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Friction stir welding of maraging steel (Grade-250)
Accepted Manuscript Technical report Friction stir welding of Maraging Steel (Grade-250) Suresh D. Meshram, G. Madhusudhan Reddy, Sunil Pandey PII: DOI: Reference:
S0261-3069(13)00029-0 http://dx.doi.org/10.1016/j.matdes.2013.01.016 JMAD 5076
To appear in:
Materials and Design
Received Date: Accepted Date:
28 September 2012 7 January 2013
Please cite this article as: Meshram, S.D., Madhusudhan Reddy, G., Pandey, S., Friction stir welding of Maraging Steel (Grade-250), Materials and Design (2013), doi: http://dx.doi.org/10.1016/j.matdes.2013.01.016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Friction stir welding of Maraging Steel (Grade-250)
Suresh D. Meshram Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India Pin 500058 [email protected] [email protected]
G. Madhusudhan Reddy Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India Pin 500058 [email protected] [email protected]
Sunil Pandey Department of Mechanical Engineering Indian Institute of Technology Delhi Hauz Khas, New Delhi, India. Pin 110016, [email protected]
Corresponding author
Suresh D. Meshram Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India Pin 500058 [email protected]
[email protected]
Phone No. +91-4024586433
Fax No:+91-4024342697
1
Abstract The feasibility of friction stir (FS) welding of maraging steel is demonstrated using W- Mo base tool. The FS weldements were evaluated by performing hardness, impact toughness and metallographic evaluations. The base metal has a microstructure consisting of martensite and austenite islands. Refined grains of the martensite phase in friction stir zone was observed, and attributed to dynamic recrystalization. A significant improvement in toughness was observed in friction stir welds compared to base metal in both aswelded (27%) and aged (114%) condition. Although more development work is needed, particularly to improve tool materials, the present results indicate that FS welding does indeed have potential for joining maraging steels in aerospace applications.
KEYWORDS: Maraging steel; friction stir welding; impact toughness 1. Introduction Maraging steels are a class of very low carbon high alloy steel first developed in the 1960s for applications requiring ultra-high strength (tensile strength greater than 1380 MPa) combined with good fracture toughness [1]. The alloy gains its strength from the precipitation of intermetallic phases in the martensite matrix [2]. The morphology and crystal structure of the precipitates depend on the composition of the alloy, aging temperature, and time. Typically, these steels are solution treated in the fully austenitic (γ) region in the range of 815-9000C and quenched to produce a complete BCC martensite (α’) matrix, and then aged at temperature within the range 4000C to 6000C. The unique property of being weldable in the solutionised condition followed by post weld maraging treatment at a relatively low temperature (4800C) makes these steels attractive for fabrication of large structures [3]. Welding of maraging steel can be carried out without preheat by processes ranging from electron beam to submerged arc [4-6]. In common practice, however, gas tungsten arc welding is widely employed in view of the consistency of weld quality and overall economy [7]. In most cases, tensile efficiency over 90% is attained. However, the main concern has been toughness of the fusion zone due to microsegregation of alloying elements. The toughness of the fusion zone is considerably lower than that of the parent metal and this has been attributed to the presence of reverted austenite which forms due to microsegregation of alloying elements [8-9]. It is interesting that in maraging steel weld-metal reverted austenite decreases toughness [9-10]. It has also been suggested that, as the austenite reversion in maraging steels is on the martensitic lath boundaries, its influence on the progress of fracture would considerably differ from that of the finely distributed retained austenite which would result in the blunting of cracks [11]. Reverted austenite is a decomposition product, which forms on heating iron-nickel martensite [12-13]. 2
Owing to its high nickel (18%) content, it does not transform to martensite on cooling to room temperature, and results in incomplete precipitation hardening on aging. Nickel content above 33% will not allow to transform in to martensite on cooling to room temperature. Sufficient segregation has been found in the fusion zone to produce reverted austenite on heating to the normal aging temperature of 4800C, but it forms most readily at 6500C [9-13]. Formation of reverted austenite is particularly enhanced when alloying elements segregate to the cell and dendrite boundaries. In an attempt to reduce such segregation effects, Kenyon [9] used filler materials in which titanium and molybdenum contents were reduced (with an increase in cobalt content, which is known not-to-segregate). Significant increase in toughness was observed when segregation and the associated formation of austenite pools were reduced. The experience of the authors is that in spite of using modified welding consumables, formation of reverted austenite persists in the fusion zone of maraging steel. Most of the steel used in aerospace applications are joined as structural assemblies. A substantial quantity of sheets are as thin as 1.3 mm. Production experience indicates that weld distortion increases substantially as component thickness decreases below 10 mm. This led to increased fabrication costs, due to the labour involved in distortion mitigation or correction. Friction stir welding (FSW), is a relatively new solid state joining process and has been the focus of constant attention in joining of material to overcome solidification and distortion related issues [14]. While most of FSW efforts to date have involved joining of softer materials such as lead, zinc, magnesium, and a range of aluminum alloys [14], there is a considerable interest in extending the technology to other materials, including steel and titanium alloys [15-17]. FSW can be regarded as autogenous joining technique without creation of liquid metal. The consolidated weld material is thus free of typical fusion welding defects such as porosity, segregation and solidification cracking. Low distortion, cost effective, FSW joints are produced with excellent mechanical properties being achieved in several aluminum alloys. Continuing investigation suggest that FSW of steel will also become commercially attractive for such applications as missile casings, submarine hulls, etc. The main obstacle to use FSW for welding high strength and high melting point materials is the development of tool materials capable of surviving the high temperatures and forces generated by the process. Considerable advances have been made, mainly through improved materials selection and tool design [18-21]. Tool material development work has looked primarily at refractory metal, polycrystalline boron nitride, and tungsten carbide.
