Effect of cooling rate on plasma nitrocarburised compound layers for pure iron

Plasma nitrocarburising was carried out with various CO gas compositions with 3 mbar at 570°C for 3 hours. After treatment, different cooling rates (fast cooling, slow cooling and gas cooling) were used to investigate their effect on the structure of the compound layer, the ε phase being the outer part of the compound layer and the γ′ phase being the inner part The CO gas composition influenced the constitution of the compound layer produced, i.e. high CO contents were essential for the production of an ε phased compound layer, as did a variation of the cooling rate, with a large amount of γ′ phase transformed from the ε phase during slow cooling. The transformation was reduced with an increasing cooling rate and prevented by a fast cooling rate of approximately 18.5°C/sec at 500°C, at which temperature this transformation began.     

Development of plasma nitrocarburizing and post-oxidation technology for the replacement of Cr<Superscript>6+</Superscript> plating for application to vehicle engine shafts

Plasma nitrocarburizing and post-oxidation treatments were performed to improve the wear and corrosion resistance of S45C steel. Plasma nitrocarburizing was conducted for 3 h at 570°C in a nitrogen, hydrogen and methane atmosphere to produce the ε-Fe₂₋₃(N,C) phase. It was found that the compound layer produced by plasma nitrocarburising was predominantly composed of the ∈-phase with traces of the γ′-Fe₄(N,C) phase. The thickness of the compound layer was approximately 12 μm and the diffusion layer was approximately 300 μm in thickness. Plasma post oxidation was performed on nitrocarburized samples with various oxygen/hydrogen ratios at a constant temperature of 500°C for 1 h. The very thin magnetite (Fe₃O₄) layer 1 μm to 2 μm in thickness on top of the compound layer was obtained by plasma post oxidation. It was also confirmed that further improvement of the corrosion characteristics of the nitrocarburized compound layer was possible with an application of the superficial magnetite layer. Finally, throttle valve shafts of S45C steel were treated under optimum plasma processing conditions. Accelerated life time test results using a throttle body assembled with a shaft treated by plasma nitrocarburising and post oxidation showed that plasma nitrocarburizing and plasma post-oxidation processes could be a viable technology in the very near future in place of Cr⁶ plating.     

High resolution electron microscopy characterization of nitrides of iron and iron alloys

A study of nitride-phase formation in alloys nitrided by glow discharge plasma nitriding was performed by high-resolution electron microscopy. The iron and steel samples were nitrided above and below the eutectoid transformation point (590 ‡C). After nitriding, the samples were annealed for several treatment times at 400 ‡C. The microstructure and identification of the iron nitrided phases formed by nitriding and annealing were investigated by x-ray diffraction (XRD), optical microscopy (OM) and mainly high resolution electron microscopy (HREM) in a JEOL-4000EX high resolution microscope at 400 kV. The results of the characterization show a surface ε compact nitride layer, which is transformed into ε + γ during cooling. The tempering process affects the surface hardness of the samples. After an initial increase relative to the untreated sample, the microhardness diminishes after long treatment times. This behavior is correlated with the αt’’ phase appearance, growth, and transformation.     

Precipitation and residual stress relaxation kinetics in shot-peened Inconel 718

Mechanical surface treatment by shot peening followed by aging at 700 and 740 °C was performed on Inconel 718. A previously proposed XRD method (Ref 10) for the quantitative phase analysis of Inconel 718 allowed for the determination of the precipitation kinetics of the γ″ phase in the shot-peened layer and the matrix, respectively. The residual compressive stress field induced by shot peening and its relaxation behavior during aging were also determined. The relaxation process can be described by the Zener-Wert-Avrami function. The precipitation rate in the γ″ phase in the shot-peened layer is greatly accelerated, which causes differences in the γ″ phase amounts between the skin and the core during aging, especially during the initial stage. The high precipitation rate of the γ″ phase in the shot-peened layer can be interpreted by the nonequilibrium segregation of niobium.     

