COMPARATIVE TENSILE STRENGTH AND SHEAR STRENGTH OF DETACLAD EXPLOSION CLAD PRODUCTS Curtis Prothe Technical Manager DMC Clad Metal Mt. Braddock, PA 15465 Email:
[email protected] Phone: 1.724.277.9710 ext 304 Fax: 1.724.277.9711 Andy Vargo Mechanical Engineer DMC Clad Metal Mt. Braddock, PA 15465 Email:
[email protected] Phone: 1.724.277.9710 ext 324 Fax: 1.724.277.9711 ABSTRACT The tensile strength of an explosion clad interface can be an important consideration when structural components are to be welded to the clad surface of equipment and for tube sheets in certain shell and tube heat exchanger designs. A considerable amount of data has been published on the bond shear strength features of explosion clad plates. In contrast, minimal data has been presented on the tensile strength of explosion welded interfaces (tested in the throughthickness direction). In general it is relatively easy to perform shear strength tests on clad plates; however, since cladding layers are typically thin, it is relatively difficulty to perform through-thickness tensile tests. A testing program has been undertaken to establish data on the interface tensile strength properties of explosion-clad plates and to compare the results with the interface shear strengths and the bulk mechanical properties of the cladding and base metal components. Results and indicative relationships are presented and discussed. KEYWORDS Clad, explosion welding, explosion clad, mechanical properties, tensile testing, shear test, ram tensile, bond interface, bond strength, base metal, cladding metal.
INTRODUCTION Explosion welded clad materials are employed in a wide range of applications, and depending on the alloy, the clad material is typically purchased in accordance with one of the internationally accepted clad specification such as ASTM/ASME A262, A263, A264, or B898. [1] Shear test requirements are defined by these standards, and the shear test is the most commonly specified bond strength test. In fact, these specifications do not define a through thickness clad tensile test. For most applications, the clad metal is simply too thin to produce a meaningful clad tensile specimen. Specification Mil-J-24445A for aluminum-steel bonded joints defines a “ram” tensile test specimen. [2] This design has been used for testing other material combinations, but this specimen also has limited applicability. If the cladding metal is too thin, the ram will shear through the cladding metal without breaking the bond zone. In addition, because of the specimen geometry, the tensile strength measured is not equivalent to the tensile strength measured by an ASTM A370 specimen. [3] The shear test specimen geometry is shown in figure 1 and test requirements are shown in table 1. The ram tensile test specimen is shown in figure 2. The explosion welding process is characterized by the formation of bond waves at the interface. As a result, test specimen must be machined carefully to ensure accurate test results. A typical explosion weld interface is shown in figure 3 and a properly machine shear test specimen in figure 4. FIGURE 1. Shear Test Specimen in Accordance with ASTM B898[4]
TABLE 1. Shear Strength Specification Requirements [4-8] Clad Specification ASTM/ASME A263, A264, A265 ASTM B-432 ASTM B-898
Materials Stainless Steel and Nickel Alloys Copper and Copper Alloy Reactive Metals (Ti, Zr)
Minimum Shear Strength (Mpa, ksi) 140 (20) 85 (12) 137.9 (20)
FIGURE 2. Ram Tensile Test Specimen in Accordance with MIL-J-24445A [2]
FIGURE 3. Typical Explosion Weld Interface
FIGURE 4. Properly Machined Shear Test Specimen after Testing (shear lug broken off)
Because of the absence of specification requirements, and due to the practical testing difficulties, minimal data has been presented on the tensile strength of explosion welded materials. Nevertheless, the tensile strength of an explosion clad interface can be an important consideration when structural components are to be welded to the clad surface of equipment and for tube sheets in certain shell and tube heat exchanger designs. One such application is the crude distillation column (figure 5) where multiple internal components are attached to the column walls. The crude tower may operate under vacuum or atmospheric conditions. Because of the lack of clad tensile data, the designer can only estimate stresses for components attached to the clad surface. Most companies allow welding of catalyst tray support rings and beams directly to explosion welded clad surfaces, but reactor beams must usually be welded to the base metal. Availability of clad tensile strength test data would strengthen the design analysis, and may lead to extending the range of applications where direct attachment to EXW clad is permitted. A testing program has been undertaken to establish data on the interface tensile strength properties of explosion-clad materials and to compare
the results with the interface shear strengths and the bulk mechanical properties of the cladding and base metal components.
