Influence of warm overloading on stresscorrosion cracking of welded structures

ELSEVIER

ht. J. Pm. Ves. & Piping 69 (1996) 91-96 Copyright 0 1996 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0308-0161/96/$09.50

0308-0161(95)00037-2

Influence of warm overloading on stresscorrosion cracking of welded structures Zhang Lianghai, Centre of Boiler

Shen Xuemong,

Jin Hengyun

& Pressure Vessel Inspection & Research, The Ministry People’s Republic of China

Chen Liangshan, Institute

Dong Xiuzhong,

of Metal Research, Academia

of Labour

of China, Beijing,

Si Zhongyao

Sinica, People’s

Republic

of China

(Received 9 April 1995;accepted 25 April 199.5) Based on simulating the material, overloading stress, residual stress and operating stressof warm overloaded pressurevessels,a specialstresscorrosion cracking (SCC) test routine was conducted. The test results shows that overloading is of benefit to improving SCC resistanceand controlling the general operating stressof overloaded pressurevessels.Copyright 0 1996 Elsevier ScienceLtd.

1 INTRODUCTION

respective stages of warm overloading; the test medium adopted was a medium commonly used in conventional SCC testing.

Prior warm overloading is an effective method for reducing residual stress and the risk of brittle and fatigue failure of pressure vessels.1-6 Unfortunately, few works have been done on the influence of overloading on stress corrosion cracking (SCC) of a pressure vessel.‘-3,6 For the sake of assessing the safety margin of eight trial warm overloaded 50-100 m3 liquefied petroleum gas cylindrical pressure vessels, which were warm overloaded during manufacturing by the present authors in the early 1990s and had been put in use without stress-relief heat treatment, a comprehensive research project on the influence of overloading on SCC and operating stress to the overloaded vessels was carried out at IMR. 2 EXPERIMENTAL

2.1 SCC test plate The test plates are 500mm X 400mm X 22mm and were butt-welded by a qualified welder with the same welding material (5507 basic welding electrode) and welding procedure specifications as the warm overloaded pressure vessels at Shenyang Chemical Equipment General Factory as shown in Fig. 1. The material of the test plate is the same as the warm overloaded pressure vessels, i.e. 16MnR hot-rolled steel, 22 mm thick. The nominal chemical compositions and mechanical properties of 16MnR steel, welded seam and welded joint are shown in Table 1. 2.2 Loading methods

METHOD

The rather large size test plates in which a welding residual stress field could be established were adopted for the SCC testing. During testing, the applied stress values on the test plate were approximately the same stress levels as the different zones of the warm overloaded liquefied petroleum gas cylindrical pressure vessels by simulating the actual strains of the vessels during

Four-point bend loading was used in the testing. Two coupled test plates composed a set of loading systems as shown in Fig. 1. Four bolts at each end of the test plates applied a controllable stress to them. The span lengths between loading points were 250 mm and 60 mm respectively. The centre zone with a length of 240 mm at the middle of the test plate was the testing zone. 91

92

Chen Liangshan

ral Factory in the early 1990s a rather high overload factor, around 2.00-2.20, should be adopted in order to obtain a desired hoop membrane stress of the trial pressure vessel of 0*85YS during overloading. Under such an overload factor, the maximum strain measured at the high-strain zone of the manhole compensation plate of the trial warm overloaded pressure vessels reached some 8000-14 000 p and unloaded elastic strain was 2700-3000 p, while the minimum circumferential strain at the girth weld of the closed end and head just reached 550 p under overloading, the latter corresponding to an overloaded stress of 130 MPa, or equal to a strain of 620 p under axial loading of test plate. For this reason, an overload factor of 2.10 was selected in this SCC testing. For the sake of simulating the stresses at the different zone of the warm overloaded pressure vessels, the following five composite loading sets were used in the SCC testing. Set A test plates adopted a short span four point bend loading for simulating the high strain zone (1100-5500 p) of the warm overloaded pressure vessels. In Set A test plates, the applied strain value within the support points was 5500 ,x, while the strain values within the testing zone but outside the support points changed from 1100 EL. to 5500,~~ continuously; the overloading strain and the followed simulating operating strain of the test plates were measured and controlled by strain gauge measurement as shown in Fig. 1. Because a higher SCC resistance was expected in the testing zone of Set A test plates, where a strain above 5500~ was produced during warm overloading, the SCC testing was focused on the regions where the overloaded strains were less than 1100 g. Set B, C and D test plates adopted a long span four point bend loading for simulating the regions having 1000, 800 and 500 ,LLof the warm overloaded pressure vessels respectively, because there was a uniform strain within the testing zone

Testing zone 240 mm A I

I

I

5

<,

1

Fig. 1.

