Microduplex Stainless Steel Research Articles

A new grain refinement route for duplex stainless steels: Micro-duplex stainless steel matrix composites processed by laser powder bed fusion

The employment of grain refinement approaches is regarded as a favorable strategy to increase mechanical properties of duplex stainless steels (DSSs). In this work, a new grain refinement route was successfully developed via laser powder bed fusion (LPBF) of micro-duplex stainless steel (SS) matrix composites along with a subsequent quenching process. For as-built composites, TiCxNy nanoparticles were first formed in-situ via the introduction of submicron/micron-sized TiC particles with low cost into 2205 DSSs, and fine equiaxed high-temperature ferrite (δ-ferrite) grains were formed due to the ultra-high cooling rate of LPBF and heterogeneous nucleation sites provided by in-situ formed TiCxNy nanoparticles. The formation of TiCxNy nanoparticles inhibited the epitaxial growth of δ-ferrite grains, and no obvious textures were found in as-built composites. Subsequently, fine austenite grains were produced at grain boundaries of fine unrecrystallized δ-ferrite during quenching treatment of as-built composites, which enabled heat-treated composites to achieve an ensemble of outstanding ultimate tensile strength and uniform elongation. The δ-ferrite recrystallization was mainly inhibited by the existence of TiCxNy nanoparticles. This study can provide a groundwork for designing grain refinement methods to produce high-performance DSS parts with complex geometry based on LPBF technique.

Effect of Ni and Mn on the Mechanical Properties of 22Cr Micro-duplex Stainless Steel

The tensile properties of 22Cr–2Ni–4Mn–0.2N micro-duplex stainless steels with different Ni and Mn contents were investigated. Duplex stainless steels were vacuum induction melted and hot rolled, then annealed at 1,000–1,100 °C, at which temperature both the austenite and ferrite phases were stable. The volume fraction of the ferrite phase was markedly affected by the alloying elements of Mn and Ni; 1 wt% of Mn was equivalent to 0.4 wt% of Ni. All of the steels tested at room temperature showed the common strain-hardening behavior, while the steels tested at lower temperatures (−30 or −50 °C) showed a distinct inflection point in their stress–strain curves, which resulted from the transformation of the austenite to strain-induced martensite. The onset strain (ε 0) of the inflection point in the stress–strain curve depended on the M d30 value of the steel. Testing at lower temperatures resulted in smaller ε 0 and consequently higher strengths and fracture strains (ε f). The tensile behavior was examined from the perspective of austenite stability of the micro-duplex stainless steels with the different Ni and Mn contents.

Electrochemical Study of Diffusion Bonded Joints between Micro-Duplex Stainless Steel and Ti6Al4V Alloy

In the present study, corrosion behavior of a diffusion bonded interface formed between micro-duplex stainless steel (MDSS) and a mixed titanium alloy (Ti6Al4V) formed at 900°C for 60 minutes under 4MPa uniaxial pressure in vacuum has been investigated in 1M HCl and 1 M NaOH solutions using various electrochemical measurements such as Equilibrium Potential (EP), Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PD). For comparison, corrosion behavior of base metal alloys, MDSS and Ti6Al4V have also been also characterized. Bonded interface has been characterized in light optical microscopy and scanning electron microscopy using back scattered electron. The layer wise σ phase and λ+FeTi phase mixture has been observed at the bond interface and the bond tensile strength and shear strength were ~556.4MPa and ~420.2MPa, respectively. The corrosion rates of the bonded joint are intermediate to the corrosion rates of MDSS and Ti6Al4V alloy.

Diffusion Bonding of Microduplex Stainless Steel and Ti Alloy with and without Interlayer: Interface Microstructure and Strength Properties

