WELDABILITY OF NIOBIUM MICROALLOYED STEELS FOR STRUCTURAL APPLICATION

Transcription

1 WELDABILITY OF NIOBIUM MICROALLOYED STEELS FOR STRUCTURAL APPLICATION BY THIAGO DE SOUZA AMARAL*; LEONARDO MAGALHÃES SILVESTRE^ SYNOPSIS Niobium microalloyed steels have been shown to be an excellent solution for structure fabrication. Employing concepts developed for the oil and gas industry, the niobium microalloyed steels deliver high strength and high toughness with considerably low carbon content. However, the definition of the real welding needs and the benefits of applying this family of steels is not well described in the welding specifications most commonly used in the construction sector. This paper demonstrates an example of real weld application in a HSLA steel class 65 ksi and a HSLA steel class 50 ksi. It was possible to determine using thermal simulations the recommended energy range of the weld more accurately, including the need or not of preheating, and show that they were comparable with real welds. The use of niobium HSLA steel shows benefits that include cost savings resulting from the elimination of preheating, reduction in material use and increase in the process speed. Keywords: HSLA Steel, Niobium, Welding, Cost Effective, Standards. * Quality Engineer, CBMM, Araxá, Brazil ^ Manager - Structural Materials Technology, CBMM, São Paulo, Brazil 1

2 Introduction: According to the World Steel association, about half of all steel produced worldwide goes to the construction industry. An increase in industrial activities, infrastructure projects driven by urbanization, population growth and replacement of aging infrastructure will ensure that strong demand for steel will continue. Indeed, the world steel consumption is expected to rise from 1500 million tons in 2000 to approximately 2800 million tons by the year The use of a higher strength steel can enable substantial savings in structural weight and material costs. Although high strength steels are more expensive than conventional structural steels, the price increase is less than the rate at which the strength increases. The reduction in weight leads to cost savings related to foundations, welding, fabrication, transportation and assembling in place. Due to this reality, high strength structural steels are increasingly being applied in modern construction. Advances in metallurgy over recent decades involving carbon reduction, microalloying and thermomechanical processes control have led to steel grades with higher strength, higher toughness and improved weldability for structural applications. This improved generation of steels provide a superior answer to current challenges worldwide, especially in leaning structures, reducing emissions, increasing safety and saving costs. E.g., microalloyed structural steels employed in building construction, result in leaner structures and faster construction with lower raw material demands. Or steels with high toughness at temperatures down to -60 C allow application of such steels in low temperature areas without risk of brittle fracture while saving money for expensive Ni-alloyed grades instead [1]. High strength microalloyed steels are steels whose properties have been modified by adding a small amount of an alloying element (usually less than 0.10%). Niobium is the solution when both increased strength and improved toughness are required. The economic benefits associated with using such small additions that confer significant improvements to mechanical properties have led to the growing popularity of microalloyed steels in the market. However, conservatisms in design codes often hinder full exploitation of these steels. The following examples will help user to recognize advantages, not only in terms of economic gain but also in terms of sustainable and environmental friendly building. The latter becoming more and more important for emerging countries with high growth rates. Past works and results in structures: A first construction study was performed by Silvestre et al.[2] in 2012 concluding that the niobium steels used to construct an industrial building for the new Sintering plant in CBMM facilities located in Araxá, Minas Gerais, Brazil provided the following benefits: Weight savings = 22%. Carbon dioxide emissions savings = 21.7% reduction or kg of CO 2. Energy savings = 21% reduction or GJ. Cost savings = 17% of cost of the structure. 2

