TMCP Steel Research Articles

Effect of Welding Heat Input on the Simulated Heat-Affected-Zone Toughness for Yield Strength 390MPa Class TMCP Steel Plate

In this study, the influence of heat input on the microstructure and toughness of the simulated heat-affected zone (HAZ) of YP390 MPa grade steel with a thickness of 80 mm was investigated. The weld coarse-grained HAZ was simulated using a Gleeble 3500 simulator, with heat input levels ranging from 30 to 650 kJ/cm. Following the HAZ simulation test, the microstructures and Charpy impact properties were examined. For heat inputs ranging from 30 to 50 kJ/cm, the microstructure consisted of bainitic ferrite/acicular ferrite, resulting in an impact toughness exceeding 200 J at -20 ℃. Conversely, in the higher heat input range of 130 to 650 kJ/cm, the simulated HAZ exhibited a lower impact toughness value due to the formation of coarse grain boundary ferrite. A high proportion of acicular ferrite with minimal grain growth was found to be the key metallurgical factor contributing to the improvement of impact toughness.

Effect of Welding Process on Microstructure and Impact Toughness of Welds in High Nitrogen TiN Steel

The effect of welding process on the microstructure and impact toughness of high-nitrogen TiN EH36-TM steels was evaluated. In the weld metals of SSAW and TSAW, GBF was created along PAGB, and AF and a small amount of PF were observed in the grain. In TSAW with low basicity and high heat input, GBF was remarkably developed, and intragranular AF was more coarse. In the EGW weld metal, GBF was hardly generated and composed of intragranular AF and PF. As the heat input increased, the size of PAGB was increased and PF was developed at the FL, and in FL+1, the grain size of AF and PF was coarsened. Meanwhile, the FL of EGW showed a similar size without grain size growth compared with FL+5. Among the three welding processes applied, weld metals of SSAW and EGW well met the values required by the classification for the EH36 steel, but the weld metal of TSAW was insufficient. HAZ of high-nitrogen TMCP steels satisfy the value required by the classifications without significant change even when welding at various input heat conditions, and in particular, the weld metal of the high heat-input EGW showed relatively stable values from FL to FL+5.

Investigation of fracture behavior and deformation capacity of high-strength steel butt-welded joints

Performance-based design of high-strength steel welded structures requires accurate prediction of their load-bearing capacity as well as deformation capacity. There have been several studies undertaken on the load-bearing capacity of high-strength steel butt-welded (HSSBW) joints, but very few studies are performed on their deformation capacity. This study investigates the fracture behavior as well as the deformation capacity of double-V HSSBW joints with strength-matching welding materials, taking into account the effects of different heat inputs, steel strengths, and steel processing methods. To evaluate the deformation capacity, fracture initiation criterion, and fracture propagation model of four zones within the butt-welded joints, including hardened heat-affected zone (HHAZ), softened heat-affected zone (SHAZ), welding material zone (WM), and base metal zone (BM), are first calibrated from experimental results and numerical analysis results. The calibrated material models are validated against the test data. Numerical analyses are then performed to predict the fracture behavior and deformation capacities of the welded joints utilizing the verified material models. According to the analysis results, heat inputs no more than 1.9 kJ/mm impose a limited influence on the fracture behavior of each zone within the double-V HSSBW joints. If the heat input is below 1.9 kJ/mm, QT steel welded joints exhibit a higher deformation capacity than TMCP steel welded joints. When compared to the tension specimens made of pure base metal, the reduction of the deformation capacity of the double-V HSSBW joints was even more than 30 % and it is more significant than the reduction of load-bearing capacity, therefore, evaluation of the deformation capacity of the welded joints is very essential for the performance-based design of high-strength steel welded structures.