FIGURE 5. Schematic of Crude Distillation Column
EXPERIMENTAL PROCEDURE The clad material for this testing program was drawn from production plates produced at Dynamic Materials Corporation, Clad Metal Division, from January July 2007. Only material with clad metal thickness >= 9.5mm (0.375”) was selected to enable clad tensile testing. Clad material combinations tested are shown in table 2. While this list of clad combinations tested is incomplete – there are many more possible combinations – the test material is representative of each group. The test material heat treat condition is representative of the final delivered condition for the production clad plate. For example, all of the titanium clad was stress relieved after the explosion welding operation. Subsize tensile coupons, 6.35mm (0.250”) gauge diameter, were machined with the clad interface in the specimen gage section. Because of the size of the test specimen, only the tensile strength was measured. Pictures of the fixture and subsize specimen test setup are found below in Figures 6, and 7. Standard shear test and base metal tensile test coupons were machined from the same test block. All of the coupons were tested on an Instron Satec Universal Testing Machine.
Table 2. Clad Test Materials Cladding Metals Stainless Steels / Nickel Alloys: 304L, 316L, 410S, 2205, 2507 Duplex SS Copper Alloys: Naval Brass, 70-30 CuNi Titanium: Grade 1, 11, 17
FIGURE 6. Test Fixture and Test Specimen
FIGURE 7. Naval Brass / SA516-70 Subsize Through-Thickness Clad Tensile Test Specimen
Base Metals SA 516-70 Carbon Steel SA 516-70 Carbon Steel SA 516-70 Carbon Steel SA 266-4 Carbon Steel Forging SA 240-304L and 316L SS
TEST RESULTS AND DISCUSSION Figure 8 shows all of the through thickness clad tensile test data plotted against the shear strength results. The data in this plot is grouped by cladding metal type. The solid data points represent test samples that broke in either the base metal or in the clad metal, and for the hollow data points the test sample broke at the bond zone. FIGURE 8. Clad Tensile Strength vs. Shear Strength
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Shear strength (Mpa) Stainless/Nickel Alloy - Base Metal Copper Alloy - Bond Zone Titanium Gr 1 / CS - Bond Zone SA516-70 Minimum Tensile Strength SB171-464 Cu Alloy Tensile Minimum
Copper alloys - Base Metal Copper Alloy - Clad Metal Titanium Gr1 / SS - Bond Zone SB265-1, Ti Gr 1 Tensile Minimum
The diagonal line in figure 8 represents points where tensile and shear strength are equal. The first observation is that all of the results fall above this line which indicates that for 100% of the samples tested, the clad tensile strength exceeded the shear strength. The second observation is that the stainless and nickel alloy clad shear strength ranges from ~350-550 Mpa (51-80 ksi) and is significantly higher than the Cu and Ti alloy clad shear strength of ~200-325 Mpa (29-47 ksi). These results are consistent with typical data from other production clad materials with thinner cladding metals.
