Phase evolution of AISI 321 stainless steel during directional solidification

Abstract

The phase evolution of AISI 321 stainless steel was studied by directional solidification and quenching techniques. Two interfaces, solid/liquid and the peritectic reaction interface, were found to exist in the directional solidification structure. With increasing growth velocity the solid/liquid interface changed in the sequence of planar, cellular, dendritic and the primary phase changed from austenite to ferrite. The phase and morphology selection was verified by the interface response functions (IRFs) and the maximum growth temperature criterion. The ferritic island banding structure was observed, not only in the austenite cellular primary growth condition (3 μm s^?1), but also in the dendritic ferrite primary growth one at relatively low growth velocity (5 μm s^?1). It is deemed that the former resulted from the nucleation of ferrite in the continuous matrix of austenite phase, yet the latter is the residual primary ferrite attributed to the growth of austenite. Both of them do not come from the nucleation near the solid/liquid interface.     

An analytical model for optimal directional solidification using liquid metal cooling

In what follows, a model is developed that describes the optimal processing parameters for directional solidification using liquid metal cooling (LMC). The model considers a sample with a flat geometry and, as a first approximation, can be used to treat the flat sections of a turbine blade. The model predicts (1) the optimal withdrawal rate of the casting from the hot zone, (2) the temperature gradient in the liquid at the solidification interface, and (3) the temperature profile along the length of the casting. The model is then used to perform a sensitivity analysis of the LMC process. Cooling bath temperature, baffle thickness, shell thickness, and shell thermal conductivity are shown to have a strong influence on system performance.     

高碳铬钢凝固及热处理组织演变规律

摘要：系统研究了高碳铬钢复合轧辊工作层材质在凝固及热处理过程的组织演变规律。利用Thermo Calc软件进行平衡相图计算,阐明了平衡凝固过程及冷却过程中碳化物析出行为。通过热膨胀仪精确测定了奥氏体化温度对合金相变点的影响,获得了珠光体等温转变动力学曲线。结合扫描电镜、能谱分析和X射线衍射分析,确定了热处理工艺对最终显微组织的影响,明确了凝固过程中形成的一次碳化物M7C3和二次碳化物对热处理组织的影响规律,为高碳铬钢复合轧辊工作层材料的实际热处理工艺制定提供理论依据。     

Local properties of undermatched steel weld metal

Samples from two undermatched, multipass welds on 50.8-mm-thick HY-100 steel were tested using a novel microtensile test machine and the local material properties were investigated using a chemical analysis, metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The microtensile test technique allowed samples from individual weld beads and weldmetal heat-affected zones to be tested in three orthogonal directions. Relationships between local microhardness and tensile properties were established. The filler metals for the two welds were MIL-70S and MIL-100S. The MIL-70S weld formed ferritic microstructures; the weld-metal heat-affected sites were predominantly polygonal ferrite, while the as-deposited regions were a mixture of lath and polygonal ferrite. This weld showed a large variation in properties from the central weld bead to the outer ones. The outermost site exhibited significant anisotropy in strength that was not revealed by microhardness measurements. The yield strength specification was 483 MPa, while the average at the center of the weld was 675 MPa and the outer sites had an average of 445 MPa. Elongation for the samples from the center was significantly lower as well, 5 pct as compared to 18 pct for the outer sites. The yield strength showed a strong correlation with the size of inclusions measured by TEM. Microprobe analysis found no dilution of the base metal alloying additions into the weld metal. The MIL-100S filler formed predominantly fine acicular ferrite throughout the weld. The strength was much more uniform; the yield strength specification was 690 MPa, while the center of the weld was 756 MPa and the outer sites had an average of 616 MPa. The inclusion size did not play an important role in the variation in mechanical properties.     

Effect of welding variables and solidification substructure on weld metal porosity

Porosity is defined as cavity-type discontinuities formed by gas entrapment during solidification. Causes of porosity in fusion welds are the dissolved gases in weld metal and welding process variables that control the solidification rate. To study the mechanisms of porosity formation in weld metal, single-pass gas tungsten-arc weld metal was produced using the bead-on-plate technique on three nickel-copper alloys (80 wt pct Ni-20 wt pct Cu, 65 wt pct Ni-35 wt pct Cu, 35 wt pct Ni-65 wt pct Cu). Four different welding speeds were used under various amounts of nitrogen content in argon-shielding atmosphere. A qualitative model was proposed to characterize the effect of welding variables and solidification substructure on bulk and interdendritic porosity formation. Increasing amounts of nitrogen gas (from 0.2 pct to 6.0 pct in volume) introduced in argon-shielding atmosphere increased the amount of porosity in weld metal. The amount of bulk and total porosity increased as the solubility of nitrogen in the weld metal alloy decreased. The solidification rate of the weld pool is the most important factor controlling the mechanism of porosity formation. The observed amount of bulk pores in this study increased with the increase of welding speed; that is, if the time is insufficient for dissolved and evolved gases to escape during solidification, porosity will result. However, a decrease in the amount of interdendritic pores was observed with increasing welding speed in the 80Ni-20Cu and 35Ni-65Cu alloys. This decrease can be related to the effect of solidification rate on the balance between the disjoining pressure, resistance of the liquid film to be disrupted, repulsion of the bubble from the solidification front, and the hydrodynamic force resisting the movement of the bubble. This balance determines the ability of the cellular solidification front to “equilibrium” capture the pores. Furthermore, the observed decrease of interdendritic porosity with increasing welding speed (80Ni-20Cu and 35Ni-65Cu alloys) can also be related to the time for nucleation and growth of pores in the molten weld metal and their entrapment in the interdendritic channels of a dendritic solidification front. This phenomenon is considered a “nonequilibrium capture” of pores. On the other hand, the 65Ni-35Cu alloy that exhibited a structural transition in solidification substructure with the variation of welding speed showed a slight increase in the amount of interdendritic pores. This increase was correlated to the change of pore-capture mechanism from an equilibrium to a nonequilibrium mode as the solidification substructure changed from cellular to cellular dendritic. To substantiate that the controlling mechanism of interdendritic porosity formation is the nonequilibrium capture, a good correlation between the measured mean pore radius and the interdendritic arm spacing was found.     

