Volume Fraction Of Δ-ferrite Research Articles

The effect of weld heat input on the microstructure and mechanical properties of wire arc additively manufactured 15-5PH stainless steel

Precipitation hardening (PH) stainless steels, such as 15-5PH, have a high strength combined with excellent corrosion resistance. These properties make them valuable in critical industries such as defence, construction, aerospace, energy and maritime. Recent advancements in additive manufacturing (AM) technology enable the rapid and cost-effective production of components. In the case of 15-5PH components manufactured using wire arc additive manufacturing (WAAM), the as-deposited mechanical properties are not suitable at present for industrial applications. This paper explores the mechanical properties of this process and alloy combination without post weld heat treatment with the aim of eventual adoption in this condition by industry. The impact of weld heat input on the microstructure and mechanical properties of stainless steel 15-5PH produced using WAAM was investigated. The microstructure was examined using hardness testing in addition to optical and electron microscopy. Furthermore, mechanical properties were measured with tensile and impact testing. Investigations were conducted on material produced using weld heat inputs of 0.223 kJ/mm and 0.565 kJ/mm. These results indicate that reducing the weld heat input leads to a minor decrease in strength but an 80% increase in impact toughness. This reduction in weld heat input is correlated with a 50% reduction in volume fraction of δ-ferrite while also noting a 55% increase in carbide precipitates. In addition, the fracture surfaces were predominantly cleavage or quasi-cleavage in morphology.

Effect of morphology and volume fraction of δ-ferrite on hydrogen embrittlement of stainless steel produced by electron beam additive manufacturing

The authors studied the influence of volume fraction and morphology of δ-ferrite on hydrogen embrittlement of austenitic stainless steel 08Kh19N9T obtained by electron beam additive manufacturing. It is experimentally shown that in additively-manufactured samples, long lamellae of δ-ferrite form a dense “net” of interphase boundaries (austenite/δ-ferrite, the volume fraction of the δ-phase is 20 %) and contribute to the hydrogen accumulation. Also, being the “easy” ways for the diffusion of hydrogen atoms, the dendritic lamellae of ferrite provide hydrogen transport deep into the samples. Post-production solid-solution treatment (at T = 1100 °C, 1 h) leads to a significant decrease in the fraction of δ-ferrite in steel (up to 5 %) and partial dissolution of dendritic lamellae. A decrease in the volume fraction of ferrite and a change in its morphology hinder the diffusion of hydro­gen deep into the samples and its accumulation during electrolytic hydrogen-charging and subsequent deformation. It contributes to a decrease in the total concentration of hydrogen dissolved in the steel samples. Despite the lower concentration of dissolved hydrogen in the solid-solution trea­ted samples, the solid-solution strengthening by hydrogen atoms is higher (\(\Delta \sigma _{0.2}^{\rm{H}}\) = 73 MPa) than for the initial samples with a high content of δ-ferrite (\(\Delta \sigma _{0.2}^{\rm{H}}\) = 55 MPa). The solid-solution treated samples are characterized by a smaller thickness of the brittle surface hydrogen-charged layer and a lower hydrogen embrittlement index compared to the post-produced samples (DH = 55 ± 12 µm, IH = 32 % for initial samples and DH = 29 ± 7 µm, IH = 24 % for samples after post-production solid-solution treatment).

The effects of sintering temperature and hold time on densification, mechanical properties and microstructural characteristics of binder jet 3D printed 17-4 PH stainless steel

In this study, the influence of sintering temperature and hold time on densification, mechanical properties, and microstructural characteristics of 17–4 PH stainless steel samples produced by binder jet 3D printing (BJ3DP) was investigated. An ultimate tensile strength (UTS) of 1050 MPa, an elongation at break (A) of >4% and a hardness of ≥330 HV10 through sintering at 1300 °C for 90 min were achieved. The tensile properties are predominantly influenced by porosity as opposed to the grain size or the volume fraction of δ-ferrite. A layered porosity appears in the sintered parts, which is attributed to the layer-wise BJ3DP process. Shrinkage in z-direction is about 2% higher than in x- and y-direction and is therefore anisotropic. Fluctuations in green density of up to 1.2% affect sintered density throughout the study and are compensated best by extended hold times. Microstructural analyses show the expected grain growth and increase in δ-ferrite fraction by increasing sintering temperature and more extensive hold times, while the comparison of fringe and core microstructures reveals little differences of the grain size but significant differences of the fraction of δ-ferrite and porosity.

The influence of intergranular and interphase boundaries and δ-ferrite volume fraction on hydrogen embrittlement of high-nitrogen steel

We study the effect of grain size of austenitic and ferritic phases and volume fraction of δ-ferrite, which were obtained in different solution-treatment regimes (at 1050, 1100, 1150 and 1200 °C), on hydrogen embrittlement of high-nitrogen steel (HNS). The amount of dissolved hydrogen is similar for the specimens with different densities of interphase (γ-austenite/δ-ferrite) and intergranular (γ-austenite/γ-austenite, δ-ferrite/δ-ferrite) boundaries. Despite, the susceptibility of the specimens to hydrogen embrittlement, depth of the hydrogen-assisted surface layers, hydrogen transport during tensile tests and mechanisms of the hydrogen-induced brittle fracture all depend on grain size and ferrite content. The highest hydrogen embrittlement index IH = 32%, the widest hydrogen-affected layer and a pronounced solid-solution hardening by hydrogen atoms is typical of the specimens with the lowest fraction of the boundaries. Even though fast hydrogen transport via coarse ferritic grains provides longer diffusion paths during H-changing, the width of the H-affected surface layer in the dual-phase structure of the HNS specimens is mainly determined by the hydrogen diffusivity in austenite. In tension, hydrogen transport with dislocations increases with the decrease in density of boundaries due to the longer dislocation free path, but stress-assisted diffusion transport does not depend on grain size and ferrite fraction. The contribution from intergranular fracture increases with an increase in the density of intergranular and interphase boundaries.

