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EXPLOSIVE WELDING OF STELLITE TO STAINLESS STEEL
EXPLOSIVE W E L D I N G OF STELLITE TO STAINLESS STEEL F. W . TRAVIS and
W . JOHNSON
Department of Mechanical Engineering, University of Manchester Institute of Science and Technology SUMMARY Experiments are described in which stellite is explosively welded to 1 3 % chromium stainless steel by means of an intermediate thin layer, or shim, of copper. The velocity required to be imparted to a stellite flyer plate to approach conditions for welding to a stainless steel base plate is found to be too high to avoid severe distortion and cracking of the stellite. By ñrst explosively cladding a thin copper shim to the stellite, the velocity of the stellite flyer plate to ensure a (copper to stainless steel) weld is substantially less. The shear strength of such welds is found to be high in comparison with values which would be obtained using the simplest alternative technique of brazing. Photomicrographs of typical specimens are presented and discussed. INTRODUCTION
Over the last ten years, the welding and cladding of sheet metals by the use of high explosive charges has been the subject of increasing interest and study. A typical set-up consists in setting the two plates to be bonded at a slight inchnation to each other, as shown in Fig. 1, and using the action of a detonated high explosive charge to drive them together RUBBER
SHEET
BUFFER
DETONATOR
EXPLOSIVE
FLYER TO PARENT
STEEL
PLATECOUNTER
CLAMP
PLATE
ANVIL
TO
^
FIG. 1.
BATTERY
Schematic view of set-up for explosive welding.
at high velocity. Inertia stresses are developed which are very greatly in excess of the yield stresses of the materials used and under some conditions a jet of metal may be projected from the point of contact of the two plates. The existence of such jets has been conñrmed by sub-microsecond flash radiographs taken during the welding operation.
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F . W . TRAVIS and W . JOHNSON
by lined cavity shaped charges by treating the colhding metal plates as non-viscous fluids and applying Bernoulli's equation.) The importance of, and necessity for, jetting in securing a good explosive weld have been well discussed and a number of different views have been advanced.^^' In one instance the jetting is considered to have the beneficial action of removing the surface layers (which
FIG. 2.(a) Failure of stellite to stainless steel weld using a i in. thick explosive charge, (b) Stei nte to stainless steel weld, using a copper insert and a i in. thick explosive charge.
Explosive Welding of Stellite to Stainless Steel
1321
may be oxidised and contaminated) from the colliding plates, so that under the ensuing high pressures, virgin surfaces are brought into very close contact to secure a strong metal lurgical bond. Bahrani et alS^^ have advanced the work of Abrahamson^"^) and Cowan and Holtzman<8) using the jetting model to provide a convincing mechanism for the character istic wavy interface between explosively welded plates. On the other hand, in examining the explosive welding of plates under conditions in which, from theoretical considerations, the occurrence of jetting is unlikely, Otto<^> has proposed a solid state mechanism in which elements of the material at the interface rotate in a periodic pattern under the drag of the flyer plate. The respective arguments advanced by these authors are too extensive for inclusion here, but both account for the observed phenomena and the results of experimental investigations are provided in both cases in support of the proposed mechanism. The above account is presented to illustrate that there is much that is not yet understood of the funda mentals of the operation. Industrially, the operation is now suflftciently integrated into present-day practice for the du Pont Company (U.S.A.) to be able to quote for explosive welds and clads to customers' requirements. Clad areas of up to 20 χ 7 ft have been produced by this company, using explosive charges of up to 3000 lb weight, Whilst a wide range of combinations of metals may be explosively welded, some com binations are welded only with diflliculty and others not at all. It is reported in ref. 10 that an explosive weld of "Haynes Stellite alloy 6B" to carbon steel, or "300 series" stainless steel, has been produced, but details of the weld and of the set-up are not given. Although during preliminary tests carried out by the authors in the explosive welding of wrought Haynes Stellite 6B to 13% chromium stainless steel using the basic set-up of Fig. 1 some degree of bonding was secured, a large number of failures similar to and including that shown in Fig. 2(a) were encountered. In Fig. 2(a) pockets of stellite detached from the main body are seen to be retained within the crested wave formations of the stainless steel. This feature was attributed to the high impact velocity required of the stellite flyer plate in order to approach bonding conditions. The aim of the present work was to examine a means by which the stellite to stainless steel weld could be accomplished under less severe impact conditions. Essentially the method consists of explosively cladding the stellite with a thin copper shim so that the bond required subsequently is then between copper and stainless steel, the conditions for the explosive welding of which are less exacting. The reader is referred to ref. 11 for a full and detailed examination of high explosive materials and to ref. 12 for the application of explosive materials to the working of metals. A survey of explosive welding and a discussion of some of the more important parameters are provided in refs. 13 and 14.
