Influence of Sigma Phase on Pitting Resistance Depending on Solidification Mode in AISI316L Weld Metal

The pitting corrosion resistances were investigated in the AISI 316L stainless steel weld metals with respect to the sigma phase precipitation in a 0.1 M NaCl solution. The modified flux-cored arc welding filler wires were fabricated at various chromium and nickel equivalent ratios using the flux-cored arc welding process. As the Cr/Ni equivalent ratio increased, the precipitated σ phase content increased in the temperature range of 923 K to 1123 K (650 °C to 850 °C), and the specimen that was aged at 1123 K (850 °C) precipitated the σ phase rapidly. The hardness increased with increasing sigma contents. During the potentiodynamic anodic polarization test, the specimens that were aged at 923 K to 1123 K (650 °C to 850 °C) exhibited lower pitting potentials than the as-weld metal. Additionally, the specimens that were aged for longer times exhibited lower pitting potentials. The pits occurred preferentially in the ductility dip cracking in specimen 1, whereas intergranular pits occurred in the sigma phase regions along the vermicular ferrite and acicular ferrite grain boundaries in specimens 2 and 3.     

Effect of Nitrogen Content on the Tensile and Stress Corrosion Cracking Behavior of AISI Type 316LN Stainless Steels

This paper deals with the effect of nitrogen on the tensile and stress corrosion cracking (SCC) behavior of type 316LN stainless steel. Yield stress (YS) and ultimate tensile stress (UTS) increased while the ductility [% total elongation (% TE)] decreased with increasing nitrogen content. Evaluation by conventional assessment parameters, such as ratios of UTS, % TE and SCC susceptibility index, derived by SCC testing using the slow strain rate testing (SSRT) technique indicated an improvement in SCC resistance on increasing the nitrogen content. However, crack growth rates, calculated from ratios of fracture stress from the SSRT tests in liquid paraffin and boiling 45 % magnesium chloride in SSRT tests, and the constant load tests at loads corresponding to 20 % YS in boiling 45 % magnesium chloride conclusively established that the SCC resistance of type 316LN stainless steel decreased with increasing nitrogen content.     

308L和347L焊缝金属的氢致滞后断裂行为

奥氏体不锈钢 30 8L和 347L的焊缝金属能发生氢致滞后断裂 ,而且比 30 4L母材更敏感。用单边缺口试样动态充氢法测出的氢致滞后断裂门槛应力强度因子 KIH随可扩散氢浓度 C0 的对数而线性下降。 3种材料氢致滞后断口的形貌与 KI 以及 C0 有关 ,当 KI 较高或 C0 较小时是韧窝断口 ;当 KI 较低或 C0 较高时是脆性断口     

Stress Corrosion Cracking Behavior at Inconel and Low Alloy Steel Weld Interfaces

Three-dimensional microstructure observations, macro- to micro-scopic residual stress measurements by three methods and creviced bent beam SCC tests were performed for Inconel/low alloy steel (LAS) weld samples. The possible reasons for the suppression of SCC crack propagation near the weld interface found at a nuclear power plant were estimated to include the crack branching at the grain boundary (GB) parallel to the interface, i.e., Type II GB, compressive residual stresses in the LAS region and crack tip oxidation in the LAS at the interface. The formation mechanism of Type II GB and stress gradient in individual grains in the Inconel are also discussed.     

Effect of Microstructure of 304L SS and Thermal Aging of 304L SS Weld on the Corrosion Behavior in Simulated High Level Waste

The effect of oxidizing ions present in simulated high level liquid waste (HLW) on the corrosion behaviour of solution annealed, sensitized 304L SS and 304L SS weld was investigated by potentiodynamic anodic polarization technique. Potentiodynamic anodic polarization studies showed a higher passivation current density and increased corrosion potential (E_corr) in simulated HLW when compared to 3 M HNO₃ in all the specimen mentioned above. In addition, the effect of microstructure of thermally aged 304L SS weld on the corrosion behavior was investigated in simulated HLW medium. Thermal ageing of 304L SS weld was carried out at 1023 K/100 h. Optical microscopy confirmed step microstructure for solution annealed and ditch microstructure for the sensitized specimens. The weld region was found to possess delta-ferrite distributed in austenite matrix. Thermal ageing resulted in fragmentation/dissolution of delta-ferrite and transformation of delta-ferrite to carbides and sigma phases. The thermally aged 304L SS weld specimen showed only a marginal decrease in corrosion resistance in simulated HLW when compared to the 304L SS weld.     

