Relationship between solidification sequence and toughness of carbon steel weld metal

https://doi.org/10.1016/j.matchar.2020.110402Get rights and content

Highlights

  • Total segregation amount strongly depends on solidification sequence.

  • Solidification segregation of S tends to be relaxed in δ-single solidification.

  • S continue to segregate during cooling after the solidification for γ-single.

  • The DBTT increased, and the vEshelf decreased only in γ- single containing sulfur.

  • High final S content induces intergranular brittle fracture with poor toughness

Abstract

This work aimed to investigate the segregation sequence of impurity elements in the weld metal of carbon steel with various solidification sequences, such as δ single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase solidification. Moreover, the relationship between the solidification sequence and toughness of the weld metal of carbon steel was studied. The solidification sequence was controlled by the nickel content of the base metals. Various amounts of sulfur and phosphorous were added to the base metals as impurity elements, and the segregation amount of the impurity elements in the weld metals increased in the order of δ-single-phase, hypo-peritectic, hyper-peritectic, and γ-single-phase. The amount of sulfur was higher than that of phosphorous in each solidification sequence. In δ-single-phase solidification, sulfur segregated during solidification and relaxed during cooling after solidification. In contrast, in γ-single-phase solidification, sulfur continued to segregate during and after solidification. The segregated sulfur was in the grain boundaries of the prior austenite at low temperatures. Charpy impact test results revealed that the ductile-brittle transition temperature (DBTT) increased, and the upper shelf energy decreased with an increase in the nickel content, specifically, only for the specimen of γ-single-phase solidification with S addition. In particular, the tendency in the specimens containing sulfur was more significant compared to the specimens containing phosphorous. Moreover, the intergranular fracture surface was only observed in the γ-single-phase solidification specimen, and sulfur was concentrated on the surface. Therefore, the superposition of segregation during and after solidification induced intergranular embrittlement and deterioration of toughness.

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 Fesingle bondNi system was adopted because the composition range of the peritectic reaction was wider than that of the Fesingle bondC 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 Fesingle bondNi 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%, Mnsingle bondNi 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:

  • 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.

References (28)

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