Abstract
To produce high-quality fasteners for motor group components of automobiles it is necessary to follow the increased requirements to calibrated rolled stock in terms of surface defects. Therefore, the goal of this paper is to study the reasons, types and depth of the surface defects on the calibrated rolled stock from steel 38KHGNM Ø 12.0 mm on the basis of the metallographic analysis. Before cold upsetting, the hot-rolled products are subjected to metal flow and removal of unacceptable surface defects by means of expensive turning operation during which the screw cuts and cracks might appear. It has been defined, that the hot-rolled stock from steel, grade 38KHGNM, diameter 12.0 мм has nonuniform mechanical properties, grooves, laps and partial decarburization on the surface. The heat treatment of the rolled stock with a decarburized layer on the surface contributes to its further decarburization. Poor alignment of calibrated stock during its turning at the turning machine does not enable to completely remove the decarburized layer with minimum skinning of rolled stock. It has been shown that the use of rolled stock from steel 38KHGNM with surface defects and unreasonably high decarburized layer on the surface increases its rejection by 8% and raises the consumption of rolled stock for manufacturing of important fasteners for the motor group of automobiles.
Highlights
The automotive fasteners made from calibrated rolled stock by cold upsetting refer to the most important components in mechanical engineering [4,5,6]
The increased requirements in terms of surface defects - their depth and the presence of a decarburized layer are applied to calibrated rolled stock used for cold die forging [7,8,9]
A negative defect in the hot-rolled and calibrated rolled stock surface is the presence of a decarburized layer [12], which significantly worsens its mechanical properties, since the surface becomes susceptible to the formation of grooves and scratches [13]
Summary
The most important task in the production development is the improvement of quality of the initial metal products and parts made therefrom, increase of their performance, reliability, durability, as well as bringing of these indicators to the world levels in order to ensure the products competitiveness at the world market [1,2,3]. The increased requirements in terms of surface defects - their depth and the presence of a decarburized layer are applied to calibrated rolled stock used for cold die forging [7,8,9]. It is relevant for fasteners used in the components of the motor group of automobiles [10, 11]. GOST norm 10702-78 “Profiles made of carbon and alloy high-quality structural steel for cold extrusion and upsetting” specifies the requirements to the presence of defects and decarburized layer on the rolled product surface and their permissible values.
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