Outside of the testing zone, the plate was peened in order to avoid SCC in the non-testing region. According to the previous work of the present authors,7a the influence on residual stress of peening is negligible 10 mm away from the peened zone. 2.3 Selection of overload factor and strain Overload factor is defined as the ratio of applied overloading stress (or pressure) to the design operating stress (or pressure) of a pressure vessel. According to the test results achieved in the eight experimental warm overloaded cylindrical pressure vessels with different volumes at Jinxi Chemical Equipment Plant, Machinery Plant under Yanshan Petroleum-Chemical Company, and Shenyang Chemical Equipment GeneTable

1: Chemical

compositions

(nominal)

et al.

and mechanical

properties

of test plates

Mechanical property

Chemical composition (%)

16MnR steel 5507weld Welded joint HAZ (16MnR + J507)

and NDT

NDT “C

c

Si

Mn

S

P

YS MPa

TS MP9

El. %

so.2

0.2-0.6

1.2-1.6

so.035

so.035

362 451

538 541

33 31

-25 -35 -20

Influence

of the test plate during long-span four-point bend loading, only one set of strain gauge measurement was needed. The simulated operating strains of the test plate were the unloaded elastic strain values divided by the overload factor of 2.1. Set E test plates used an overload factor of 1.25 and followed the same simulated operating load as the aforementioned short span loading after unloading from overload (Set A) in order to explore the influence of conventional hydrostatic testing (usually having an overload factor of 1.25) on SCC of pressure vessels. The relevant overloading strain values at the test zone were in the range of 1600-600 p. 2.4 SCC testing In order to accelerate the SCC process and decrease the test period, a corrosive medium consisting of 60%Ca (No~)~ + 3%NH,NO, + 37%H,O was used in the SCC testing. The temperature of the test medium during SCC testing was 125-130°C. After a prior overloading, unloading from the overloaded load, and simulated operation loading condition the test plate or the test plates of a composite loading set was immersed in the test medium. The test zone was periodically examined (each 24 hrs), by means of dyepenetration testing, for cracking. The stipulated test period was lOTc, where Tc was defined as the SCC time period of the as-welded test plate under the conditions of no prior overload and no applied simulated operating stress. The

6

0 Overloaded strain (overload factor: 1.83-Z. 16) 0 Strain during SCC testing .& Strain after unloading from overloading load

XC

testing

zone

4

93

of warm overloading on welded structures

’ Ulstance apart tram the centre line of test plate mm I t 1

test should be terminated if a crack is found at either test plate surface of a composite test-plate set, as cracking will affect the stress level of the untracked plate. 3 RESULTS

AND

DISCUSSION

3.1 Stress and strain distributions on the surface of the composite test plates The measured surface strain distributions of a short span composite test plate under overload, unloading and the experimental simulated operating load are shown in Fig. 2. It can be seen from Fig. 2 that the test plate of Set A has simulated basically the strain conditions of the actual warm overloaded pressure vessels. The surface strain between the support points reaches a maximum value either in overload, unloading or under the simulated operating loading, and the maximum overloaded strain is 5472 p. The strain decreases sharply at the outside of the support points. At the load points (225 mm from the centre line of the test plate), the strain value is zero. A linear distribution of strain with distance is found at the elastic bend loading zone adjacent to the loading points. Residual plastic deformation occurs after unloading at the region where an overloaded strain value exceeds 1500 ,LL. The bigger the overloaded strain value, the bigger the residual plastic strain will be. A residual compressive stress would form in such a region due to plastic deformation. The actual overloaded strains at the the testing zone of Set A composite test plates are in the range of 1100 to 5472 p; the elastic strains after unloading are in the range of of 1100 to 2424 p; the operating strains are in the range of 600 to 1116 p. There is a slight difference of overloaded factor at the different zones of the test plate, changing from 1.83 to 2.16. The strain level of the test plate can roughly simulate a high-stress zone and cylinder of the warm overloaded pressure vessels. The calculated outer surface stress distribution of Set A composite test plate is shown in Fig. 3. A yield point of 350 MPa of ideal plastic material (i.e. neglecting work hardening) and a YS value of original welding residual stress was supposed in the calculation (see curve 1 in Fig. 3). The calculated overloaded stress, operating stress and residual stress distribution curves after overload-

