Fatigue strength a b s t r a c t Modification of spraying fused MSF of plasma arc as heat source was used to improve the fatigue per-formance of welded joint, which both fundamentally red
Trang 1Research on fatigue behavior of welded joint spraying fused by low
transformation temperature alloy powder
Xiaohui Zhaoa,⇑, Dongpo Wangb, Caiyan Dengb
a
Key Laboratory of Automobile Materials, School of Materials Science and Engineering, Jilin University, Changchun 130025, China
b
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
Article history:
Received 16 May 2013
Accepted 9 July 2013
Available online 29 July 2013
Keywords:
D Welded joint
C Modification of spraying fused
E Fatigue strength
a b s t r a c t
Modification of spraying fused (MSF) of plasma arc as heat source was used to improve the fatigue per-formance of welded joint, which both fundamentally reduced stress concentration at weld toe and achieved metallurgical bond between spraying fused coating and welding The low transformation tem-perature alloy powder was applied to the method of MSF After spraying fusion, especially spraying fused joint by low transformation temperature alloy powder, the distribution of residual stress is more difficult
to be obtained Finite element (FE) simulation as an important tool was used to determine the stress field and temperature field of spraying fused joint Simulated results show that as-welded joint and welded joint spraying fused by conventional nickel base alloy powder (Conventional-joint) present tensile stress The stress of welded joint spraying fused by low transformation temperature alloy powder (LTT-joint) is compressive stress Fatigue test results indicated that under the condition of 2 106cycles, the fatigue strength of as-welded joint is 135 MPa, while that of Conventional-joint and LTT-joint is 218 MPa and
235 MPa, respectively The fatigue strength of Conventional-joint increases by 61.48%, and fatigue strength of LTT-joint increases by 74.07%
Crown Copyright Ó 2013 Published by Elsevier Ltd All rights reserved
1 Introduction
Welded joints are widely used in the engineering structure
Most of welded joints work under the condition of cyclic loading
Therefore, fatigue failure is one of the most common and
danger-ous failure ways[1] There exists serious structural stress
concen-tration and a certain number of welding defects at weld toe
Welding defects is equal to the natural fatigue crack source, greatly
reducing the time of crack initiation So as to the fatigue life is
mainly decided by the crack propagation Consequently, it is very
meaningful to take technologic treatment to reduce the stress
con-centration and eliminate defects for the improvement of fatigue
strength[2–5]
The existing technique of prolonging mainly consists of
TIG-dressing[6,7], welding toe grinding and ultrasonic peening
treat-ment (UPT) [8–11] TIG-dressing and welding toe grinding are
important industrial techniques, but they are faced with the risk
of reducing the strength of welded joint due to the change of
cross-sectional area UPT[12–17]mainly rely on the presence of
compressive residual stresses to improve fatigue lives of welded
joint Tensile residual stresses in welding area weaken the tensile
load capacity As a result, the fatigue life is shortened For that
reason we attempt to reduce or at least eliminate these harmful
stresses By means of periodic impacts or peens with frequencies above 20 kHz, UPT creates a deformation in tensile zone on the sur-face of weld and/or weld toe Tensile stresses can be relieved by this action Moreover, cracks and voids are closed, and weld toe geometry is modified Compressive residual stress is induced by the regional plastic deformation as well However, UPT also has some shortcomings For example, the residual compressive stress will be released with the increase of stress level Thus, UPT will
be hard to maintain its ability to resist fatigue In general, UPT are not suitable for structures operating at applied stress ratios
R > 0.5 or maximum applied stresses above around 80% yield
In this study, modification of spraying fused (MSF) as a new method to improve the fatigue strength of cruciform welded joint was proposed Deng et al also studied this new technique[18] MSF fundamentally reduces stress concentration at weld toe Meanwhile, metallurgical bond between spraying fused coating and weld can make welded joint bear much larger plus alternating load In this paper, the heat source of MSF is a plasma flame, which has excellent performance In addition, the low transformation temperature alloy powder was applied to the method of MSF, which will make the spraying fused coating form compressive residual stress Such, the MSF with low transformation tempera-ture alloy powder both reduce the stress concentration and form the residual compression stress, which will further improve the fa-tigue performance of welded joint
0261-3069/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd All rights reserved.
