The nonunjform distribution of inclusions in low-alloy steel weld deposits

Nonmetallic inclusions in low-alloy steel welds have an important effect on the microstructure and properties of weld deposits. This work is an attempt at understanding the factors controlling the spatial distribution of such inclusions, with particular emphasis on the uniformity of the distribution and the effect of solidification mode during manual-metal-arc welding. The solidification mode has been controlled by using unusual combinations of base plates and experimental electrodes. It is found that the first phase to solidify (in the form of columnar grains) is delta-ferrite (δ) when a medium carbon electrode is deposited onto a low carbon substrate, but that it is austenite (γ) when a low carbon electrode is deposited onto a high carbon substrate. Relatively large inclusions have been found to position themselves preferentially, during solidification, to the columnar grain boundaries of the first phase to solidify, whether this is 8-ferrite or austenite. The results can be understood qualitatively in terms of a surface tension driven Marangoni effect, or in terms of the pushing of solid inclusions by the solidification front. Both mechanisms drive the larger inclusions into cusps in the interface while smaller ones are passively trapped. The implications of the observed nonuniform distribution of inclusions are more severe for solidification with austenite as the primary phase, since the larger inclusions are in that case located in the weakest region of the weld where they also do not contribute to the intragranular nucleation of acicular ferrite.     

An Investigation of the Massive Transformation from Ferrite to Austenite in Laser-Welded Mo-Bearing Stainless Steels

A series of 31 Mo-bearing stainless steel compositions with Mo contents ranging from 0 to 10 wt pct and exhibiting primary δ-ferrite solidification were analyzed over a range of laser welding conditions to evaluate the effect of composition and cooling rate on the solid-state transformation to γ-austenite. Alloys exhibiting this microstructural development sequence are of particular interest to the welding community because of their reduced susceptibility to solidification cracking and the potential reduction of microsegregation (which can affect corrosion resistance), all while harnessing the high toughness of γ-austenite. Alloys were created using the arc button melting process, and laser welds were prepared on each alloy at constant power and travel speeds ranging from 4.2 to 42 mm/s. The cooling rates of these processes were estimated to range from 10 K (°C)/s for arc buttons to 10⁵ K (°C)/s for the fastest laser welds. No shift in solidification mode from primary δ-ferrite to primary γ-austenite was observed in the range of compositions or welding conditions studied. Metastable microstructural features were observed in many laser weld fusion zones, as well as a massive transformation from δ-ferrite to γ-austenite. Evidence of epitaxial massive growth without nucleation was also found when intercellular γ-austenite was already present from a solidification reaction. The resulting single-phase γ-austenite in both cases exhibited a homogenous distribution of Mo, Cr, Ni, and Fe at nominal levels.     

Spatially resolved X-ray diffraction mapping of phase transformations in the heat-affected zone of carbon-manganese steel arc welds

Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite; however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld.     

Fatigue crack propagation in austenitic stainless steel weldments

The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6) compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed areas in their fractures.     

The effect of quenching on the solidification structure and transformation behavior of stainless steel welds

Weld solidification structure of three different types of stainless steel,i.e., 310 austenitic, 309 and 304 semiaustenitic, and 430 ferritic, was investigated. Welds of each material were made without any quenching, with water quenching, and with liquid-tin quenching during welding. The weld micro-structure obtained was explained with the help of the pseudobinary phase diagrams for Fe-Cr-Ni and Fe-Cr-C systems. It was found that, due to the postsolidification 5 → γ phase transformation in 309 and 304 stainless steels and the rapid homogenization of microsegregation in 430 stainless steel, their weld solidification structure could not be observed unless quenched from the solidification range with liquid tin. Moreover, the formation of acicular austenite, and hence, martensite, at the grain boundaries of 430 stainless steel welds was eliminated completely when quenched with liquid tin. The weld solidification structure of 310 stainless steel, on the other hand, was essentially unaffected by quenching. Based upon the observations made, the weld microstructure of these stainless steels was summarized. The effect of cooling rate on the formation of primary austenite in 309 stainless steel welds was discussed. Finally, a simple method for determining the relationship between the secondary dendrite arm spacing and the solidification time, based on welding speeds and weld pool configurations, was suggested.     

