δ-Ferrite Formation and Its Effect on the Mechanical Properties of Heavy-Section AISI 316 Stainless Steel Casting

Metallurgical and Materials Transactions A - δ-Ferrite formation across the thickness of a heavy-section AISI 316 casting slab and the effect of its decomposition on the bending properties...     

Effect of Cr–Mo Addition on the Microstructure and Mechanical Properties of Medium-Carbon Steel

Herein, the effects of Chromium–Molybdenum (Cr–Mo) addition on the microstructural evolution and mechanical properties of medium-carbon steel after spheroidization annealing are systematically studied through scanning electron microscopy, electron backscatter diffraction, and tensile testing. Cr–Mo addition hinders the proeutectoid ferrite + pearlite transformation, thereby promoting the bainite transformation. Moreover, it refines the pearlite lamellar spacing as well as decreases the average carbide diameter, increases the number of carbides per unit area, and hinders ferrite recrystallization. Compared with those in the B1 steel annealed for 8 h, the size of carbides and their number per unit area in the CM1 steel are 30% lower and 2.2-fold higher, respectively. Due to finer ferrite grains, smaller carbides, and a higher amount of carbides, the strength of steel improves, and the plasticity slightly reduces after Cr–Mo addition. After 2 h of annealing, the yield strengths of Cr–Mo steels are 77.5–109.5 MPa higher than those of base steels; the elongations are above 20%. The contributions of the strengthening mechanism of steel to the yield strength are as follows (from high to low): grain boundary, precipitation, solid solution, and dislocation strengthening.     

Effect of Grain Size on Mechanical Properties of NickelFree High Nitrogen Austenitic Stainless Steel

The fine grained structures of nickelfree high nitrogen austenitic stainless steels had been obtained by means of cold rolling and subsequent annealing. The relationship between microstructure and mechanical properties and gain size of nickelfree high nitrogen austenitic stainless steels was examined. High strength and good ductility of the steel were found. In the grain size range, the HallPetch dependency for yield stress, tensile strength, and hardness was valid for grain size ranges for the nickelfree high nitrogen austenitic stainless steel. In the present study, the ductility of cold rolled nickelfree high nitrogen austenitic stainless steel decreased with annealing time when the grain size was refined. The fracture surfaces of the tensile specimens in the grain size range were covered with dimples as usually seen in a ductile fracture mode.     

Effect of Homogenization Treatment on Microstructure and Eutectic Precipitates in Ce-Containing 15Cr–22Ni–1Nb Austenitic Heat-Resistant Stainless Steel

The effects of homogenization temperature and time on microstructure and eutectic precipitates in Ce-containing heat-resistant stainless steel are studied. The increase in the homogenization temperature and time promotes the diffusion of alloying elements and the transition from dendrite structure to austenitic grain. Laves phase particles are fully dissolved after the homogenization of the ingots regardless of the cerium contents and homogenization temperature and time, except for the case of the homogenization at 1130 °C for 4 h. Honeycomb Laves phases gradually dissolve and become clusters of small blocky during homogenization treatment. The amount of eutectic NbC dissolved into the matrix is increased with the increase in the homogenization temperature and time. The solubility of niobium and carbon in austenite is increased with increasing the cerium content, which is beneficial to the dissolution of eutectic NbC. The distribution homogeneity of alloying elements in the austenitic matrix is increased with the increase in the cerium content. The homogenization temperature for reaching full diffusion of alloying elements into austenitic matrix is decreased with increasing the homogenization time. The decrease in the secondary dendrite arm spacing by cerium addition is favorable to the homogenization of alloying elements during homogenization treatment.     

