Effect of Carbon Partitioning, Carbide Precipitation, and Grain Size on Brittle Fracture of Ultra-High-Strength, Low-Carbon Steel after Welding by a Quenching and Partitioning Process

Abstract

To improve the weld zone properties of Advanced High Strength Steel (AHSS), quenching and partitioning (Q&P) has been used immediately after laser welding of a low-carbon steel. However, the mechanical properties can be affected for several reasons: (i) The carbon content and amount of retained austenite, bainite, and fresh martensite; (ii) Precipitate size and distribution; (iii) Grain size. In this work, carbon movements during the partitioning stage and prediction of Ti (C, N), and MoC precipitation at different partitioning temperatures have been simulated by using Thermocalc, Dictra, and TC-PRISMA. Verification and comparison of the experimental results were performed by optical microscopy, X-ray diffraction (XRD), Scanning Electron Microscop (SEM), and Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive Spectroscopy (EDS) and Electron Backscatter Scanning Diffraction (EBSD) analysis were used to investigate the effect of martensitic/bainitic packet size. Results show that the increase in the number density of small precipitates in the sample partitioned at 640 °C compensates for the increase in crystallographic packets size. The strength and ductility values are kept at a high level, but the impact toughness will decrease considerably.

Advanced High-Strength Steels for Automotive Applications: Arc and Laser Welding Process, Properties, and Challenges

“…Forouzan et al [44] studied the brittle fracture behavior of ultra-high-strength, lowcarbon steel welds after a quenching and partitioning (Q&P) process. This study performed laser welding on 'DOMEX 960' steel.…”

“…As shown in Figure 4, many particles, such as nitrides and carbides, can be seen in the HAZ. The authors observed Ti (C, N) particles nucleated and coarsened in the HAZ and the growth of nucleated carbides at the grain boundary [44]. In this regard, the welded joints of HSS and AHSS generally have lower m properties than the parent materials because the heat generated during welding the original microstructures in the FZ and the HAZ significantly.…”

“…The phenomenon of huge amounts of the alloying elements being picked up by the weld metal from the BM due to the high dilution effect while employing current welding methods was given special attention. The behavior of austenite grain development in the HAZ and how it influences later phase transformation and toughness qualities was also of great interest [44]. Due to the potentially negative consequences that have been considered in the literature, the involvement of microalloying elements in solid solution or precipitate and their influence on HAZ toughness are of particular interest in this phase transformation process.…”

“…Welding AHSS is frequently required to realize the full potential of these steels and to provide designers with more options. Numerous investigations by various authors [14,15,34,36,37,39,44,[64][65][66] have been conducted into welding AHSS, particularly on the use of gas GMAW and laser welding. The weldability of these steels was examined under various situations utilizing various welding methods, including laser welding, ultra-narrow gap laser welding, GTAW, controlled mode of metal transfer (CMT) during GMAW, and hybrid welding.…”

High Strain Rate Mechanical Behavior of Advanced High Strength Steels

“…The histograms of dimple size distribution for samples tested at different strain rates are plotted in Fig. It should be noted that quasi-cleavage and brittle fracture were also reported for some Q&P processed steels [304]. The fracture surface of the Q&P steels is to great extent determined by the tempered condition of the martensitic matrix.…”