Effects of AISI 316L corrosion products in in vitro bone formation

Rat bone marrow cells were cultured in experimental conditions that favour the proliferation and differentiation of osteoblastic cells (i.e., 2.52 x 10(-4) mol l(-1) ascorbic acid, 10(-2) mol l(-1) beta-glycerophosphate and 10(-8) mol l(-1) dexamethasone) in the absence and in the presence of stainless-steel corrosion products, for a period of 18 days. An AISI 316L stainless-steel slurry (SS) was obtained by electrochemical means and the concentrations of the major metal ions, determined by atomic absorption spectrometry, were 8.78 x 10(-3) mol l(-1) of Fe, 4.31 x 10(-3) mol l(-1) of Cr and 2.56 x 10(-3) mol l(-1) of Ni. Bone marrow cells were exposed to 0.01, 0.1 and 1% of the SS and at the end of the incubation period, control and treated cultures were evaluated by histochemical assays for the identification of the presence of alkaline phosphatase and also calcium and phosphate deposition. Cultures were further observed by scanning electron microscopy. Levels of total and ionised calcium and phosphorus in the culture media collected from control and metal exposed cell cultures were also quantified. Histochemical staining showed that control cultures presented a strong reaction for the presence of alkaline phosphatase and exhibited formation of calcium and phosphates deposits. The presence of 0.01% SS caused no detectable biological effects in these cultures, 0.1% SS impaired osteoblastic behaviour and, 1% SS resulted in cell death. In the absence of bone cells, levels of total and ionised calcium and phosphorus in the control and metal added culture medium were similar throughout the incubation period. A significant decrease in the levels of ionised calcium and phosphorus were observed in the culture medium of control cultures and also in cultures exposed to 0.01% SS after two weeks of incubation, an event related with the formation of mineral calcium phosphate deposits in these cultures. In cultures grown in the presence of 0.1 and 1% SS corrosion products, levels of calcium and phosphorus were similar to those observed in the absence of cells. Results showed that stainless-steel corrosion products above certain concentrations may disturb the normal behaviour of osteoblast-like rat bone marrow cell cultures.     

Process Parameter Optimization of AISI 316L(N) Weld Joints Produced using Flux-Cored Arc Welding Process

Bead on plate welds were carried out on AISI 316L(N) austenitic stainless steel using flux cored arc welding process. The bead on plates weld was conducted as per L₂₅ orthogonal array with statistical design of experiment technique. In this paper, the welding parameters will be optimized based on the weld bead geometry such as depth of penetration, bead width and weld reinforcement. Grey relational analysis and desirability approach are used to optimize the input parameters like wire feed rate, voltage, travel speed and torch angle while considering the multiple output variables simultaneously. Confirmation experiment has also been conducted to validate the optimized parameters.     

Effect of Chemical Composition on Susceptibility to Weld Solidification Cracking in Austenitic Weld Metal

The influence of the chemical composition, especially the niobium content, chromium equivalent Cr_eq, and nickel equivalent Ni_eq, on the weld solidification cracking susceptibility in the austenite single-phase region in the Schaeffler diagram was investigated. Specimens were fabricated using the hot-wire laser welding process with widely different compositions of Cr_eq, Ni_eq, and niobium in the region. The distributions of the susceptibility, such as the crack length and brittle temperature range (BTR), in the Schaeffler diagram revealed a region with high susceptibility to solidification cracking. Addition of niobium enhanced the susceptibility and changed the distribution of the susceptibility in the diagram. The BTR distribution was in good agreement with the distribution of the temperature range of solidification (ΔT) calculated by solidification simulation based on Scheil model. ΔT increased with increasing content of alloying elements such as niobium. The distribution of ΔT was dependent on the type of alloying element owing to the change of the partitioning behavior. Thus, the solidification cracking susceptibility in the austenite single-phase region depends on whether the alloy contains elements. The distribution of the susceptibility in the region is controlled by the change in ΔT and the segregation behavior of niobium with the chemical composition.     

