Fusion Boundary Region Research Articles

Role of water chemistry and microstructure in stress corrosion cracking in the fusion boundary region of an Alloy 182-A533B low alloy steel dissimilar weld joint in high temperature water

Stress corrosion cracking (SCC) in the fusion boundary (FB) region of an Alloy 182-low alloy steel (LAS) dissimilar weld joint in 288 °C water was investigated by experiments and finite element simulation. Creviced bent beam and crack growth rate (CGR) experiments showed that, while the FB was a barrier to SCC growth, further crack growth into LAS was activated by a combined effect of sulfate and dissolved oxygen in water. Finite element simulation suggested that a positive gradient of hardness as the crack approached to the FB in dilution zone caused decreased CGR. Role of microstructure and water chemistry in SCC was discussed.

Microstructure and stress corrosion cracking of the fusion boundary region in an alloy 182-A533B low alloy steel dissimilar weld joint

Stress corrosion cracking (SCC) in the fusion boundary (FB) region of an Alloy 182-A533B low alloy steel (LAS) dissimilar weld joint in high temperature water doped with sulfate was studied following a microstructure characterization of the FB region. The microstructure characterization suggested the type-II and type-I boundaries in the dilution zone (DZ) adjacent to the FB had lower resistance to SCC growth than the FB. Crack propagating perpendicular to the FB in the DZ was observed to be blunted by pitting at the FB, followed by the reactivation from the pitting by localized oxidation along the grain boundary in LAS.

Microstructure and mechanical property of the fusion boundary region in an Alloy 182-low alloy steel dissimilar weld joint

Characterizations of the microstructure and mechanical property of the fusion boundary region of an Alloy 182-A533B low alloy steel (LAS) dissimilar weld joint were conducted. The existence of type-II boundary that parallels to the fusion boundary in the dilution zone (DZ) of Alloy 182 within a distance of about 50 μm was observed. The chemical composition transition was found in the narrow zone between the type-II and the fusion boundaries. Highest hardness was also found in this narrow zone in the fusion boundary region, implying a high residual strain level in the narrow zone. Lath martensite was observed in both sides of the type-II boundary in DZ, suggesting the localized enrichment of Fe and C adjacent to the type-II boundary due to the diffusion from LAS to DZ. Massive precipitations of carbides were observed on the type-II boundary but not on the fusion boundary. In addition, the orientation relationship at the fusion boundary between the lath martensite in the heat-affected zone in LAS and the austenite in the DZ was found to be Bain, K–S and N–W relationships.

Effects of base and filler chemistry and weld techniques on equiaxed zone formation in Al–Zn–Mg alloy welds

Equiaxed zone (EQZ) formation in Al–Zn–Mg alloy welds as affected by base metal, filler metal chemistry and weld techniques is studied. Filler metal chemistry and welding techniques have great influence on the formation of EQZ microstructure as base metal composition has. In an effort to characterise the equiaxed grain zone formation in Al–Zn–Mg alloy welds two commercial Al alloys AA7018 and RDE40 were selected. Gas tungsten arc welding in continuous current, pulsed current and arc oscillation mode were applied to weld the base materials. The influence of Sc containing fillers have been studied and compared with the commercial filler material. Mechanical and metallurgical characterisation were carried out in the EQZ. Intergranular corrosion in EQZ was studied according to ASTM G 110-92. Results reveals that RDE40 with low solute contents showed wider EQZ but relatively better corrosion and mechanical properties compared to AA7018 EQZ. Gas tungsten arc welding in pulsed and arc oscillation mode fusion boundary region exhibits better corrosion and mechanical properties compared to continuous current mode welds. Addition of Sc to the AA5556 filler combined with pulsed mode resulted in elimination of EQZ, better corrosion and mechanical properties compared to welds made with conventional AA5556 filler and also the presence of Sc within the EQZ so called unmixed zone has been observed.

