Analysis and prevention of cracking phenomenon occurring during cold forging of two AISI 1010 steel pulleys

Abstract

This study is concerned with the effects of microstructural parameters on the cracking phenomenon occurring during cold forging of two AISI 1010 steels that were fabricated by converter steel making and electric furnace steel making, respectively. This allowed a comparison between microstructures that contained a small or large amount of nitrogen. Detailed microstructural analyses of the cracked region showed that a number of adiabatic shear bands, along which cracks initiated and propagated, were formed in the top interior part of the cold-forged pulley. Dynamic torsional tests were conducted using a torsional Kolsky bar in order to investigate the dynamic deformation behavior during cold forging, and then the test data were compared via microstructures, mechanical properties, adiabatic shear banding, and fracture mode. From the dynamic shear stress-strain curves, the steel containing a considerable amount of nitrogen showed a smaller shear strain of 0.2 at the maximum shear stress point, after which the shear stress decreased rapidly prior to fracture, whereas the other steel containing a smaller amount of nitrogen showed relatively homogeneous shear deformation. This dynamic torsional behavior correlated well with the cracking and adiabatic shear banding behavior, together with the yield-point phenomenon occurring in the steel containing more nitrogen. Because the cracking occurring during cold forging was associated with the adiabatic shear banding and the yield-point phenomenon, the minimization of nitrogen and the fast cooling rate after hot rolling were suggested to prevent the cracking.

Dynamic deformation behavior of ultrafine-grained low-carbon steels fabricated by equal-channel angular pressing

“…The thermal instability caused by the abrupt temperature rise further accelerates the inhomogeneous deformation and causes a concentration of shear deformation in the central area of the specimen. [15][16][17][18][19]21] The local concentration of plastic deformation reduces the load-carrying capacity of the area, thereby leading to a rapid decrease of shear stress in the shear stress-shear strain curves of Figure 4, followed by final fracture.…”

“…It plays a role in initiating a partial or total fracture of materials, which is not desirable in the perspective of fracture resistance under dynamic loading. [15][16][17][18][19] Consequently, it is desirable to prevent the formation of adiabatic shear bands by microstructural control through alloy design or heat treatment.…”

“…In this localized area, the hardness increases greatly and the load-carrying capability seriously deteriorates, thereby causing defects or eventual failures during high-speed processing. [15][16][17][18][19] Therefore, studies on microstructural modification and high-speed process controlling are essential in order to improve the dynamic properties of the ultrafine-grained materials.…”

Dynamic Torsional Deformation Behavior of Ultra-Fine-Grained Dual-Phase Steel Fabricated by Equal Channel Angular Pressing

“…It further accelerates localized shear deformation at the gage center, which reduces the load carrying capacity of the central area, thereby leading to a rapid decrease of shear stress in shear stress-shear strain curves and final fracture. [27][28][29][30] Adiabatic shear bands are frequently found in materials having low strain hardenability, low strain rate sensitivity, low thermal conductivity, and high thermal softening rate. [23][24][25] Dynamic torsional deformation of ductile metals develops in the following three stages: (1) homogeneous deformation stage before reaching maximum shear stress, (2) inhomogeneous deformation stage starting from the maximum shear stress point, and (3) initiation and development stage of an adiabatic shear band due to shear localization.…”

“…[12,23] It plays a role in initiating a partial or total fracture, which is not desirable in the perspective of the fracture resistance under dynamic loading. [12,23,[27][28][29] Consequently, it is desirable to prevent the formation of adiabatic shear bands by the microstructural control through alloy design or heat treatment.…”