The interface microstructure and strength properties of solid state diffusion bonding of microduplex stainless steel (MDSS) to Ti alloy (TiA) with and without a Ni alloy (NiA) intermediate material were investigated at 1173 K (900 °C) for 0.9 to 5.4 ks in steps of 0.9 ks in vacuum. The effects of bonding time on the microstructure of the bonded joint have been analyzed by light optical microscopy and scanning electron microscopy in the backscattered mode. In the direct bonded joints of MDSS and TiA, the layer-wise σ phase and the λ + FeTi phase mixture were observed at the bond interface when the joint was processed for 2.7 ks and above holding times. However, when NiA was used as an intermediate material, the results indicated that TiNi3, TiNi, and Ti2Ni are formed at the NiA-TiA interface, and the irregular shaped particles of Fe22Mo20Ni45Ti13 have been observed within the TiNi3 intermetallic layer. The stainless steel-NiA interface is free from intermetallics and the layer of austenitic phase was observed at the stainless steel side. A maximum tensile strength of ~520 MPa, shear strength of ~405 MPa, and impact toughness of ~18 J were obtained for the directly bonded joint when processed for 2.7 ks. However, when nickel base alloy was used as an intermediate material in the same materials, the bond tensile and shear strengths increase to ~640 and ~479 MPa, respectively, and the impact toughness to ~21 J when bonding was processed for 4.5 ks. Fracture surface observations in scanning electron microscopy using energy dispersive spectroscopy demonstrate that in MDSS-TiA joints, failure takes place through the FeTi + λ phase when bonding was processed for 2.7 ks; however, failure takes place through σ phase for the diffusion joints processed for 3.6 ks and above processing times. However, in MDSS-NiA-TiA joints, the fracture takes place through NiTi2 layer at the NiA-TiA interface for all bonding times.

Electrochemical Study of Diffusion Bonded Joints between Micro-duplex Stainless Steel and Ti6Al4V Alloy

In the present study, corrosion behavior of diffusion bonded joints formed between micro-duplex stainless steel (MDSS) and Ti6Al4V alloy (TiA) (at 900 °C for 60 min under 4 MPa uniaxial pressure in vacuum) was investigated in 1 mol/L HCl and 1 mol/L NaOH solutions using various electrochemical measurements such as open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PD). For comparison, corrosion behavior of base metals (MDSS and TiA) was also evaluated. Bonded joint was characterized by light optical microscopy and scanning electron microscopy using backscattered electron mode. The layer wise σ phase and λ + FeTi phase mixture has been observed at the bond interface and the bond tensile strength and shear strength were ∼556.4 MPa and ∼420.2 MPa, respectively.

Effect of Alloying Composition and Plastic Deformation on the Microstructure of 22Cr Micro-Duplex Stainless Steel

Effect of Alloying Composition and Plastic Deformation on the Microstructure of 22Cr Micro-Duplex Stainless Steel

Interfacial reactions and strength properties in dissimilar titanium alloy/Ni alloy/microduplex stainless steel diffusion bonded joints

The interfacial reactions and strength properties of a titanium alloy/ microduplex stainless steel diffusion bonded joint using Ni alloy as an intermediate materials was investigated in the temperature range of 800 to 950°C for 75min and at 950°C for 15–90min in vacuum. The microstructure observations at the stainless steel/Ni alloy interface indicate the presence of γ-Fe layer up to 900°C for 75min and at 950°C up to 45min. Ni3Ti, NiTi and NiTi2 intermetallic layers were observed at the Ni alloy/titanium alloy interface at the same processing parameters and irregular shaped particles were present within the TiNi3 intermetallic layer. Beyond these processing parameters, the layer wise Fe–Ti or Fe–Cr–Ti base intermetallic compounds were observed at the microduplex stainless steel/Ni alloy interface. Maximum tensile strength of ∼639.9MPa and shear strength of ∼477.8MPa along with ∼9.3% ductility were obtained for the diffusion couple processed at 900°C for 75min.

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950°C for 45min under the compressive stress of 4MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni3Ti, NiTi and NiTi2 intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe22Mo20Ni45Ti13 was observed in the Ni3Ti intermetallic layer. At 950°C processing temperature, black island of β-Ti phase has been observed in the NiTi2 intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi2 phase at the NiA–TiA interface when bonding was processed up to 900°C, however, failure takes place through NiTi2 and β-Ti phase mixture for the diffusion joints processed at 950°C. Joint strength was evaluated and maximum tensile strength of ∼560MPa and shear strength of ∼415MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900°C for 45min.

