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Microstructure and residual stress distribution of similar and dissimilar electron beam welds – Maraging steel to medium alloy medium carbon steel
Materials and Design 31 (2010) 749–760
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Microstructure and residual stress distribution of similar and dissimilar electron beam welds – Maraging steel to medium alloy medium carbon steel P. Venkata Ramana a,*, G. Madhusudhan Reddy b,1, T. Mohandas b,2, A.V.S.S.K.S. Gupta c,3 a
Mahatma Gandhi Institute of Technology, Gandipet, Hyderabad 500 075, India Defence Metallurgical Research Laboratory, Hyderabad 500 058, India c Jawaharlal Nehru Technological University, Hyderabad 500 085, India b
a r t i c l e
i n f o
Article history: Received 8 June 2009 Accepted 2 August 2009 Available online 7 August 2009 Keywords: A. Maraging steel A. Medium alloy medium carbon steel G. Residual stress
a b s t r a c t The influence of parent metal heat treatment condition on the residual stress distribution in dissimilar metal welds of maraging steel to quenched and tempered medium alloy medium carbon steel has been investigated. It has been observed that the residual stress distribution would be more compressive if the maraging steel is in soft condition. This is attributed to stress absorbing nature of highly yielding soft maraging steel. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Several high-technology applications, such as aircraft and aerospace, need to possess ultrahigh strength coupled with fracture toughness in order to meet the requirement of minimum weight while ensuring high reliability [1]. In many applications, dissimilar combinations of steels are necessary for technical and economic reasons. In such applications materials such as maraging steel and medium alloy medium carbon steel are used. Maraging steels are a class of ultrahigh strength martensitic steels, which develop strength due to the precipitation of intermetallic compounds [2–9]. They exhibit a unique combination of properties that include ultrahigh strength and excellent fracture toughness. Medium alloy medium carbon structural steels with ultrahigh strength and reasonable ductility, considered to be inexpensive and attractive substitute for maraging steel [1]. Another important feature of these two steels is that they exhibit good weldability [10,11]. These steels are therefore important candidate materials for critical applications such as rocket motor cases, submarine hulls, connecting rods, landing gears and bridge layer tanks [12,13]. These materials are used extensively and individually but not much is reported about the dissimilar combination.
* Corresponding author. Mobile: +91 98480 25631. E-mail addresses: [email protected] (P. Venkata Ramana), [email protected] (G. Madhusudhan Reddy), [email protected] (A.V.S.S.K.S. Gupta). 1 Tel.: +91 040 2458 6426. 2 Tel.: +91 040 2458 6424. 3 Mobile: +91 98494 27331. 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.08.007
The adoption of dissimilar-metal combination provides possibilities for the flexible design of the component by using each material efficiently i.e., benefiting from the specific properties of each material to meet functional requirements. The dissimilar metal joints are characterized particularly by compositional gradients and microstructural changes, which yield large variations in physical and mechanical properties across the joint. These variations may lead to metallurgical incompatibility, e.g., the formation of brittle phases, the segregation of high and low melting phases due to chemical mismatch, and possibly large residual stresses from the physical mismatch. The joining of dissimilar metals is, therefore, far more complex than the joining of similar metals [14,15]. Fusion welding is one of the most widely employed fabrication processes for these steels. Electron beam welding (EBW) is a highenergy density fusion welding process which is extensively employed in the aerospace and defence applications [16]. EBW is considered advantageous over other fusion welding processes in joining dissimilar metals as it has high heating and cooling rates which takes care of the difference in the melting temperatures of the materials being welded. A low total-heat input per unit length of weld reduces the residual stresses substantially. The small weld bead size minimizes mixing of dissimilar metals thus limiting the brittle zones arising from chemical mismatch, to some extent [14]. Residual stress is the stress that exists within a material without application of an external load [17], or it can be described as the stress which remains in a body that is stationary and at equilibrium with its surroundings. Residual stresses can arise in materials in almost every step of processing. The origins of residual stresses in a component may be classified as: mechanical, thermal,
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and chemical. Mechanically generated residual stresses are often a result of manufacturing processes that produce non-uniform plastic deformation. They may develop naturally during processing or treatment, or may be introduced deliberately to develop a particular stress profile in a component. Examples of operations that produce undesirable surface tensile stresses or residual stress gradients are rod or wire drawing, welding, machining and grinding. Knowledge of residual stress field becomes essential, as it should be used as initial state of stress of the load carrying structure. Residual stresses are associated with any metal joining process in general, and with fusion welding processes in particular. It is widely recognized that residual stresses in welded joints are highly significant in practical terms. Residual stresses are produced in the vicinity of the weld due to non-uniform expansion and shrinkage of differently heated zones during thermal transient of a weld pass [18]. Residual stresses are not cyclic, but they may augment or detract from applied stresses depending on their respective sign. Tensile residual stresses may increase the rate of damage by fatigue or creep and reduce the load carrying capacity. Compressive stresses are generally considered beneficial, but cause a decrease in the allowable buckling load [17,19–22]. The beneficial effects of compressive stresses have been widely recognized in industry, as these are believed to increase fatigue strength of the components and reduce stress corrosion cracking and brittle fracture etc. In several practical applications these are deliberately introduced through post manufacturing treatment such as shot peening or water jet peening etc. life limiting residual stresses can be some times reduced by post-weld heat treatment, but this may be impractical with large or inaccessible components. The present study is on the residual stress distribution in the dissimilar metal electron beam weldments of maraging steel to medium alloy medium carbon ultrahigh strength steels. Generally these steels are supplied in soft condition. They attain their ultrahigh strength after respective heat treatments. The aim of the present study is to investigate the influence of starting parent metal strength on the residual stress distribution in the as-welded and subsequent to different post-weld heat treatment conditions, as strength levels are reported to influence the magnitude of residual stresses in similar metal welding [23]. A detailed study of microstructure and its correlation with the hardness and residual stress distribution attempted in this investigation assumes significance due to the availability of limited data on the subject in this dissimilar-metal combination of welding.
Fig. 1. Weld coupon design and test plate assembly. (a) Section of test plate assembly prior to welding and (b) test plate assembly after welding.