94

Chen Liangshan

0.8 -

Distance apart from the centre line of test plate mm

Fig. 3.

ing are shown in curve 2, curve 3 and curve 4 in Fig. 3 respectively. It should be noted that a distinctive residual compressive stress has formed in the high-strain zone is curve 4. Curve 5 represents the distribution of general stress (defined as the sum of residual stress and operating stress in a test plate or a pressure vessel) at the surface of the test plate. A distinct characteristic of general stress is that it decreases along with the increasing overloaded strain. At the maximum strain zone, the general stress is minimum. That is to say, a maximum safety margin will be expected at the zone. Obviously, the warm overloaded pressure vessels might possess the same features as the aforementioned characteristics of the SCC simulated test plate. The general stresses at the testing zone of Set A test plates are in the range of 0.2-0.69 YS. The general stress distributions of Set B, Set C Original 1.0 General

welding residual \ stress of No. 10,

et al.

and Set D test plates are shown in Fig. 4. The distributions of general stress at each testing zone of them are uniform but at different levels due to using long span four-point bend loading. The general stresses in them are 0.71 YS, 0.8 YS and 0.87 YS respectively. The surface stress distributions of Set E test plates are shown in Fig. 5. Comparing with Fig. 3, the residual stress after overloading (curve 4) is higher and the general stress (curve 5) is distinctly higher than that of the results shown in Fig. 3 at the same simulated operating stress condition (curve 3) owing to adopting a lower overload factor. The general stresses in the testing zone are in the range of 0.83-0.91 YS. The as-welded test plate without prior overloading and tested in free condition (i.e. without restraint) is deemed as a zero overloading strain, a zero operating strain and an arbitratory overload factor loading pattern (hereinafter referred as Set F). The general stress in it is equal to the welding residual stress, i.e. 1.0 YS. The general stress levels of the mentioned different composite loading sets increase with the following order: A-B-C-D-E-F. 3.2 SCC testing results The stress and strain parameters and the results of SCC testing are shown in Table 2. The five test plates of set F tested in free condition were carried out simultaneously with other sets of composite test plates in different SCC test periods. All of the five test plates of set F cracked within 24 hrs of SCC testing with a mean SCC incubation period of 15.5 hrs. Figure 6

stress

ldem No. 7, No. 8,

0.2 -

XC I 50

0

testing

zone I 4

I I 150 200 I ,Distance apart from the centre , lme of test plate mm

100

SCC testing

zone ;

I

I

& Fig. 4.

Distance apart from the centre line of test plate mm

Fig. 5.

InJIuence of warm overloading Table Test set No.

Test plate No.

Loading method

Overload strain P

2: Test parameters Unloading strain F

Applied strain P

95

on welded structures

and results of SCC testing Overload factor

General stress level

Test period (hrs)

(fl/a.,)

Remarks

Cracks Max. length (mm)

Amount

01 00

free

0

0

0

1

18

50 45

4I

i:;

F

02 03 04

free free free

0 0 0

0 0 0

0 0 0

1 1 1

24 22 23

60 40 60

2 1 3

(b) (b) (b)

E

12

(cl

1644

1497 600

1193 480

1.24

0.83 0.91

24

55 0

i 0

(b)

600

13

A

05 06

(cl

5472 1100

2424 1100

1116 600

2.17 1.83

0.2 0.69

230

0 0

0 0

B

07 08

(4

987

987

456

2.16

0.71

142

0 55

0 1

C

14 15

Cd)

841

841

400

2.02

0.80

229

0 0

0 0

10

(4

500

500

247

2.10

0.87

238

0 0

0 0

D

11

(a) No crack was found

after having immersed for 13 hrs: the cracks originate (b) Cracked at the first examination (24 hrs); crack(s) originates at weld. (c) Short span, four points bend loading. (d) Long span, four points bend loading. (e) No crack was found after having immersed for 118 hrs: the crack originates

gives some appearances of SCC cracks on the surface of a set F test plate. Among the test plates having the same overload factor [around 2.11 set A, set B, set C and set D, no crack is found after 230 hrs of SCC testing (i.e. 14.8Tc) except No. 8 test plate of set B (with an overloaded strain of 1000 p) which cracked after 142 hrs (902Tc) of SCC testing. The appearance of SCC crack of No. 8 test plate is shown in Fig. 7. The crack originates at the welding heat-affected zone (HAZ) and has propagated to the base metal in the distance at one end and just entering into the weld at the other. This is the sole crack found among the

