Influence of Inclusion and Specimen Orientations on Intergranular Corrosion Testing of AISI 316LN Stainless Steel

Fabricated components must be free from sensitization for using these in critical applications in aggressive environments. During fabrication of a hollow bar from solid bar, deep hole drilling was employed which introduces residual stresses. Stress-relieving heat treatment was employed by heating the hollow bar from room temperature to 1,065°C @ 40°C/h and soaking at 1,065°C for 1 h followed by cooling @ 40°C/h as well as 70°C/h. To detect the susceptibility to IGC, specimens were taken from both circumferential direction as well as longitudinal direction and subjected to ASTM A 262 Practice E test. In U bend, the specimens from the circumferential direction failed whereas longitudinal specimens did not fail. However specimens of both orientations showed Step structure in Practice A test indicating that no carbide has nucleated during the stress-relieving heat treatment ensuring that the cooling rates are faster than the critical cooling rates and the material is not susceptible to IGC. EDAX studies indicated the presence of numerous MnS inclusions enriched in chromium which might have led to chromium depletion around the inclusions resulting in poor passivity at these locations. This study presents the influence of orientation of MnS inclusions in causing failure in U bend test. The need to select specimens of correct orientation during IGC testing is emphasized in this investigation.     

Thermal Simulation Analysis of Microstructure and Hardness for CrMoV with PWHT in Type IV Region

Welded components of CrMoV steam pipe exhibit a pernicious form of type IV cracking after long-term service at elevated temperature. To investigate the cracking mechanism, the type IV microstructure and hardness were characterized after thermal simulation of post-weld heat treatment. Below 1098 K (825 °C), loss of carbon from the pearlite region was apparent, and the work zone exhibited a slightly lower hardness than the parent material because of a minor amount of austenite transformation. In addition, for peak temperatures above 1133 K (860 °C), additional transformation into austenite occurred and was followed by retransformation into ferrite upon further increasing the temperature. The pearlite formed at 1173 K to 1223 K (900 °C to 950 °C) resulted in an increase of the volume fraction of pearlite and microstructural refinement, which yielded a remarkable increase of hardness in the work zone. For the peak temperature of 1573 K (1300 °C), previous austenite grains were coarsened and alloy carbides were dissolved in the austenite, which significantly hardened the work zone.     

High Strain Rate Compression of Martensitic NiTi Shape Memory Alloy at Different Temperatures

The compressive response of martensitic NiTi shape memory alloy (SMA) rods has been investigated using a modified Kolsky compression bar at various strain rates (400, 800, and 1200 s^?1) and temperatures [room temperature and 373 K (100 °C)], i.e., in the martensitic state and in the austenitic state. SEM, DSC, and XRD were performed on NiTi SMA rod samples after high strain rate compression in order to reveal the influence of strain rate and temperature on the microstructural evolution, phase transformation, and crystal structure. It is found that at room temperature, the critical stress increases slightly as strain rate increases, whereas the strain-hardening rate decreases. However, the critical stress under high strain rate compression at 373 K (100 °C) increase first and then decrease due to competing strain hardening and thermal softening effects. After high rate compression, the microstructure of both martensitic and austenitic NiTi SMAs changes as a function of increasing strain rate, while the phase transformation after deformation is independent of the strain rate at room temperature and 373 K (100 °C). The preferred crystal plane of the martensitic NiTi SMA changes from (\( 1\bar{1}1 \))_M before compression to (111)_M after compression, while the preferred plane remains the same for austenitic NiTi SMA before and after compression. Additionally, dynamic recovery and recrystallization are also observed to occur after deformation of the austenitic NiTi SMA at 373 K (100 °C). The findings presented here extend the basic understanding of the deformation behavior of NiTi SMAs and its relation to microstructure, phase transformation, and crystal structure, especially at high strain rates.     