Microstructural evolution of DS CM186LC during creep and thermal exposure

Thermal exposure tests were carried out at the same temperature and duration as those of creep tests. Growth of γ′ either in the matrix or in the interdendritic regions was observed during high temperature thermal exposure. The growth of γ′ caused an isolated γ island in the γ/γ′ eutectics within which fine HfC particles precipitated. Some directional coarsening of γ′ was found in the specimens exposed at 1050°C. MC carbide reaction with the y matrix was observed in crept and thermally exposed specimens. Needle-like precipitate, a kind of TCP, was found in the specimen crept at 1050°C for 400 hours.     

A study of the corrosion properties of plasma nitrocarburized and oxidized AISI 1020 steels

Plasma nitrocarburized AISI 1020 steels were oxidized for 15, 30 and 60 min to evaluate their corrosion and microstructural properties. After plasma nitrocarburizing for 3 h at 570°C in a gas mixture comprising 85 vol.% N₂, 12vol.% H₂ and 3 vol.% CH₄, the compound layer composed of ɛ-Fe_2–3(N,C) and γ’-Fe₄(N,C) phases and the diffusion layer above the matrix were observed. The top oxide layer, consisting mainly of magnetite (Fe₂O₄) and hematite (Fe₂O₃) phases, forms after post-oxidation treatment at 500°C. However, the oxide layer was severely degraded by spallation as a result of increases in post-oxidizing time. The difference in corrosion resistance should be attributed to the thickness of the top oxide layer, which was governed by post-oxidizing time.     

Composition and heat-treatment effects on the adhesion strength of Sn-Zn-Al solders on Cu substrate

Intermetallic layer growth at the solder/substrate interface during soldering and subsequent aging in service can affect bond strength and dimensional stability. In this work, interfacial structure and bonding strength studies were performed on lead-free, Sn-Zn-Al solders as a function of composition and aging time. With the x value of Sn-x(5Al-Zn) solder increasing from 5 wt.% to 40 wt.%, the adhesion strength of bonding decreased from 11.2±0.5 MPa to 3.3±0.9 MPa while the thickness of the γ-Cu₅Zn₈ intermetallic compound layer increased from less than 1 μm to about 2 μm. Simultaneously, the adhesion strength of Sn-x(5Al-Zn) solders with x=5, 9, 20, 30, and 40 decreased as the aging time at 150°C increased from 0 to 1,000 hours. For more information, contact M.-C. Wang, National Kaohsiung Institute of Technology, Department of Mechanical Engineering, 415 Chien-Kung Road, Kaohsiung 80782, Taiwan; telephone 886-6-258-5663; fax 886-6-250-2734.     

Stress-induced martensitic transformation in metastable austenitic stainless steels: Effect on fatigue crack growth rate

This paper addresses the influence of cyclic stress-induced martensitic transformation on fatigue crack growth rates in metastable austenitic stainless steels. At low applied stress and mean stress values in AISI type 301 stainless steel, fatigue crack growth rate is substantially retarded due to a cyclic stress-induced γ-α′ and γ-ε martensitic transformation occurring at the crack-tip plastic zone. It is suggested that the transformation products produce a compressive residual stress at the tip of the fatigue crack, which essentially lowers the effective stress intensity and hence retards the fatigue crack growth rate. At high applied stress or mean stress values, fatigue crack growth rates in AISI type 301 steels become almost equal to those of stable AISI type 302 alloy. As the amount of transformed products increases (with an increase in applied or mean stress), the strain-hardening effect brought about by the transformed martensite phase appears to accelerate fatigue crack growth, offsetting the contribution from the compressive residual stress produced by the positive volume change of γ → α′ or ε transformation.     

