Ti-containing steel weld metals with boron addition contents of 0-85 ppm were prepared, and their microstructural characteristics as well as the impact toughness were investigated. The results show that in these microstructures, compared to the weld metal without boron, the addition of 22-39 ppm boron results in a remarkable increase in the amount of acicular ferrite at the expense of grain boundary ferrite, idiomorphic ferrite and side-plate ferrite. However, with a further increase in the boron content up to 61-85 ppm, the bainitic ferrite is formed, accompanied with a drop in the amount of acicular ferrite. In the acicular ferrite, the size of martensite-austenite (M/A) islands is much smaller, and the amount is much lower than those found in the bainitic ferrite. In the case of the weld metals primarily composed of acicular ferrite, during the fracture of the impact specimens, the crack propagation path is more bent in comparison with the weld metals with large amounts of grain boundary ferrite, idiomorphic ferrite, side-plate ferrite or bainitic ferrite, which that the presence of acicular ferrite improves the toughness of the weld metals. The coarse martensite-austenite islands readily induce micro-cracks at the interface between martensite-austenite islands and ferrite matrix, deteriorating the toughness. The weld metals with B contents of 22-39 ppm exhibit outstanding impact toughness because of high amount of acicular ferrite, accompanied with fine martensite-austenite islands.
Se han preparado soldaduras de acero aleadas con titanio con contenidos de boro de 0-85 ppm, y se investigó su microestructura, así como su tenacidad al impacto. Los resultados muestran que, en estas microestructuras, en comparación con la soldadura sin boro, la adición de 22-39 ppm de boro da como resultado un aumento notable en la cantidad de ferrita acicular a expensas de la ferrita que se forma en el límite de grano, la ferrita idiomórfica y la ferrita de placa lateral. Sin embargo, con un mayor aumento en el contenido de boro hasta 61-85 ppm, se forma la ferrita bainítica, acompañada de una caída en la cantidad de ferrita acicular. En la ferrita acicular, el tamaño de las islas de martensita-austenita (M/A) es mucho menor, y la cantidad es inferior comparado con la que se forma en la ferrita bainítica. En el caso de las soldaduras compuestos principalmente de ferrita acicular, durante la fractura de las muestras sometidas a impacto, la ruta de propagación de la grieta muestra una ruta más zigzagueante en comparación con las soldaduras con grandes cantidades de ferrita en el límite de grano, ferrita idiomórfica, ferrita de tipo placa lateral o ferrita bainítica, lo que demuestra que la presencia de ferrita acicular mejora la tenacidad de los metales de soldadura. Las islas groseras de martensita-austenita inducen fácilmente micro-grietas en la interfaz entre las islas de martensita-austenita y la matriz de ferrita, lo que deteriora la tenacidad. Las soldaduras con contenidos de B de 22-39 ppm exhiben una excelente tenacidad al impacto debido a la alta cantidad de ferrita acicular, acompañada de finas islas de martensita-austenita.
The microstructures of low carbon low alloy steel weld metals are usually composed of different amounts of coarse-grained transformation products (i.e., grain boundary ferrite (GBF), idiomorphic ferrite (IF) and side-plate ferrite (SPF)), bainitic ferrite (BF), and fine acicular ferrite (AF), depending on the chemical compositions of weld metals, welding processes, cooling conditions, etc. The GBF and IF are the proeutectoid ferrite (PF) nucleated respectively at prior austenite grain boundaries and the inclusions within austenite (
AF nucleates intragranularly in the form of independent plates on the inclusions (
Boron (B) element is another additive used to increase the hardenability, and even a small amount of B can also enhance the hardenability of the weld metal. Free B dissolved in austenite is known to segregate strongly to the austenite grain boundaries, lowering the interfacial energy. On the basis of the transformation thermodynamics, the decrease in the interfacial energy raises the energy barrier for ferrite nucleation at the austenite grain boundaries and, as a result, suppresses the transformation products preferentially nucleated at prior austenite grain boundaries, such as GBF and SPF (
In the present study, the weld metals with a certain amount of Ti, but different B contents were prepared by a submerged-arc welding process. In these weld metals, expensive elements, such as Ni and Mo, were not added, and Mn content was lower. The microstructures in the weld metals with different B contents were analyzed, and the microstructure-toughness relationship was investigated. This work aims at that in the case of no additions of Ni, Mo, etc., in the weld metals, a high amount of AF and an excellent impact toughness can also be obtained only by B addition, and the alloying cost is decreased.