3 This already demonstrate the continuous pursue of more sustainable constructions, being safer against fracture and generating lower costs while keeping up with modern design developments. The process started, however requires more steps and applications to convince designers worldwide. A second work was presented in HSLA steel seminar in 2015 [3] applying ASTM A572 grade 65 steel in combination with grade 50 as an optimized engineering solution for CBMM s new industrial building to house the company s dephosphorization plant in Araxá, Minas Gerais, Brazil. As a result, it was shown an additional benefit of: Weight savings = 10%. Carbon dioxide emissions = 9% reduction. Energy savings = no preheating welding process necessary. Cost savings = 5% of the cost of the structure. Most of the grade 65 (YS > 65 ksi) steel was applied in the main columns since it delivers advantages related to high load levels and good interlocking provided by platforms. In the platforms, calculations also demonstrated a benefit of using grade 65 in the main beams with sections up to 600 mm in height. The grade 65 plates were supplied by a Brazilian producer using thermomechanical controlled processing with a fast cooling rate to produce a microstructure composed primarily of bainite, fine pearlite and polygonal ferrite (Figure 1). Figure 1: Microstructure of ASTM A572 grade 65, nital 4% and 500x magnification Additional benefits in reduction of costs due to logistic, plant assembling and maintenance are expected but not yet fully measured. Not only cost but safety can also be improved when reducing the capacity need of the equipment when assembling the parts, like cranes, as well as the possible fatigue occurred during transportation or vibration during the structure life. The higher stability of the structure in emergency fire case is also an important must. 3

4 The benefits of Niobium in microalloyed steels: Strengthening mechanism of niobium microalloying Niobium effectively controls the microstructure of steel and small amounts of this element can refine the grain size of hot rolled steels. The effects of niobium as a microalloying element are schematically illustrated in Figure 2 [4] for reheating temperatures up to 1200 C. Figure 2: Niobium precipitation at each stage of heating, rolling and cooling and its effect on refining ferrite grains and precipitation hardening. [4] Achieving higher strength Fine grain size is an essential requirement for steels to obtain strength and toughness properties. Figure 3 shows the strong effect that grain size (d) has on mechanical yield strength (σy) in carbon-manganese steels. 4

5 Figure 3: Relationship between grain size and yield strength. [5] Reducing grain size generates a robust increase in strength for all carbon contents considered. This is even stronger with niobium microalloying due to its effect of preventing recrystallization during controlled rolling. Additionally, niobium precipitates as very fine particles, further contributing to increased strength. High strength and increased toughness simultaneously A study of ASTM 992 beam (S355) based on industrial heats led to the commercialization of low-carbon niobium-bearing beam in place of a vanadiumbearing beam [6]. The addition of niobium refines the grain and improves toughness. Structural beams containing niobium microalloy exhibit double the impact strength at room temperature compared to a vanadium microalloy system at similar condition (carbon, sulfur, phosphorous and nitrogen levels and cooling rates) as illustrated in Figure 4. 5

6 Figure 4: Charpy V-notch impact strength comparison niobium vs. vanadium. [6] Welding high strength low alloy steels (HSLA): The process of welding a HSLA steel is not new and has been described in different standards like ISO : Quality requirements for fusion welding of metallic materials - Part 3: Standard quality requirements or in some other welding groups like American Welding Society with the AWS D1.1/D1.1M: Structural Welding Code-Steel. An important point in the welding standards is the need to group materials in classes related to properties, especially according to different mechanical properties. However, these methods bring a more complicated approach when working with materials with different production methods like TMCP (Thermo-Mechanically Controlled Processed) or QT (Quenched & Tempered) and same classification or even with the variation of alloying elements levels. The differences can also impact some manufacturing procedures needs in welding as preheating. Borba et al. [7] showed some interesting results comparing two steels for wind tower application - Sincron EN S355 M (TMCP) and ASTM A572 grade 50 (QT) being welded for same process of high energy. It is showed that the microstructure of a TMCP steel is much refined and has a lower carbon and alloying elements content, without loss of the mechanical properties in the base metal. Once they were welded, the impact Charpy-V results showed a big gap in the Heat Affected Zone (HAZ) absorbing much more energy in the TMCP steel then in the QT steel as showed in Figure 5. As explained by Borba et al. the result is related to a higher heterogeneity in the tested samples and being the notch test with small opening less sensitive to the different microstructure in the HAZ. The result is related to the region with lower toughness when it has at least 30% of the notch. 6