⇑Corresponding author Tel./fax: +86 0431 85094687.
E-mail address: zhaoxiaohui@jlu.edu.cn (X Zhao).
Contents lists available atScienceDirect Materials and Design
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / m a t d e s
Trang 22 Experiments
2.1 Testing specimens and method
The basic principle of MSF (seeFig 1) is to spray fusing a
met-allurgical bonding layer of high curvature appearance on the
sur-face of the whole welded joint, which can greatly reduce the
stress concentration of welded joint[18] In order to reduce defects
and enhance the bonding strength, this paper use plasma flame
in-stead of oxygen acetylene flame to MSF
Specimens were cruciform welded joints (carbon-dioxide arc
welding with substrate material of low-carbon steel Q235B and
welding material of H08Mn2Si), whose geometrical characteristics
are shown inFig 2.Table 1 is mechanical properties of Q235B
steel Spraying fused materials was nickel-base alloy powder
(Ni65Cr16B3.1Si4.5), also including low transformation
tempera-ture alloy powder They have good oxidation resistance, impact
toughness, corrosion and heat resistance
2.2 The choice of low transformation temperature alloy powder
According to actual circumstances, the author assumes that:
800 °C is a plastic transition point, there is no thermal contraction
stress under the condition of this temperature The relationship between Msand stress under the constraint condition is analyzed
Fig 1 Basic principle of MSF.
Fig 2 Geometric dimensioning of as-welded joint.
Table 1
Mechanical properties of Q235B steel.
Material Yield
strength
rs (MPa)
Tensile strengthrb
(MPa)
Elongation (d)
Elasticity modulus E (GPa)
Poisson’s ratio (m) Q235B 267.4 435.5 26% 206 0.3
Fig 3 Development of stress during constrained cooling.
Fig 4 Finite element model of as-welded joint.
Trang 3Literature[19]shows that different combinations of Ni and Cr will
be of different linear expansion coefficient and strain, etc
Tensile stress (rtensile) is formed due to volume shrinkage of
material during the cooling process since temperature decreases
from 800 °C Phase transformation stress (rcompressive) generated
due to volume expansion caused by martensite phase
transforma-tion when the temperature drops to phase transitransforma-tion point (Ms)
The interaction of phase transformation stress (rcompressive) and
previous tensile stress form the final residual stress (r):
r¼rtensileþrcompressive ð1Þ
where E is the average elastic modulus; Del is strain shrinkage
caused by heat bilges cold shrink of material; Dep is expansion
strain caused by martensite phase transformation
From formula(1)–(3), Stress–Temperature curves of five kinds
of low transformation temperature alloy powder can be gained
(seeFig 3)
Fig 3shows that the largest residual compressive stress was
obtained at room temperature when Msis about 200 °C Based on
the above analysis, the phase transition point of alloy powder
should be controlled at around 200 °C The chemical composition
of alloy system (wt%) is: C < 0.12, Cr = 5–8, Ni = 8, Mn = 1.0,
Si = 0.8, Mo = 0.5, Ti = 0.07, Cu = 0.3, S = 0.005, P = 0.005, B = 0.8,
also including a moderate amount of rare earth elements,
allow-ance for Fe
2.3 Method and parameters of MSF
In this paper, plasma flame is used to modification processing
Before spraying, the surface of welded joint was pre-grinded,
which not only eliminates oxide in the surface of welded joint,
but also maintains certain roughness of the surface, thus
improv-ing the adhesion strength between coatimprov-ing and weldimprov-ing The
pro-cess of modification of plasma spurt spraying only need to
change horizontal swing of spray jet into circular orbit rotation
on the basis of spraying process, which can be completed by robot
