with some forgings exhibiting more data scatter than others. Such a variation in toughness is not unexpected because of the inherent inhomogeneous nature of such large masses of steel. The variation of toughness through the thickness (outside diameter to inside diameter) was investigated for three forgings. With the exception of the outer surface region (3-4 in. from the outside diameter), the toughness did not vary consistently as a function of depth from the o.d. For example, most of the data at 0°F in forging, 124K406 (Fig. 5(b)). represent samples taken systematically from near surface to near center, and within the scatter shown, there was no consistent variation attributable to location. Thus, when making practical use of data from any given forging, it appears necessary to employ an appropriate lower bound limit representative of the poorest toughness found in the forging. t bf Specimen size requirements

The extensive ICI, testing program involved in this investigation provided a good opportunity to assess the applicability of the specimen size criterion which is presently advocated[5-71 for plane-strain fracture toughness testing. This specimen size criterion is currently based on both the crack length ‘a’ and the thickness B being equal to or greater than 2.5 (K,,/avs)*, where (+r.s is the O-2 per cent offset yield strength at the test temperature. All of the test results shown in Figs. 2-5 are from specimens which satisfied this criterion. However, considerable additional data, not shown in Figs. 2-5, were obtained for the various forgings; numerous test specimens failed to satisfy the size criterion. In general, the K, values obtained from the specimens which were too small (a and B in the range of l-5-2.5 times (ly&.rYs)*), were higher than those obtained with the larger specimens which did satisfy the size requirements. Typical examples for two forgings (HV9241 and 3178) are provided in Fig. 11 where arrows point to symbols which represent the results of specimens with a and B = i-5 to 2.5 (&~I(TY,?; all other symbols satisfy the criterion, a and B 2 2.5 (K,claws)2. Hence, the results of this investigation support the current recommendations that specimens with dimensions of a and B equal to or greater than 2.5 (K,,/or,s)2 are required to provide valid plane-strain fracture toughness measurements. The exactness with which a coefficient of 2-S can be applied to assessing the validity of results for other materials, or for these forging materials at higher temperatures is not well established. but those data[2,5,7], which are available. indicate that the present size criterion should be sufficient to assure plane-strain conditions. A few additional tests of larger size specimens at the higher temperatures are currently being conducted for some of these forgings to confirm that a coefficient of 2.5 is satisfactory for the engineering purposes for the higher portions of the temperature range of practical interest. (cl Fracture toughness measurements and transition temperature tests An analysis of the parameters of the drop weight test for nil-ductility transition temperature (NDT) has been reported [ I21 and the following expression was developed for approximating the dynamic crack toughness at NDT:

where lYId is the dynamic crack toughness for a running crack at NDT and oYd is the dynamic uniaxial yield strength at N DT.

Fracture toughness of turbine-generator

rotor forgings

671

I

I HV 9241 NlCrMoV

All Data. kept ”

ar Noted.

Satisfy Recommended

EO

Y

-

60

I

0' 2aor

I

I

I

I

/

,

I

I

3178 N!CrMaV

I I --2M)

l

31 WOL

.

41 WOL -220

MI

01

-300

I -200

I -100

t

t

I

I

Temperature.

I 0

I

loo

- a, i

XI

4

Fig. 11.The effect of insufficient specimen size on the measured values of K,,.

The results of the present investigation representing ten forgings of three different alloys provide a good opportunity to test the expressions for N DT because the following data are available: K,, (static), uYS (static) and NDT. The KI, and cYS data extend over a wide range of temperatures encompassing NDT. To utilize the above expressions for estimating Kid at NDT, one must convert the measured u Y.Yto estimated uYd at the NDT temperature. For a first approximation, uYd = 1.4 oYS was employed. For the temperature and strain rate range of concern, the 1a4 factor corresponds to the approximate increase in oyS that may be derived from the strain rate-temperature parameters usually employed[12-141. This resulted in estimated Kid values for NDT which were considerably

160 I

-.

I

I

I

I

I

I

1

_

160

_

80 -

60 -

/ 4DL

/

,

I

I

I

I

I

160

m K,d=

-1 .lEinch oyD

ooVD=4S 60-

/

40& a0

Fig.