时效对新型Al-Zn-Mg-Cu合金力学及应力腐蚀性能的影响

通过力学性能和电导率测试、慢应变速率试验(SSRT)以及显微组织TEM分析,研究了不同时效制度对新型Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr合金力学及应力腐蚀性能的影响。结果表明,合金的力学性能和应力腐蚀性能与时效制度密切相关。T6状态下,晶内析出相弥散细小,晶界析出相呈连续分布,合金的强度最高,抗应力腐蚀性能最差;经T7双级过时效处理后,晶界析出相粗化呈离散分布,出现明显宽化的晶间无析出带,合金的抗应力腐蚀性能得到明显提高,但其强度损失较多。经三级时效处理后,合金的组织综合了T6态和T7态的优点,使合金既有高的强度又有良好的抗应力腐蚀性能,合金的极限抗拉强度、屈服强度、伸长率和电导率分别达到580,570 MPa,16.7%和23.3 MS.m-1。     

Mechanical Properties and Metallurgical Characterization of Dissimilar Welded Joints between AISI 316 and AISI 4340

In the present study, the influence of six different process parameters and three interactions on joint tensile strength, toughness, fusion zone microhardness variation are studied during dissimilar tungsten inert gas welding between austenitic stainless steel AISI 316 and alloy steel AISI 4340. Detailed experimental study using fractional factorial experimental design and subsequent statistical analysis show that higher tensile strength, toughness can be achieved using ER 309 filler material and suitably selecting the other process parameters and heating conditions. Addition of small proportion of hydrogen in shielding gas increases the heat transfer efficiency, melting and subsequent penetration. Preheating of AISI 4340 material reduces the chance of solidification cracking and post-heating helps to improve the joint mechanical property. Microstructural observations show that improper selection of process parameters may lead to micro-pores and degrade the joint quality. Successful joining of the dissimilar materials greatly depends on the selection of optimum process parameters, filler material and shielding gas.     

Threshold Stress Intensity of Hydrogen-Induced Cracking and Stress Corrosion Cracking of High Strength Steel

Stress corrosion cracking (SCC) of highstrength steels in an aqueous solution has been wide-ly investigated[1— 8] .It has long been accepted thatSCC of high strength steel in water is one kind ofhydrogen- induced cracking (HIC) [1— 5] .Because ofhydrolysis of the metal ions,the value p H of the lo-cal environment within a pit,crevice or crack on asteel surface can be decreased to about 3.5 [1] .Be-cause of crack- tip acidification,local conditions arealways favorable for release of hydro…     

Mass Transfer and Weld Appearance of 316L Stainless Steel Covered Electrode During Shielded Metal Arc Welding

The mass transfer and the weld appearance of 316L stainless steel covered electrodes during shielded metal arc welding were investigated. According to the experimental measurements on the deposited metal and the observations on the welding process, the mass transfer coefficient of the nickel was found to be in the range of 88.09 to 99.41 pct, while those of molybdenum, chromium, manganese, and silicon are in the ranges of 84.60 to 92.51 pct, 71.59 to 77.64 pct, 20.88 to 30.15 pct, and 6.72 to 10.47 pct, respectively. Some relationships between the mass transfer and the flux coating ingredient/welding current were established. The formability properties of the weld, including the spreadability, spattering, slag detachability, and oxidation tint on the weld surface, were also discussed based on the tested data and the observations.     

