Abstract
In this research, the large-heat-input welding conditions optimized to use the rear plate and the high-performance steel of SM570TMCP, a new kind of steel suitable for the requirements of prospective customers, are proposed. The goal of this research is to contribute to securing the welding fabrication optimized to use the high-strength steel and rear steel plates in the field of construction industry in the future. This research is judged to contribute to securing the welding fabrication optimized to use the high-strength steel and rear steel plates in the field of construction industry in the future.
1. Introduction
Since the rigidity and strength required for members increase according to recent trends of the large space and high rise such as diagrid [1–4] of architectural structures and the large span of steel bridges, steel materials gradually tend to aim at high strength, extremely thick plate, and high performance, and its application is introduced by Schroeter and Lehnert [5]. Such steel materials, while structured, require mutual connection between members and inevitably accompany welding during the work. Thus, in order to secure the safety of structures and the efficiency of work, it is essential to improve welding quality and secure reliability.
According to the questionnaire survey conducted by Korea Research on the basis of a contract with POSCO and Research Institute of Industrial Science and Technology (RIST) regarding the demand analysis of high-performance steel types [6] targeting over 500 experts from academia, structural designers, and construction companies, most of the experts preferred, as performance standards of high-performance steel kinds, (i) a high design standard strength; (ii) no reduction in the design standard strength in the over 40 mm thickness; and (iii) excellent weldability. Out of these preferences, the first and second preferences are fundamental requirements which may cause troubles in securing the expected performance of steel kinds, while the third one may be regarded as a secondary requirement which can be preferentially improved on the basis of the weldability data of existing general steels.
In response to such requirements, POSCO has developed PILAC-BT45 steel in 2004, which is denoted as SM570TMC, a 600 MPa-class new steel material, for application to architectural structures, whose weldability is improved by reducing the carbon equivalent a great deal, while raising the design standard strength of steel materials, with the thermomechanically controlled (TMC) process. As representative researches related to SM570TMC, Chang et al. [7] introduced experimental and numerical investigations on residual stresses of SM570TMC. Lee et al. [8] provided characteristics of high temperature tensile properties and residual stresses in welded joints.
In general, for high strength and extremely thick steel materials, alloying elements are added, which accompanies the quality nonuniformity of steel materials. In the welding work of using such steel materials, the reduction in the welding constructability and the structural performance of welded areas has been known to be inevitable. Lately, in case of fabricating steel structures, the application of such a large-heat-input welding method for which Kojima et al. [9] developed high HAZ toughness steel plates of box columns, as SAW, has increased to improve the productivity. In case of applying steel types of high strength and extremely thick plate under increasing demand, the verification of such weldability is always essential. The rear plate is mostly used as a structure assembled by cutting and welding and so its weldability should sufficiently be considered. In addition, in case of welding at low temperatures, the welding method should also be reviewed. Meanwhile, though the large-heat-input welding method has an advantage that it improves work efficiency, the amount of heat input, groove shape, and welding method should be sufficiently reviewed in selecting large-heat-input steel materials.
In this research, experiments were conducted by taking the amount of heat input, including the numerical value specified for the rear plate by the fabricator, and the kind of steel (SM490TMC, SM570TMC) as variables and applying the welding methods of ESW, SAW, and FCAW, in which Kou [10] dealt with fusion welding processes of steel, Schwartz [11] described metal joining manual in advance, and Song et al. [12] introduced single-side resistance spot welding as a basis of welding methods, and the large-heat-input welding conditions are presented in accordance with the experimental results. SM570TMC as newly high strength steel is alternative which is the most appropriate to build mega steel structures, as it may save material quantity such as reduction of steel thickness. For the purpose, it is necessary to execute large-heat-input welding, but there is rarely detailed information of the welding condition of SM570TMC.
In this research, in order to present the optimum large-heat-input welding conditions of SM570TMC high-performance steel, the following sequential verification processes were performed in comparison with the existing characteristics of SM490TMC: cutting inspection of steel materials, review of existing welding conditions, amount of heat input for SAW and FCAW, establishment and verification of large-heat-input welding conditions through tensile, flexural, impact, macro, and hardness performance experiments, and preparation of large-heat-input welding manual.