of controlling nozzle line The basic parameters for this test is: turning radius of spray jet of 200 mm; swing Angle of about 5°; oscillation frequency of 1 times/s The feeding switch is off after forming overall shape, the non-transferred arc of plasma arc is used to adjust edge position of spray welding layer at a small scale Meanwhile, for different alloy powder, the process parameters can
be appropriate adjusted
2.4 The temperature field and stress field of spraying fused joint The residual stress of cruciform welded joint is difficult to be measured accurately by the traditional measuring method because
of its complicated structure After spraying fusion, especially spraying fused joint by low transformation temperature alloy pow-der, the distribution of residual stress is more difficult to be ob-tained Finite element (FE) simulation has become an important tool for the prediction of residual stresses of welded joint 3D FE modeling of a single pass cruciform welded joint and spraying fused joint are set up by ANSYS According to geometric dimen-sioning of as-welded joint (seeFig 2), the modeling of as-welded joint is showed inFig 4 According to geometric dimensioning of spraying fused coating, the modeling of spraying fused joint is showed inFig 5.Fig 5shows that the minimum thickness of coat-ing is 0.5 mm, and knuckle radius (q) is 20 mm
The method of coupled thermal stress analysis is adopted Firstly a transient thermal analysis is finished to predict the tem-perature filed of the whole joint and spraying fused coating Then the results of the thermal analysis are applied as a thermal body load in a transient structural analysis to calculate stresses In this process, the technique of ‘element birth and death’ is used for mod-eling of the welding and coating material Heat source is applied in the form of heat flux density
Heat flux density is calculated according to the actual parame-ters of spraying fused coating and welding The total quantity of heat source can be obtained through the current (I), voltage (U) and thermal efficiency (g) The total quantity divided by the total volume is per unit volume of heat, and Heat flux density (q) is calculated according to the formula of Qv= qt(q), t is the time corresponding to a loading step during the process of simulation The size of heat flux density is q = 7.90 1010w/m3s in this simulation
Table 2
Hardness and Young’s modulus of Conventional-joint at three different positions.
Conventional-joint Coating Transition zone Welding
Hardness H (GPa) 6.417 3.829 1.944
Young’s modulus E (GPa) 225.221 182.55 196.209
Fig 6 The bonding condition of Conventional-joint between coating and welding.
Trang 43 Fatigue test
Specimens were divided into three groups: as-welded joint,
welded joint spraying fused by conventional nickel base alloy
powder (Conventional-joint) and welded joint spraying fused by
low transformation temperature alloy powder (LTT-joint)
All fatigue tests were carried out under constant amplitude
load, in term of stretch bending combined loads, with stress ratio
(R) of 0.1 Tests were carried out on 100 kN HF Fatigue Testing
Machine, whose static load error for full measuring range is
±0.2% Besides, error of the dynamic load is ±2%
4 Results and discussion 4.1 The mechanical property of spraying fused joint The purpose of MSF is to improve fatigue strength of welded joint The adhesion strength between coating and welding which greatly affects the fatigue strength is very important.Fig 6showed the bonding condition between coating and welding of Conven-tional-joint From Fig 6 we can see that coating and welding achieved perfectly metallurgy bonding There is no impurities and air hole exit Therefore, it is difficult for fatigue crack source
to form at the transition zone
Fig 8 The cooling process of spraying fused coating.