Experimental Investigation and Analytical Prediction of σ-Phase Precipitation in AISI 316L Austenitic Stainless Steel

The phase precipitation in industrial AISI 316L stainless steel during aging for up to 80,000 hours between 823 K and 1073 K (550 °C and 800 °C) has been studied using transmission electron microscopy, scanning transmission electron microscopy, and carbon replica energy-dispersive X-ray microanalysis. Three phases were identified: Chromium carbides (M₂₃C₆), Laves phase (η), and σ-phase (Fe-Cr). M₂₃C₆ carbide precipitation occurred firstly and was followed by the η and σ-phases at grain boundaries when the aging temperature is higher than 873 K (600 °C). Precipitation and growth of M₂₃C₆ create chromium depletion zones at the grain boundaries and also retard the σ-phase formation. Thus, the σ-phase is controlled by the kinetic of chromium bulk diffusion and can appear only when the chromium reaches, at grain boundaries and at the M₂₃C₆/γ and M₂₃C₆/η/γ interfaces, content higher than a critical value obtained by self-healing. An analytical model, based on equivalent chromium content, has been established in this study and successfully validated to predict the time–temperature–precipitation diagram of the σ-phase. The obtained diagram is in good agreement with the experimental results.     

Corrosion Behaviour of Chrome–Manganese Austenitic Stainless Steels and AISI 304 Stainless Steel in Chloride Environment

Electrochemical behaviour of chrome–manganese austenitic stainless steels (Cr–Mn ASS) and AISI 304 stainless steel (SS) is evaluated in various chloride (Cl^?) concentrations (Cl^? free to 20,000 ppm) to simulate rural, industrial and marine environment. Potentiodynamic polarization and electrochemical impedance spectroscopy has clearly shown that with increase in Cl^? concentration, the corrosion rate of both Cr–Mn ASS and AISI 304 SS increases and polarization resistance decreases. Comparatively, Cr–Mn ASS is more affected by Cl^? concentration than AISI 304 SS. This is attributed to relatively low Cr content and lack of Ni. The findings have been explained with the help of point defect model. However, in less aggressive environment of up to 100 ppm Cl^? concentration, Cr–Mn ASS may be a candidate material as a cheaper substitute of AISI 304 SS. Ways of improving corrosion resistance of Cr–Mn ASS by alloying with various elements have also been discussed. It is argued that a dedicated effort is needed to improve corrosion resistance of Ni-free or low-Ni Cr–Mn ASS.     

Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, A1 1.3, and Nb-kV 0.13 （mass%） with annealed martensite （TAM steel） were investigated using optical microscopy, field emission gun scanning electronic microscope （FEG SEM）, transmission electron microscopy （TEM）, X-ray diffraction （XRD）, and mechanical testing. The mierostructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix （TPF steel）, the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain harden- ing behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.     

Effect of Cold Rolling on as–ECAP Interstitial Free Steel

Ti-stabilized interstitial free steel subjected to eight passes, route B_C room temperature equal channel angular pressing (ECAP) additionally was cold rolled (CR) up to 95 pct thickness reduction. Electron back-scattering diffraction and transmission electron microscopy characterized microstructural refinement and microtexture evolution, whereas the mechanical properties were assessed by uniaxial tensile tests. After 95 pct CR, the average high-angle grain boundary spacing reduces to 0.14 μm, whereas the high-angle boundary fraction increases to ~81 pct. The ECAP negative simple shear texture components rotate by ~15 deg around the transverse direction toward the rolling direction for up to 50 pct CR, with typical rolling textures observed at 95 pct CR. The decrease in boundary spacing produces a ~500 MPa gain in 0.2 pct proof stress, a ~600 MPa increase in ultimate tensile strength (UTS), and a ~4 pct loss in total elongation after 95 pct CR. Similar rates of decrease in work hardening correspond to comparable rates of cross and/or multiple slip events irrespective of the processing regime and substructural refinement. The fracture mode of the tensile samples changes from ductile to brittle type between ECAP and 95 pct CR and is attributed to the reduced work hardening capacity of the latter. The modified Hall–Petch equation shows that the convergence of high-angle boundary spacing values with their low-angle counterparts results in an increased contribution via boundary strengthening to the 0.2 pct proof stress and UTS.     