The compositions of weld metals were determined by a Shimadzu OES-5500 optical emission spectrometer except for oxygen and nitrogen, which were analyzed using a Leco TC-436 N/O analyzer. The compositions of the welding wire and weld metals are listed in
C | Mn | Si | S | P | Al |
---|---|---|---|---|---|
0.05 | 0.86 | 0.06 | 0.022 | 0.02 | 0.01 |
C | Mn | Si | S | P | Al | Ti | B | O | N |
---|---|---|---|---|---|---|---|---|---|
0.062 | 1.55 | 0.25 | 0.028 | 0.035 | 0.015 | 0.021 | 0 | 380 | 70 |
0.056 | 1.53 | 0.27 | 0.024 | 0.033 | 0.012 | 0.019 | 22 | 390 | 68 |
0.061 | 1.58 | 0.19 | 0.027 | 0.031 | 0.015 | 0.022 | 39 | 385 | 69 |
0.053 | 1.49 | 0.22 | 0.031 | 0.028 | 0.011 | 0.024 | 61 | 395 | 67 |
0.058 | 1.52 | 0.26 | 0.022 | 0.032 | 0.013 | 0.021 | 85 | 400 | 73 |
(B, O, and N: in ppm)
The specimens for metallographic microstructure observation were cut from the weld metals, and the examined planes were vertical to the welding direction. After mechanically polished, the specimens were etched with 4% nital solution and LePera reagent, respectively. The microstructural morphologies along with M/A islands were examined under a Leica DMIRM image analyzer, and the amounts of different ferritic phases were measured quantitatively.
According to
The fracture surface and the cross-sectional region beneath the fracture surface coated by nickel were observed using an FEI Quanta 600 SEM to investigate the fracture morphology and crack propagation during fracture.
(a) 0 ppm, (b) 22 ppm, (c) 39 ppm, (d) 39 ppm under amplification, (e) 61 ppm, and (f) 85 ppm
B contents | Constituents fractions (area %) | Impact absorbed energy (J) | |||
---|---|---|---|---|---|
IF+GBF | SPF | AF | BF | ||
without B | 39 | 35 | 26 | 0 | 35 |
22 ppm | 20 | 4 | 76 | 0 | 67 |
39 ppm | 0 | 0 | 100 | 0 | 75 |
61 ppm | 0 | 0 | 59 | 41 | 45 |
85 ppm | 0 | 0 | 0 | 100 | 23 |
Additionally, the B content also affects the amount and size of the M/A islands in the microstructures. The characteristics of M/A islands etched in LePera reagent are shown in
(a) 39 ppm and (b) 85 ppm
In the microstructures of low carbon low alloy steel weld metals, different ferritic phases form at their specific temperature ranges during continuous cooling. The formation temperatures for IF, GBF and SPF are higher than that for AF (
(a)-(e) fracture morphologies and (f) impact absorbed energy
The impact toughness of the weld metals is closely correlated with their weld metal microstructures, as shown in
(a) without B, (b) with 39 ppm B and (c) with 85 ppm B
AF is known to consist of fine-grained lath-like ferrite separated by the grain boundaries with the misorientation of 15° or more (i.e., high angle grain boundary) (
Frequently deflecting the crack paths resulted from the impediment of HAGBs can consume more crack propagation energy, leading to an improved impact toughness (
Furthermore, M/A islands in the microstructure also have a significant effect on the impact toughness.
The addition of 22-39 ppm B in the steel weld metals containing Ti results in a remarkable increase in the amount of AF at the expense of GBF, IF and SPF in the microstructures. However, with further increase in the B content up to 61-85 ppm, BF is formed, accompanied with a drop in the amount of AF. In the weld metals with a mainly AF microstructure, the size of the M/A islands is much smaller, and the amount is much lower than those microstructures containing BF.
In the case of the steel weld metals which microstructure is primarily composed of AF, the crack propagation path is more bent in comparison to the weld metals with large amounts of GBF, IF and SPF, or BF. The coarse M/A islands readily induce microcracks at the interface between M/A islands and ferrite matrix, deteriorating the toughness.
The weld metals with B contents of 22-39 ppm exhibit excellent impact toughness because of having a high amount of AF and fine M/A islands in the microstructure.
This work was financially supported by a Project of Education Department of Liaoning Province (grant Nº L2016132). Authors are grateful to Drs. H.Y. Wu and W.N. Zhang (State Key Laboratory of Rolling & Automation of Northeastern University, China) for providing helps in SEM analyses works.