Cryogenic S–N Fatigue and Fatigue Crack Propagation Behaviors of High Manganese Austenitic Steels

In the current study, the S–N fatigue and the fatigue crack propagation (FCP) behaviors of high manganese austenitic steels, including Fe24Mn and Fe22Mn, were studied, and the results were compared with STS304 (Fe-1Si-2Mn-20Cr-10Ni). The S–N fatigue tests were conducted at 298 K and 110 K (25 °C and ?163 °C), respectively, and at an R ratio of 0.1 under a uniaxial loading condition. The FCP tests were conducted at 298 K and 110 K (25 °C and ?163°C), respectively, and at R ratios of 0.1 and 0.5, respectively, using compact tension specimens. The resistance to S–N fatigue of each specimen increased greatly with decreasing temperature from 298 K to 110 K (25 °C to ?163 °C) and showed a strong dependency on the flow stress. The FCP behaviors of the austenitic steels currently studied substantially varied depending on testing temperature, applied ΔK (stress intensity factor range), and R ratio. The enhanced FCP resistance was observed for the Fe24Mn and the Fe22Mn specimens particularly in the near-threshold ΔK regime, while the enhancement was significant over the entire ΔK regimes for the STS304 specimen, with decreasing temperature from 298 K to 110 K (25 °C to ?163 °C). The S–N fatigue and the FCP behaviors of high manganese austenitic steels are compared with STS304 and discussed based on the fractographic and the micrographic observations.     

Analysis of the Deformation Behavior in Tension and Tension-Creep of Ti-3Al-2.5V (wt pct) at 296 K and 728 K (23 °C and 455 °C) Using In Situ SEM Experiments

The deformation behavior of a Ti-3Al-2.5V (wt pct) near-α alloy was investigated during in situ deformation inside a scanning electron microscopy (SEM). Two plates with distinct textures were examined. Tensile experiments were performed at 296 K and 728 K (455 °C) (~0.4T _m), while a tensile-creep experiment was performed at 728 K (455 °C) and 180 MPa (σ/σ _ys = 0.72). The active deformation systems were identified in the α phase using electron backscattered diffraction based slip-trace analysis and SEM images of the surface. Prismatic slip deformation was the dominant slip mode observed for all the experiments in both plates, which was supported by a critical resolved shear stress (CRSS) ratio analysis. However, due to the texture of plate 1, which strongly favored the activation of prismatic slip, the percentages of prismatic slip activity for specimens from plate 1 tested at 296 K and 728 K (23 °C and 455 °C) were higher than the specimens from plate 2 under the same testing conditions. T1 twinning was an active deformation mode at both 296 K and 728 K (23 °C and 455 °C), but the extent of twinning activity decreased with increased temperature. T1 twinning was more frequently observed in specimens from plate 2, which exhibited a higher fraction of twinning systems favoring activation at both 296 K and 728 K (23 °C and 455 °C). The tension-creep experiment revealed less slip and more grain boundary sliding than in the higher strain rate tensile experiments. Using a previously demonstrated bootstrapping statistical analysis methodology, the relative CRSS ratios of prismatic, pyramidal 〈a〉, pyramidal 〈c+a〉, and T1 twinning deformation systems compared with basal slip were calculated and discussed in light of similar measurements made on CP Ti and Ti-5Al-2.5Sn (wt pct).     

Fracture Behavior of High-Nitrogen Austenitic Stainless Steel Under Continuous Cooling: Physical Simulation of Free-Surface Cracking of Heavy Forgings

18Mn18Cr0.6N steel was tension tested at 0.001 s^?1 to fracture from 1473 K to 1363 K (1200 °C to 1090 °C, fracture temperature) at a cooling rate of 0.4 Ks^?1. For comparison, specimens were tension tested at temperatures of 1473 K and 1363 K (1200 °C and 1090 °C). The microstructure near the fracture surface was examined using electron backscatter diffraction analysis. The lowest hot ductility was observed under continuous cooling and was attributed to the suppression of dynamic recrystallization nucleation.     

Correlation Between Pre-annealing Temperature and {110}〈001〉 Annealing Texture in C- and Al-Free Fe-3 Pct Si-0.1 Pct Mn-0.002 Pct S Electrical Steel

In C- and Al-free electrical steel, the increase in primary grain size with increasing pre-annealing temperature causes the transition in annealing texture after final annealing from {110} + {100} to {110}. The strip pre-annealed at 1073 K (800 °C) shows a low magnetic induction B₈(T) of 1.784 T after final annealing. The strip pre-annealed at 1223 K (950 °C) shows a sharp {110}〈001〉 Goss texture, producing a high magnetic induction B₈(T) of 1.914 T comparable to that of the conventional electrical steels.     