Numerical analysis of role of melting rate on electroslag remelting continuous directional solidification of a die steel

The effect of melting rate on the temperature distribution,velocity field,macrosegregation and dendrite arm spacing during electroslag remelting continuous dire...     

冷却速率对奥氏体不锈钢凝固过程影响的原位观察

通过采用激光共聚焦扫描显微镜对AISI304奥氏体不锈钢的凝固过程进行了原位动态观察研究.发现当冷却速率为0.05℃·s^-1时,奥氏体不锈钢以胞状晶方式凝固,其凝固模式为FA模式,即δ铁素体相先从液相中形核并长大,γ相在1 448.9℃时通过与液相发生包晶反应(L+δ→γ)在δ铁素体相界形成,当温度降到1 431.3℃时液相消失,δ铁素体相通过固态相变转变为γ相,富Cr贫Ni的残留铁素体位于胞状晶之间.当冷却速率为3.0℃·s^-1时,奥氏体不锈钢以枝晶方式生长,冷却到1346.4℃时包晶反应在液相与δ铁素体相界之间进行,其残留铁素体位于枝晶干,与冷却速率为0.05℃·s^-1时相比,其残留铁素体的数量增多,残留铁素体富Cr贫Ni的程度减轻.     

Statistical analysis of the disorder of two-dimensional cellular arrays in directional solidification

By introducing a weighted Wigner-Seitz construction and, for the first time in directional solidification, using the minimal spanning tree (MST) approach and the (m,σ)-diagram, the statistical analysis of the topological defects and disorder of two-dimensional (2-D) cellular arrays has been carried out. For “standard” growth of massive Pb-30 wt pct Tl alloys, the underlying honeycomb has been brought out, which was rather unexpected, as the percentage of defects is so high that the cellular arrays are “melted” by the defects, with a structure close to that of a 2-D liquid. Furthermore, disorder can be described by a Gaussian noise applied on an array of hexagonal cells, Lewis's law is satisfied, and for a sufficiently large number of cells, a peak is evidenced in the primary spacing distribution, which indicates that, in some way, the cell size is selected. Nevertheless, the associated standard deviation is rather large. It is conceivable that the defects can have a leading role in the selection of the primary spacing.     

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 increasingM _d (metal-d levels) in traversing from a dendrite core to an adjacent interdendritic volume. In alloys forming a terminal solidification TCP constituent, the calculatedM _d values in interdendritic regions were greater than the criticalM _d values for formation ofσ as stated by Morinagaet al. Implications of the correlation between TCP phase formation andM _d in the prediction of weld metal solidification microstructure, prediction of potential hot-cracking behavior, and applications in future alloy design endeavors are discussed.     

相场法在金属凝固过程的应用现状

相场法是在计算材料学中发展最快的一种强大的计算方法,以基本的热力学和动力学为输入,可用于模拟和预测材料的介观尺度形貌和微观结构的演变。首先总结了相场模型的历史发展、物理基础、数学表达及其数值求解,其次分析了相场法在纯物质、二元合金、多组分系统以及定向凝固、增材制造等领域中的应用情况,最后对相场法进行了总结及展望,并指出相场法发展应趋向于超大尺度相场模拟技术,更高效算法的开发,相场模型与热力学、动力学数据库的结合,工业应用的探索以及与实验观察技术的进一步结合。     

Investigation of solidification and microsegregation of near-net-shape cast carbon steel

Different near-net-shape casting techniques are investigated in terms of solidification parameters, microstructure, and microsegregation of manganese in carbon steels based on experimental simulation methods: ingot casting of thick and thin slab samples, strip samples, and thin strip. The as-cast thicknesses were between 1.9 and 150 mm. By calculations and measurements data have been determined which lead to the solidification structure like solidification- and cooling-rate, secondary dendrite arm spacings, and the concentrations of microsegregations. Finally, some literature data of material properties of steel strip produced by the investigated methods are given.     

