Solidification microstructures and phase transformations in Al-Ti-Si-Mn deoxidized steel weld metals

The present investigation is concerned with basic studies of solidification mechanisms in Al-Ti-Si-Mn deoxidized steel weld metals. Assessment of the weld metal solidification micro-structures was done on the basis of optical microscopy in combination with secondary ion mass spectrometry (SIMS), while both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for characterization of indigenous oxide inclusions. It is shown that nonmetallic inclusions play a critical role in the development of the weld metal columnar grain structure by acting as inert substrates for nucleation of delta ferrite ahead of the advancing interface. The nucleation potency of the oxides increases in the order SiO₂-MnO, Al₂O₃-Ti₂O₃-SiO₂-MnO, A1₂O₃, reflecting a corresponding increase in the inclusion/liquid interfacial energy. Moreover, a shift in the peritectic reaction (which facilitates growth of the austenite grains across the phosphorus-rich boundaries of the primary delta ferrite phase) has been observed in the presence of A1₂O₃ inclusions. Indications are that the resulting decrease in the local phosphorus concentrations at the austenite grain boundaries will strongly alter the kinetics of the subsequent solid-state transformation reactions by promoting growth of grain boundary ferrite sideplates at the expense of intragranularly nucleated acicular ferrite.     

Characterization of the solidification structures within the dendritic core of M2 high speed steel

The phases which are present within the dendritic core region during the solidification of AISI type M2 high speed steel (6 W, 5 Mo, 4 Cr, 2 V) were studied and characterized by metallography and quantitative microprobe analysis. A series of samples were quenched from various temperatures during solidification, “freezing in” the different solidification phases. The first phase observed to solidify in M2 is ferrite which contains very little carbon. As the solidification process continues, most of the liquid surrounding the ferrite transforms to austenite by virtue of a peritectic reaction which initiates at 1330°C,(L + F →A). The ferritic cores also transform at around 1330°C into an austenite plus car-bide aggregate. By the time the ingot cools to 1255°C, the carbides at the center of the dendrites dissolve completely, leaving an austenitic phase of uniform carbon and alloy content. At temperatures below the solidus, very fine carbides precipitate from the aus-tenite. No eutectoid decomposition by products such as those commonly observed in Tl tool steel were observed in these specimens nor in samples from a commercial ingot. JAMES McLAUGHLIN, formerly Graduate Assistant at Lehigh University     

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.     

耐蚀合金中TiN析出机制

摘要：TiN颗粒尺寸及其分布对耐蚀合金性能有明显的影响,因此有必要对TiN在铸坯中的分布及其析出行为进行研究。采用扫描电镜(SEM)、金相显微镜（OM）观察了TiN夹杂物在铸坯中的分布、尺寸及其形貌;基于热力学和动力学理论分析了耐蚀合金铸坯中TiN夹杂物的析出时机及其尺寸,结合试验结果和理论计算明确了TiN夹杂物在凝固后铸坯中的位置和尺寸与析出时机的关系,为控制TiN夹杂物提供理论指导。结果表明,冶炼过程中析出的TiN夹杂物尺寸较大,在凝固过程中被枝晶吞没,位于铸坯枝晶内和等轴晶内;微观偏析计算结果表明,在凝固分数为0.55时,TiN开始析出,最开始析出TiN夹杂物的逐渐长大,长大后的TiN易于被二次枝晶吞没,最终位于铸坯中的枝晶间和等轴晶内,后期析出的TiN则在枝晶间和等轴晶间。固相中析出的TiN夹杂物长大较慢,尺寸细小,最终位于奥氏体晶界。     

850级钢熔化焊焊缝的组织特征

利用金相显微镜、扫描电子显微镜及附带EDS系统和透射电子显微镜研究850 MPa级焊缝金属的微观组织,并通过分析焊缝金属凝固和相变过程,研究组织形成机制。发现原δ铁素体柱状晶晶界附近的锰和镍含量高于其心部含量。原δ铁素体柱状晶晶界附近组织由平行板条马氏体组成,板条宽度约为300 nm,原δ铁素体柱状晶心部组织由"交织状"板条马氏体组成,板条宽度约为400 nm。分析认为造成原δ铁素体柱状晶晶界附近和心部组织差异的重要原因是锰和镍的偏析,而焊缝金属良好的冲击韧性是因为存在30%"交织状"马氏体和一定量残余奥氏体。     

