Estimates of local tensile strength of welded joints

ELSEVIER

ht. J. Pres. Ves. & Piping 65 (1996) 41-45 0 1995 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0308-0161/%/$09.50

0308-0161(94)00157-X

Estimates of local tensile strength of welded joints Y. W. Shi, Z. X. Han & N. N. Zhou School of Mechanical

Engineering,

Xi’an

Jiao-tong

University,

Xi’an,

Shaanxi

Province

710049, People’s

Republic

of China

(Received 10 September 1994; accepted30 October 1994)

In the present paper two methods are introduced to estimate the tensile properties of welded joints. The studiesindicate that single-edgeprecracked three-point bend specimentests can be successfullyusedto estimatethe local yield strength of material sampledby crack tip in weldedjoints. The constraint factor of the specimensmay be experimentally calibrated by weld metal or basemetal. Moreover, microsheartestscan be conveniently used to estimate the ultimate tensile strength and tensile yield strength of materials in each zone of welded joints, and the cost of such tests is low. The tensile yield strength and ultimate tensile strength estimatedby either method will play an important role in fracture mechanicstests of welded joints and integrity assessmentof welded structures. Furthermore, the present studies on the welded joints of Weiten 80C pressurevesselsteel and API X65 welded pipe steel indicate that the methodsfor estimating the tensile properties of welded joints are successful.

An exception to this rule is made for the defects in HAZ, because it is difficult to measure the properties in the regions. Therefore, it is very necessary to determine the local tensile characteristics and their distribution in welds for studying deformation and fracture behaviour of welded joints. The tensile properties of weld metal and base metal can be determined by conventional small-scale tests on specimens extracted from the corre,sponding locations, but the determination of tensile properties of HAZ is difficult, as it is not possible to extract a tensile specimen from regions of HAZ due to the continuous change of microstructures and mechanical properties in the HAZ. In the case of solid-phase welding, such as friction welding or hot pressure welding, there only exists the regions of interface and HAZ in the weldments. In the case of brazing or diffusion bonding, the brazed metal or the interficial layer is very narrow, and their chemical compositions and mechanical properties usually differ from their filler metals. In these situations, the fracture behaviour of joints is controlled by the interface or HAZ. Thus, the tensile characteristics of interface and HAZ are essential, and the determination of their tensile properties by

1 INTRODUCTION

By their very nature welded joints are highly inhomogeneous. The microstructures and tensile characteristics vary in different regions of welded joints including the weld metal, heat affected zone (HAZ) and base metal. The values of the local yield strength sampled by crack tip are of particular interest for determining the elasticplastic fracture toughness of welded joints, since the values of yield strength are necessary for determination of the J-integral blunting line and solution of the crack tip opening (CTOD) formulae. Also, knowledge of the values of yield strength is necessary for determination of validity conditions for the fracture toughness measurement. When the finite element method is used to investigate the onset of yielding and plastic strain distribution of welded joints and the stress/strain singularity at a crack tip, the tensile properties of materials in different regions of welded joints have to be taken into account. As far as quantitative defect assessment is concerned, the procedure PD6493 recommends the use of tensile properties for the region of the welded joint for which the defect assessment is to be conducted.’ 41

Y. W. Shi, Z. X. Han, N. N. Zhou

42

means of conventional tension tests is difficult. Moreover, it must be emphasized that the mechanical properties of local hardening or softening regions of HAZ in fusion welding differ from the results obtained from welding thermosimulated bulk specimens: this is because the mechanical properties of local materials are strongly influenced by the surrounding materials in the practical welds. Similar situations occur in the joints of solid-phase welding and brazing, etc. If no tensile test data exist, then for carbon, carbon-manganese, and low-alloy steels the room-temperature yield strength may be estimated from Vicker’s hardness (Hv) 10 kgf data using the formulas as follows:2 a? = 3.25 Hv - 349 MPa a, = 3.15 Hv - 168 MPa

(Base metal) (Weld metal)

(1) (2)

If there exists a similar regressive correlation, estimates of the yield and ultimate strength of HAZ may also be made from the hardness tests. Sometimes the average of the yield strength of the base metal and weld metal is simply used for the calculation of the CTOD, when the crack is located at the fusion boundary or in the HAZ.’ It should be noted that the estimates of tensile properties from hardness tests must be done with care, as the regressive correlations are influenced by the chemical compositions, solidification, and heat treatment etc. of materials in welded joints. In this paper, two methods are introduced for (a) C-Mn-Mo-B-Ti

filler

Distance

(b)

