Verification of numerical model to predict microstructure of austenitic stainless steel weld metal using synchrotron radiation and trans varestraint testing

Abstract

A numerical model to predict the microstructure of austenitic stainless steel weld metal is proposed, and spatially resolved X-ray diffraction measurements using synchrotron radiation have been carried out for Fe–20Cr–(9·8–14·4)Ni weld metals, quenched in liquid Sn, to verify the validity of the numerical model. X-ray diffraction analysis of Fe–20Cr–11·5Ni quenched weld metal, solidifying in the ferritic–austenitic mode, showed that the secondary γ phase crystallised in a eutectic growth mode down to a temperature drop of 6 K from the initiation of solidification. Also, from X-ray diffraction analysis of Fe–20Cr–12·7Ni quenched weld metal, which solidified in the austenitic–ferritic mode, it was found that the secondary δ phase crystallised in a eutectic growth mode within the temperature drop range between 15 and 21 K from the initiation of solidification. The crystallisation temperatures predicted by the numerical model for secondary γ and δ phases in Fe–20Cr–11·5Ni and Fe–20Cr–12·7Ni weld metals agreed with experimental data. Furthermore, it was found that the effect of Ni content on the solidification cracking susceptibility of Fe–20Cr–(9·8–14·4)Ni weld metal, determined via trans varestraint testing, agreed with the results calculated using the model. These agreements support the validity of the developed numerical model.     

Microstructure and hot cracking susceptibility of high conductivity Al–Fe–Si alloys

Aluminium–silicon based casting alloys have been extensively utilised in various industrial applications, but their relatively low electrical and thermal conductivities make them unsuitable for high conductivity parts. In this research, Al–Fe–Si based high conductivity alloys containing limited silicon content were investigated. Al–0·5Fe–xSi alloys with silicon ranging from 0·5 to 2% showed significantly higher electrical conductivity than conventional Al–Si based alloys. The hot cracking susceptibility of Al–Fe–Si alloys became seriously high as the Si content increased up to 1·5%, then susceptibility rapidly reduced with the further increase in Si. The relationship between solidification characteristics and hot cracking susceptibility of Al–0·5Fe–xSi alloys was discussed based on the thermal and cooling curve analyses and microstructural observations.     

Analysis of crystallisation and growth behaviour of NbC on solidification process of SUS347H weld metal

Abstract

In order to investigate the solidification cracking susceptibilities of SUS347H weld metal containing high niobium and carbon contents, the crystallisation behaviours of niobium carbide as well as those of and γ phases during welding solidification were numerically analysed to compare with the results from in situ observations of the behaviour of the phase selection for γ, and niobium carbide phases for Fe–18Cr–0·2C–1Nb–(5–12)Ni weld metals. The in situ observation was carried out during tungsten inert gas welding using synchrotron radiation. On the other hand, to evaluate the segregation in liquid phase, which has significant influences on the solidification cracking susceptibility, a numerical model to calculate the segregation in liquid phase considering the crystallisation of γ, and niobium carbide phases was developed. The developed numerical model was verified by the comparison with experimental results from the in situ observation. It was suggested that the solidification cracking susceptibilities of Fe–18Cr–0·2C–1Nb–(5–12)Ni weld metals estimated from the results of the developed numerical model coincided with the experimental results evaluated by transverse Varestraint test.     

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract

Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.     

Synergistic effect of Ni and N on improvement of pitting corrosion resistance of high nitrogen stainless steels

Abstract

The pitting corrosion resistance of Fe18Cr10Mn(0·33–0·69)N, Fe18Cr10Mn1Ni(0·33–0·84)N, and Fe18Cr10Mn0·35N(0–3)Ni alloys were investigated. The pitting potential increased as the N content increased in both Fe18Cr10Mn(0·33–0·69)N and Fe18Cr10Mn1Ni(0·33–0·84)N alloys. The rise in the pitting potential was more pronounced in Fe18Cr10Mn1Ni(0·33–0·84)N alloys than in Fe18Cr10Mn(0·33–0·69)N alloys. However, it was found that Ni alone had no effect on the pitting corrosion resistance of Fe18Cr10Mn0·35N based alloys. Thus, it was concluded that the alloyed N worked synergistically with Ni to promote the pitting corrosion resistance in Fe18Cr10Mn based alloys. Analyses of passive films of Fe18Cr10Mn(0·33–0·69)N and Fe18Cr10Mn1Ni(0·33–0·84)N alloys revealed that N was incorporated into the passive film, with N enriched at the film/metal interface. However, the alloyed N increased the Cr cation fraction in passive films of Fe18Cr10Mn1Ni(0·33–0·84)N alloys, whereas N decreased in that of Fe18Cr10Mn(0·33–0·69)N alloys. This difference was considered as the reason for the synergistic effect between N and Ni in Fe18Cr10Mn based alloys.     