The effect of grain size and retained austenite on the ductile-brittle transition of a titanium-gettered iron alloy

The effect of microstructural changes on the ductile-brittle transition temperature (DBTT) was studied in a titanium-getter ed Fe-8Ni-2 Mn-0.15 Ti alloy. A fairly strong grain size dependence of the transition temperature, 8°C/mm^?1/2, was found. Grain size refinement from 38 μm (ASTM #6.5) to 1.5 μm (ASTM #15.5) through a four-step thermal treatment lowered the transition temperature by 162°C. A small amount of retained austenite was introduced into this grain-refined microstructure, and the transition temperature was reduced by an additional 120 ~ 150°C. The reduction of the DBTT due to retained austenite was smaller when the austenite was in a large-grained structure (64°C). The distribution and stability of retained austenite were also studied.     

Control of the Mechanical Asymmetry in an Extruded MN11 Alloy by Static Annealing

An extruded Mg-1 wt pct Mn-1 wt pct Nd (MN11) alloy with a recrystallized microstructure and a weak texture was subjected to different thermal treatments at temperatures ranging from 548 K to 673 K (275 °C to 400 °C) for time intervals between 1 and 45 hours. Room-temperature mechanical tests were carried out in tension and compression at 10^?3 s^?1 in order to investigate the effect of annealing on the mechanical behavior. Microstructural examinations revealed that both the annealing temperature and time have little effect on the grain size and on the texture, which are mainly controlled by the presence of thermally stable Mn-containing particles and by the segregation of Nd to the grain boundaries. However, the composition and distribution of the Nd-containing particles vary significantly for the different annealing conditions. The annealed bars exhibit a subtle change in the tensile and compressive yield stress relative to the as-extruded bar and a somewhat larger mechanical asymmetry. The present results suggest that the Nd-containing phases, as well as Nd solute atoms, play a significant role in the mechanical behavior of the MN11 alloy by changing the relative critical resolved shear stress of the different deformation modes.     

Mechanical Characterization of Austempered Ductile Iron Obtained by Two Step Austempering Process

Austempered ductile iron (ADI) is known to have a good combination of mechanical properties due its unique ausferrite microstructure. The strength of ADI is mainly a function of the austempering temperature and the stability of ausferrite matrix. To increase the stability of the ausferritic matrix, two stage austempering processes was developed. During this investigation, in the Ist step, ductile iron specimens were austenitized at 900 °C for 60 min followed by quenching to 250 °C in salt bath. In the IInd step, after quenching at 250 °C, the salt bath was gradually heated to 350 °C, 400 °C and 450 °C respectively where specimen were soaked for 120 min. The tensile strength and impact strength were evaluated according to ASTM standards. The results were compared with that obtained by conventional austempering process by quenching directly into salt bath at 400 °C for 120 min. Both tensile and impact strength were found to have improved by two step austempering process. During Ist stage of austempering, martensite was observed while during IInd stage of austempering microstructures revealed acicular ferrite and carbon stabilized austenite. The fractographic examination revealed mixed type of fracture mode and intergranular fracture was seen under SEM. It was further observed that the tensile strength decreased whereas the impact strength increased with IInd stage of austempering temperature.     

Cavitation Erosion Behavior of Nitrogen Ion Implanted 13Cr4Ni Steel

Nitrogen ions (N⁺) with five different energies (100–600 keV) were implanted on the 13Cr4Ni steel (base) samples under high vacuum at temperature <100 °C. The base and implanted samples were also annealed at 600 °C for 6 h at high vacuum (~10^?9 bar). Energy dependent change in structure and mechanical properties of implanted samples were observed after annealing process. Structural study suggested formation of nitrides and implantation induced surface segregation of nitrogen. The nano-indentation hardness and elastic modulus were increased from 5 to 13 and 183 to 314 GPa respectively with increasing N⁺ energy. The N⁺ implantation process had significantly enhanced the cavitation erosion resistance of the base steel. The minimum cumulative weight loss and maximum erosion resistance were obtained for the sample implanted at 600 keV energy. The roughness values of surfaces at various erosion periods were correlated with erosion process to understand erosion mechanism. The lowest roughness values (R_a = 164.42 nm, R_q = 214.75 nm) after 12 h of cavitation erosion test were obtained for the sample implanted at 600 keV energy.     