Characterization,Tribological and Mechanical Properties of Plasma Paste Borided AISI 316 Steel

The AISI 316 steel was treated by the plasma paste boriding by using a gas mixture of 70%H₂–30%Ar with a boron source of 100% B₂O₃ in the temperature range of 700–800 °C for 3, 5 and 7 h. The boride layers formed on the samples were observed by scanning electron microscope. The iron borides were also identified by the use of an X-ray microanalyzer, equipped with energy dispersive X-ray spectroscopy. The XRD analysis was carried out to identify the iron and metallic borides present inside the boride layer. Based on the kinetic data, the value of boron activation energy for the AISI 316 steel was estimated as 118.12 kJ mol^?1 and compared with the data available in the literature. A regression model based on ANOVA analysis was used to predict the boride layers’ thicknesses depending on the boriding parameters: the treatment time and the boriding temperature. A good correspondence was obtained between the experimental values and those predicted by the regression model. Furthermore, the wear behavior of the sample borided at 750 °C for 5 h was investigated. The significant increase in wear resistance of plasma borided layer was observed in comparison with the untreated AISI 316 steel. The nanomechanical properties of the sample, plasma paste borided at 700 °C for 7 h, were examined using the nanoindenter with a Vickers diamond tip. The load–displacement curves, as well as, Young’s moduli and hardness were shown for the selected measurements. The obtained results depended on the phase composition of the tested area.     

Influence of the Carbo-Chromization Process on the Microstructural,Hardness, and Corrosion Properties of 316L Sintered Stainless Steel

We report on the changes on the microstructural, hardness, and corrosion properties induced by carbo-chromization of 316L stainless steel prepared by Spark Plasma Sintering technique. The thermo-chemical treatments have been performed using pack cementation. The carburizing and chromization were carried out between 1153 K (880 °C)/4 h to 1253 K (980 °C)/12 h and 1223 K (950 °C)/6 h to 1273 K (1000 °C)/12 h in a solid powder mixture of charcoal/BaCO₃ and ferrochromium/alumina/NH₄Cl, respectively. The obtained layers were investigated using X-ray and electron diffraction, optical and scanning electron microscopies, Vickers micro-hardness, and potentiodynamic measurements. The thickness of the carbo-chromized layer ranges between 300 and 500 μm. Besides the host γ-phase, the layers are mainly constituted of carbides (Fe₇C₃, Cr₂₃C₆, Cr₇C₃, and Fe₃C) and traces of α′-martensite. The average hardness values decrease smoothly from 650 HV at the sample surface down to 200 HV at the center of the sample. The potentiodynamic tests revealed that the carbo-chromized samples have smaller corrosion resistance with respect to the untreated material. For strong chromization regimes, the corrosion rate is increased by a factor of four with respect to that of the untreated material, while the micro-hardness of the layer is three times larger. Such materials are suited to be used in environments where good corrosion resistance and wear properties are required.     

Grain-size effects on tensile behavior of nickel and AISI 316L stainless steel

The aim of this work is to provide experimental results to understand the grain-size effects on tensile hardening of fcc polycrystalline materials. The contribution of grain size on hardening rate is discussed in terms of backstress (X) and effective-stress (Σ _ef) evolutions in the different hardening stages. Based on this stress partition, the origin of the classical Hall-Petch relationship is clarified at the different levels of microstructural heterogeneities. If the backstresses and effective stresses verified the Hall-Petch formulation, however, the effective stress is less dependent on grain size than the backstress. The grain-size effect on short-range internal stresses (effective stress) is well explained in terms of a mean path length using classical dislocation modeling. The backstress dependence on grain size seems to be mainly the result of intergranular plastic-strain incompatibilities in relation with the formation of a grain-boundary layer in stage I. In others stages (higher plastic strain), the interactions between intergranular and intragranular long-range internal stresses have been illustrated. The degree of these interactions remains unclear.     

