Operational characteristics of steel structures of long term service life

Abstract

Introduction. The process of natural ageing of metal structures under influence of long-term mechanical loading and environmental factors, leads to changes in physical, chemical and mechanical properties of metals. Nowadays, a considerable number of industrial and civil buildings with metal structures having service life in the range of 30 to 120 years are in operation all over the world. Most of the scientific work on the aging of metals has been done in mechanical engineering, wear resistance of pipelines, machine parts, etc. In this connection, undoubtedly, the development of scientific direction connected with the study of peculiarities of changes in the structure, physical and mechanical characteristics of steel structures of a long period of their operation becomes actual. This is caused by the necessity of safe functioning of these constructions during the whole service life of a building or a structure, and also by the necessity of prediction of their behavior in fire and fire-resistant conditions.
 
 Materials and methods. The results of research concerning a steel structure specimen with a service life of 86 years have been presented. A set of experimental research methods was used: optical emission method to determine the chemical composition; metallographic method to study the microstructure, identify material defects and evaluate non-metallic inclusions in the structure of the sample; static tensile tests to determine the quality and performance characteristics of materials, in particular strength and ductility.
 
 Results. The results show that the chemical composition of the steel in general remains essentially unchanged during operation. A comparative analysis of the microstructure of steel St3 and steel with a lifetime of 86 years indicates slight changes in the structure related to the presence of multi-grain and fragmentary restructuring of structural elements, which may be indicative of the beginning of the destruction of the structure and reduce the strength characteristics of steel. The steel structure revealed the presence of non-metallic inclusions (point oxides and manganese sulfides) which serve as stress and strain concentrators in the matrix, causing local material failure at lower average strain, which may also lead to a reduction in the tensile strength of steel structures. According to the results of metallographic analysis insignificant transformations in the structure characterized by a change in perlite morphology, the presence of heterogeneity as well as the presence of non-metallic inclusions in the form of point oxides and manganese sulfides can be demonstrated. The significant influence of uniform and ulcerous corrosion process flowing into corrosion cracking going deep into the base metal on mechanical characteristics of steel is revealed. The obtained values of operational characteristics of the considered steel structure, having surface corrosion damages shows the lowering of ultimate strength by 15 %, yield strength by 10 % and relative elongation by 12 % from the normative values.
 
 Conclusions. Characteristic changes in the structure of steel with a long service life have been established. A comparative analysis of the results of static tensile tests on specimens from sections of the structure without corrosion damage and specimens cut out at the section acutely affected by corrosion has been made. The influence of corrosion process on reduction of operational characteristics of steel structure is shown. It is supposed that corrosion in course of time turns to a more aggressive type from even corrosion to corrosion cracking and spreads deep into the base metal which leads to considerable loss of mechanical properties of steel structures and to reduction of their fire-resistance limits.