Martensite microstructure of 9–12%Cr steels weld metals

Abstract

Several new grades of steel have been developed in recent years, which have been based on the high-strength 9–12%Cr steels. These steels were developed to meet the proposed demand by power generation companies to increase efficiency to operating at higher temperatures by and thus burning less fossil fuel, principally coal, and thereby reducing costs and also meeting the increasingly stringent environmental requirements.

The 9–12Cr steels used for high temperature components in power plants are generally required to possess good mechanical properties, corrosion resistance, creep strength and fabricability. Although such steels normally have a fully martensite microstructure, they are also susceptible to the formation of delta ferrite, mainly during the welding process. Delta ferrite has several detrimental effects on such properties as creep, ductility and toughness. Thus, it is important to avoid its formation.

In this study the formation of delta ferrite in the weld metal of high-strength 9–12%Cr steels has been analysed for several samples with variations of key alloying elements. The results indicate that the most effective way to avoid delta ferrite in the weld metal is to reduce the chromium equivalent value to as low a value as possible. A fully martensite microstructure is obtained when both the Schneider chromium equivalent is lower than 13.5 and the difference between the chromium and nickel equivalent is lower than 8.

Introduction

Ferritic steels with a tempered martensite structure, include 2Cr, 9Cr and 12Cr steels, together with new high-strength steel grades, based on 9–12%Cr. Such steels have comparatively good corrosion resistance and can be cost effective substitutes for austenitic 18Cr–8Ni steels, even though higher creep strength and fracture toughness are available from austenitic steels. Some of the physical properties of austenitic steels, principally the lower thermal conductivity and higher coefficient of expansion relative to ferritic steels, mean that thick section boiler components cannot be operated with maximum flexibility if made in such steels. In addition austenitic steels have a relatively expensive alloy content and a more costly conversion route to tubular products. The high-strength 9–12%Cr based steels on the other hand allow reduced wall thickness of tubing components, and an improved oxidation and corrosion resistance, when compared with conventional low alloy steels. The 9–12%Cr steels also offer improved creep and fracture toughness over the low alloyed steels which are still used extensively in power generation components. Their higher creep strengths offer the possibility of achieving power generation at higher efficiency levels than current solid fossil fuel fired units. The improvements in efficiency in thermal power plants by decreasing fuel cost and CO₂ emissions require the elevating steam conditions to even higher ranges of temperature and pressure.

Many heat-resistant steels for power boilers, have been developed based on 9Cr–1Mo steel (T9) and 12Cr steel (AISI 410), which was put into service around 1940. The development focused on the optimisation of chemical compositions such as C, V, and Nb; 9Cr–1Mo–Nb–V steel (E91) and 12Cr–Mo–V steel. The subsequent development identifying alloy designs using W for solid solution strengthening and martensitic structures with MX precipitates (where M is Nb or V and X is C or N); 9Cr–1Mo–1W–Nb–V–N steel (E911). The development of power plants with steam temperatures of 625 °C or higher, are expected using new alloy designs employing Co and other elements (Ir, Rh, etc.) in addition to W for strengthening and stabilising the martensite matrix for high temperature and high pressure applications [1].

The 9–12%Cr steels required a careful balance of ferrite forming and austenite forming elements. This is to achieve a satisfactory solidification process, a fully austenitic and hot workable microstructure at primary processing, tubemaking and normalising heat treatment temperatures and also to provide high creep resistance. Alternatively, the equilibrium phase shows that the addition of high-Cr or high-Si causes the formation of delta ferrite during austenitisation at around 1100 °C [2]. This suggests that the addition of austenite stabilising elements is required to prevent the formation of delta ferrite in steels containing Cr in concentration higher than a 9% or Si higher than 0.5%. Ni, Cu, N and Co have been used as austenite stabilising elements in high-Cr ferritic steels [2]. For strengthening and stabilising the martensite matrix, the austenite stabilising elements should reduce diffusion rates and the martensite start transition temperature (M_s) and should increase Young's modulus of ferritic steels.

For 9–12%Cr alloy steels, it is generally accepted that weld consumables of matching composition to the parent material should be used [3], [4]. The consumables developed for Steel E91 provided the basis for the development of E911 and other 9–12%Cr steels consumables. For weld metal of the 9–12%Cr steels, the requisite time independent (tensile) and time dependent (creep) properties use be obtained by post weld heat treatment (PWHT). The temperature control requires particular attention to avoid a reduction in the toughness and ductility properties.

Although the 9–12%Cr steels normally have fully martensitic microstructures, they have some tendency to the formation of delta ferrite, mainly during the welding process. The presence of delta ferrite in martensitic stainless and high-strength 9–12%Cr steel welds have been reported to have several detrimental effects on mechanical properties. It has been shown to impair toughness due to the notch sensitivity of the delta ferrite phase [1], [5], enhance solidification cracking [6], promote sigma-phase precipitation and consequently embrittlement at intermediate service temperatures [7], and reduce the creep ductility at high service temperatures [1]. Thus, it is important to avoid its formation.

This paper analyses the weld metal microstructures produced by shielded metal arc welding (SMAW) using electrodes with different composition of Cr and other alloy elements such as Mo, W, Cu and Co. The aim is to develop consumables with different composition to obtain martensitic microstructure free of delta ferrite in the fusion zone for the SMAW of high-strength 9–12%Cr steels.