Welded Connections in High Strength Cold-Formed Steels

Transcription

1 Welded Connections in High Strength Cold-Formed Steels by Gregory Hancock 1, Tim Wilkinson 2, Lip Teh 3 Centre for Advanced Structural Engineering University of Sydney NSW, Australia ABSTRACT Cold-formed steel is being used more widely for routine structural steel design including portal frames composed of open and/or closed sections. In Australia, cold-formed channel and Z- sections made of G450 (zinc coated 450 MPa yield) sheet steel are used to construct frames by welding. The cold-formed steel structures are normally designed to the Australian/ New Zealand Cold-Formed Steel Structures Standard AS/NZS 4600:1996 which is based on the 1996 Edition of the AISI Specification. The design rules for welded connections in the AISI Specification and AS/NZS 4600 are based mainly on testing of mild steel (300 MPa) connections at Cornell University in the 1970s and so may not be applicable to high strength steels. Cold-formed tubular sections made of C450 steel (In-line galvanised 450 MPa yield) are also used to construct portal frames with welded and sleeved knee connections. They are designed to the Australian Steel Structures Standard AS which is similar to the AISC LRFD Specification. Recent research by Wilkinson and Hancock reported in the Journal of Structural Engineering of the ASCE (March 2000) has shown that fracture may occur in the heat affected zone of connections of this type. The paper describes the results of two ongoing research programs investigating welded connections in high strength cold-formed steel sections. The results of butt welds and fillet welds are described at this stage. The nature and stress of fracture in the heat affected zone are described in detail. The effect of heat input has also been investigated and is described. 1 BHP Steel Professor of Steel Structures, Centre for Advanced Structural Engineering, University of Sydney, Australia, Lecturer, Centre for Advanced Structural Engineering, University of Sydney, Australia, Senior Researcher, Centre for Advanced Structural Engineering, University of Sydney, Australia,

2 1. INTRODUCTION This paper describes research on the effects of welding on the strengths of G450 Sheet Steel manufactured to the Australian Standard AS Steel Sheet and Strip Hot Dipped or Aluminium/Zinc Coated (1) and C450 Steel to the Australian Standard AS Structural Steel Hollow Sections (2). The steels are quite different in their methods of manufacture and so need to be studied independently for the effect of welding. However, both steels suffer degradation in the tensile strength in the Heat Affected Zone (HAZ) of the welds and so their structural behaviour is quite similar when tested for a welded connection in tension and/or shear. Consequently, although the paper describes similar phenomena, it is based on two separate research studies. The full study for the Strength of Fillet Welded Connections in G450 Sheet Steel can be found in Teh and Hancock (3) and the full study of the Effect of GMAW on the Mechanical properties of In-Line Galvanised Cold-Formed Steel can be found in Wilkinson and Hancock (4). This paper separates the descriptions from the two studies, and concentrates on material properties in the HAZ. During the welding process, the grains of the cold-worked steel recrystallise, and the heat affected zone will soften compared to the cold-formed hardness. Consequently, the ultimate tensile strength (f u ) in the heated affected zone (HAZ) may be less than the yield stress of the parent material. There are several instances in which a steel structure has to demonstrate ductile behaviour. In plastic design, the plastic hinges must rotate sufficiently for moment redistribution to take place in the structure, in order to obtain the strength increase afforded by plastic design. For seismic design, deformation capacity is essential to dissipate the energy caused by the earthquake motion. In such cases, the joints of a steel structure are required to show ductile behaviour. However, if there is a small HAZ in a welded joint, where the ultimate tensile strength is less than the yield stress in the adjacent unaffected steel, the HAZ will fracture before significant plastic deformations occur near the joint. This renders the structure unsuited for plastic design or seismic applications. A previous investigation examining the suitability of portal frame knee joints for use in a plastically designed structure constructed from cold-formed rectangular hollow sections (RHS), found that under opening bending moment, the connection fractured in the HAZ before large plastic deformation occurred. It should be noted that the connection displayed adequate strength, as opposed to ductility, which means that it was still suitable for use in elastic design. 1.1 G450 Cold-Reduced Zinc-Coated Steel to AS 1397 In Australia and New Zealand, the design rules for cold-formed steel members including connections are specified in AS/NZS 4600 (5) which is similar to the AISI Specification (6). The design equations for welded connections in thin sheet steels less than 3.0 mm (2.5 mm for fillet welds) specified in the standard are adapted from the AWS D1.3 Structural Welding Code (7), which is based on the testing results of Pekoz & McGuire (8) on double-lap welded connections in mainly mild sheet steels. Since the welds in thin sheet steels are generally as thick as or thicker than the sheets, and the weld metal must be at least as strong as the weaker of the sheets being joined, these equations use the sheet material strength and the sheet thickness (rather than the weld metal strength and the weld throat size) in determining the nominal capacity of the connections. Unfortunately, it is not clear how applicable the equations are to welded connections in high-strength sheet steels manufactured to AS In Clause of AS/NZS 4600, it is stated that The effect of welding on the mechanical properties of a member shall be determined on the basis of test on the full section containing the weld within the gauge length. Any necessary allowance for such effect shall be made in the structural use of the 362