SSAW 용접기법이 강재의 용접부 미세조직에 미치는 영향

This study investigated the effects of self-shielded arc welding on the microstructures of welded parts in laminar-cooled-thermo-mechanical-controlled-process (LC-TMCP), titanium nitride (TiN), and B grade steels. The microstructure of the fusion zone was the same in all three samples, consisting of ferrite (grain boundary ferrite (GBF)+acicular ferrite+ferrite side plate) and pearlite phases, and the LC-TMCP and TiN steels had a higher volume fraction of ferrite and smaller grain sizes than those of the B grade steel. In all three samples, the microstructure near the fusion line consisted of ferrite and pearlite phases with a small amount of bainite, and the grain sizes of the LC-TMCP and TiN steels were smaller than those of the B grade steel. In addition, the TiN steel was smaller than the TMCP steel, and the volume fraction of ferrite (GBF, etc.) was the highest in the B grade steel and lowest in the TiN steel. The microstructure of the heat-affected zone included a phase in which a small amount of bainite was present in the ferrite and pearlite phases near the fusion line (1 mm) in all three samples. However, the shape was different from that of the base material, and the grain size was the largest in the B grade steel and smallest in the TiN steel. The volume fraction of ferrite (GBF, etc.) was the highest in the B grade steel and lowest in the TiN steel. All three samples had the same shape as the base metal phase at 5 mm from the fusion line. However, grain was growth.

CTOD Evaluation of API X80MO PSL2 TMCP Steel Welded Joints Used in Offshore Structures Based on API RP 2Z

Offshore structures and equipment, used both for oil extraction and wind power generation, are constantly exposed to extreme application conditions, such as low temperatures, corrosion, and cyclic loading. Thus, to ensure the integrity of such components, the use of materials with excellent mechanical properties combined with good weldability is required. In this sense, the main committees, institutes, and normative societies have created pre-qualifications, codes, and programs to evaluate and certify the weldability of steels applied in the offshore industry. API RP 2Z prequalification is one such standard that is designed to ensure that steels developed for these applications have excellent fracture toughness in the heat affected zone (HAZ). In this context, the effect of heat input on the HAZ toughness of welded joints of API X80MO steel plates was evaluated, using the CTOD (Crack Tip Opening Displacement) parameter. It was found that this steel, of 37 mm thickness, welded with heat inputs of 0.8 and 4.5 kJ/mm, presented weldability characteristics and fracture toughness behavior suitable for API RP 2Z standard specification. Therefore, API X80MO steel plates, produced by controlled rolling followed by accelerated cooling, is an option for application in the construction of offshore structures and components.

Butt welding of thin sheets of S960MC steel

The paper presents the application of MAG welding to TMCP steels (thermo-mechanically controlled processed) grade S960MC and 3 mm thick. In the analyzed joints, the research focused on their mechanical properties and changes in the heat-affected zone (HAZ) that occur in this type of steels. The hardness and tensile strength tests carried out showed a significant decrease in the properties of the joint compared to the declared values of the base material and the filler material used in the tests. In the case of hardness, it was a decrease of 34% in HAZ and by 15-21% in relation to the strength limit. Changes in HAZ properties of a joint correlate with changes in its structure.

Texture Evolution During Cold Forming of Rectangular Hollow Section

This work aims to study the effect of cold forming on the texture evolution during the manufacturing of rectangular hollow sections. Conventional TMCP steel and a direct-quenched steel in 420 MPa strength level are compared. The texture was characterized at the centerline (S=0) and both surfaces (S=0.8) for base material, four flat side and one corner samples of the rectangular hollow section. The results show that the flat sides of both steels have the minor intensity of ∼{554}<225> and ∼{112}<110> texture components and an intense texture component of ∼{001}<110> in the centerline. Generally, any significant difference between four flat size samples was not found and texture intensities of the conventional TMCP steel were slightly sharper compared to direct-quenched steel. The most important change is observed with the inner corner samples, where the randomly oriented texture in the base material and flat side samples changes to the {110}<111>/<112> shear texture components.

