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TECHNICAL BRIEFS

# Dependence of the Yield and Fatigue Strength of the Thread Rolled Mild Steel on Dislocation Density

[+] Author and Article Information

Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iranagazad@yahoo.com

Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran

1

Corresponding author.

J. Manuf. Sci. Eng 129(1), 216-222 (Jun 21, 2006) (7 pages) doi:10.1115/1.2401631 History: Received May 23, 2005; Revised June 21, 2006

## Abstract

Dependence of the yield and fatigue strength of steel bolts with composition in accordance to AISI 1035 manufactured by thread rolling and machining process on dislocation density were investigated. The results indicate that the fatigue strength of the rolled bolts are 55% higher than the machined bolts and by full annealing at $850°C$, it reduced to the extent of machined specimen. Partial annealing of the thread rolled bolts at $680°C$ caused a reduction of fatigue strength by approximately 61% due to reduction in the dislocation density. Fatigue strength was improved by deformation rate (i.e., rolling speed), which is also due to the increasing dislocation density. Yield stress of the studied specimens followed the same pattern as fatigue strength. Considering the obtained results from the low and high speed, partial and full annealed thread rolled specimens, yield stress of the thread rolled bolts has been modeled based on the dislocation density. The obtained results from the model are in good agreement with the experimental results. The contribution to fatigue strength by thread rolling stems from the strain hardening effect which would facilitate the formation of compressive residual stress near the surface layer. The strengthening may be attributed to increasing dislocation density in the ferrite phase (i.e., substructure formation), in addition to the formation of a fine layered structure consisting of elongated pearlite colonies and ferrite grains.

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## Figures

Figure 8

Pearlite lamellar and dislocation substructure in a fully annealed thread rolled specimen

Figure 9

Pearlite interlamellar spacing in a thread rolled specimen

Figure 10

TEM micrograph illustrating pearlite lamellar and dislocation substructure in a partially annealed thread rolled specimen

Figure 11

Dislocation substructure in low speed thread rolled specimen

Figure 12

Schematic representation of composite structure of a thread rolled bolt

Figure 13

S-N curves of thread rolled and annealed bolt

Figure 14

S-N curves of thread machined and annealed bolt

Figure 15

Substructure developed in the specimens fatigued at 300MPa: (a) in high speed thread rolled (fatigue life at 8.7×105cycles), (b) low speed thread rolled (fatigue life at 2.1×104cycles), (c) thread rolled partially annealed (fatigue life at 9.1×102cycles), and (d) thread rolled full annealed (fatigue life at 8.8×10cycles)

Figure 16

A crack formed at the crown of a thread during thread rolling

Figure 1

Dimension of the tensile and fatigue specimens

Figure 2

A schematic showing the preparation method of thin foils for TEM studies

Figure 3

Microhardness pattern of the rolled sample

Figure 4

Microstructure of thread rolled specimen illustrating the inhomogenous deformation pattern consist of ferrite grains and pearlite colonies

Figure 5

Microstructure of deformed area showing stretched ferrite grains and elongated pearlite colonies

Figure 6

Figure 7

Dislocation substructure in a high speed thread rolled specimen

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