Effect of Hot Induction Bending on Microstructure, Precipitate Characteristics, and Mechanical Properties in Thick-Walled X90 Bend

The microstructure and precipitate characteristic in the thick-walled X90 hot bend related to mechanical properties were investigated in this paper. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the microstructure characteristic and precipitation behavior. The tensile test and low-temperature impact test were used to measure the strength and impact toughness of the X90 hot bend. The results show that the X90 mother pipe is composed of quasi-polygonal ferrite, acicular ferrite (AF), lath bainite (LB) and a small amount of martensite/austenite (M/A) constituents. The width of the bainite lath is about 0.2-0.3 mu m. After hot induction bending, the grains coarsened obviously and AF phase is absent in the bend zone. In the outer arc side, the width of LB coarsened to 0.53-1.34 mu m, and sharp M/A constituents formed along the prior austenite grain boundaries. Compared to the X90 mother pipe, the strength of the X90 bend decreased 30-80 MPa, and the Charpy impact energy increased 20 J. The outer arc side has the weakest low-temperature impact toughness, is 153 J. The main precipitate in the outer arc side is NbC with a small amount of TiC, possibly (Ti, Nb) C, and the size is about 15 nm. The fraction of the high angle grain boundaries and the kernel average misorientation (KAM) value of the outer arc side is 21% and 0.62 degrees, respectively. Angular shaped M/A constituents and high KAM value are the main reason to deteriorate the toughness.

Automated summary

The microstructure and precipitate characteristics of the X90 hot bend play a crucial role in determining its mechanical properties. After hot induction bending, the microstructure in the bend area undergoes significant changes, resulting in a decrease in strength but an increase in impact toughness compared to the mother pipe.