Fig. 6.

at weld.

at HAZ.

whole SCC test plates where a crack has originated in the HAZ. It is common knowledge that the weld usually possesses the highest SCC tendency in a welded joint for its cast structure. A crack originating in the HAZ is abnormal: maybe it relates to some hidden defects in the base metal of the No. 8 test plate. The overloaded strains of sets A, B, C, D test plates cover a wide range of 500-5472 p. Therefore, those zones having produced an overloaded strain exceeding 500 lu. and an overload factor of 2.1, will possess an obviously improved SCC resistance. A 500 p strain under axial loading corresponds to 103 MPa of overloading stress, i.e.

Fig. 7.

96

Chen Liangshan

et al.

operating processes. While the overload factor is 2,l and overloaded strain exceeds 500 p, the general stress of the overloaded pressure vessel will not exceed 0.87 YS. Under such a stress level, the SCC resistance of the pressure vessel will be significantly improved. Conventional hydrostatic tests (usually having an overload factor of 1.25) can not improve SCC resistance of pressure vessel effectively. SCC may be avoided effectively by peening treatment. Fig. 8.

ACKNOWLEDGEMENT corresponding to 420 or.tangent strain of bi-axial stress in a cylindrical pressure vessel. As mentioned above, the minimum strain of the trial warm overloaded pressure vessels is 550 p, and it is reasonable that the warm overloaded pressure vessels will possess very good SCC resistance during operation. The composite test plates of set E used an overload factor of 1.25. One crack was found at No. 12 test plate during the first examination after 24 hrs of SCC testing (Fig. 8) but no crack was found at the other test plate. The test results show that comparing with the as-welded nonoverloaded test plate, after overloading with an overload factor adopted in conventional hydrostatic tests (1.25), SCC tendency will be decreased but the SCC incubation period is not itself prolonged. No crack is found at the peened area outside the testing zone of all plates tested in this SCC testing. It can be concluded that peening is an effective measure for preventing SCC. 4 CONCLUSIONS The tensile surface on the 500 mm X 400 mm X 22 mm test plate loaded at four points by bending loads with different spans could approximately simulate the stress and strain level of overloaded and pressure vessels both in overloading

The authors would like to express their appreciation to the Jinxi Chemical Equipment Plant, Machinery Plant under Yinshan Petroleum Chemical Company and Shenyang Chemical Equipment General Factory for their devoted support to this research project. REFERENCES 1. Nichols, R. W., The use of overstressing techniques to reduce the risk of subsequent brittle fracture, Part 1 and Part 2, British Welding Journal, (Jan/Feb 1968), 21-42, 75-83. 2. Pickles, B. W. & Cowan, A., A review of warm prestressing studies, Int. J. for Pressure Vessels and Piping, 14 (1983) 95-131. 3. Smith, D. J. & Garwood, S. J., The significanceof prior overload on fracture resistance- a critical review, Znt. J. Pressure Vessels & Piping 41, (1990), 15.5-196. 4. Joint Working Group IIW Commission IX and X: Recommendationfor PWHT of Welded Joint in Steel PressureVesselsand Other Heavy Duty Structures, IIW Dot X-1227-91/1X-1656-91. 5. Hrinak, I., Lances, J. & Vejvoda, S., Relaxation overstressingof huge spherical storage vesselsrepaired by welding, Stress Relieving Heat Treatment of Welded Steel Constructions, Proc. Int. Conf , Sofia, Bulgaria, 6-7 July 1987,189-197. 6. UnpublishedTechnical Reports, CBPVI, Beijing, China. 7. Chen Liangshan, Dong Xiuzhong, Determining residual stressby means of impacted dent, Proc. 7th National Welding Technique Conference, 5, 1993. 8. Wang Yanyan, Chen Liangshan, Relaxing welding residual stress of welded joint by peening, Proc. 7th National

Welding

Technique

Con&, 5, 1993.