3 member. However, no significant research has been conducted on welded connections in coldreduced high-strength sheet steels such as G450, G500 and G550 steels, which are manufactured to AS Zhao and Hancock (9) have pointed out that as the tensile strength of the steel is increased by cold working, the heat-affected-zone (HAZ) may play a more important role in the strength of welded connections. It is noteworthy that with regard to milder steels including cold-formed tubular sections, it has previously been concluded that welding does not affect the steel properties significantly (Wardenier & Koning (10)). This conclusion supports the existing design equation for the nominal capacity of a transverse fillet welded connection in sheet steel, as specified in Clause of AS/NZS It is also consistent with the statement of Pekoz & McGuire that a butt or transverse fillet welded connection can be expected to develop the full strength of the sheet. However, recent research by Chen et al. (12) shows that the tensile strength of the heataffected-zone (HAZ) of G550 sheet steel drops substantially from a nominal value of 550 MPa to about 450 MPa. This considerable decrease in tensile strength due to welding puts into question the applicability of current design equations to welded connections in cold-reduced high-strength sheet steels such as G450, G500 and G550 sheet steels. Additionally, there is a concern about the effect of reduced ductility especially of G550 steel on the ability of a (long) welded connection to redistribute the stresses prior to fracture in the stress concentration area. It may be noted that with regard to the tensile strength assumed in the design of bolted connections in G550 sheet steel, liberalisation of the design rule which requires that the yield and ultimate strengths be reduced to 75% was recently proposed by Rogers and Hancock (13). This paper describes the laboratory tests conducted on full width transverse fillet welded connections in 1.5-mm and 3.0-mm G450 sheet steels, which are cold-reduced high-strength steels having a design yield strength of 450 MPa and a design tensile strength of 480 MPa. These thicknesses represent the minimum and the maximum thicknesses commonly available, respectively, for G450 sheet steel. The use of these thicknesses ensures that any proposed design rules are applicable to the whole range of thicknesses available to the designer. The G450 sheet steel materials used in the laboratory tests, which have a trade name GALVASPAN, were manufactured and supplied by BHP Coated Products, Port Kembla. The coating class designation is Z350, which indicates zinc coating of a nominal mass density of 185 g/m 2 on each side of the sheet steel. Tensile testing of the specimens was performed using a 2000-kN capacity Dartec servo-controlled testing machine manufactured in Stourbridge, England, and an MTS Teststar digital controller. Tensile loading of all specimens was in the rolling direction of the G450 sheet steel. The main objective of the tests was to determine the tensile strength in the HAZ of the G450 steel. 1.2 C450 In-line Galvanised DuraGal Section Steel A recent innovation in steel products in Australia is the DuraGal range of cold-formed in-line galvanised hollow and open sections produced by BHP Structural and Pipeline Products (formerly known as Tubemakers) (14). The typical steel strip used in the manufacturing process has a nominal yield stress (f y ) of 300 MPa. After cold-forming and in-line galvanising, the final product has a nominal yield stress in the range MPa, depending on the exact process and the shape of the product. If the sections are tubular, they can be designed to the Australian Steel Structures Standard AS (15) where the weld strength is usually based on weld metal strength. If the sections are open sections, they can be designed to AS/NZS 4600:1996 where the weld strength is 363

