Ductility of stabilized ferritic stainless steel welds

An investigation was made into the mechanism of ductility loss in low interstitial 18 Cr-2Mo ferritic stainless steel welds stabilized with Ti and Nb. It was found that stabilizing TiN or Nb(C,N) precipitates are dissolved during the welding process, resulting in a finer distribution of precipitates in the weld metal than in the base metal. Furthermore, the FATT was found to increase by more than 200°C, leading to decreased room temperature ductility. Such an increase in FATT may not be explained solely in terms of grain growth. Internal friction measurements indicate that no free nitrogen is present in the weld metal, yet wet chemical analysis reveals that the nitrogen is present in a soluble form. Kinetic arguments suggest that the stabilized nitrogen dissolved during welding tends to reprecipitate during solidification in the form of a chromium rich nitride phase.     

Austenitic solidification mode in austenitic stainless steel welds

The micro- and macrostructures of about 50 different stainless welds of the AISI/AWS 300 series are analyzed. The results indicate that in welding condition corresponding to a typical SMA welding those and only those welds in which the ratio Cr_eq/Ni_eq≲1.48, where Ni_eq and Cr_eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify with the austenite as the primary or leading phase and the delta ferrite, if any, formed from the rest melt between growing cells or cellular dendrites of the austenite. At room temperature these welds are characterized by a regular general microstructure, soft forms of the ferrite and relatively large compositional differences mainly caused by solidification. T. TAKALO, formerly Research Staff Member, University of Oulu     

Microstructure development in high-strength low-alloy steel welds

The reliability of steel welds becomes more critical issue with increasing steel strength,because brittle phases are more likely to form in the weld metals and heat-affected zone(HAZ) and thereby the toughness and ductility of the welds are degraded.Therefore,refinement of microstructure and minimization of the brittle phases are necessary to improve the reliability of the high-strength steel welds.In this presentation,microstructure formation that controls the toughness of weld metals and HAZ in high-strength low-alloy(HSLA) steel welds is reviewed and possible routes to the improvement of the weld microstructure and weld toughness are discussed.     

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.     

STEM Analysis of primary austenite solidified stainless steel welds

Scanning transmission electron microscopy (STEM) of welds in Fe-Ni-Cr ternary alloys was used to study the nature of partitioning during primary austenite solidification. It was found that concentration profiles obtained by STEM analysis can be used to distinguish between primary ferrite and primary austenite solidified weld structures. Measured alloy concentrations within the cellular structure were also used to estimate the solidification path and tie lines for solidification by primary austenite.     

Carbide precipitation in welds of two-phase austenitic-ferritic stainless steel

Transmission electron microscopy (TEM) and microanalytical chemistry were performed on sensitized samples of duplex welds that exhibited both skeletal ferrite microstructures and lath ferrite microstructures. The objective was to understand why welds with lath ferrite, contrary to a theoretical prediction, are not immune to sensitization. Most of the ferrite-austenite (α-γ) interphase boundaries in the welds with skeletal ferrite were curved and incoherent, while those in welds with lath ferrite were predominantly planar and semicoherent. The density of carbide precipitation on incoherent boundaries was much greater than that on semicoherent boundaries. Carbide precipitates on incoherent boundaries were typically equiaxed, while those on semicoherent boundaries had very high aspect ratios and appeared to form along ledges in the interphase boundary. During sensitizing heat treatments, the chromium-depleted zone on the ferrite side of the interphase region transformed to austenite, causing the α-γ interphase boundary to move into the ferrite region. This markedly increased the width of the chromium-depleted zone in the austenite phase and extended the time of heat treatment required to replenish the zone with chromium. It is proposed that migration of the α-γ interphase boundary, which occurs to a much greater extent in the welds with lath ferrite, is responsible for their unexpected susceptibility to sensitization at 550°C.     

Single-phase ferritic solidification mode in austenitic-ferritic stainless steel welds

The macro- and microstructures of about 50 different stainless welds of the AISI/AWS 300 series are analyzed. The results indicate that under conditions corresponding to a typical SMA welding, those and only those welds in which the ratio Cr_eq/Ni_eq?1.95, where Ni_eq and Cr_eq are the nickel and chromium equivalents on the Schaeffler diagram, solidify as single-phase ferrite: Austenite subsequently forms from ferrite in the solid state by a Widmanstätten mechanism. The microstructure at room temperature (RT) is characterized by straight phase boundaries and a distinct orientation relationship between the austenite and ferrite. The compositional differences observed at RT are mainly a consequence of the solid state transformation, not of solidification.     