For the titanium alloy clad, all of the samples broke at the titanium / base metal interface. It is interesting that the measured clad tensile strength, which ranged from ~370 - 470 Mpa (54-58 ksi), is significantly higher than the titanium minimum tensile strength of 240 Mpa (35 ksi). [4] In addition, both the shear and tensile test results for titanium explosion welded to stainless steel base metals fell within the same range. The copper alloy clad test results exhibited variability in the test specimen failure location. Both (2) of the 70-30 CuNi samples and two (2) of the naval brass samples broke in the cladding metal. Two (2) additional naval brass samples broke in the carbon steel base metal (figure 9), and the remaining four (4) naval brass samples broke at the bond interface. From this data it appears that for naval brass/steel explosion clad, the strength of the clad metal, the base metal and the bond interface is fairly balanced. The through thickness tensile strength for the copper alloys ranged from ~395-550 Mpa (58-80 ksi) which exceeds the cladding metal minimum tensile strength of 345 Mpa (50 ksi). [8] FIGURE 9. Naval Brass / SA-516-70 Tensile Specimen after Testing
Figure 10 shows only the stainless steel and nickel alloy clad test results, along with the corresponding base metal tensile test. All of these through thickness clad tensile specimen broke in the base metal. Significantly, for the stainless and nickel alloy clad materials tested, the strength of the explosion weld always exceeded the strength of the base metal. These tests were essentially short
transverse tensile tests of the base metal. The SA516-70 minimum tensile strength is shown in figure 10, and it is clear that both the standard base metal tensile test and the short transverse test values exceed this requirement. [9]
FIGURE 10. Stainless / Nickel Alloy Clad Tensile Strength, Base Metal Tensile Strength vs. Shear Strength 650
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EVALUATION OF RAM TENSILE SPECIMEN As discussed in the introduction, the ram tensile specimen will not yield tensile strength results equivalent to those measured by an ASTM A370 specimen due to the differences in specimen geometry. While the ram tensile specimen design was not used in this study to evaluate clad strength, preliminary testing was performed to evaluate use of this specimen for future studies. Six (6) subsize round tensile specimen and six (6) ram tensile specimen were machined from the same carbon steel plate. Test results for the subsize round tensile specimen averaged 498.7 Mpa (72 ksi), and tests performed with the ram tensile specimen averaged 538.1 Mpa (78 ksi). The apparent tensile strength as measured with the ram tensile specimen was ~8% higher than measured with a standard round specimen. Additional comparative testing should be performed using clad materials, but the geometry effect indicated by these tests with solid material should be conservative.
CONCLUSIONS The interface tensile strength properties of explosion-clad plates were evaluated by directly testing the through thickness properties of a wide range of production clad materials. Results were compared with the shear strengths and the bulk mechanical properties of the cladding base metal components. These test results support the following conclusions: • •
• •
The clad tensile strength exceeded the shear strength for 100% of the materials tested. For explosion welded stainless steel and nickel alloy clad, the tensile strength of explosion weld o Exceeds the tensile strength of the steel base metal and o Meets the base metal minimum tensile strength requirement. The through thickness tensile strength of explosion welded titanium alloy clad ranged from ~370-470 Mpa (54-68 ksi). These values are well above the cladding metal minimum tensile strength of 240 Mpa (35 ksi). The through thickness tensile strength of explosion welded copper alloy clad ranged from ~395-550 Mpa (58-80 ksi). These values exceed the cladding metal minimum tensile strength of 345 Mpa (50 ksi).
Plans for future investigation include extending the range of clad alloys tested and performing additional tests to improve the statistical basis for each class of clad materials. REFERENCES 1. Patterson A, “Fundamentals of Explosion Welding”, ASM Handbook, Vol. 6, Welding, Brazing and Soldering, 1993, pp 160-164. 2. “Joint, Bimetallic Bonded, Aluminum to Steel”, Military Specification Mil-J24445A, Naval Sea Systems Command. 3. “Standard Test Methods and Definitions for Mechanical Testing of Steel Products”, Specification ASTM A 370, ASTM International, West Conshocken, PA. 4. “Standard Specification for Reactive and Refractory Metal Clad Plate, Specification ASTM B 898, ASTM International, West Conshocken, PA. 5. “Standard Specification for Corrosion-Resisting Chromium Steel-Clad Plate, Sheet, and Strip”, Specification ASTM A 263, ASTM International, West Conshocken, PA. 6. “Standard Specification for Stainless Chromium-Nickel Steel-Clad Plate, Sheet, and Strip”, Specification ASTM A 264, ASTM International, West Conshocken, PA. 7. “Standard Specification for Nickel and Nickel Base Alloy-Clad Steel Plate”, Specification ASTM A 265, ASTM International, West Conshocken, PA.
8. “Standard Specification for Copper and Copper Alloy Clad Steel Plate”, Specification ASTM B 432, ASTM International, West Conshocken, PA. 9. “Specification for Pressure Vessel Plates, Carbon Steel, for Moderate- and Lower- Temperature Service”, Specification SA-516, ASME Boiler and Pressure Vessel Code, Section II, Part A, ASME, New York, NY.