Electron Beam Welding of TMCP steel S700MC

The article discusses tests aimed to determine the effect of electron beam welding on the properties of butt welded joints made in 10 mm thick TMCP steel S700MC. The welding process was performed in the flat position (PA) using an XW150:30/756 welding and surface processing machine (Cambridge Vacuum Engineering). The joints obtained in the tests were subjected to non-destructive tests including visual tests and magnetic particle tests. The joints were also subjected to destructive tests including static tensile tests, bend tests, toughness tests (performed at a temperature of -30°C), hardness tests as well as macro and microscopic metallographic tests. The destructive tests revealed that the joint represented quality level B in accordance with the PN EN ISO 13919-1 standard. The analysis of the destructive test results related to the electron beam butt welded joint (made in steel S700MC) revealed its high mechanical and plastic properties. The toughness tests revealed a decrease in toughness in the HAZ (27 J/cm2) in comparison with that of the base material (50 J/cm2). In addition, the hardness of the HAZ and of the weld increased up to approximately 330 HV; the hardness of the base material amounted to 280 HV.

Development of the PC-GMAW welding technology for TMCP steel in accordance with welding thermal cycle, welding technique, structure, and properties of welded joints

In this paper, the effect of the welding thermal cycle (WTC) on the structure and properties of the S460M steel heat affected zone (HAZ) metal has been studied. In particular, the changes in the mechanical properties vs the HAZ metal cooling rate in the 600÷500 °C temperature range was studied for S460M model samples, heat-treated in accordance with WTCs. The results from the experimental data refers to the static strength, ductility, and impact toughness values at the level of the base metal, as well as to structural changes in the seam. The weld metal and HAZ welded joints of S460M steel, made by pulsed arc welding, have shown adequately high resistance to brittle fracture, and at 20% greater strength. Thus, based on the obtained results, it is recommended to apply pulse-current gas metal welding of the thermo-mechanical cold processingsteel instead of conventional welding.

Critical assessment of the fatigue crack growth rate sensitivity to material microstructure in ferrite-pearlite steels in air and marine environment

This paper presents a critical assessment of the effect of microstructure on the fatigue crack growth rate (FCGR) in ferrite-pearlite (α-P) steels in air and marine environment. This was done by evaluating the resistance of three subgrades of S355 steel, produced by normalized-rolling, NR (S355J2+N) and thermo-mechanical control process, TMCP (S355G8+M and S355G10 + M), to fatigue crack growth (FCG) in air and seawater. The data from the tests were then compared with results of several studies on α-P steels of similar compositions and microconstituents in the literature. It was found that microstructure strongly affected the Paris region of the da/dN vs. ΔK sigmoidal curve in both media. This is contrary to the current belief that microstructure has little or a negligible effect in the linear region of the FCGR curve in air. TMCP and quenched-and-tempered (QT) steels offered better resistance to FCG than NR steel in both media. TMCP steel offered the highest resistance to FCG. The crack path was non-planar with a complex crack front. The tendency for crack deviation, branching, wedging action by metal crumbs and perhaps interlocking majorly explained why the FCGRs of the TMCP and QT steels are lower than that of NR. The result also suggests that ductility and yield stress do not have a strong retarding effect on FCG in α-P steels at low stress intensity factor range (SIFR). This finding has a fundamental implication for analytical and numerical models that wholly depend on the mechanical properties of the steel and assumes plane horizontal crack propagation front in predicting FCGR in steels. Such models are likely to fail in predicting FCGR even in a subgrade of the same grade of steel.

Gas Metal Arc Welding of Thermo-Mechanically Controlled Processed S960MC Steel Thin Sheets with Different Welding Parameters

In this paper are presented results of mechanical properties evaluation of the thin sheets welds made of the S960MC TMCP steel, which were executed using the GMAW procedure with different process parameters. The microstructural changes in the heat affected zone (HAZ) were evaluated, as well. The microstructural observation revealed significant changes in the HAZ and the three main zones, coarse grain, fine grain and intercritical (CGHAZ, FGHAZ and ICHAZ) were identified in the HAZ for both sets of tested welding parameters. Evaluation of the micro-hardness showed significant reduction of the micro-hardness in the ICHAZ, for both tested states, and the ICHAZ was identified as the most critical area of the whole welded joint. Results of the tensile tests revealed significant reduction of mechanical properties regardless of the welding parameters.