Trang 5The Hardness and Young’s modulus of Conventional-joint at
three different positions (coating, transition zone and welding)
based on nano-indentation were shown inTable 2 FromTable 2,
the value of Hardness and Young’s modulus gradually increases
from welding to coating
In addition, the Hardness and Young’s modulus of LTT-joint are
basic the same with Conventional-joint
4.2 Simulated results
Because of the distribution of residual stress is decided by
tem-perature profile, we first study the distribution of temtem-perature
field Stress field of LTT-joint is simulated by changing the linear
expansion coefficient of Conventional-joint, therefore,
Conven-tional-joint and LTT-joint share a temperature field
Fig 7is temperature profile of spraying fused coating at some
time In the process of simulation, cooling process of temperature
field is very important The sample finally is used at room
temper-ature, so the temperature of the spray welding layer should be
reduced to room temperature.Fig 8is cooling process of spraying
fused coating Fig 8a shows that the distribution of isotherm is
uniform during the cooling process of spraying fused coating The
larger temperature gradient does not exist at each location
Fig 8b is the final cooling effect, and the whole temperature of
spraying fused coating is around 22 °C, basically achieving the
de-sired cooling effect
Residual tensile stress of welded joint will reduce fatigue
strength of joint Thus, the distribution of residual stress of
spray-ing fused joint is very important The extraction path of residual
stress was shown inFig 9 Because the loading direction of fatigue
specimen is the direction of X axis, we only analyzed stress
distri-bution of direction of X axis.Fig 10 is stress cloud chart of
as-welded joint.Fig 11is stress cloud chart of welded joint spraying
fused by conventional nickel base alloy powder
(Conventional-joint).Fig 12is stress cloud chart of welded joint spraying fused
by low transformation temperature alloy powder (LTT-joint)
Fig 13is stress distribution of X-direction of As-welded joint,
Con-ventional-joint and LTT-joint along path P1 in the same coordinate
system, and Fig 14 is Stress distribution of X-direction of three
kinds of joints along path P2 in the same coordinate system
FromFig 13we can see that as-welded joints present large
ten-sile stress in this direction Maximum value of tenten-sile stress
reached 294 MPa The stress of Conventional-joint is also tensile
stress at weld toe But compared to as-welded joint, the value of
tensile stress is smaller, only is 107 MPa The stress of LTT-joint
is compressive stress at weld toe of coating, and the maximum
value of compressive stress reached 126 MPa The residual com-pressive stress in the direction of fatigue loading will significantly increase the fatigue life of welded joint FromFig 14we can see that as-welded joint and Conventional-joint present tensile stress
Fig 11 Stress cloud chart of Conventional-joint.
Fig 12 Stress cloud chart of LTT-joint.
Trang 6of 300 MPa in this direction The stress of LTT-joint is compressive stress
Therefore, the fatigue crack sources of as-welded joint and most
of Conventional-joints formed at weld toe for residual tensile stress However, fatigue crack source of LTT-joint has been transferred to the base metal for residual compressive stress 4.3 S–N curve
In order to further verify the accuracy of simulation results and residual compressive stress effects on fatigue life of welded joint, high cycle fatigue tests were done For high cycle fatigue, the max-imum stress is always lower than yield strength of material When stress ratio is 0.1, stress range is also certain lower than yield strength Fatigue life corresponding to different stress range can
be gained by fatigue tests S–N curves (The authorized surviving fraction is 50%) of welded joints (as-welded joint, Conventional-joint, LTT-joint) are shown inFig 15 Fatigue strength of three kinds
of joint corresponding to 2 106cycles were shown inTable 3 Through calculation, it is clear that under the condition of
2 106cycles, the fatigue strength of as-welded joint is 135 MPa, while that of Conventional-joint and LTT-joint is 218 MPa and
235 MPa, respectively The fatigue strength of Conventional-joint increases by 61.48%, and fatigue strength of LTT-joint increases
by 74.07%
4.4 Fracture analysis For as-welded joint, the fatigue crack sources of fractured spec-imens are all at weld toe For welded joint with the spray coating (Conventional-joint, LTT-joint), the locations of fatigue crack source of fractured specimens are shown inFig 16 FromFig 16
we can see that, most of crack sources of Conventional-joints ap-pear in the weld toe between spray coating and base metal (see Fig 16a), and most of crack sources of LTT-joints appear in the base metal (see Fig 16b) Meanwhile, there is also a small part of Conventional-joint fractured in the base metal and a small part
of LTT-joint fractured in the weld toe between spray coating and base metal
5 Conclusions Modification of spraying fused (MSF) of plasma arc as heat source both fundamentally reduced stress concentration at weld toe and achieved metallurgical bond between spraying fused coat-ing and weldcoat-ing This method also can form compressive residual stress on the surface of welded joint by means of alloy powder with low phase transition point, which will significantly improve the fatigue performance of welded joint The application of numer-ical simulation method plays an important role for understanding mechanism and characteristics of MSF Fatigue testing results showed that under the condition of 2 106 cycles, the fatigue
Table 3
Fatigue strength of three kinds of joints (R = 0.1).
Type of joint Fatigue strength
(2 10 6 ) Dr(MPa)
Slope (m) Increase
degree (%) As-welded joint 135 6.23 –
Conventional-joint 218 9.88 61.48
Fig 16 The locations of fatigue crack source.
