Surface integrity and its effects on the fatigue life of the nickel-based superalloy GH33A

Int J Fatigue 13 No 4 (1991) pp 322-326

Surface integrity and its effects on the fatigue life of the nickel-based superalloy GH33A Qi H u a n g and Jing Xin Ren

This paper presents a study of the formation of surface roughness, residual stress and surface hardening produced by cutting and grinding, and then compares the different effects of surface integrity on the high-cycle and low-cycle fatigue lives of the nickel-based superalloy GH33A at room temperature and at 550°C. For the purposes of enhancing the fatigue life and reliability of machined components and controlling the machining parameters, the experimental results offer a useful reference point. It is concluded that, under the condition of low-cycle fatigue, surface roughness becomes the main factor and surface hardening has an unfavourable influence on the fatigue life, while the residual stress can be neglected because of its relaxation under the applied loads. The influence of the integrity of the surface on the highcycle fatigue life has a different mechanism from that on the low-cycle fatigue life.

Key words: machined surface integrity; nickel-based superalloy; high-cycle fatigue life; low-cycle fatigue life

The nickel-based superalloy GH33A is a key structural material used in the aeronautical industry. J Components made of GH33A normally function under severe conditions of high temperatures and large cyclic loads. An analysis of their service lives and failure under working loads shows that almost all the fatigue failures are initiated on the surface or near the surface of components. The machined-surface integrity, including surface roughness, residual stress and surface hardening, has a significant influence on the types of fatigue behaviour of the components. 2-5 It is regrettable that all the studies in the past focused on the influence of the surface integrity on high-cycle stress-controlling fatigue. "-11 As a matter of fact, many components in industry, such as aeroengine turbine discs, landing gears and high-pressure vessels, work under low-cycle alternate load conditions. It is now required to obtain information about the low-cycle fatigue performance of these components. Therefore, it becomes essential to study the effects of the surface integrity created by machining on the low-cycle fatigue life. ~2'~3 In order to widen the application of the superalloy GH33A and improve its fatigue behaviour, the object of the present work is to analyse and compare the different effects of surface roughness, surface residual stress and surface hardening produced by cutting and grinding on the high-cycle and the low-cycle fatigue lives and then to establish the relationship between the surface integrity and the fatigue life. Furthermore, reasonable machining parameters can be selected to improve the fatigue life.

The experimental conditions The nickel-based superalloy GH33A belongs to the class of age-strengthened alloys; its chemical composition and mechanical properties are given in Table 1. The specimens

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were designed as tensile-compressive axial cylinders. The procedures for the preparation of the specimens are as follows: double-vacuum melting, forging a round flat blank, cutting the blank into the specimens with a grinding wheel, standard heat treatment, working out the standard specimens and machining the working section of the specimens to achieve different surface qualities with different machining methods. Figure l(a) shows the machining process for these specimens. All the tests were performed on a MAYES servohydraulic test system. The high-cycle fatigue conditions were: R = - I , the stress amplitude ~ = 5 1 0 MPa, a triangular waveform and the cyclic frequency is 10 Hz. Referring to ASTM 606-80,14 we carried out the low-cycle fatigue test following the procedure adopted in GB6399-86.~5 The low-cycle fatigue conditions were R = - 1, a triangular waveform and a strain rate ~=4 x I0- 3 s--~. The three nominal strain ranges ~l~T were 0.6, 0.9 and 1.2%, respectively, so the cyclic frequency was equal to ½~/~l~T × 60 (per minute). Conducted at room temperature and 550 °C, each test was repeated five times to determine the average fatigue life so as to compare the varying effects of the different surface states produced by the four machining methods on the fatigue life.

Formation of surface integrity As listed in Table 2 a specific surface quality can be formed with different machining methods. The formation of surface roughness, residual stress and surface hardening depends heavily on the tool geometry and cutting parameters as shown in Fig. l(b). When the tool material is 643 carbide alloy, the rake angle ",/0=5°, the clearance angle ix0=5 °, the edge angle ×r-×~,-45 ° and the angle inclination M - 0 °, by using the

0142-1123/91/040322-05 © 1991 Butterworth-Heinemann Ltd Int J Fatigue July 1991

Table 1. Chemical compositions and mechanical properties of the Ni-based superalloy GH33A at room temperature