Evaluation of interface microstructure and mechanical properties of the diffusion bonded joints of Ti–6Al–4V alloy to micro-duplex stainless steel

In the present investigation, Ti–6Al–4V and micro-duplex stainless steel was diffusion bonded in vacuum. The layer wise σ phase and λ + FeTi phase mixture were observed at the bond interface when bonded joints was processed at and above 850 °C for 90 min and at 800 °C for 120 min and longer bonding time. Effect of bonding temperature and time on the strength properties at room temperature were evaluated. The maximum tensile strength of ∼510.1 MPa and shear strength of ∼397.5 MPa along with 6.5% elongation were obtained for the diffusion couple processed at 850 °C for 90 min. Fracture surface observation in SEM using EDS demonstrates that, failure takes place through λ + FeTi phase when bonding was processed at 850 °C for 90 min and at 800 °C for 120 min, however, failure takes place through σ phase for the diffusion couples processed at and above 900 °C for 90 min and at 800 °C for 120 min and longer bonding time.

Interface microstructure and strength properties of Ti–6Al–4V and microduplex stainless steel diffusion bonded joints

The diffusion bonding of Ti–6Al–4V alloy and micro-duplex stainless steel was carried out in the temperature range of 850–1000 °C for 45 min in vacuum. The influence of bonding temperature on the microstructural development, micro-hardness and strength properties across the joint region was determined. The layer wise σ phase, λ + FeTi and λ + FeTi + β-Ti phase mixtures were observed at the bond interface when the joint was processed at 900 °C and above temperature. The maximum tensile strength of ∼520.1 MPa and shear strength of ∼405.5 MPa along with 6.8% elongation were obtained for the diffusion couple processed at 900 °C. Fracture surface observation in scanning electron microscopy (SEM) using energy dispersive X-ray spectroscope (EDS) demonstrates that, failure takes place through λ + FeTi phase when bonding was processed at 900 °C, however, failure takes place through σ phase for the diffusion joints processed at and above 950 °C.

Influence of Bonding Temperature on Structure and Strength Properties of Titanium and Micro-duplex Stainless Steel Diffusion Bonded Joints

In the present study, diffusion bonding of titanium and micro-duplex stainless steel was investigated in vacuum. Diffusion interfaces were characterized using light microscopy, scanning electron microscopy and X-ray diffraction technique. The inter-diffusion of the chemical species across the diffusion interfaces were evaluated by electron probe microanalysis (EPMA). The maximum tensile strength of ~97% and shear strength of ~80% of those of Ti along with 6.9% elongation were obtained for the diffusion couple processed at 900°C joining temperature. Fracture surface observation in SEM using EDS demonstrates that, failure takes place through β-Ti phase when bonding was processed up to 850°C, however, failure takes place through λ+FeTi+β-Ti for the diffusion couples processed at 900°C and above bonding temperature.

Diffusion bonding between commercially pure titanium and micro-duplex stainless steel

Diffusion-bonded joints between commercially pure titanium and micro-duplex stainless steel were prepared in the temperature range of 800–950 °C for 1.5 h under 3 MPa uniaxial load in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy. The composition of the reaction products were determined by energy dispersive spectroscopy. Up to 850 °C, α-Fe + λ and λ + FeTi phase mixtures were formed at diffusion interface; however α-Fe + λ, λ + FeTi and FeTi + β-Ti phases mixtures were formed at 900 °C and above. The presence of these intermetallics was confirmed by X-ray diffraction technique. The maximum tensile strength of ∼96% of Ti and shear strength of ∼81% of Ti along with 6.9% ductility were obtained for the diffusion couple processed at 850 °C due to the finer width of intermetallic phases. With a rise in the joining temperature the bond strength drops owing to an increase in the width of reaction products.

Effect of transient liquid-phase bonding variables on the properties of a micro-duplex stainless steel

Abstract The TLP bonding process has been used to join a 2205 micro-duplex stainless steel using a pure copper interlayer and a foil based on the Fe–B–Si ternary system. Joints were produced by isothermal solidification when using these interlayers, and the rapid heating and cooling cycles used during bonding suppressed the formation of σ-phase within the joint region. Metallographic examination of the bonds show that a homogeneous microstructure with grain growth and good α/γ phase balance across the bond-line was achieved, and this gave the joint mechanical and corrosion-resistant properties similar to that of the parent alloy.

