The 𝜎 phase which exists in various series of stainless steels is a significant subject in steels science and engineering. The precipitation of the 𝜎 phase is also a widely discussed aspect of the science and technology of stainless steels. The microstructural variation, precipitation mechanism, prediction method, and effects of properties of 𝜎 phase are also of importance in academic discussions. In the first section, a brief introduction to the development and the precipitation characteristics (including morphologies and precipitation sites) of 𝜎 phase in stainless steels is presented. In the second section, the properties effect, prediction method, processing effect, elemental addition, retardation method and Thermo-Calc simulation of the 𝜎 phase in stainless steels are highlighted.

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Overview of Intermetallic Sigma () Phase Precipitation in Stainless Steels

A brief overview is given of the·current understanding of the solidification microstructure, the effect of aging and precipitation processes, and the factors controlling the mechanical and high temperature properties of some AISI 300 series weld metals. The effects of alloy chemistry on the distribution and morphology of the carbide and intermetallic phases in the weld metal are discussed. It is shown that the precipitation behaviour depends strongly on the carbon, silicon, chromium, and molybdenum content of the weld metal and the rate of dissolution of the δ-ferrite. The precipitation reactions are considered to be of primary importance in controlling the mechanical properties of the weld metals, and can result in significant reduction in weld metal ductility and toughness. Much of the deterioration in properties can be directly attributed to the formation of secondary phases such as M23C6 and the intermetallic σ and χ phases. A much more complex situation is found to prevail with respect to cre...

Influence of microstructure and composition on mechanical properties of some AISI 300 series weld metals

Understanding the behavior of weldment at elevated temperatures and especially their corrosion behavior has recently become an object of scientific investigation. Investigation has been carried out on friction-welded AISI 4140 and AISI 304 under Na2SO4 + V2O5 (60%) environment at 500 to 600°C under cyclic condition. The resulting oxide scales in the weldment have been characterized systematically using surface analytical techniques. Scale thickness on the low alloy steel side was found to be higher and was also prone to spalling. Weld area was found to be more susceptible to degradation than in base metals. The influences of welding parameters on the hot corrosion are discussed.

An assessment of hardness, impact strength, and hot corrosion behaviour of friction-welded dissimilar weldments between AISI 4140 and AISI 304

The effects of various reduction ratios on the precipitation behavior of the σ phase in 19Cr–9Ni–2Mn and 18Cr–0.75Si stainless steels as hot-rolled at 800 °C have been investigated in this study. The morphology, quantity and chemical composition of the σ phase were analyzed by using optical microscopy (OM), X-ray diffractometry (XRD), image analysis, scanning electron microscopy (SEM) and wavelength dispersive spectrometer (WDS), respectively. The σ phase showed a dendrite-like morphology in the as-received materials. A hot-rolling process changed the morphology of the σ phase from dendrite-like to globular, especially at higher reduction ratio. The amounts of σ phase in the stainless steels also increased gradually at 800 °C with increasing the reduction ratio from 0 to 75%. The XRD analyses showed that a higher reduction ratio enhanced the conversion of δ-ferrite (1 1 0) to σ phase (5 4 2). The σ phase was identified to be a Cr- and Mo-rich phase by WDS analysis, which indicated that although Cr and Mo elements are stabilizers of δ-ferrite in stainless steels as hot-rolled at 800 °C, these elements partition to the σ phase at high temperature.

Precipitation behavior of σ phase in 19Cr–9Ni–2Mn and 18Cr–0.75Si stainless steels hot-rolled at 800 °C with various reduction ratios

This article deals with the effect of the microstructural changes, due to transformation of delta ferrite, on the associated variations that take place in the tensile and stress corrosion properties of type 316 L stainless steel weld deposits when subjected to postweld heat treatment at 873 K for prolonged periods (up to 2000 hours). On aging for short durations (up to 20 hours), carbide/ carbonitride was the dominant transformation product, whereas sigma phase was dominant at longer aging times. The changes in the tensile and stress corrosion behavior of the aged weld metal have been attributed to the two competitive processes of matrix softening and hardening. Yield strength (YS) was found to depend predominantly on matrix softening only, while sig-nificant changes in the ultimate tensile strength (UTS) and the work-hardening exponent, n, occurred due to matrix hardening. Ductility and stress corrosion properties were considerably affected by both factors. Fractographic observations on the weld metal tested for stress-corrosion cracking (SCC) indicated a combination of transgranular cracking of the austenite and interface cracking.

Effect of ferrite transformation on the tensile and stress corrosion properties of type 316 L stainless steel weld metal thermally aged at 873 K

This paper deals with the microstructural changes and consequent deterioration in the room temperature tensile properties of type 316L stainless steel weld metal when exposed to elevated temperatures (773 to 973 K) for prolonged periods (up to 5000 hours). The microstructure-property correlation derived in this study is based on a variety of techniques: Magne-Gage, electrochemical extraction, X-ray diffraction, tensile testing, and both optical and electron microscopy. It has been established that the amount and morphology of the sigma phase are the key factors in determining the changes in the strength levels, total elongation, and extent of work hardening. The amount and morphology of sigma, in turn, is seen to depend on the relative kinetics of the various transformations, such as dissolution of delta-ferrite, growth of carbides,etc., shape changes in sigma, and the relative stabilities of the phases at the corresponding temperature of aging. The complicated dependence of the tensile properties on the microstrutural changes has been explained with direct quantitative evidence.

T. P. S. Gill

Papers

On microstructure-property correlation of thermally aged type 316L stainless steel weld metal