3
2. Experimental procedure
Friction stir welds were produced in position control mode on plates of 5.5 mm thick maraging steel MDN 250 plates (solution treated condition) with welding direction parallel to the rolling direction of the plates using indigenously designed and developed computer numerical controlled (CNC) machine, Fig.1. The composition of the base material is shown in Table 1. Welds were made in square groove-butt joint configuration on samples of 200mm in length and 100mm width. Tool materials included tungsten carbide (WC), tungsten-iron (WFe) and tungstenmolybdenum (WMo). Schematic diagram of tool with dimension and tool is shown in Fig.2. Prior to welding each tool was characterized dimensionally using digital profilometry and height gauge measurements. Initial trials were made to get defect free welds by keeping welding speed constant at 25mm/min and varying the rotation speed. Welding speed was kept constant and at lower value so that tool damage can be minimized. Low and high rotational speed resulted in either surface or internal defects respectively as shown in Fig.3 (a,c). It was found that spindle speed 600 rev/min and travel speed of 25 mm/min can produce void free welds with a smooth surface finish with WMo tool material Fig.3 (d,e). All three tool material types were used for welding maraging steel using exactly the same parameters and processing conditions. Weld cross section and shape of tool after welding, shown in Table 2, indicates intense wear and penetration of the tool material in the joint area of welds made using WC and WFe tools. All welding was performed without primary or secondary shielding. Post weld ageing of some of the joints was carried out at 480 OC for 3 hrs followed by air cooling. Transverse section of these welds were mounted and polished as per standard metallographic procedure and etched with modified Fry’s reagent (50 ml HCl, 25 ml HNO3, 1 g CuCl2 and 150 ml water) to reveal the macro-and microstructures. The phases present in the parent metal and welds in various conditions were identified by X-ray diffraction. For this purpose Philips PW 3020 X-ray diffraction machine is used with Cu K radiation. Mechanical properties i.e., micro-hardness and Charpy ‘V’-notch impact toughness of welds and parent metal in different heat-treated conditions were evaluated. Charpy impact test was carried out on sub size samples of 2.5 mm thickness as per ASTM standards, ASTM:E23-07ae1. Micro hardness survey was carried out across the joint interface at the centre on a Matsuzawa micro-hardness tester using Vickers indenter. Fractography of impact samples was carried out on a LEO scanning electron microscope (SEM). The impact samples were sectioned in mid section perpendicular to the notch to examine crack path under an optical microscope.
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3 Results and discussion 3.1 Optical microscopy and XRD analysis The microstructure of the parent metal is shown in Fig.4. The solution annealed material exhibits martensite with fine grain size corresponding to a nominal ASTM grain size of 11µm (mean linear intercept grain size). Friction stir maraging steel weld consists of five distinct bands. They have been labelled (Fig.5) as A, B, C, D and E. Region ‘A’ is stir zone. Region ‘B’, adjoining the weld represents the coarse grain martensitic phase. During welding the parent material adjacent to the weld interface is heated to high temperature in the austenite region where considerable grain growth occurs. On cooling, the austenite transforms to martensite and inherits the coarse austenite grain size. Region ‘C’ is a light etching martensite that has been heated due to weld thermal cycle in the austenite region but not high enough to cause grain growth. Region ‘D’ is a dark etching region where martensite phase experiences peak temperatures in the range of 590-7300C [3]. Some reverted austenite will therefore form in this region in a martensitic matrix. XRD analysis of this region in the as-weld and post weld aged condition, shown in Fig.6, indicates presence of two-phase microstructure. Region ‘E’ is unaffected parent metal during welding thermal cycle (Fig.5). Austenite transforms to martensite during cooling in the stir zone so that the microstructure of as-welded stir zone resembles that of the martensitic parent material. During welding of maraging steel through conventional fusion welding process the microstructure at fusion zone consists of austenite pool after ageing treatment due to microsegregation of alloying element resulting in low impact toughness. In friction stir welds, since there is no melting, microsegregation of alloying element is least resulting in no reverted austenite visible at optical microscopy. 3.2 Hardness Micro-hardness survey across the weld in the as-welded and aged conditions shown in Fig.7 revealed five different regions. Hardness variations across the interfaces as a result of welding could be attributed to high strain rate and thermal cycle during welding. Hardness of stir region is nearly equal to that of parent metal (Region 'A'). Immediately away from the interface region hardness has a drooping trend, probably due to softening of these regions due to exposure to high temperatures during welding leading to grain coarsening (Region 'B'), while the highest hardness towards the outer edge of the thermo mechanically affected zone (Region 'C') could be due to aging of the martensite microstructure of the parent maraging steel which is in solution treated condition prior to welding. Region ‘D’ represents dark band region and exhibited lowest hardness due to the