C ~<0.07

Concentration (%) Nb Fe

Cr

Ti

AI

19-22

2.5-3.0

0.7-1.2

1.15-1.65

B

~<1.5

~<0.01

Ca ~<0.07

Mn

Ni

~<0.35

base

~b (kgf mm 2)

~s (kgf mm -2)

5 (%)

qJ (%)

e~k (kg m cm -2)

123-126

87-92

27-30

28-34

3.5-7.8

Table 2. Different surface qualities produced by three machining methods Maching methods

Surface roughness Rz (l~m)

Rough turning Smooth turning Fine turning Grinding

35-80 6-16 1.5-2 0.8-2

Residual stress ~r (MPa)

700-800 120-200 140-250 550-900

Surface hardening Degree N (%)

Depth Hm (~m)*

30-40 15-20 30-40 20-30

140-200 80-150 90-120 35-65

*Hm is the depth of the surface hardening distribution

A-A

7o Z Workpiece

,

I

- V -

~

f

"

Workpiece

j,

A v

f ~ ~ rf

\

Cuttingtool

Standardworkpiece a

b

Fig. 1 Tool geometries for the preparation of the specimens. In (a) the left-hand side shows the tool used for cutting and the righthand side the tool used for grinding

orthonormal linear optimization to deal with the experimental data, we can achieve the relation between the surface roughness Rz and the residual stress ~;r as functions of the cutting speed v, the feed per revolution f and the back engagement ap, expressed as R~. = 134.8 v - ° ° s 2 f °'672 a °'°44 I~m

(1)

crr = 1019.8 v - ° ° 9 s f °221 ap°399 MPa

(2)

Here, the residual stresses were measured by X-ray stress analysing equipment (RIGRAKU MSF-2905: tube voltage, 30 kV; tube current, 7.5 mA).

Int J Fatigue July 1991

Equations (1), (2) and Table 2 show that when selecting larger f or ap we can obtain a rough surface with a large residual stress. A higher cutting speed will be favourable to form the surface quality because a fine and low residual stress surface can be formed at this speed. In addition, a larger back engagement ap or feed per revolution f is still the main factor that increases the surface hardening, for example, at v=63.3 mm s -1, when f increases from 0.07 to 0.245 mm/rev, and ap from 0.1 to 0.5 ram, the degree N% of surface hardening, which is defined as [(Hv-H,.o)/Hvo] × 100%, where Hv is the surface microhardness of hardening and Hv0 is the microhardness of the base

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material, will increase from 14.12 to 31.82%, while the surface hardening depth H,1 increases from 33 to 144 ~m. The same conclusions as above have been proved when other tools with different geometries have been selected for investigation. Experiments 16 have shown that the main tool geometrical factors affecting the surface roughness are the rake angle % and edge angle ×r, that the factors affecting residual stress are the clearance angle o~0 and the rake angle %, and further, that the factors affecting the surface hardening are the clearance angle el0, the corner radius r, and the rounded cutting edge radius rn. Unlike a surface produced by turning, a ground surface shows stochastic features in its contours and has a thinner layer where deterioration has taken place. The grit size, the hardness of the wheel and the type of grinding fluid all have a great influence on the formation of the surface quality. The surface roughness and the surface hardening depend mainly on radial feed and then on the speed of the workpiece. An increase of the wheel speed will cause the surface roughness and surface hardening to decrease; on the other hand, it will cause the surface tensile residual stress to increase.

The effect of surface integrity on the highcycle fatigue life of GH33A Table 3 gives the results of stress controlling the high-cycle fatigue life of the specimens produced by the rough-, smoothand fine-turning methods at room temperature and at 550 °C. The surface machining coefficient 13 is the ratio of the mean fatigue life N~ of the specimens produced by another turning method to the mean fatigue life Nf of the specimens produced by the rough turning method, that is 13=NdN~r. A large value of 13 indicates that as compared with the fatigue life of the

Table 3. Effect of turning methods on high-cycle fatigue life of GH33A superalloy Experimental temperature

