Evolution of the Structural State and Properties of 32CrB4 Steel during the Production of High-Strength Fasteners

Alexey Stepanov,Nataliya Arutyunyan,Alexander Zaitsev,Anton Koldaev

doi:10.3390/met12020366

Alexey Stepanov, Nataliya Arutyunyan + Show 2 more

Open Access

https://doi.org/10.3390/met12020366

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Abstract

For the production of high-strength fasteners, bar sections are used, which must withstand significant cold plastic deformation and, therefore, are subjected to a special heat treatment. In the present work, the dependences of the microstructure and mechanical properties of round bars with diameters of 17 mm and 21 mm made of 32CrB4 steel on the heat treatment modes are established. During the study, the methods of optical and scanning electron microscopy, mechanical tests, and hardness measurements were used. Dependences of the degree of spheroidization of the ferrite–pearlite structure and mechanical properties on the spheroidizing annealing mode have been established. It is shown that preliminary obtainment of a bainitic or martensitic structure of rolled products after hot-rolling of steel accelerates the process of pearlite spheroidization. Additional heat treatment, including austenitizing, quenching, and tempering, allows obtaining fasteners of various strength classes: 8.8, 9.8, and 10.9. However, with a guaranteed receipt of strength characteristics for all strength classes, including 12.9, the problem arises of achieving the required values of the relative elongation of steel.

Highlights

High-strength fasteners are widely used in mechanical engineering, automotive, shipbuilding, and construction industries and other branches of technology
Particular attention is paid to the processes of obtaining a spheroidized pearlite structure of steel, which controls its manufacturability during cold forging, as well as the possibility of achieving mechanical properties of the resulting fasteners corresponding to various strength classes
The results of the study of the microstructure showed that the samples of round bars have a ferrite–pearlite microstructure with a 50/50 ratio of pearlite and ferrite

Summary

Introduction

High-strength fasteners are widely used in mechanical engineering, automotive, shipbuilding, and construction industries and other branches of technology. In real-world practice, for the production of high-strength bolts, medium-carbon and alloyed steels containing boron, manganese, and chromium are usually used, for example, 10B21, 20MnTiB, 32CrB4, and 34Cr4 [4]. These alloying elements increase strength and hardenability. The choice of the optimal modes for preliminary treatment and spheroidizing annealing is still carried out in relation to specific steel grades and production conditions For boron steels, this information is limited, and for 32CrB4 steel, it is not available. Particular attention is paid to the processes of obtaining a spheroidized pearlite structure of steel, which controls its manufacturability during cold forging, as well as the possibility of achieving mechanical properties of the resulting fasteners corresponding to various strength classes

Materials and Methods

Mechanical Properties and Microstructure of Hot-Rolled Round Bars

Dis1c7ussion

Conclusions