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presence of fine dispersion of retained austenite in martensite. Region ‘E' represents the parent metal. Micro-hardness survey across the weld bead in the aged condition presented in Fig.7 shows that hardness of all the regions except region ‘D’ increases almost to the hardness of the aged parent metal. From Fig.7 it is evident that hardness of the dark band region did not acquire full hardness during subsequent ageing; therefore this region remained softer than parent metal. Based on the available literature it is inferred that this soft zone may not be of practical significance, if the zone size is narrow [9]. Percentage increase in hardness after ageing treatment at different locations of weld is shown in Table 3. 3.3 Impact toughness Impact toughness data of parent metal and welds normalized as per ASTM sub size standard specimen are presented in Table 4. Friction stir welds exhibited higher toughness as compared to parent metal irrespective of weld condition. The impact toughness is in the order of as-welded > solutionized base metal > post weld aged > base metal aged conditions. The crack paths for different samples were observed by cutting the sample perpendicular to the notch at the center. Typical crack path features in as-welded and post weld aged conditions are presented in Fig.8. Improvement in toughness was reflected in crack path. With improvement in toughness the crack path was more tortuous. These observations are in conformity with similar trends reported in maraging steel weldment [8,10,11 and 22]. Maximum crack path length noticed in as-welded condition is 10.6 mm as compared to 8.5 mm of directly aged condition. It has been reported that the toughness of weldments depends on volume fraction of second phases, inclusions, and cell size [23]. As the samples are taken from the same weldment, difference in inclusion content or cell sizes are not expected, at the same time since the samples (welds and base metal) were aged at same temperature together there should not be any difference in volume fraction of precipitates. The above observations indicate that the main microstructural features influencing toughness of the stir zone is grain size. The extent of grain refinement observed in friction stir welding cannot be achieved through any other fusion welding process [7,8]. It was reported well in the literature that, poor impact toughness of fusion zone in maraging steel welds are attributed to the presence of reverted austenite in the martensite microstructure which leads to void formation in the austenite during deformation [9-11]. Austenite cracks preferentially because slip can take place in it at stresses, which are too low to produce slip in the matrix. These observations are in agreement with the reported trends, whenever a two phase structure is loaded, the soft phase will deform preferentially and may reach its critical strain for
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fracture at an early stage by setting up a series of micro-cracks in the harder matrix [9]. The observed fracture features (Fig.9) are in tune with impact toughness trends, i.e in the as-welded condition the dimples are fine while in aged conditions they are grown up. The fine grain microstructure at weld zone coupled with absence of reverted austenite could be attributed to higher impact toughness of maraging steel friction stir welds compared to fusion welds [11].
4. Conclusions Maraging steel (MDN-250) of 5.5 mm thick was successfully friction stir welded using WMo tool. In the present work, the microstructural characteristics and governing mechanical properties such as hardness and impact toughness of the maraging steel friction stir welds were examined. Since friction stir welding is a solid state welding process, there is no melting and hence microsegregation of alloying element and formation of reverted austenite is absent. Fine grain microstructure at the weld nugget coupled with absence of reverted austenite results in superior impact toughness as compared to base metal. Acknowledgement We express our gratitude to Defence Research and Development organization for the financial support to carry out this program. The authors are thankful to Dr. G. Malakondaiah, Distinguished Scientist & Director DMRL and Dr. Amol A. Gokhale, outstanding scientist for their continued encouragement and support. We would like to thank all those who have either directly or indirectly extended their help in carrying out the studies. REFERENCES 1. Mahmoudi A, Zamanzad Ghavidel MR, Hossein Nedjad S, Heidarzadeh A, Nili Ahmadabadi M. Aging behavior and mechanical properties of maraging steels in the presence of submicrocrystalline Laves phase particles. Materials Characterization 2011 ;62:976-81. 2. Viswanathan UK, Dey GK, Asundi MK. Precipitation Hardening in 350 Grade Maraging Steel. Metall Trans A 1993;24A:2429-41. 3. Sundaresan S, Manirajan M, Nageswara Rao B. On the fracture toughness evaluation in weldments of a maraging steel rocket motor case. Materials and Design 2010;31:492-26. 4. Boniszewski T, Kenyon DM. Examination of electron beam welds in 18% Ni/Co/ Mo maraging steel. British Weld J 1966;13:415-35. 5. Duffy FD, Sutar W. Submerged arc welding of 18% Nickel maraging steel. Weld J 1965;44:251s-63s. 6. Venkata Ramana P, Madhusudhan Reddy G, Mohandas T, Gupta AVSSKS. Microstructure and residual stress distribution of similar and dissimilar electron beam welds-Maraging steel to medium alloy medium carbon steel. Materials and Design 2010;3:
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749-60. 7. Venkata Ramana P, Madhusudhan Reddy G, Mohandas T. Microstructure, hardness and residual stress distribution in maraging steel gas tungsten arc weldments. J Sci Technol Weld Joining 2008;13(4):388-94. 8. Venkateswara Rao V, Madhusudhan Reddy G, Sitarama Raju AV. Influence of post weld heat treatment on the microstructure and mechanical properties of maraging steel electron beam weldments. Steel Grips 2010;8:434-40. 9. Kenyon N. Effect of austenite on the toughness of maraging steel welds. Weld J 1968; 47:193s-98s. 10. Venkateswara Rao V, Madhusudhan Reddy G, Sitarama Raju AV. Microstructure, hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel. Materials Science and Technology 2010 ;26(12):1503-12. 11. Venkateswara Rao V, Madhusudhan Reddy G, Sitarama Raju AV. Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments. Materials Science and Technology 2010;26(12)1459-68. 12. ASM International. ASM Handbook Volume 1, Properties and Selection: Irons, Steels, and th High Performance Alloys. 10 ed. Ohio: ASM International; 1990. 13. Atsmon N, Rosen A. Reverted austenite in maraging steel. Metallography 1981;14:163-7. 14. Nandan R, DebRoy T, Bhadeshia HKDH. Recent advances in friction-stir welding – Process weldment structure and properties. Progress in Materials Science 2008;53:9801023. 15. Bhadeshiad HKDH, DebRoy T. Critical assessment: friction stir welding of Steels. J Sci Technol Weld Joining 2009;14(3):193-6. 16. Cam G. Friction stir welded structural materials: beyond Al-alloys. International Materials Reviews 2011;56(1):1-48. 17. Fazel-Najafabadi M, Kashani-Bozorg SF, Zarei-Hanzaki A. Joining of CP-Ti to 304 stainless steel using friction stir welding technique. Materials and Design 2010;31:4800-07. 18. Sorensen CD, Nelson TW, Packer SM. Tool material testing for FSW of high-temperature alloys, Proceedings of the Third International Symposium on Friction Stir Welding, Sept 2001 (Kobe, Japan), TWI, paper on CD 6. 19. Reynolds AP, Tang W, Posada M. DeLoach J. Friction stir welding of DH36 steel. J Sci Technol Weld Join 2003;8:455-60. 20. Collier M, Steel R, Nelson T, Sorensen C, Packer S. Grade development of polycrystalline cubic boron nitride for friction stir processing of ferrous alloys. Mater Sci Forum 2003;426432(4):3011-16. 21. Song KH, Fujii H, Nakata K. Effect of welding speed on microstructural and mechanical properties of friction stir welded Inconel 600. Materials and Design 2009;30:3972–78. 22. Rajasekhar A, Reddy GM, Mohandas T, Murti VSR. Influence of post-weld heat treatments on microstructure and mechanical properties of AISI 431 martensitic stainless steel friction welds. Material Science and Technology 2008;24:201-12. 23. O. Grong. Metallurgical Modeling of welding. 2nd ed. London: The Institute of Materials; 1997.
Figure Caption Fig.1 Indigenously designed and developed friction stir welding unit Fig.2 (a) WMo FSW tool b) Schematic diagram showing tool dimensions
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Fig.3 Friction stir welded maraging steel plates using WMo tool: (a) at 400 rpm with surface defect (b) at 800 rpm showing no surface defects (c) transverse section at 800 rpm showing internal defects (d) at 600 rpm without surface defects (e) transverse section at 600 rpm without internal defects Fig.4 Optical microstructure of maraging steel, 250 grade (Solutionized condition) Fig.5 Microstructure at different locations of friction stir welded maraging steel weld in aswelded and post weld aged conditions Fig. 6 XRD analysis of friction stir maraging steel weldments in as-weld and post weld aged condition at the dark band region Fig.7 Microhardness survey across interface of friction stir welded joints in as-welded and post weld aged condition Fig.8 Crack path features of charpy impact toughness samples in as-welded and post weld aged condition Fig.9 Fracture features of charpy impact toughness samples of friction stir welded joints (a) as welded (b) post weld aged
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Table 1 Composition of parent metal, maraging steel (MDN-250)
Elements Wt %
C 0.01
Ni 18.9
Co 8.3
Mo 4.6
Ti 0.41
Al 0.15
Fe Bal.
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Table 2 Different tools used for welding maraging steel showing extent of tool wear
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Table 3 percentage increase in hardness at different locations of weld
As-welded Post weld aged % increase in hardness
Region A Region B Region C Region D Region E 364 360 500 350 365 663 663 715 588 625 82 84 43 68 71
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Table 4 Impact toughness of parent metal and friction stir welds in different heat treated condition
Material condition Parent metal Friction stir welds % increase in toughness
Un-aged Impact energy (Joules ) 110 140
After ageing Impact energy (Joules ) 42 90
27
114
% decrease in toughness 38 64
13
Figure-1
Figure-2
Figure-3
Figure-4
Figure-5
Figure-6
Figure-7
Figure-8
Figure-9
Highlights (for review)
Ultra high strength steel welded using friction stir welding for the first time. Established tool material to weld maraging steel. Impact toughness of friction stir welds is superior to parent metal.
