S700MC Steel Research Articles

Experimental studies on mechanical properties of S700 MC steel at elevated temperatures

At elevated temperatures, the mechanical properties of high strength steel (HSS) reported in the literature have an observable scatter. They differ from design code values that are mainly based on experiments on mild steel. In this paper, the mechanical properties of S700 MC at elevated temperatures were investigated with steady state and transient state tensile tests. The reduction factors of yield strength started to decrease from 100 °C onwards, while the EN 1993-1-2 values have no decrease up to 400 °C. The reduction factors of proportional limit were below the code values up to 200 °C. The modulus of elasticity was consistent when using both monotonic and repeated loading but differed from the code values. Further comparisons of the mechanical properties of steels with the same strength grade with the values in the literature confirm the importance of testing HSSs produced with different manufacturing processes. The constitutive equations and representative material models proposed for S700 MC also support the implementation of a performance-based approach to structural fire safety design.

Numerical Verification of Tests on the Influence of the Imposed Thermal Cycles on the Structure and Properties of the S700MC Heat-Affected Zone

The article presents the results of studies on the influence of simulated thermal cycles parameters on the structure and properties of the heat-affected zone (HAZ) of thermo-mechanically rolled S700MC steel. For this purpose, resistance heating tests of the tested samples were carried out to determine the effect of maximum temperatures of the imposed thermal cycles with different maximum temperatures at a constant cooling time in the temperature range between 800 and 500 °C (t8/5) and to study the influence of changes of this time on the structure and hardness as well as the tensile strength, elongation and toughness of the simulated HAZ in S700MC steel. The results of the tests, were supported by the results of finite element method (FEM) analyses in the VisualWeld (SYSWELD Code) software of the ESI Group. Selected heat distributions during heating, distributions of individual metallurgical phases and hardness were compared with results from real tests. On the basis of the results presented, an attempt was made to explain the decrease in mechanical and plastic properties in the HAZ area caused by the influence of the welding heat cycle.

Assessment and Comparison of the Mechanical Properties of Laser Welded Joints in Docol 1200M and Strenx S700MC Steel Alloy Grades Under Impact Loads

The article evaluates the strength and ductility of laser butt joints made of 2 mm Docol 1200 M martensitic steel sheets based on the hardness, quasi-static and dynamic tensile tests. Technological research of laser welding process was carried out on welding cell using IPG fiber – based welding source with 6 kW maximum power. The tests were carried out for parallel and perpendicular orientation of specimens by rolling direction. In addition, the obtained results were compared with the analogous results obtained during the Strenx S700 MC steel tests. Dynamic tests were performed using the tensile split Hopkinson pressure bar technique with strain rates of 103 s-1. The obtained results showed that the strength of Docol 1200 M under dynamic tensile test conditions are similar to the material strength under static tensile test conditions. However, due to the breaking of the specimens in the heat affected zone, the strength of the welded joint is much lower than in base material, which was not observed during the Strenx S700 MC steel tests.

Investigation of the effect of isothermal heat treatments on mechanical properties of thermo-mechanically rolled S700MC steel grade

The effect of isothermal heat treatments (1 hour at 200, 400, 600 and 800°C) on mechanical properties of thermo-mechanically rolled S700MC steel has been investigated by extensive mechanical characterizations. Treatments at 600°C increase yield and tensile strength and decrease impact energy. Below 600°C the steel retains its bainitic structure. Precipitation kinetics simulations indicate that this secondary hardening effect arises from the nucleation of fine (Nb,Ti)C particles, indicating that the bainitic structure is unstable above 600°C due to its high supersaturation with respect to C, Nb and Ti. These results can help to optimize the operating practices for post-weld heat treatments.

Assessment of the Effect of Laser Welding on the Properties and Structure of TMCP Steel Butt Joints.

The research work and related tests aimed to identify the effect of filler metal-free laser beam welding on the structure and properties of butt joints made of steel 700MC subjected to the TMCP (thermo-mechanically controlled processed) process. The tests involved 10-mm thick welded joints and a welding linear energy of 4 kJ/mm and 5 kJ/mm. The inert gas shielded welding process was performed in the flat position (PA) and horizontal position (PC). Non-destructive testing enabled classification of the tested welded joints as representing the quality level B in accordance with the requirements set out in standard 13919-1. Destructive tests revealed that the tensile strength of the joints was 5% lower than S700MC steel. The results of tensile tests and changes in structure were referred to joints made using the MAG (Metal Active Gas) method. The tests of thin films performed using a high-resolution scanning transmission electron microscope revealed that, during laser beam welding, an increase in dilution was accompanied by an increase in the content of alloying microadditions titanium and niobium, particularly in the fusion area. A significant content of hardening phases in the welded joint during cooling led to significant precipitation hardening by fine-dispersive (Ti,Nb)(C,N) type precipitates being of several nanometres in size, which, in turn, resulted in the reduction of plastic properties. An increase in the concentration of elements responsible for steel hardening, i.e., Ti and Nb, also contributed to reducing the weld toughness below the acceptable value, which amounts to 25 J/cm2. In cases of S700MC, the analysis of the phase transformation of austenite exposed to welding thermal cycles and the value of carbon equivalent cannot be the only factors taken into consideration when assessing weldability.

