TENSION TESTS ON WELDED THREADED STUDS WITH A TENSILE STRENGTH OF 800N/mm²

TENSION TESTS ON WELDED THREADED STUDS WITH A TENSILE STRENGTH OF 800N/mm² Dieter Ungermann and Stephan Schneider Dortmund University of Technology, Institute of Steel Construction, August Schmidt Straße 6, 44227 Dortmund, Germany stahlbau@tu-dortmund.de Rainer Trillmich KÖCO Köster & Co GmbH Spreeler Weg 32, 58256 Ennepetal, Germany r.trillmich@koeco.net ABSTRACT Welding studs have to consist of a weldable material because of the joining process. It is for this reason that currently only a few austenitic steels and carbon steels out of strength class 4.8 according to EN ISO 898-1, with a maximum carbon content of C = 0,20%, are accredited for welding studs. If higher ultimate tensile strengths are required, the manufacturer used in the past the cold headed steel 20MnB4, which achieves ultimate tensile strengths close to strength class 8.8 due to a thermally post tempering. Welding of the 20MnB4 with a maximum carbon content of C = 0,23% is suitable only to a limited extent. By reason of additional costs for thermally post-tempering processes the 20MnB4 studs has not prevailed in the market. Another alternative material is the cold headed steel 8MnSi7 (1.5113), which was used for the first time from the manufacturer KÖCO to produce high strength studs. It has a maximum carbon content of C = 0,10% which suspects a good weldability. The ultimate tensile strength of 8MnSi7 wire rod is approximately between 520 to 620N/mm² and in addition by cold forging it reaches a level of 800N/mm², so that studs made of 8MnSi7 could be classified as strength class 8.8. To verify the weldability and the classification of the strength class 91 axial tension tests on threaded studs with a statically load were performed in the course of a German research project. The present paper gives an overview of the results of the research project. 1. INTRODUCTION 1.1 Cause and subject of the research project Stud welding has its roots in ship building in the early 20 th century and nowadays it is also being used in many other applications like composite constructions, mechanical and civil engineering. Stud welding is a highly sophisticated, very economic fastening technology but due to the joining process it is limited to weldable materials with a low carbon content respectively carbon equivalent. The strengths for approved materials are generally below 450N/mm². Against the background of the increasing use of high strength material for steel and composite constructions the need for welded threaded studs out of high strength steel is

obvious. 8MnSi7 is a weldable cold headed steel which reaches the strength of class 8.8 by cold forging. In this research project the weldability of studs type PD (threaded stud) and RD (threaded stud with reduced shank) made of 8MnSi7 were tested in 91 tension tests with static load. Furthermore the applicability of the design rules for the tension resistance of the threaded part and the shank based on EC3-1-8 was verified. In case of thin steel plates the ultimate load was limited by the shear strength of the steel plate material and did not achieve the tension resistance of the studs. To cover this failure mode, a design approach was suggested, founded on the results of the tests with shear failure in the steel plate material. 8MnSi7 was used for the first time from the manufacturer KÖCO to produce high strength studs. Due to this KÖCO supported the research project with the studs and the welding equipment. Since October 2010 KÖCO is worldwide the only manufacturer with a general technical approval (DIBt Z-14.4-585) using 8MnSi7 for studs with a strength comparable to strength class 8.8 and sell them under the brand name K800. Further trade mark rights on the European level are submitted. 1.2 Welding processes and stud types Arc stud welding can be divided into two main welding processes: stud welding with tip ignition on the one hand and drawn arc stud welding on the other hand. Based on the welding time, the weld pool backup and the source of the welding energy EN ISO 4063 defines different subcategories of arc stud welding. The most common welding process for applications in civil engineering is drawn arc stud welding with ceramic ferrule or shield gas (reference number 783 according to EN ISO 4063). Figure 1.1 illustrates the basic principles of drawn arc stud welding. The stud is placed against the steel plate and then the stud is lifted while the current is flowing and these lights up the drawn arc which melts the surface of the steel plate and the tip of the stud. Afterwards the stud is plunged in the weld pool and a cross sectional joint is achieved. Legend: L lifting P excess length X time Y current Z distance between stud and steel plate surface Figure 1.1: Procedure of drawn arc stud welding according to EN ISO 14555 The geometry of the studs is standardized in EN ISO 13918. Three exemplary selected threaded studs are shown in Figure 1.2. The associated welding process is given in the caption. A cone shaped stud end is characteristic for the drawn arc stud welding. The point angle α indicates the welding time, the smaller the point angle the shorter is the welding time. Studs for welding with tip ignition have a small tip which lights up the drawn arc when lifted up from the steel plate instead the cone shaped end. The welding time, the weld pool backup and the peak current determine the limitations of