Effect of solidification conditions on the solidification sequence of austenitic chromium-nickel stainless steels

Unlike the well-known effect of alloy elements in promoting the ferritic or austenitic solidification of stainless and acid-resisting chromium-nickel steels, kinetic effects have as yet not been so widely looked into. For this reason, the impact of the solidification rate on the ratio of the amounts of ferritic and austenitic liquid solidification was investigated for the steels of grades X8CrNiTi18.10 and X8CrNiMoTi18.11. A microanalysis for the determination of the primary ferrite content of samples taken from ingots of different size and at different distances from the ingot surface for a total of 161 heats revealed the following:

• – Increasing solidification rate causes the primary ferrite content produced during solidification to rise for steels with peritectic solidification sequence due to the resultant approach of the distribution coefficient to unity.
• – Increasing solidification rate causes the austenite content to rise for steels with a primary simultaneous crystallization of austenite and ferrite due to a low total segregation in case of austenite crystallization as compared with ferrite crystallization.
• – The effect of an elevated solidification rate is qualitatively equivalent to a shift of the saturation lines of the three-phase space l+δ+γ in the Fe—Ni—Cr ternary system for liquid and γ-crystals in the direction S with the two saturation lines approaching each other. Hence, contrary to what is expected according to the equilibrium diagram of Schürmann and Brauckmann, austenitic Cr—Ni steels solidify primarily in peritectic mode and, in the area of the line of the double-saturated liquid, through a primary simultaneous crystallization of austenite and ferrite.
• – The boundary composition between primary ferritic and primary austenitic crystallization changes with an increase in cooling rate by seven orders of magnitude from 1.25 to 1.70 as expressed in the ratio of the Cr—Ni equivalents according to Hammar and Svensson.

     

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.     

Grain structures in gas tungsten-arc welds of austenitic stainless steels with ferrite primary phase

The grain structures were investigated in full penetration gas tungsten-arc (GTA) welds in sheets of 304 and 321 austenitic stainless steels for a range of welding conditions. In type 321 steel welds, fine equiaxed ferrite dendrites were observed in the ferrite phase. The equiaxed structure was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions present in this steel, since these inclusions were observed at the origin of equiaxed dendrites. In type 304 welds, the ferrite grains were columnar, except in less complete penetration specimens, where a few coarse equiaxed dendrites appeared to originate from the weld surface. The secondary austenitic grain structure was columnar in both steels. In type 304 steel, the columnar austenitic grain structure did not necessarily correspond to the primary ferrite grains. In type 321 steel, the secondary austenite was columnar despite the equiaxed structure of the primary ferrite. Factors which affect the columnar-to-equiaxed transition (CET) are discussed. The failure to form equiaxed austenitic grains in type 321 steel is ascribed to austenite growing across the space between ferrite grains instead of renucleating on the primary equiaxed ferrite.     

Catalyst effects in heterogeneous nucleation of acicular ferrite

The present investigation is concerned with basic studies of the mechanisms of acicular ferrite (AF)’formation in low-alloy steel weld metal. It is confirmed experimentally that different types of orientation relationships exist between AF and specific cubic inclusion constituent phases(i.e., γ-Al₂ MnOAl₂O₃, and TiN). Since the majority of these falls within the Bain orientation region, it is concluded that the associated reduction of the energy barrier to nucleation is the primary cause for the ferrite nucleus to develop orientation relationships with both the substrate and the austenite. Theoretical calculations show that about 12 pct of the inclusions will contain a cubic phase that lies within the Bain region purely by chance if they are randomly orientated in space. This intrinsic density of heterogeneous nucleation sites is sufficiently high to promote the formation of fine, interlocking AF laths in the weld metal during they- to- a transformation.     