Microstructure and mechanical properties of low power pulsed Nd:YAG laser welded S700MC steel

Thermomechanically controlled processed steels have gained attention increasingly by many industries. In this research S700MC steel is successfully welded using low power pulsed Nd:YAG laser and the microstructure and mechanical properties are investigated. It is shown that the average power and overlapping factor both affect the weld geometry. Full penetration with double-sided welding achieved on 2 mm thick plates autogenously. Optical metallographic methods and SEM/EDS were used to evaluate the resulting microstructures. The evaluations revealed that the weld metal microstructure contains different morphologies of ferrite such as acicular, allotriomorphic and Widmanstatten as well as bainaite and martensite structure in the weld zone. Also, no noticeable heat affected zone was detected near the fusion zone of the weldments. In addition to microstructures investigation, micro hardness and tensile tests were performed to evaluate mechanical properties. Hardness measurement results exhibit higher hardness values in weld zone than that of in the base metal. The tensile test revealed a ductile fracture behavior which happened in the base metal, due to proper weld zone microstructure. The strength and elongation of the prepared joints were 774 ± 14 MPa and 26.5 ± 2.5%, respectively.

Weldability of S700MC steel welded with the hybrid plasma + MAG method

The paper describes the microstructure of welded joints produced by the plasma+MAG (Metal Active Gas) method of S700MC high yield strength steel (700 MPa). Welded joints of thermomechanical steel have been made with different values of heat input. The results of metallographic research of welded joints, microstructure of the weld and heat affected zone, hardness distribution and impact toughness are presented. The heat affected zone consists of two sub-zones with different grain size and lowered hardness. The tensile test show that strength of welded joints was slightly reduced and the bending test revealed no crack formation in the weld. The impact toughness of measured welded samples with V-notch in HAZ (heat affected zone) reached high values that are higher comparing to samples with notch placed in the weld area. The investigation results show that the use of plasma concentrated heat source together with MAG welding arc does not significantly change the structure and deteriorate properties of welded S700MC thermomechanically treated high strength steel. The hybrid plasma+MAG welding method has a potential to become a beneficial alternative to other welding processes due to its high efficiency, reduced amount of weld metal content or limited requirements for a preparation of edges of welded joints.

Fatigue properties of laser and hybrid laser-TIG welds of thermo-mechanically rolled steels

The hybrid laser-tungsten inert gas welding technology was applied to butt weld 3-mm-thick S460MC and S700MC high-strength low-alloy steel sheets. The intent of low-current arc addition was to preheat the material to reduce extremely fast cooling rate accompanying laser welding. High-cycle fatigue tests were performed to evaluate the effect of welding conditions on fatigue behavior of S460MC and S700MC laser and hybrid welds. Both base materials exhibit approximately the same fatigue limit. However, corresponding samples with the weld behaved significantly different under cyclic loading. We demonstrated that the drop of their fatigue lifetime is caused by the combination of microstructural changes and especially by the presence of surface notches resulting from the welding process. The two series of both steels were tested to reveal the contribution of induced surface roughness and microstructural changes itself. The first one was in the as-welded condition. The second one had a smooth surface resulting from surface layer grinding after the welding. It was found, that the geometric notches created by the welding process are the determining factor in case of both tested steels. Testing of the ground samples showed, that in case of S460MC steel, the application of pre-heat did not improve the fatigue properties. On the other hand, the performed fatigue tests showed improved fatigue limit of ground series samples of the pre-heated S700MC steel, when compared with the conventional laser welding.

Determination of Grain Growth Kinetics and Assessment of Welding Effect on Properties of S700MC Steel in the HAZ of Welded Joints

The welding of fine-grained steels is a very specific technology because of the requirement for the heat input limit value. Applying temperature cycles results in an intense grain growth in a high-temperature heat-affected zone (HAZ). This has a significant effect on the changing of strength properties and impact values. The intensity of grain coarsening in the HAZ can be predicted based on the experimentally determined activation energy and material constant, both of which define grain growth kinetics. These quantities, together with real measured welding cycles, can be subsequently used during experiments to determine mechanical properties in a high-temperature HAZ. This paper shows a methodical procedure leading to the obtainment of the material quantities mentioned above that define the grain growth, both at fast and slow temperature cycles. These data were used to define the exposure temperature and the soaking time in a vacuum furnace to prepare test samples with grain sizes corresponding to the high-temperature HAZ of welded joints for the testing procedures. Simultaneously, by means of the thermo-mechanical simulator Gleeble 3500, testing samples were prepared which, due to a temperature gradient, created conditions comparable to those in the HAZ. The experiments were both carried out with the possibility of free sample dilatation and under a condition of zero dilation, which happens when the thermal expansion of a material is compensated by plastic deformation. It has been found that shape of the temperature cycle, maximal achieved cycle temperature, cooling rate, and, particularly, the time in which the sample is in the austenite region have significant effects on the resulting change of properties.