7 Figure 5: Hardness mapping in the region of Charpy-V notch samples and absorbed energy in weld metal at -20 C (MS) and HAZ at -30 C (LF1 and LF5) [7] In applications, especially those operating in low temperatures, this difference can be of great importance and the explanation for a better result in the TMCP steel according to Borba et al. [7] refers to a smaller prior austenitic grain size and lower carbon and carbon equivalent content. Another influence occurs due to the fragile second phase constituents formed in bigger austenitic grains size. The extension of the modified region is also very important for the properties, being showed in the Figure 6 that there is a visible difference in the Coarse Grain Heat Affected Zone (CGHAZ) occurred due to the different start and ending in metallurgical transformations of steels with different compositions. 7

8 Figure 6: Crystallographic orientation map in evaluated steels HAZ by EBSD. [7] Stalheim & Muralidharan [8] also showed in their different Continuous Cooling Transformation (CCT) simulations how much the elements can affect the phase transformation in steels. As an example, the carbon increase moves the ferrite, perlite and bainite transformation areas to the right (increasing the time for these transformations) allowing a higher amount of martensite to be formed. The addition of elements as molybdenum, nickel and niobium reduce the kinetics of ferrite and perlite formation allowing a higher amount of bainite to be formed. Each element affects the CCT diagram in a different way, by nature or magnitude. Real case studies: The majority of structural steel buildings in Brazil are made of conventional carbon manganese steels (ASTM A36) or grade 50, reaching up to 345 MPa of minimum yield strength. Therefore, a new project, applying grade 65 steel with minimum yield strength of 450 MPa required an evaluation of weldability and a qualification of the welding procedures. The H-beams section need to be produced using the submerged arc welding process (SAW). The as-rolled sections had not previously been produced with grade 65 in the Brazilian market. The joints of the SAW process (sections) and FCAW process (repair) were projected and qualified on a Welding Procedure Specification (WPS) according to AWS D1.1 Structural Welding Code Steel Standard. The welders were requalified according to new consumables and materials, which are assumed as critical variables, and which changed from groups into the welding standard. The consumables, for example, shift from 70 ksi class in the grade 50 to 80 ksi class in the grade 65. 8

9 A flux-cored arc welding (FCAW) process was used for the connections between grade 65 and 50 at the base of the sections conform to the existing welding procedures since the process for welding dissimilar materials follows the requirements for the lower specification material that were currently performed. ASTM A572 grade 65 The plate supplied by the Brazilian company was submitted to thermal simulations using a dilatometer (DIL805A/D from TA Instruments) and a thermomechanical simulation (Gleeble 3500 from Dynamic Systems Inc.) in order to understand the possible problems during the production of the H-beams and real welds were also performed. This bainitic steel present a very big welding range due to the low carbon, low alloy and TMCP production process. No concern was raised after the analysis, but two main questions need to be answered: 1 Is this steel more difficult to weld? The characteristics of the Grade 65 steel is described in tables 1 and 2. A remark is that the Cu content was residual and not added. Table 1: Typical composition of low carbon low alloy grade 65 steel studied (wt %) C (%) Si (%) Mn(%) P (%) S (%) Nb (%) Cu (%) N (%) V (%) Ti (%) CEV Table 2: Mechanical properties of the low carbon low alloy grade 65 steel studied YS (MPa) TS (MPa) El (%) (J) Based on the chemical composition presented in table 1, it is possible to start the evaluation of the steel weldability according to the Granville diagram (Figure 7), which evaluates how susceptible a certain composition is to present welding defects from hydrogen and/or cold cracking. The grade 65, does not present high susceptibility to have hydrogen induced cracking or high hardness cracking as can be seen from Figure 7 (steel classified in Zone I). The responsible for this favorable behavior is the grain refinement strengthening that allows a reduction of absolute carbon content level and consequently, the reduced carbon equivalent. It is achieved using a niobium alloy design and TMCP process in ASTM A572 grade 65 production. 9