Hydrogen induced ductility losses in austenitic stainless steel welds

The effect of hydrogen on the tensile behavior of austenitic stainless steel welds was studied in two AISI 300 series alloys and two nitrogen strengthened alloys. The microstructure of these welds typically contained several percent ferrite in an austenite matrix. Hydrogen was found to reduce the ductility of all welds; however, the severity of ductility loss increased with increasing tendency to deform via a planar slip mode. In materials exhibiting large degrees of slip planarity, 304L and 308L, hydrogen changed the fracture mode from dimple rupture to a mixed mode of ductile and brittle fracture associated with the austenite-ferrite interface. The two alloys, 22-13-5 and 309S, which tend to deform by cross slip mechanisms, showed smaller losses in ductility even though hydrogen assisted the ductile rupture process by aiding void growth and coalescence, without changing the fracture mode. Varying the amount of ferrite from approximately one to 10 pct had no significant effect on performance in hydrogen.     

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.     

镍含量对2205双相不锈钢焊接接头的影响

采用光学显微镜观察了不同镍含量下2205双相不锈钢焊接接头的金相组织,通过点腐蚀试验测量腐蚀速率,并采用电化学工作站对焊接接头的点蚀电位进行测量.结果表明,1.2 m m厚度规格2205双相不锈钢焊接接头为铁素体加奥氏体的两相组织,点蚀坑主要出现在热影响区和焊缝.镍含量高的样品金相组织中奥氏体含量高,对应点蚀电位也要高...     

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.     

Microstructural origin of the skeletal ferrite morphology of austenitic stainless steel welds

Scanning transmission electron microscopy (STEM) was conducted on welds exhibiting a variety of skeletal, or vermicular ferrite morphologies in addition to one lathy ferrite morphology. These ferrite morphologies result from primary ferrite solidification followed by a solid state transformation upon cooling. During cooling, a large fraction of the ferrite transforms to austenite leaving a variety of ferrite morphologies. Comparison of composition profiles and alloy partitioning showed both the skeletal and lathy ferrite structures result from a diffusion controlled solid state transformation. However, the overall measured composition profiles of the weld structure are a result of partitioning during both solidification and the subsequent solid state transformation.     

Effects of nitrogen and helium shielding mixtures on semiaustenitic stainless steel welds

Abstract

The effect of nitrogen-helium atmos pheres used as a shielding gas for welding of some semiaustenitic stainless steels was studied. Nitrogen, being a strong austenite stabilizer, has the tendency to suppress the amount of retained delta ferrite in the weldment. The addition of 1.0 per cent nitrogen by volume to the helium shielding gas increased the tensile strength of the weldment approximately 4000 psi to within about 1 per cent of the parent metal, and the ductility approximately 33 per cent to within 95 per cent of the parent metal.

The additions of nitrogen to the shielding gas significantly reduced the a mo unt of retained ferrite. However, increasing the nitrogen in excess of 1 per cent by volume drastically reduced the tensile strength and ductility. It is suggested that this reduction in strength and ductility is the result of pertinent alloying components being used up in the formation of nitrides.

Résumé

Les auteurs ont étudié 1'influence de mélanges d'hélium et d'azote utilisés comme gaz de couverture en soudage d'aciers inoxydables semi austénitiques. Stabilisateur puissant de 1'austénite, l'azote tend à diminuer la teneur en fer delta des soudures. En utilisant un gaz renferment 1% en volume d'azote, la résistance à la traction des éprouvettes accuse une augmentation de 4000 psi, la relevant ainsi jusqu'à un écart aussi petit que 1% de la résistance du métal de base; concomitamment, l'allongement s'améliore de 33% et ainsi s'approche jusqu'à 95% de l'allongement du métal de base. L'influence de l'azote, comme moyen de réduire la teneur en fer delta est significative. Toutefois, dès que la quantité d'azote dépasse 1% en volume, la résistance à la traction et l'allongement fléchissent considérablement. Les auteurs avancent que le fléchissement est le résultat de la fixation d'éléments d'alliage par l'azote.     