MATERIALS, EQUIPMENT, PROCEDURE
Explosive Material Throughout the tests, the explosive material used was "Metabel", which is P.E.T.N. based, and supplied by I.C.I, in the form of rubber-hke sheets, | in. thick X 10 in. χ 5 in. It is reported
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F. W. TRAVIS and W. JOHNSON
Test Specimens The dimensions of the stelhte plates were 0Ό80 in. thickness χ 1 in. X 6 in., and the dimensions of the stainless steel plates were 0-41 in. thickness χ 2 in. χ 6\ in. The percent age chemical compositions of these materials were: Haynes Stellite 6B-Wrought: Μ C, 30 Cr, 4-5 W, 3 Ni, 1-5 Mo, 2 Si, balance Co. Stainless Steel: 0-18-0-25 C, 12-14 Cr, 0-5 Ni, 0-5 Si, 1-0 Μη, > 0-045 S, > 0-045 P. A 4-in. length of each stellite plate was cranked, using a vice, to the required angle, a , as shown in Fig. 1. Except where stated otherwise, α was retained at 5° throughout the tests. The copper shim used was of commercial purity, in the fully annealed condition and of 0-008 in. thickness. Velocity Measurement Tests were carried out to find the velocity imparted to the flyer plate under different experimental conditions. The bared ends of two thin leads were set below the flyer plate, as indicated in Fig. 1, at a distance of J to ^ in. apart, measured normal to the direction of the flyer plate. By supplying the latter with 10 volts from a battery and running the leads to the "start" and "stop" terminals of a microsecond counter, the mean velocity of the collapsing flyer plate at this station over the displacement interval covered was established. Experimental Procedure Prior to testing, the surfaces to be bonded were wire brushed and degreased. Where a buffer was used, this was then stuck on to the back of the flyer plate with Evo-stik adhesive and the assembly made up as shown in Fig. 1. The explosive material was cut to the width of the flyer plates and buffer, and to such length that its end could be folded around the detonator subsequently, to ensure rehable detonation. The assembly was then placed in open ground, well clear of the buildings in which the firing and other equipment were housed. Where velocity tests were performed, the settings of the leads were checked and the detonator then introduced. Connections to the assembly from the battery and the microsecond counter were made from within the buildings and only immediately prior to the instant of firing. RESULTS
A N D
DISCUSSION
Micro-examination of the Welded Interface In all the photomicrographs presented, the direction of welding is from right to left, with the flyer plate uppermost, consistent with Fig. 1. Etching reagents used were am monium hydroxide/hydrogen peroxide and ferric chloride. For the first test using a copper shim, the latter was simply laid on top of the stainless steel parent plate prior to testing. With two thicknesses of Metabel and no buffer, both a stellite/copper and a copper/stainless steel weld were secured, as shown in Fig. 2(b). From the measurement made, the velocity of the flyer plate in the test was estimated to have been about 3600 ft/sec. A number of features associated with this high flyer plate velocity are evident in Fig. 2(b). The copper/stainless steel interface wave formations are of large amplitude which, it is reported,
1323
Explosive Welding of Stellite to Stainless Steel
)f shear is responsible for the production of molten pockets velding operation. Attempts are usually made to avoid the impounds during explosive welding, or at least to confine )pposed to allowing them to form a continuous band—on the
at these points during the formation of intermetallic them to local pockets—as grounds that they are often
ο·οιο
FIG. 3. (a) Copper to stellite clad, using a flyer plate angle of 10°, a J in. thick explosive charge and a i in. thick rubber buffer, (b) A copper/stellite clad formed as in Fig. 3(a), welded to stainless steel using a i in. thick explosive charge.
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F . W . TRAVIS and W . JOHNSON
hard and brittle and may be detrimental under certain conditions, i.e. where the weld has to withstand impact, vibration or thermal cycling, etc.<3) When the velocity of the flyer plate was reduced, by the use of a J in. thick rubber buffer or by restricting the charge to one thickness of Metabel, only a copper/stainless steel weld was obtained, so that further attempts in this direction were discontinued.
FIG. 4 . (a) As for Fig. 3(b), but using a i in. thick explosive charge, (b) Copper to stainless steel clad, as for Fig. 3(a).
Explosive Welding of Stellite to Stainless Steel
1325
In the next method attempted, a preliminary test was performed to explosively clad the jtelhte whh the copper shim, after which a second test was performed to weld the copper Jad stellite on to the stainless steel. One thickness of Metabel was far in excess of the charge equired to accelerate the copper flyer plate, and in the ñrst few tests, the copper was welded )n to the stellite more in the form of a metal spray rather than as one continuous strip of netal. When the rubber buffer was increased to J in. thickness, satisfactory cladding was ichieved at a flyer plate velocity of about 5000 ft/sec, as shown in Fig. 3(a). It was found that I less continuous intermetalhc compound was formed when the flyer plate angle, a , was ncreased to 10°, which is in agreement with previous observations.^^^^ When the stellite/ :opper clad was welded on to the stainless steel using two thicknesses of Metabel, as in the est of Fig. 2(b), similar results to those shown in the latter figure were obtained. The exensive copper/stainless steel intermetalhc compound produced, again with large shrinkage lavities, is shown in Fig. 3(b). By reducing the charge to one thickness of Metabel however.
C O P P E R / C O P P E R
S T E L L I T E / C O P P E R
I N T E R F A C E
I N T E R F A C E
C O P P E R / S T E E L INTERFACE
FIG. 5.
A copper/stellite clad formed as in Fig. 3(a), welded on to a copper/stainless steel clad also formed as in Fig. 3(a), using a i in. thick explosive charge.