High power laser welding of Nb microalloyed steel plates

Abstract

Laser welding studies on 12 mm thick Nb microalloyed steels were done using a 25 kW CO₂ laser at welding speeds of up to 3 m min^?1 with the aim of identifying the influences of Nb and carbon on laser weldability and weld properties. Welds were examined for solidification flaws, penetration characteristics, microstructure and mechanical properties. Solidification cracking did not appear to be influenced by carbon or Nb in the ranges studied when welding at power levels between 22 and 25 kW. The area fraction of martensite was shown to increase with decreasing energy input and increasing carbon content. Weld metal toughness was improved by reducing carbon content in the range 0·08–0·05%C but was little influenced by Nb. Weld metal and heat affected zone hardness levels were reduced, as expected, by reducing carbon content. The results show that increasing Nb content leads to higher strength material without significant loss of toughness, while lowering carbon content can improve toughness without loss of strength. Thus low carbon Nb microalloyed steels with about 0·05%C can achieve a good combination of parent plate and laser weld properties and appear to be suitable for high power laser welding situations.     

Influence of Gd and B on solidification behaviour and weldability of Ni–Cr–Mo alloy

Abstract

The influence of Gd and B on the solidification behaviour and weldability of Ni–Cr–Mo alloy UNS N06455 has been investigated by Varestraint testing, differential thermal analysis and microstructural characterisation. These alloys are currently being developed as structural materials for nuclear criticality control in applications requiring transportation and disposition of spent nuclear fuel owned by the US Department of Energy. The Gd containing alloys were observed to solidify in a manner similar to a binary eutectic system. Solidification initiated with a primary L→y reaction and terminated at ~1258°C with a eutectic type L→y+Ni₅Gd reaction. The solidification cracking susceptibility of the Gd containing alloys reached a maximum at ~1 wt-%Gd and decreased with both higher and lower Gd additions. Low cracking susceptibility at Gd concentrations below ~1 wt-% was attributed to a relatively small amount of terminal liquid that existed over much of the crack susceptible solid+liquid zone. Low cracking susceptibility at Gd concentrations above ~1 wt-% was attributed to a reduced solidification temperature range and backfilling of solidification cracks. The addition of B above the 230 ppm level leads to the formation of an additional eutectic type reaction at ~1200°C and the secondary phase within the eutectic type constituent was tentatively identified as Mo₃B₂. The B containing alloys exhibited a three step solidification reaction sequence consisting of primary L→y solidification, followed by the eutectic type L→y+Ni₅Gd reaction, followed by the terminal eutectic type L→y+Mo₃B₂ reaction. Boron additions had a strong, deleterious influence on solidification cracking susceptibility. The high cracking susceptibility was attributed to extension of the crack susceptible solid+liquid zone induced by the additional eutectic type L→y+Mo₃B₂ reaction and extensive wetting of the grain boundaries by the solute rich liquid. Simple heat flow equations were combined with solidification theory to develop a relation between the fraction liquid f _L and distance x within the solid+liquid zone. Information on the phenomenology of crack formation in the Varestraint test were coupled with the calculated f _L–x curves and were shown to provide useful insight into composition–solidification–weldability relations.     