4 based on parent metal strength as described in 1.1 above. C450 tubular sections are specified to the Australian Standard AS 1163 Structural Steel Hollow Sections (2). This paper summarises the initial portion of a research project on C450 steel examining the strength in the HAZ of butt welded connections. 2. TENSILE STRENGTHS IN THE HAZ 2.1 G450 Cold-Reduced Zinc Coated Steel to AS 1397 Each specimen was a double-lap transverse fillet welded connection consisting of two mm hot-rolled plates of Grade 450, manufactured to AS/NZS 3678 (16), abutted together and joined by two mm G450 sheets as illustrated in Fig. 1. The weld length is the same as the sheet width so that the tensile stresses are assumed to be uniform in the cover sheets. As mentioned previously, the tensile load, which was transverse to the welds, was in the rolling direction of the cover sheets. Each specimen was gripped at the hot-rolled plates on both ends, and the distance between the two grips was approximately 400 mm. Such a set-up was also used for subsequent double-lap connection specimens used to verify the reliability of existing design equations and described fully in Teh and Hancock (3). Although it is not the purpose of the present work to find the optimum welding procedure for G450 sheet steel, two different electrodes and two different shielding gases were used for the specimens. The two electrodes are 0.8-mm ES6-GC/M-W503AH wire and 0.9-mm ES4-GC/M- W503AH wire, both of which are manufactured to AS/NZS (17) and are pre-qualified welding consumables for gas metal-arc welding (GMAW) of G450 sheet steel according to Clause of AS/NZS (18). Both shielding gases are argon and carbon-dioxide based, with one containing helium. The settings of the GMAW machine were varied from specimen to specimen while ensuring that acceptable welds were produced. The welding voltage, current and time were recorded using a WeldPrint monitoring machine (19). Grade 450 hot-rolled plate Fillet weld 100mm 130mm Fillet weld G450 sheet cover Figure 1: Diagram of a HAZ Specimen 10mm The welding procedure for each HAZ specimen is given in Appendix 1 of Teh and Hancock (3). All the specimens failed in the HAZs of the cover sheets rather than in the welds, as illustrated in Fig. 2, so it can be inferred that the weld fusion and penetration of each specimen were 364

5 satisfactory. Hydrogen cracking was not a concern as G450 sheet steel does not have a sensitive microstructure and the double-lap joints were not highly constrained. It may also be noted that both the electrodes used in the welding are hydrogen controlled as denoted by the letter H at the end of the classifications. Figure 2: HAZ Failure in 3.0-mm G450 Sheet Steel The HAZ tensile strength f uh of each specimen is computed from the ultimate test load P t and the actual dimensions of the cover sheets. The actual dimensions are the average sheet width and the average base metal thickness (with the zinc coating removed). The ultimate test loads listed in Tables 1 and 2 were obtained using a stroke rate of 0.2 mm/minute, which translates to strain rates of the order of 10-5 per second for the cover sheets. The average tensile strength of the HAZs in the 1.5-mm sheet steel was found to be 488 MPa, and that in the 3.0-mm sheet steel was found to be 495 MPa. Table 1. Strength of HAZs in 1.5-mm G450 Sheet Steel Arc energy (kj/mm) 1.1 Dimensions 1.2 (mm 2 ) P t (kn) f uh (MPa) f uh /f un HAZ HAZ HAZ HAZ HAZ HAZ HAZ HAZ HAZ HAZ