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F . W . TRAVIS and W . JOHNSON
where the flyer plate velocity was estimated to have been about 1900 ft/sec, the satisfactory weld of Fig. 4(a) was produced. Only a small amount of intermetallic compound is formed at each interface and the wave formations are regular and not excessive in amplitude. A third method was investigated in which both the stellite and the stainless steel were explosively clad with copper, prior to welding. The initial copper/stainless steel clad is
FIG. 6. (a) Sheared face of test specimen of weld of Fig. 4(a). (b) Shear face of test specimen of weld of Fig. 5.
shown in Fig. 4(b), and the final weld in which one thickness of Metabel was used, with no buffer, in Fig. 5(a). The wavy copper/copper interface may be detected in the latter figure, and the indications are that a satisfactory weld has been formed. It will be noted that over the short length of copper/stainless steel interface shown, the periodic wave pattern is not
Explosive Welding of Stellite to Stainless Steel
1327
3resent and that the intermetallic compound is irregularly dispersed. This defect could 3 r o b a b l y be eliminated v^ith some slight alteration in the buffer thickness or in the flyer 3late inclination during the initial copper/stainless steel clad, although it is reported^i^) that
here are noticeable effects due to entrapped air when thin gauge flyer plates are used. The lolution may then be to carry out the tests in a vacuum and reference can be made to some previously reported^^^^ observations on the use of explosives in a vacuum chest. Strength of the Weld Short lengths of the welded specimens of Figs. 4(a) and 5 were tested in shear, in the iirection of the length of the weld. The stellite side of the bonded specimens was ground down ;o the stainless steel excepting for an area of about \ in. square, which was left proud. The stainless steel parent plate was placed between the compression platens of a universal esting machine, gripped in a vice so that it could not rotate, and a hardened steel bar then ised to push off the stellite plate, care being taken to ensure that the direction of the applied oad was in the plane of the weld. For the specimen of the weld of Fig. 4(a), the maximum oad recorded was 4-64 tonf, corresponding to a shear stress of 14-2 tonf/in.^ and it will be loted from Fig. 6(a) that, excepting for a very small area towards the right hand side, 'ailure has occurred entirely through the body of the copper. The maximum load of 3*28 onf for the specimen of the weld of Fig. 5 provides a value for the shear stress of 11-5 :onf/in.2, but in this case, as shown in Fig. 6(b), partial separation at the copper/copper inter'ace occurred. Micro-hardness indentations were made, using a l o a d of 20 g, at the stellite/copper nterface and the copper/stainless steel interface of the specimen of Fig. 5, and these are ;hown in Figs. 7(a) and (b) respectively. The hardness of the intermetallic compound at 3 0 t h these interfaces is seen to be intermediate between the hardnesses of its parent mat erials, from which it may be deduced that its presence is not necessarily detrimental to the veld. From measurement of the indentations, Vickers hardness numbers for the stellite, •tellite/copper intermetallic compound, copper, copper/stainless steel intermetallic com)ound, and stainless steel were 773, 391, 155, 258, and 420 respectively. These figures are ntended for p u r p o s e s of comparison, since when the hardness of the stellite, copper and itainless steel were determined in the vicinity of the welded interface using a 1 kg load in order that errors due to polishing, i.e. the Bielby layer, be reduced) the respective v a l u e s )btained were 515, 129 and 276 HV respectively. The hardness v a l u e s of the stellite and he stainless steel before testing were 383 and 261 HV30 respectively, from which it is noted hat the stellite has been substantially hardened. This c o u l d have been anticipated on L c c o u n t of the deformation suffered by the flyer plate during the welding operation. Use of a Buffer The main reason for the use of a buffer in explosive welding is to protect the surface of he flyer plate. In addition it is reported^^) that it will attenuate the peak pressure of the hock wave produced by the explosion and this may be particularly advisable in the present vork in view of the tendency of the carbide particles of the stellite to crack. It will be ippreciated, however, that the presence of a buffer reduces the velocity imparted to the iyer plate. In reference 13 it is reported that the surface of a buffer must be smooth, othervise the surface markings are embossed on the flyer plate. Such effects are well illustrated η Fig. 8, which presents three copper clad stellite plates, where, from top to bottom, the | in.
1328
F . W . TRAVIS and W . JOHNSON
thick buffers were of smooth rubber, patterned rubber, and cardboard. The exaggerated buffer pattern on the surface of the clad—in view of the small thickness of the copper flyer plates—has encouraged the authors to speculate that the buffer pattern may be translated into differences in velocity imparted over the surface of the flyer plate, which cause the pattern to be reproduced when the flyer plate suffers impact with the stellite parent plate.
FIG. 7. (a) Stellite/copper interface of welded specimen of Fig. 5. (b) Copper/stainless steel interface of welded specimen of Fig. 5.
Further Considerations A number of deleterious features of the explosive welding process are shown in Figs. 9(a) and (b). A particularly severe form of "edge defect" is demonstrated in (a), where the outer I in. or so of the stellite flyer plate has broken off*. Such edge defects are characteristic
Explosive Welding of Stellite to Stainless Steel
1329
)f explosive welding and it has been reported
FIG. 8. Copper clad stellite, formed using a flyer plate angle of 10°, a i in. thick explosive charge and a i in. thick buffer, where the material of the buffer, from top to bottom was, smooth rubber, patterned rubber, and cardboard, respectively.
iliminated in the
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F . W . TRAVIS and
W . JOHNSON
welding of steel is a common phenomenon. Similarly, if care is not taken, the detonation wave of the explosive charge may cause damage to the plate to be welded, or to any fixtures used. In this respect there could be advantage in the use of an explosive material having a sub-sonic detonation velocity, i.e. lower than the speed with which elastic stress waves are
FIG. 9. (a) Showing "edge defect" in failure of stellite to stainless steelweld, using a i in. thick explosive charge, a i in. thick rubber buffer, and a sand base, (b) Showing spalling of parent plate in failure of stellite to stainless steel weld, using a nickel insert, a i in. thick explosive charge, and a sand base.
