Towards a dislocation-based model for strain path effects in bainitic pipeline steels

Albert Van Bael,Marc Seefeldt,Matthias Bönisch,Nuria Sanchez,Steven Cooreman

doi:10.25518/esaform21.2403

Abstract

Modern pipeline steels exhibit complex microstructures that cause mechanical anisotropy in various respects. For instance, strain path effects under non-monotonic loadings are exceptionally pronounced in these steels. Crystallographic texture and morphological anisotropy are the main contributors to strength and hardening directionality in pipeline steels under monotonic loading. In contrast, the dislocation substructure is seen as the primary source for Bauschinger and cross effects during complex non-monotonic loading, e.g. during pipe forming. The Bauschinger effect for example may arise from pile-ups formed at obstacles such as intragranular shear bands, and homo- or heterophase boundaries. The dislocation-based model by Peeters et al. [Acta Mater., 49 (2001), pp. 1607-1619] developed for coarse-grained ferritic steel allows for complex strain path effects through the accumulation of dislocations at micro-shear bands. However, it struggles to reproduce the large Bauschinger effect of ~250MPa in fine-grained bainitic pipeline steel [Bönisch et al., Procedia Manuf., 47 (2020), pp. 1434-1441]. Considering the microstructural differences between the two steel varieties, a promising way to improve the model predictions - especially for the Bauschinger effect - is to incorporate dislocation interactions with phase and/or grain boundaries. In the present work, we introduce this approach and demonstrate the basic capabilities of such a grain boundary-extended Peeters model. By accounting for the formation of pile-ups at grain boundaries the Bauschinger effect is enlarged. Furthermore, by explicitly considering the grain boundary spacing, the model can deliver grain size (Hall-Petch) strengthening.

Highlights

Bainitic high-strength low-alloy (HSLA) steels are widely used for the construction of pipelines for long-distance liquid and gas transport
Despite the success of Peeters' model, our recent work suggested that incorporation of dislocation interactions with phase and/or grain boundaries are important to reliably reproduce the large Bauschinger effect exhibited by X70 pipeline steels [3]
The presented work demonstrates the development of a novel dislocation-based crystal plasticity hardening model for fine-grained bainitic steel

Summary

Introduction

Bainitic high-strength low-alloy (HSLA) steels are widely used for the construction of pipelines for long-distance liquid and gas transport. Modern pipeline steels consist of different constituents, such as ferrite (polygonal, acicular, granular, bainitic), martensite-austenite, and non-metallic inclusions [2, 4,5,6,7] These complex microstructures abound with obstacles, such as intragranular shear bands, homo- or heterophase boundaries, which in turn impede dislocation motion, leading to pile-up formation. Dislocation-based models which explicitly include the dislocation substructure and pile-up formation are favored, since substructure and pile-ups are the major sources of the Bauschinger effect This law is still to be developed and, based on our experience, should incorporate intragranular shear bands and grain boundaries as directional, planar glide obstacles including their crystallographic alignments.

Dislocation-based hardening models for cubic materials

Present work

Proof-of-concept

Findings

Conclusions