Austenitic Stainless Steel Base Metal Research Articles

Microstructure, properties and fracture failure mechanism of MAG welded joints of four types of stainless steels

In this study, four types of austenitic stainless steels (SUS 301L, SUS 304, SUS 316L and SUS 321) with different chemical compositions were butt welded by using MAG (metal active gas) welding. Unique information was gained by studying the solidification mode, microstructure and mechanical properties of the four welded joints under the same welding process, and their fracture mechanisms were analyzed from the perspective of crystallography. The results indicate that the microstructure of the four austenitic stainless steel base metals is uniformly sized equiaxed austenite with δ-ferrite distributed inside or at the boundary of austenite grains in 301L and 304 base metals. The welds of the four kinds of stainless steels are solidified in Austenitic-Ferrite (AF) solidification mode, and the final microstructures of the 301L, 304 and 316L weld are equiaxed grains while the 321 stainless steel weld grows into a dendritic structure due to its low temperature gradient. The microstructures of the fusion zone (FZ)/fine equiaxed zone (FQZ) and heat affected zone (HAZ) of the four types of stainless steels are closely related with the solidification mode of FZ, i.e. small FZ and HAZ regions result from Ferrite (F) solidification mode present in the 301L and 321 welded joints. However, larger FQZ regions generate in the 304 and 316L welded joints due to the Ferrite-Austenitic (FA) solidification mode. The 304 and 316L stainless steel tend to fracture at the HAZ positions due to the formation of FQZ; large angle grain boundary formed at the weld of 301L stainless steel is the reason for its fracture, and the good anisotropy of the weld and heat affected zone of 321 stainless steel also makes it easier to fracture in the base metal (BM). The section hardness distribution characteristics of the four stainless steels welded joints also verify their fracture tendency.

Isothermal and thermomechanical fatigue behaviour of type 316LN austenitic stainless steel base metal and weld joint

Thermomechanical fatigue (TMF) studies were carried out on type 316 LN austenitic stainless steel (SS) base metal (BM) and its weld joint (WJ) with a temperature interval of 623–873 K under in-phase (IP) and out-of-phase (OP) combinations of mechanical strain and temperature. Tests were performed under mechanical strain control mode using the strain amplitudes in the range, ±0.25% to ±0.6%. Besides, isothermal low cycle fatigue (IF) tests were also conducted on the weld joint at 873 K. TMF cycling resulted in a slightly higher cyclic stress response (CSR) compared to IF loading for the weld joint. The IP TMF cycling yielded a higher plastic strain amplitude in comparison to OP TMF for both the base metal and the weld joint. Cyclic life of the weld joint is found to be inferior to that of the base metal. The fatigue lives exhibited by the base metal and the weld joint varied in the following sequence: IP TMF < IF < OP TMF. Preferential crack initiation in the weld region owing to embrittlement of transformed δ-ferrite was noticed with an increase in the mechanical strain amplitude under both TMF and IF cycling conditions. Crack initiation and propagation generally occurred in transgranular mode under IF and OP TMF and mixed (trans-plus intergranular) mode under IP TMF cycling. Electron back scattered diffraction (EBSD) measurements were employed to investigate the substructural development in terms of local misorientation distribution, inverse pole figure (IPF) maps and boundary dislocation density (ρ) in the weld metal region, under IF and IP TMF cycling. The dislocation density was found to decrease for both IF and IP TMF cycling compared to the as-welded condition, with the decrease being higher for IF cycling. The TMF data generated for the type 316LN SS base metal and weld joint at different strain ranges in the present study clearly demonstrated a lack of conservatism associated with the current design practice which is based on the IF data at the maximum temperature (Tmax) of the service cycle.

A-TIG welding of dissimilar P92 steel and 304H austenitic stainless steel: Mechanisms, microstructure and mechanical properties

The present study focuses on the dissimilar joining of P92 steel and 304H austenitic stainless steel plates of thickness 8 mm using A-TIG welding with aim to select a proper flux for through thickness penetration. The effect of four different oxide fluxes named: Cr2O3, MoO3, SiO2 and TiO2 on weld bead geometry was studied. A detailed investigation on possible mechanism for increase in depth of penetration has been carried out. Out of four oxide fluxes, TiO2 provided through thickness penetration. Further, the integrity of weld joint developed using TiO2 flux was assessed in light of microscopy, hardness test, tensile test and impact toughness test. Microstructure study of the weld joint showed the formation of untempered martensite, polygonal austenite grains and ferrite stringers in different zones of weld joint. In tensile testing, the specimens were failed from 304H austenitic stainless steel base metal. The tensile properties were found as tensile strength 688.6 MPa, yield strength 562.2 MPa and elongation 37.4%. The V-notch Charpy impact toughness of the weld joint (30 J) was quite lower than the base metals due to the formation of untempered martensitic structure.