S0261-3069(13)00029-0 http://dx.doi.org/10.1016/j.matdes.2013.01.016 JMAD 5076
To appear in:
Materials and Design
Received Date: Accepted Date:
28 September 2012 7 January 2013
Please cite this article as: Meshram, S.D., Madhusudhan Reddy, G., Pandey, S., Friction stir welding of Maraging Steel (Grade-250), Materials and Design (2013), doi: http://dx.doi.org/10.1016/j.matdes.2013.01.016
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Friction stir welding of Maraging Steel (Grade-250)
Suresh D. Meshram Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India Pin 500058 [email protected] [email protected]
G. Madhusudhan Reddy Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India Pin 500058 [email protected] [email protected]
Sunil Pandey Department of Mechanical Engineering Indian Institute of Technology Delhi Hauz Khas, New Delhi, India. Pin 110016, [email protected]
Corresponding author
Suresh D. Meshram Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad, India Pin 500058 [email protected]
[email protected]
Phone No. +91-4024586433
Fax No:+91-4024342697
1
Abstract The feasibility of friction stir (FS) welding of maraging steel is demonstrated using W- Mo base tool. The FS weldements were evaluated by performing hardness, impact toughness and metallographic evaluations. The base metal has a microstructure consisting of martensite and austenite islands. Refined grains of the martensite phase in friction stir zone was observed, and attributed to dynamic recrystalization. A significant improvement in toughness was observed in friction stir welds compared to base metal in both aswelded (27%) and aged (114%) condition. Although more development work is needed, particularly to improve tool materials, the present results indicate that FS welding does indeed have potential for joining maraging steels in aerospace applications.
KEYWORDS: Maraging steel; friction stir welding; impact toughness 1. Introduction Maraging steels are a class of very low carbon high alloy steel first developed in the 1960s for applications requiring ultra-high strength (tensile strength greater than 1380 MPa) combined with good fracture toughness [1]. The alloy gains its strength from the precipitation of intermetallic phases in the martensite matrix [2]. The morphology and crystal structure of the precipitates depend on the composition of the alloy, aging temperature, and time. Typically, these steels are solution treated in the fully austenitic (γ) region in the range of 815-9000C and quenched to produce a complete BCC martensite (α’) matrix, and then aged at temperature within the range 4000C to 6000C. The unique property of being weldable in the solutionised condition followed by post weld maraging treatment at a relatively low temperature (4800C) makes these steels attractive for fabrication of large structures [3]. Welding of maraging steel can be carried out without preheat by processes ranging from electron beam to submerged arc [4-6]. In common practice, however, gas tungsten arc welding is widely employed in view of the consistency of weld quality and overall economy [7]. In most cases, tensile efficiency over 90% is attained. However, the main concern has been toughness of the fusion zone due to microsegregation of alloying elements. The toughness of the fusion zone is considerably lower than that of the parent metal and this has been attributed to the presence of reverted austenite which forms due to microsegregation of alloying elements [8-9]. It is interesting that in maraging steel weld-metal reverted austenite decreases toughness [9-10]. It has also been suggested that, as the austenite reversion in maraging steels is on the martensitic lath boundaries, its influence on the progress of fracture would considerably differ from that of the finely distributed retained austenite which would result in the blunting of cracks [11]. Reverted austenite is a decomposition product, which forms on heating iron-nickel martensite [12-13]. 2
Owing to its high nickel (18%) content, it does not transform to martensite on cooling to room temperature, and results in incomplete precipitation hardening on aging. Nickel content above 33% will not allow to transform in to martensite on cooling to room temperature. Sufficient segregation has been found in the fusion zone to produce reverted austenite on heating to the normal aging temperature of 4800C, but it forms most readily at 6500C [9-13]. Formation of reverted austenite is particularly enhanced when alloying elements segregate to the cell and dendrite boundaries. In an attempt to reduce such segregation effects, Kenyon [9] used filler materials in which titanium and molybdenum contents were reduced (with an increase in cobalt content, which is known not-to-segregate). Significant increase in toughness was observed when segregation and the associated formation of austenite pools were reduced. The experience of the authors is that in spite of using modified welding consumables, formation of reverted austenite persists in the fusion zone of maraging steel. Most of the steel used in aerospace applications are joined as structural assemblies. A substantial quantity of sheets are as thin as 1.3 mm. Production experience indicates that weld distortion increases substantially as component thickness decreases below 10 mm. This led to increased fabrication costs, due to the labour involved in distortion mitigation or correction. Friction stir welding (FSW), is a relatively new solid state joining process and has been the focus of constant attention in joining of material to overcome solidification and distortion related issues [14]. While most of FSW efforts to date have involved joining of softer materials such as lead, zinc, magnesium, and a range of aluminum alloys [14], there is a considerable interest in extending the technology to other materials, including steel and titanium alloys [15-17]. FSW can be regarded as autogenous joining technique without creation of liquid metal. The consolidated weld material is thus free of typical fusion welding defects such as porosity, segregation and solidification cracking. Low distortion, cost effective, FSW joints are produced with excellent mechanical properties being achieved in several aluminum alloys. Continuing investigation suggest that FSW of steel will also become commercially attractive for such applications as missile casings, submarine hulls, etc. The main obstacle to use FSW for welding high strength and high melting point materials is the development of tool materials capable of surviving the high temperatures and forces generated by the process. Considerable advances have been made, mainly through improved materials selection and tool design [18-21]. Tool material development work has looked primarily at refractory metal, polycrystalline boron nitride, and tungsten carbide.