固溶温度对Ti-62A合金拉伸性能和断裂韧性的影响

Ti-62A合金是一种新型高强高韧损伤容限型钛合金,研究了固溶温度对Ti-62A合金30 mm厚板材的显微组织、拉伸性能以及断裂韧性的影响规律。研究结果表明:Ti-62A合金Φ720 mm铸锭经单相区和两相区多道次大变形轧制后所得的30 mm厚板材组织为典型的片层组织,由片层状的α相和β转变组织构成,组织均匀,片层状α相平均宽度约为2.5μm,长度在40～65μm之间。两相区固溶+时效处理后,合金的组织类型为片层状组织,即片层状的初生α相(αp)相与β转变组织,随固溶温度升高,合金中的初生α相(αp)相含量显著减少,β转变组织逐渐增多,次生α相(αs)片层宽度增大,同时合金的强度下降,塑性上升,当接近相变点时这种趋势变缓。单相区固溶+时效处理获得魏氏组织,晶粒粗大,晶界平直而清晰,其拉伸强度高于920℃和940℃固溶时的片层组织,但塑性显著降低;与900℃固溶时相比强度和塑性均降低。合金的断裂韧性随固溶温度的上升而逐渐升高,单相区固溶并时效后的魏氏组织的断裂韧性明显优于两相区固溶并时效后的片层组织。     

Effect of Crack-Tip Stresses on Stress Corrosion Cracking Behavior

It is well documented that the applied load for a given material/environment system has a significant effect on the stress corrosion cracking (SCC) behavior in terms of K _ISCC values as well as on the K-da/dt relation. Traditionally, the role of crack-tip stress has not received proper attention but rather the crack-tip strain that is supposedly critical in cracking the passive oxide exposing the fresh metal surface at the crack tip to the environment. This article discusses a different point of view pertinent to crack-tip stress calculations and their role on SCC behavior. We have examined different continuum mechanics solutions with respect to the role of blunting on crack-tip stress. Both solutions where free traction boundary conditions are satisfied on a sharp and blunt crack are analyzed and discussed. It was shown that the stress component perpendicular to the crack plane at the crack tip for the plateau region remains essentially the same due to blunting even with the increase in applied load. Constant crack-tip stress would also result in rather constant crack-tip strain or creep rate, and as such, the crack growth rate remains the same or constant with respect to applied stress. Presented stress analysis is based on available solutions taken from the literature and assumes that a chemical potential at the crack tip is fixed.     

Role of Slip Mode on Stress Corrosion Cracking Behavior

We have examined the incubation times in two alloys, 7075-T651 aluminum alloy and 4140 steel, as a function of applied K, using the published data in aqueous environment. The role of overloads was compared with the results from those without overloads, for a given environment. Effect of environment (NaCl vs deionized water) was also examined. The results show that in a constant K test, the incubation time increases with decreasing K. When a single overload cycle was applied, the time increased with percent overload for a constant background K, indicating that overload cycle affected the crack tip driving forces. These effects varied with the environment. The changes in the incubation times are analyzed considering one-to-one correspondence between the crack tip driving force and the times. Overloads contributed to compressive residual or internal stresses, thereby affecting the crack tip driving force. The stresses are related to changes in the plastic zone (PZ) sizes formed before and after the overloads. The effective stress intensity due to internal stress, K _int, is defined and is shown to be a function of PZ size. Similarly, condition for crack initiation is expressed as K _total = K _app ± K _int ≥ K _Iscc. A detailed methodology for the determination of K _int is outlined.     

Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion Cracking Behavior

Transmission electron microscopy (TEM) was employed to investigate the microchemistry and microstructure of grain boundary precipitates in Al 7075 aged at room temperature for several hours, at 393 K (120 °C) for 12 hours (under aged), at peak aged (T651) and over aged (T73) conditions. High resolution TEM analysis of precipitates at grain boundaries and fine probe energy dispersive spectrometry showed that the grain boundary precipitates at peak and over aged conditions are hexagonal η phase with stoichiometry Mg(Cu _x Zn_1?x )₂. Considerable increase in Cu content in the grain boundary η in the over aged condition compared to the peak aged condition was observed. The average Cu content in the over aged condition was found to be 20 at. pct. The higher Cu content of the precipitate is associated with a lower stress corrosion cracking plateau velocity.