Effect of Flowing Lead–Bismuth Eutectic on the Mechanical and Structural Properties of Stainless Steel 304L

Stainless steel 304L is being considered as a structural material for a component in a critical facility in an environment of molten lead–bismuth eutectic (LBE) at a temperature between 250 and 350 °C. This paper gives results on the effect, of 10,000 h exposure to non-isothermal liquid LBE at temperatures of 250 and 350 °C, on the mechanical and structural properties of SS 304L. The dissolved oxygen concentration in the molten eutectic was 4 × 10^?10 wt% and flow velocity was 16 cm/s. In order to assess the changes in mechanical properties tensile tests were carried out in air at 25 °C and fractography of fractured surface was observed by scanning electron microscopy. The results showed no change in the mechanical properties of SS 304L after 10,000 h exposure to LBE at 250 and 350 °C. Electron probe microanalysis of the interface of SS 304L with LBE showed that there was no penetration of LBE into the grain boundaries nor preferential dissolution of any of the components of steel in LBE after 10,000 h exposure at either temperature. No oxide layer was observed on the surface of SS 304L.     

Effect of Yttrium on Mechanical Properties and Microstructure of Ceramets

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Effect of Mn Content on Microstructure and Mechanical Properties of Fe–Mn–Cr–C High-Manganese Steels Produced Using Laser-Directed Energy Deposition

The microstructure and mechanical properties of high-manganese steel (HMnS) fabricated using laser-directed energy deposition (LDED) with Fe–Mn–4Cr–0.4C alloys with Mn contents of 13, 18.5, and 24 wt% are investigated. Additionally, the effect of annealing heat treatment on the microstructure and mechanical properties of the deposited HMnS is examined. Regardless of the manganese content, the deposited HMnS exhibits a fine microstructure without any defects (cracks or voids) and a strong fibrous texture along the <001>//building direction of the primary austenite phase. In addition, regardless of the manganese content, the grain size increases during annealing heat treatment, and the hardness decreases as the annealing temperature increases. The strength tends to decrease as the Mn content increases in the as-built state. In addition, regardless of the Mn content, the yield strength and ultimate tensile strength tend to decrease owing to the effect of annealing heat treatment. Although the maximum elongation of 18.5Mn and 24Mn does not change significantly upon heat treatment, the maximum elongation of 13Mn is greatly reduced by annealing. The deformation behavior of HMnS is characterized by transformation-induced plasticity (TRIP) for 13Mn and both TRIP and twinning-induced plasticity for 18.5Mn and 24Mn.     

Effect of Bonding Temperature on Microstructure and Mechanical Properties of WC–Co/Steel Diffusion Brazed Joint

In this work, the diffusion brazing of AISI 4145 steel to WC–Co cemented carbide using RBCuZn-D interlayer with bonding temperature values of 930, 960, 990 and 1020 °C was studied. The microstructure of the joint zone was evaluated by scanning electron microscope (SEM) and X-ray diffraction (XRD). Vickers microhardness and shear strength tests were performed to investigate mechanical behaviors of the brazed joints. The XRD and SEM results indicated that with increase of bonding temperature, the elements readily diffused along the interface and formed various compounds such as γ, α and β and Co₃W₃C. The results also showed that with the increase of bonding temperature from 930 to 960 °C, a sound metallurgical bond was produced, however in higher bonding temperatures (990 and 1020 °C) a decrease in mechanical properties of the joints was observed which could be due to the excessive zinc evaporation, interface heterogeneity and voids formation. The maximum shear strength of 425 MPa was obtained for the bond made at 960 °C.     

Effect of Rare Earth Composite Modification on Microstructure and Properties of a New Cast Hot-Work Die Steel

The effects of RE modification on structure and the properties of a new cast hot-work die (CHD) steel were investigated. The grains of the CHD steel are refined by RE modification. With the increase of RE addition, both grain size and inclusion amount are reduced. Appropriate amount of RE results in decrease in inclusion amount and formation of spheroidal inclusions uniformly - distributed in steel, so that the morphology and distribution of inclusions are improved. RE composite modification favors the formation of bainite, austenite and fine lath martensite with dense dislocation. When the residual RE content reaches 0.02 %, no obvious changes in strength and hardness are found, while fracture toughness and threshold of fatigue crack growth are increased. The impact toughness, elongation and reduction of cross sectional area are increased by a factor of two, and thermal fatigue resistance is also improved.     