Solidification and solidification cracking in nitrogen-strengthened austenitic stainless steels

The solidification behavior of three heats of nitrogen-strengthened austenitic stainless steel was examined and was correlated with solidification mode predictions and with hot cracking resistance. The heat of NITRONIC* 50 solidified by the austenitic-ferrite mode, and the NITRONIC 50W and NITRONIC 50W - Nb heats solidified by the ferritic-austenitic mode. This behavior was in good agreement with predictions based on Espy’s formulas for Cr and Ni equivalents. Both the NITRONIC 50W and NITRONIC 50W + Nb welds contained primary delta-ferrite, with the latter weld and the NITRONIC 50 weld also containing some eutectic ferrite. Solute profiles in austenite near the eutectic ferrite showed decreasing Fe and increasing Cr, Ni, Mn, and Mo relative to austenite in the dendrite cores. Numerous Nb-rich precipitates were found on the eutectic ferrite/austenite interfaces and within the eutectic ferrite. The precipitates were mainly Nb(C, N), with some Z-phase, a Nb-rich nitride, also detected. One instance of the transformation of eutectic ferrite to sigma-phase was observed to have occurred during cooling of the NITRONIC 50 weld. Hot cracking was seen in the NITRONIC 50 and NITRONIC 50W + Nb welds and resulted from the formation of a niobium carbonitride eutectic in the interdendritic regions. In the absence of Nb, the NITRONIC 50W heat formed no observable eutectic constituents and did not hot crack. The presence of hot cracks in the NITRONIC 50W + Nb weld indicates that solidification by the ferritic-austenitic mode did not counteract the effects of small Nb additions.     

Structure of a cast high-strength corrosion-resistant 05Kh20AG10N3MF austenitic steel containing 0.40 or 0.53% nitrogen

The phase composition and fine structure of a high-strength corrosion-resistant 05Kh20AG10N3MF austenitic steel containing 0.40 or 0.53% N are studied by X-ray diffraction and electron microscopy. In the as-cast state, this steel has a structure containing austenite, δ ferrite, and dispersed CrV(C, N) carbonitrides. The δ ferrite is represented by layers between austenite grains, the dislocation density in which is lower than in the δ ferrite. After quenching from 1100, 1150, and 1200°C, the structure of the steel with 0.53% N has no δ ferrite and the structure of the steel with 0.40% N has a low δ-ferrite content and χ-phase precipitates.     

Effect of cooling after welding on microstructure and mechanical properties of 12 Pct Cr steel weld metals

The microstructure of three 12 pct cr steel weld metals with different nickel and nitrogen contents was studied in as-welded condition and after postweld heat treatment with and without intercooling. Tensile strength and impact toughness of the weld metals were investigated in different postweld heat treatment conditions. In weld metals heat treated without intercooling, austenite decomposed by a eutectoid reaction that resulted in M₂₃C₆ aggregates around retained δ-ferrite. Two morphologies of M₂N and MN precipitates were found in a low-dislocation α-ferrite. It was concluded that these phases were also transformed from austenite. In weld metals heat treated with intercooling, M₂₃C₆ precipitates were smaller and more homogeneously distributed. Different MN precipitates were found in the tempered martensite. The fracture mode of the weld metals at room temperature was mainly transgranular cleavage with some fibrous fracture. Intercooling treatment improved Charpy impact toughness of the 12 pct Cr steel weld metals substantially. It was found that the important microstructural factors affecting the impact toughness of the weld metals which were heat treated without intercooling were the sizes of the α-ferrite grains, nonmetallic inclusions, and M₂₃C₆ aggregates. For the weld metals heat treated with intercooling, the factors which affect the toughness of the weld metals were the sizes of martensite packets and nonmetallic inclusions.     

Solidification modeling and solid-state transformations in high-energy density stainless steel welds

The solidification and solid-state transformations which occur during the high-energy density (HED) welding of austenitic stainless steel were studied. Comparisons were made between structures observed in gas tungsten arc (GTA) welds and those of electron beam (EB) and laser welds using Fe-Ni-Cr ternary alloys with Cr/Ni ratios ranging from 1.5 to 1.85. Weld solidification and microsegregation was modeled using a finite difference analysis and compared with experimental results. These calculations were also used to help interpret the origin of the observed microstructures. Calculations showed that little solid-state diffusion occurs during the solidification and cooling of primary austenite solidified welds, whereas structures which solidify as ferrite may become almost completely homogenized as a result of diffusion. A change in solidification mode from primary austenite to primary ferrite was found to occur at higher Cr/Ni ratios with the HED welds than with GTA welds and is attributed to dendrite tip undercooling. A nearly segregation-free, single-phase austenite structure which appears to be unique to the rapid solidification velocities and cooling rates of HED welds was also observed. It is suggested that this structure is a product of ferrite solidification which transforms to austenitevia a massive transformation.     