Explosive Welding of Stellite to Stainless Steel
1331
propagated through metals, but at the present time such materials are not available in Britain. APPENDIX
Measurement of Flyer Plate Velocity Although not intended for use in the present tests (this work was carried out in 1963), an attempt was made by the authors to develop a re-useable set-up for the determination of flyer plate velocity, in which the block shown in Fig. 10 replaced the parent plate in Fig. 1. wtym
FIG. 10.
PLATE
Block constructed for use in flyer plate velocity tests.
Insulated hardened silver steel pins, forming part of an electrical circuit, were set into the case-hardened steel block, the tops of the pins being ground flush with the surface of the block. Signals generated upon contact of the flyer plate with the pins were displayed on an oscilloscope screen and photographed, and from this record, the velocity was determined. (This method is basically that of pin-contactors, ^^s) which is used extensively for the mea surement of velocity in dynamic metal forming. ( ^ ^ 0 It was found in tests using 0-020 in. thick copper plates that above flyer plate velocities of about 400 ft/sec, excessive depression of the pins required the block to be reground after each test, and above velocities of about 900 ft/sec the tops of the pins were broken off under the forward drag of the (necessarily unwelded) flyer plate. With aluminium plates of 0-020 in. thickness, however, velocities in excess of 1000 ft/sec caused no damage to the pins, and it may be that with such softer materials the method could find application. ACKNOWLEDGEMENTS
The financial support of the Science Research Council and Associated Electrical Industries, Manchester, is gratefully acknowledged. The authors wish also to thank the latter for their agreement to publish this paper. REFERENCES HoLTZMAN, A. H . and RUDERSHAUSEN, C . G . Recent advances in metal working with explosives, Sheet Metal Industries, 39, 422, 1962. 2 BiRKHOFF, G., MACDOUGALL, D . P . , PUGH, E . M . and TAYLOR, G . I. Explosives with lines cavities, /. Appl. Phys. 19, 563, 1948. 3. WRIGHT, E . S . and BAYCE, A. E. Current methods and results in explosive welding, Proc. NATO Advanced Study Institute on High Energy Rate Working of Metals, Sandefjord and Lillehammer, Norway, Sept. 1964. 4. HOLTZMAN, A. H . Explosion clads, Ibid. 1.
1332 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
F . W . TRAVIS and W . JOHNSON BUCK, G . and HORNBOGEN, E . Metallographic investigation of shock welded metals by electron microscopy, Ibid. BAHRANI, A . S., BLACK, T . J. and CROSSLAND, B . The mechanics of wave formation in explosive welding, Proc. Roy Soc, A 296, 1 2 3 - 1 3 6 , 1967. ABRAHAMSON, G . R . Permanent periodic surface deformation due to a travelling jet, / . Appl. Mech. 28, E, 5 1 9 , 1 9 6 1 . COWAN, G . R . and HOLTZMAN, A. H . Flow configurations in colliding plates: Explosive bonding, /. Appl. Phys. 3 4 , No. 4 , 9 2 8 , 1963. OTTO, H . G . Aspects relating to the welding mechanism, Proc. NATO Advanced Study Institute on High Energy Rate Working of Metals, Sandefjord and Lillehammer, Norway, Sept. 1964. POCALYKO Α., Explosion clad plate for corrosion service, Proc. NACE North Central Region Conf., Sept. 1964. COOK, M . A. The Science of High Explosives, Reinhold, New York, 1958. RINEHART, J. S . and PEARSON, J. Explosive Working of Metals. Pergamon, 1963. BAHRANI, A, S. and CROSSLAND, B . Explosive welding and cladding: An introductory survey and pre liminary results, Proc. Inst. Mech. Engrs. 179, part 1 , 1 9 6 4 - 6 5 . BAHRANI, A. S. and CROSSLAND, B . Further experiments on explosive welding and cladding with particular reference to the strength of the bond, Proc. Inst. Mech. Engrs. 180, part 31, 1 9 6 5 - 6 6 . CROSSLAND, B . and BAHRANI, A. S. Some observations on explosive cladding and welding. Preprint AD66-112, A.S.T.M.E. 1966. JOHNSON, W . and TRAVIS, F . W . Peeling, hot forming of tungsten and mild steel discs and the use of a vacuum chest with explosives, Proc. 6th Int. M.T.D.R. Conf, Manchester, Sept. 1965. DIETER, G . E . Metallurgical effects of high intensity shock waves in metals. Response of Metals to High Velocity Deformation, Interscience Publishers, New York, 1960. MiNSHULL, S. Properties of elastic and plastic waves determined by pin-contactors and crystals. /. Appl. Phys. 26, 4 6 3 , 1955. JOHNSON, W . , KORMI, K . and TRAVIS, F . W . An investigation into the explosive deep drawing of circular blanks using the plug cushion technique, Int. J. Mech. Sci. 6, 2 8 7 , 1 9 6 4 .