Metal Oxide Nanoparticle-Based Coating as a Catalyzer for A-TIG Welding: Critical Raw Material Perspective

Besides a wide application in corrosion protection, wear resistance increase, providing thermal properties and power conversion, oxide coatings have found an alternative application in welding technology as catalysts of the tungsten inert gas (TIG) welding process. In this paper, the novel approach of fabricating a coating containing nanoparticles based on nanosized SiO2 and TiO2 and their mixtures was applied to the austenitic stainless-steel base metal. It was found that coatings increased depths of penetration, enabling a consumable-free welding. Using this method, the use of several critical and near-critical raw materials (e.g., Si and Cr), as well as the relatively expensive Ni can be completely avoided. The most effective coating in terms of weld penetration consisted of a mixture of nanoparticles, rather than unary oxide coatings based on nanoparticles. A model for liquid weld metal flow is proposed based on the metallographic examination of recrystallized grains and microhardnesses measured near the weld metal, supporting the reversed Marangoni convection theory.

高圧水素ガス中におけるオーステナイト系ステンレス鋼溶接金属317LのSSRT特性と疲労寿命特性

In order to study the hydrogen embrittlement behavior of austenitic stainless steel weld metal, slow strain rate tensile (SSRT) tests and fatigue life tests were performed in high-pressure hydrogen gas. Tensile and fatigue life specimens, in which whole of the gauge section consists of weld metals, were machined out from a TIG welded round bar. The base metal of multi-pass welded bar was SUS316 (hi-Ni), and the filler metal was 317L. Two series of weld metals were tested; one was an as-welded metal having coarse columnar gain with dendrite and δ-ferrite, and the other was a post-welded solution-treated metal. The relative reduction of area, RRA, was 0.55 for 317L as-welded metal, and approximately 0.9 for SUS316 (hi-Ni) base metal and 317L post-welded solution-treated metal. The fracture surface of 317L as-welded metal included lamellar weld (LW) fracture surface, whereas those of SUS316 (hi-Ni) base metal and 317L post-welded solution-treated metal were entirely covered with dimples. Fractographic observation in combination with EDS manifested that δ-ferrite was responsible for the hydrogen-enhanced crack growth leading to the formation of LW, and therefore the post-welded solution-treated metal without δ-ferrite exhibited excellent resistance to hydrogen. Regardless of the heat treatment, the fatigue limit of 317L weld metal was not degraded in hydrogen gas as well as the austenitic stainless steel base metal.

Study of Additional Compensation Shielding Gas Technique for Pulsed MIG Seam Welding

This paper proposes a novel additional compensation shielding gas technique for seam welding as a method to overcome weld defects caused by atmospheric exposure of high-temperature solid and liquid state welding seams. This technique can be used to appropriately modify the existing pulsed MIG welding system through the addition of a gas control branch channel, allowing for control of compensation shielding gas flow to the system. A plate overlay experiment was carried out with 18-8 austenitic stainless steel base metal and a modified pulsed MIG welder with a modified a gas control branch channel. The results suggest that this technique can be used to meliorate weld defects, improve welding seam shape and enhance welding efficiency.

Reducing hot cracking tendency of dissimilar weld overlay by magnetic arc oscillation

Nickel filler metals are used for joining and repair of dissimilar metal welds in nuclear power plants. However, with some compositions of austenitic stainless steel base metals, weld cracking is observed. In the present work, the solidification cracking behaviour of 52M overlay on stainless steel with and without magnetic stirring was studied. Single, double and triple bead on plate experiments with 52M filler wire were performed on a type 303 stainless steel plate cladding with a single layer of ER308LSi stainless steel. Weldings were then performed with and without magnetic stirring. Although cracking tendency was observed in all experiments, it was seen that magnetic stirring significantly reduced the cracking tendency. Confirmatory electron backscattered diffraction analyses confirmed the grain refinement in 52M beads with magnetic stirring.