3
2. Experimental procedure
Friction stir welds were produced in position control mode on plates of 5.5 mm thick maraging steel MDN 250 plates (solution treated condition) with welding direction parallel to the rolling direction of the plates using indigenously designed and developed computer numerical controlled (CNC) machine, Fig.1. The composition of the base material is shown in Table 1. Welds were made in square groove-butt joint configuration on samples of 200mm in length and 100mm width. Tool materials included tungsten carbide (WC), tungsten-iron (WFe) and tungstenmolybdenum (WMo). Schematic diagram of tool with dimension and tool is shown in Fig.2. Prior to welding each tool was characterized dimensionally using digital profilometry and height gauge measurements. Initial trials were made to get defect free welds by keeping welding speed constant at 25mm/min and varying the rotation speed. Welding speed was kept constant and at lower value so that tool damage can be minimized. Low and high rotational speed resulted in either surface or internal defects respectively as shown in Fig.3 (a,c). It was found that spindle speed 600 rev/min and travel speed of 25 mm/min can produce void free welds with a smooth surface finish with WMo tool material Fig.3 (d,e). All three tool material types were used for welding maraging steel using exactly the same parameters and processing conditions. Weld cross section and shape of tool after welding, shown in Table 2, indicates intense wear and penetration of the tool material in the joint area of welds made using WC and WFe tools. All welding was performed without primary or secondary shielding. Post weld ageing of some of the joints was carried out at 480 OC for 3 hrs followed by air cooling. Transverse section of these welds were mounted and polished as per standard metallographic procedure and etched with modified Fry’s reagent (50 ml HCl, 25 ml HNO3, 1 g CuCl2 and 150 ml water) to reveal the macro-and microstructures. The phases present in the parent metal and welds in various conditions were identified by X-ray diffraction. For this purpose Philips PW 3020 X-ray diffraction machine is used with Cu K radiation. Mechanical properties i.e., micro-hardness and Charpy ‘V’-notch impact toughness of welds and parent metal in different heat-treated conditions were evaluated. Charpy impact test was carried out on sub size samples of 2.5 mm thickness as per ASTM standards, ASTM:E23-07ae1. Micro hardness survey was carried out across the joint interface at the centre on a Matsuzawa micro-hardness tester using Vickers indenter. Fractography of impact samples was carried out on a LEO scanning electron microscope (SEM). The impact samples were sectioned in mid section perpendicular to the notch to examine crack path under an optical microscope.
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3 Results and discussion 3.1 Optical microscopy and XRD analysis The microstructure of the parent metal is shown in Fig.4. The solution annealed material exhibits martensite with fine grain size corresponding to a nominal ASTM grain size of 11µm (mean linear intercept grain size). Friction stir maraging steel weld consists of five distinct bands. They have been labelled (Fig.5) as A, B, C, D and E. Region ‘A’ is stir zone. Region ‘B’, adjoining the weld represents the coarse grain martensitic phase. During welding the parent material adjacent to the weld interface is heated to high temperature in the austenite region where considerable grain growth occurs. On cooling, the austenite transforms to martensite and inherits the coarse austenite grain size. Region ‘C’ is a light etching martensite that has been heated due to weld thermal cycle in the austenite region but not high enough to cause grain growth. Region ‘D’ is a dark etching region where martensite phase experiences peak temperatures in the range of 590-7300C [3]. Some reverted austenite will therefore form in this region in a martensitic matrix. XRD analysis of this region in the as-weld and post weld aged condition, shown in Fig.6, indicates presence of two-phase microstructure. Region ‘E’ is unaffected parent metal during welding thermal cycle (Fig.5). Austenite transforms to martensite during cooling in the stir zone so that the microstructure of as-welded stir zone resembles that of the martensitic parent material. During welding of maraging steel through conventional fusion welding process the microstructure at fusion zone consists of austenite pool after ageing treatment due to microsegregation of alloying element resulting in low impact toughness. In friction stir welds, since there is no melting, microsegregation of alloying element is least resulting in no reverted austenite visible at optical microscopy. 3.2 Hardness Micro-hardness survey across the weld in the as-welded and aged conditions shown in Fig.7 revealed five different regions. Hardness variations across the interfaces as a result of welding could be attributed to high strain rate and thermal cycle during welding. Hardness of stir region is nearly equal to that of parent metal (Region 'A'). Immediately away from the interface region hardness has a drooping trend, probably due to softening of these regions due to exposure to high temperatures during welding leading to grain coarsening (Region 'B'), while the highest hardness towards the outer edge of the thermo mechanically affected zone (Region 'C') could be due to aging of the martensite microstructure of the parent maraging steel which is in solution treated condition prior to welding. Region ‘D’ represents dark band region and exhibited lowest hardness due to the