the welding processes. For drawn arc stud welding the limitations are summarized in Table 1.1. Welding process: drawn arc stud welding Stud welding with tip ignition PD Stud (M6 M24) RD Stud (M6 M24) PT Stud (M3 M8) Figure 1.2: Threaded studs according to EN ISO 13918 Table 1.1: Limitations for drawn arc stud welding according to EN ISO 14555 stud diameter peak current welding time workpiece thickness workpiece weld pool backup surface d [mm] I [A] t W [ms] t min [mm] drawn arc stud welding 3 to 25 300 to 3.000 > 100 ceramic ferrule (CF) or shield gas (SG) blank, rolling layer, initial rust 1/4d (CF), 1/8d (SG), 1mm Further information for other welding processes can be found in Annex A of EN ISO 14555. 2. EXPERIMENTAL RESEARCH 2.1 Test program overview and material properties The experimental program consisted of two test series with threaded studs type PD and RD with nominal diameter of M8 and M20, see Figure 1.2. Currently the 8MnSi7 wire rod is limited to a maximum diameter of 18,15mm and due to this studs with a nominal diameter above M20 are not available. The studs were tested in tension tests according to EN ISO 14555. The test setup and detailed information of the test procedure are given in chapter 2.2 of this paper. To verify the weldability in the first test series (VS series) a steel plate with a thickness of 40mm was used. The large thickness of the steel plate leads to a disadvantageous thermal gradient in the heat affected zone (HAZ) and with it the hardness increases. In the second test series (VT series) the thickness of the steel plate depends on the nominal diameter of the studs according to the minimum requirements for the draw arc stud welding with ceramic ferrule, Table 1.1. In the VS series the predicted failure mode was a tensile fracture in the threaded part of the PD studs respectively in the reduced shank of the RD studs. Due to the very small thickness of the steel plate in the VT series a shear fracture of the steel plate should occur along the outer boundary of the welding bead, Table 2.1.

Table 2.1: Test series and predicted failure mode Test series stud type No. of workepiece studs predicted failure mode tests material thickness material diameter VS - Series PD-stud 20 tensile fracture in the thread S690QL 40mm 8MnSi7 M8 - M20 RD-stud 20 tensile fracture in the reduced shank VT - Series PD / RD 30 shear failure in the workpiece SZBS800 2mm 8MnSi7 M8 PD / RD 21 S690QL 5mm M20 All steel plates used for the investigations are made of high strength steels with nominal yield strength of approximately 690N/mm². In case of the 5 and 40mm steel plates it was a high strength quenched and tempered, fine grained steel according to EN 10025, part 6 and the 2mm steel plates are made of a thermo-mechanical rolled, microalloyed steel. The chemical composition of the steel plates and the studs are given in Table 2.2 and 2.3. Table 2.2: Chemical compositions of the steel plates No. 01 S690QL t = 40mm No. 02 SZBS800 t = 2mm No. 03 S690QL t = 5mm C Si Mn P S N B Cr Cu 0,17 0,29 1,17 0,0110 0,0042 0,0051 0,0048 0,331 0,035 0,086 0,53 1,90 0,0100 0,0013 0,0076 0,0011 0,034 0,013 0,15 0,29 1,36 0,013 0,0040 0,0070 0,0021 0,042 0,034 Mo Nb V Ti Ni Zr As Sn Sb No. 01 S690QL t = 40mm 0,207 0,028 0,0025 0,0200 0,048 0,0031 0,0043 0,0100 0,0110 No. 02 SZBS800 0,033 0,0050 0,157 0,0058 t = 2mm Nb + V + Ti = 0,195 0,338 0,0034 0,0150 0,0044 0,023 No. 03 S690QL t = 5mm 0,110 0,027 0,0040 0,013 0,043 0,0031 0,017 0,0051 0,0082 In general the weldability depends on the carbon content and should be less than C = 0,20%. As shown in Table 2.2 and 2.3 all material fulfills these requirements. The effect of other alloyed elements on the weldability can be reviewed by the carbon equivalent PCM, based on the work of Ito and Bessyo. A good weldability could be assumed, if the carbon equivalent is less than PCM = 0,45, calculated according to Equation (2.1). PCM Si Mn + Cu + Cr Ni Mo V = C + + + + + + 5 B 30 20 60 15 10 (2.1) The carbon equivalent varied between PCM = 0,20 for the stud material and PCM = 0,29 for the 40mm thick steel plate made of S690QL, therefore a good weldability could be assumed. Table 2.3: Chemical compositions of the studs material: 8MnSi7 C Si Mn P S V Cr Ni W No. 04 studs M8 (w ire rod 7,06mm) No. 05 studs M20 (wire rod 18,15mm) 0,074 1,03 1,68 0,014 0,0052 0,018 0,028 0,081 1,03 1,70 0,015 0,0051 0,018 0,024 0,017 0,034 0,014 0,033

