Introduction: FeMnAl steel in this work belongs to the Fe-Mn-Al-C steels which were first developed in 1958 to replace chromium and nickel containing austenitic stainless steels according to US Navy’s request. The driving force of the development of Fe-Mn-Al-C steels before 1980s was to reduce the content of costly chromium and nickel in stainless steels. In the past two decades, the Fe-Mn-Al-C steels were studied for the exceptional properties of low density, corrosion resistance, high strength, high ductility, high strain hardening and good elongation compared to almost any other steels. Welding is widely used in joining metal parts in manufacturing. Understanding the weldability of Fe-Mn-Al-C steels is important in the welding investigation of Fe-Mn-Al-C steels and becomes indispensable for a wide application of Fe-Mn-Al-C steels in industry. There is limited research has been reported on the weldability of Fe-Mn-Al-C steels. This project studied the welds of two heats of FeMnAl steel. The weld cross section was characterized and cracks was found in the HAZ of certain welds. The hot ductility test was conducted to understand the HAZ cracking in the real welds. Tensile strength test, side face bend test and toughness test were conducted.
Materials and Experiments: In this welding investigation, two heats of FeMnAl steel were used, heat-1 (A1132) and heat-2 (A1133). The two heats are both in the form of plate with a thickness 0.5 inch. The gas metal arc welding (GMAW) was used to weld the two heats of FeMnAl steel with two designs: bead-on-plate (BOP) weld and double-V-groove weld. Different filler wires were used: ER316, ER308, Inco 82, ER120s, ER420, low temperature phase transformation (LTPT) wires 1766 and 1764.
The BOP and double-V-groove weld samples prepared for metallographic characterization under optical microscope and SEM. EDAX analysis was conducted to reveal the chemical composition distribution. EBSD analysis was used to reveal the phase composition of certain samples. 3500 Thermal-mechanical simulator Gleeble™ was used for the hot ductility test. The samples are the round solid rods with a diameter 0.25 inch, a length 4.0 inch, with threads on two ends. Between the two copper grips, it is 1.0 inch free span of the rod samples. Temperature was controlled and recorded through a thermocouple wire percussion welded at the middle of the sample. The Nil-strength temperature (NST), Nil-ductility temperature (NDT) and the ductility recover temperature (DRT) were tested in the Gleeble system.
Results and Discussion: The FeMnAl heat-1 (A1132) base metal has large grains with twinning within the grains, with the average grain size ~190 µm. The FeMnAl heat-2 (A1133) base metal has the grain with the average size ~27 µm, much finer than the heat-1. The hardness of heat-1 and heat-2 base metals are respectively 466±23 HV0.1 and 423±14 HV0.1.
GMAW welds of FeMnAl heat-1
The optical metallography characterization revealed that all the GMAW BOP welds and double-V-groove welds of heat-1 contain cracks in the HAZ adjacent to the fusion boundary and they are all intergranular cracks. Some of the cracks have obvious re-solidified liquid features around the cracks along the grain boundaries adjacent to the fusion boundary. This is due to the liquid metal penetration along the grain boundary from the fusion zone.
GMAW welds of FeMnAl heat-2
The cracking issue of FeMnAl heat-2 welds is much less than the heat-1. There is still liquation features in the HAZ, but they did not cause cracking. The liquation features are found along the prior grain boundaries, but during welding, grain boundary migration occurred due to grain growth and moved off the prior grain boundaries. The grain boundaries in the heat-2 HAZ are tortuous. White secondary phase “pinned” at a grain boundary was found in the HAZ. EBSD analysis shows that the white secondary phase is ferrite and it should be δ-ferrite. These features can explain why there is little cracking issues in the heat-2.
Comparison of the cracking situation of heat-1 and heat-2
No matter for BOP or double-V-groove weld design, the heat-2 welds have much fewer or none cracks than the heat-1, no matter in terms of the number of cracks or the total length of cracks in each single weld. The welding results show that the heat-1 is more susceptible to HAZ cracking than heat-2, or heat-2 has better weldability than heat-1.
Hot ductility test
For FeMnAl heat-1, the tested nil-strength temperature (NST) is 1261°C. The on cooling curve drops dramatically from 60% ductility and the ductility recovery temperature (DRT) is 1198°C. The on heating curve determines the nil-ductility temperature (NDT) which is 1215°C. The liquation cracking temperature range (LCTR) is the difference between NST and DRT. The LCTR of heat-1 is 63°C. The NST, NDT and DRT of FeMnAl heat-2 are respectively 1341°C, 1243°C and 1201°C. The LCTR of heat-2 is 140°C. The on heating and on cooling curves of heat-2 both dramatically drop to zero at NDT and DRT.
Conclusions: The weldability of two heats of FeMnAl steel were studied. FeMnAl heat-1 and heat-2 are both austenitic microstructure with high hardness and high strength: heat-1 466±23 HV0.1 and heat-2 423±14 HV0.1; heat-1 YS: 142.5 ksi and heat-2 YS: 131 ksi. The toughness of heat-1 is low, much smaller than heat-2.
With GMAW, all the bead-on-plate welds and double-V-groove welds of FeMnAl heat-1 contain intergranular cracks in the HAZ adjacent to the fusion boundary. For FeMnAl heat-2, the cracking issue is much reduced or none.
The hot ductility test shows the liquation cracking temperature range of heat-1 is smaller than the LCTR of heat-2, but heat-1 welds are more susceptible to HAZ cracking than heat-2 welds. There are three probable reasons:
The grain boundaries of heat-2 migrated to a new location without elements segregation. These migrated grain boundaries do not melt at a lower temperature.
The formation of δ-ferrite “islands” “pinned” the grain boundaries of heat-2 samples and hinder the cracking propagation along the grain boundaries.
The grain size of heat-2 HAZ is small and the grain boundaries are short and tortuous, which are unfavorable for cracking.