Table 1 Welding parameters for electron beam welding. Machine settings
Parameters
Gun to work distance, mm Accelerating voltage, kV Beam current, mA
283 55 30 mA (for initial first pass for preheating) and 65 mA (for penetration) Slightly above the surface 1 10 4 mbar and less 214.5
Focus Speed, m/min Vacuum level, mbar Heat input, J/mm
2. Experimental procedures 2.1. Parent metals The materials investigated are 18% Ni (250 grade) maraging steel and medium alloy medium carbon steels in the form of 5.2 mm thick sheets. In order to investigate the influence of pre weld heat treatment on microstructure, residual stress distribution and hardness, the materials were given respective heat treatments. Maraging steel was solutionised at 815 °C for 1 h followed by air cooling and then aged at 480 °C for 3 h followed by air cooling. Medium alloy medium carbon steel was austenised at 925 °C for 35 min followed by air cooling and then tempered at 295 °C for 45 min followed by air cooling. The as-received maraging steel is in solutionised condition. The details of weld coupon preparation and test plate assembly are shown in Fig. 1. Electron beam welding was performed in butt joint configuration with welding parameters shown in Table 1. Similar and dissimilar-metal combinations were welded in different parent metal heat treatment conditions as shown in Table 2. The yield stress values of parent material in different heat treatment conditions are presented in Table 3 for ready reference.
Fig. 2. Schematic sketch of hardness traverse across the weldment.
2.2. Metallography Analysed composition of the parent materials is given in Table 4. The weldment microstructures of similar and dissimilar joints were studied by metallography of various regions using Leitz optical microscope. 2% Nital (2 ml HNO3 and 98 ml methanol) was used to etch medium alloy medium carbon steel weld and modified Fry’s reagent (50 ml HCl, 25 ml HNO3, 1 g CuCl2 and 150 ml water) was used to etch maraging steel weld. The respective etchants were used to etch fusion zone, heat affected zone and parent metal regions.
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Weldment with parent metal condition
Post-weld condition
1 2 3 4 5
Maraging steel (solutionised and aged) to maraging steel (solutionised and aged) Medium alloy medium carbon steel (quenched and tempered) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (solutionised and aged) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered)
As-welded As-welded As-welded As-welded Aged
Table 3 Parent material yield stress values. S.No.
Material
Condition
YS (MPa)
UTS (MPa)
El. (%)
1 2 3 4
Maraging steel Maraging steel Medium alloy medium carbon steel Medium alloy medium carbon steel (quenched and tempered)
As received Solutionised and aged Quenched and tempered Aged
950 1600 1458 1322
1000 1750 1815 1675
12 7.5 12.01 13.17
Table 4 Composition of parent materials. Material
Element (wt.%) C
Ni
Co
Mo
Ti
Al
Cr
Si
Mn
Fe
Maraging steel Medium alloy medium carbon steel
0.01 0.33
18.9 2.8
8.3 <1.0
4.6 <1.0
0.41 –
0.15 –
– 0.85
– 1.8
– 0.35
Balance Balance
2.3. Stress measurement Residual stress measurement was carried out with AST3000 Xray stress analyser employing CrKa radiation. X-ray techniques measure stress indirectly by measuring the surface strain, which is indicated by the position of a diffracted peak h for a crystal plane oriented at various angles to the surface of a specimen as is de-
scribed in the literature [17,19,24–26]. The stress is given by the gradient of a plot of diffraction angle 2h against sin2 w, where w is the angle between the diffracting planes and the specimen surface. The instrument uses a pair of solid-state detectors located on each side of the main beam. Diffraction peaks are captured by the individual pixels in the detectors, giving rapid data capture without mechanical movement. Residual stresses were evaluated
Fig. 3. Optical microstructure of similar metal weld of maraging steel.
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in this analyser using multiple exposure sin2 w technique based on the diffraction from (300) planes in austenitic welds and from (211) planes in ferritic/martensite welds. The residual stress measurement comprise of at least fourteen measurements of lattice spacing over a range of w orientations ( 45° to +45°) to the surface of the specimen. Residual stress measurements were carried out across the weldment (i.e., perpendicular to welding direction). The accuracy of stress measurements is approximately ±20 MPa. When some anisotropy was encountered, more trials were performed. In weld/HAZ/parent metal where the grain size influenced the measurements, the X-ray goniometer was oscillated by ±2°. For computation of stresses from measurement of strain data, appropriate X-ray elastic constants were used. Prior to measurement of stress, the surfaces of the spots in each location were chemically cleaned with acetone or carbon tetrachloride solution. Following this, the spots were electro polished to a depth of 0.2 mm using electro polishing kit with 20% perchloric acid in ethanol, cooled to 0 °C prior to measurement. 2.4. Hardness measurement Micro-hardness survey was conducted across the weld beads of all the welded coupons employing Knoop micro-hardness testing machine. All of the hardness readings were obtained at a load of 300 gf. The distance between two consecutive indentations is 0.5 mm. But this distance varied when the hardness was taken at specific regions such as fusion boundary. A schematic diagram of the hardness survey is shown in Fig. 2. 3. Results and discussion 3.1. Microstructure Figs. 3 and 4 show the optical microstructure of similar metal welds of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel, respectively, whereas Fig. 5
shows the optical microstructure of dissimilar weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel. The optical microscopy revealed that the similar metal welds exhibited symmetrical fusion zone and heat affected zones, whereas the dissimilar metal welds exhibited unsymmetrical fusion zone and heat affected zones. This unsymmetrical nature may be due to the difference in the thermal conductivity of the materials. It is evident from the Fig. 6 that the width of heat affected zone of medium alloy medium carbon steel is observed to be more than that of the maraging steel because the thermal conductivity of medium alloy medium carbon steel is higher than the thermal conductivity of maraging steel. The heat affected zone of quenched and tempered steel medium alloy medium carbon steel has two regions, A – coarse-grained heat affected zone, and B – fine-grained heat affected zone, partially transformed region (soft zone) and tempered region (Fig. 6). In both similar and dissimilar welds the fusion zone exhibited cellular/dendritic structure in the entire weld zone. The size of cells and spacing at the top and bottom of the fusion zone are found to be smaller. In the middle portion of the weld, a mixture of cellular and dendritic structure is predominant. This trend is also observed at the fusion boundaries of similar metal welds. In dissimilar welds the microstructure at the fusion boundaries is found to be dependant on the magnitude of thermal conductivity. The interface microstructure of maraging steel shows cellular structure, while the interface microstructure of quenched and tempered medium alloy medium carbon steel shows epitaxial grains. This is due to higher thermal conductivity of the medium alloy medium carbon steel. The rate of heat flow being more in the direction perpendicular to the weld, the grains tend to grow in that direction resulting in columnar grains. The microstructure of the dissimilar weld of as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition is shown in Fig. 7. The microstructure is similar to that of the dissimilar weld of
Fig. 4. Optical microstructure of similar metal weld of medium alloy medium carbon steel.