Diffusion bonding between Ti -6Al -4V alloy and microduplex stainless steel with copper interlayer

In the present study, diffusion bonding was used to join Ti -6Al- 4V alloy to a microduplex stainless steel using a pure copper interlayer. The effects of heating rate and holding time on microstructural developments across the joint region were investigated. After bonding, microstructural analysis including metallographic examination and energy dispersive spectroscopy (EDS), microhardness measurements, and shear strength tests were carried out. From the results, it was seen that heating rate and holding time directly affect microstructural development at the joint, especially with respect to the formation of TiFe intermetallic compounds, and this in turn affects the shear strength of the bonds. A sound bond was obtained with a heating rate of 100 K min -1 and holding time of 5 min, and this was related to the small amount of TiFe intermetallics formed close to the duplex stainless steel side at this bonding condition. Although Ti2Cu and TiFe intermetallics were formed in all specimens, it was seen that the most deleterious intermetallic was TiFe. As the heating rate was decreased and holding time increased the amount of TiFe intermetallics increased, and consequently shear strength decreased. As a result, from the microstructural observations, EDS analysis, microhardness measurements, and shear strength tests, it was concluded that a high heating rate and a short holding time must be used in the diffusion bonding of Ti-6Al- 4V to a microduplex stainless steel when pure copper interlayers are used.

Diffusion bonding of a microduplex stainless steel to Ti–6Al–4V

In this study, a microduplex stainless steel (AVESTA 2205) and Ti–6Al–4V alloy were bonded for the first time. The bonding variables were chosen on the basis of the importance of grain boundary diffusion in α-phase of the titanium alloys. The study showed that the formation of Fe–Cr–Ti intermetallics is more important than σ-phase formation during bonding.

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization. A continuous increase in α subgrain boundary misorientation is obtained by the present processing where grain boundary sliding does not occur and the effect of increasing boundary misorientation with cumulative strain is comparable to those observed in dynamic continuous recrystallization of superplastic aluminium alloys. The increase in boundary misorientation is accounted for by the absorption of dislocations into subgrain boundaries during annealing, dislocations which had operated to accommodate the plastic strain incompatibility of the neighboring phases undergoing slip deformation. The present results show that grain boundary sliding is not indispensable but the difference in accommodation deformation between adjacent subgrains is of great importance for the dynamic continuous recrystallization during superplastic deformation.

Microstructural Evolution during High Strain Rate Superplastic Deformation in Microduplex Stainless Steel

Microstructural Evolution during High Strain Rate Superplastic Deformation in Microduplex Stainless Steel

Change in Boundary Misorientation with Subgrain Growth in a Microduplex Stainless Steel

Change in Boundary Misorientation with Subgrain Growth in a Microduplex Stainless Steel

Subgrain growth and misorientation of the α matrix in an ( α + γ) microduplex stainless steel

The change of the ( α + γ) microduplex structure during annealing in an Fe-26%Cr-7%Ni alloy has been investigated with emphasis on the change in the misorientation of the α matrix subgrain boundaries with subgrain growth in a large range of subgrain size. A long time holding up to 360 ks at 1273 K caused significant structure coarsening, and the growth of the α matrix subgrains and γ particles obeyed the third power law. On the other hand, the average misorientation of the α subgrain boundaries changed insignificantly, even though the α subgrain size markedly increased from 2.6 to 15.6 μm. This insignificance in the misorientation resulted from the fact that the variation of the local lattice rotation in the α matrix is not monotonic but periodic, and no long range lattice curvature exists.

Effect of initial structure on recrystallization of the α matrix in an (α+γ) microduplex stainless steel

The high temperature annealing behavior at 1,273K after 80% cold-rolling of the {alpha} matrix in ({alpha} + {gamma}) microduplex structures in a Fe-26Cr-7Ni alloy has been studied with particular attention to the effects of the size of initial structure and the initial orientation of {alpha}. The main results obtained are as follows: (1) No recrystallization of the {alpha} matrix takes place when the initial structure before rolling is fine; the {gamma} particle size is smaller than 9 {micro}m and the {alpha} subgrain size is not larger than 10 {micro}m. This result deduces the conclusion that the {alpha} matrix in ({alpha} + {gamma}) microduplex structures is very hard to recrystallize. (2) Recrystallization occurs in the {alpha} matrix with the {l_brace}111{r_brace} initial orientation when the initial structure is coarse; the {gamma} particle size is 13.8 {micro}m and the {alpha}subgrain size is 15.6 {micro}m. The {l_brace}001{r_brace} initially-oriented regions, by contrast, do not recrystallize even in the coarse initial structure. These results indicate that they recrystallization of the {alpha} matrix in ({alpha} + {gamma}) microduplex structures strongly depends on the initial orientation of {alpha} as well as the size of initial structure.