Effect of the Chemical Homogeneity of a Quenched and Tempered C-Mn Steel Pipe on the Mechanical Properties and Phase Transformations

Abstract

Seamless steel pipes for application in the oil and gas industries are manufactured from quenched and tempered steel. Aiming to meet the required characteristics, it is necessary that the microstructure and the mechanical properties are homogeneous along the wall thickness of the pipes, which is even more critical for thick wall pipes. Considering the seamless steel pipe manufacturing process, any chemical segregation along the thickness could affect the phase transformation kinetics during heat treatments and, consequently, microstructure and mechanical properties. In this context, this pioneer work when evaluating the effect of the chemical homogeneity of a seamless pipe on the mechanical properties and phase transformations. The studied pipe was manufactured by a C-Mn steel designed for oil and gas industry applications. Two regions of the pipe wall were analyzed-a region close to the inner surface and another near the outer surface. The steel was subjected to quenching and tempering heat treatments. Scanning electron and optical microscopy techniques were used to characterize the resultant microstructures. Microhardness and Charpy impact tests were performed aiming to analyze the pipe mechanical behavior in the studied regions. In addition, dilatometric tests were performed in order to determine the continuous cooling transformation diagrams of these regions.

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.…”

Effects of Cooling Rate during Quenching and Tempering Conditions on Microstructures and Mechanical Properties of Carbon Steel Flange

“…The mechanical properties of thick steel plates used for large structures such as flanges are controlled using post-heat treatment [ 6 , 7 , 8 , 9 ]. Quenching is used for the purpose of improving strength but it has a detrimental effect on low temperature toughness due to the formation of a hard secondary phase.…”

“…However, when a thick steel plate is subjected to a given T TEM and t TEM , different parts of it cool at different rates; this variation causes non-uniformity of microstructures and variation in final mechanical properties [ 8 , 9 , 15 , 16 , 17 ]. After hot-forging of the flange, a post-heat treatment process can also be used to control its mechanical properties, but the combination of high strength and high toughness is difficult to obtain due to the non-uniform microstructure and the complicated effects of the parameters of the tempering process.…”