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Fig. 5. Optical microstructure of dissimilar weld of maraging steel in solutionised and aged condition and medium alloy medium carbon steel in quenched and tempered condition.
Fig. 6. Widths of heat affected zones in dissimilar weld of aged maraging steel and quenched and tempered steel.
as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the as-welded condition (Fig. 8). The only noticeable difference is that the presence of coarse martensitic structure in the parent metal of medium alloy medium carbon steel evolved due to its response to the aging treatment. 3.2. Residual stresses Transverse residual stress distribution across the similar metal weld of age hardened maraging steel in the as-welded condition is shown in Fig. 9. It is observed that the residual stresses are compressive in nature, both in the weld and parent metal. They are more compressive ( 406 MPa) in the weld than in the parent metal ( 141 MPa). In the similar metal weld of quenched and tempered medium alloy medium carbon steel in the as-welded condition, the nature
of residual stresses is tensile in the weld as well as in the parent metal (Fig. 10). The magnitude of the stresses is more in the weld (+406 MPa) compared to that in the parent metal (+108 MPa). Fig. 11 shows the residual stress distribution across the dissimilar weld joint of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel in the as-welded condition. In this joint, compressive stresses were observed in the maraging steel and tensile stresses were observed in medium alloy medium carbon steel. Maximum compressive residual stress is observed at the weld centre. At the fusion boundary of the maraging steel the residual stresses are observed to be more compressive compared to the adjacent heat affected zone. The reduction in the magnitude of stress value in the heat affected zone compared to that at the fusion boundary is explained as follows: Dissolution of strengthening precipitates takes place at the fusion boundary of the age hardened maraging steel as it is subjected to high temperatures during weld thermal cycle resulting in soft behaviour of the material. This soft material absorbs more stresses generated due the hindrance of shrinkage and the magnitude of residual stresses, compressive in nature is high. In addition to this soft region close to fusion boundary, due to dilution of medium alloy medium carbon steel, resulted in duplex structure consisting of martensite and austenite. Phase transformation stresses are predominant here than the shrinkage stresses which resulted in more compressive stresses. This is mainly due to low temperature phase transformation phenomenon. The reduction in the magnitude of the residual stresses in the heat affected zone is due to the fact that this zone is close to the un-affected parent material which is hard in condition and reduces
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Fig. 7. Optical microstructure of the dissimilar weld of as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition.
the stress absorbing capacity of the adjacent material of heat affected zone. Contrary to the high tensile residual stresses in the weld and fusion boundary of similar metal welds of medium alloy medium carbon steel, the residual stresses at the fusion boundary of dissimilar weld on the medium alloy medium carbon steel side are found to be compressive. This may be due to the high compressive residual stresses present in adjacent maraging steel. The residual stresses in the heat affected zone and parent metal of medium alloy medium carbon steel are found to be tensile in nature. Fig. 12 shows the residual stress distribution across the postweld aged dissimilar weld joint of maraging steel in as-received condition and quenched and tempered medium alloy medium carbon steel. The residual stresses are found to be more compressive at the fusion boundary of maraging steel and less compressive at the fusion boundary of medium alloy medium carbon steel, compared to the residual stresses at the weld centre. In the maraging steel the stresses are more compressive in the heat affected zone compared to the fusion boundary and parent metal. The residual stresses in the medium alloy medium carbon steel side are tensile in nature from fusion boundary to parent metal. Residual stress in weld zone and un-affected parent material is shown in the Table 5. From the residual stress distribution in similar metal welds it is observed that maraging steel welds exhibit compressive stresses, while quenched and tempered steel welds exhibit tensile stresses (Figs. 9 and 10). The magnitude of the stresses is maximum in the weld region in that maximum compressive stresses are observed in the maraging steel weld while maximum
tensile stresses in the quenched and tempered steel weld. These trends are as per the trends reported in the literature. The difference is mainly attributed to BCC (Body-centered cubic) martensite in maraging steel and BCT (Body-centered tetragonal) martensite is the quenched and tempered steel. In the dissimilar metal welds, if the starting parent metals are in their respective heat treatment conditions the magnitude of residual stresses (either compressive or tensile) are lower (Fig. 11). If the maraging steel is in soft condition and quenched and tempered steel is in hardened condition the weld experiences higher compressive stress in the as-welded (Fig. 13) and aged condition (Fig. 12). However, post-weld aging results in higher compressive stresses in the maraging steel side of the weld and tensile stresses on the quenched and tempered steel side of the weld while the parent quenched and tempered steel experiences lower tensile stresses after aging (compare Figs. 12 and 13). Reduction in the magnitude of tensile stresses in quenched and tempered steel after aging can be attributed to the coarsening of martensite during aging as the aging temperature is higher than the tempering temperature of the steel. These trends are reflected in hardness reduction in the quenched and tempered steel after aging. The high stresses in the dissimilar metal weld region subsequent to aging could be attributed to inter diffusion of elements as seen from the EPMA elemental scan (Figs. 14 and 15) that made the weld respond to heat treatment similar to quenched and tempered steel. In general the compressive nature of the residual stresses in dissimilar metal welds is due to dilution effects that lead to a composition that does not result in fully BCC and BCT martensite. Reduction
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Fig. 8. Optical microstructure of the dissimilar weld of as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the as-welded condition.
Fig. 9. Transverse residual stress distribution across the similar metal weld of age hardened maraging steel in the as-welded condition.
Fig. 10. Transverse residual stress distribution across the similar metal weld of quenched and tempered medium alloy medium carbon steel in the as-welded condition.
in the stresses in the quenched and tempered steel side when the maraging steel is in soft condition can be attributed to stress absorbing capacity of the soft maraging steel. However, the stress in the weld region after aging is enhanced marginally but with in the compressive regime. These observations are in conformity with that
reported in similar metal welds in that starting parent metal heat treatment and correspondingly the strength condition would have a bearing on residual stress distribution in welds [23]. Using maraging steel in soft condition is similar to the established trends of usage of undermatching fillers to absorb stresses [27].
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Fig. 11. Transverse residual stress distribution across the dissimilar weld joint of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel in the as-welded condition.
Fig. 12. Transverse residual stress distribution across the post-weld aged dissimilar weld of maraging steel in as-received condition and quenched and tempered medium alloy medium carbon steel.