Thermodynamic assessment of the Hg-Sn system

A thermodynamic assessment of the Hg-Sn system has been carried out using the CALPHAD method. The comprehensive assessment covers the extensive phase diagram data as well as the enthalpy, activity, and vapor pressure data. Two cases of intermetallic compounds in the Hg-Sn system are considered. Case 1 considers the intermetallic compounds β, γ, and HgSn₄ as having no solubility and can thus be treated as the stoichiometric phases β-HgSn₃₈, γ-HgSn₁₂, and HgSn₄. Case 2 uses a sublattice model to more accurately describe a solubility of the γ phase; it also considers the stoichiometric δ-HgSn₇ phase. The ε phase was considered to be metastable and neglected in the thermodynamic assessment. Thermodynamic parameters have been optimized using all the assessed experimental thermodynamic and phase equilibrium data. Both calculated phase diagrams of the Hg-Sn system (Cases 1 and 2) and the thermodynamic data are reasonable and satisfactory when compared with literature data. Future crucial experiments are suggested.     

Thermodynamic assessment of the Hg-Sn system

A thermodynamic assessment of the Hg-Sn system has been carried out using the CALPHAD method. The comprehensive assessment covers the extensive phase diagram data as well as the enthalpy, activity, and vapor pressure data. Two cases of intermetallic compounds in the Hg-Sn system are considered. Case 1 considers the intermetallic compounds β, γ, and HgSn₄ as having no solubility and can thus be treated as the stoichiometric phases β-HgSn₃₈, γ-HgSn₁₂, and HgSn₄. Case 2 uses a sublattice model to more accurately describe a solubility of the γ phase; it also considers the stoichiometric δ-HgSn₇ phase. The ε phase was considered to be metastable and neglected in the thermodynamic assessment. Thermodynamic parameters have been optimized using all the assessed experimental thermodynamic and phase equilibrium data. Both calculated phase diagrams of the Hg-Sn system (Cases 1 and 2) and the thermodynamic data are reasonable and satisfactory when compared with literature data. Future crucial experiments are suggested.     

Phase transformations of 24Cr-14Ni-0.7Si stainless steel under different aging conditions

This study discusses the development of a phase transformation in 24Cr-14Ni-0.7Si stainless steel after aging under various aging temperatures, times, and N₂/Air ratios. The observation of OM indicated that the initial state of δ-ferrite in the test material appeared as complete dendrite structures at short aging times and then exhibited lacy and dispersed structures when the aging time increased. This led to a gradual austenitization transformation as the nitrogen/air ratio increased, accelerating the δ/σ phase transformation and retarding the δ/γ phase transformation at the same time. The δ/σ phase transformation was dominant when the aging temperature was 800 °C. A line scanning analysis of the EPMA showed that the X-ray spectrum of Cr at the δ/γ interphase boundary was raised. In addition, Si showed lower X-ray spectrum energy after the δ/γ phase transformation. Clearly, Si had a stabilizing effect on the δ-ferrite and σ-phase. Furthermore, it had the fastest precipitation ratio for the δ/σ phase transformation at 800°C among all aging temperatures.     

Diffusion and phase transformation on interface between substrate and NiCrAlY in Y-PSZ thermal barrier coatings

NiCrAlY/Y₂O₃-Y-PSZ (yttria-partially stabilized zirconia) thermal barrier coatings were developed on a superalloy (Ni-10Co-9Cr-7W-5Al, wt.%) surface. The superalloys were first coated with a bond coat of Ni-19Cr-8Al-0.5Y (wt.%) alloy that was deposited by low-pressure plasma spraying and then covered with a top coat of ZrO₂-8wt.%Y₂O₃ by air plasma spraying. The microstructure near the interface was analyzed using an optical microscope, a scanning electron microscope, microhardness measurements, and x-ray diffraction, and the phases of composition were measured using an electron probe microanalyzer after exposure at 1100°C for different times in air or a vacuum. The reaction processes also were simulated using diffusion-controlled transformation (DICTRA) software in which diffusion was considered as being only the γ phase, and the γ′ phase was treated as spheroidal particles in γ. From the authors’ results, it can be concluded that a γ′-phase layer is observed at the interface between substrate and bond coat, and its thickness increases with increasing exposure times in air at 1100 °C. This layer showed good cohesion with the substrate and bond coat. It can also be concluded that the formation of the γ′-phase layer can be predicted from DICTRA simulation. The simulation also shows the same trend of the composition profiles as experimental data.     

