Per: GERALDO LUCIO DE FARIA (Universidade Federal de Ouro Preto), Paulo Henrique Cirilo (Universidade Federal de Ouro Preto), Paulo Sérgio Moreira (Universidade Federal de Ouro Preto), José Marcio da Rocha (Universidade Federal de Ouro Preto)
Abstract:
The OCTG steel pipes classified as API 5CT, specifically the grade K55, are specified for casing pipes in oil and gas exploration wells. The steels used as raw material for manufacturing this type of product are of high mechanical strength and low alloy. Generally, they are quenched and tempered, therefore possessing a microstructure mainly consisting of tempered martensite. However, some studies point to the possibility of using biphasic microstructures composed of ferrite and bainite, aiming to improve the mechanical strength-toughness relationship of this steel type. In this context, considering the feasibility of producing a biphasic pipe through austempering heat treatment after intercritical autenitizing, this work investigated, through computational simulations and dilatometric tests, the effects of intercritical autenitizing on the isothermal bainitic transformation of an OCTG steel, with potential application in the oil and gas industry. It was concluded that, for the studied steel, it is possible to obtain a promising ferrite-bainite biphasic microstructure through adequate planning of intercritical autenitizing and austempering conditions. It was shown that decreasing the austempering temperature results in significant microstructural refinement of bainite, leading to a considerable hardness increase. Intercritical autenitizing condition at 750°C associated with bainitic transformation temperatures of 400°C and 350°C proved to be the most promising for the desired application.