Elaboration of Thermomechanical Treatment Conditions of Ti-V and Ti-Nb-V Microalloyed Forging Steels

Abstract: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.

Section: Resultsmentioning

confidence: 99%

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

Górka

2018

“…The use of advanced technologies in metallurgical processing as well as a new look at the significance and the role of alloying elements used in steels have enabled the fabrication of various groups of steels characterised by vast ranges of mechanical and plastic properties [1]. The development of new steel grades, particularly high-strength low alloys (HSLA) having ferritic, ferritic-pearlitic, ferritic-bainitic, bainitic, or tempered martensitic structures, has made it possible to significantly reduce the weight of structural elements and that of entire structures [2][3][4]. The reduction of thicknesses of steels manufactured in thermo-mechanical control processes (TMCP) (dictated by the needs of the automotive, ship-building, and petroleum industries) without compromising previously obtained performance characteristics has made it possible to achieve significant savings resulting from lower material processing and transport-related costs [5][6][7][8][9][10][11].…”

Section: Introductionmentioning

confidence: 99%

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

Górka

Stano

2018

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.

“…The selected temperature and grain size may seem to be too low. However, the aim was to take into account a grain size after plastic deformation, which is usually successfully decreased during multistep deformation tests [21,24]. The pearlite transformation starts after ca.…”

Section: Thermodynamic Calculationsmentioning

confidence: 99%

“…It should be taken into account that plastic deformation shifts diffusion-controlled transformations to a shorter time [31,32]. JMatPro software does not enable the user to take into take into account a grain size after plastic deformation, which is usually successfully decreased during multistep deformation tests [21,24]. The pearlite transformation starts after ca.…”

Section: Thermodynamic Calculationsmentioning

confidence: 99%

Phase Equilibrium and Austenite Decomposition in Advanced High-Strength Medium-Mn Bainitic Steels

Grajcar

Zalecki

Burian

et al. 2016

Abstract:The work addresses the phase equilibrium analysis and austenite decomposition of two Nb-microalloyed medium-Mn steels containing 3% and 5% Mn. The pseudobinary Fe-C diagrams of the steels were calculated using Thermo-Calc. Thermodynamic calculations of the volume fraction evolution of microstructural constituents vs. temperature were carried out. The study comprised the determination of the time-temperature-transformation (TTT) diagrams and continuous cooling transformation (CCT) diagrams of the investigated steels. The diagrams were used to determine continuous and isothermal cooling paths suitable for production of bainite-based steels. It was found that the various Mn content strongly influences the hardenability of the steels and hence the austenite decomposition during cooling. The knowledge of CCT diagrams and the analysis of experimental dilatometric curves enabled to produce bainite-austenite mixtures in the thermomechanical simulator. Light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to assess the effect of heat treatment on morphological details of produced multiphase microstructures.