Melt flow in a mushy zone – barrier effect of intermetallic phases

Abstract

Iron and manganese are common impurity elements in cast aluminium alloys, especially in secondary aluminium. During casting Fe/Mn-containing intermetallics are formed between the aluminium dendrites, which cause porosity and shrinkage defects. In this paper an experimental study on the influence of controlled convection during solidification on the spatial arrangement of intermetallic phases and their interaction with the dendritic microstructure in Al–7Si–1Fe (AlSiFe) and Al–7Si–1Mn (AlSiMn) alloys (wt-%) is presented. Forced convection is induced by a rotating magnetic field. The alloys are solidified directionally over a range of constant solidification velocities (0·015–0·18 mm s^–1) at a constant temperature gradient G of 3 K mm^–1. The results indicate that the primary spacing and the secondary dendrite arm spacing are affected by the presence of Fe and Mn intermetallic phases. In samples solidified under forced convections the primary dendrite arm spacing did not depend on the solidification velocity and no obvious fluid flow effect on the secondary spacing could be detected. These observations are in contrast to Fe and Mn free alloys. It seems that the intermetallics act as a barrier for the flow into the mushy zone.     

Evaluation of solidification cracking susceptibility of Fe–18Mn–0·6C steel welds

Solidification cracking susceptibilities of high Mn steel welds were evaluated in the present study. A longitudinal Varestraint technique was utilised to assess the solidification cracking behaviours of the fusion zone. High Mn steel welds were more susceptible to solidification cracking than 304 and 202 austenitic stainless steel welds, however, they were less susceptible than 310S austenitic stainless steel welds. Extensive segregations of Mn and C took place at the dendritic and grain boundaries in the weld metal, and accordingly contributed to the increase of the hot cracking susceptibility of high Mn steel by the enlargement of solidification temperature range. Further, continuous γ-(Fe,Mn)₃C eutectic phases formed at 1090°C along the grain boundary primarily resulted in the increase of solidification cracking sensitivity in high Mn steel.     

Determining hot cracking index of Al–Si–Cu–Mg casting alloys calculated using effective solidification range

The possibility of determining the hot cracking index using the calculated value of the effective solidification range is investigated for multicomponent cast aluminium alloys based on the system Al–Si–Cu–Mg with Mn, Ni, Fe and Zn additives. The upper limit of the effective solidification range was calculated as the temperature of formation of 65 wt-% solid phase using Sheil model. The linear relationship of the hot cracking index and the effective solidification range in the industrial and experimental multicomponent alloys based on the Al–Si–Cu–Mg system is demonstrated.     

Determining hot cracking index of Al–Mg–Zn casting alloys calculated using effective solidification range

The possibility of determining the hot cracking index using the calculated value of the effective solidification range is investigated for multicomponent cast aluminium alloys based on the Al–Mg–Zn system with Mn, Ni, Fe and Si additives. The upper limit of the effective solidification range was calculated as the temperature of formation of a 65?wt-% solid phase using the Sheil model. The linear relationship of the hot cracking index and the effective solidification range in the industrial and experimental multicomponent alloys based on the Al–Mg–(Zn) system is demonstrated.     

Microcracking in multipass weld metal of alloy 690 Part 3 – Prevention of microcracking in reheated weld metal by addition of La to filler metal

Abstract

The effect of addition of La to a filler metal on microcracking (ductility dip cracking) in the multipass weld metal of alloy 690 was investigated with the aim of improving its microcracking susceptibility. The susceptibility to ductility dip cracking in the reheated weld metal could be greatly improved by adding 0·01–0·02 wt-%La to the weld metal. Conversely, excessive La addition to the weld metal led to liquation and solidification cracking in the weld metal. Hot ductility of the weld metal at the cracking temperature was greatly improved by adding 0·01–0·02 wt-%La to the weld metal, implying that the ductility dip cracking susceptibility was decreased as a result of the desegregation of impurity elements of P and S to grain boundaries due to the scavenging effect of La. The liquation and solidification cracking resulting from excessive addition of La to the weld metal is attributed to the formation of liquefiable Ni–La intermetallic compound. A multipass welding test confirmed that microcracks in the multipass weldment were completely prevented by using a filler metal containing an addition of 0·01 wt-%La.     