MAG welding of S700MC steel used in transport means with the operation of low arc welding method

Manufacturers of welding equipment strive to develop the most efficient, cost-effective and easy to process welding methods. This necessity is also related to welding of new, often hard-to-weld steel types. The article aims to present the possibility of welding a high-strength S700MC steel with an increased yield point using MAG (135) process and a blowtorch with an intelligent arc control. The blowtorch allows to reduce input energy and reduce splinters while maintaining the mechanical properties of the material. The S700MC steel has been selected for the tests purposefully, as it may create welding problems in order to maintain high strength and increased yield point. The applied technology provided lower power consumption compared to traditional welding machines and joints with very good mechanical properties were achieved.

Numerical Simulations of Laser and Hybrid S700MC T-Joint Welding.

This article presents examples of numerical simulations done based on the real experiments of S700MC steel T-joint laser and hybrid welding. Presented results of numerical analyses carried out using SYSWELD show the possibilities offered to contemporary engineers by modern software used to make numerical analyses of production processes. After calibration of a heat source models on the chosen examples of S700MC steel 10-mm-thick T-joint laser and hybrid welding, distributions of temperature fields, thermal cycles, distributions of individual metallurgical phases and hardness, and strains and plastic deformations in simulated processes were calculated for one selected joint from both mentioned methods. The results of the analysis allow determining both the differences in the stress distributions and their minimal and maximal values. This article also presents the benefits resulting from the use of such analyses, due to the significant savings in time and resources to be spent on the development of correct technologies for joining modern construction materials such as thermomechanically treated steels, especially given that some of the results are unavailable or very difficult to collect using conventional measurement methods.

Fatigue response of high-strength steel bolted connections under in-plane shear and out-of-plane bending loading modes

The use of high-strength steel (HSS) permits weight reduction in heavy-duty applications such as trucks, trailers, and agricultural machinery, where significant fatigue resistance is a requirement. Fatigue cracks are often initiated at locations of stress concentration such as the joining of mechanical parts, the most widely used methods being welding and mechanical fastening. It is known that HSSs do not only have higher yield values but also perform better in terms of fatigue. Yet, the weld fatigue resistance does not improve with an increase in the base material property due to the stress concentrations and imperfections introduced by welding. The aim is to explore the possibilities of using conventional mechanical joining techniques for HSSs and assess the fatigue properties under typical loading modes. Mechanical joining introduces an additional failure mechanism called fretting fatigue resulting from the oscillatory relative displacement between the surfaces in contact. This failure mode is reproduced by the experiments and the effects of pre-tension, hole-making procedure, washer configuration, and surface condition are analyzed for steel grades S500MC, S700MC, and S960MC. Accompanying finite-element simulations are employed to develop a deeper understanding of the underlying physics. The results might lead to various guidelines with respect to design rules and best practices for HSS bolted connections subjected to fatigue loading.

Tow truck frame made of high strength steel under cyclic loading

The paper presents experimental results from test on tow truck frame made of the high strength steel S700MC subjected to fatigue loading. The mounting manner of the frame with anti-vibration platform and connection with a coupling ball to apply the loading are presented. Cracks features captured by means of macrophotography and microscopic observations are illustrated. Reasons for the crack occurrence are discussed by the use of fractography, hardness tests, microstructural analysis of the parent material, Heat Affected Zone and weld.

Influence of High Frequency Impact Treatment (HiFIT) on fatigue strength of welded joints of high-strength steel S700MC for bridges applications

The paper deal with experimental fatigue test of arc welded joints of thermomechanicaly rolled structural steel S700MC, uses for heavy duty application like steel bridges structures. The aim of the work was to compare the fatigue strength of welded joints in two conditions, i.e. as-welded and after HiFIT process as a post-weld treatment and set the influence of HiFIT process on fatigue strength improvement. For this purpose three different welded joints have been prepared. The experimental test series were consist of butt welded joints, T-joints with longitudinal and transverse stiffeners. In the paper the fatigue characteristics S-N of welded joints have been presented as well as calculated fatigue category FAT. The study shows that the fatigue strength of welded joints of S700MC steel grade after HiFIT treatment is much more higher in each cases than the fatigue strength of non-treated welded joints. The obtained results can be used by engineers at the design stage or estimating the remaining fatigue life of the welded steel structure made of similar high-strength steel used for bridges application.