W . JOHNSON
Department of Mechanical Engineering, University of Manchester Institute of Science and Technology SUMMARY Experiments are described in which stellite is explosively welded to 1 3 % chromium stainless steel by means of an intermediate thin layer, or shim, of copper. The velocity required to be imparted to a stellite flyer plate to approach conditions for welding to a stainless steel base plate is found to be too high to avoid severe distortion and cracking of the stellite. By ñrst explosively cladding a thin copper shim to the stellite, the velocity of the stellite flyer plate to ensure a (copper to stainless steel) weld is substantially less. The shear strength of such welds is found to be high in comparison with values which would be obtained using the simplest alternative technique of brazing. Photomicrographs of typical specimens are presented and discussed. INTRODUCTION
Over the last ten years, the welding and cladding of sheet metals by the use of high explosive charges has been the subject of increasing interest and study. A typical set-up consists in setting the two plates to be bonded at a slight inchnation to each other, as shown in Fig. 1, and using the action of a detonated high explosive charge to drive them together RUBBER
SHEET
BUFFER
DETONATOR
EXPLOSIVE
FLYER TO PARENT
STEEL
PLATECOUNTER
CLAMP
PLATE
ANVIL
TO
^
FIG. 1.
BATTERY
Schematic view of set-up for explosive welding.
at high velocity. Inertia stresses are developed which are very greatly in excess of the yield stresses of the materials used and under some conditions a jet of metal may be projected from the point of contact of the two plates. The existence of such jets has been conñrmed by sub-microsecond flash radiographs taken during the welding operation.
1320
F . W . TRAVIS and W . JOHNSON
by lined cavity shaped charges by treating the colhding metal plates as non-viscous fluids and applying Bernoulli's equation.) The importance of, and necessity for, jetting in securing a good explosive weld have been well discussed and a number of different views have been advanced.^^' In one instance the jetting is considered to have the beneficial action of removing the surface layers (which
FIG. 2.(a) Failure of stellite to stainless steel weld using a i in. thick explosive charge, (b) Stei nte to stainless steel weld, using a copper insert and a i in. thick explosive charge.
Explosive Welding of Stellite to Stainless Steel
1321
may be oxidised and contaminated) from the colliding plates, so that under the ensuing high pressures, virgin surfaces are brought into very close contact to secure a strong metal lurgical bond. Bahrani et alS^^ have advanced the work of Abrahamson^"^) and Cowan and Holtzman<8) using the jetting model to provide a convincing mechanism for the character istic wavy interface between explosively welded plates. On the other hand, in examining the explosive welding of plates under conditions in which, from theoretical considerations, the occurrence of jetting is unlikely, Otto<^> has proposed a solid state mechanism in which elements of the material at the interface rotate in a periodic pattern under the drag of the flyer plate. The respective arguments advanced by these authors are too extensive for inclusion here, but both account for the observed phenomena and the results of experimental investigations are provided in both cases in support of the proposed mechanism. The above account is presented to illustrate that there is much that is not yet understood of the funda mentals of the operation. Industrially, the operation is now suflftciently integrated into present-day practice for the du Pont Company (U.S.A.) to be able to quote for explosive welds and clads to customers' requirements. Clad areas of up to 20 χ 7 ft have been produced by this company, using explosive charges of up to 3000 lb weight, Whilst a wide range of combinations of metals may be explosively welded, some com binations are welded only with diflliculty and others not at all. It is reported in ref. 10 that an explosive weld of "Haynes Stellite alloy 6B" to carbon steel, or "300 series" stainless steel, has been produced, but details of the weld and of the set-up are not given. Although during preliminary tests carried out by the authors in the explosive welding of wrought Haynes Stellite 6B to 13% chromium stainless steel using the basic set-up of Fig. 1 some degree of bonding was secured, a large number of failures similar to and including that shown in Fig. 2(a) were encountered. In Fig. 2(a) pockets of stellite detached from the main body are seen to be retained within the crested wave formations of the stainless steel. This feature was attributed to the high impact velocity required of the stellite flyer plate in order to approach bonding conditions. The aim of the present work was to examine a means by which the stellite to stainless steel weld could be accomplished under less severe impact conditions. Essentially the method consists of explosively cladding the stellite with a thin copper shim so that the bond required subsequently is then between copper and stainless steel, the conditions for the explosive welding of which are less exacting. The reader is referred to ref. 11 for a full and detailed examination of high explosive materials and to ref. 12 for the application of explosive materials to the working of metals. A survey of explosive welding and a discussion of some of the more important parameters are provided in refs. 13 and 14.