Effect of Arc Welding Processes on the Weld Attributes of Type 316LN Stainless Steel Weld joint

The present study aims at understanding the effect of various arc welding processes on the evolution of microstructure, mechanical properties, residual stresses and distortion in 9 mm thick type 316LN austenitic stainless steel weld joints. Weld joints of type 316LN stainless steel were fabricated by three different arc welding processes which were commonly employed in the nuclear industry. All the weld joints passed radiographic examination. Microstructural characterization was done using optical and scanning electron microscope. Volume fraction of δ-ferrite was lowest in the A-TIG weld joint. The A-TIG welded joint exhibited adequate strength and maximum impact toughness values in comparison to that of weld joints made by SMAW and FCAW processes. The A-TIG weld joint was found to exhibit lowest residual stresses and distortion compared to that of other welding processes. This was attributed to lower weld metal volume and hence reduced shrinkage in the A-TIG weld joint compared to that of weld joints made by FCAW and SMAW processes which involved v-groove with filler metal addition. Therefore, type 316LN stainless steel A-TIG weld joint consisting of lower δ-ferrite, adequate strength, high impact toughness, lower residual stresses and distortion was suited better for elevated temperature service compared to that of SMAW and FCAW weld joints.

9-12% Cr강의 용접부에 미치는 δ-ferrite의 영향

Abstract As the energy consumption increases rapidly, power generation needs the high energy efficiency continuously.To achieve the high efficiency of power generation, the materials used have to endure the higher temperatureand pressure. The 9-12%Cr steels possess good mechanical properties, corrosion resistance, and creep strength in high temperature due to high Cr contents. Therefore, the 9-12%Cr steels are widely used for the high-temperature components in power plants. Even though the steels usually have a fully martensitic microstructure, they are susceptible to the formation of δ-ferrite specifically during the welding process. The formation of δ-ferrite has several detrimental effects on creep, ductility and toughness. Therefore, it is necessary to avoid its formation. As the volume fraction of δ-ferrite is less than 2% in microstructure, it hasthe isolated island morphology and causes no significant degradation on mechanical properties. For δ-ferrite above 2%, it has a polygonal shape affecting the detrimental influence on the mechanical properties. The formation of δ-ferrite is affected by two factors: a chemical composition and a welding heat input. The most effective ways to get a fully martensite microstructure are to reduce the chromium equivalent less than 13.5, to keep the difference between the chromium and nickel equivalent less than 8, and to reduce the welding heat input.Key Words : 9-12%Cr steel, Delta ferrite, Austenite, Carbide, Heat input, Cooling rate

Microstructural behavior of 8Cr-ODS martensitic steels during creep deformation

Oxide dispersion strengthened (ODS) steels show a high anisotropy in their creep behavior because of the δ-ferrite grain elongated in the hot-rolled direction and the characteristic formation of creep cavities. In this work, the relationship between the δ-ferrite grain and the growth of creep cavities in 8Cr-ODS steels was investigated. The samples of two ODS steels with different δ-ferrite volume fractions were machined parallel and perpendicular to the hot-rolled direction. Creep rupture tests and interrupted tests were performed at 700°C and about 197MPa. Cavities formed in the martensite along δ-ferrite grains during creep deformation. The area fraction of the cavities of all specimens increased in proportion to the cube root of test time. When the volume fraction of δ-ferrite was high and δ-ferrite grains elongated parallel to the load direction, δ-ferrite then obstructed the propagation of cracks. However, when the volume fraction of δ-ferrite was low and δ-ferrite grains elongated perpendicular to the load direction, δ-ferrite grains had little effect on crack propagation.

Creep behavior of oxide dispersion strengthened 8Cr–2WVTa and 8Cr–1W steels

Microstructures and creep behavior of two martensitic oxide dispersion strengthened (ODS) steels 8%Cr–2%W–0.2%V–0.1%Ta (J1) and 8%Cr–1%W (J2) with finely dispersed Y 2Ti 2O 7 have been investigated. Creep tests have been carried out at 670, 700 and 730 °C. Creep strength of J1 is stronger than that of any other ODS martensitic steels and the hoop strength of the ferritic ODS steel cladding. At the beginning of creep test, shrinkage was frequently observed for J1. This is one of the reasons for high creep strength of J1. The δ-ferrite, which is untransformed to austenite at hot isostatic press and hot rolling temperatures, was elongated along the rolling direction, and volume fraction of δ-ferrite in J1 is larger than J2. Although the elongated δ-ferrite affects the anisotropy of creep behavior, the extent of anisotropy in J1 is not so large as that of the ferritic ODS steel.