Coking and Dusting of Fe–Ni Alloys in CO–H2–H2O Gas Mixtures

Metal dusting of Fe–Ni alloys was investigated in a CO–H₂–H₂O–Ar gas corresponding to a _C = 19.6 at 650 °C. Thermogravimetric analysis showed that increasing the nickel content in the alloy decreased the initial rate of carbon uptake. A uniform Fe₃C scale formed on pure iron, a layer with mixed structures of Fe₃C, γ and α-Fe developed on ferritic Fe–5Ni, and small amounts of Fe₃C developed at the surface of an austenite layer grown on two-phase (α + γ) Fe–10Ni. At nickel levels above 10%, no carbide appeared. These observations are shown to be broadly consistent with local equilibrium according to the Fe–Ni–C phase diagram. However, the failure of higher nickel austenitic alloys to form the (Fe,Ni)₃C expected at high carbon activities indicates a barrier to nucleation and growth of this phase. Graphite deposition was catalysed by (Fe,Ni)₃C on ferritics and by the metal itself on austenitics. The rates of carbon deposition on Fe–60Ni corresponded to the existence of three parallel and independent paths: the synthesis gas, the Boudouard and the carbon methanation reactions.     

On the effect of strain on peritectic reactions and transformations in Fe–Ni and Fe–Cu binary alloys

Abstract

Differential thermal analysis (DTA) experiments conducted on Fe–Ni and Fe–Cu alloys showed undercooling below the equilibrium peritectic temperatures, T_P . The intervals between the observed liquidus and peritectic temperatures were on average 11°C and 8°C larger than the intervals obtained from equilibrium phase diagrams of Fe–Ni and Fe–Cu respectively. The transformation from δ-Fe to γ-Fe during the peritectic reaction is associated with density change and strain build up at the δ-Fe/γ-Fe interface. Thermodynamic calculations showed that by introducing the strain energy at the δ-Fe/γ-Fe interface, T_P dropped 9 K below its equilibrium value and the increase in the liquidus-to-peritectic temperature interval was in reasonable agreement with the experimental observations. The growth rate of γ-Fe during a peritectic transformation was calculated based on the strain-induced undercooling in T_P and the results showed partial agreement with observations obtained from CSLM directional solidification experiments conducted earlier on Fe–Ni alloys.     

Solidification and welding metallurgy of Thermo-Span alloy

Abstract

The welding metallurgy of Thermo-Span alloy has been evaluated and compared with that of two common superalloys, alloy 909 and alloy 718. The solidification behaviour and fusion zone hot cracking tendency were evaluated using differential thermal analysis and varestraint testing. Gleeble thermal cycle simulations were used to assess the hot ductility of the alloy during weld thermal cycles. Solidification microstructures were characterised by optical and electron microscopy. Differential thermal analysis indicated that melting of wrought Thermo-Span initiates at ternperatures near 1265°C and continues to ～1413°C. In a manner similar to that of alloy 909, Thermo-Span solidifies with the formation of primary austenite followed by a terminal eutecticlike constituent near 1225°C. Varestraint testing indicates that the hot cracking tendency of Thermo-Span is similar to that of alloys 718 and 909. Hot ductility testing revealed that Thermo-Span displays ductilities on heating and cooling similar to those of alloy 909 and other nickel based superalloys. Solidification microsegregation patterns were determined by electron probe microanalysis, and the terminal solidification phases were identified by analytical electron microscopy and backscattered electron Kikuchi patterns to be an austenite–Laves eutecticlike constituent.     

Effect of Cr and Ni Contents on the Oxidation Behavior of Ferritic and Austenitic Model Alloys in Air with Water Vapor

Ferritic and austenitic model alloys with various contents of Cr and Ni ranging between 10–20% and 0–30%, respectively, were oxidized in air + 10% water vapor during 1 hr cyclic oxidation at 650°C and 800°C. Depending on the alloy composition and temperature, either a thin protective oxide scale was observed or accelerated attack occurred which sometimes included spallation. For austenitic model alloys, increasing either the Cr or Ni contents delayed the accelerated attack. For lower Cr and Ni contents at 800°C, accelerated attack, including spallation, occurred at short exposure times. No spallation was observed for the ferritic model alloys. However, accelerated attack can occur quickly with low Cr contents. Increasing the temperature delayed the breakaway observed on ferritic alloys whereas it reduced the protective-oxide-growth stage for austenitic alloys.     