10 Figure 7: Granville diagram In order to demonstrate the weldability of the steel with higher strength, complementary tests were performed according to table 3. A general overview of the results is presented in table 4 and some details are given in the following Figures 8 to 12. Table 3: Welding process details Process Joint detail Details SAW butt joint Heat input = 70 kj/cm Interpass < 270 C SAW T joint Heat input = 12 kj/cm 8 mm Interpass < 240 C FCAW butt joint Heat input = 30 kj/cm Interpass < 240 C 10

11 Table 4: Test results and conclusions of weldability tests in high strength steel Test Result Conclusion Reduction in area >60% Greater than 30%, no lamellar (Through-thickness tearing susceptibility in the steel. tensile test) Tensile strength 570 MPa (SAW) Matches A572 Grade 65 Hardness Bending test Impact test (at 23 ºC) 600 MPa (FCAW) 203 HV (SAW HAZ) 213 HV (FCAW HAZ) SAW and FCAW no fracture in the weld 293 J (SAW HAZ) specification (minimum 550 MPa). The hardness results are below 350 HV and do not indicate hard zones and susceptibility to cracks from martensitic formation. Even when bent, the welded joint does not crack, qualifying the welding procedure. While not a requirement in structural steels in Brazil, the result shows the welded material has good toughness at typical regional temperatures (Araxá, Brazil). Hardness mapping indicated that there was no hard zone formation after SAW and FCAW (Figures 8-10). Figure 8: Hardness measurement of grade 65 specimen submitted to SAW Figure 9: Hardness measurement of grade 65 specimen submitted to FCAW Figure 10: Hardness map of SAW T joint specimen in grade 65 11

12 Macro and microstructural analyses were performed in SAW and FCAW specimens, Figures 11 and 12, showing that no segregation or martensitic formation was found in the weld. Figure 11: Macro and microanalysis of the SAW welded regions Figure 12: Macro and microanalysis of the FCAW welded regions 2 - A preheating procedure, described as necessary in the standard AWS D1.1 (used as reference by the structure manufacturer) is really necessary in the low carbon steel? To answer this question two coupons with ASTM A572 grade 65 with 25.4 mm thickness were prepared and welded with SAW butt joint process (described in table 3) in the same environmental conditions. The only difference was the preheating performed till 150ºC in one of the plates. The result is presented in the table 5. 12

13 With preheating Without preheating Table 5: Microstructure and mechanical results in welds of ASTM A572 grade 65 with and without preheating Microstructure Mechanical results Hardness HV0,7 = 170 Charpy C = 160 J Transition temperature 27J= - 52 C Tensile Strength: 565 MPa Hardness HV 0,7 = 180 Charpy C = 150 J Transition temperature 27J = - 33 C Tensile Strength: 575 MPa The expected difference in the two procedures is the change in the cooling time due to a higher heat already existing in the sample. This heat change and impact tend to be higher in a real beam situation as the dimensions and heat transfer will be bigger but due to the high heat input and speed of the SAW weld it can still be comparable. The results are aligned with the thermal simulation results where the material preheated presented in the end bigger ferritic grain size in the CGHAZ and also more perlite volume due to a slower cooling and more time for the austenitic grain growth. The TS and hardness are not changed due to the high weldability of the steel due to the low carbon and carbon equivalent content, but is important to note the big drop in the toughness and transition temperature. This result shows that not only the preheating does not seem appropriate in terms of process efficiency (there was no change in the possibility of excessive softening or MA formation), but can also be deleterious if the structure is later submitted to cyclic efforts or low temperature environment. The preheating procedure for this steel is therefore not necessary even if indicated in the AWS D1.1 standard in the preapproved tables. It is also important to remark that the Annex I of AWS D1.1 has a methodology to identify preheating needs according to the real carbon content and procedures variables that confirm the results of this study. 13

