Yutaka S. Sato

Preferential precipitation site of sigma phase in duplex stainless steel weld metal

Duplex stainless steels are characterized by favorable combination of mechanical and corrosion properties, consisting roughly of equal parts of austenite ({gamma}) and ferrite ({alpha}). But exposure to elevated temperatures brings partial decomposition of ferrite to austenite and sigma phase, which deteriorates their properties. Sigma phase forms often at ferrite/austenite ({alpha}/{gamma}) interfaces through nucleation process. The heterogeneous nucleation of sigma phase at an {alpha}/{gamma} interface depends on the chemical driving force and the interfacial energy. Many studies have examined the effect of chemical driving force on sigma phase formation in duplex and austenitic stainless steel weld metals with different chemical compositions, but no detailed report has described the influence of {alpha}/{gamma} interfacial energy on sigma phase nucleation. The Kurdjumov-Sachs (K-S) orientation relationship is accepted to bring a coherent and low energy {alpha}/{gamma} interface in duplex stainless steels. The coherency of {alpha}/{gamma} interface can affect the sigma phase formation. The present study has examined the effect of crystallographic orientation relationship at {alpha}/{gamma} interface on sigma phase formation in a duplex stainless steel weld metal where the chemical element distribution is relatively uniform because of rapid cooling during weld thermal cycle.

Interlayer growth at interfaces of Ti/Al–1%Mn, Ti/Al–4·6%Mg and Ti/pure Al friction weld joints by post-weld heat treatment

Abstract The interlayer growth at interfaces of Ti/Al–1%Mn and Ti/Al–4·6%Mg weld joints was studied by postweld heat treatment. The heating temperatures ranged from 676 to 873 K (400–600°C) and maximum heating time was 360 ks (100 h). The basic mechanism of interlayer growth for pure Ti/pure Al friction weld joint was also estimated. The interlayer growth rate of Ti/Al–4·6%Mg joint was much faster than for the Ti/ Al–1%Mn joint. The interlayer mainly consisted of (Al,Si)3Ti for the Ti/Al–1%Mn joint, and Al18Mg3Ti2 for the Ti/Al–4·6%Mg joint. While the interlayer grew from Al alloy substrate to the Ti side for the Ti/Al–1%Mn joint, it grew from the Ti substrate to the Al alloy side for the Ti/Al–4·6%Mg joint. The interlayer growth stopped for several hours on heating for 36 ks (10 h). Neither linear nor parabolic time-dependence relations could be exactly fit to the interlayer growth rate for both joints. The interlayer growth of Ti/Al–1%Mn was proportional to heating time raised to approximately 0·85. The crystal direction of Al3Ti interlayer growth of the Ti/Al joint was close to 〈001〉 and 〈111〉 directions obtained by OIM method. Nucleation and nuclei growth were observed at the interface of the Ti/Al joint. The nucleation and the nuclei growth are the reason for the phenomena (time dependence) described above.

Microstructures in Friction Stir Welded 304 Austenitic Stainless Steel

Friction stir welding was applied to 304 austenitic stainless steel. The microstructural evolution and hardness distribution in the weld were investigated. The stir zone (SZ) and thermomechanically affected zone (TMAZ) showed dynamically recrystallised and recovered microstructures, respectively. The hardness of the SZ was higher than that of the base material and the maximum hardness was located in the TMAZ. The higher hardness in TMAZ was attributed to high density of dislocations and sub-grains. Electron microscopic observations revealed that ferrite and sigma phase were formed in austenite matrix in the SZ depending on the cooling rate during FSW.

Grain Boundary Engineering for Intergranular Corrosion Resistant Austenitic Stainless Steel

Optimum parameters in the thermomechanical treatment during grain boundary engineering (GBE) were investigated for improvement of intergranular corrosion resistance of type 304 austenitic stainless steel. The grain boundary character distribution (GBCD) was examined by orientation imaging microscopy (OIM). The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation (EPR) and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of coincidence-site-lattice (CSL) boundaries indicated a maximum at the small pre-strain. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanical-treated specimen than in the base material for long time sensitization. The optimum thermomechanical treatment introduced a high frequency of CSL boundaries and the clear discontinuity of corrosive random boundary network in the material, and resulted in the high intergranular corrosion resistance arresting the propagation of intergranular corrosion from the surface.

