Relationship between solidification sequence and toughness of carbon steel weld metal
Introduction
The toughness of the weld metal of carbon steel deteriorates because of various factors, such as the heterogeneity of the microstructure, grain coarsening of prior austenite, formation of the brittle phase, and segregation during the welding thermal cycle. Notably, impurity elements, such as sulfur and phosphorous, segregate at the prior austenite grain boundary in the weld metal and cause intergranular embrittlement and a significant deterioration of the toughness [[1], [2], [3]]. This segregation is a superposition of segregation during solidification [[4], [5], [6], [7]] and intergranular segregation during cooling after solidification [[8], [9], [10]]. Moreover, the impurity elements tend to segregate because of a low partition coefficient during solidification [[11], [12], [13], [14]]. Thus, the solidification sequence is critical in determining the total amount of segregated impurity elements because the partition coefficient depends on the solidified phase (mainly the crystal structure). Intergranular segregation during cooling after solidification also strongly depends on the solubility limit and the diffusion coefficient of the impurity element in the matrix phase [15,16]. Therefore, to improve the toughness of the weld metals of carbon steel, it is necessary to understand and control the influence of the impurity elements on the toughness, that is, the segregation behaviour of the elements from the start of solidification to low temperatures.
However, the microstructure evolution of carbon steel is intricate as it involves a peritectic reaction during solidification and solid-solid phase transformations during cooling. Thus, there are few reports on the segregation phenomenon from the start of solidification to low temperatures because. The solidification sequence of carbon steel can be categorized into δ-ferrite single-phase, hypo-peritectic, hyper-peritectic, and austenite single-phase solidification, corresponding to the chemical composition [[17], [18], [19], [20], [21]]. After solidification, the solid state transformation occurs from austenite into ferrite, pearlite, and martensite. In addition, the solid solubility limits and the partition coefficients of sulfur and phosphorous are different between the δ-ferrite and austenite phases. Therefore, the solidification sequence and the transformation behaviour have a strong influence on the solidification segregation and intergranular segregation after solidification.
In the present study, the influence of the solidification sequence on the segregation behaviour from the start of solidification to low temperatures, and the toughness of the weld metals of carbon steel, were investigated. The relationship between the microstructural evolution consisting of solidification and solid state transformations and the segregation behaviour was studied using specimens with various solidification sequences. Nickel was added to control the solidification sequence of the specimen, and the contents of sulfur and phosphorous as the impurity elements were varied. The FeNi system was adopted because the composition range of the peritectic reaction was wider than that of the FeC system, and it was possible to determine the morphology of the primary phase (cell structure) from the solidification segregation of nickel.
Section snippets
Specimen preparation
The specimens were of the FeNi system, and the nickel content controlled the solidification sequence. The contents of carbon, silicon, and manganese were fixed and set to 0.015 mass% (hereafter %), 0.05%, and 0.05%, respectively. Based on a pseudo-binary phase diagram of the Fe-0.015% C-0.05% Si-0.05%, MnNi system was calculated using Thermo-Calc. The nickel content was 1.0% and 3.0% for δ-ferrite single solidification (δ-single), 3.6% and 4.0% for hypo-peritectic solidification (hypo-p), 4.4%
Microstructure of weld metal
Fig. 2 shows OM images and IPF maps of the weld metals of the weld specimens containing 100 ppm S. The image and the map were taken from identical areas in each specimen. Coarse ferrite grains are observed in δ-single. Coarse ferrite grains, ferrite side plates, and intergranular ferrites are found in hypo-p and hyper-p. Thus, more and larger ferrite side plates and intergranular ferrites form with an increase in the nickel content. However, for γ-single, martensite and bainite are observed in
Discussion
The microstructural analysis of the quenched specimens revealed that the segregation behaviour of the impurity elements, such as sulfur and phosphorous, strongly depended on the solidification sequence. Phosphorous segregated during solidification in the specimens containing phosphorous of various solidification sequences except for δ-single; however, the solidification segregation was relaxed during cooling (solid state) after solidification. In the specimens containing sulfur, solidification
Conclusions
The solidification sequence on the segregation behaviour from the start of solidification to low temperatures, and the toughness of the weld metal of carbon steel was investigated. Moreover, the segregation behaviour of the impurity elements of sulfur and phosphorous during solidification and cooling after solidification was examined under various solidification sequences. The results are as follows:
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The total segregation amount increased in the order of δ-single, hypo-p, hyper-P, and γ-single,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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