C-Mn-Si-Mo-Ti

filler

wire

from

weld

centre

(mm)

weld

centre

(mm)

wire

(a) C-Mn-Mo-B-Ti

filler

wire

Distance (b) C-Mn-Si-Mo-Ti

-25

from

filler

from

Fig. 1. Ultimate tensile strength of welds estimated by microshear test: (a) C-Mn-Mo-B-Ti filler wire: (b) C-Mn-Si-Mo-Ti filler wire.

centre

(mm)

wire

-5

-15 Distance

from

5 weld

centre

15

25

(mm)

Fig. 2. Yield strength of welds estimated by microshear test: (a) C-Mn-Mo-B-Ti filler wire; (b) C-Mn-Si-Mo-Ti filler wire.

estimating the local tensile data of welded joints. The estimating methods may be simple and reliable in application. 2 ESTIMATES OF YIELD STRENGTH FROM PRECRACKED BEND TEST It is known that the Charpy-size single-edgenotch three-point bend specimen test results may be used to estimate yield strength values when the test loading record reveals that general yielding has occurred.3 From theoretical study the constraint factor, c, in plane strain is defined asI4 2M (3) C=k(W-a)2 where M is the applied bending moment per unit thickness, W is the specimen width, a is the notch or crack depth, and k is the shear yield strength of materials. In addition, uniaxial yield strength can be inferred from the shear yield strength using either a Tresca (sv = 2k) or Mises (a? = fi k) yield criterion. Substituting for the applied three-point bending moment M at general yield for a span-to-width ratio of four, eqn (3) can be rearranged as: 4PGYW

Distance

weld

uy =cB(W -a)’ 2ti POW

TV =cqw

-

a)2

(Tresca)

(4)

(Mises)

(5)

Estimates

of local tensile strength of welded joints

where B is the specimen thickness, a, is the yield strength, Pcy is the general yield load. Only plane strain Tresca is considered below, as the Mises gives 1.155 times the Tresca limit load. If the tup indenter is assumed to be a flat punch and could not sink into the material, and half indenter width is 1 mm, the slip-line fields analysis can give the c value of 1.279 for the precracked Charpy specimen5 Substituting this for c in eqn (4), the following expression for a precracked Charpy-size specimen is given:

PGYW a?= c’ B(W_ a)2 where c’ = 4/c = 3.13. It should be noted that the solution in eqn (6) is only an approximation for real materials since the slip-line fields analyses treat the material as showing no strain hardening. In addition, if the specimen is not the Charpy size the constraint factor may be changed. Furthermore, the assumption of zero root radius for the precracked specimen is probably inadequate due to the crack tip blunting during loading process. In the present work the factor c’ in eqn (6) is determined by an experimental calibration of a single-edge precracked bend specimen, in which the specimen size and geometry and loading manner are identical to those of the specimens to be used in the following practical fracture mechanics measurements. The material of the calibration test may be chosen to be base metal, as the tensile properties of base metal can be directly determined by the uniaxial tensile test. If the general yield load PGy is known in the calibration test, the factor c’ in eqn (6) can be determined substituting for the specimen size, crack size and the general yield load obtained. One may have good reason to say that the c’ value determined by this way represents the crack tip constraint in following practical fracture mechanics specimens tests. If the c’ value is known the local yield strength of the welds may be estimated by the specimens in which size and loading manner is the same as the calibrated base metal specimens. In the present studies the yield strengths of a high strength low alloyed steel Welten SOC welded joints are estimated. The steel plate was heat-treated by water cooling and then tempering. Yield strength and ultimate tensile strength of the steel being 845 MPa and 899 MPa. The welded joints were made by manual metal arc

