Dendrite Core Research Articles

A study of the weldability and weld related microstructure of cabot alloy 214

The weldability and weld metal microstructure of Cabot Alloy 214 have been investigated with a variety of experimental and analytical techniques. These include Varestraint hot crack testing, hot ductility testing, pulsed Nd:YAG laser welding, scanning and analytical electron microscopy, electron microprobe analysis, and X-ray diffraction. A heat of Alloy 214 containing intentionally alloyed B (0.003 wt pct) and Zr (0.07 wt pct) was much more sensitive to both fusion zone hot cracking as quantified by the Varestraint test and to simulated heat-affected-zone (HAZ) cracking as quantified by hot ductility testing than a heat of Alloy 214 containing no intentionally added B (0.0002 wt pct) or Zr (0.02 wt pct). Scanning electron microscopy of the high B and Zr alloy showed the presence of dendritically-shaped, Zr-rich constituents in interdendritic regions in the gas-tungsten-arc (GTA) welds. Electron microprobe analysis of these welds revealed a segregation pattern of Cr, Al, Mn, and Zr enrichment in interdendritic regions and Ni and Fe enrichment in dendrite core regions. Analytical electron microscopy revealed the presence of ZrX (X = B, C, N, O), M23C6, andγ′ in the fusion zone of GTA weld specimens,γ′ was also found in the as-received base metal and in the GTA weld HAZ. X-ray diffraction analysis of extractions from the high B and Zr GTA weld metal also indicated the presence of a ZrX-type constituent. The results of this study are in qualitative agreement with earlier work performed on alloys such as NIMONIC 90 and INCONEL 718∗ relative to the detrimental effect of B and Zr additions on fusion zone and HAZ hot cracking susceptibility.

SOLIDIFICATION OF SUPERCOOLED FE-NI ALLOYS

ABSTRACT The effect of supercooling on the solidification microstructure and solute distribution in Fe-Ni alloys containing 7, 10, 15, 35, and 45 wt.% Ni were investigated. A levitation technique was used to melt and supercool the alloys up to 320 K. X-ray microanalysis and optical techniques were used to determine the fraction of partitioniess solidification as a function of supercooling and alloy composition. It was found that supercooling the alloys below the γ-To curve, but above the metastable extension of the δ-To curve, results in partitioniess solidification of γ in the form of solute-rich crosses at the dendrite cores. On the other hand, when the supercooling was below the metastable δ-To extension, the crosses were not found and a martensitic structure was observed instead of the solute-rich crosses. We attribute this phenomenon to the activation of the alternative δ-Fe phase, instead of γ-Fe, when the supercooling is below the extended δ-To.

The use of new PHACOMP in understanding the solidification microstructure of nickel base alloy weld metal

The weld metal microstructures of five commercial nickel base alloys (HASTELLOYS* C-4, C-22, and C-276, and INCONELS* 625 and 718) have been examined by electron probe microanalysis and analytical electron microscopy. It has been found that solidification terminates in many of these alloys with the formation of a constituent containing a topologically-close-packed (TCP) intermetallic phase(i.e., σ, P, Laves). Electron microprobe examination of gas-tungsten-arc welds revealed a solidification segregation pattern of Ni depletion and solute enrichment in interdendritic volumes. New PHACOMP calculations performed on these segregation profiles revealed a pattern of increasingMd (metal-d levels) in traversing from a dendrite core to an adjacent interdendritic volume. In alloys forming a terminal solidification TCP constituent, the calculatedMd values in interdendritic regions were greater than the criticalMd values for formation ofσ as stated by Morinagaet al. Implications of the correlation between TCP phase formation andMd in the prediction of weld metal solidification microstructure, prediction of potential hot-cracking behavior, and applications in future alloy design endeavors are discussed.

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.

Solidification of M2 high speed steel

The freezing process in AISI type M2 high speed tool steel (6 pct W, 5 pct Mo, 4 pct Cr, 2 pct V, 0.8 pct C) was studied by metallographic and thermal analysis techniques. Unidirectional solidification of small laboratory melts in a modified crystal growing apparatus was employed to provide metallographic sections of known macroscopic growth direction. Also cooling curves were obtained on 40 g specimens solidified in thimble crucibles. X-ray microradiography, electron probe scanning techniques, and quantitative microanalysis of dendrites and interdendritic carbides were extensively used to supplement conventional metallography. Carbon and vanadium contents of M2 were varied in order to observe the effect of an austenite and ferrite stabilizer on the thermal analysis curves and microstructure. The nonequilibrium freezing process in M2 includes three major liquid-solid reactions: 1) Liquid → Ferrite, 1435°C; 2) Liquid + Ferrite → Austenite, 1330°C; 3) Liquid → Austenite + M6C + MC, 1240°C. These reactions account for the as-cast structure of the commercial alloy. The addition of carbon depresses the liquidus (1) and solidus temperatures (3) and narrows the gap between the liquidus (1) and peritectic transformation (2). This gap is eliminated at > 1.39 wt pct C, where the initial freezing reaction is the crystallization of austenite. The accompanying microstructural change is the elimination of σ eutectoid dendrite cores. The addition of vanadium promotes ferrite formation by strongly depressing the peritectic reaction and thus widening the gap between the liquidus and the peritectic.