3.3. Hardness Hardness survey across the similar metal weld in the transverse section of the aged maraging steel (Fig. 16) shows that the hard-
ness in the weld (Region A) is low compared to that of parent metal (Region E) because the weld is in as-welded condition whereas the parent metal is in heat treated condition (solutionised and aged). During the welding process the material adjacent to weld is subjected to higher peak temperature and fast cooling rates resulting in coarse-grained heat affected zone (Region B). The region away from the fusion boundary and adjacent to heat affected zone is exposed to a lower peak temperature resulting in fine-grained structure (Region C). Thus there is a gradual increase in the hardness of heat affected zone from the fusion boundary to the fine-grained region. It is observed that there is a small dip in the hardness value in the weldment adjacent to fine-grained region. This is a very narrow dark etched soft region (Region D) that is reported to be of not much practical significance [10]. The average hardness of the parent metal is 575 HK. Fig. 17 shows the hardness survey across the similar metal weld in the transverse section of the quenched and tempered medium alloy medium carbon steel. It is observed that there is small difference in the hardness of weld and adjoining parent metal. The weld hardness is marginally lower than that of parent metal. The average hardness of the weldment is 600 HK. Near heat affected zone exhibited marginally higher hardness than the weld and the parent metal. The high hardness of this region is due to expected grain coarsening although not predominant in low heat input process like electron beam welding. The hardness distribution across the dissimilar metal weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel is shown in Fig. 18. The weld exhibited hardness gradient with a low hardness (350 HK) on the maraging steel and high hardness (500 HK) in the quenched and tempered steel side. The parent metal maraging steel adjacent to the weld experienced softening as a result of exposure to temperature higher than the solution treatment temperature for maraging steel. The hardness gradient is opined to be due to composition gradient in the weld region, in that the weld on maraging steel side would have lower carbon than that in the quenched and tempered steel. Due to carbon dilution the quenched and tempered steel adjacent to the weld could not be hardened equal to that of parent metal, in other words, the hardenability of the weld region on the quenched and tempered steel side is lower than the corresponding parent metal. The hardness traverse across the dissimilar weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition is shown in Fig. 19. It is clear from the figure that the weld hardness is almost equal to that of maraging steel, as it is subjected post-weld aging treatment. The hardness of the medium alloy medium carbon steel is observed to be lower than the weld and maraging steel. Far heat affected zone in the quenched and tempered steel exhibited lower hardness as compared to the parent metal.
Table 5 Residual stress in weld zone and un-affected parent material. S.no.
Weldment
Condition
Residual Stress (MPa)
1
Maraging steel (solutionised and aged) to maraging steel (solutionised and aged) Medium alloy medium carbon steel (quenched and tempered) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (solutionised and aged) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered)
As-welded
406
As-welded
+405
Weld
2 3 4 5
As-welded
276 to
As-welded Aged
Maraging steel 141 – 151
Medium alloy medium carbon steel – +107
141
+93
401
286
+190
400 to +100
254
+26
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A comparison of hardness distribution across the welds shown in Figs. 19 and 20 reveals that post-weld aging leads to eliminate steep hardness gradients. This can be attributed to compositional homogenisation and correspondingly better response to age hardening. The lower hardness observed in the far heat affected zone of quenched and tempered steel is due to aging at a temperature higher than the tempering temperature that results in coarsening of martensite laths. 4. Conclusions
Fig. 13. Transverse residual stress distribution across the dissimilar weld of maraging steel in as-received condition and quenched and tempered medium alloy medium carbon steel, in as-welded condition.
The influence of parent metal strength on microstructure, residual stress distribution and hardness in similar and dissimilar welds has been investigated. Similar metal welds exhibited symmetrical fusion zone and heat affected zones, whereas the dissimilar metal welds exhibited unsymmetrical fusion zone and heat affected zones due to the difference in the thermal conductivity of the materials. In the fusion zone, residual stresses are compressive in similar welds of maraging steel and tensile in similar metal welds of medium alloy medium carbon steel. If one of the parent metals in the dissimilar metal welds is in soft condition the magnitude of stresses is lowered due to stress absorbing nature of the softer parent metal. The benefit of soft parent metal prevailed even after
Fig. 14. Electron probe micro analysis (EPMA) scan across the dissimilar metal weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel.
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Fig. 15. Electron probe micro analysis (EPMA) scan across the dissimilar metal weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition.
Fig. 16. Hardness survey across the similar metal weld in the transverse section of the solutionised and aged maraging steel.
Fig. 17. Hardness survey across the similar metal weld in the transverse section of the quenched and tempered medium alloy medium carbon steel.
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Fig. 18. Hardness distribution across the dissimilar metal weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel.
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Fig. 20. Hardness traverse across the dissimilar weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the aswelded condition.
Ramana) thanks the management of Mahatma Gandhi Institute of Technology, Hyderabad for permission and encouragement to carryout this work.
References
Fig. 19. Hardness traverse across the dissimilar weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the postweld aged condition.
post-weld aging treatment. Coarsening of martensite plates during aging enables to reduce the stresses in quenched and tempered steel. In the dissimilar welds, due to the presence of BCC and BCT martensite at the fusion boundaries of maraging steel and medium alloy medium carbon steel, respectively, the hardness trend showed a low to high variation from maraging steel to medium alloy medium carbon steel. The microstructural changes are reflected in lower hardness trends after aging. Acknowledgements Financial assistance from Defence Research Development Organization (DRDO) is gratefully acknowledged. The authors would like to thank Dr. G. Malakondaiah, Director, Defence Metallurgical Research Laboratory, Hyderabad for his continued encouragement and permission to publish this work. The authors also thank Structural Failure Analysis Group and Metal Working Group for help in metallography and heat treatment. One of the authors (P.Venkata
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[20] Lee Chin-Hyung, Chang Kyong-Ho. Numerical analysis of residual stresses in welds of similar or dissimilar steel weldments under superimposed tensile loads. Comput Mater Sci 2007;40:548–56. [21] Dong P. Residual stresses and distortions in welded structures: a perspective for engineering applications. Sci Technol Weld Join 2005;10(4):389–98. [22] Lee Chin-Hyung, Chang Kyong-Ho. Prediction of residual stresses in welds of similar and dissimilar steel weldments. J Mater Sci 2007;42:6607–13. [23] Venkata Ramana P, Madhusudhan Reddy G, Mohandas T. Residual stress distribution in high strength low alloy steel weldments. J Non Destr Test Eval 2007;6(1):33–40.