Studies on Welding and Sensitization of Chrome–Manganese Austenitic Stainless Steel

The standard composition of austenitic stainless steel (ASS) includes Cr and Ni. However, due to rising cost of Ni, search for suitable alternative is always on. Manganese, on account of being an austenite stabilizer, can be considered as a replacement of Ni. Chrome–manganese steel (part of 200 series of ASS) contains chromium (≤15%) and low nickel. However, there is not enough confidence about their service life. Present work is an attempt to study the effect of welding and sensitization characteristics of this steel. The heat affected zone (HAZ) is identified microstructurally as per ASTM standard A262 Practice A test. Time temperature sensitization (TTS) diagram is established and critical cooling rate (CCR) is estimated. double loop electrochemical potentiokinetic reactivation (DLEPR) technique is used to quantify degree of sensitization (DOS) for heat treated and welded samples. The DOS for thermally aged sample at 750°C for 180 min is found to be 32.56% whereas for welded sample it is 31.30%. The results are discussed and mechanism of micro-structural changes due to welding of such steel is suggested.     

Thermal Scratch on Surface of SUS430 Stainless Steel Strip in Cold Rolling Process

The thermal scratch significantly influences the surface quality of the stainless steel in cold strip rolling.The thermal scratch has a close relation to the rolling parameters,the rolls surface and the emulsions used in rolling.In order to explain the thermal scratch on the strip surface,the cold rolling process of SUS430stainless steel strip was investigated in the laboratory.The thermal scratch defect occurs frequently in the second rolling pass(maximum reduction in height is 32.3%),especially on the lower surface of strips.When concentration and temperature of the emulsion are the same,the thermal scratch on the surface of the strip is aggravated with increasing the roll surface roughness.With the same roll surface roughness and emulsion concentration,the thermal scratch is obviously more severe at an emulsion temperature of 63℃than 55℃.With the same roll surface roughness and emulsion temperature,the thermal scratch is distinctly weaker at the emulsion concentration of 6%than that of 3%.     

Effect of Friction Stir Processing on Microstructure and Mechanical Properties of a Cast-Magnesium–Rare Earth Alloy

Single-pass friction stir processing (FSP) was used to increase the mechanical properties of a cast Mg-Zn-Zr-rare earth (RE) alloy, Elektron 21. A fine grain size was achieved through intense plastic deformation and the control of heat input during processing. The effects of processing and heat treatment on the mechanical and microstructural properties were evaluated. An aging treatment of 16 hours at 200 °C resulted in a 0.2 pct proof stress of 275 MPa in the FSP material, a 61 pct improvement over the cast + T6 condition.     

Effect of Ageing at 700 ℃ on Microstructure and Mechanical Properties of S31042 Heat Resistant Steel

 To investigate the effect of high temperature ageing on the microstructure and mechanical properties of S31042 steel, solid solution treatment at 700 ℃ was carried out for various time from 10 to 6000 h. Experimental results showed that the change of mechanical properties is closely related to the amount of precipitated phases. During ageing from 10 to 300 h, precipitation in the tested steel increases rapidly, and correspondingly, the high temperature yield strength and room temperature hardness of tested steel increase rapidly. Meanwhile, the thickness of the secondary phase on grain boundaries widens sharply and the room temperature Charpy impact absorb energy decreases. Ageing beyond 300 h, the precipitation in the steel increases gradually and the precipitates coarsen to a certain extent. The high temperature yield strength of the steel keeps stable, and the room temperature Charpy impact energy and hardness decrease slowly. Ageing beyond 3000 h, the mechanical properties of the steel tend to be stable. The main precipitates are M23C6, NbCrN and NbC in the tested steel.     

Effect of Coiling Temperature on the Microstructure and Mechanical Properties of Hot-Rolled Ti–Nb Microalloyed Ultra High Strength Steel

A novel low carbon Ti–Nb microalloyed hot rolled steel with minimum yield strength of 700 MPa and good balance of stretch-flangeability and impact toughness has been developed by controlled thermo-mechanical processing following thin slab direct rolling route. In the present work, the effects of two coiling temperatures on the resulting microstructure, micro-texture and mechanical properties on this Ti–Nb microalloyed steel have been studied. It is observed that increase in coiling temperature from 520 to 580 °C significantly affects the mechanical properties. Higher dislocation density and increased precipitation along with slightly smaller grain size is observed for 580 °C coiling temperature resulting in about 50 MPa increase in yield and tensile strengths as compared to 520 °C coiling temperature.