The mechanical properties of the wire rod and the studs where measured in coupon tests. The results are summarized in Table 2.4. It is particularly noticeable, that the yield strength and ultimate tensile strength of the studs is lower than the strength of the corresponding wire rod and the M20 studs did not reach the minimum strength requirements of strength class 8.8 according to EN ISO 898-1. Further investigations on PD-studs M20 have shown that the yield and ultimate tensile strength did not depend on the diameter of the test specimen. For this reason an inhomogeneous distribution of the yield and ultimate tensile strength over the cross section could be excluded. The drop in strength can only be assumed in the manufacturing process of the studs, but no further investigations were made in the frame of this research project. Table 2.4: Mechanical properties of the wire rod and the studs measured values requirement acc. to EN ISO 898-1 R p0,2 R m f u / f y E R p0,2 R m [N/mm²] [N/mm²] [-] [N/mm²] [N/mm²] [N/mm²] wire rod ( 7,06mm) 852 884 1,04 184.000 PD / RD stud M8 671 842 1,25 180.000 wire rod ( 18,15mm) 770 824 1,07 175.000 PD / RD stud M20 627 777 1,24 178.000 640 660 800 830 2.2 Welding parameters and tension test setup Previously to the VS and VT series a small series of test studs were welded in the downhand position (Figure 2.1) using different welding times and peak currents. In order to obtain suitable welding parameters these studs were subjected to bending test according to EN ISO 14555 and if the studs did not fail by a brittle fracture in the welding area, the used welding parameters are acceptable for the stud type. Due to the different diameter at the cone shaped end of PD and RD studs the welding parameters differ slightly. The average values for studs M8 are: t W = 300ms and I W = 600A and for studs M20: t W = 750ms and I W = 1.400A. It should be noted that all welding parameters are setting values at the stud welding system. Figure 2.1: Sample picture from stud welding Figure 2.2: Hydraulic jack

The tests of the VS series were carried out with a hydraulic jack as shown in Figure 2.2. The elongations of the studs were measured by two displacement transducers, which were placed oppositely. The tension force was applied on the test studs by a threaded rod which was fixed at the top of the jack. To exclude effects of load eccentricities, a hinge bearing was placed between the jack and the load cell. All test of the VT series were performed in a 630kN hydraulic testing machine with a comparable measurement system. In both tests series packing pieces were used with the aim to ensure a uniform ratio of drilling diameter d i and welding beat diameter d 3, Figure 2.3. To avoid a significant bending of the thin steel plates in test series VT a small drilling diameter was chosen (d 3 /d i 0,65). Due to the bending resistance of the 40mm thick steel plate in the VS test series the drilling diameters of the packing piece does not need to be as small as in the VT series, therefore a ratio of d 3 /d i 0,30 was used. Figure 2.3: Detailed sketch of the test setup During the test the measurements were recorded by a data acquisition system from HBM and stored in ASC II files for further evaluations. 2.3 Results of VS test series In the VS series 40 tests were performed with thread studs type PD and RD with different length of the studs. With exception of one test stud M20x35-RD (No. 63) which failed by a brittle fracture in the welding area all test studs shows the expected failure modes: tensile fracture in the threaded part (PD studs, see Figure 2.4) respectively tensile fracture in the shank (RD studs). The test results and the theoretical tension resistance are summarized in Table 2.5. The theoretical tension resistance was computed based on EC3-1-8 by equation (2.2) and (2.3). The comparisons of the ultimate test loads (mean value of five tests) with the theoretical tension resistance indicate the applicability of both approaches. F u, theo. = k 2 ASp fu fracture in the threaded part where: 9 u theo. u k = 0, (2.2) 2 F, = A f fracture in the unthreaded shank (2.3)