6 The last columns of Tables 1 and 2 show the ratios of the measured HAZ tensile strengths f uh to the nominal design tensile strength f un of 480 MPa specified in AS/NZS 4600 for G450 sheet steel. It is evident that irrespective of the arc energy and the welding procedures, the tensile strengths of the HAZs do not differ significantly from the nominal tensile strength, although they are significantly lower than the actual tensile strengths of the corresponding coupons cut from the same sheets. The average tensile strength of the 1.5-mm G450 sheet steel in the rolling direction was found to be 596 MPa, and that of the 3.0-mm G450 sheet steel was found to be 529 MPa. Thus the close agreement between the tensile strengths of the HAZs and the nominal tensile strength of 480 MPa used in the design of fillet welded connections is fortuitous. Table 2. Strength of HAZs in 3.0-mm G450 Sheet Steel Arc energy (kj/mm) Dimensions (mm 2 ) P t (kn) f uh (MPa) f uh /f un HAZ HAZ HAZ HAZ HAZ HAZ HAZ HAZ HAZ More research is required to correlate the tensile strengths of HAZs in G450 sheet steel to the virgin strength and the welding procedures used to produce the fillet welds. It is also noted that while the average virgin strength of the 1.5-mm sheet steel is higher than that of the 3.0-mm sheet steel, the reverse is true with regard to their average HAZ strengths. 2.2 C450/C400/C350 In-line Galvanised DuraGal Section Full strength butt welds were used to test the C400 and C350 steel Steel Properties Rather than testing an entire butt welded connection between two RHS, it is more convenient to perform a tension test on the flat faces of the RHS that have been welded together in the same manner. To further simplify the production of test specimens and to avoid having to cut the flat faces from the RHS, two flat bars of cold-formed DuraGal steel were connected. Currently, there is no Australian Standard applicable to the manufacture of cold-formed open profiles, and hence the sections are manufactured to an internal BHPSPP Specification, TS100. In most respects, the properties of the cold-formed open profile sections are the same as those for the coldformed hollow sections manufactured to AS

7 2.2.2 Weld Metal Properties Two types of welding wire were used in the GMAW process. Autocraft LW1 (f yn = 390 MPa, f un = 500 MPa) and Autocraft Mn-Mo (f yn = 530 MPa, f un = 630 MPa), to AS/NZS were used. More details on the wire properties can be found in CIGWELD (20) Typical Welding Parameters Two methods of GMAW were employed, the dip-transfer and spray-transfer modes. Generally, the spray transfer method requires a higher wire speed and higher current, and consequently a higher heat input. It is not possible to include the full details of all welding procedures in this paper, due to length requirements, however full details are given in Wilkinson and Hancock (4) Separate flatbar sections 12.5 Butt weld Figure 3: Dimensions and Location of Tensile Coupon within Welded Plate (all dimensions in millimetres) Test Procedure Two 150 mm long plates were butt welded together. The sections were either 100 mm wide (3.8 mm thick specimens) or 150 mm wide (8 mm thick sections). Different tests were performed in accordance with AS 2205 (21). A tensile coupon was cut longitudinally from the plate in accordance with AS (22) as shown in Figure 3. The butt weld was located transversely at the middle of the coupon. The tensile coupons were prepared and tested to AS 1391 (23). An extensometer was used to measure strain. The coupons were tested in a 300 kn capacity SINTECH Testing Machine with friction grips to apply the loading. A constant strain rate of approximately s -1 was used. In some cases the weld reinforcement was removed so that a completely flat coupon was tested. In the remaining cases the weld reinforcement remained. 367

8 The properties of the weld metal itself were obtained by performing an all-weld-metal tensile test to AS (24). Properties of the unwelded steel were determined to AS Macro specimens were also cut from the specimens. However the results of the macro section examination and Vickers Hardness tests are not presented in this paper Test Results The values of yield stress and ultimate stress can also be seen in Figures 4 to 7. Since the steel is cold-formed there is no well-defined yield stress, and the yield stress quoted is the dynamic 0.2% proof stress. The term dynamic is used since the stress was determined while the testing machine was loading at a constant rate of stroke Stress (MPa) mm DuraGal Flatbar Butt Welded LW1 Ultimate Stress (fu) Yield Stress (fy) Stress (MPa) mm DuraGal Flatbar Butt Welded LW1 Ultimate Stress (fu) Yield Stress (fy) Nominal Parent Weld Spray Spray reinforced Dip no reinforced Nominal Parent Weld Dip no reinforced Figure 4: Results mm steel, LW1 Figure 5: Results - 8 mm steel, LW1 Stress (MPa) Nominal Parent Weld 3.8 mm DuraGal Flatbar Butt Welded Ultimate Stress (fu) Yield Stress (fy) reo reo Stress (MPa) Nominal Parent Weld 8 mm DuraGal Flatbar Butt Welded Ultimate Stress (fu) Yield Stress (fy) reo reinforced Figure 6: Results mm steel, Mn-Mo Figure 7: Results - 8 mm steel, Mn-Mo Discussion Several observations can be made from the test results. There is considerably more variation in the results of the 3.8 mm steel compared to the 8.0 mm steel. There is a statistically significant drop in yield and ultimate stresses in the welded 3.8 mm steel, compared to the unwelded material. The change in properties for the 8 mm steel is small. The measured properties of the unwelded 3.8 mm steel are significantly higher than the nominal properties. This is very common, but as a result, the strength of the 3.8 mm steel is higher than that of the commonly used welding wire, Autocraft LW1. It is usual practice to match the strength of the welding consumable to that of the parent metal. In two instances (3.8 mm, dip 368