Novel laser surface treatment approach to suppress sensitisation in modified type 316(N) stainless steel weld metal

Welded components are subjected to solution annealing heat treatment for achieving full stress relief and restoration of mechanical properties and corrosion resistance. During such heat treatments, optimum cooling rate has to be selected because very slow cooling rate will result in sensitisation and susceptibility to intergranular corrosion whereas fast cooling will result in reintroduction of residual stress. For 316 LN stainless steel which is welded using modified E316-15 electrodes (0·045–0·055%C), critical cooling rate above which there is no risk of sensitisation is 75 K h−1. This paper presents a novel laser surface treatment which suppresses sensitisation in weld metal, even at a slower cooling rate of 65 K h−1. Experiments involving laser surface melting were carried out with 150 W average power pulsed Nd:YAG laser and 10 kW CO2 laser, in both continuous wave and pulse modulated (100 Hz) modes. Best results were obtained when surface melting was performed with high frequency pulse modulated CO2 laser beam. The processed weld metal remained unsensitised after solution annealing followed by slower rate of cooling at 65 K h−1. Numerical simulation study was performed with ANSYS 7·0 software to understand the physical reason behind the difference in sensitisation behaviour of CO2 laser melted specimens under continuous wave and high frequency pulse modulated conditions and the predictions were validated using results of electron backscattered diffraction studies. Weld metal specimens treated with high frequency pulse modulated CO2 laser clearly showed evolution of fine grains near the fusion boundary region which enhanced sensitisation resistance.

Fusion boundary structures in a laser welded duplex Fe-Mn-Al-C alloy

The microstructure and composition of the fusion boundary region in a laser-welded duplex (austenite plus ferrite) Fe-Mn-Al-C alloy were investigated, and a new weld region named as the incompletely melted zone (IMZ) was postulated. In this zone, the austenite matrix was totally melted and resolidified; however, the ferrite plate was melted partially. The mechanism for the formation of IMZ was also discussed. In addition, some martensite and/or austenite precipitations with different morphologies were located within the ferrite plates both in the IMZ and the heat-affected zone, suggesting that they were formed during quenching from high temperature.

Formation of stainless steel layer on mild steel by welding arc cladding

The discrete microstructural characterization and the formation of stainless steel layer on mild steel where produced in cladding deposits, and fusion boundary region were investigated using tungsten inert gas (TIG) arc, high current pulsed arc and constricted plasma arc. The experimental procedure involved making bead-on-plate method for controlled travel speed, employing filler metal by using tungsten inert gas arc, pulsed current gas tungsten arc and transferred plasma arc, with subsequent sectioning and examination of the reaction interface. For TIG arc cladding, using filler metal of small diameter the deposit does not become stainless steel, but on using 3.2 mm diameter filler metal it becomes stainless steel with less than 50% dilution. For pulsed arc cladding, the complete stainless steel is not obtained on account of the existence of an incomplete mixture, particularly at the fusion boundary region. However, on using a large diameter filler metal at a pulse frequency of 500 Hz, the complete stainless steel microstructure has been accomplished. The plasma arc cladding can be achieved in such a way that the conversion into stainless steel on the mild steel surface — which is the microstructures of cellular austenite in cladding deposit and cellular dendritic austenite containing δ or σ-phase in fusion boundary region — is possible irrespective of the melt penetration and the dilution. The following conditions were found to be beneficial for the formation of stainless steel microstructure layer on the mild steel: using large diameter filler metal, below 50% dilution, and further rendering arc localized and constricted.

Microstructural surface modification of mild steel by TIG and plasma arcs

This letter addresses the microstructures of the fusion zone (FZ), fusion boundary region (FBR) and heat-affect zone (HAZ) of mild steel produced by TIG and plasma arcs, and the possibility of converting the mild steel surface-melting portion into stainless steel