In case of a failure in the threaded part the reduction factor k 2 = 0,90 is needed to cover the scatter in the tension resistance. If the failure occurs in the unthreaded shank of the stud, a reduction factor is negligible. Table 2.5: Test results of VS test series thickness of the steel plate t = 40mm stud type geometry and cross sectional area of the studs material strength tension resistance d 1 l 2 d 2 A A sp A sp / A f yb f ub F u,theo. acc. to equation (2.2) / (2.3) test results mean value ultimate load standard deviation comparison F u,m / F u,theo. [-] [mm] [mm] [mm²] [mm²] [-] [N/mm²] [N/mm²] [kn] F u,m [kn] SD [kn] [-] PD M8 M20 20 30,8 0,76 1,11 7,19 40,6 36,6 0,901 673 845 27,8 40 28,4 0,88 1,02 60 200,0 1,48 1,17 18,38 265,3 245 0,923 634 774 170,7 100 190,9 0,87 1,12 RD 20 28,8 0,78 1,14 M8 6,2 30,2 36,6 1,212 669 838 25,3 40 28,3 0,52 1,12 35 196,1 3,17 1,18 M20 16,5 213,8 245 1,146 619 780 166,8 100 192,6 1,70 1,15 Except the test stud No. 63, which failed by a brittle fracture caused by gas pore in the welding area, all test studs shows a ductile load bearing behavior, Figure 2.5. Possible explanations for the critical porosity in the welding area of stud No. 63 are an insufficient preparation of the steel plate surface or a not perfect fitted ceramic ferrule. This question could not be finally answered, but it was just one of more than 40 welded studs in the VS series and therefore it should not be overrated. As mentioned in chapter 2.1, the large thickness of the steel plate in the VS series was chosen in order to generate a disadvantageous thermal gradient in the heat affected zone. The fast heat dissipation in the welding zone increased the local hardness, which should be less than 380HV according to EN ISO 15614-1. To measure the Vickers hardness macrosections were made on selected test studs including stud No. 63, see Figure 2.6. 35 VS-series - PD / RD-Studs M8 30 25 Force F [kn] 20 15 10 5 M8x20-PD M8x40-PD M8x20-RD M8x40-RD 0 0,00 1,00 2,00 3,00 4,00 Displacement Δu [mm] Figure 2.4: Test stud M20x100-PD; failed in threaded part of the stud Figure 2.5: Load displacement curves from selected studs M8-PD and M8-RD

Figure 2.6: Macrosection of test No. 63, M20x35-RD (location of measurements Figure 2.7: Measured hardness of test No. 63, M20x35-RD (Measurement 01) The Vickers hardness varied in all three measurements between 280HV01 for the steel plate made of S690QL and maximum values of approximately 490HV01 in the HAZ, as shown in Figure 2.7. Because the exceeding of the hardness limit value given by EN ISO 15614-1 was detected in all measurements, it could be assumed, that a local hardness above the limit did not reduce the load bearing capacity and the ductility of the stud welding in case of studs made of 8MnSi7. This estimation should not be assigned to stud welding in general without further investigations. 2.3 Results of VT test series To verify the weldability of studs made of 8MnSi7 on thin steel plates 51 tension tests were performed in the VT series. According to EN ISO 14555 the minimum requirement for drawn arc stud welding using ceramic ferrules as weld pool backup is tmin = 1/4d, therefore steel plates with a thickness 2mm (M8) respectively 5mm (M20) were used. In each stud welding of the VT series the penetration was less than the steel plate thickness, see Figure 2.8. The required welding energy to achieve a cross sectional joint for studs made of 8MnSi7 is comparable to studs of strength class 4.8 and due to this the minimum thickness requirement of tmin = 1/4d is applicable. Figure 2.8: Back side of steel plate A1 and macrosections of studs from the VT series In general the studs failed by a shear fracture in the steel plate. Only in one subset with studs M8x40-RD three tests failed in the welding area caused by gas pores, which are a result of a slight underestimation of the peak current and the welding time. Therefore ten additional studs M8x40-RD with modified welding parameters were tested and all failed by a shear fractures in the steel plate.