Investigation on the Effect of Sulfur and Titanium on the Microstructure of Lamellar Graphite Iron

The goal of this work was to identify the inclusions in lamellar graphite cast iron in an effort to explain the nucleation of the phases of interest. Four samples of approximately the same carbon equivalent but different levels of sulfur and titanium were studied. The Ti/S ratios were from 0.15 to 29.2 and the Mn/S ratios from 4.2 to 48.3. Light and electron microscopy were used to examine the unetched, color-etched, and deep-etched samples. It was confirmed that in irons with high sulfur content (0.12 wt pct) nucleation of type-A and type-D graphite occurs on Mn sulfides that have a core of complex Al, Ca, Mg oxide. An increased titanium level of 0.35 pct produced superfine interdendritic graphite (~10 μm) at low (0.012 wt pct) as well as at high-S contents. Ti also caused increased segregation in the microstructure of the analyzed irons and larger eutectic grains (cells). TiC did not appear to be a nucleation site for the primary austenite as it was found mostly at the periphery of the secondary arms of the austenite, in the last region to solidify. The effect of titanium in refining the graphite and increasing the austenite fraction can be explained through the widening of the liquidus-eutectic temperature interval (more time for austenite growth) and the decrease in the growth rate of the graphite because of Ti absorption on the graphite. The fact that Ti addition produced larger eutectic cells supports the theory that Ti is not producing finer graphite because of a change in the nucleation potential, but because of lower growth rate of the graphite in between the dendrite arms of a larger fraction of austenite. In the presence of high-Ti and S, (MnTi)S star-like and rib-like inclusions precipitate and act as nuclei for the austenite.     

Solidification mechanism of martensitic stainless steel

Abstract

In an unidirectional solidification experiment, an 8 kg stainless steel ingot with the composition 0·25%C, 17%Cr, and 1%Mn was solidified under continuous casting conditions. The dwell time of primary cooling was varied, followed by secondary spray cooling. Metallographic investigation, heat transfer, and segregation were carried out to study the solidification mechanism. The partition ratio of the elements present in ferrite and in austenite (martensite) was determined. It was indicated that the solidification follows: L → L + δ → L + δ + γ → δ + γ + carbides. Under high cooling rates γ austenite solidifies as a leading phase. The beginning of spray cooling has the main effect in controlling the obtained microstructures. Carbide thickening is observed in the rapidly cooled zone between the ferrite and the martensitic matrix. Tempered martensite increases by lowering the cooling rate, which gives more time for carbide dissolution and for carbon to diffuse into the ferrite, eventually increasing the austenite (martensite) fraction in the final matrix at the expense of ferrite.     

Alternating grain orientation and weld solidification cracking

A new mechanism for reducing weld solidification cracking was proposed, based on the concept of the crack path and resistance to crack propagation, and its effectiveness was verified in magnetically oscillated GTA welds of a rather crack susceptible material 2014 aluminum alloy. This mechanism,i.e., alternating grain orientation, was most pronounced in welds made with transverse arc oscillation of low frequency and high amplitude, and solidification cracking was dramatically reduced in these welds. The effect of the arc oscillation pattern, amplitude, and frequency on the formation of alternating columnar grains and the reduction of solidification cracking in GTA welds of 2014 aluminum alloy was examined and explained. The present study demonstrated for the first time that columnar grains can, in fact, be very effective in reducing solidification cracking, provided that they are oriented favorably.     

Direct observations of the <Emphasis Type="Italic">α</Emphasis> → <Emphasis Type="Italic">γ</Emphasis> transformation at different input powers in the heat-affected zone of 1045 C-Mn steel arc welds observed by spatially resolved X-ray diffraction

