Influence of Operating Conditions of the Steel Cord on the Structure and Selected Mechanical and Technological Properties of High Carbon Steel
Abstract
When buying tires, we would like them to serve us as long as possible. Experts of the independent organisation Dekra, based on case studies found that after six years the risk of tire failure increases drastically, which is why they recommend replacement of tires after six years of operation at the latest. The results of the presented studies have shown that operation of the car tires in such extreme conditions as continuous contact with ground, frequent braking and repeated hitting against bumps in the road leads to delamination of the steel material of belting and decrease in its mechanical properties by about 35-48% compared to the reference material.
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Introduction
Car tire is the outer part of the wheel of open section, applied to the wheel rim and filled with pressurised air. It is a part of the vehicle contacting with the ground, can carry the weight of the vehicle, the driving, braking and turning forces, as well as provide the basic shock absorption during the ride [14,15]. Build of the car tire is complex and its main parts are: tread, belt, matrix, filler and bead wire. The tread is responsible for tractive adhesion and driving a car. The foot consists of a bead wire and a filler and it enables assembling a tire at the wheel rim and provides for tightness of that junction. The matrix is the primary load-bearing part in the whole structure, providing for dimensional stability and resistance to tearing. The other, extremely important part is the belt, the basic function of which is stiffening of the tire front and providing resistance to devastating effects of the centrifugal force at high speeds. The belt consists of one or several layers of steel cord, the layer of resistant wires of 0,2-0,5 mm diameter, made of pearlite steel [10, 5, 11].
The eutectoid steels containing from about 0,8 to 0,95% of C belong to the group of unalloyed steels of the quality class designated for cold drawing or rolling [1,2]. They have found application mainly as the wires for production of steel cord used for reinforcing tires (PN-EN 10323:2005 (U)), hoses (PN-EN 10324:2006) or production of ropes (PN-EN 102641:2005) [4,8]. Their chemical composition and mechanical properties should comply with the PN-EN 10323:2005 (U) standard [10,4]. However, it should be remembered that mechanical properties, and mainly the tensile strength, are highly dependent on the percentage content of steel components, especially on contents of chromium, silicon and manganese. In the annealed state the eutectoid steels have the highest strength in relations to the remaining unalloyed steels [4]. At the same time the steels are characterised with particularly low share of non-metallic inclusions and limited contents of chromium and nickel, which affect the prolongation of pearlite transformation time [4,8].
Conclusion
The results of microstructure analysis of the steel belt wires have shown that during tire operation the material is destroyed. The material structure delaminations observed in the longitudinal sections were visible exclusively in the after operation wires and oriented in the direction of the stress distribution in those components of the tire.
At the same time the analysis of the results of the mechanical property tests have shown that the consequence of the car tire operation is clear decrease in the strength and technological properties of the steel belt wires by some 35-48% in relation to the reference material.
Operation of car tires in such extreme conditions as continuous contact with ground, frequent braking and repeated hitting bumps in the road leads to delaminations of the steel belt material and changes in its mechanical properties [5,12,13]. The necking and the tensile strength directly influence the increase in Young’s modulus value, and by that the decrease in stiffness of the steel belt wires which, as a consequence, causes reduction in stiffness of the tire forehead and decrease in the resistance to the devastating effects of centrifugal force at high speeds.
When buying tires, we would like them to serve us as long as possible. Experts of the independent organisation Dekra, based on case studies found that after six years the risk of tire failure increases drastically, which is why they recommend replacement of tires after six years of operation at the latest. The results of the presented studies confirm that each kilometre of the trip distance is not without an impact on the quality of the steel belt and thereby the properties of car tires and most importantly on the driving safety.