Correlation Between Microstructures and Tensile Properties of Strain-Based API X60 Pipeline Steels

Abstract

The correlation between the microstructures and tensile properties of strain-based American Petroleum Institute (API) X60 pipeline steels was investigated. Eight types of strain-based API X60 pipeline steels were fabricated by varying the chemical compositions, such as C, Ni, Cr, and Mo, and the finish cooling temperatures, such as single-phase and dual-phase regions. In the 4N and 5C steels, the volume fractions of bainitic ferrite (BF) and the secondary phases increased with the increasing C and adding Cr instead of Ni. In the 5C and 6NC steels, the volume fractions of acicular ferrite (AF) and BF decreased with increasing C and adding Ni, whereas the volume fractions of polygonal ferrite (PF) and the secondary phases increased. In the 6NC and 6NM steels, the volume fraction of BF was increased by adding Mo instead of Cr, whereas the volume fractions of PF and the secondary phases decreased. In the steels rolled in the single-phase region, the volume fraction of polygonal ferrite ranged from 40 to 60 pct and the volume fraction of AF ranged from 20 to 40 pct. In the steels rolled in the dual-phase region, however, the volume fraction of PF was more than 70 pct and the volume fraction of AF was below 20 pct. The strength of the steels with a high volume fraction of AF was higher than those of the steels with a high volume fraction of PF, whereas the yield point elongation and the strain hardening exponent were opposite. The uniform elongation after the thermal aging process decreased with increasing volume fraction of PF, whereas the uniform elongation increased with increasing volume fraction of AF. The strain hardening exponent increased with increasing volume fraction of PF, but decreased with increasing volume fraction of AF and effective grain size.

Archives of Metallurgy and Materials

“…The yield strength and tensile strength of the cT 570 and cT 630 steels meet the aPi requirements for X70 (485 MPa minimum yield strength and 570 MPa minimum tensile strength) in all directions. The cT 570 and cT 630 steels in all directions show discontinuous yielding behavior because PF is the dominant microstructure in both steels [20][21][22]. The tensile test data for the CT 630 specimen indicate higher yield strength and tensile strength than the cT 570 specimen at both 0° and 90°.…”

Effect of Heat Input on Microstructure and Tensile Properties in Simulated CGHAZ of a V-Ti-N Microalloyed Weathering Steel

“…Furthermore, the uE increases significantly with the increase in E j from 15 to 75 kJ/cm. The uE is closely related to slip of dislocation: once dislocations slip over long distances without encountering barriers, high uE is generated [50,51]. When the E j = 15 and 35 kJ/cm, there are many DTs and ferrite lath boundaries in the microstructure, and the dislocation density is high (Figure 4a,b and Figure 7e,f).…”

Effect of Microstructural Constituents on Hydrogen Embrittlement Resistance of API X60, X70, and X80 Pipeline Steels

“…it has been reported that an increase in the strength level deteriorates the hydrogen embrittlement resistance [10,11]. However, it is challenging to understand the effect of each microstructural constituent on the hydrogen embrittlement resistance of pipeline steels owing to their very complicated microstructures [12][13][14][15][16]. Thus, it is necessary to investigate the correlation between characterized microstructural constituents and the hydrogen embrittlement resistance to develop aPi pipeline steels possessing an excellent combination of strength and hydrogen embrittlement resistance.…”