Impact Toughness and Softening of the Heat Affected Zone of High Heat Input Welded 390 MPa Yield Strength Grade TMCP Steel

The Charpy impact toughness of the heat affected zone (HAZ) of electro gas welded 390 MPa yield strength grade steel, manufactured by a thermo mechanically controlled process, was investigated. The effects of added Nb on the toughness of the steel and the factors influencing scatter in toughness are discussed in the present work. It was observed that adding Nb to the steel led to the deterioration of HAZ toughness. The presence of soluble Nb in the HAZ increased its hardenability and resulted in a larger amount of low toughness bainitic microstructure. Microstructural observations in the notch root area revealed the significant role of different microstructures in the area. In the presence of a larger amount of bainitic microstructures, the HAZ exhibited a lower Charpy toughness with a larger scatter in toughness. A softened zone with a lower hardness than the base metal was formed in the HAZ. However, theoretical analysis revealed that the presence of the zone might not be a problem in a real welded joint because of the plastic restraint effect enforced by surrounding materials. Key words: alloys, welding, toughness, impact test, niobium

Fatigue behavior and microstructural characterization of a high strength steel for welded railway rails

Railway rails are subjected to complex dynamic loading, which promotes the fatigue crack propagation phenomenon. As newer demands arise for increasingly faster and more heavily burdened trains, the need for rails with improved mechanical properties increases as well. In this work, a high strength TMCP steel of S700MC grade aimed for rail production is evaluated regarding welding ability.Steel joints were MAG welded and characterised regarding fatigue life. Fatigue tests were carried out using 0.1 stress ratio. Paris Law was assessed using Digital Image Correlation method. The weld seam and heat affected zone were characterised regarding microhardness variation throughout, and results were interpreted based on microstructural features.The produced welds show different microstructure depending on the cooling rate from weld pass temperature. In the centre of the seam, weld root presents fine grain bainitic structure with HV0.1 around 336, while weld face shows coarse grain ferritic structure with HV0.1 around 307. Experimental data from fatigue tests were used to validate a numerical simulation; a difference below 7% was obtained. Experimental data were used to evaluate a case study regarding crack propagation in a railway rail. Numerical simulation showed that only 718,320 cycles are required to increase a crack with initial length 40 mm in 4 mm.The results obtained from the joint experimental and numerical approach show that assessing the material properties and correlating them with the material microstructure is fundamental to develop applications for new materials, while simulation of the crack propagation phenomenon can be used to compare material performance.

Materials selection for XL wind turbine support structures: A corrosion-fatigue perspective

The continued growth of the offshore wind industry will depend essentially on reductions in wind energy production cost. Large cost reductions can be achieved through efficient, economic and optimised wind turbine support structures. To achieve maximum offshore wind adoption beyond 2020, significant industrial and research efforts are being made in optimised material selection and application. Fatigue and corrosion damage are the greatest challenges today in design and life estimation of wind turbine support structures. S355 steel is currently used in fabrication of most wind turbine monopile support structures. Clear understanding of their corrosion-fatigue properties and accurate steel selection will support the optimisation and economic design of extra-large wind turbines. This paper presents the fatigue crack growth test results of advanced S355 TMCP steel in air and seawater, and compares the results with studies on commonly available S355 steel. The results show that S355 TMCP steels generally offer higher fatigue damage tolerance than normalised S355 steels in air and the factor decreases and tends towards a common value with increase in stress intensity factor range. However, in seawater there is no significant difference in fatigue crack growth rates for all the S355 ferritic steels considered in this study.

