Evaluation of Stationary Creep Rate in Heat-Affected Zone of Martensitic 9–12% Cr Steels

Fevzi Kafexhiu,Jaka Burja

doi:10.3390/met10121612

Abstract

The purpose of the present study was to evaluate the contribution of distinct regions of the simulated heat-affected zone (HAZ) to the overall creep behavior of welded joints in the X20 and P91 steels. The HAZ was simulated by means of dilatometry at four peak temperatures (900, 1000, 1200, and 1350 °C) with a consequent tempering at 650 °C. Microstructure features of the four simulated HAZ regions including precipitates, prior austenite grains, and subgrains were quantified by means of electron microscopy. The quantified parameters and the measured hardness were used in three physical models for evaluation of the stationary creep rate (ε˙ at 170 MPa and 580 °C. The resulting ε˙ values fall within the range 10−8–10−7 s−1, being in good agreement with the experimental data with a similar thermal history, but an order of magnitude lower than the measured values for the parent metal of the studied steels (10−7–10−6 s−1). Depending on the model utilized, their output can be linearly related to hardness, subgrain size, or interparticle spacing. The model relating ε˙ to hardness was the most consistent one in prediction, being always lower for higher peak temperatures.

Highlights

Steels with 9–12 wt.% Cr content and microstructure of tempered martensite are used in vital components of thermal power plants such as evaporators, headers, and main steam pipelines, which operate at elevated temperatures and high pressures
The rate by which the creep deformation proceeds in a material depends on the applied stress and temperature and on the microstructure state, especially at critical locations of power plants’ vital components such as bends and welded joints. The latter are especially susceptible to deterioration because welding strongly influences the microstructure and properties of the parent metal through severe thermal cycles, which induce the creation of the so-called heat-affected zone (HAZ)
A phase transition can be noticed on cooling from 650 ◦ C, but only after the lower two simulation peak temperatures (900 and 1000 ◦ C)

Summary

Introduction

Steels with 9–12 wt.% Cr content and microstructure of tempered martensite are used in vital components of thermal power plants such as evaporators, headers, and main steam pipelines, which operate at elevated temperatures and high pressures. Materials operating at such conditions are subjected to a combination of deteriorating mechanisms such as corrosion, erosion, thermal fatigue, and most importantly creep, which is a time-dependent and thermally activated permanent deformation that proceeds mainly by dislocations glide and climb. HAZ is usually a few millimeters wide and contains a continuous gradient of regions with different prior austenite grain (PAG) and subgrain size, and different precipitates size and distribution, providing different creep resistance [1]

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