12. Comparison

, 60

of the estimated

I I I I 80 100 120 140 MearuredFraclure Toughness _ Static.Klc

dynamic fracture toughness. NDT temperature. 672

I

160

K ,d. with the static

measured

K,, at the

Fracture

toughness

of turbine-generator

rotor forgings

673

higher than the measured K1, (static). A more conservative estimate of the dynamic elevation of the yield strength ((TYd= l-2 cY?;) based on another reference [ I.51 was then employed with similar results. Finally, it was assumed that any dynamic effect on uniaxial yield strength. did not carry over into a situation where plane-strain (triaxial Yd= crys). In this latter case, a better correspondence of stress) conditions prevail (CF Kid to K,, was observed. The three Kid estimates for all levels of cryd are provided in Fig. 12. With the exceptions of the case where mYd= crYs was assumed, the estimated Kid at NDT was higher than the measured static K,,. Since the dynamic crack toughness, Kid, is normally thought to be less than the static K,, for strain-rate-sensitive material, it appears that the Kid = 0.78 (in.) crYd expression results in an overestimation of Kfri at NDT for the forging steels. A similar overestimation has also been reported for two other materials [ 161. However, this subject requires further detailed consideration relative to the influence of the heat-affected-zone on the effective defect size of the drop-weight specimen and the actual elevation of the uniaxial yield strength by dynamic loading. Refinements in these considerations may result in an improvement of the observed correlations. On the other hand, the current observations could be interpreted as suggesting that dynamic loading effects, as measured in uniaxial tension tests. are not directly transferable to plane-strain fracture toughness tests where essentially tri-axial stresses prevail. These aspects appear worthy of further study in view of the increased interest in the effects of dynamic loading on fracture toughness.

flf (2) (3) (4) (5)

(6)

(7) (8)

6. CONCLUSIONS Valid KI, data are presented over a range of temperatures for four Ni-MO-V forgings. three Cr-MO-V forgings and three Ni-Cr-MO-V forgings. K,, data obtained from compact tension tests, correlate remarkably well with similar data obtained from spin burst test rotors, notched by fatigue cracking. K,, increases relatively rapidly in the temperature range of practical interest for turbine-generator rotor applications (O”F-200°F). Transition temperatures, particularly FATT. provide a rough index of the temperature range in which KI, rises rapidly with increasing temperature. Although the calculated critical flaw sizes for several of the forgings appear to be very large at the normal stress levels operative in turbine-generator rotors, they may be quite small for the Cr-MO-V forgings and the high strength Ni-CrMO-V disc, especially when one takes into account crack growth under cyclic loading and the likely detrimental effects of inclusion arrays. The specimen size criterion currently advocated for plane-strain fracture toughness testing, a and B 2 2.5 (Kl,/a,Y appears to be satisfactory for the materials tested. With the exception of the outer surface region (3-4 in. from o.d.), fracture toughness does not vary consistently as a function of depth from the o.d. The proposed expression Kid = O-78 (In.) ’ mYd (at NDT) results in an overestimation of Kid.

Acknoriedgement-The writers wish to acknowledge Evaluation Department. in particular R. B. Stouffer. are extended to G. 0. Sankey for the contribution of supported by the Large Turbine Division and Large Electric Corporation.

Eml

Vd

1. Na

4-G

the invaluable assistance of the Materials Test and L. J. Ceschini, and A. J. Bush. In addition. thanks unpublished spin burst test data. This program was Rotating Apparatus Division of the Westinghouse

674

H.

D.

GREENBERG.

F.

1. WESSEf.

and W.

Ii.

PRYLF

REFERENCES [II

R. W. Renner. H. D. Greenberg and W. G. Clark. Jr.. Ultrasonic and metaflurgical evaluations oftlau~ in large rotor forgings. Presented at the 1967 Int. Mtg qfASNT at Montrerrl. Cnnrrdrr ( 19h7). PI E. T. Wessei, State of the Art of the WOL specimen for li,,. fracture toughness testing. Ettgtr2i: prt>crlcut> &fecl?. I. 77 (196X). 131 G. 0. Sankey. Spin tests to determine brittle fracture under plane strain. SESA Paper 135hA. Presented at 1968 SESA Spring Mtg, Albany, New k’ork f 1968). [41 A. M. Sullivan. New specimen design for plane-strain fracture toughness tests. Mater. NW. .Srtrud. 4. ( 1964). 151 W. F. Brown. Jr. andJ. E. Srawley. Plane strain crack toughness testing of high strength metallic materials. AS TM Sppc. Tech. Publ. No. 4 IO ( 1967). [61 ASTM Committee E-74. Re~~~/~z~?letldedPrucfice j&r Pfune Srrcrin Frwrrrre Toughness Testing I$

High Strength ~et~~li~ M~t~riul~~ Usitrg LI Fatigue-Crucked

171 J. @I [91 ilO1 [I 11 113 1131 [I41 IiS1 1161

E. Srawley.

B~~dSp~~irnejt.