The test results of the VT series, excluding the first subset of M8x40-RD studs, are summarized in Table 2.6. As a first approximation the shear resistance F u,theo. was computed by equation (2.4) using the diameter of the stud shank (d 2 ) to define the shear stressed area. F = A f 3 where: = π d t (2.4) u, theo. v u A v 2 A good accordance of the theoretical shear resistance and the test results was given only for studs M20-PD. Depending on the stud type and nominal diameter the theoretical approach leads to a significant underestimation (M8-PD, M8-RD) or overestimation (M20-RD) of the shear resistance. By close examination of the fracture pattern the reason for this is obvious, see Figure 2.9. For studs M8-PD the shear fracture occurs along the outer edge of the welding bead and if the shear resistance was calculated with d 3 instead of d 2 the deviation is less than 27% (computed with the nominal yield strength!). In all tests with RD studs the fracture pattern scatter significantly. It is particularly noticeable in Figure 2.9, that the area of the shear fracture surface decrease with an increasing stud diameter. For this reason the theoretical approach leads to an overestimated shear resistance for studs M20-RD. Table 2.6: Test results of VT test series thickness of the steel plate t = 1/4d 1,nom stud type PD RD geometry of the studs and the welding bead d d 3,nom d 1 l 2 2 (welding (shank) bead) material and geometry properties of the test results work piece comparison material shear resistance mean strength value standard A v = F u,theo. = t ultimate deviation f y f u π *d 2 *t f y *A v / 3 load F u,m /F u,theo. [ ] [mm] [mm] [mm] [N/mm²] [N/mm²] [mm] [mm²] [kn] F u,m [kn] SD [kn] [-] 20 31,4 0,69 1,77 M8 7,19 10,0 680 800 2 45,2 17,7 40 (1) 29,9 0,58 1,69 60 129,2 4,68 1,00 M20 18,38 24,5 776 834 5 288,7 129,4 100 132,5 9,24 1,02 20 18,8 2,35 1,23 M8 6,2 9,0 680 800 2 39,0 15,3 40 18,9 2,22 1,24 35 79,0 6,80 0,68 M20 16,5 23,0 776 834 5 259,2 116,1 100 84,1 6,15 0,72 (1) 3 of 5 studs failed by a tensil fraction in the threaded part of the stud Figure 2.9: Detail photo from PD and RD studs tested in the VT series

The comparison of the measured ultimate loads of RD studs with those of PD studs with the same nominal diameter reveals a constant ratio of approximately 0,62. Therefore equation (2.4) could be enlarged by an additional coefficient k that covers the reduced shear fracture surface of RD studs: F = k π d f 3 where: u, theo. 2 u 1,00 k = 0,62 PD studs RD studs (2.5) The modified approach given in equation (2.5) should be verified by additional tests using studs with a nominal diameter between M8 and M20 to ensure that the suggested linear interpolation did not overestimate the shear resistance. 3. CONCLUSIONS In the course of the research project the weldability of high strength studs made of 8MnSi7 was verified. The basic principles for drawn arc stud welding according to EN ISO 14555 could be utilized for the high strength studs. Further statistical evaluations which are not part of the presented paper have shown that the drop in strength from the studs with a nominal diameter of M20 did not affect the safety level, even if the nominal ultimate strength was used to determine the tensile resistance. Due to this, the high strength studs could be classified as strength class 8.8. To cover shear failure in case of thin a thin steel plate, a design approach was proposed, founded on the test results. By reason of the scattering of the fracture pattern and the supposed linear interpolation for stud diameters between M8 and M20 additional tests should be performed. 4. ACKNOWLEDGEMENTS The research work was preformed in the course of a German research project with the financial support of the FOSTA (www.fosta.de) and KÖCO Köster & Co. GmbH (www.koeco.net) and will be summarized in the report number P787, published by the FOSTA. All studs used in the tests were produced and welded by KÖCO.

6. REFERENCES CEN (2010) EN 1993, Design of steel structures Part 1-8: Design of joints; German version EN 1993-1-8:2010 CEN (2002) EN 10263, Steel rod, bars and wire for cold heading and cold extrusion - Part 2: Technical delivery conditions for steels not intended for heat treatment after cold working; German version EN 10263-2:2001 CEN (2009) EN ISO 898, Mechanical properties of fasteners made of carbon steel and alloy steel - Part 1: Bolts, screws and studs with specified property classes - Coarse thread and fine pitch thread, German version EN ISO 898-1:2009 CEN (2000) EN ISO 4063, Welding an allied processes Nomenclature of processes and reference numbers, German version EN ISO 4063:2000 CEN (2008) EN ISO 13918, Welding Studs and ceramic ferrules for arc stud welding, German version EN ISO 13918:2008 CEN (2006) EN ISO 14555, Welding Arc stud welding of metallic materials, German version EN ISO 14555:2006 Trillmich, R., Welz, W.: Bolzenschweißen Grundlagen und Anwendung; Fachbuchreihe Schweißtechnik, Band 133; DVS Verlag, Düsseldorf; 1997 Ungermann, D., Schneider, S.: Welded threaded studs with a tensile strength of 800N/mm², Schlussbericht FOSTA Forschungsvorhaben P787; published by FOSTA