New insights into intragranular ferrite in a low-carbon low-alloy steel
Introduction
Intragranular ferrite formed in association with non-metallic inclusions has attracted much attention since its discovery in weld metal because it leads to a good combination of strength and toughness. Intragranular plates are particularly desired due to their shape, size and distribution, which allow the development of complex, interlocked microstructures with a considerable density of crystallographic discontinuities [1], [2], [3]. Propagating cracks are therefore forced to adopt tortuous paths, resulting in better toughness [1]. This is important particularly given the increasing importance of productive welding techniques, where the heat inputs during fabrication have to be large. Conventional steels then suffer from the coarse austenite grains that develop adjacent to the weld; if nucleation only occurs on austenite grain boundaries, then the coarse grains tend to transform into detrimental phases, such as martensite, during cooling [3]. This is unacceptable from a toughness point of view. The stimulation of intragranular ferrite on inclusions within the coarse austenite grains is a solution to this problem and may lead to enhancements in technology in the construction of ships, pressure vessels and pipelines [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].
Intragranular ferrite and its relation with inclusions in steels have been investigated extensively [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. The mechanisms of inclusion stimulated nucleation are: (i) solute depletion in the vicinity of a non-metallic inclusion [12], [13], [14], [15]; (ii) reduced interfacial energy between ferrite and inclusions [9], [18]; (iii) thermal strain energy due to the different expansion coefficients of the inclusion and matrix [18] and (iv) provision of an inert surface [19], [20]. However, the subject could still benefit from clarification, e.g. of the details of whether single or multiple nucleation occurs at an individual site, and how the interlocked plate microstructures (so-called acicular ferrite) are formed. Features such as the three-dimensional morphologies, orientation relationship and micromechanical properties of the intragranular ferrite need further research to reveal the mechanisms involved. The present study was undertaken with these specific purposes in mind.
Section snippets
Experimental
The steel was prepared by vacuum induction melting utilizing high-purity electrolytic iron, graphite and manganese; the chemical composition of the alloy is listed in Table 1. A 50 kg ingot of the steel was hot-rolled at 1000 °C into a plate of 50 mm thickness and subsequently cold rolled with a reduction ratio of 70%. Specimens 10 × 10 × 0.35 mm in size were austenitized at 1250 °C under a purified argon atmosphere and isothermally reacted in a salt bath at temperatures in the range 610–690 °C for
Three-dimensional morphology of intragranular ferrite at elevated transformation temperatures
Fig. 1a and b shows optical micrographs of ferrite idiomorphs formed by isothermal transformation at 690 °C for 40 s. The arrowed black dots are inclusions on which the ferrite initiates. Though the formal mineralogical definition of an idiomorph is a mineral form with an external shape consistent with its crystal lattice, it is used loosely to describe precipitates with approximately equiaxed shapes [21]. In the present work, the ferrite idiomorphs have this equiaxed morphology.
The shape of the
Nucleation of intragranular ferrite
Much progress has been made in understanding the nucleation of intragranular ferrite [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Most believe that one or more nucleation mechanisms play a role in the nucleation of intragranular ferrite. In the present work, vanadium and phosphorus were deliberately added to the steel to form the inclusions responsible. SEM observations and EDS analyses of intragranular ferrite grains and inclusions demonstrated that a layer of MnS was
Conclusions
The nucleation, three-dimensional morphology, orientation relationship and nanohardness of intragranular ferrite in a low-carbon low alloy steel are investigated by means of serial sectioning, computer-aided 3-D reconstruction, EBSD and nanohardness indentation techniques. The following conclusions are drawn:
- (1)
Intragranular ferrite grains are nucleated on inclusions at higher, intermediate and lower transformation temperatures. One-to-one nucleation of ferrite idiomorphs on inclusions is observed
Acknowledgements
The authors gratefully acknowledge the support from NSFC (National Natural Science Foundation of China) and Baosteel under Grant Nos. 50471107 and 50734004.
References (29)
- et al.
Acta Mater
(2008) - et al.
Mater Sci Eng A
(2006) - et al.
Mater Sci Eng A
(2006) - et al.
Mater Charact
(2008) - et al.
Acta Mater
(1997) - et al.
Acta Mater
(1999) - et al.
Acta Mater
(2001) - et al.
Acta Mater
(1999) - et al.
Mater Charact
(2004) - et al.
Mater Sci Eng A
(2003)
Bainite in Steels
Mater Sci Tech
Trans ISIJ
Tetsu-to-Hagané
Cited by (84)
Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel
2024, Materials Science and Engineering: ACrystallographic characteristics of acicular ferrite nucleated on inclusions in a HSLA steel
2024, Journal of Materials Research and TechnologyWeld morphology and grain growth characteristics of driven moving arc hollow stud welding
2023, Journal of Materials Research and TechnologyNew insights of heterogeneous nucleation and anisotropic growth of acicular ferrite on non-metallic inclusion
2022, Materials and DesignCitation Excerpt :Among different types of microstructure evolution, acicular ferrite (AF) is a typical microstructure that precipitates in the interior of the original γ austenite-grain through the heterogeneous nucleation on non-metallic inclusions (NMIs). Although the ‘acicular’ term denotes a needle-like morphology, it has been experimentally confirmed that AF is in form of a lenticular plate with very thin thickness rather than a rod [4] (see Fig. 1). Due to its chaotic ordering leading to a fine ‘basket weave’ appearance, the material microstructure achieves a considerable improvement in strength and toughness [5–6].
Mechanical property and strengthening mechanism on DH36 marine steel after laser surface melting
2021, Surface and Coatings Technology