2. Fabrication of Large Heat Input Test Specimen
2.1. Fabrication of Welding Specimen for SAW and FCAW Welding Test
Thick steel plates are basically used as the steel plates for large-heat-input welding test specimen for SAW and FCAW [13]. The thicknesses of SM490TMCP 40 mm and 80 mm and SM570TMCP steels are 40 mm, 80 mm, and 100 mm, respectively. For welding parts, single V Groove, root opening 10 mm, root face 0, and angle 30° are applied. For preheating [14], normal temperature is applied to specimen with thickness below 50 mm, and temperature above 50°C is applied to thickness 100 mm, respectively. Welding test steel plates, shapes of welding specimen, and specification of specimen are in accordance with AWS D 1.1 and KS D 3503 code [15], and these are shown in Figures 1 and 2, respectively.
2.2. Fabrication of ESW Welding Specimen
Thick steel plates are basically used as the steel plates for large-heat-input welding test specimen for ESW. The thicknesses of SM490TMCP and SM570TMCP steels are 35 mm. Welding parts details are in accordance with AWS D 1.1 and KS code, which are shown in Figure 3.
For ESW, a CES welding machine as shown in Figure 4(a) is used, and its status of attachment before welding and after welding are shown in Figures 4(b) and 4(c).
(a) CES welding machine
(b) Status of attachment before welding
(c) Status of attachment after welding
3. Experimental Results of Large-Heat-Input Welding Specimen
3.1. Large-Heat-Input Welding Test Details and Test Set-Up Procedures
As a consideration during planning of large-heat-input welding, it is necessary to confirm the weldable thickness in case of ESW [16, 17]. For the case of TAG Company of Japan, in case of using two nozzles, a weldable thickness of 100 mm has been reported to be feasible. In Korea, the optimum weldable thickness of CES welder [18] is 60 mm, the limit of quality assurance. In this research, the thickness of parent materials is based on 35 mm.
The conditions of large-heat-input welding tests established on the basis of the above contents in this research are given in Table 1.
Large-heat-input welding test according to AWS D 1.1 [19] includes visual inspection, NDE, cutting inspection, sectional tensile test, side face bending test, impact test, and macro and hardness test. Test types carried out in this study are summarized in Table 2.
Tables 3 and 4 show the specimens for large-heat-input welding of SM490TMCP and SM570TMCP which are carried out in this study.
3.2. Tensile Test Results
The results for tensile tests of large-heat-input welding for SM490TMCP and SM570TMCP are shown in Tables 5 and 6, respectively. In case of ESW, SM570TMCP requires 1.5 to 2 times the amount of heat input of SM570TMCP and has shown satisfactory yield strength. The elongation at break of SM570TMCP, high strength steel, is found to be about 25, lower than that of SM490TMCP 33, a general steel type.
It has been found that, in case of SAW, an automatic welding, at the same amount of heat input of 50 or 70 KJ/cm, as the plate thickness of SM490TMCP increases from 40 mm to 80 mm, the tensile strength falls from about 550 to about 520. It has also been found that, at the same amount of heat input of 70 KJ/cm, as the plate thickness of SM570TMCP increases from 40 mm to 80 mm, the tensile strength falls from about 660 to about 620. Therefore, in case of SAW, it is judged that as the thickness increases, it is necessary to increase the amount of heat input from 70 to about 100 in the aspect of securing the safety of welded areas.
In case of FCAW, a semiautomatic welding, both SM490TMCP and SM570TMCP show the tensile strength without deviations regardless of the plate thickness, demonstrating the performance stability for welding fabrication of specimens. And, in all cases, breakups occurred at the base or welded areas.
Figure 5 shows the tensile test result of SAW and FCAW for SM570TMCP with the thickness of 80 mm and 100 mm. As can be seen, all tensile test specimens collapse at the welding part, not the parent metal part, after making plastic hinge behaviors. It verifies the superiority of welding connections.
(a) SM570TMCP-80 mm-SAW
(b) SM570TMCP-80 mm-FCAW
(c) SM570TMCP-100 mm-SAW
(d) SM570TMCP-100 mm-FCAW
3.3. Side Face Bending Test Results
According to varying input heat and thickness, side face bending test results of SM490TMCP and SM570TMCP are shown in Figure 6. These results are satisfactory in general; however, welding defects are found in SM490TMCP-40 mm-SAW input heat 70 and it shows manufacturing defect of the specimen.
(a) SM570-80 mm-SAW input heat 70
(b) SM490-35 mm-ESW input heat 60
(c) SM570-35 mm-ESW input heat 80
(d) SM490-40 mm-SAW input heat 70
(e) SM490-80 mm-FCAW input heat 50
(f) SM490-40 mm-SAW input heat 70
As can be seen in Figures 6(a), 6(d), and 6(f), large heat input of 70 can be used for the thick and high strength SM570TMCP specimen of SAW, but SM490TMCP of SAW may produce welding error owing to too large heat input and relatively thin specimen condition.