MATERIALS, EQUIPMENT, PROCEDURE
Explosive Material Throughout the tests, the explosive material used was "Metabel", which is P.E.T.N. based, and supplied by I.C.I, in the form of rubber-hke sheets, | in. thick X 10 in. χ 5 in. It is reported
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F. W. TRAVIS and W. JOHNSON
Test Specimens The dimensions of the stelhte plates were 0Ό80 in. thickness χ 1 in. X 6 in., and the dimensions of the stainless steel plates were 0-41 in. thickness χ 2 in. χ 6\ in. The percent age chemical compositions of these materials were: Haynes Stellite 6B-Wrought: Μ C, 30 Cr, 4-5 W, 3 Ni, 1-5 Mo, 2 Si, balance Co. Stainless Steel: 0-18-0-25 C, 12-14 Cr, 0-5 Ni, 0-5 Si, 1-0 Μη, > 0-045 S, > 0-045 P. A 4-in. length of each stellite plate was cranked, using a vice, to the required angle, a , as shown in Fig. 1. Except where stated otherwise, α was retained at 5° throughout the tests. The copper shim used was of commercial purity, in the fully annealed condition and of 0-008 in. thickness. Velocity Measurement Tests were carried out to find the velocity imparted to the flyer plate under different experimental conditions. The bared ends of two thin leads were set below the flyer plate, as indicated in Fig. 1, at a distance of J to ^ in. apart, measured normal to the direction of the flyer plate. By supplying the latter with 10 volts from a battery and running the leads to the "start" and "stop" terminals of a microsecond counter, the mean velocity of the collapsing flyer plate at this station over the displacement interval covered was established. Experimental Procedure Prior to testing, the surfaces to be bonded were wire brushed and degreased. Where a buffer was used, this was then stuck on to the back of the flyer plate with Evo-stik adhesive and the assembly made up as shown in Fig. 1. The explosive material was cut to the width of the flyer plates and buffer, and to such length that its end could be folded around the detonator subsequently, to ensure rehable detonation. The assembly was then placed in open ground, well clear of the buildings in which the firing and other equipment were housed. Where velocity tests were performed, the settings of the leads were checked and the detonator then introduced. Connections to the assembly from the battery and the microsecond counter were made from within the buildings and only immediately prior to the instant of firing. RESULTS
A N D
DISCUSSION
Micro-examination of the Welded Interface In all the photomicrographs presented, the direction of welding is from right to left, with the flyer plate uppermost, consistent with Fig. 1. Etching reagents used were am monium hydroxide/hydrogen peroxide and ferric chloride. For the first test using a copper shim, the latter was simply laid on top of the stainless steel parent plate prior to testing. With two thicknesses of Metabel and no buffer, both a stellite/copper and a copper/stainless steel weld were secured, as shown in Fig. 2(b). From the measurement made, the velocity of the flyer plate in the test was estimated to have been about 3600 ft/sec. A number of features associated with this high flyer plate velocity are evident in Fig. 2(b). The copper/stainless steel interface wave formations are of large amplitude which, it is reported,
1323
Explosive Welding of Stellite to Stainless Steel
)f shear is responsible for the production of molten pockets velding operation. Attempts are usually made to avoid the impounds during explosive welding, or at least to confine )pposed to allowing them to form a continuous band—on the
at these points during the formation of intermetallic them to local pockets—as grounds that they are often
ο·οιο
FIG. 3. (a) Copper to stellite clad, using a flyer plate angle of 10°, a J in. thick explosive charge and a i in. thick rubber buffer, (b) A copper/stellite clad formed as in Fig. 3(a), welded to stainless steel using a i in. thick explosive charge.
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F . W . TRAVIS and W . JOHNSON
hard and brittle and may be detrimental under certain conditions, i.e. where the weld has to withstand impact, vibration or thermal cycling, etc.<3) When the velocity of the flyer plate was reduced, by the use of a J in. thick rubber buffer or by restricting the charge to one thickness of Metabel, only a copper/stainless steel weld was obtained, so that further attempts in this direction were discontinued.
FIG. 4 . (a) As for Fig. 3(b), but using a i in. thick explosive charge, (b) Copper to stainless steel clad, as for Fig. 3(a).
Explosive Welding of Stellite to Stainless Steel
1325
In the next method attempted, a preliminary test was performed to explosively clad the jtelhte whh the copper shim, after which a second test was performed to weld the copper Jad stellite on to the stainless steel. One thickness of Metabel was far in excess of the charge equired to accelerate the copper flyer plate, and in the ñrst few tests, the copper was welded )n to the stellite more in the form of a metal spray rather than as one continuous strip of netal. When the rubber buffer was increased to J in. thickness, satisfactory cladding was ichieved at a flyer plate velocity of about 5000 ft/sec, as shown in Fig. 3(a). It was found that I less continuous intermetalhc compound was formed when the flyer plate angle, a , was ncreased to 10°, which is in agreement with previous observations.^^^^ When the stellite/ :opper clad was welded on to the stainless steel using two thicknesses of Metabel, as in the est of Fig. 2(b), similar results to those shown in the latter figure were obtained. The exensive copper/stainless steel intermetalhc compound produced, again with large shrinkage lavities, is shown in Fig. 3(b). By reducing the charge to one thickness of Metabel however.
C O P P E R / C O P P E R
S T E L L I T E / C O P P E R
I N T E R F A C E
I N T E R F A C E
C O P P E R / S T E E L INTERFACE
FIG. 5.
A copper/stellite clad formed as in Fig. 3(a), welded on to a copper/stainless steel clad also formed as in Fig. 3(a), using a i in. thick explosive charge.