Fig 15 S–N curves of three kinds of joints.
Fig 14 Stress distribution of X-direction of three kinds of joints along path P2.
Trang 7strength of as-welded joint is 135 MPa, while that of
Conventional-joint and LTT-Conventional-joint is 218 MPa and 235 MPa, respectively The
fati-gue strength of Conventional-joint increases by 61.48%, and fatifati-gue
strength of LTT-joint increases by 74.07%
In conclusion, modification of spraying fused (MSF) of plasma arc
as heat source will have more widely practical value by adjusting
the different alloy powder Study of MSF will play a promoting role
for the development of life-extending technology of welded joint
Acknowledgements
This work was financially supported by the National Science
Foundation of Tianjin through Grant No 50805102 The authors
wish to thank Professor D.P Wang and associate professor C.Y
Deng for their experimental supports
References
[1] Lixing Huo Fracture behavior and assessment of welded
structure Beijing: China Machine Press; 2000
[2] Dahle T Design fatigue strength of TIG-dressed welded joints in high-strength
steels subjected to spectrum loading Int J Fatigue 1998;20(9):677–81
[3] Janosch JJ, Koneczny H, Debiez S, Statnikov ES, Tryufyakov VI, Mikheev PP.
Weld World 1996;37:72–83
[4] Statnikov ES, Muktepavel VO, Blomqvist A Weld World 2002;46(3/4):20–32
[5] Manteghi S, Maddox SJ Methods for fatigue life improvement of welded joints
in medium and high strength steels IIW Doc XIII-2006-04; 2004.
[6] Ramalho Armando L, Ferreira José AM, Branco Carlos AGM Fatigue behaviour
of T welded joints rehabilitated by tungsten inert gas and plasma dressing.
Mater Des 2011;32(10):4705–13
[7] Morisada Yoshiaki, Fujii Hidetoshi, Inagaki Fuminori Development of high frequency tungsten inert gas welding method Mater Des 2012;44:12–6 [8] Yildirim Halid Can, Marquis Gary B Fatigue strength improvement factors for high strength steel welded joints treated by high frequency mechanical impact Int J Fatigue 2012;44:168–76
[9] Mori T, Shimanuki H, Tanaka M Effect of UIT on fatigue strength of web-gusset welded joints considering service condition of steel structures Weld World 2012;56(9–10):141–9
[10] Abdullah Amir, Malaki Massoud, Eskandari Ahmad Strength enhancement of the welded structures by ultrasonic peening Mater Des 2012;38:7–18 [11] Yang Xinjun, Zhou Jianxin, Ling Xiang Study on plastic damage of AISI 304 stainless steel induced by ultrasonic impact treatment Mater Des 2011;36:477–81
[12] Huther I, Lieurade HP, Souissi R, Nussbaumer A, Chabrolin B, Janosch JJ Analysis of results on improved welded joints Weld World 1996;37(5):242–66
[13] Branco CM, Maddox SJ, Infante V, Gomes EC Fatigue performance of TIG and plasma welds in thin sections Int J Fatigue 1999;22(6):589–602
[14] Kirkhope KJ, Bell R, Caron L, Basu RI, Ma K-T Weld detail fatigue life improvement techniques Part 2: application to ship structures Mar Struct 1999;12:477–96
[15] Kirkhope KJ, Bell R, Caron L, Basu RI, Ma K-T Weld detail fatigue life improvement techniques Part 1: review Mar Struct 1999;12:447–74 [16] Marquis G Failure modes and fatigue strength of improved HSS welds Eng Fract Mech 2010;77(11):2051–62
[17] Roy S, Fisher JW, Yen BT Fatigue resistance of welded details enhanced by ultrasonic impact treatment (UIT) Int J Fatigue 2003;25:1239–47
[18] Zhao Xiaohui, Dongpo Wang, Deng Caiyan J Mater Process Technol 2011;211:2039–44
[19] Wenxian Wang Study on low transformation temperature welding electrode and its application to improve fatigue performance of welded joints PhD thesis, Tianjin University, Tianjin; 2002.