Similar and dissimilar welds of dual-phase (DP) and high strength low alloy (HSLA) steels were made by fiber laser welding (FLW). The welds were characterized with respect to microstructure, micro- and nano-hardness, and tensile properties. The fusion zone (FZ) in the DP welds consisted of fully martensitic structure; whereas HSLA and dissimilar weld FZ microstructure were mixture of martensite and bainite. Analytical transmission electron microscopy (TEM) confirmed bainite structures containing bainitic ferrite laths with intralath and interlath cementite. Precipitation of single variant carbides inside the bainitic ferrite laths were confirmed by measuring the interplanar spacing. The cooling rate in the FZ, estimated using Rosenthal equation, and continuous-cooling-transformation diagrams corroborated the microstructure formed. Nanoindentation was used to verify the hardness of these individual microconstituents, since a much lower nano-hardness for bainite (4.11 GPa) was observed compared to martensite (6.57 GPa) phase. Tensile failure occurred in the tempered area of the heat affected zone (HAZ) in the DP steel welded, which was confirmed by typical cup-like dimple fracture; likewise failure in the HSLA base metal, which occurred in dissimilar and HSLA welds, indicated distinctive dimple and shear dimple ductile morphology.

Microstructure-properties correlation in fiber laser welding of dual-phase and HSLA steels

The heat affected zones (HAZ) of two Dual Phase steels spot welds, DP450 and DP980, were investigated experimentally with a Gleeble 3500 thermomechanical simulator. The thermal cycles experienced locally were identified by finite element analysis of the resistance spot welding process and their evolution with an increasing sheet thickness was highlighted within a usual range of [1.0–3.0] mm. The cooling rates are significantly lower in the case of thick sheets and this promotes the occurrence of diffusional phase transformations. HAZ microstructures and constitutive behaviours could be characterized with the Gleeble specimens. Experimental simulations were run in the subcritical and the coarse grain temperature ranges (700°C and 1200°C respectively) in order to address the main transformations for both steels. DP450 shows little sensitivity to the subcritical thermal cycles while DP980 exhibits significant softening. This is in accordance with their respective base metal martensite content and the occurrence of martensite tempering. On the contrary, DP450 microstructures and mechanical properties are strongly sensitive to the investigated range of coarse grained HAZ thermal cycles while the evolution is less pronounced in the case of DP980, which is related to their respective hardenability.

HAZ Microstructures and Local Mechanical Properties of High Strength Steels Resistance Spot Welds

https://uwaterloo.ca/centre-advanced-materials-joining/sites/ca.centre-advanced-materials-joining/files/uploads/files/jmad_2013.pdf

Effects of weld line position and geometry on the formability of laser welded high strength low alloy and dual-phase steel blanks

The increased adoption of martensite-containing advanced high strength steels, such as martensitic and dual-phase steels, into automotive applications has led to concerns among practitioners with respect to softening during rapid tempering cycles such as those experienced during laser welding. Past studies on rapid tempering have successfully modeled the rapid tempering process; however, the activation energies and softening rates calculated did not match the classic literature values associated with martensite tempering. The present study examined rapid tempering data for a martensitic steel and separated the softening process into two stages: carbide nucleation and carbide coarsening or growth. The activation energies calculated for each process were found to be consistent with classic literature values for diffusion controlled nucleation and growth of carbides during martensite tempering.

/pdf/decoupling-of-the-softening-processes-during-rapid-tempering-208fib84q3.pdf

Decoupling of the softening processes during rapid tempering of a martensitic steel

Dual-phase steel with ferrite-martensite-bainite microstructure exhibited secondary hardening in the subcritical heat affected zone during fiber laser welding. Rapid isothermal tempering conducted in a Gleeble simulator also indicated occurrence of secondary hardening at 773 K (500 °C), as confirmed by plotting the tempered hardness against the Holloman–Jaffe parameter. Isothermally tempered specimens were characterized by analytic transmission electron microscopy and high-angle annular dark-field imaging. The cementite (Fe3C) and TiC located in the bainite phase of DP steel decomposed upon rapid tempering to form needle-shaped Mo2C (aspect ratio ranging from 10 to 25) and plate-shaped M4C3 carbides giving rise to secondary hardening. Precipitation of these thermodynamically stable and coherent carbides promoted the hardening phenomenon. However, complex carbides were only seen in the tempered bainite and were not detected in the tempered martensite. The martensite phase decomposed into ferrite and spherical Fe3C, and interlath-retained austenite decomposed into ferrite and elongated carbide.

/pdf/mechanism-of-secondary-hardening-in-rapid-tempering-of-dual-1onkkxj5pj.pdf

Mechanism of Secondary Hardening in Rapid Tempering of Dual-Phase Steel

Flash butt welding of 3.7 mm thick Mn-Cr-Mo dual phase steel is carried out at different final jaw distances, where various post weld cooling systems such as the normal machine cooling, forced air cooling and water spray cooling are used. The energy input during welding such as the current and voltage are kept constant. The intention of this investigation is limited to study the influence of various weld thermal cycle on the behaviour of phase transformation at weld centre and different regions of HAZ. Under different conditions of welding the weld thermal cycle is analysed and the behaviour of phase transformation at different regions of the weldment is determined with the help of welding CCT diagram. A detail microstructural study is carried out under optical and scanning electron microscopes. A special attention is paid to study the behaviour of tempered martensite region of HAZ, which is identified earlier as the weakest region of HAZ, under different welding conditions. The weakening of tempered martensite region is estimated by measuring its microhardness under different conditions of welding.

P. K. Ghosh

Papers

Thermal Cycle and Microstructure of Heat Affected Zone (HAZ) of Flash Butt Welded Mn-Cr-Mo Dual Phase Steel