Clarification of micromechanism on Brittle Fracture Initiation Condition of TMCP Steel with MA as the trigger point

Brittle fracture of carbon steel has a serious effect on the safety of steel structures. In case of bainitic microstructure it is well known that MA works as a triggering point of brittle fracture. However, the brittle fracture prediction procedure has not been fixed yet because of its complexity of the microstructure.The target of this series of research is to clarify the selection rule of the MA which causes brittle fracture from a number of MAs in the bainitic steel manufactured by TMCP process. Double-notch tests was carried out to identify the true trigger point of brittle fracture comparing with a simple whole cleavage fracture surface observation. Then, with respect to the nucleus of fracture occurrence, orientation information data of crystals acquired by EBSD was investigated after surface processing using FIB polishing. As a result, it was revealed that brittle crack occurred from the MA located in the much more coarse crystal grain than the surrounding grains. Furthermore, according to the GROD (Grain Reference Orientation Deviation) map, a large amount of piled-up dislocations is observed around MA which worked as the trigger of brittle crack initiation. This observation will help an establishment of quantitative model which can explain the behavior of brittle fracture in TMCP steel. Authors also established numerical simulation model of DWTT test used in quality assurance in line pipe field.

Properties and Structure of Resistance Short-Circuit Welded Joints in TMCP Steel S700MC

Properties and Structure of Resistance Short-Circuit Welded Joints in TMCP Steel S700MC

Analytical Study on Microstructure and Mechanical Property in HAZ-Softened Weld Joint with High Heat Input of Low Carbon TMCP Steel

Three types of low-carbon TMCP steels with different strength of the same carbon equivalent were welded by the flux-cupper back for SAW(FCB) method in this work. The microstructure, hardness and tensile strength of the FCB welded joints were studied. Softening phenomenon occurs in the heat affected zone of low-carbon content and low carbon-equivalent TMCP EH36 steel after high heat input welding. The softened zone is mainly depended on the strength of the base metal (BM), which appears on the fine-grain zone and incomplete crystallize zone of BM with relative low strength, and on the coarse grain zone of BM with high strength. The ratio of the tensile strength between each FCB joint and BM is the same of 0.98 as the same carbon and carbon equivalent content of 0.5% and 0.315% of BM, which is almost independent of the strength of BM. The tensile strength of the incomplete crystallize zone depends on the strength of the BM, which results in the improved strength of the whole softened zone with the increasing strength of base metal.

Bending Strength of Welded Joints in TMCP Steel Square Tubular Profiles “T” Connexions

The use of DOMEX 700 MCTM steel weldments is still little explored, due to some concern of the validity of the rules imposed by several standards and Codes for this class of steel. This material has low ductility and consequently the relation between tensile strength and yield strength is significantly lower than ordinary structural steels. For this reason, the instability phenomena are more critical than the instability phenomena of ordinary structural steels. Therefore, the aim of this study was to obtain detailed data on the mechanical efficiency of joints welded by GMAW. Six different heat inputs were used on square tubular profiles of TMCP steel. The tubular profiles were placed as a column/beam weldment with transverse and longitudinal welds positioned in relation to the loading axis. Twelve welded structures were instrumented with extensometer and tested in simple bending. Comparing the obtained data, it was verified that longitudinal welded joints presented higher bending strength than transversal welded joints. In the case of longitudinal joints, two weld beads were subjected to bending efforts, and in the case of transverse joints, only one weld bead resisted bending forces.

Laser Beam Welding of 10 mm Thick T-joints Made of TMCP Steel

Laser Beam Welding of 10 mm Thick T-joints Made of TMCP Steel

TMCP강을 적용한 해상용 풍력타워의 용접 공정에 따른 기계적 물성 평가

FCAW(Flux Cored Arc Welding), SAW(Submerged Arc Welding), EGW(Electro Gas Welding), and three-pole SAW are applicable in manufacturing the offshore wind tower. In this paper, mechanical properties of these welded-joints for TMCP steels were evaluated in all above welding processes. The tensile strength of welded-joints for all the welding methods satisfied the standard guideline (KS D 3515). No cracking on weldment was found after the bending test. Changes of weldedments hardness with welding processes were observed. In a weld HAZ (heat-affected zone), a softened HAZ-zone was formed with high heat input welding processes (SAW and EGW). However, the welded-joint fractures were found in the base metal for all cases and small decrease in welded-joint strength was caused by a softened zone. The multi-pole SAW welds exhibited similar mechanical properties comparing to the one with one-pole SAW process.