M. H. Jones and W.

F. Brown. Jr.. Determination of plane strain fracture toughnes\. ASTM Muter. Res. Stcmd. 7.362-766 ( 1967). W. 0. Clark. and L. J. Ceschini. Fatigue precracking of spin-burst toughness specimens. SESA Paper 1357. presented at IY68 SESA Spring Mfg. Albany. Ne+t* Ynrk ( 1968). G. 0. Sankey. Spin test to determine brittle fracture under plane strain. SESA Paper 1356A. Presented at 1968 SESA Spring Mtg, Aihuny, New York f 1968). Fracture toughness testing and its application.~S~~ Spct.. Trth. Publ. No. 3X t (1965).

R. E. Peterson. N. L. Mochel. J. D. Conrad and D. W. Gunther. L.arge rotor forgings for turbines and generators. ASME Paper 55-A 2 15. Presented at ASME A. Mtn. Chicogo. Illinois (1955). G. R. Irwin ct ~1..Basic aspects of crack growth and fracture. NRL Rep No. 6598. p. 38. ( 1967). G. R. Irwin, The leading edges of fracture mechanics. ASME Truru. ( 1966). H. T. Corten and A. K. Shumaker. Fracture toughness of structural steels as a function of the rate parameter. 7 log (A/E). ASME Puper No. 66WA-Met-8 t 1966). N. Perrone. Impulsively loaded strain-rate sensitive plates. ASME Puper No. 67-ATM-F f 1967). A. K. Shumaker and S. T. Rolfe. Static and dynamic low temperature R,,. behavior of steels. Paper presented at ASMESymp. Dynamic Fructrwe Toughness Testing. Chicctgct ( 1968).

(Recw’wd 4 MN?. 1969) Resume-On etablit un programme complet pour I’application de la technologie de la mecanique des ruptures aux grands rotors des turbo-gt&rateurs. Un aspect de ce programme concerne la determination de la resistance Q la rupture en force plane (k’,,) pour une gamme de temperatures et pour divers aciers de rotors. Le\ informations obtenues interessent dix moulages de grande serie. representant trois alliages, employant divers types de specimens d‘essais compact K,,. et de rupture par rotation a grande vitesse. Ces resultats montrent que des informations valables de K,,. peuvent etre obtenues pour des types d’aciers a resistance moyenne et a haute resistance, dans la gamme de temperatures d’intiret pratique. Les informations indiquent que la resistance a la rupture sous I’effet d’une force plane de ces aciers augmente rapidement avec la temperature et est assez elevee (K&r,, > 5 mm’% dans la gamme d’application. En resultat. les defauts critiques entrainant des defaillances catastrophiques pour un seul cycle de chargement sont relativement grands. Les mesures de&stance& la rupture sous une force plane, ainsi que l’application de ces donnCes. sont pr&entees et discuttes.

ZusammenfassungEs wird ein umfassendes Programm in Bezug auf die Anwendung der Bruchmechaniktechnologie auf grosse Turbinenlaufer durchgefiihrt. Ein Teilgebiet dieser Arbeit betrifft die Bestimmung der Bruchzhhigkeit im ebenen Dehnungszustand (K,,) innerhalb eines Temperaturbereiches fiir verschiedene Lauferstahle. Unter Verwendung verschiedener Arten von kompakten K,,-und Drallbruch-probekiirpern wurden Messwerte fur zehn grosstechnische Schmiedestiicke. die drei verschiedene Legierungen darsteflen. erhalten. Diese Ergebnisse zeigen, dass mit diesen mittelfesten. hochzahen St&Men. in dem fdr praktische Zwecke belangreichen Temperaturbereich, brauchbare i(,, Messwerte erhalten werden kiinnen. Die Messwerte zeigen, dass die ebene Dehnungszahigkeit dieser Stlhle mit steigender Temperatur rapid ansteigt und im Anwendungsbereich recht hoch liegt (K&r., > 5 mm’% Demzufolge sind die kritischen Rissgrossen fur katastrophale Zerstorung bei einem einzelnen Belastungszyklus VerhiiltnismGsig gross. Die Messungen der Bruchzahigkeit im ebenen Dehnungszustandes. sowie die Anwendung dieser Messwerte, werden dargelegt und besprochen.