Precipitation Behavior of VN in High Nitrogen and Vanadium Micro-alloyed Low Carbon Weathering Steel

In this study, the high nitrogen and vanadium micro-alloyed low carbon weathering steel (0.0320%N–0.096V) with excellent mechanical properties was produced. TEM and EDS tests of the hot rolled specimens were conducted and it was found that a great number of VN precipitates were generated and the size of VN particles was in the range of 10–300 nm, which exceeded the critical size of 7.89 nm for pinning dislocation line. Thermodynamic calculation results showed that, at high temperature, the precipitation performance of VN remarkably increased due to high nitrogen content. For the experimental steel, the initial precipitation temperature of VN reached 1133 °C and the precipitation rate of V exceeded 90% at the finish rolling temperature of 850 °C. The precipitation temperature time (PTT) curves drawn from kinetic data showed that the fastest precipitation temperature was about 950 °C. The total precipitation time was shortened from 110 to 10 s at 850 °C due to the high dislocation density caused by deformation, and the VN particles could thus be completely precipitated with a brief relaxation process. High nitrogen and vanadium micro-alloying was beneficial for improving the grain refinement and precipitation strengthening effect of vanadium.     

Influence of Temperature and Holding Time on the Interaction of V,Al, and N in Microalloyed Forging Steels

A medium-carbon vanadium microalloyed steel (38MnSiVS5) with three different aluminum levels (0.006, 0.020, and 0.03 wt pct) was used to examine the interaction of vanadium, aluminum, and nitrogen during the heating and cooling cycle for forging. The thermal cycle was simulated using a Gleeble^® 1500. Hold times varied from 5 to 45 minutes and temperature varied from 1323 K to 1523 K (1050 °C to 1250 °C). Thermal simulation specimens and as-received material were characterized by quantitative metallography, hardness, and chemical analysis of electrolytically extracted precipitates. The hardness was observed to be relatively constant for all aluminum levels after all thermal simulations at and above 1423 K (1150 °C). Hardness, pearlite fraction, and austenite grain size decreased with increasing aluminum content at the two lowest temperatures examined, which were 1323 K and 1373 K (1050 °C and 1100 °C). The amount of vanadium precipitated in the lowest aluminum steel was very consistent, approximately 70 pct, for the thermal simulations. The amount of precipitated vanadium decreased with increasing amount of aluminum nitride for the 0.03 wt pct Al level.     

Fabrication of Ti-6Al-4V Scaffolds by Direct Metal Deposition

Direct metal deposition (DMD) is a rapid laser-aided deposition method that can be used to manufacture near-net-shape components from their computer aided design (CAD) files. The method can be used to produce fully dense or porous metallic parts. The Ti-6Al-4V alloy is widely used as an implantable material mainly in the application of orthopedic prostheses because of its high strength, low elastic modulus, excellent corrosion resistance, and good biocompatibility. In the present study, Ti-6Al-4V scaffold has been fabricated by DMD technology for patient specific bone tissue engineering. Good geometry control and surface finish have been achieved. The structure and properties of the scaffolds were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and tension test. The microstructures of laser-deposited Ti-6Al-4V scaffolds are fine Widmanstätten in nature. The tensile and yield strengths of the as-deposited Ti-6Al-4V were 1163 ± 22 and 1105 ± 19 MPa, respectively, which are quite higher than the ASTM limits (896 and 827 MPa) for Ti-6Al-4V implants. However, the ductility of the as-deposited sample was very low (～4 pct), which is well below the ASTM limit (10 pct). After an additional heat treatment (sample annealed at 950 °C followed by furnace cooling), both strength (UTS ～ 1045 ± 16, and YS ～ 959 ± 12 MPa) and ductility (～10.5 ± 1 pct) become higher than ASTM limits for medical implants.     

