Elsevier

Acta Materialia

Volume 57, Issue 13, August 2009, Pages 3754-3762
Acta Materialia

New insights into intragranular ferrite in a low-carbon low-alloy steel

https://doi.org/10.1016/j.actamat.2009.04.045Get rights and content

Abstract

Single and multiple nucleation events on non-metallic inclusions have been observed, leading to the intragranular formation of ferrite as a function of transformation temperature. Three-dimensional shapes have also been characterized. When the ferrite forms at elevated temperatures it is in the form of isolated idiomorphs, whereas larger undercoolings are associated with the multiple nucleation of plates emanating from the inclusions. The plates grow at a fixed orientation to the parent austenite. Nanohardness tests indicate that the idiomorphs are softer when compared with the plates. The formation of intragranular ferrite laths or plates can facilitate the attainment of fine-grained microstructures.

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.

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