43

welding and automatic Ar-10%C02 gas metal arc welding processes. For the manual metal arc welding the electrodes used were Japanese LSO, and AWS E10015, diameter 4mm. For the automatic gas shielded metal arc welding, the electrode used was OK Antrod 13*29(Ni-Mo), diameter l-2 mm. Cross-section of three-point bend specimens extracted W = 2B = 24 mm, and a/W = 0.5. Span of the specimens was 96 mm. Experimental calibration of the factor c’ is taken from the test results in six specimens of Welten 80C base metal with T-L and L-T orientations. The average of c’ values was 3.86. For the weld metal tests, the through-thickness notch was located around the middle of the weld, and for the HAZ tests the notch was located at the coarse grained zone apart from the vertical side of the half K weld. The yield strength of the weld metal and HAZ tested can be estimated from eqn (6) with the calibrated c’ value. Thus, the yield strength of the welded joints at room temperature can be estimated and are given in Table 1. It should be noted that the yield strength of base metal estimated by this method, as given in Table 1, is 833 MPa, which is approaching to the value given by the results of the tensile specimens extracted from the base metal. All the results are average of three or four tests. In addition, the yield strength of base metal and welded joints at the different test temperatures may also be estimated by the same method with the c’ value determined at room temperature. The results for base metals are given in Table 2. When the microstructures and fracture performance of Welten 80C steel undergoing welding thermo-simulation are investigated, and the fracture tests are carried out by precracked Charpy specimens, the yield strength of the simulated materials at room temperature with changing the thermo-simulation cooling time from 800°C to 500°C (&) may be estimated by this estimating method. The results are given in Table

1. Yield

strength of Welten 8OC welded room temperature

L80 weld metal OK Antrod weld metal HAZ welded by L80 electrode HAZ welded by El0015 electrode Base metal

717 663 752 729 833

joints MPa MPa MPa MPa MPa

at

Y. W. Shi, Z. X. Han, N. N. Zhou

44 Table 2. Yield

strength

of Welten temperatures

20 T-L L-T

specimens specimens

832 833

8oC

steel

Test temperatures 0 -20 -40

(“C) -60

-80

849 869

863 893

836 845

834 859

832 864

at test

Table 3. For making a comparison, the Vicker’s hardness test results are also given in Table 3. It is clear that this method is very useful for estimating the yield strength of materials, and especially the local yield strength of material sampled by crack tip in welding heterogeneous bodies. 3 ESTIMATES OF TENSILE PROPERTIES FROM MICROSHEAR TEST The tensile properties of materials can be successfully reckoned by microshear test by using the correlation between the microshear test and the conventional tensile test.6 In general the microshear specimens have a square cross-section of 1.5 X l-5 mm and length of 30 to 50 mm. One end of the specimen is fixed in a clamp, and a shear tool cuts through the outstanding part. During the test, force and displacement of the tool are measured, thus a shear force vs displacement curve can be established. The ultimate shear strength (rU), and shear yield strength (r,) are defined as follows: (7)

where P,,,,, is the maximum shear force, and A, is the original cross-section of the specimen. Py is the shear yield force, which may be also defined as the load P0.2 corresponding to the plastic displacement equal to 0.2% of original specimen width. Table

3. Yield

strength of Welten 8oC undergoing thermo-simulation

welding

fR/S(set)

9

18

27

45

100

240

myWW

754 357

706 322

683 306

677 299

663 266

651 253

Hv (10 kgf)

Previous studies indicate that the microshear test shows very good repeatability of measurements for different materials.6,7 In order to establish the correlation between the conventional tensile strength and microshear strength, a wide range of steels from carbon steel, carbon-manganese steels, to low alloyed steels have been tested. The regressive correlations of strength between tensile and microshear tests are given as follows: a, = 2.182, - 30843 (MPa)

R = O-998

uY = 1.832, - 13.62 (MPa)

R = O-984 (10)

(9)