Direction of grain-boundary migration in the weld metal of an austenitic stainless steel

The moving direction of the grain boundary (GB), after solidification in the weld metal of AISI310S stainless steel, was examined through a computer simulation technique using the vertex dynamics model and by observing the microstructure. The results are as follows. (1) The grain-growth exponent in the vertex model was fitted to describe the experimental data. (2) The vertex dynamics model can predict the moving direction of a grain boundary in the weld metal after solidification.     

Low temperature aging behavior of type 308 stainless steel weld metal

The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M₂₃C₆ carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.     

Characteristics of a pulsed-current, vertical-up gas metal arc weld in steel

The performance of the pulsed-current gas metal arc welding (GMAW) process for vertical-up weld deposition of steel has been found to be superior over the use of the short-circuiting arc GMAW process with respect to the tensile, impact, and fatigue properties of the weld joint. The microstructure, weld geometry, and mechanical properties of a pulsed-current weld joint are largely governed by the pulse parameters, and correlate well to the factor φ, defined as a summarized influence of pulse parameters such as peak current, base current, pulse-off time, and pulse frequency. The increase of φ has been found favorable to refine the microstructure and enhance the tensile strength, C _v toughness, and fatigue life of a weld joint. The fatigue life of a short-circuiting arc weld joint has been found to be markedly reduced due to the presence of an undercut at the weld toe and incomplete side-wall fusion of the base material.     

A thermomechanical model for simulation of carbon steel solidification in mould in continuous casting

Abstract

The 17th Annual Conference of the Sheffield Metallurgical and Engineering Association (SMEA), held on 17–18 June 2008 at the Endcliffe Conference Centre, University of Sheffield, attracted 120 delegates. The aim of the conference was to explore the effect of hot rolling and forging processes on the quality and performance of steels and high duty alloys. Five technical sessions and 20 presentations explored process developments and their influence on product quality, how the hot working process can be optimised to influence microstructure and properties, and the influence of new technologies.     

Cellular-to-dendritic transition during the directional solidification of binary alloys

The transition from a cellular to dendritic microstructure during the directional solidification of alloys is examined through experiments in a transparent system of succinonitrile (SCN)-salol. In a cellular array, a strong coupling of solute fields exists between the neighboring cells, which leads not only to multiple solutions of primary spacing, but also includes multiple solutions of amplitude, tip radius, and shape of the cell. It is found that these multiple solutions of different microstructural features in a cellular array, obtained under fixed growth conditions and compositions, play a key role in the cell-dendrite transition (CDT). The CDT is controlled not only by the input parameters of alloy composition (C ₀), growth rate (V), and thermal gradient (G), but also by microstructure parameters such as the local primary spacing. It is shown that the CDT is not sharp, but occurs over a range of growth conditions characterized by the minimum and maximum values of V/G. Within this transition range, a critical spacing is observed above which a cell transforms to a dendrite. This critical spacing is given by the geometric mean of the thermal, diffusion, and capillary lengths and is inversely proportional to composition in weight percent.     

Study of the formation and intergrowth of granular eutectic in austenitic steel matrix composite by directional solidification technology

The formation and intergrowth of granular eutectic in austenitic steel matrix composite has been studied by directional solidification technology. The results indicate that the modifying element Si enhances the dendritic segregation of C and Mn. The surface active elements, such as Y and Ca, concentrate highly ahead of the solid-liquid (S-L) interface of the composite due to the nonequilibrium solidification. As a result, the S-L interface of the composite is unstable during solidification. The spatiotemporal condition of the formation and the growth of the granular eutectic is the formation of granular eutectic between austenitic dendrite arms at the end of solidification and its growth restricted by the austenitic dendrites. By the simulating eutectic growth of the granular eutectic by Fe−C−Mn alloy, the Si, Ca, and Y adsorb and enrich on the growing surface of the eutectic during crystallization, which makes the crystallization model of the eutectic turn from facet/nonfacet to nonfacet/nonfacet. The intergrowth of the eutectic can be explained by (1) the influence of the modifying elements on the crystallization of the eutectic, (2) the coarse solidification growth interface of the eutectic and the same growth rate for austenite and cementite ((Fe, Mn)₃C), and (3) the austenite and cementite ((Fe, Mn)₃C) have not lateral branch during eutectic growth.     

Analysis of inclusions and precipitates in tough weld metal of X80 steel

Submerged arc welding(SAW)and gas metal arc welding(GMAW)experiments of Nb-bearing X80 steel were conducted with high-toughness wires.The inclusions in weld metals were analyzed in terms of their types and sizes.In GMAW,the inclusions are primarily Ti,Ca,Si,Al,and Mg compounds with no Nb and are generally less than 0.8 pm in size,whereas,in SAW weld,the inclusions are larger,mostly approximately 2-5 μm in size,and are cored with Ca and Ti,exhibiting obvious oxidation metallurgical features.The SAW joint was hot-deformed,and Nb-bearing nano precipitates were newly found in the weld metal through transmission electron microscopy,and Nb-free core-shell inclusion was found through scanning electron microscopy.The inclusions and precipitates were dispersed in or on the boundaries of acicular ferrite,contributing to acicular ferrite nucleation and grain refinement.