Evolution of Strain and Microstructure During Creep of Wrought AE42 and ZE10 Magnesium Alloys

Wrought magnesium alloys have been extensively used in the aerospace, electronics and automotive industries, where component weight is of concern and ambient temperatures remain below 100 °C. Undesirable creep relaxation of the wrought alloys above this temperature has been generally attributed to grain boundary sliding and plastic deformation leading to intergranular failure. The objective of this study was to investigate the compressive creep performance and microstructure of two wrought magnesium alloys (AE42 and ZE10) developed for high temperature applications. The total deformation of the AE42 and ZE10 alloys was 2.4 and 0.2 %, respectively, after 24 h creep test at 175 °C and 50 MPa. The poor creep performance of the AE42 alloy was explained via neutron diffraction studies which revealed that the elastic compressive response of $ (10\bar{1}0),\;(10\bar{1}1)\;{\text{and}}\;(2\bar{1}\bar{1}0) $ planes was significantly more anisotropic in the AE42 than in the ZE10 alloy. Further, microstructural analysis revealed ~10 % increase in grain size due to creep, with additional $ (10\bar{1}2) $ and $ (11\bar{2}1) $ twinning in the AE42 alloy. Precipitation of β-Mg₁₇Al₁₂ phase in the AE42 alloy possibly contributed to grain boundary sliding and high plastic strain during creep testing.     

Compositional effects on the high temperature ductility of 1 Cr-1.25 Mo-0.25 V Steel

This paper describes the results of slow strain rate (ε = 4.4 × 10^-5 s^-1) tensile tests performed at temperatures between 25 and 700 °C on a high purity CrMoV steel containing various dopants. The materials all had a bainitic microstructure, a hardness of R^C28, and a grain size of ASTM 0. Some samples were step cooled prior to tensile testing. Four different compositions were tested: undoped (HP), Mn + P doped (MnP), P doped (P), and Sn doped (Sn) materials. All four materials failed in a low ductility cleavage mode at low temperatures and by a low ductility grain boundary cavitation mode at high temperatures. At intermediate temperatures, around 500 °C, the MnP material showed the highest ductility, the HP and Sn materials showed the lowest, and the P material was intermediate. The beneficial effects of both Mn and P on the creep ductility are rationalized in terms of their control of the sulfur concentration on prior austenite boundaries. In addition, it is suggested that P on the grain boundaries can reduce the cavitation rate by reducing the grain boundary self diffusion rate.     

Study on Properties of Hyper-duplex Stainless Steel 7A of ASTM A-890 (CD3MWN)

Hyper-duplex stainless steel castings of grade 7A of ASTM A-890 are comparatively new in duplex series, and their regular production has not yet been started like other super-duplex grades 5A and 6A. In view of this, a study has been undertaken to find out the properties of this grade under different experimental conditions. Tensile properties and hardness are found to be higher than other super-duplex grades; however, impact values at − 50 °C are lower than grade 6A. Pitting corrosion, critical pitting temperature and intergranular corrosion tests were carried out as per ASTM standard. Potentiodynamic polarization scan was carried out and compared with 6A. Deleterious phases were formed by holding samples for a shorter time at 1000 °C with a corresponding increase in hardness and drop in ferrite content. 475 °C embrittlement effect was studied and compared with other duplex grades. Re-dissolution of harmful precipitates was carried out at 550 °C by varying time to study any change in properties. Some of the properties of grade 7A were compared with wrought product of hyper-duplex grade, UNS S32707. Low-temperature impact values of hot-worked cast 7A test block were improved considerably in both longitudinal and transverse direction. Dry sliding wear resistance test was carried out as per ASTM standard, and values were found to be better than grade 6A.     