9 method, LW1), fracture occurred in the weld rather than in the parent metal. For the corresponding case using the spray method (higher heat input), failure occurred in the HAZ, indicating that the higher heat input had reduced the strength of the HAZ by a greater amount compared to the dip method. Welding produces a greater percentage reduction in strength for the 3.8 mm steel, compared to the 8 mm steel. The 3.8 mm steel is more heavily cold-worked to produce its higher nominal strength compared to the 8 mm steel. Consequently, there is greater scope for strength reduction in the HAZ. This is similar to the G450 steel described in Section 2.1. The higher strength electrode (Mn-Mo) produced greater capacity in the 3.8 mm sections compared to the LW1 electrode despite a similar heat input. The Mn-Mo electrode had an almost negligible effect on the 8 mm steel, compared to the results of the LW1 electrode. The higher heat input method of spray transfer compared to dip transfer produces a larger reduction in yield and ultimate stresses. There are several instances in which the ultimate strength of the welded 3.8 mm specimens drops below the yield stress of the parent material. Consequently, a welded connection of this type would not be able to provide the amount of ductility required for seismic or plastic design applications. The ultimate strength of the welded 8 mm samples did not fall below the yield stress of the parent material. It should be noted that to utilise the available feedstock most efficiently, BHPSPP use virgin strip with yield stress f yn = 360 MPa for the 3.8 mm flat bar, and a different strip with yield stress f yn = 300 MPa for the 8.0 mm flat bar. This is the most likely cause of the 3.8 mm steel exhibiting strength considerably higher than the nominal properties. It is possible that BHPSPP may change the feedstock, so that all DuraGal flatbars are produced from the 300 MPa strip. It is possible that flat bar made from this material will not experience as significant changes in the strength of the HAZ, compared to the product tested. This paper has considered the preliminary results of the initial stage of this project. Future examinations will consider microhardness determination, macro cross section examination, and fillet welded specimens. 3. CONCLUSIONS Based on the following test results of the specimens, the following conclusions can be made. For sections of G450 sheet steel which were transverse fillet welded to a Grade 450 plate. The tensile strength of the HAZ in G450 steel is significantly lower than that of the virgin G450 steel for 1.5 mm and 3.0 mm sheets but is generally higher than the nominal tensile strength of 480 MPa. Hence they may still produce reliable designs as discussed by Teh and Hancock (3). For sections of cold-formed flats in C450 steel which were butt-welded together using either the dip transfer method or the spray transfer method using either LW1 or Mn-Mo electrode. There was a small reduction in the yield and ultimate stresses in the welded 8.0 mm steel compared to the unwelded steel. The 3.8 mm samples displayed a more significant drop in yield and ultimate stresses when welded, and the drop in strength was greater when the higher 369