Metallography of a pulsed Nd: YAG laser weld in a RS/PM Al8Fe2Mo alloy

The microstructure of a pulsed Nd:YAG laser weld in a rapid solidification/powder metallurgy (RS/PM) Al-8.0 wt.% Fe-2.3 wt.% Mo (Al8Fe2Mo) alloy was investigated using light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. This analysis revealed significant microstructure variations across the weld fusion zone (FZ). Near the fusion boundary, a light-etching FZ microstructure was observed to contain submicron dispersoids “entrapped” in a matrix of fine-sized dendritic alpha aluminum. At the center of the FZ, the presence of relatively coarse-sized intermetallic particles that served as growth centers for coarser dendritic alpha aluminum promoted a dark-etching microstructure. In the boundary between successive melt zones, both a heat-affected zone (HAZ) containing acicular dispersoids and a fusion boundary region (FBR) containing irregular-shaped particles in a coarse-grained dendritic alpha aluminum matrix were observed. A HAZ comprised of slightly coarsened dispersoids was also observed in the base metal bounding the FZ. Knoop hardness traverses across the FZ indicated a maximum hardness of KHN 260 in the light-etching FZ and a minimum hardness of KHN 135 in the HAZ between successive melt zones.

A microstructural study of local melting of grey cast iron by a static plasma arc

The microstructural changes produced by plasma arc local-melting method in the fusion region or deposited metal, fusion boundary region and heat-affected zone (HAZ) of flake graphite cast iron were investigated. Cylindrical base metal specimens were locally melted at fixed time intervals under a stationary plasma torch using argon plasma gas and Ar + 10% H2 shielding gas. The welds were produced autogenously and with filler metals. The cooling rate in the fusion region was recorded. Evaluation of the fusion boundary area included metallurgical analysis, microhardness and electron probe microanalysis. In the absence of filler metal, the structure of fusion region where completely melted was ledeburite. In the centre of the fusion region the structure appears to be that of hypoeutectic white cast iron, and in the fusion boundary of this region the structure appears to be fined ledeburite. In the fusion boundary region where supercooled phenomena can occur, fine martensite precipitates appear along the fusion line for small heat input and secondary graphite is seen for large heat input. The HAZ is composed of white martensitic, dark martensitic and martensitic-fine pearlitic layers for small heat input, and of finely laminated pearlitic and finely laminated pearlitic-ferritic layers for large heat input. Using filler metal, the structure of the deposited metal is found to be that of a nickel austenitic matrix precipitating tiny graphite nodules or slim graphite for nickel and Ni-Fe filler metals, but to be a pearlitic matrix for iron filler metal. In the fusion boundary region, nickel-martensite, eutectic or slim graphite and fine nickel-martensite are precipitated from the nickel system filler metal, and the columnar structure of ferrite and pearlite is obtained from iron filler metal. The HAZ is composed of thin ledeburitic, martensitic, martensitic-fine pearlitic and finely laminated pearlitic layers for the nickel filler metal, of ledeburitic and finely laminated pearlitic layers for the Ni-Fe filler metal, and of thick ledeburitic, eutectic graphite-crystallized and finely laminated pearlitic layers for iron filler metal. The hardness of the ledeburitic layer and the nickel-martensitic portion is very high where the liquid existed. The diffusion of nickel from the deposited metal into the HAZ can occur at least until the fused base metal of HAZ.

Local melting of nodular cast iron by plasma arc

The microstructural changes in the fusion boundary area of nodular cast iron (NCI) were investigated by plasma arc local-melting with and without a filler metal. The NCI cylinder specimen was locally melted by the static plasma torch for a very short time, adopting the non-keyhole method. The microstructures produced in the fusion region, fusion boundary region and heat-affected zone (HAZ) were examined metallographically. In the absence of filler metal, the fusion boundary area is composed of ledeburite in the fusion region, and martensitic, troostitic—ferritic layers for the short arc-time and sorbitic, sorbitic—ferritic layers for long arc times in HAZ. Although the formation of white iron in the fusion region may be caused by a relatively high cooling rate, the major cause is due to supercooling effects produced by the decrease of magnesium content in the molten iron directly under the plasma arc. Using nickel and Ni—Fe filler metals, Nimartensite appears in the deposited fusion boundary, and the HAZ region is composed of ledeburitic and martensitic or sorbitic layers. The ledeburitic layer thickness varies with the melting point of the filler metal. The diffusion of nickel from the deposit metal to the HAZ occurs at least until the fused base metal in the HAZ.