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F . W . TRAVIS and W . JOHNSON
where the flyer plate velocity was estimated to have been about 1900 ft/sec, the satisfactory weld of Fig. 4(a) was produced. Only a small amount of intermetallic compound is formed at each interface and the wave formations are regular and not excessive in amplitude. A third method was investigated in which both the stellite and the stainless steel were explosively clad with copper, prior to welding. The initial copper/stainless steel clad is
FIG. 6. (a) Sheared face of test specimen of weld of Fig. 4(a). (b) Shear face of test specimen of weld of Fig. 5.
shown in Fig. 4(b), and the final weld in which one thickness of Metabel was used, with no buffer, in Fig. 5(a). The wavy copper/copper interface may be detected in the latter figure, and the indications are that a satisfactory weld has been formed. It will be noted that over the short length of copper/stainless steel interface shown, the periodic wave pattern is not
Explosive Welding of Stellite to Stainless Steel
1327
3resent and that the intermetallic compound is irregularly dispersed. This defect could 3 r o b a b l y be eliminated v^ith some slight alteration in the buffer thickness or in the flyer 3late inclination during the initial copper/stainless steel clad, although it is reported^i^) that
here are noticeable effects due to entrapped air when thin gauge flyer plates are used. The lolution may then be to carry out the tests in a vacuum and reference can be made to some previously reported^^^^ observations on the use of explosives in a vacuum chest. Strength of the Weld Short lengths of the welded specimens of Figs. 4(a) and 5 were tested in shear, in the iirection of the length of the weld. The stellite side of the bonded specimens was ground down ;o the stainless steel excepting for an area of about \ in. square, which was left proud. The stainless steel parent plate was placed between the compression platens of a universal esting machine, gripped in a vice so that it could not rotate, and a hardened steel bar then ised to push off the stellite plate, care being taken to ensure that the direction of the applied oad was in the plane of the weld. For the specimen of the weld of Fig. 4(a), the maximum oad recorded was 4-64 tonf, corresponding to a shear stress of 14-2 tonf/in.^ and it will be loted from Fig. 6(a) that, excepting for a very small area towards the right hand side, 'ailure has occurred entirely through the body of the copper. The maximum load of 3*28 onf for the specimen of the weld of Fig. 5 provides a value for the shear stress of 11-5 :onf/in.2, but in this case, as shown in Fig. 6(b), partial separation at the copper/copper inter'ace occurred. Micro-hardness indentations were made, using a l o a d of 20 g, at the stellite/copper nterface and the copper/stainless steel interface of the specimen of Fig. 5, and these are ;hown in Figs. 7(a) and (b) respectively. The hardness of the intermetallic compound at 3 0 t h these interfaces is seen to be intermediate between the hardnesses of its parent mat erials, from which it may be deduced that its presence is not necessarily detrimental to the veld. From measurement of the indentations, Vickers hardness numbers for the stellite, •tellite/copper intermetallic compound, copper, copper/stainless steel intermetallic com)ound, and stainless steel were 773, 391, 155, 258, and 420 respectively. These figures are ntended for p u r p o s e s of comparison, since when the hardness of the stellite, copper and itainless steel were determined in the vicinity of the welded interface using a 1 kg load in order that errors due to polishing, i.e. the Bielby layer, be reduced) the respective v a l u e s )btained were 515, 129 and 276 HV respectively. The hardness v a l u e s of the stellite and he stainless steel before testing were 383 and 261 HV30 respectively, from which it is noted hat the stellite has been substantially hardened. This c o u l d have been anticipated on L c c o u n t of the deformation suffered by the flyer plate during the welding operation. Use of a Buffer The main reason for the use of a buffer in explosive welding is to protect the surface of he flyer plate. In addition it is reported^^) that it will attenuate the peak pressure of the hock wave produced by the explosion and this may be particularly advisable in the present vork in view of the tendency of the carbide particles of the stellite to crack. It will be ippreciated, however, that the presence of a buffer reduces the velocity imparted to the iyer plate. In reference 13 it is reported that the surface of a buffer must be smooth, othervise the surface markings are embossed on the flyer plate. Such effects are well illustrated η Fig. 8, which presents three copper clad stellite plates, where, from top to bottom, the | in.
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F . W . TRAVIS and W . JOHNSON
thick buffers were of smooth rubber, patterned rubber, and cardboard. The exaggerated buffer pattern on the surface of the clad—in view of the small thickness of the copper flyer plates—has encouraged the authors to speculate that the buffer pattern may be translated into differences in velocity imparted over the surface of the flyer plate, which cause the pattern to be reproduced when the flyer plate suffers impact with the stellite parent plate.
FIG. 7. (a) Stellite/copper interface of welded specimen of Fig. 5. (b) Copper/stainless steel interface of welded specimen of Fig. 5.
Further Considerations A number of deleterious features of the explosive welding process are shown in Figs. 9(a) and (b). A particularly severe form of "edge defect" is demonstrated in (a), where the outer I in. or so of the stellite flyer plate has broken off*. Such edge defects are characteristic
Explosive Welding of Stellite to Stainless Steel
1329
)f explosive welding and it has been reported
FIG. 8. Copper clad stellite, formed using a flyer plate angle of 10°, a i in. thick explosive charge and a i in. thick buffer, where the material of the buffer, from top to bottom was, smooth rubber, patterned rubber, and cardboard, respectively.
iliminated
1330
F . W . TRAVIS and
W . JOHNSON
welding of steel is a common phenomenon. Similarly, if care is not taken, the detonation wave of the explosive charge may cause damage to the plate to be welded, or to any fixtures used. In this respect there could be advantage in the use of an explosive material having a sub-sonic detonation velocity, i.e. lower than the speed with which elastic stress waves are
FIG. 9. (a) Showing "edge defect" in failure of stellite to stainless steelweld, using a i in. thick explosive charge, a i in. thick rubber buffer, and a sand base, (b) Showing spalling of parent plate in failure of stellite to stainless steel weld, using a nickel insert, a i in. thick explosive charge, and a sand base.