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presence of fine dispersion of retained austenite in martensite. Region ‘E' represents the parent metal. Micro-hardness survey across the weld bead in the aged condition presented in Fig.7 shows that hardness of all the regions except region ‘D’ increases almost to the hardness of the aged parent metal. From Fig.7 it is evident that hardness of the dark band region did not acquire full hardness during subsequent ageing; therefore this region remained softer than parent metal. Based on the available literature it is inferred that this soft zone may not be of practical significance, if the zone size is narrow [9]. Percentage increase in hardness after ageing treatment at different locations of weld is shown in Table 3. 3.3 Impact toughness Impact toughness data of parent metal and welds normalized as per ASTM sub size standard specimen are presented in Table 4. Friction stir welds exhibited higher toughness as compared to parent metal irrespective of weld condition. The impact toughness is in the order of as-welded > solutionized base metal > post weld aged > base metal aged conditions. The crack paths for different samples were observed by cutting the sample perpendicular to the notch at the center. Typical crack path features in as-welded and post weld aged conditions are presented in Fig.8. Improvement in toughness was reflected in crack path. With improvement in toughness the crack path was more tortuous. These observations are in conformity with similar trends reported in maraging steel weldment [8,10,11 and 22]. Maximum crack path length noticed in as-welded condition is 10.6 mm as compared to 8.5 mm of directly aged condition. It has been reported that the toughness of weldments depends on volume fraction of second phases, inclusions, and cell size [23]. As the samples are taken from the same weldment, difference in inclusion content or cell sizes are not expected, at the same time since the samples (welds and base metal) were aged at same temperature together there should not be any difference in volume fraction of precipitates. The above observations indicate that the main microstructural features influencing toughness of the stir zone is grain size. The extent of grain refinement observed in friction stir welding cannot be achieved through any other fusion welding process [7,8]. It was reported well in the literature that, poor impact toughness of fusion zone in maraging steel welds are attributed to the presence of reverted austenite in the martensite microstructure which leads to void formation in the austenite during deformation [9-11]. Austenite cracks preferentially because slip can take place in it at stresses, which are too low to produce slip in the matrix. These observations are in agreement with the reported trends, whenever a two phase structure is loaded, the soft phase will deform preferentially and may reach its critical strain for
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fracture at an early stage by setting up a series of micro-cracks in the harder matrix [9]. The observed fracture features (Fig.9) are in tune with impact toughness trends, i.e in the as-welded condition the dimples are fine while in aged conditions they are grown up. The fine grain microstructure at weld zone coupled with absence of reverted austenite could be attributed to higher impact toughness of maraging steel friction stir welds compared to fusion welds [11].
4. Conclusions Maraging steel (MDN-250) of 5.5 mm thick was successfully friction stir welded using WMo tool. In the present work, the microstructural characteristics and governing mechanical properties such as hardness and impact toughness of the maraging steel friction stir welds were examined. Since friction stir welding is a solid state welding process, there is no melting and hence microsegregation of alloying element and formation of reverted austenite is absent. Fine grain microstructure at the weld nugget coupled with absence of reverted austenite results in superior impact toughness as compared to base metal. Acknowledgement We express our gratitude to Defence Research and Development organization for the financial support to carry out this program. The authors are thankful to Dr. G. Malakondaiah, Distinguished Scientist & Director DMRL and Dr. Amol A. Gokhale, outstanding scientist for their continued encouragement and support. We would like to thank all those who have either directly or indirectly extended their help in carrying out the studies. REFERENCES 1. Mahmoudi A, Zamanzad Ghavidel MR, Hossein Nedjad S, Heidarzadeh A, Nili Ahmadabadi M. Aging behavior and mechanical properties of maraging steels in the presence of submicrocrystalline Laves phase particles. Materials Characterization 2011 ;62:976-81. 2. Viswanathan UK, Dey GK, Asundi MK. Precipitation Hardening in 350 Grade Maraging Steel. Metall Trans A 1993;24A:2429-41. 3. Sundaresan S, Manirajan M, Nageswara Rao B. On the fracture toughness evaluation in weldments of a maraging steel rocket motor case. Materials and Design 2010;31:492-26. 4. Boniszewski T, Kenyon DM. Examination of electron beam welds in 18% Ni/Co/ Mo maraging steel. British Weld J 1966;13:415-35. 5. Duffy FD, Sutar W. Submerged arc welding of 18% Nickel maraging steel. Weld J 1965;44:251s-63s. 6. Venkata Ramana P, Madhusudhan Reddy G, Mohandas T, Gupta AVSSKS. Microstructure and residual stress distribution of similar and dissimilar electron beam welds-Maraging steel to medium alloy medium carbon steel. Materials and Design 2010;3:
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749-60. 7. Venkata Ramana P, Madhusudhan Reddy G, Mohandas T. Microstructure, hardness and residual stress distribution in maraging steel gas tungsten arc weldments. J Sci Technol Weld Joining 2008;13(4):388-94. 8. Venkateswara Rao V, Madhusudhan Reddy G, Sitarama Raju AV. Influence of post weld heat treatment on the microstructure and mechanical properties of maraging steel electron beam weldments. Steel Grips 2010;8:434-40. 9. Kenyon N. Effect of austenite on the toughness of maraging steel welds. Weld J 1968; 47:193s-98s. 10. Venkateswara Rao V, Madhusudhan Reddy G, Sitarama Raju AV. Microstructure, hardness and residual stress distribution of dissimilar metal electron beam welds: maraging steel and high strength low alloy steel. Materials Science and Technology 2010 ;26(12):1503-12. 11. Venkateswara Rao V, Madhusudhan Reddy G, Sitarama Raju AV. Influence of post-weld heat treatments on microstructure and mechanical properties of gas tungsten arc maraging steel weldments. Materials Science and Technology 2010;26(12)1459-68. 12. ASM International. ASM Handbook Volume 1, Properties and Selection: Irons, Steels, and th High Performance Alloys. 10 ed. Ohio: ASM International; 1990. 13. Atsmon N, Rosen A. Reverted austenite in maraging steel. Metallography 1981;14:163-7. 14. Nandan R, DebRoy T, Bhadeshia HKDH. Recent advances in friction-stir welding – Process weldment structure and properties. Progress in Materials Science 2008;53:9801023. 15. Bhadeshiad HKDH, DebRoy T. Critical assessment: friction stir welding of Steels. J Sci Technol Weld Joining 2009;14(3):193-6. 16. Cam G. Friction stir welded structural materials: beyond Al-alloys. International Materials Reviews 2011;56(1):1-48. 17. Fazel-Najafabadi M, Kashani-Bozorg SF, Zarei-Hanzaki A. Joining of CP-Ti to 304 stainless steel using friction stir welding technique. Materials and Design 2010;31:4800-07. 18. Sorensen CD, Nelson TW, Packer SM. Tool material testing for FSW of high-temperature alloys, Proceedings of the Third International Symposium on Friction Stir Welding, Sept 2001 (Kobe, Japan), TWI, paper on CD 6. 19. Reynolds AP, Tang W, Posada M. DeLoach J. Friction stir welding of DH36 steel. J Sci Technol Weld Join 2003;8:455-60. 20. Collier M, Steel R, Nelson T, Sorensen C, Packer S. Grade development of polycrystalline cubic boron nitride for friction stir processing of ferrous alloys. Mater Sci Forum 2003;426432(4):3011-16. 21. Song KH, Fujii H, Nakata K. Effect of welding speed on microstructural and mechanical properties of friction stir welded Inconel 600. Materials and Design 2009;30:3972–78. 22. Rajasekhar A, Reddy GM, Mohandas T, Murti VSR. Influence of post-weld heat treatments on microstructure and mechanical properties of AISI 431 martensitic stainless steel friction welds. Material Science and Technology 2008;24:201-12. 23. O. Grong. Metallurgical Modeling of welding. 2nd ed. London: The Institute of Materials; 1997.
Figure Caption Fig.1 Indigenously designed and developed friction stir welding unit Fig.2 (a) WMo FSW tool b) Schematic diagram showing tool dimensions
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Fig.3 Friction stir welded maraging steel plates using WMo tool: (a) at 400 rpm with surface defect (b) at 800 rpm showing no surface defects (c) transverse section at 800 rpm showing internal defects (d) at 600 rpm without surface defects (e) transverse section at 600 rpm without internal defects Fig.4 Optical microstructure of maraging steel, 250 grade (Solutionized condition) Fig.5 Microstructure at different locations of friction stir welded maraging steel weld in aswelded and post weld aged conditions Fig. 6 XRD analysis of friction stir maraging steel weldments in as-weld and post weld aged condition at the dark band region Fig.7 Microhardness survey across interface of friction stir welded joints in as-welded and post weld aged condition Fig.8 Crack path features of charpy impact toughness samples in as-welded and post weld aged condition Fig.9 Fracture features of charpy impact toughness samples of friction stir welded joints (a) as welded (b) post weld aged
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Table 1 Composition of parent metal, maraging steel (MDN-250)
Elements Wt %
C 0.01
Ni 18.9
Co 8.3
Mo 4.6
Ti 0.41
Al 0.15
Fe Bal.
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Table 2 Different tools used for welding maraging steel showing extent of tool wear
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Table 3 percentage increase in hardness at different locations of weld
As-welded Post weld aged % increase in hardness
Region A Region B Region C Region D Region E 364 360 500 350 365 663 663 715 588 625 82 84 43 68 71
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Table 4 Impact toughness of parent metal and friction stir welds in different heat treated condition
Material condition Parent metal Friction stir welds % increase in toughness
Un-aged Impact energy (Joules ) 110 140
After ageing Impact energy (Joules ) 42 90
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114
% decrease in toughness 38 64
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Highlights (for review)
Ultra high strength steel welded using friction stir welding for the first time. Established tool material to weld maraging steel. Impact toughness of friction stir welds is superior to parent metal.