[24] Noyan IC, Cohen JB. Residual stress measurement by diffraction and interpretation. New York: Springer-Verlag; 1987. [25] Jones AM. Residual stresses: a review of their measurement and interpretation using X-ray diffraction, Report AERE-R-13005. Harwell, United Kingdom Atomic Energy Authority; 1989. [26] Cullity BD, Stock SR. Elements of X-ray diffraction. 3rd ed. New Jersy, USA: Prentice hall; 2001. [27] Gooch TG. Effect of deposited weld metal hydrogen content on risk of cracking when welding a medium carbon low alloy steel with austenitic consumables, Report 3505/1/76. The Welding Institute: Cambridge, UK; 1977.
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Microstructure and residual stress distribution of similar and dissimilar electron beam welds – Maraging steel to medium alloy medium carbon steel P. Venkata Ramana a,*, G. Madhusudhan Reddy b,1, T. Mohandas b,2, A.V.S.S.K.S. Gupta c,3 a
Mahatma Gandhi Institute of Technology, Gandipet, Hyderabad 500 075, India Defence Metallurgical Research Laboratory, Hyderabad 500 058, India c Jawaharlal Nehru Technological University, Hyderabad 500 085, India b
a r t i c l e
i n f o
Article history: Received 8 June 2009 Accepted 2 August 2009 Available online 7 August 2009 Keywords: A. Maraging steel A. Medium alloy medium carbon steel G. Residual stress
a b s t r a c t The influence of parent metal heat treatment condition on the residual stress distribution in dissimilar metal welds of maraging steel to quenched and tempered medium alloy medium carbon steel has been investigated. It has been observed that the residual stress distribution would be more compressive if the maraging steel is in soft condition. This is attributed to stress absorbing nature of highly yielding soft maraging steel. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Several high-technology applications, such as aircraft and aerospace, need to possess ultrahigh strength coupled with fracture toughness in order to meet the requirement of minimum weight while ensuring high reliability [1]. In many applications, dissimilar combinations of steels are necessary for technical and economic reasons. In such applications materials such as maraging steel and medium alloy medium carbon steel are used. Maraging steels are a class of ultrahigh strength martensitic steels, which develop strength due to the precipitation of intermetallic compounds [2–9]. They exhibit a unique combination of properties that include ultrahigh strength and excellent fracture toughness. Medium alloy medium carbon structural steels with ultrahigh strength and reasonable ductility, considered to be inexpensive and attractive substitute for maraging steel [1]. Another important feature of these two steels is that they exhibit good weldability [10,11]. These steels are therefore important candidate materials for critical applications such as rocket motor cases, submarine hulls, connecting rods, landing gears and bridge layer tanks [12,13]. These materials are used extensively and individually but not much is reported about the dissimilar combination.
* Corresponding author. Mobile: +91 98480 25631. E-mail addresses: [email protected] (P. Venkata Ramana), [email protected] (G. Madhusudhan Reddy), [email protected] (A.V.S.S.K.S. Gupta). 1 Tel.: +91 040 2458 6426. 2 Tel.: +91 040 2458 6424. 3 Mobile: +91 98494 27331. 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.08.007
The adoption of dissimilar-metal combination provides possibilities for the flexible design of the component by using each material efficiently i.e., benefiting from the specific properties of each material to meet functional requirements. The dissimilar metal joints are characterized particularly by compositional gradients and microstructural changes, which yield large variations in physical and mechanical properties across the joint. These variations may lead to metallurgical incompatibility, e.g., the formation of brittle phases, the segregation of high and low melting phases due to chemical mismatch, and possibly large residual stresses from the physical mismatch. The joining of dissimilar metals is, therefore, far more complex than the joining of similar metals [14,15]. Fusion welding is one of the most widely employed fabrication processes for these steels. Electron beam welding (EBW) is a highenergy density fusion welding process which is extensively employed in the aerospace and defence applications [16]. EBW is considered advantageous over other fusion welding processes in joining dissimilar metals as it has high heating and cooling rates which takes care of the difference in the melting temperatures of the materials being welded. A low total-heat input per unit length of weld reduces the residual stresses substantially. The small weld bead size minimizes mixing of dissimilar metals thus limiting the brittle zones arising from chemical mismatch, to some extent [14]. Residual stress is the stress that exists within a material without application of an external load [17], or it can be described as the stress which remains in a body that is stationary and at equilibrium with its surroundings. Residual stresses can arise in materials in almost every step of processing. The origins of residual stresses in a component may be classified as: mechanical, thermal,
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and chemical. Mechanically generated residual stresses are often a result of manufacturing processes that produce non-uniform plastic deformation. They may develop naturally during processing or treatment, or may be introduced deliberately to develop a particular stress profile in a component. Examples of operations that produce undesirable surface tensile stresses or residual stress gradients are rod or wire drawing, welding, machining and grinding. Knowledge of residual stress field becomes essential, as it should be used as initial state of stress of the load carrying structure. Residual stresses are associated with any metal joining process in general, and with fusion welding processes in particular. It is widely recognized that residual stresses in welded joints are highly significant in practical terms. Residual stresses are produced in the vicinity of the weld due to non-uniform expansion and shrinkage of differently heated zones during thermal transient of a weld pass [18]. Residual stresses are not cyclic, but they may augment or detract from applied stresses depending on their respective sign. Tensile residual stresses may increase the rate of damage by fatigue or creep and reduce the load carrying capacity. Compressive stresses are generally considered beneficial, but cause a decrease in the allowable buckling load [17,19–22]. The beneficial effects of compressive stresses have been widely recognized in industry, as these are believed to increase fatigue strength of the components and reduce stress corrosion cracking and brittle fracture etc. In several practical applications these are deliberately introduced through post manufacturing treatment such as shot peening or water jet peening etc. life limiting residual stresses can be some times reduced by post-weld heat treatment, but this may be impractical with large or inaccessible components. The present study is on the residual stress distribution in the dissimilar metal electron beam weldments of maraging steel to medium alloy medium carbon ultrahigh strength steels. Generally these steels are supplied in soft condition. They attain their ultrahigh strength after respective heat treatments. The aim of the present study is to investigate the influence of starting parent metal strength on the residual stress distribution in the as-welded and subsequent to different post-weld heat treatment conditions, as strength levels are reported to influence the magnitude of residual stresses in similar metal welding [23]. A detailed study of microstructure and its correlation with the hardness and residual stress distribution attempted in this investigation assumes significance due to the availability of limited data on the subject in this dissimilar-metal combination of welding.