Machining methods

Fatigue life average value, Nf

Machining

coefficent, 13

550 °C

Rough turning Fine turning

111 050 218240

1 1.96

Room

Rough turning Smooth turning

254836 401 136

1 1.57

rough-turned specimen, a specimen produced by another turning method can give a long fatigue life. As shown in Fig. 2(a) the machined surface can be considered to be a composition of many micronotches created by cutting processes. It has been found during the cycleloading process that microcracks first appear at the micronotch roots in Fig. 2(b). The effect of those micronotches on fatigue can be represented by the effective stress concentration coefficient Kt. The deeper the wave valley of the surface roughness or the smaller the curvature of the micronotch root radius, the larger the coefficient Kf of the effective stress concentration. The stress amplitude is normally selected to be less than the yield limit at the high-cycle fatigue condition. As a result the whole specimen surface shows an elastic deformation, but the stress concentration K~ caused by those micronotches on rough surfaces may increase the local stress and strain of the surface. The rougher the surfaces, the larger the local stress and strain on the surface. Consequently, the larger the local elastic-plastic state on the surface that can be caused. This reduces the initial fatigue life, and the fatigue cracks are quickly initiated into their growth. As illustrated in Table 3, at a temperature of 550 °C, the high-cycle fatigue life in rough-turned specimens is always lower than in the others. Because at high temperature the relaxation of the residual stress and surface hardening will take place as shown in Fig. 3, the relaxation greatly reduces the effect of the residual stress and the surface hardening on the fatigue life; only the surface roughness remains as an important factor. Since fine turning produces a surface with the least roughness, the highcycle fatigue life of the fine-turned specimens increases by a factor of 13-1 =96% over that of rough-turned specimens. The effect of surface residual stress on high-cycle fatigue can be equivalently considered as that of a mean stress applied on the surface. 4'7 If the tensile residual stress ~r>0, which is applied on the surface, increases, the fatigue life will reduce because the maximum and minimum applied loads on the surface increase. The hardened surface displays a fine subgrain structure and a high dislocation density that raise the surface strength and thus improve the high-cycle fatigue life. Consequently, at room temperature, by combining the influence of both residual stress and surface roughness, the fatigue life of smooth-turned specimens increases by a factor of 13-1=57% over rough-turned specimens. The reason for this is that the rough turning process produces the larger value of Kt and the larger tensile residual stress on the surface, which reduces the high-cycle fatigue life. Although the surface hardening is favourable for improving the fatigue life, the

Fig. 2 (a) Surface contours produced by cutting. (b) Microcracks formed under fatigue loads

324

Int J Fatigue July 1991

Holding time, t (h) 03

400

0

I

3 I

i

6 I

12 I

i

E

E

o.

~ ~

300

600

550 A

u~ (D t-

200

500

v

cO

100

o_

V

E

Q) O 03 1::

tO

I

o

J

t

I

I

200 400 600 Heating temperature, T(°C)

J

I 800

co

a

450

400 0

i

I

~

i

I

i

[

50 100 150 Depth below surface, H (tim)

200

b

Fig. 3 (a) The relaxation of residual stress: ~ , t = 3 h, o-r varies with T; V , T = 550 °C, ~ varies with t. (b) The relaxation of surface hardening: A , holding time of 5 h; V , holding time of 10 h

effect of roughness and residual stress on fatigue life is greater than the effect of surface hardening. T h e e f f e c t of s u r f a c e i n t e g r i t y on t h e l o w cycle f a t i g u e life of G H 3 3 A Along with the applied stress amplitude increasing, the surface roughness will gradually change its effect on the fatigue life. The stress concentration will be progressively replaced by the effect of strain concentration on fatigue life. Under the condition of strain-controlled low-cycle fatigue, the whole specimen surface displays an elastic-plastic state. The reason why the large surface roughness reduces the low-cycle fatigue life is that a large strain concentration is created on the rough surface. The larger the surface roughness, the larger the strain concentration. Consequently, surface roughness is the main factor affecting the low-cycle fatigue life; at 550 °C the influences of rough turning, smooth turning and fine turning on the low-cycle fatigue life are as listed in Table 4. Under the low-cycle fatigue loading, surface residual stress satisfies stress relaxation, 9-17 that is when at R - - - l , err+era>Ors, the relaxation will occur. If, for example, a small strain level ACT=0.6% is selected, the relevant stress amplitude