14 ASTM A572 grade 65/50 A weld joint using both steels grade 65 and grade 50 was also performed in order to understand possible effects in the weldability of both steels (25.4 mm plate). The grade 50 has a higher carbon content as it is a QT steel comparing to the TMCP steel grade 65. The chemical composition is showed in the table 6 and 7. Table 6: Composition of low carbon low alloy grade 50 steel studied (wt %) C (%) Si (%) Mn(%) P (%) S (%) Nb (%) Cu (%) V (%) Ti (%) CEV ND Table 7: Composition of low carbon low alloy grade 65 steel studied (wt %) C (%) Si (%) Mn(%) P (%) S (%) Nb (%) Cu (%) V (%) Ti (%) CEV ND As a consequence, there is a higher peak in hardness in the grade 50 when welded, as showed in the figure 13. This higher hardness represents a zone with the lowest toughness and with high risk of cracks due to possibility of formation of martensitic islands. Figure 13: Hardness measure of FCAW welded regions in different ASTM A572 grades (left Grade 65 and right Grade 50) The most important point in this case is to understand that having a lower grade does not mean an easier weld or a steel that can support better high heat inputs. Conclusion: The correct use of welding procedures is very important in the project and fabrication of structures. It has been shown how the material technology has changed in the past few years and how the new niobium steels can allow advantages in terms of leaner structures and easy welded joints. 14

15 As a comparison in the project described by Leonardo et al. [3] and using as a reference the steel structure manufacturing data and IPCC guidelines, in a project of roughly 450 tonnes of welded beams, the non-use of preheating procedure when using SAW for the H beans of ASTM A572 Grade 65 steel reduced the consumption of 233,030 m 3 of oxygen and 146 tonnes of LPG resulting process economy of roughly US$ 250,000.00, reduction in beam welding time and a non-emission of ton of CO 2 e (carbon dioxide equivalent) increasing the sustainable value of the building in addition to all points already described in the introduction as the 7% total material cost reduction comparting with a ASTM A572 Grade 50 structure. Much has to be done and the correct knowledge coming to all levels in the structural supply chain is one of the challenges to be overcome in the next years. The technologies already exist and in some levels are just not being used in the most effective way. References: 1. P. Langenberg, Quality matters material requirements in Eurocode 3 (EN1993) in view of component safety, Value-added Niobium microalloyed 2. L. Silvestre, R. Pimenta, High Strength Steel as a Solution for the Lean Design of Industrial Buildings, Journal of Materials Research and Technology, May, 2012 Brazil. 3. L. Silvestre, P. Langenberg, T. Amaral, M. Carboni, M. Meira, A. Jordão, Use of Niobium High Strength Steels with 450 MPa Yield Strength for Construction, HSLA Steels 2015, Microalloying 2015 & Offshore Engineering Steels 2015, 2015, Hangzhou, Zhejiang Province. 4. K. Ichikawa, M. Fujioka, R. Uemori and A. Yoshie, Progress in Thermomechanical Control Process Steel Plates, 2011 International Symposium on the Recent Developments in Plate Steels, AIST, June, 2011, Winter Park, Colorado, USA. 5. W. B. Morrison, Overview of Microalloying in Steel, The Proceedings of the Vanitec Symposium, Guilin, China S. G. Jansto, Niobium-Bearing Plate Steels for the 21st Century, 2011 International Symposium on the Recent Developments in Plate Steels, AIST, June, 2011, Winter Park, Colorado, USA. 7. T. M. D. Borba, R. S. Oliveira, H. R. Gama, M. F. O. Caizer, L. O. Turani, S. G. Jansto, Avaliação da Soldabilidade do Aço Sincron EN S355M Aplicado na Fabricação de Torres Eólicas com Processo de Soldagem de Alta Deposição Soldagem & Inspeção. 2017; 22(4): , Brazil. 8. D. G. Stalheim, G. Muralidharan, The Role of Continuous Cooling Transformation Diagrams in Material Design for High Strength Oil and Gas 15

16 Transmission Pipeline Steels. Proceedings of IPC 2006, 6th International Pipeline Conference, Calgary