Study of the Ni–Si–Be system as a base to create boron-free brazing filler metals

The investigation results of alloys of the Ni–Si–Be system with a various content of Si and Be are presented. Two low-melting eutectics with melting temperatures of the order of 1100 and 915°C have been found out by the methods of differential thermal and X-ray phase analysis and metallography. Two alloys have been selected for the development of brazing filler metals: Ni–5Si–3Be and Ni–6Si–5Be alloys. Three phases have been shown to be components of the binary and ternary eutectics in the Ni–Si–Be system from the side of the nickel angle of the phase diagram. A Ni–5Si–3Be ribbon and fragments of the Ni–6Si–5Be ribbon have been obtained from the selected alloys by rapid quenching. Filler metal of the Ni–5Si–3Be system has been used for brazing samples of the 12Cr21Ni5Ti austenitic–ferritic corrosion-resistant steel at 1150°C for 30?min.     

Synergistic alloying effect on microstructural evolution and mechanical properties of Cu precipitation-strengthened ferritic alloys

We report the influence of alloying elements (Ni, Al and Mn) on the microstructural evolution of Cu-rich nanoprecipitates and the mechanical properties of Fe–Cu-based ferritic alloys. It was found that individual additions of Ni and Al do not give rise to an obvious strengthening effect, compared with the binary Fe–Cu parent alloy, although Ni segregates at the precipitate/matrix interface and Al partitions into Cu-rich precipitates. In contrast, the co-addition of Ni and Al results in the formation of core–shell nanoprecipitates with a Cu-rich core and a B2 Ni–Al shell, leading to a dramatic improvement in strength. The coarsening rate of the core–shell precipitates is about two orders of magnitude lower than that of monolithic Cu-rich precipitates in the binary and ternary Fe–Cu alloys. Reinforcement of the B2 Ni–Al shells by Mn partitioning further improves the strength of the precipitation-strengthened alloys by forming ultrastable and high number density core–shell nanoprecipitates.     

Processing and microstructural characterization of liquid phase sintered tungsten–nickel–cobalt heavy alloys

In this study, the effects of composition and sintering temperature on the microstructural characteristics of liquid phase sintered 90W–Ni–Co alloys were investigated. 90W–Ni–Co alloys having Ni/Co ratios of 3/1, 4/1 and 6/1 were examined. It was found that the alloys studied have reached almost to full density when sintered at and above 1475 °C. The microstructures of the alloys were typical of liquid phase sintered alloys, which consisted of rounded, nearly pure W grains embedded in a ternary Ni–Co–W binder matrix phase. The binder matrix phase in these alloys was observed to dissolve up to 42 wt.% W. The relative amount of the binder matrix phase and the average size of the W grains were found to increase with increasing sintering temperature. The activation energies for grain coarsening are determined for the investigated alloys by assuming that the coarsening process is mainly governed by Ostwald ripening mechanisms in the liquid state. The calculated activation energies, which were within 113–162 kJ/mol range, were found to be in rather close agreement to the literature data given for W–Ni–Fe alloys. This indicates that grain coarsening in W–Ni–Co and W–Ni–Fe alloys most probably takes place through similar diffusional processes.     

The Effect of Water Vapor on the Oxidation Behavior of CVD Iron-Aluminide Coatings

The oxidation behavior of iron-aluminide coatings, Fe₃Al or (Fe,Ni)₃Al, produced by chemical-vapor deposition (CVD) was studied in the temperature range of 700–800°C in air + 10 vol.% H₂O. A typical ferritic steel, Fe–9Cr–1Mo, and an austenitic stainless steel, 304L, were coated. For both substrates, the as-deposited coating consisted of a thin (<5μm), Al-rich outer layer above a thicker (30–50 μm), lower-Al-content inner layer. In addition to coated and uncoated Fe–9Cr–1Mo and 304L, cast Fe–Al model alloys with similar Al contents (13–20 at.%) to the CVD coatings were included in the oxidation exposures for comparison. The specimens were cycled to 1000 1 hr cycles at 700°C and 500 1 hr cycles at 800°C, respectively. The CVD coating specimens showed excellent performance in the water-vapor environment at both temperatures, while the uncoated alloys were severely attacked. These results suggest that an aluminide coating can substantially improve resistance to water-vapor attack under these conditions.