at room temperature will be 686-735 MPa, which approaches cr0.2-~crs=843 MPa. If 686-735 MPa is added to the residual stress value listed in Table 2, Crr+~ra will be absolutely greater than (r~. It is concluded that residual stress has relaxed at the beginning and, consequently, its effect can be neglected. The surface prestrain caused by surface hardening uses up the plasticity of the original material when it is under elastic-plastic loading; the plastic deformation caused by surface hardening causes, to some extent, a decrease in the low-cycle fatigue life. Table 5 indicates the effect of surface hardening on the A~T=0.9% low-cycle fatigue life. The cutting tool with corner radius r~=2.5 mm can produce more serious surface hardening during machining than can a cutting tool with a sharp-pointed corner radius. Thus, the workpiece machined by the cutting tool with r~=2.5 mm has the shorter low-cycle fatigue life at room temperature, but at high temperature, both results are nearly the same because of the relaxation of surface hardening as shown in Fig. 3. As a result both the surface roughness and surface hardening will influence the low-cycle fatigue life at room temperature. As seen in Table 4 the value of Nf is lower in rough turning, while there is a higher value for fine turning. The residual stress caused by grinding is more than that produced by fine turning but the influence of the residual

Table 4. The effect of surface integrity produced by different machining methods on low-cycle fatigue life of G H 3 3 A superalloy

Temperature

Average low-cycle life, Nf

Machining methods 0.6%

550 °C

Room, 20 °C

13"

0.9%

13

1.2%

13

Rough turning Smooth turning Fine turning Grinding

9842 12568 15365 13904

1

1240 1415 1846 1996

1 1.141 1.489 1.610

644

1

1.277 1.561 1.413

787 742 -

1.152 -

Rough turning Smooth turning Fine turning Grinding

24602 25816 40595

1 1.049 1.650 -

3715 4398 4830 4058

1 1.184 1.300 1.092

804 1426 1315 -

1 1.774 1.636 -

1.222

*The applied strain ranges A~T are 0.6, 0.9 and 1.2% and I~ is the machining coefficient

Int J Fatigue July 1991

325

Table 5. Effect of surface microhardness produced by t w o turning m e t h o d s on low-cycle fatigue life Turning techniques

Surface roughness

550 °C

Room temperature

Chen Yongmao for their obliging and useful help in doing the experiments. We are also grateful to Professor Zhang Juncheng and Mr Hu Puzhan for their suggestions for this paper.

Rz (~m) N,

13

Nf

13

Fine t u r n i n g (r,=2.5 mm)

8-14

1331 1

3095

1

Fine t u r n i n g

6-16

1448 1.088

4398

1.421

"13 is the ratio of the mean fatigue life of fine-turned specimens to that of r,=2.5 mm fine-turned specimens

stress can be neglected owing to its relaxation. Furthermore, the surface roughness produced by grinding is nearly the same as that caused by fine turning. The results from both machining methods are therefore almost equal. The small difference between these results may be caused by the different degrees of surface hardening. As discussed above the effect of surface integrity on the low-cycle fatigue life can be illustrated as in Table 4. Rough turning gives the shortest low-cycle fatigue life, next is smooth turning, fine turning, and grinding producing the longest lowcycle fatigue life.

Conclusions The machined surface integrity, including the surface roughness, residual stress and surface hardening, has different effects on the high-cycle and low-cycle fatigue lives of GH33A superalloy. In addition different fatigue loading patterns and different temperatures also have some effect. Under the high-cycle stress-controlling fatigue condition the main factors affecting the fatigue life are surface roughness and residual stress. Under the low-cycle strain-controlling fatigue condition, the main factor affecting the fatigue life is the surface roughness. Surface hardening is an unfavourable factor to fatigue life. Residual stress has no effect on fatigue life because of its relaxation. When the experimental temperature and heating time increase, the relaxation of residual stress and surface hardening will take place and consequently residual stress and surface hardening will have greatly decreased effects on the fatigue life. Since nickel-based superalloy GH33A components work in conditions of high temperature and high cyclic loads, a good surface quality is a surface state with low surface roughness and small surface hardening. For these purposes, feed per revolution f, back engagement ap, the rake angle "v0 and edge angle Xr will need to be strictly controlled, while the cutting speed v can increase. Residual stress has its influence only on high-cycle fatigue at room temperature; therefore, only those components that work in high-cyclic loads require a low or no residual stress surface.

Acknowledgements The authors express their heartfelt gratitude to Professor Hua Dinan, senior engineers Liang Hongshu, Chui Shuiling and

326

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Authors

Qi Huang and Jing Xin Ren are with the Department of Aeronautical Manufacturing Engineering, Northwestern Polytechnic University, Xi'an 710072, People's Republic of China.

Int J F a t i g u e J u l y 1991