Recently, the importance of pipelines as a long-distance transportation means for crude oil, natural gas, and the like is emerging. Up to now, the American Petroleum Institute (API) standard X80 or less has been applied to long-distance transport trunk line pipes. However, to improve the transport efficiency through the increase of the transportation pressure and the improvement of the laying efficiency through the reduction of the diameter and weight of the line pipes, the increase in strength is accelerating. In particular, the X120 grade line pipe, which exhibits a tensile strength of over 900 MPa and can withstand approximately twice the internal pressure of X65, can transport about twice as much gas as compared to a lower grade line pipe of the same size [1, 2].
Compared to the conventional methods of increasing the pressure enduring ability of the line pipe by increasing the pipe wall thickness, the use of high-specific strength line pipe saves the cost of materials, transportation, and on-site welding work, leading to overall construction cost savings. These enhanced properties achieved due to unique microstructure which is mainly composed of martensite/bainite mixture.
The austenite transformation structure is classified into ferrite, pearlite, bainite, and martensite [3, 4]. Martensite phase forms through diffusionless transformation when austenite phase is quenched at a very high speed. According to the carbon content, martensite can be classified into lath/plate martensite . When the austenite is rapidly cooled, the carbon saturation concentration is reduced and the alloy phase supersaturated with carbon, and the deformed region has high strength due to the high dislocation density. However, since martensite has low elongation compared to strength, it is used by adding alloying elements such as manganese (Mn) and chromium (Cr) or increasing the carbon content.
According to the existing literature, the alloy of high strength-high ductility combination mainly developed using TRIP phenomenon reported by Zackay et al. . It has ductile ferrite matrix which is strengthened by bainite which gives it excellent mechanical properties such as dimensional stability, fatigue strength, and impact toughness owing to the combination of uniform elongation generated by the transformation from austenite to martensite[7 ~ 9]. A small amount of retained austenite remains even in two-phase steels, which have been studied as high-strength-high-ductility steels since 1970s, and its reported that the ductility could be improved through transformation . In the late 1980s, various studies have been conducted to improve ductility by increasing the retained austenite fraction in the structure of abnormally structured steels [11, 12]. The volume fraction, distribution of various phases, especially the transformation rate and stability of retained austenite are very important in the mechanical properties of TRIP steel [7, 9].
As mechanical properties can be determined according to the fraction of retained austenite, observation of microstructure in steel is very important. The microstructure observation of steel is generally divided into Black (pearlite, martensite), White (ferrite), Gray (martensite, bainite) phases using Nital etching. However, in steel it is difficult to accurately distinguish images due to problems of contrast, especially intermediate colors such as gray. It is well reported that LePera etching is capable of distinct phase analysis compared to Nital etching. Because LePera etching divides the phases into Blue/Green (ferrite), Brown (bainite), and White (martensite) [13–15]. According to the research result of Tsipouridis , cooling rate has a large influence on the martensite structure. In the case of a slow cooling rate, the inside of unrecrystallized martensite grains is transparent white while for the fast-cooling rate, the inside of the coarsened grains appeared as dark brown.
This study aims to analyze the M/A phase of Fe-Nb alloy specimens through LePera etching of the retained austenite and martensite phases which do not appear distinctly while using Nital etching.