10 heat input spray method was used. The 3.8 mm steel had a higher nominal strength than the 8.0 mm steel due to more cold working in the manufacturing process, so it is not unexpected that this steel experienced a greater drop in strength when welded. Significantly, there were some occasions in which the ultimate strength of the welded 3.8 mm specimens dropped below the yield stress of the parent material. 4. REFERENCES (1) Standards Australia, 1993, Sheet Steel and Strip-Hot-Dipped or Aluminium/Zinc-Coated, AS , Standards Australia, Sydney, Australia. (2) Standards Australia, 1991, Structural Steel Hollow Sections, AS , Standards Australia, Sydney, Australia. (3) Teh, LH and Hancock, 2000, "Strength of Fillet Welded Connections in G450 Sheet Steels," Research Report No R802, Department of Civil Engineering, University of Sydney, Sydney, Australia. (4) Wilkinson T and Hancock GJ, 2000, Effect of GMAW on the Mechanical Properties of In-line Galvanised Cold-Formed Steel, IIW Asia Pacific International Congress, Melbourne, November. (5) Standards Australia/New Zealand Standards, 1996, Cold-Formed Steel Structures, AS/NZS 4600:1996, Standards Australia, Sydney, Australia. (6) American Iron and Steel Institute, 1996, Specification for the Design of Cold-Formed Steel Structural Members, Washington DC. (7) American Welding Society, 1989, Structural Welding Code: Sheet Steel, AWS D1.3. (8) Pekoz, T and McGuire, W, 1980, Welding of Sheet Steel, Proceedings, Fifth International Specialty Conference on Cold-Formed Steel Structures, St Louis, Missouri, pp (9) Zhao, X-L and Hancock, GJ, 1996, Welded Connections in Thin Cold-Formed Rectangular Hollow Sections, Connections in Steel Structures III, editors R Bjorhovde, A Colson and R Zandonini, Pergamon, Oxford, pp (10) Wardenier, J and Koning, CHM, 1975a, Static Tensile Tests on T-Joints in Structural Hollow Sections, Research Report, IBBC-TNO and TH-Delft. (11) Wardenier, J and Koning, CHM, 1975b, Ultimate Static Strength of Welded Lattice Girder Joints in Rectangular Hollow Sections, Research Report, IBBC-TNO and TH- Delft. (12) Chen, YW, Dunne, D, Norrish, J and Szalla, J, 1990, Effect of GMA Welding on Microstructure and Mechanical Properties of G550 Sheet Steel, Research Report, Department of Materails Engineering, University of Wollongong. (13) Rogers, CA and Hancock, GJ, 1997, Ductility of G550 Sheet Steels in Tension, Journal of Structural Engineering, Vol 123, pp (14) BHP Structural Pipeline Products, 1999, Structural Cold Formed Hollow Sections and Profiles, Technical Information, BHP Structural and Pipeline Products, Mayfield, Newcastle, Australia. (15) Standards Australia, Steel Structures, AS , Standards Australia, Sydney, Australia. (16) Standards Australia/Standards New Zealand, Structural Plates Hot-rolled Plates and Slabs, AS/NZS 3678:1996, Standards Australia, Sydney, Australia. (17) Standards Australia/Standards New Zealand, 1996, Welding Electrodes Gas Metal Arc Ferritic Steel Electrodes, AS/NZS 2717:1996, Standards Australia,Sydney, Australia. 370

11 (18) Standards Australia/Standards New Zealand, 1995, Structural Steel Welding Welding of Steel Structures, Amendment No 1 to AS/NZS :1995, Standards Australia, Sydney, Australia. (19) Welding Technology Institute, 2000, WeldPrint, Build 2.70EN, University of Sydney, Australia. (20) CIGWELD, (1993), Welding Consumable Guide, Part No. WCGUIDE, Preston, Victoria, Australia. (21) Standards Australia, 1997, Methods for Destructive Testing of Welds in Metal; Method 1: General Requirements for Tests, AS , Standards Australia, Sydney, Australia. (22) Standards Australia, 1997, Methods for Destructive Testing of Welds in Metal; Method 2.1: Transverse Butt Tensile Test, AS , Standards Australia, Sydney, Australia. (23) Standards Australia, 1991, Methods for Tensile Testing of Metals, AS 1391, Standards Australia, Sydney, Australia. (24) Standards Australia, 1997, Methods for Destructive Testing of Welds in Metal; Method 2.2: All-weld-metal Tensile Test, AS Standards Australia, Sydney, Australia. 371