Characterization of Austenite Decomposition in Steels with Different Chemical Concepts and High Potential to Manufacture Seamed Pipes for Oil and Gas Industry

Abstract

This work presents the characterization of austenite decomposition kinetics in two steels during continuous cooling considering non-deformed austenite grains. Two different chemical concepts of steels were evaluated: low carbon-high manganese and a relatively new concept based on low carbon-low manganese-high niobium contents. Dilatometric experiments, microstructural characterization procedures and microhardness tests were carried out. Experimental and calculated CCT diagrams were plotted and the adapted JMAK model was applied aiming to predict the austenite decomposition kinetics. The low carbon-high manganese steel was the most sensitive to the applied cooling rates and, due to that, presented the higher grain refinement and hardness increasing after continuous cooling. These results highlight that the Mn content decreasing may promote a potential loss of grain refinement in procedures of continuous cooling where the structure recrystallization does not occur, as in some welding procedures or conventional heat treatments applied in the seamed pipe manufacturing process.

Proposition of an Empirical Functional Equation to Predict the Kinetics of Austenite to Ferrite Transformation in a Continuous Cooled IF-Ti-Stabilized Steel

“…As the austenite to ferrite transformation is a diffusional mechanism, the cooling rate increase can retard the elements diffusion, increasing its content on austenite until lower temperatures, retained it metastable and delaying its decomposition. This behavior was observed in the IF-Ti studied steel, but due to its low carbon and low alloying element contents, the effect of increasing cooling rate on the austenite undercooling is not so intense as the described for alloyed or microalloyed steels [38][39][40][41][42] . Higher cooling rates decrease the austenite to ferrite transformation temperature, increasing the austenite decomposition driving force.…”

“…Higher cooling rates decrease the austenite to ferrite transformation temperature, increasing the austenite decomposition driving force. As consequence, the increased cooling rates should promote the reduction of the critical radius for ferrite nucleation, favoring the ferrite nucleation rate and promoting a ferrite grain refinement 38,39 .…”

“…Firstly, two parallel lines were drawn as Figure 5a presents. Then, an isotherm was traced and, by applying the specific volume variation method (lever rule) 38,39,43 , the austenite fraction was calculated as a temperature function. In order to sample points along the entire dilatometric curve, the austenite fraction was calculated several times for different temperature values.…”

“…For each studied cooling rate, the austenite fractions were plotted as a function of instantaneous temperature, as exemplified in Figure 5b. It is known, and widely described by many authors, that the volumetric fraction of the transformed phase can be fitted by the adapted JMAK model, as far as diffusion processes are concerned and for which the transformed fraction is time dependent [8][9][10][37][38][39] . For the phases whose formation is diffusionless and the transformation rate depends on the temperature and do not on the time, as the martensitic one, the Koistinen and Marburger equation is used 44 .…”

Investigation into the Impact Toughness of API 5L X80 Steel Weldments and its Relationship with Safe Welding Procedures

“…Sharma et al 1 reviewed the possible causes of failures in pipelines and observed that the critical issues and challenges in welding of pipelines are corrosion, hydrogen embrittlement, residual stresses, weld repairing and a deteriorated heat affected zone which adversely affects the performance of a welded structure. The welding thermal cycle promotes changes in the microstructure and it can contribute to local brittleness in the Heat Affected Zone (HAZ) [5][6][7][8][9][10][11][12][13] .…”