Ferritic-austenitic solidification mode in austenitic stainless steel welds

The macro-and microstructures of about fifty different stainless welds of the AISI/ AWS 300 series are analyzed. The results indicate that under conditions corresponding to a typical shielded metal arc (SMA) welding the welds with a ratio in the range 1.48≾Cr _eq /Ni _eq ≾1.95, where Ni _eq and Cr _eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify in accordance with a duplex mode with the delta ferrite as the primary (leading) phase. The austenite forms between ferrite dendrites through a three-phase reaction between liquid, ferrite and austenite, and subsequently grows into the ferrite by either an equiaxial or an acicular mechanism, resulting in a drastic decrease in the volume fraction of the delta ferrite. The micro-structure at room temperature is characterized by a general irregularity and the varied morphology of the ferrite. The compositional differences observed at room temperature are a consequence both of the solidification and the solid state transformation. Formerly Research Staff Member, Laboratory of Physical Metallurgy, University of Oulu.     

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.     

Microstructural evolution in a 17-4 PH stainless steel after aging at 400 °C

The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e., after solution heat treatment, tempering at 580 °C, and long-term aging at 400 °C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution-treated specimen consists largely of martensite with a small fraction of δ-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the δ-ferrite. After tempering for 4 hours at 580 °C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 hours aging at 400 °C, the martensite spinodaly decomposes into Fe-rich α and Cr-enriched α′. In addition, fine particles of the G-phase (structure type D8 _a , space group Fm m) enriched in Si, Ni, and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.     

Microstructural investigation of a rapidly solidified 12Cr-Mo-V steel

Rapidly solidified martensitic stainless steel (11.59Cr-0.98Mo-0.28V (in wt pct) ribbons have been produced by the melt-spinning process. The microstructure of the ribbons showed three distinct zones: a columnar, a cellular, and a cellular-dendritic zone. The height of the columnar grain zone is independent of the process parameters such as the wheel material or the wheel velocity. Due to a high level of undercooling and a high growth velocity of the solid/liquid interface, the rapid solidification process is found to suppress the formation of δ-ferrite and enhance the formation of austenite. The austenite is transformed into martensite upon cooling. In comparison with conventional solidification, a reduction in the initial austenite grain size has been found to result in a very fine lath martensite (M) structure. Investigations of the texture within the ribbons along the growth direction show a weak fiber texture. Transmission electron microscopy (TEM) has revealed a [111]_M1 ‖ [001]_M2 and (011)_M1 ‖ (110)_M2 orientation relationship between two neighboring martensite laths. The observed orientation relationship is a result of a superposition of both the Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (N-W) orientation relations.     

Phase stability and atom probe field ion microscopy of type 308 cre stainless steel weld metal

Improvement in high-temperature creep-rupture properties of type 308 stainless steel welds due to the controlled addition of boron is related to microstructural evolution during welding and thermal phase stability at creep service temperatures. The microstructure of boron-containing type 308 austenitic stainless steel welds, in the as-welded state, consisted of 8 to 10 pct ferrite in an austenite matrix. Atom probe field ion microscopy studies revealed segregation of boron and carbon to ferriteaustenite boundaries in the as-welded state; the segregation level was less than one monolayer coverage. On aging at 923 K for 100 hours, M₂₃C₆ carbides precipitated at ferrite-austenite boundaries. On further aging at 923 K for 1000 hours, the ferrite transformed into σ phase. Similar microstructural evolution was observed in a type 308 stainless steel weld without boron addition. The volume fractions of M₂₃C₆ carbides were identical in boron-containing and boron-free welds. Atom probe results from the welds with boron addition in the aged condition showed that the boron dissolved in the M₂₃C₆ carbides. However, lattice parameter analysis showed no apparent difference in the extracted carbides from the welds with and without boron. Creep property improvement due to boron addition could not be related to any change in the volume fraction of carbides. However, the results suggest that the incorporation of boron into M₂₃C₆ carbides may reduce the tendency for cavity formation along the M₂₃C₆ carbide-austenite boundaries and hence improve the resistance to creep fracture. The observed microstructural evolution in welds is consistent with thermodynamic calculations by THERMOCALC software.     