Fig. 1. Weld coupon design and test plate assembly. (a) Section of test plate assembly prior to welding and (b) test plate assembly after welding.
Table 1 Welding parameters for electron beam welding. Machine settings
Parameters
Gun to work distance, mm Accelerating voltage, kV Beam current, mA
283 55 30 mA (for initial first pass for preheating) and 65 mA (for penetration) Slightly above the surface 1 10 4 mbar and less 214.5
Focus Speed, m/min Vacuum level, mbar Heat input, J/mm
2. Experimental procedures 2.1. Parent metals The materials investigated are 18% Ni (250 grade) maraging steel and medium alloy medium carbon steels in the form of 5.2 mm thick sheets. In order to investigate the influence of pre weld heat treatment on microstructure, residual stress distribution and hardness, the materials were given respective heat treatments. Maraging steel was solutionised at 815 °C for 1 h followed by air cooling and then aged at 480 °C for 3 h followed by air cooling. Medium alloy medium carbon steel was austenised at 925 °C for 35 min followed by air cooling and then tempered at 295 °C for 45 min followed by air cooling. The as-received maraging steel is in solutionised condition. The details of weld coupon preparation and test plate assembly are shown in Fig. 1. Electron beam welding was performed in butt joint configuration with welding parameters shown in Table 1. Similar and dissimilar-metal combinations were welded in different parent metal heat treatment conditions as shown in Table 2. The yield stress values of parent material in different heat treatment conditions are presented in Table 3 for ready reference.
Fig. 2. Schematic sketch of hardness traverse across the weldment.
2.2. Metallography Analysed composition of the parent materials is given in Table 4. The weldment microstructures of similar and dissimilar joints were studied by metallography of various regions using Leitz optical microscope. 2% Nital (2 ml HNO3 and 98 ml methanol) was used to etch medium alloy medium carbon steel weld and modified Fry’s reagent (50 ml HCl, 25 ml HNO3, 1 g CuCl2 and 150 ml water) was used to etch maraging steel weld. The respective etchants were used to etch fusion zone, heat affected zone and parent metal regions.
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Weldment with parent metal condition
Post-weld condition
1 2 3 4 5
Maraging steel (solutionised and aged) to maraging steel (solutionised and aged) Medium alloy medium carbon steel (quenched and tempered) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (solutionised and aged) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered)
As-welded As-welded As-welded As-welded Aged
Table 3 Parent material yield stress values. S.No.
Material
Condition
YS (MPa)
UTS (MPa)
El. (%)
1 2 3 4
Maraging steel Maraging steel Medium alloy medium carbon steel Medium alloy medium carbon steel (quenched and tempered)
As received Solutionised and aged Quenched and tempered Aged
950 1600 1458 1322
1000 1750 1815 1675
12 7.5 12.01 13.17
Table 4 Composition of parent materials. Material
Element (wt.%) C
Ni
Co
Mo
Ti
Al
Cr
Si
Mn
Fe
Maraging steel Medium alloy medium carbon steel
0.01 0.33
18.9 2.8
8.3 <1.0
4.6 <1.0
0.41 –
0.15 –
– 0.85
– 1.8
– 0.35
Balance Balance
2.3. Stress measurement Residual stress measurement was carried out with AST3000 Xray stress analyser employing CrKa radiation. X-ray techniques measure stress indirectly by measuring the surface strain, which is indicated by the position of a diffracted peak h for a crystal plane oriented at various angles to the surface of a specimen as is de-
scribed in the literature [17,19,24–26]. The stress is given by the gradient of a plot of diffraction angle 2h against sin2 w, where w is the angle between the diffracting planes and the specimen surface. The instrument uses a pair of solid-state detectors located on each side of the main beam. Diffraction peaks are captured by the individual pixels in the detectors, giving rapid data capture without mechanical movement. Residual stresses were evaluated
Fig. 3. Optical microstructure of similar metal weld of maraging steel.
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in this analyser using multiple exposure sin2 w technique based on the diffraction from (300) planes in austenitic welds and from (211) planes in ferritic/martensite welds. The residual stress measurement comprise of at least fourteen measurements of lattice spacing over a range of w orientations ( 45° to +45°) to the surface of the specimen. Residual stress measurements were carried out across the weldment (i.e., perpendicular to welding direction). The accuracy of stress measurements is approximately ±20 MPa. When some anisotropy was encountered, more trials were performed. In weld/HAZ/parent metal where the grain size influenced the measurements, the X-ray goniometer was oscillated by ±2°. For computation of stresses from measurement of strain data, appropriate X-ray elastic constants were used. Prior to measurement of stress, the surfaces of the spots in each location were chemically cleaned with acetone or carbon tetrachloride solution. Following this, the spots were electro polished to a depth of 0.2 mm using electro polishing kit with 20% perchloric acid in ethanol, cooled to 0 °C prior to measurement. 2.4. Hardness measurement Micro-hardness survey was conducted across the weld beads of all the welded coupons employing Knoop micro-hardness testing machine. All of the hardness readings were obtained at a load of 300 gf. The distance between two consecutive indentations is 0.5 mm. But this distance varied when the hardness was taken at specific regions such as fusion boundary. A schematic diagram of the hardness survey is shown in Fig. 2. 3. Results and discussion 3.1. Microstructure Figs. 3 and 4 show the optical microstructure of similar metal welds of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel, respectively, whereas Fig. 5
shows the optical microstructure of dissimilar weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel. The optical microscopy revealed that the similar metal welds exhibited symmetrical fusion zone and heat affected zones, whereas the dissimilar metal welds exhibited unsymmetrical fusion zone and heat affected zones. This unsymmetrical nature may be due to the difference in the thermal conductivity of the materials. It is evident from the Fig. 6 that the width of heat affected zone of medium alloy medium carbon steel is observed to be more than that of the maraging steel because the thermal conductivity of medium alloy medium carbon steel is higher than the thermal conductivity of maraging steel. The heat affected zone of quenched and tempered steel medium alloy medium carbon steel has two regions, A – coarse-grained heat affected zone, and B – fine-grained heat affected zone, partially transformed region (soft zone) and tempered region (Fig. 6). In both similar and dissimilar welds the fusion zone exhibited cellular/dendritic structure in the entire weld zone. The size of cells and spacing at the top and bottom of the fusion zone are found to be smaller. In the middle portion of the weld, a mixture of cellular and dendritic structure is predominant. This trend is also observed at the fusion boundaries of similar metal welds. In dissimilar welds the microstructure at the fusion boundaries is found to be dependant on the magnitude of thermal conductivity. The interface microstructure of maraging steel shows cellular structure, while the interface microstructure of quenched and tempered medium alloy medium carbon steel shows epitaxial grains. This is due to higher thermal conductivity of the medium alloy medium carbon steel. The rate of heat flow being more in the direction perpendicular to the weld, the grains tend to grow in that direction resulting in columnar grains. The microstructure of the dissimilar weld of as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition is shown in Fig. 7. The microstructure is similar to that of the dissimilar weld of
Fig. 4. Optical microstructure of similar metal weld of medium alloy medium carbon steel.
