The effect of heating and thermocycling on the mechanical properties of carbon steel rope wire

Heating and thermal cycling of carbon steel rope wire significantly affect the mechanical properties and durability of the entire rope under operating conditions. However, despite a large amount of studies aimed at determination of the reasons for destruction of the ropes of foundry cranes at metallurgical plants and development of measures for their elimination, accidents attributed to sudden rupture unfortunately occur. Therefore, the problem is still urgent and requires further research. Nowadays, new exhaust air suction systems for hot air streams mounted in converter shops of metallurgical enterprises reduce dangerously high temperatures to T = 240 – 300°C thus providing the possibility to avoid overheating of the ropes. However, the rope metal is exposed to a strong impact of blue brittleness – phenomenon, which causes reduction of the metal plasticity in this temperature range. We present the results of studying the impact of the heating temperature and number of cycles in thermocycling on changes in the characteristics of the mechanical properties of steel 70KK wire ropes in tensile tests. Tests of the wire samples are carried out at room temperature and after their exposure to high-intensity heating and thermal cycling at different temperatures and number of cycles. confirmed Presence of zones of blue brittleness is proved experimentally for the wire rope with a diameter of 1.65 mm in a temperature range of 200 – 240°C. It is shown, that the largest reduction in the relative narrowing (21%) and increase in the ultimate tensile stress by 4.8% occurred at a temperature of 240°C. This phenomenon should be taken into account when assessing the structural strength of steel. It is shown that there are reserves to increase the life cycle of ropes, taking into account the data on diagnostics of the mechanical and magnetic characteristics in industrial conditions.

PROCESSING METHOD FOR ENLARGING PARTIAL DIAMETER OF STEEL SHAFT HEATED LOCALLY BY HIGH FREQUENCY INDUCTION

In the present works effective application of partial preheating by high frequency induction to enlarge the partial diameter of middle carbon steel was experimentally investigated. The partial reheating was carried out by varying preheating times under the same electric power conditions. As all other conditions other than temperature remained constant during the diameter enlargement experiments, the partial preheating effects were clarified to be estimative using a parameter of the σ/σ y , which was found to be dependent on temperature regardless of constant compressive axial stress. For heating times of less than 50 sec, the preheating effect for promoting partial diameter-enlargement did not appear. In fact the reverse effect appeared in times ranging from 30 sec to 50 sec due to blue brittleness generated at temperatures from about 523K to about 623K. Also, low cycle fatigue cracks must be avoided when initiating at the filet end of an enlarged shaft while passing through the temperature range of blue brittleness during the cooling process. Therefore, the preheating temperature found to be most effective for promoting the diameter enlargement was greater than about 856K so as to increase easily the D f /D0 to above 2 times without generating damage cracks from fatigue.