The microstructure of an Fe-Mn-Si-Cr-Ni stainless steel shape memory alloy

The microstructure and phase stability of the Fe-15Mn-7Si-9Cr-5Ni stainless steel shape memory alloy in the temperature range of 600 °C to 1200 °C was investigated using optical and transmission electron microscopy, X-ray diffractometry (XRD), differential scanning calorimetry (DSC), and chemical analysis techniques. The microstructural studies show that an austenite single-phase field exists in the temperature range of 1000 °C to 1100 °C, above 1100 °C, there exists a three-phase field consisting of austenite, δ-ferrite, and the (Fe,Mn)₃Si intermetallic phase; within the temperature range of 700 °C to 1000 °C, a two-phase field consisting of austenite and the Fe₅Ni₃Si₂ type intermetallic phase exists; and below 700 °C, there exists a single austenite phase field. Apart from these equilibrium phases, the austenite grains show the presence of athermal ɛ martensite. The athermal α′ martensite has also been observed for the first time in these stainless steel shape memory alloys and is produced through the γ-ɛ-α′ transformation sequence.     

Precipitation in CF-8M duplex stainless steel welds

Welds of CF-8M, a cast 316-type stainless steel which normally solidifies as primary delta-ferrite, were induced to solidify as primary austenite by the addition of nitrogen to the shielding gas used during gas tungsten arc welding. Those welds which experienced a shift in solidification mode formed eutectic ferrite during the terminal transient stage of solidification. Primary delta-ferrite and eutectic ferrite are differentiated by their location in the dendritic microstructure. The shape of the ferrite/austenite interface tends to be rounded for primary delta-ferrite and more angular for eutectic ferrite. Elemental profiles were plotted from STEM/EDS measurements across the two types of ferrite, and showed differences between the composition of the austenite immediately adjacent to the primary delta-ferrite, as opposed to the eutectic ferrite. In addition, while the primary delta-ferrite/austenite interfaces are largely devoid of precipitation, the eutectic ferrite/austenite interfaces are densely covered with small precipitates ofx-phase. The mean stoichiometry of this phase has been calculated from STEM/EDS data on extraction replicas, and approximates Fe₅₀Cr₃₂Mo₁₃Ni₅. Intragranular inclusions were also examined and found to be complex, with most of them containing varying quantities of Mn, Si, and S.     

Effect of Rolling and Subsequent Annealing on Microstructure,Microtexture, and Properties of an Experimental Duplex Stainless Steel

A lean duplex stainless steel (LDSS) has been prepared with low-N content and processed by different thermo-mechanical schedules, similar to the industrial processing that comprised hot-rolling, cold-rolling, and annealing treatments. The microstructure developed in the present study on low-N LDSS has been compared to that of high-N LDSS as reported in the literature. As N is an austenite stabilizer, lower-N content reduced the stability of austenite and the austenite content in low-N LDSS with respect to the conventional LDSS. Due to low stability of austenite in low-N LDSS, cold rolling resulted in strain-induced martensitic transformation and the reversion of martensite to austenite during subsequent annealing contributed to significant grain refinement within the austenite regions. δ-ferrite grains in low-N LDSS, on the other hand, are refined by extended recovery mechanism. Initial solidification texture (mainly cube texture) within the δ-ferrite region finally converted into gamma-fiber texture after cold rolling and annealing. Although MS-brass component dominated the austenite texture in low-N LDSS after hot rolling and cold rolling, that even transformed into alpha-fiber texture after the final annealing. Due to the significant grain refinement and formation of beneficial texture within both austenite and ferrite, good combination of strength and ductility has been achieved in cold-rolled and annealed sample of low-N LDSS steel.     

Effect of Aging on the Fracture Behavior of Lean Duplex Stainless Steels

The influence of aging in the range of 550 °C to 850 °C for 5 to 120 minutes on the impact fracture behavior of 2101 and 2304 lean duplex stainless steels (DSS) was investigated in the present study. The 2304 steel displayed ductile behavior irrespective of aging conditions. In contrast, the 2101 steel displayed a ductile behavior only in the case of aging for 5 minutes at 550 °C and 650 °C, whereas in all other cases, it fractured in a brittle manner. The brittle fracture behavior of the 2101 steel has been attributed to the precipitation of small black particles at the α/α and α/γ grain boundaries (nitrides), which form paths for easy crack propagation. In the 2304 steel, such particles precipitated at 750 °C and 850 °C, but they were located inside the austenitic grains because of the formation of secondary austenite. They therefore did not embrittle the steel. The larger Ni content of the 2304 steel favored the formation of the secondary austenite that is absent in the 2101 steel.