Structure and Properties of Hybrid Laser Arc Welded T-Joints (Laser Beam – MAG) in Thermo-Mechanical Control Processed Steel S700MC of 10 mm thickness

Structure and Properties of Hybrid Laser Arc Welded T-Joints (Laser Beam – MAG) in Thermo-Mechanical Control Processed Steel S700MC of 10 mm thickness

Assessment of the Weldability of T-Welded Joints in 10 mm Thick TMCP Steel Using Laser Beam.

The article presents tests aimed to verify the possibility of Thermomechanically Controlled Processed (TMCP) steels T-joints laser welding. The 10 mm thick high-yield-point steel S700MC obtained in an industrial manufacturing process was used for tests of laser welding. The joints made during the tests were single- and double-sided. Subsequent nondestructive tests revealed that the laser-welded joints represented quality level B in accordance with PN-EN ISO 13919-1. Single-sided welding performed at the output laser beam power of 11 kW provided the penetration depth of just 8 mm without visibly deforming of the joint. The double-sided welded joints were characterized by proper geometry and the presence of gas pores in the welds not compromising the requirements of quality level B (strict requirements). The identified weld structure was bainitic-ferritic. The weld hardness was by approximately 60 HV1 higher than that of the base material (280 HV1). The HAZ (Heat Affected Zone) area was slightly softer than the base material. The tests of thin foils performed using a high-resolution scanning transmission electron microscope revealed that, during welding, an increase in the content of the base material in the weld was accompanied by an increase in contents of alloying microagents Ti and Nb, particularly near the fusion line. The above-named alloying microagents, in the form of fine-dispersive (Ti,Nb)(C,N) type precipitates, could reduce plastic properties of joints.

Assessment of Steel Subjected to the Thermomechanical Control Process with Respect to Weldability

The study is concerned with the assessment of the weldability of steel S700MC subjected to the thermomechanical control process (TMCP) and precipitation hardening and characterised by a high yield point. Appropriate mechanical and plastic properties of steel S700MC were obtained using the thermomechanical control process through precipitation, solution, and strain hardening as well as by using grain-refinement-related processes. Constituents responsible for the hardening of steel S700MC include Ti, Nb, N, and C. The hardening is primarily affected by (Ti,Nb)(C,N)-type dispersive precipitates sized from several nanometres to between ten and twenty nanometres. The welding process considerably differs from TMCP conditions, leading to the reduction of plastic properties both in the heat-affected zone (HAZ) and in the weld area. This study demonstrates that in cases of TMCP steels, where the effect of precipitation hardening is obtained through titanium and niobium hardening phases, the carbon equivalent and phase transformation γ–α cannot constitute the basis of weldability assessment. The properties of welded joints made from the above-named group of steels are primarily affected by the stability of hardening phases, changes in their dispersion, and ageing processes. The most inferior properties were identified in the high-temperature and coarse-grained HAZ area, where the nucleation of hardening phases in the matrix and their uncontrolled reprecipitation in the fine-dispersive form lead to a sharp decrease in toughness.

Microstructure and Properties of Hybrid Laser Arc Welded Joints (Laser Beam-MAG) in Thermo-Mechanical Control Processed S700MC Steel

The article presents the microstructure and properties of joints welded using the Hybrid Laser Arc Welding (HLAW) method laser beam-Metal Active Gas (MAG). The joints were made of 10-mm-thick steel S700MC subjected to the Thermo-Mechanical Control Process (TMCP) and characterised by a high yield point. In addition, the welding process involved the use of solid wire GMn4Ni1.5CrMo having a diameter of 1.2 mm. Non-destructive tests involving the joints made it possible to classify the joints as representing quality level B in accordance with the ISO 12932 standard. Destructive tests of the joints revealed that the joints were characterised by tensile strength similar to that of the base material. The hybrid welding (laser beam-MAG) of steel S700MC enabled the obtainment of good plastic properties of welded joints. In each area of the welded joints, the toughness values satisfied the criteria related to the minimum allowed toughness value. Tests involving the use of a transmission electron microscope and performed in the weld area revealed the decay of the precipitation hardening effect (i.e., the lack of precipitates having a size of several nm) and the presence of coagulated titanium-niobium precipitates having a size of 100 nm, restricting the growth of recrystallised austenite grains, as well as of spherical stable TiO precipitates (200 nm) responsible for the nucleation of ferrite inside austenite grains (significantly improving the plastic properties of joints). The tests demonstrated that it is possible to make welded joints satisfying quality-related requirements referred to in ISO 15614-14.

Possibilities of Increasing the Fatigue Strength of Welded Joints in Steel S700MC through High Frequency Impact Treatment (HiFIT)

Possibilities of Increasing the Fatigue Strength of Welded Joints in Steel S700MC through High Frequency Impact Treatment (HiFIT)

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