3.4. Impact Test Results
Charpy impact characteristics of welding parts of weld metal (WM), hard zone (HZ), and fusion line (FL), in case SM570TMCP for both SAW and FCAW meet the requirements in the standards of the red dash line, which are shown in Figure 7. As can be seen, SM570TMCP with thickness of 100 mm also produces large impact energy at WM, HZ, and FL.
(a) SAW
(b) FCAW
Figure 8 shows impact energies of SM570TMCP and SM490TMCP of ESW at WM, HZ, and PL. As can be seen, impact energies of WM and HZ of the red circle line are not satisfactory to the standards. It can be found that welding conditions such as heat input, in especial, for ESW should be controlled to take manufacturing quality, regardless of steel type.
(a) SM570TMCP of ESW
(b) SM490TMCP of ESW
Figure 9 shows the impact test result of SM570TMCP specimen. Requested requirements by the standard are those for SM490TMCP 27 J and, SM570TMCP 47 J.
(a) SM570TMCP-40 mm-SAW
(b) SM570TMCP-40 mm-FCAW
3.5. Results for Macro and Hardness Tests
The cross sections of specimens for macro test results of ESW welding for SM490TMCP and SM570TMCP, to which heat input amounts (kJ/cm) of 56 and 120, respectively, were applied, are shown in Figure 10.
Figure 11 shows the results for hardness distribution of the base metal (BM), HAZ, and welded metal (WM) areas during SAW, FCAW, and ESW welding of SM490TMCP and SM570TMCP. As shown in Figure 11, in case the amounts of heat input for SM570TMCP and SM490TMCP are large, the upper limit of hardness for HAZ part turned out to be large, and in case the amount of heat input are the same, the thicker the plate, the larger the calculated hardness value. It is natural to obtain these results, which means that the provided welding conditions are appropriate overall, demonstrating that the welding fabrication by SAW, FCAW, and ESW is excellent.
(a) SM570-40 mm-SAW (70 kJ/cm)
(b) SM570-40 mm-SAW (100 kJ/cm)
(c) SM570-80 mm-SAW (70 kJ/cm)
(d) SM570-100 mm-SAW (70 kJ/cm)
(e) SM570-40 mm-FCAW (60 kJ/cm)
(f) SM570-80 mm-FCAW (60 kJ/cm)
(g) SM570-100 mm-FCAW (60 kJ/cm)
(h) SM570-35 mm-ESW (60 kJ/cm)
(i) SM490-40 mm-SAW (50 kJ/cm)
(j) SM490-40 mm-SAW (70 kJ/cm)
(k) SM490-80 mm-SAW (50 kJ/cm)
(l) SM490-80 mm-SAW (70 kJ/cm)
(m) SM490-40 mm-FCAW (50 kJ/cm)
(n) SM490-80 mm-FCAW (50 kJ/cm)
4. Conclusions and Remarks
In this research, embodied are the proper welding conditions of large-heat-input welding as an efficient welding method for performance improvement required for high strength and high performance of architectural steel materials. Welding fabrication tests were conducted for SM490TMCP, a steel kind which is in general used a lot, and SM570TMCP, high strength steel. An optimum amount of heat input, especially, was quantitatively analyzed for the case of rear plates with high strength properties.
In this research, conducted was the performance evaluation of the welded areas based on the welding fabrication methods of SAW, FCAW, and ESW, and proposed are the large-heat-input welding conditions optimized to use the rear plates and high performance steel of SM570TMCP, a new steel kind suitable for the requirements of prospective customers.
In especial, welding conditions such as heat input for ESW including SAW and FCAW have to be controlled to take manufacturing quality, regardless of steel type. Large-heat-input condition can be appropriately used for the thick and high strength SM570TMCP steel construction, but normal strength steel such as SM490TMCP may produce welding error owing to too large heat input and relatively thin thickness condition.
This research is judged to contribute to securing the welding fabrication optimized to use the high strength steel and rear steel plates in the field of construction industry in the future.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This research was supported by the MKE (The Ministry of Knowledge Economy), Korea, under the Convergence-ITRC (Convergence Information Technology Research Center), supervised by the NIPA (National IT Industry Promotion Agency), (NIPA-2013-H0401-13-1003) and a grant (code# 2010-0019373, 2011-0010300, 2012R1A2A1A01007405, 2013R1A1A2057502, and 2014R1A1A3A04051296) from the National Research Foundation of Korea (NRF) funded by the Korea government.