Crystallographic Reconstruction Study of the Effects of Finish Rolling Temperature on the Variant Selection During Bainite Transformation in C-Mn High-Strength Steels

The effect of finish rolling temperature on the austenite-(γ) to-bainite (α) phase transformation is quantitatively investigated in high-strength C-Mn steels using an alternative crystallographic γ reconstruction procedure, which can be directly applied to experimental electron backscatter diffraction mappings. In particular, the current study aims to clarify the respective contributions of the γ conditioning during the hot rolling and the variant selection during the phase transformation to the inherited texture. The results confirm that the sample finish rolled at the lowest temperature [1102 K (829 °C)] exhibits the sharpest transformation texture. It is shown that this sharp texture is exclusively due to a strong variant selection from parent brass {110} \( \left\langle {1\bar{1}2} \right\rangle \) , S {213} \( \left\langle {\bar{3}\bar{6}4} \right\rangle \) and Goss {110}〈001〉 grains, whereas the variant selection from the copper {112} \( \left\langle {\bar{1}\bar{1}1} \right\rangle \) grains is insensitive to the finish rolling temperature. In addition, a statistical variant selection analysis proves that the habit planes of the selected variants do not systematically correspond to the predicted active γ slip planes using the Taylor model. In contrast, a correlation between the Bain group to which the selected variants belong and the finish rolling temperature is clearly revealed, regardless of the parent orientation. These results are discussed in terms of polygranular accommodation mechanisms, especially in view of the observed development in the hot-rolled samples of high-angle grain boundaries with misorientation axes between 〈111〉γ and 〈110〉γ.     

Effect of Nd Additions on Extrusion Texture Development and on Slip Activity in a Mg-Mn Alloy

Two Mg-1 wt pct Mn alloys containing 0.5 wt pct and 1 wt pct Nd have been processed by indirect extrusion at temperatures ranging from 548 K (275 °C) to 633 K (360 °C) and speeds between 2.8 and 11 mm/s. The microstructure and the texture of the extruded bars were analyzed in order to understand the effect of the processing parameters and of the rare-earth (RE) alloying additions on the texture development. Increasing the Nd content results in weak textures in which the predominant orientations are a function of the extrusion conditions. This may be explained by the occurrence of particle pinning of grain boundaries and by the nucleation of grains with a wider range of orientations. Mechanical tests were carried out in tension and in compression in all the processed samples at 10^?3 s^?1 and room temperature. It was found that larger RE amounts give rise to the disappearance of the yield asymmetry and to an anomalously high activity of tensile twinning, especially at the lowest extrusion temperatures. This has been attributed to an increase of the critical resolved shear stress of basal slip due to the presence of Mg₃Nd coherent and semi-coherent intermetallic prismatic plates.     

The influences of impurity content,tensile strength,and grain size on in-service temper embrittlement of CrMoV steels

The influences of impurity levels, grain size, and tensile strength on in-service temper embrittlement of CrMoV steels have been investigated. The samples for this study were taken from several steam turbine CrMoV rotors which had operated for 15 to 26 years. The effects of grain size and tensile strength on embrittlement susceptibility were separated by evaluating the embrittlement behavior of two rotor forgings, which were made from the same ingot, after giving an extended step-cooling treatment. The results reveal that among the residual elements in the steels, only P produces a significant embrittlement. The variation of P and tensile strength of the steels in the ranges investigated has no effect on in-service temper embrittlement susceptibility, as measured by the shift in fracture appearance transition temperature (FATT). However, the prior austenite grain size plays a major role on in-service embrittlement. The fine grain steels with a grain size of ASTM No. 9 or higher are virtually immune to in-service embrittlement. In steels having duplex grain sizes, the embrittlement susceptibility is controlled by the size of coarser grains. For a given steel chemistry, the coarse grain steel is more susceptible to in-service embrittlement, and a decrease in ASTM grain size number from 4 to 0/1 increases the shift in FATT by 61°C (110°F). It is demonstrated that long-term service embrittlement can be simulated, except in very coarse grain steels, by using the extended step-cooling, treatment. The results of step-cooling studies also show that the coarse grain rotor steels take longer time during service to reach a fully embrittled state than the fine grain rotor steels. This difference in the kinetics of embrittlement is believed to be related to the variations in Mo content in the matrix and the grain size of the steels.