Microstructure of TiB2/carbon steel surface-alloyed materials fabricated by high-energy electron beam irradiation

The processing and the microstructural analysis of TiB₂/carbon steel surface-alloyed materials using the irradiation of a high-energy electron beam were investigated in this study. The mixtures of TiB₂ powders and flux were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these mixtures using an electron beam accelerator. The microstructure of the irradiated surface layer was composed of a melted region, an interfacial region, a coarse-grained heat-affected zone (HAZ), and a fine-grained HAZ. A few residual micropores were found in the melted region of the specimen processed without flux because of irregular thermal transfer, but their number was decreased in the specimens processed with a considerable amount of flux. As a result of irradiation, the Ti content was homogeneously maintained throughout the melted region, whose hardness was greatly improved. This was associated with the microstructural modification including the segregation of Ti and B along solidification cell boundaries and the formation of fine Ti(C, N) particles. The proper flux mix ratio was 15 to 30 pct to obtain excellent surface alloying and a homogeneous microstructure.     

304L不锈钢焊缝在混凝土模拟孔隙液中的点蚀行为

采用动电位极化曲线、电化学阻抗谱、Mott-Schottky曲线等电化学方法研究了以308 L为焊丝的304 L不锈钢焊接接头在不同氯离子含量的混凝土模拟孔隙液中腐蚀行为和电化学规律.随Cl-增加,304 L不锈钢焊接接头的三个区域(母材、焊缝和热影响区)在混凝土模拟孔隙液中的自腐蚀电位、点蚀电位及电荷转移电阻降低,钝化膜中载流子密度和焊接接头的点蚀坑数量增加.在同浓度的腐蚀溶液中,308 L的焊缝区域耐蚀性最佳,热影响区次之,304 L基体表现出低的电荷转移电阻和高的掺杂浓度使得母材的耐蚀性最差.      

新型高碳马氏体不锈钢的组织与性能

高碳马氏体不锈钢因其高硬度、优异的耐磨性以及适中的耐蚀性,被广泛应用于刀剪行业。主要通过金相,扫描电镜,硬度、冲击韧性、耐磨性能、耐蚀性能以及抗菌性能检测等方法,对新型刀具用高碳马氏体不锈钢6Cr16MoVRE进行微观组织表征与性能研究,并与5Cr15MoV和9Cr18MoV钢对比。研究发现,6Cr16MoVRE的碳化物尺寸细小且分布均匀。相比于另2种材料,6Cr16MoVRE具有高硬度与最佳冲击韧性,良好的耐磨性,优异的耐蚀性。此外,Ag的添加使6Cr16MoVRE具有极好的抗菌性。新型6Cr16MoVRE性能优异,为国内生产高档刀具提供了材料保障,有利于中国刀剪行业转型升级。     

Analysis of solidification microstructures in Fe-Ni-Cr single-crystal welds

A geometric analysis technique for the evaluation of the microstructures in autogenous single-crystal electron beam welds has been previously developed. In the present work, these analytical methods are further extended, and a general procedure for predicting the solidification microstructure of single-crystal welds with any arbitrary orientation is established. Examples of this general analysis are given for several welding orientations. It is shown that a nonsymmetric cell structure is expected in transverse micrographs for most welding geometries. The development of steady-state conditions in the weld pool is also examined in terms of the weld pool size, its shape (as revealed by the dendritic growth pattern), and the size of the dendritic cells. It is found that steady state is established within a few millimeters of the beginning of the weld. Furthermore, steady state is achieved faster in welds made at higher welding speeds. A general analysis of the three-dimensional (3-D) weld pool shape based on the dendritic structure as revealed in the two-dimensional (2-D) transverse micrographs is also developed. It is shown that in combination with information on the preferred growth direction as a function of the solidification front orientation, the entire dendritic growth pattern in single-crystal welds can be predicted. A comparison with the actual weld micrographs shows a reasonable agreement between the theory and experiment. Finally, the theoretical analysis of the dendrite tip radius is extended from binary systems to include the case of ternary systems. The theoretical dendrite trunk spacing in a ternary Fe-Ni-Cr alloy is calculated from the dendrite tip radius and is compared with the experimental values for several weld conditions. Good agreement between experiment and theory is found. Formerly Visiting Scientist, Solid State Division, Oak Ridge National Laboratory.