Comparison of creep rupture behaviour of type 316L(N) austenitic stainless steel joints welded by TIG and activated TIG welding processes

► In the present study, Creep rupture behaviour of 316L(N) base metal, and its weld joints were investigated at 923 K. ► Both the weld joints possessed lower creep rupture strength than the base metal. ► A-TIG joint had higher creep rupture strength than the MP-TIG joint. ► Minimum steady state creep rates of the base metal, A-TIG and MP-TIG weld joints were comparable. ► Reduced cavitation in A-TIG joint has been attributed to lower δ-ferrite content and lesser extent of strength heterogeneity. ► The columnar grains along the applied stress direction in weld metal leading to improvement in creep strength of A-TIG joint. Creep rupture behaviour of type 316L(N) austenitic stainless steel base metal and its weld joints fabricated both by single-pass activated TIG (A-TIG) and multi-pass conventional TIG (MP-TIG) welding processes were studied at 923 K over a stress range of 160–280 MPa. Both the weld joints possessed lower creep rupture lives than the base metal. The A-TIG weld joint displayed higher rupture lives than the MP-TIG weld joint. Failure in the weld joints occurred in the weld metal. Progressive localization of creep deformation in the weld metal of both the joints led to the premature failure. Accumulation of creep deformation at higher rate was observed in the weld metal of the MP-TIG joint than in the A-TIG joint. Finer microstructural features and higher amount of δ-ferrite was observed in the weld metal of MP-TIG joint than in the A-TIG weld joint. Orientation of the columnar grains and δ-ferrite was nearly transverse to the welding direction in the A-TIG joint, whereas it was towards short transverse to the welding direction in the MP-TIG weld joint. TEM investigation of creep exposed weld metal showed the extensive formation of M 23 C 6 carbides, σ-phase and Laves phase along the boundaries in MP-TIG joint, which were less prevalent in the A-TIG joint. With creep exposure, the δ-ferrite transformed to σ and Laves phases, and creep cavitation was found to be associated with the intermetallic phases. Creep cavitation was more pronounced in the MP-TIG weld joint than in the A-TIG weld joint.

ＳＵＳ３０４ＭＩＧ溶接金属中の酸素と破壊じん性に関する研究

In the gas shield arc welding with SUS304 austenitic stainless steel base metal and, JISY308L type electrod, the effect of the welding condition on the oxygen content and microstructure of weld metal has been investigated. Next, the effect of the oxygen on the mechanical properties, mainly on the fracture toughness, were studied. The main results are summarized as follows;1) The oxygen content of weld metal is increased with dicreasing the welding current (from 200A to 400A) and with increasing the arc voltage (from 20V to 30V). At the constant welding atmosphere, there is a linear correlation between the oxygen content and a parameter of A/V which is established by the authors, where A is welding current and V is arc voltage. The oxygen content decreases with increasing the parametre A/V.2) The most of the oxygen exists in the γ region as the spherical oxide of Mn-Silicate. The number of oxide per unit volumu is related to the welding heat input. The number decreases with increasing the heat input.3) Charpy impact values and Jin values decrease with increasing the oxide volume fraction.4) The relationship between Charpy impact value and oxide volume fraction is represented by linear line on the log-log cordinate. At the constant oxide voulme fraction, the distribution (number and size) of oxide has little effect to Charpy impact value.

Volumetric inspection of moderately thick austenitic stainless steels by multifrequency eddy currents

This paper describes the initial phase of a project to develop eddy-current methods to inspect welds joining sections of austenitic stainless steel pipe having walls up to 13 mm (0.5 in.) thick. The objective of this phase was to demonstrate the feasibility of detecting and characterizing flaws in austenitic stainless steel base metals. These materials and welds present challenging eddy-current problems because of their relatively large thickness and ferromagnetism. Multiparameter analysis shows that a reflection coil probe operated with three discrete driver frequencies and phase detection can locate and size a cracklike defect in a single conductor in the presence of variations in conductor resistivity, permeability, and thickness and in the probe-conductor spacing (liftoff). Experiments were performed with a modular three-frequency instrument. Flat-plate specimens of types 304L and 347 stainless steel machined to 12.7 to 15.9 mm thickness simulated pipe walls; saw-cut slots 10 to 30% of nominal specimen thickness simulated cracklike defects. The same slots were used in duplicate experiments as near-side (directly under the test probe) or far-side (in the face opposite the probe) defects. Flaw detection and characterization capability was demonstrated by a series of experimental measurements fitted to specimen properties by least squares techniques. The quality of the fit determined the expected accuracy of measurement. Comparison of accuracy estimates determined the best choice of operating frequencies. From the 1,2,5 sequence of frequencies between 0.5 and 20 kHz, the optimum set of operating frequencies was selected to be 0.5, 2, and 10 kHz. Estimates of measurement accuracy for combined near- and far-side defect cases were: plate thickness, 0.74 mm; probe liftoff, 0.03 mm; defect location (depth of material above defect), 3.48 mm; and defect size (vertical slot depth), 1.09 mm. A few property values were back-calculated from instrument readings; the errors in these values were somewhat larger than the measurement accuracy estimates because of instrument drift and the absence of calibration circuits.