where a, and uY are the ultimate tensile strength and tensile yield strength, respectively; R is the correlation coefficient. In the range of tested materials, the tensile yield strength is changed from 302sOMPa to 1064.3 MPa. For the tested materials the ratio of tensile yield strength to shear yield strength u.“/r, = 1.738 to l-808. The value approaches the Mises yield criterion in which u,/r, = fi. Thus, it may be expected that the tensile yield strength and ultimate tensile strength of materials can be accurately estimated from eqns (9) and (10). At the same time, due to the very small cross-section of the microshear specimens, the tensile properties of each zone of interest in welded joints can be determined by incremental sectioning. As the shear tests can be made within tiny regions and very short distances, the distribution of tensile properties of all zones transverse to the welded joints can be conveniently determined. In the present work, distribution of ultimate tensile strength and tensile yield strength of welded joints of spiral welded pipes is estimated by the microshear test. The plate with a thickness of 7 mm follows the API X65 grade made in Thyssen, Germany. The values of uY and a, are 496 and 578 MPa, respectively. The spiral seam of welded pipe was made with submerged arc welding, a single pass on each side. The welding speed was 15OOmm/min. Heat input of backing pass and final pass was 6.2 and 10.1 kJ/cm. Diameter of filler wires was 4mm. The alloy systems of the filler wires were C-Mn-Si-Mo-Ti and C-Mn-Mo-B-Ti, respectively. SiO,-TiO,CaO-MgO-ALO,-MnO-CaF, type agglomerated flux was used in the submerged arc welding. The microshear specimens were extracted from the upperside of the welded seam and transverse to the seam. The test with shear speed of

Estimates

of local tensile strength of welded joints

(a) C-Mn-Mo-B-Titiller wire

local yield strength of material sampled by crack tip in welded joints. Microshear tests are a very convenient method to estimate the ultimate tensile strength and tensile yield strength of materials in any zone of welded joints. The cost of the tests is low. Present studies on the welded joints of Welten 8OC pressure vessel steel and API X65 welded pipe steel indicate that the methods of estimating the tensile properties of welded joints are successful.

260r

Distancefromweldcentre(mm) (b) C-Mn-Si-Mo-Titiller wire

260r

8

ACKNOWLEDGEMENT

180 160

-25

I -15

,I W.M. 1,

-5

5

I

IS

I

25

Distancefromweld centre(mm) Fig. 3. Vicker’s hardnessof welds: (a) C-Mn-Mo-B-Ti filler wire; (b) C-Mn-Si-Mo-Ti filler wire.

O-05 mm/set was conducted at room temperature. Spacing of the two neighbouring cutting points was 2 mm and three microshear specimens with different initial cutting positions were used for each welded joint. The ultimate tensile strength and yield strength of the welds estimated are shown in Figs 1 and 2. The results of Vicker’s hardness test are shown in Fig. 3 as a reference. The above test results indicate that the strength a, and uY of the weld metal with C-Mn-Si-Mo-Ti filler wire is higher than that with C-Mn-Mo-B-Ti filler wire; the trend of these results are clearly coincident with the hardness examination. As the steel plates are produced by the thermomechanical control process, the softening of the HAZ can be seen by the fall of strength and hardness. Thus, it is clear that this is very convenient method to estimate the tensile properties of welded joints, and the cost of the tests is low.

The authors are grateful for the financial support given by the National Natural Science Foundation of China. REFERENCES 1. PD 6493 : 1991, Guidance acceptability

CONCLUSIONS

Single-edge precracked three-point bend specimen tests can be successfully used to estimate the

on methods for assessing the of flaws in fusion welded structures, British

StandardsInstitution, 1991. 2. Dawes, M. G., Pisarski,H. G. & Squirrell, S. J., Fracture mechanics tests on welded joints, nonlinear fracture mechanics:Volume 2, Elastic-plastic fracture, ASTM STP 995, Eds J. D. Landes er al., ASTM, Philadelphia, 1989,191-213. 3. Server. W. L., General yielding of Charpy V-notch and precracked Charpy specimens,Journal of Engineering Materials

4. 5. 6.

7. 4

45

and Technology,

Transactions

of the ASME,

100, (1978), 183-188. Green, A. P. & Hundy, B. B., Initial plastic yielding in notch bend tests,Journal of the Mechanics and Physics of Solids, 4, (1956), 128-144. Ewing, D. J. F., Calculations on the bending of rigid-plastic notched bars, Journal of the Mechanics and Physics of Solids, 16, (1968), 205-213. Dorn, L., Niebuhr, G. & Wawer, G., Information of microtensile and microshear tests concerning strength and ductility of steels, Schweissen und Schneiden, 29, (1977) 246-249. Dorn, L., Determination of weld structure properties by means of the microshear test, Proceedings of the International Conference on Quality and Reliability in Welding, The Welding Institution of the Chinese

Mechanical Engineering Society, 6-8 September, Hangzhou, China, 1984,B21(1-6).