Ductile-Brittle Transition Temperature of Ultrafine Ferrite/Cementite Microstructure in a Low Carbon Steel Controlled by Effective Grain Size

Abstract

To analyze the good toughness of ultrafine ferrite/cementite steels, the concept of effective grain size (d EFF ) is applied to ductile-to-brittle transition temperature, DBTT, for ultrafine ferrite/cementite (Uf-F/C), ferrite/pearlite (F/P), quenched (Q), and quench-and-tempered (QT) microstructures in a low carbon steel. The d EFF is determined to be 8, 20, 100, and 25 mm for Uf-F/C, F/P, Q, and QT, respectively. In F/P and Q, it is in accordance with the ferrite grain size and the prior austenite grain size, respectively. In QT, the d EFF fits the martensite packet size. In Uf-F/C, the ferrite grain size has a bimodal distribution and the larger grain size corresponds to the d EFF , which is the smallest among the four microstructures. In terms of the relationship between d EFF and DBTT, the Uf-F/C, Q, and QT microstructures can be placed into the same group and the F/P to a different one. Furthermore, the Uf-F/C has the highest estimated fracture stress among the four microstructures. These might be the result of the difference in the surface energy of fracture, namely the former is estimated to have a surface energy of 34.6 J/m 2 and latter a surface energy of 7.7 J/m 2 . Thus, the excellent toughness of the ultrafine ferrite/cementite steel can be attributed to the small d EFF and the high surface energy of fracture.KEY WORDS: effective grain size; ductile-to-brittle transition temperature; toughness; fracture strength; low carbon steel; ultrafine grain structure. Fig. 1. Relationship between the change in DBTT and the change of 9.8 MPa in yield strength for different microstructural factors, i.e. dislocation strengthening, dispersion strengthening, and grain-size strengthening.

Surface imaging of metallic material fractures using optical coherence tomography

“…In the present paper we shall analyze the structure of metallic materials (their fracture surfaces) previously subjected to tests for ductile [10,11] or brittle fractures [12][13][14]. The aim is to demonstrate the capability of OCT to replace an established method of analysis, SEM, used in such studies.…”

Two Step Ductile to Brittle Transition Behavior on Ferrite + Pearlite Structure Steel Sheet

“…This model indicated the important factors that affect the relation shown in Fig. 14: the properties of structures in steel, 9,11,21) strain rate, 19) specimen thickness, 13) and shape of the notch. 13) However, the evidence for the model in this study is not sufficient.…”

“…Figure 14 shows the schematic relation among the yield stresses of ferrite and pearlite, the brittle fracture stress of pearlite, and the DBT temperatures in ferrite + pearlite DP structure steel sheet. 20,21) Pearlite, which is a harder structure in DP steel sheet, has higher yield stress and lower fracture stress than ferrite, which is a softer structure. The yield stresses of ferrite and pearlite (σ Y, α and σ Y, P , respectively) are smaller than the fracture stress of pearlite (σ F, P ) at temperatures above T 1 .…”

Analysis of the fractures of metallic materials using optical coherence tomography

“…In order to demonstrate the utility of the OCT method for fracture analysis, we have considered in [17] both ductile and brittle fractures [19,20]. In the following a different steel sample (OL44) than the ones previously investigated is presented; such a material is usually subjected to ductile fractures.…”