Cyclic cracking resistance of low-alloy and carbon structural steels hardened by tensile deformation and heat treatment

Abstract

Cold drawing deformation of structural steels increases the yield limit of the material while retaining sufficiently high ductility and toughness. Consequently, in a number of cases the users of constructional rolled metal stock give preference to hardening by cold drawing in comparison with thermal hardening. This treatment is applied to, in particular, low-alloy structural steels. Taking into account the large volume of production of these steels, it is easy to see that the production of material with high strength by this method is associated with a high consumption of scarce alloying elements. In the USSR, attempts were made in recent years to examine the process of cold deformation of standard low-alloy steels strengthened by heat treatment and the possibilities of increasing the strength of structural rolled metal stock from these steels, already subjected to preliminary thermal hardening. At the same time, there are almost no reports on the susceptibility to brittle failure after these treatments, evaluated by fracture mechanics criteria. Hardening reinforcement bars by cold drawing has a strong effect on the cyclic cracking resistance of the material, in particular on one of the most important characteristics, i.e., the growth rate of fatigue cracks. In this case, the effect of cold deformation on this parameter is ambiguous: the fatigue crack growth rate can either decrease or increase, or remain unchanged in comparison with its value in the steel in the undeformed condition [1-5]. It is also necessary to explain the problem of the behavior of a fatigue crack in cold-deformed metal, whose properties change during cyclic loading. The importance of these interrelationships for practice is evident since the capacity of the material to resist brittle failure determines in the final analysis the service life of the structure. The aim of this investigation was to obtain such data for reinforcement steels of a periodic section, and carry out comparative analysis of the variation of the properties during cyclic of loading hot-rolled and thermally hardened cold-deformed steels. Hardening these steel bars by drawing was carried out by cold tensile deformation of reinforcement rods of a periodic section 850 mm long. Tensile loading was carried out in an EUS-100 hydraulic testing machine with the travel speed of the moving clamps of 5 mm/min. The degree of plastic deformation was inspected using an Amsler mechanical strain gauge with the measurement length of 40 mm and a scale divided in 0.01 mm. The uniformity of plastic deformation was evaluated on the basis of simultaneous readings of two strain gauges fixed at the ends of the gauge length. The measurement results show that the nonuniformity of plastic deformation along the length of the rod does not exceed + 5 %. The reinforcement rods, hardened by cold drawing, were then tested by static tensile loading to determine the standard mechanical characteristics (ultimate strength, physical yield limit or proof stress, relative uniform and relative elongation). The experiments were carried out on specimens 450 mm long in the same EUS-100 testing machine. Cyclic cracking resistance was evaluated by constructing the diagrams of fatigue failure (FFD) in cantilever bending of prismatic specimens 8  16 x 60 mm in size with a side notch at a loading frequency of 11 Hz and a stress ratio of R=0. The length of the fatigue crack was measured by the method of the electrical difference of the potentials with automatic recording of the relevant parameter. The measurement error of the length of the fatigue crack did not exceed 6% and that of the force was not higher than 9%.