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Fig. 5. Optical microstructure of dissimilar weld of maraging steel in solutionised and aged condition and medium alloy medium carbon steel in quenched and tempered condition.
Fig. 6. Widths of heat affected zones in dissimilar weld of aged maraging steel and quenched and tempered steel.
as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the as-welded condition (Fig. 8). The only noticeable difference is that the presence of coarse martensitic structure in the parent metal of medium alloy medium carbon steel evolved due to its response to the aging treatment. 3.2. Residual stresses Transverse residual stress distribution across the similar metal weld of age hardened maraging steel in the as-welded condition is shown in Fig. 9. It is observed that the residual stresses are compressive in nature, both in the weld and parent metal. They are more compressive ( 406 MPa) in the weld than in the parent metal ( 141 MPa). In the similar metal weld of quenched and tempered medium alloy medium carbon steel in the as-welded condition, the nature
of residual stresses is tensile in the weld as well as in the parent metal (Fig. 10). The magnitude of the stresses is more in the weld (+406 MPa) compared to that in the parent metal (+108 MPa). Fig. 11 shows the residual stress distribution across the dissimilar weld joint of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel in the as-welded condition. In this joint, compressive stresses were observed in the maraging steel and tensile stresses were observed in medium alloy medium carbon steel. Maximum compressive residual stress is observed at the weld centre. At the fusion boundary of the maraging steel the residual stresses are observed to be more compressive compared to the adjacent heat affected zone. The reduction in the magnitude of stress value in the heat affected zone compared to that at the fusion boundary is explained as follows: Dissolution of strengthening precipitates takes place at the fusion boundary of the age hardened maraging steel as it is subjected to high temperatures during weld thermal cycle resulting in soft behaviour of the material. This soft material absorbs more stresses generated due the hindrance of shrinkage and the magnitude of residual stresses, compressive in nature is high. In addition to this soft region close to fusion boundary, due to dilution of medium alloy medium carbon steel, resulted in duplex structure consisting of martensite and austenite. Phase transformation stresses are predominant here than the shrinkage stresses which resulted in more compressive stresses. This is mainly due to low temperature phase transformation phenomenon. The reduction in the magnitude of the residual stresses in the heat affected zone is due to the fact that this zone is close to the un-affected parent material which is hard in condition and reduces
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Fig. 7. Optical microstructure of the dissimilar weld of as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition.
the stress absorbing capacity of the adjacent material of heat affected zone. Contrary to the high tensile residual stresses in the weld and fusion boundary of similar metal welds of medium alloy medium carbon steel, the residual stresses at the fusion boundary of dissimilar weld on the medium alloy medium carbon steel side are found to be compressive. This may be due to the high compressive residual stresses present in adjacent maraging steel. The residual stresses in the heat affected zone and parent metal of medium alloy medium carbon steel are found to be tensile in nature. Fig. 12 shows the residual stress distribution across the postweld aged dissimilar weld joint of maraging steel in as-received condition and quenched and tempered medium alloy medium carbon steel. The residual stresses are found to be more compressive at the fusion boundary of maraging steel and less compressive at the fusion boundary of medium alloy medium carbon steel, compared to the residual stresses at the weld centre. In the maraging steel the stresses are more compressive in the heat affected zone compared to the fusion boundary and parent metal. The residual stresses in the medium alloy medium carbon steel side are tensile in nature from fusion boundary to parent metal. Residual stress in weld zone and un-affected parent material is shown in the Table 5. From the residual stress distribution in similar metal welds it is observed that maraging steel welds exhibit compressive stresses, while quenched and tempered steel welds exhibit tensile stresses (Figs. 9 and 10). The magnitude of the stresses is maximum in the weld region in that maximum compressive stresses are observed in the maraging steel weld while maximum
tensile stresses in the quenched and tempered steel weld. These trends are as per the trends reported in the literature. The difference is mainly attributed to BCC (Body-centered cubic) martensite in maraging steel and BCT (Body-centered tetragonal) martensite is the quenched and tempered steel. In the dissimilar metal welds, if the starting parent metals are in their respective heat treatment conditions the magnitude of residual stresses (either compressive or tensile) are lower (Fig. 11). If the maraging steel is in soft condition and quenched and tempered steel is in hardened condition the weld experiences higher compressive stress in the as-welded (Fig. 13) and aged condition (Fig. 12). However, post-weld aging results in higher compressive stresses in the maraging steel side of the weld and tensile stresses on the quenched and tempered steel side of the weld while the parent quenched and tempered steel experiences lower tensile stresses after aging (compare Figs. 12 and 13). Reduction in the magnitude of tensile stresses in quenched and tempered steel after aging can be attributed to the coarsening of martensite during aging as the aging temperature is higher than the tempering temperature of the steel. These trends are reflected in hardness reduction in the quenched and tempered steel after aging. The high stresses in the dissimilar metal weld region subsequent to aging could be attributed to inter diffusion of elements as seen from the EPMA elemental scan (Figs. 14 and 15) that made the weld respond to heat treatment similar to quenched and tempered steel. In general the compressive nature of the residual stresses in dissimilar metal welds is due to dilution effects that lead to a composition that does not result in fully BCC and BCT martensite. Reduction
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Fig. 8. Optical microstructure of the dissimilar weld of as-received maraging steel and quenched and tempered medium alloy medium carbon steel, in the as-welded condition.
Fig. 9. Transverse residual stress distribution across the similar metal weld of age hardened maraging steel in the as-welded condition.
Fig. 10. Transverse residual stress distribution across the similar metal weld of quenched and tempered medium alloy medium carbon steel in the as-welded condition.
in the stresses in the quenched and tempered steel side when the maraging steel is in soft condition can be attributed to stress absorbing capacity of the soft maraging steel. However, the stress in the weld region after aging is enhanced marginally but with in the compressive regime. These observations are in conformity with that
reported in similar metal welds in that starting parent metal heat treatment and correspondingly the strength condition would have a bearing on residual stress distribution in welds [23]. Using maraging steel in soft condition is similar to the established trends of usage of undermatching fillers to absorb stresses [27].