Effect of Threads on Tool in Friction Stir Welding of Aluminum Alloys

The effect of tool threads on the weldability and mechanical properties of 5-mm thick welded aluminum plates was investigated. The simplest shape, a circular cylinder tool without threads, and an ordinary shape, the same diameter tool with threads, were used to weld 1050-H24 and 6061 -T6 aluminum alloys. The tensile strength of the joints using the tool without threads was slightly higher than or almost the same as that using the tool with threads. In the friction stir welded joints of the 1050-H24 and 6061 -T6 aluminum alloys, the tool threads have very little effect on the weldability and mechanical properties. Defect-free joints had almost the same tensile strength and Vickers hardness profile for both tools. Temperature during welding and microtexture are also almost the same for both tools.

Corrosion Properties in Friction Stir Welded 304 Austenitic Stainless Steel

A 304 austenitic stainless steel was friction stir welded using polycrystalline cubic boron nitride (PCBN) tool. Relationship between corrosion properties and microstructure was examined in the weld using several corrosion tests and microstructural observation techniques. The stir zone (SZ) had better corrosion properties than the base material (BM). The corrosion tests showed that the intergranular corrosion was developed in the heat-affected zone (HAZ) compared to the BM and SZ although the grain boundaries were not severely corroded. TEM/EDS analysis revealed that Cr depletion zone near grain boundary in the HAZ was shallow and narrow. Friction stir welding (FSW) suppressed sensitisation in the HAZ, which could be explained by short duration at sensitisation temperatures during welding. On the other hand, many grain boundaries were deeply corroded in the AS, where the corrosion resistance was significantly degraded. The microstructural observation revealed that sigma phase was formed in the AS during FSW. Sigma formation produced the wide and deep Cr depletion zone with the minimum Cr content less than 12 wt % in the vicinity of the grain boundary in the AS, which severely deteriorated the corrosion resistance in the AS.

Microstructural Evolution during Friction Stir Welding of Ultrafine Grained Al Alloys

Recently, several metallic materials with ultrafine-grained structures and characterized by high strength and toughness have been developed. When these ultrafine-grained materials are practically used, welding and joining processes are required. However, conventional fusion welding processes result in deterioration of the good mechanical properties of these ultrafine-grained materials due to the drastic grain growth of the ultrafine grains. On the other hand, friction stir welding (FSW) is a solid-state joining process having lower heat-input than fusion welding processes, enabling formation of a fine grain structure in the stir zone. Thus, this process would effectively alleviate deterioration of mechanical properties of the ultrafine-grained materials. The authors applied FSW to ultrafine-grained Al alloys and then examined the microstructural features associated with hardness in the friction stir welds. The present paper reviews microstructural evolution of ultrafine-grained Al alloys, produced by equal channel angular pressing (ECAP) and accumulative roll-bonding (ARB), during FSW.

Dependence of Carbide Precipitation on Grain Boundary Structure in Sensitized Austenitic Stainless Steel

Grain boundary carbide precipitation and intergranular corrosion in sensitized austenite stainless steel were examined by transmission electron microscopy (TEM) to clarify the effect of grain boundary structure on precipitation and corrosion. A type 304 steel, which had been solutionized at 1350 K was heat-treated at temperatures of 800-1300 K. Oxalic acid etch and Strauss tests showed that the frequency of grain boundaries with M23C6 carbide precipitation and corroded boundaries increased with holding time at sensitizing temperatures. The grain boundary carbide precipitation was observed during heat treatment at 1000 K by TEM. Grain boundaries were characterized on the basis of the Coincidence Site Lattice (CSL) theory using electron diffraction Kikuchi patterns. The observations revealed that the propensity to intergranular precipitation depends strongly on the grain boundary structure. Carbide precipitates tend to be detected at grain boundaries with higher Σ -values or larger deviation angles (Δθ) from low- Σ CSL misorientations. The border lines between precipitation and no precipitation can be drawn by a deviation parameter of Δθ/ΔθC, where Δθc is the maximum deviation angle by Brandon’s criterion. The border line of Δθ/Δθc decreased with the increase in the holding time at 1000 K. This means that the more ordered boundary needs the longer time for intergranular carbide precipitation and corrosion than less ordered or random boundaries.