Spatially resolved X-ray diffraction (SRXRD) experiments have been performed during gas tungstenarc (GTA) welding of AISI 1045 C-Mn steel at input powers ranging from 1000 to 3750 W. In-situ diffraction patterns taken at discreet locations across the width of the heat-affected zone (HAZ) near the peak of the heating cycle in each weld show regions containing austenite (γ), ferrite and austenite (α+γ), and ferrite (α). Changes in input power have a demonstrated effect on the resulting sizes of these regions. The largest effect is on the γ phase region, which nearly triples in width with increasing input power, while the width of the surrounding two-phase α+γ region remains relatively constant. An analysis of the diffraction patterns obtained across this range of locations allows the formation of austenite from the base-metal microstructure to be monitored. After the completion of the α → γ transformation, a splitting of the austenite peaks is observed at temperatures between approximately 860 °C and 1290 °C. This splitting in the austenite peaks results from the dissolution of cementite laths originally present in the base-metal pearlite, which remain after the completion of the α → γ transformation, and represents the formation of a second more highly alloyed austenite constituent. With increasing temperatures, carbon, originally present in the cementite laths, diffuses from the second newly formed austenite constituent to the original austenite constituent. Eventually, a homogeneous austenitic microstructure is produced at temperatures of approximately 1300 °C and above, depending on the weld input power.     

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.     

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.     

A Study on Microstructure and Phase Transformation in the Weld Fusion Zone of TIG-Welded IN939 with IN625 and IN718 as Filler Metal

Microstructure and phase transformations were investigated in the weld fusion zone (WFZ) of tungsten inert gas (TIG)-welded IN939 nickel-based superalloy with IN718 and IN625 alloys used as filler metal. The optical and field-emission scanning electron microscopy (FESEM) and also energy dispersive spectroscopy (EDS) were utilized for the microstructural study of the WFZ. The microstructural examination showed that the Nb and Ti elements were microsegregated in the inter-dendrite region, causing a significant concentration gradient between the core and the inter-dendrite region within the WFZ. Observations indicated the formation of the (Nb, Ti) C carbide phases in the solidification process. With a thickness of about 0.2 μm, some plate-like δ phase was observed in the WFZ of both samples upon heat treatment at 750 °C. The results of X-ray diffraction analysis (XRD) confirmed the presence of the δ phase in the WFZ. However, the δ phase disappeared at temperatures beyond 1000 °C. The occurrence of phase transformation during the solidification process was identified by differential thermal analysis (DTA). The WFZ was seen to solidify in the temperature range of 1370°C to 1270°C.

     

Quantitative Analysis of Microstructural Constituents in Welded Transformation-Induced-Plasticity Steels

A quantitative analysis of retained austenite and nonmetallic inclusions in gas tungsten arc (GTA)–welded aluminum-containing transformation-induced-plasticity (TRIP) steels is presented. The amount of retained austenite in the heat-affected and fusion zones of welded aluminum-containing TRIP steel with different base metal austenite fractions has been measured by magnetic saturation measurements, to study the effect of weld thermal cycles on the stabilization of austenite. It is found that for base metals containing 3 to 14 pct of austenite, 4 to 13 pct of austenite is found in the heat-affected zones and 6 to 10 pct in the fusion zones. The decomposition kinetics of retained austenite in the base metal and welded samples was also studied by thermomagnetic measurements. The decomposition kinetics of the austenite in the fusion zone is found to be slower compared to that in the base metal. Thermomagnetic measurements indicated the formation of ferromagnetic ε carbides above 290 °C and paramagnetic η(ε′) transient iron carbides at approximately 400 °C due to the decomposition of austenite during heating.     

Autogenous electrospark deposition of NiCoCrAlY

Abstract

Electrospark deposition is a microarc welding process that has been successfully used to deposit an autogenous ultrafine NiCoCrAlY coating. Primary solidification of the β phase shows epitaxial growth where orientation and heat flux coincide. Electron channelling contrast imaging of the deposits shows a homogeneous distribution of non-stoichiometric phases from the rapid solidification of the electrospark deposition process. Electron backscatter diffraction mapping of the deposit indicates that an ultrafine β structure can grow epitaxially where favourable conditions prevail. The equiaxed structure associated with the γ to β solidification shift was not observed and is believed to result from the homogeneous electrode and substrate compositions. Oxidation of the deposit shows advantageous alumina growth stemming from the ultrafine aluminium rich β phase. The transition from θ- to α-alumina was nearly complete in only 6 h at 1000°C in air.     

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.