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Fig. 11. Transverse residual stress distribution across the dissimilar weld joint of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel in the as-welded condition.
Fig. 12. Transverse residual stress distribution across the post-weld aged dissimilar weld of maraging steel in as-received condition and quenched and tempered medium alloy medium carbon steel.
3.3. Hardness Hardness survey across the similar metal weld in the transverse section of the aged maraging steel (Fig. 16) shows that the hard-
ness in the weld (Region A) is low compared to that of parent metal (Region E) because the weld is in as-welded condition whereas the parent metal is in heat treated condition (solutionised and aged). During the welding process the material adjacent to weld is subjected to higher peak temperature and fast cooling rates resulting in coarse-grained heat affected zone (Region B). The region away from the fusion boundary and adjacent to heat affected zone is exposed to a lower peak temperature resulting in fine-grained structure (Region C). Thus there is a gradual increase in the hardness of heat affected zone from the fusion boundary to the fine-grained region. It is observed that there is a small dip in the hardness value in the weldment adjacent to fine-grained region. This is a very narrow dark etched soft region (Region D) that is reported to be of not much practical significance [10]. The average hardness of the parent metal is 575 HK. Fig. 17 shows the hardness survey across the similar metal weld in the transverse section of the quenched and tempered medium alloy medium carbon steel. It is observed that there is small difference in the hardness of weld and adjoining parent metal. The weld hardness is marginally lower than that of parent metal. The average hardness of the weldment is 600 HK. Near heat affected zone exhibited marginally higher hardness than the weld and the parent metal. The high hardness of this region is due to expected grain coarsening although not predominant in low heat input process like electron beam welding. The hardness distribution across the dissimilar metal weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel is shown in Fig. 18. The weld exhibited hardness gradient with a low hardness (350 HK) on the maraging steel and high hardness (500 HK) in the quenched and tempered steel side. The parent metal maraging steel adjacent to the weld experienced softening as a result of exposure to temperature higher than the solution treatment temperature for maraging steel. The hardness gradient is opined to be due to composition gradient in the weld region, in that the weld on maraging steel side would have lower carbon than that in the quenched and tempered steel. Due to carbon dilution the quenched and tempered steel adjacent to the weld could not be hardened equal to that of parent metal, in other words, the hardenability of the weld region on the quenched and tempered steel side is lower than the corresponding parent metal. The hardness traverse across the dissimilar weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition is shown in Fig. 19. It is clear from the figure that the weld hardness is almost equal to that of maraging steel, as it is subjected post-weld aging treatment. The hardness of the medium alloy medium carbon steel is observed to be lower than the weld and maraging steel. Far heat affected zone in the quenched and tempered steel exhibited lower hardness as compared to the parent metal.
Table 5 Residual stress in weld zone and un-affected parent material. S.no.
Weldment
Condition
Residual Stress (MPa)
1
Maraging steel (solutionised and aged) to maraging steel (solutionised and aged) Medium alloy medium carbon steel (quenched and tempered) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (solutionised and aged) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered) Maraging steel (as-received) to medium alloy medium carbon steel (quenched and tempered)
As-welded
406
As-welded
+405
Weld
2 3 4 5
As-welded
276 to
As-welded Aged
Maraging steel 141 – 151
Medium alloy medium carbon steel – +107
141
+93
401
286
+190
400 to +100
254
+26
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A comparison of hardness distribution across the welds shown in Figs. 19 and 20 reveals that post-weld aging leads to eliminate steep hardness gradients. This can be attributed to compositional homogenisation and correspondingly better response to age hardening. The lower hardness observed in the far heat affected zone of quenched and tempered steel is due to aging at a temperature higher than the tempering temperature that results in coarsening of martensite laths. 4. Conclusions
Fig. 13. Transverse residual stress distribution across the dissimilar weld of maraging steel in as-received condition and quenched and tempered medium alloy medium carbon steel, in as-welded condition.
The influence of parent metal strength on microstructure, residual stress distribution and hardness in similar and dissimilar welds has been investigated. Similar metal welds exhibited symmetrical fusion zone and heat affected zones, whereas the dissimilar metal welds exhibited unsymmetrical fusion zone and heat affected zones due to the difference in the thermal conductivity of the materials. In the fusion zone, residual stresses are compressive in similar welds of maraging steel and tensile in similar metal welds of medium alloy medium carbon steel. If one of the parent metals in the dissimilar metal welds is in soft condition the magnitude of stresses is lowered due to stress absorbing nature of the softer parent metal. The benefit of soft parent metal prevailed even after
Fig. 14. Electron probe micro analysis (EPMA) scan across the dissimilar metal weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel.
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Fig. 15. Electron probe micro analysis (EPMA) scan across the dissimilar metal weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the post-weld aged condition.
Fig. 16. Hardness survey across the similar metal weld in the transverse section of the solutionised and aged maraging steel.
Fig. 17. Hardness survey across the similar metal weld in the transverse section of the quenched and tempered medium alloy medium carbon steel.
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Fig. 18. Hardness distribution across the dissimilar metal weld of age hardened maraging steel and quenched and tempered medium alloy medium carbon steel.
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Fig. 20. Hardness traverse across the dissimilar weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the aswelded condition.
Ramana) thanks the management of Mahatma Gandhi Institute of Technology, Hyderabad for permission and encouragement to carryout this work.
References
Fig. 19. Hardness traverse across the dissimilar weld of as-received soft maraging steel and quenched and tempered medium alloy medium carbon steel, in the postweld aged condition.
post-weld aging treatment. Coarsening of martensite plates during aging enables to reduce the stresses in quenched and tempered steel. In the dissimilar welds, due to the presence of BCC and BCT martensite at the fusion boundaries of maraging steel and medium alloy medium carbon steel, respectively, the hardness trend showed a low to high variation from maraging steel to medium alloy medium carbon steel. The microstructural changes are reflected in lower hardness trends after aging. Acknowledgements Financial assistance from Defence Research Development Organization (DRDO) is gratefully acknowledged. The authors would like to thank Dr. G. Malakondaiah, Director, Defence Metallurgical Research Laboratory, Hyderabad for his continued encouragement and permission to publish this work. The authors also thank Structural Failure Analysis Group and Metal Working Group for help in metallography and heat treatment. One of the authors (P.Venkata
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