Effect of the welding mode and filler content on the structure of microalloyed Nb/Ti steel weldments

2018

Metals

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: Methodsmentioning

confidence: 99%

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

2018

Metals

“…Laser welding, in addition to its high precision (previously quite paradoxically regarded as a disadvantage because of very precise and thus expensive preparations required prior to welding), enables the very fast welding of elements without the use of filler metals while not affecting the properties of the base material [ 4 , 5 , 6 , 7 ]. Technical and economic aspects resulting from the possibility of using high-yield-point Thermomechanically Controlled Processed (TMCP) steels in energy-saving integrated production lines and the usability of the above-named steels when making various structures, including those exposed to extremely harsh climate, are responsible for the growing scientific interest in this group of materials and improvements in their manufacturing and joining (e.g., using the laser beam) [ 8 , 9 , 10 , 11 , 12 ]. Most research works related to the weldability of TMCP steels are concerned with materials having a yield point of up to 460 MPa.…”

Section: Introductionmentioning

confidence: 99%

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

2018

Materials

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.

“…As a result, welded structures with the same load capacity are now thinner and lighter. Furthermore, the use of TMCP steels reduces welding costs by decreasing the consumption of filler metals, shortening the time of welding processes, and reducing costs related to the straightening of structures and the testing of welds [17][18][19][20][21][22]. It should also be noted that the welding thermal cycle itself significantly differs from the classical thermomechanical treatment cycle (by being considerably more intense).…”

Section: Introductionmentioning

confidence: 99%

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

2020

Materials

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.