Effects of Aging at 550℃ on α'-Martensitic Transformation of High Si-Bearing Metastable Austenitic Stainless Steel Weld Metal

The high Si-bearing 15Cr-9Ni-Nb metastable austenitic stainless steel weld metal was prepared via gas tungsten arc welding and then processed by stabilized heat treatment(SHT)at 850℃ for 3 h.The effects of 550℃ aging on the α'-martensitic transformation of the as-welded and the SHT weld metals were investigated.The results showed that the weld metal had poor thermal stability of austenite.The precipitation of NbC during the 850℃ SHT made the thermal stability of the local matrix decrease and led to the formation of a large amount of C-depleted α'-martensite.The precipitation of coarse σ-phase at the δ-ferrite led to the Cr-depleted zone and the formation of Cr-depleted α'-martensite at the early stage of 550℃ aging.The homogenized diffusion of C and Cr in the matrix during 550℃ aging led to the restoration of austenitic thermal stability and the decrease of α'-martensite content.The C-depleted α'-martensite content in the SHT weld metal decreased rapidly at the early stage of aging due to the fast diffusion rate of the C atom in the matrix,while the Cr-depleted α'-martensite decreased at the later stage of aging due to the decreased diffusion rate of the Cr.

Interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints during aging

The interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints with a nickel-based filler metal during aging was studied in detail to elucidate the mechanism of premature failures of this kind of joints.The results showed that not only a band of granular Cr_(23)C_(6)carbides were formed along the fusion boundary in the ferritic steel during aging,but also a large number of granular or plate-like Cr_(23)C_(6)carbides,which have a cube-cube orientation relationship with the matrix,were also precipitated on the weld metal side of the fu-sion boundary,making this zone be etched more easily than the other zone and become a dark etched band.Stacking faults were found in some Cr_(23)C_(6)carbides.In the as-welded state,deformation twins were observed in the weld metal with a fully austenitic structure.The peak micro-hardness was shifted from the ferritic steel side to the weld metal side of the fusion boundary after aging and the peak value increased signific-antly.Based on the experimental results,a mechanism of premature failures of the joints was proposed.

Interactive effect of thermal aging and proton irradiation on microstructural evolution and hardening ofδ-ferrite in 308L stainless steel weld metal

In the harsh service environment of high temperature and intense neutron irradiation in water-cooled nuclear reactors,the austenitic stainless steel weld overlay cladding on the inner surface of the reactor pressure vessel suffers from thermal aging and irradiation damage simultaneously,which can induce microstructural evolution and hardening of the material.Since it is quite difficult to achieve this simul-taneous process out of the pile,two kinds of combined experiments,i.e.,post-irradiation thermal aging and post-aging irradiation were performed on 308 L stainless steel weld metals in this work.The interactive effect of thermal aging and proton irradiation on microstructural evolution and hardening ofδ-ferrite in 308 L weld metal was investigated by combining atom probe tomography,transmission elec-tron microscopy and nanoindentation tests.The results revealed that thermal aging could eliminate the dislocation loops induced by irradiation and affect the phase transition process by accelerating spinodal decomposition and G-phase precipitation,thus enhancing hardening of irradiatedδ-ferrite.For the effect of irradiation on the microstructure and hardening of thermally agedδ-ferrite,however,intensive collision cascades can intensify G-phase precipitation and dislocation loop formation but decrease spinodal decomposition,leading to a limited effect on hardening of thermally agedδ-ferrite.Furthermore,the interaction of thermal aging and irradiation can promote G-phase precipitation.Meanwhile,the interaction can causeδ-ferrite hardening,which is mainly influenced by spinodal decomposition,followed by G-phase and dislocation loops,where spinodal decomposition and G-phase cause hardening by inducing strain fields.

Effect of Heat Input on Fume Generation and Joint Properties of Gas Metal Arc Welded Austenitic Stainless Steel

The effect of heat input on fume and their compositions during gas metal arc welding （GMAW） of AISI 316 stainless steel plates are investigated. Fume generation rate （FGR） and fume percentage were determined by ANSI/AWS F1.2 methods. Particle characterization was performed with SEM-XEDS and XRF analysis to reveal the particle morphology and chemical composition of the fume particles. The SEM analysis reveals the morphology of particles having three distinct shapes namely spherical, irregular, and agglomerated. Spherical particles were the most abundant type of individual particle. All the fume particle size falls in the range of less than 100 nm. Mechanical properties （strength, hardness and toughness） and microstructural analysis of the weld deposits were evaluated. It is found that heat input of 1.15 kJ/mm is beneficial to weld stainless steel by GMAW process due to lower level of welding fume emissions and superior mechanical properties of the joints.

Secondary Austenite Morphologies in Fusion Zone of Welded Joint after Postweld Heat Treatment with a Continuous Wave Laser

In order to improve the weldability of duplex stainless steels,obtaining more secondary austenite in the weld metal is an effective way.Therefore,optimizing the secondary austenite by changing its morphology,volume fraction and stability may be expected to enhance the ductility of the weld.The secondary austenite morphologies in the fusion zone of the laser continuously heat treated welds of 2205 duplex stainless steel were investigated.The secondary austenite morphologies were found to be influenced by different laser power level.The secondary austenite with penniform,freely grown and dendritic shape appeared in the course of 4,6 and 8 kW continuous heat treatment,respectively.It was found that there were three kinds of morphologies of secondary austenite in the fusion zone treated by different power,i.e.,widmannst¨atten austenite,grain boundary austenite and intragranular austenite.The results demonstrated that the mechanism of the secondary austenite formation was a displacement mechanism during the initial austenite lath formation and a diffusion mechanism during cooling.The nitrides provided the nitrogen for the transformation and at the same time acted as nucleation sites for the secondary austenite.