Explosive Welding of Stellite to Stainless Steel
1331
propagated through metals, but at the present time such materials are not available in Britain. APPENDIX
Measurement of Flyer Plate Velocity Although not intended for use in the present tests (this work was carried out in 1963), an attempt was made by the authors to develop a re-useable set-up for the determination of flyer plate velocity, in which the block shown in Fig. 10 replaced the parent plate in Fig. 1. wtym
FIG. 10.
PLATE
Block constructed for use in flyer plate velocity tests.
Insulated hardened silver steel pins, forming part of an electrical circuit, were set into the case-hardened steel block, the tops of the pins being ground flush with the surface of the block. Signals generated upon contact of the flyer plate with the pins were displayed on an oscilloscope screen and photographed, and from this record, the velocity was determined. (This method is basically that of pin-contactors, ^^s) which is used extensively for the mea surement of velocity in dynamic metal forming. ( ^ ^ 0 It was found in tests using 0-020 in. thick copper plates that above flyer plate velocities of about 400 ft/sec, excessive depression of the pins required the block to be reground after each test, and above velocities of about 900 ft/sec the tops of the pins were broken off under the forward drag of the (necessarily unwelded) flyer plate. With aluminium plates of 0-020 in. thickness, however, velocities in excess of 1000 ft/sec caused no damage to the pins, and it may be that with such softer materials the method could find application. ACKNOWLEDGEMENTS
The financial support of the Science Research Council and Associated Electrical Industries, Manchester, is gratefully acknowledged. The authors wish also to thank the latter for their agreement to publish this paper. REFERENCES HoLTZMAN, A. H . and RUDERSHAUSEN, C . G . Recent advances in metal working with explosives, Sheet Metal Industries, 39, 422, 1962. 2 BiRKHOFF, G., MACDOUGALL, D . P . , PUGH, E . M . and TAYLOR, G . I. Explosives with lines cavities, /. Appl. Phys. 19, 563, 1948. 3. WRIGHT, E . S . and BAYCE, A. E. Current methods and results in explosive welding, Proc. NATO Advanced Study Institute on High Energy Rate Working of Metals, Sandefjord and Lillehammer, Norway, Sept. 1964. 4. HOLTZMAN, A. H . Explosion clads, Ibid. 1.
1332 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
F . W . TRAVIS and W . JOHNSON BUCK, G . and HORNBOGEN, E . Metallographic investigation of shock welded metals by electron microscopy, Ibid. BAHRANI, A . S., BLACK, T . J. and CROSSLAND, B . The mechanics of wave formation in explosive welding, Proc. Roy Soc, A 296, 1 2 3 - 1 3 6 , 1967. ABRAHAMSON, G . R . Permanent periodic surface deformation due to a travelling jet, / . Appl. Mech. 28, E, 5 1 9 , 1 9 6 1 . COWAN, G . R . and HOLTZMAN, A. H . Flow configurations in colliding plates: Explosive bonding, /. Appl. Phys. 3 4 , No. 4 , 9 2 8 , 1963. OTTO, H . G . Aspects relating to the welding mechanism, Proc. NATO Advanced Study Institute on High Energy Rate Working of Metals, Sandefjord and Lillehammer, Norway, Sept. 1964. POCALYKO Α., Explosion clad plate for corrosion service, Proc. NACE North Central Region Conf., Sept. 1964. COOK, M . A. The Science of High Explosives, Reinhold, New York, 1958. RINEHART, J. S . and PEARSON, J. Explosive Working of Metals. Pergamon, 1963. BAHRANI, A, S. and CROSSLAND, B . Explosive welding and cladding: An introductory survey and pre liminary results, Proc. Inst. Mech. Engrs. 179, part 1 , 1 9 6 4 - 6 5 . BAHRANI, A. S. and CROSSLAND, B . Further experiments on explosive welding and cladding with particular reference to the strength of the bond, Proc. Inst. Mech. Engrs. 180, part 31, 1 9 6 5 - 6 6 . CROSSLAND, B . and BAHRANI, A. S. Some observations on explosive cladding and welding. Preprint AD66-112, A.S.T.M.E. 1966. JOHNSON, W . and TRAVIS, F . W . Peeling, hot forming of tungsten and mild steel discs and the use of a vacuum chest with explosives, Proc. 6th Int. M.T.D.R. Conf, Manchester, Sept. 1965. DIETER, G . E . Metallurgical effects of high intensity shock waves in metals. Response of Metals to High Velocity Deformation, Interscience Publishers, New York, 1960. MiNSHULL, S. Properties of elastic and plastic waves determined by pin-contactors and crystals. /. Appl. Phys. 26, 4 6 3 , 1955. JOHNSON, W . , KORMI, K . and TRAVIS, F . W . An investigation into the explosive deep drawing of circular blanks using the plug cushion technique, Int. J. Mech. Sci. 6, 2 8 7 , 1 9 6 4 .