Effects of cold drawing ratio on δ phase precipitation behaviors of alloy 718 wire

The 8 phase precipitation behavior of cold drawn alloy 718 has been investigated at 1116 K as a function of cold drawn ratio and aging time. The planar markings formed by cold drawing were identified as deformation twins and developed more clearly as cold drawing ratio increases. The grain boundaries and twins were played as nucleation site of δ precipitation, therefore, cold drawing enhanced the precipitation of δ particles. As the cold drawing ratio and aging time increased the amount of round-shaped δ also increased. In a specimen which was 50% cold drawn and aged for 4 hrs. the finely dispersed δ phase precipitated throughout the material. As aging time increased. The round-shaped δ phase changed to plate-like morphology, and the morphology transformation time was increased with cold drawing ratio. With a 50% cold drawn specimen, the transformation time was 7 hrs. On the other hand, for the 50% cold drawn specimen aged for one hour at 1116 K, nucleation of the strain free grain was observed.     

Thermodynamic evaluation of the Fe-Mn-Si system and the γ/ε martensitic transformation

The thermodynamic properties of the Fe-Mn-Si system are analyzed by means of thermodynamic models for the individual phases. Special attention is paid to the γ → ε martensitic transition. A complete set of parameters, from which arbitrary sections of the phase diagram as well as the M_s and A_s temperatures may be calculated, is given.     

The phase equilibria of N3Al evaluated by directional solidification and diffusion couple experiments

Three solid phases are involved in the phase equilibria of the important intermetallic compound N1₃Al near its melting point, β, γ′(Ni₃AI) and γ. The generally-accepted phase diagram involves a eutectic reaction between γ′ and γ, but some recent studies agree with an older diagram due to Schramm, 5 which has a eutectic reaction between the β and γ′ phases. This work uses diffusion couple experiments to evaluate the equilibria at temperatures just below the liquidus, and it examines the liquidus region using quenched, directional solidification experiments that preserve the microstructures formed at the solidification front. At the growth rate studied, 61 μm/s, the eutectic forms between the γ and γ′ phases, as in the Schramm diagram.     

The phase equilibria of N3Al evaluated by directional solidification and diffusion couple experiments

Three solid phases are involved in the phase equilibria of the important intermetallic compound N1₃Al near its melting point, β, γ′(Ni₃AI) and γ. The generally-accepted phase diagram involves a eutectic reaction between γ′ and γ, but some recent studies agree with an older diagram due to Schramm, 5 which has a eutectic reaction between the β and γ′ phases. This work uses diffusion couple experiments to evaluate the equilibria at temperatures just below the liquidus, and it examines the liquidus region using quenched, directional solidification experiments that preserve the microstructures formed at the solidification front. At the growth rate studied, 61 μm/s, the eutectic forms between the γ and γ′ phases, as in the Schramm diagram.     

The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the γ/β microstructure of the feedstock powder, and the VPS coating consisted of a single (γ) phase. A 3-h, 1100 °C heat treatment in vacuum converted the single-phase VPS coating to a two-phase γ/β microstructure and coarsened the γ/β microstructure of the HVOF coating. Oxidation of free-standing as-sprayed and heat-treated coatings of each type was carried out in air at 1100 °C for a duration of 100 h. Parabolic rate constant(s), K _p, were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in K _p following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower K _p values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner α-Al₂O₃ layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a single-layer oxide, α-Al₂O₃. The formation of a thin α-Al₂O₃ layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.