Study of the mechanism of structural state alteration in the strain ageing process and its influence on properties of bainitis type pipe steel

Abstract

A pipeline is refered to metal structures of increased danger. To provide a high operational safety of pipe lines it is common to follow requirements to steels by level of strength, ductility, impact strength, cold resistance, fracture strength, weldability, corrosion resistance and other properties, forming of which is influenced by a complex of different factors (both technological and operational).At present structural low carbon low alloyed steels with increased level of properties, completely meeting all complex of presented requirements are used for production of sheet rolled products for pipes. However, during longtime operation of a pipeline, properties degradation demonstration is possible, that can result in decreasing of safety and originating of a structure destroying probability. A passing of strain ageing can be one of the reasons, responsible for state alteration of longtime operation pipes metal, resulted in increasing of yield strength to ultimate strength (σ0,2/σв) relation, decreasing of impact strength and cold resistance.Results of study of strain ageing influence on complex of properties and structural state of sheet rolled product of X80 strength class presented, the product being widely used for pipes production. It was shown, that in the process of the strain ageing, apart from decreasing of complex of standard properties, an increase of relaxation effect takes place. It speaks for an increased level of local strains, arising at loading and deformation as a result of fixing of dislocations by interstitial atoms. Concentration of the atoms is increasing in the process of preliminary deformation, which is confirmed by Snuk peak arising during measuring of internal friction.Study of structural state by different methods enabled to formulate peculiarities of strain deformation mechanism, typical for a steel with a structure of low carbon bainite type (bainite ferrite) with austenite insertions. Alteration of properties during strain ageing of bainite steel is linked with simultaneous arising of two processes in structure at different scale levels, having contrary influence on resistance to deformation. Increase of yield strength as a result of strengthening takes place because of fixing of dislocations by interstitial atoms. At the same time the weakening is caused by disappearing of M/A phase islands, which is strengthening the metal at the initials state, as a result of TRIP-effect during preliminary deformation and martensite disintegration during the following heating.