Improvement of reliability and creep resistance in advanced low-alloy steels

doi:10.1016/j.msea.2008.05.056

Materials Science and Engineering: A

Volumes 510–511, 15 June 2009, Pages 234-237

https://doi.org/10.1016/j.msea.2008.05.056 Get rights and content

Abstract

The T23 and T24 steels were designed for welding membrane water walls without preheating and without post-weld heat treatment (PWHT). Results presented demonstrate that non-tempered weld joints of low-alloy creep-resistant steels T23 and T24 undergo a process of secondary hardening during long-term exposure at elevated temperatures. This process is accompanied by degradation of plastic properties, especially in the heat affected zone (HAZ). Therefore, PWHT of T23 and T24 welds is necessary to achieve suitable hardness and plastic properties of welds.

Introduction

Construction of advanced power plants requires materials with improved high-temperature strength and superior resistance to high temperature corrosion. For several decades, a permanent effort has been given for improvement of the creep resistance in modified 9–12% Cr steels. Requirement on reduction of power generators’ costs has led to the development of cheaper low alloy steels. These advanced steels should be characterized by higher creep resistance in comparison with the conventional 2.25%Cr–1%Mo steel.

As a result, the new advanced low alloy steels for membrane water walls T23 (HCM2S), T24 (7CrMoVTiB7-7) and F-2W were developed. T23, T24 and F-2W steels have excellent mechanical properties at elevated temperatures and improved weldability as a result of the enhanced alloying. In comparison with conventional 2.25%Cr–1%Mo steel, there is a small addition of V, W, Nb, Ti, N, B elements and carbon content is reduced to below 0.10%.

The main application of advanced low-alloy creep resistant steels is in construction of membrane water walls of power generators. New steels T23, T24 and F-2W were supposed to allow welding of these large components without preheating and post-weld heat treatment (PWHT). Results presented in this article show that post-weld heat treatment is quite necessary.

The creep properties of advanced low alloy steels are controlled by chemical composition and microstructure of these steels. If the chemical composition is given, the microstructure of these steels depends on the heat treatment, temperature and time of creep exposure [1]. The most important strengthening mechanisms in advanced low alloy steels, operating during high temperature creep exposure, are precipitation strengthening and solid solution strengthening [2], [3].

Precipitation strengthening in low alloy CrMoV steels is predominantly affected by the inter-particle spacing of MX particles, i.e. vanadium carbide (V₄C₃) or vanadium carbonitride VCN. It has been shown that both the proof stress at room temperature and creep rupture strength increase while inter-particle spacing of secondary phases decrease. At the same time, the creep rate decreases [2], [3].

Sometimes it is expected that solid solution strengthening of ferritic steels can be improved by increasing Mo and/or W contents in the steel. Creep rupture tests, performed on CrMoV steels containing 0.5%Cr–0.3%V and Mo content up to 1.5 wt.%, have shown that there is no reason to increase Mo content in steel beyond the soludibity limit.

Section snippets

Experimental material and procedures

Mechanical properties and microstructure of T23 and T24 steel welds depending on PWHT were investigated in the experimental programme. The chemical composition of experimental material is given in Table 1. Experimental welds were performed by manual metal arc welding (MMAW). The chemical composition of used welding materials is presented in Table 2.

One half of weld joints were tempered at 750 °C. The second half of welds were retained in “as welded” condition. Samples were aged without stress in

Results

Results of hardness and impact toughness of aged welds are presented in Fig. 1, Fig. 2, Fig. 3, Fig. 4, Fig. 5. Fig. 1, Fig. 2 show hardness curves for the overheating zone of HAZ in T23 and T24 steel during simulated operation at 550 °C.

Fig. 3, Fig. 4, Fig. 5 give results of impact toughness measurements. The effect of tempering after welding on impact toughness of the HAZ in T24 steel is depicted in Fig. 3. This figure shows a significant difference between the impact toughness of tempered and

Secondary hardening of advanced low-alloy steels

High creep resistance of steel alloyed with vanadium is caused predominantly by dispersion of fine MX particles. MX particles lead to a great increase of creep rupture strength but on the other hand, the major consequence of dispersion of MX particles is a degradation of plastic properties due to secondary hardening. This process is most significant in weld joints, where due to the welding process the degree of dissolution of dispersed particles varies. Subsequently, the correct tempering

Conclusion

Results presented in this article demonstrate, that weld joints of low-alloy creep resistant steels hardened by dispersed MX particles are subject to a process of secondary hardening during long term exposure at elevated temperatures. This secondary hardening can be expected in all advanced ferritic creep resistant steels including T23, T24 and F-2W at operating temperatures. Secondary hardening causes embrittlement of non-tempered welded joints.

The extent of secondary hardening depends on the

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