| Abstract Scope |
High mechanical performance linked with low density is the leading research direction in structural steel for the scientific and industrial communities. In recent years, combined additions of Al and Mn have been pursued in the steel development scenario. Al has the potential to decrease the weight of structural steel, and Mn addition was presented to decrease the negative influence of aluminum on decreasing Young's modulus of the material.
One notable example of such a combination is the Fe-Mn-Al-C (FeMnAl) alloys. Originally designed to replace the stainless steels, the FeMnAl steels take advantage of the abundant and, consequently, much lower cost manganese and aluminum, but also delivering good mechanical properties such as high strength and high toughness at room and low temperature: high-density reductions of about ~18% lighter than conventional steels, high corrosion resistance, tensile strength from 600 to 2,000 MPa, and elongation to failure as great as 70%. The exceptional properties of these steels are very relevant for lightweight automotive vehicles and military armor applications.
Despite extensive mechanical performance and heat treatment exploration on this alloy class, a lack of weldability studies on such alloys is evident. A critical issue related to welding is the hot cracking susceptibility of metallic alloys. Hot cracking is associated with liquid films present along grain boundaries in the fusion zone and the partially melted zone region of the heat-affected zone.
The U.S. ARMY CCDC-GVSC has been developing and optimizing a light-weight alternative to HRA armor steels currently employed by the defense industry under MIL DTL 12560. This new steel is based on the Fe-Mn-Al ternary system with additions of C to provide higher strengths. Through high additions of Al the density of the steel is decreased to allow an overall reduction in weight of armor materials without compromising its armor functionality
The heat-affected zone (HAZ) liquation cracking response of cast FeMnAl has not been fully studied and quantified, yet it has been reported by OSU and EWI based on welding mockups and simulated non-equilibrium solidification scenarios (welding). During exploratory work performed at OSU A GTAW spot weld was made on a small piece of the material, which was sectioned and analyzed using optical microscopy. It can be observed that several cracks occurred at the bottom of the weld pool, and in the HAZ of the base metal.
To investigate the liquation cracking susceptibility of the FeMnAl alloys, 4 different compositions were selected to be evaluated using spot varestraint and hot ductility tests. Combine Spot-Varestraint and Hot-Ductility tests to evaluate and quantify the liquation cracking susceptibility of four (4) experimental FeMnAl alloys. Use modeling and characterization to assess the effects of chemical and microstructural identities regarding the solidification cracking susceptibility and provide guidelines to future alloy optimization regarding weldability aspects
The spot-Varestraint test has been widely applied for evaluating HAZ liquation cracking susceptibility. During spot-Varestraint testing, a GTA spot weld is produced in the center section of a small specimen, nominally 6 X 1 X 0.25 in. After a predetermined weld time, the arc is extinguished and the specimen is forced to conform to the surface of a radiused die block. In this manner, HAZ liquation cracks can be generated on the surface of the specimen adjacent to the GTA spot weld. The applied augmented strain of the top surface of the specimen is approximated in the same manner as for the longitudinal- Varestraint test. Cracking susceptibility is determined by measuring the length of each crack on the as-tested specimen surface. The threshold level of strain to cause cracking or the degree of cracking (quantified by MCL or TCL) over a range of strain levels have been generally accepted as cracking indexes since the introduction of this test
The concept in the design of the hot-ductility test is different from the majority of weldability testing techniques. Instead of quantifying the cracking susceptibility by the degree of cracking, it characterizes the ductility of the material at elevated temperatures and relates this ductility data to cracking susceptibility.
In this study, the liquation cracking susceptibilities of FeMnAl alloys were evaluated and compared. The main conclusions are as follows. The FeMnAl steel welds were more sensitive to liquation cracking than those of 304 SS, but less susceptible than that of A286. Segregations of Mn and C took place at both the dendritic and grain boundaries during solidification. They led to an expansion of the solidification temperature range and enhanced the formation of low melting (Fe,Mn)3C at the final stage of solidification, thereby, increasing the cracking susceptibility. The formation of eutectic phases [c-(Fe,Mn)3C] at 1090degC during solidification primarily played a major role to increase the liquation cracking susceptibility of FeMnAl steel. Liquation cracking susceptibility of FeMnAl steel decreased as a function of Al content. Addition of Al improved the cracking resistance by suppressing the formation of low melting (Fe,Mn)3C eutectic at the final stage of solidification. The importance of saturated augmented strain in spot-Varestraint tests was evaluated. The saturated strain refers to the applied strain above which the maximum crack length remains constant. It was found that saturated strain conditions must be used to uniquely define the temperature above which the ductility of the material drops to essentially zero. This temperature correlates to the on-heating nil- ductility temperature in the hot-ductility test |