Fracture Toughness and Fatigue Crack Growth Characteristics of UFG Microalloyed and IF Steels Processed by Critical Phase Control Multiaxial Forging

Abstract:

The aim of the current study is to design multiaxial forging (MAF) schedules in order to achieve submicron-grained (<1μm) structure in a microalloyed (MA) steel as well as an interstitial-free (IF) steel, which could impart a good combination of yield strength and tensile ductility. At the same time, an effort has been made to evaluate the fracture toughness characteristics by conducting 3-point bend tests and computing the K_Q, K_ee and J-integral values of ultrafine grained (UFG) samples and correlating them with the microstructure, besides evaluating the other mechanical properties. Fatigue strength in the high cycle fatigue (HCF) regime were also investigated and fracture mechanisms analyzed and comparison established between differently processed samples. The microstructural analysis was performed using transmission electron microscopy (TEM) and Electron backscatter diffraction (EBSD) and results corroborated with the mechanical properties. Superior combinations of yield strength (YS), ductility (% El.), fracture toughness (K_ee) and high cycle fatigue strength (σ_f) were obtained under certain conditions, i.e., i) MA steel: intercritical (α+γ) phase regime (~A_r1) controlled and 15-cycle multiaxially forged (MAFed) (YS=1027MPa, %El.=8.3%, σ_f=355MPa and K_ee=90MPa√m), and ii) IF steel: ferritic region (<A_r1) controlled 18-cycle MAFed (YS=881MPa, %El.=11.2%, σ_f=255MPa and K_ee=97MPa√m). In the case of MA steel, an enhancement of the fatigue and fracture toughness properties can be ascertained following the formation of uniformly distributed nanosized fragmented cementite (Fe₃C) particles (~35nm size) present in the submicron sized (average ~280nm size) ferritic microstructure. In contrast, in the case of IF steel, this is ascribed to the development of submicron sized ferrite grains (average ~320nm) along with a high density of dislocation substructures. These fine dislocation cells/substructures along with the nanosized Fe₃C particles could effectively block the initiation and propagation of cracks and thereby enhance the fatigue endurance and fracture toughness of the steel. Superior fracture toughness along with high mechanical properties in submicron-grained condition render the two steels highly useful for high-strength structural applications.

• Periodical: Materials Science Forum (Volume 1016)
• Pages: 481-486
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.1016.481
• Online since: January 2021
• Authors: Sumit Ghosh*, Suhrit Mula, Mahesh Chandra Somani, Jukka I. Kömi
• Keywords: Fatigue Crack Growth, Fracture Toughness, IF Steels, Microalloyed, Multiaxial Forging

References

[1] Azushima A, Kopp R, Korhonen A, Yang DY, Micari F, Lahoti GD, Groche P, Yanagimoto J, Tsuji N, Rosochowski A, Yanagida A. Severe plastic deformation (SPD) processes for metals. CIRP Ann Manuf Technol 2008; 57:716-735.

DOI: 10.1016/j.cirp.2008.09.005

[2] Alexander DJ. New Methods for Severe Plastic Deformation Processing. J Mater Eng Perform 2007; 16:360–374.

[3] Kundu A, Kapoor R, Tewari R, Chakravartty JK. Severe plastic deformation of copper using multiple compression in a channel die. Scripta Mater 2008;58:235-238.

DOI: 10.1016/j.scriptamat.2007.09.046

[4] Ghosh S, Mula S, Improvement of fracture toughness of Ti+Nb stabilized microalloyed and interstitial free steels processed through single phase regime control multiaxial forging. Mater. Sci. Eng. A 2020;772:138817.

DOI: 10.1016/j.msea.2019.138817

[5] Ghosh S, Mula S, Fracture toughness characteristics of ultrafine grained Nb–Ti stabilized microalloyed and interstitial free steels processed by advanced multiphase control rolling. Mater. charact. 2020;159:110003.

DOI: 10.1016/j.matchar.2019.110003

[6] Hui W, Zhang Y, Shao C, Chen S, Zhao X, Dong H, Microstructural effects on high-cycle fatigue properties of microalloyed medium carbon steel 38MnVS, Mater. Sci. Eng. A 2015;640:147-153.

DOI: 10.1016/j.msea.2015.05.054

[7] Vinogradov A, Fatigue limit and crack growth in ultra-fine grain metals produced by severe plastic deformation, J Mater. Sci. 2007;42:1797-1808.

DOI: 10.1007/s10853-006-0973-z

[8] Majumdar S, Bhattacharjee D. Ray K.K, On the Micromechanism of Fatigue Damage in an Interstitial-Free Steel Sheet, Metal Materials Transactions A 2008;39:676-1690.

DOI: 10.1007/s11661-008-9537-y