Effect of Tempering Temperature on Strength and Toughness of Novel Carbide-Free Bainite/Martensite Duplex Phase Steel

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Structure of a Carbon Steel–Stainless Steel Bimetal

Bimetallic samples may be produced by casting St3 structural steel between sheets of Kh18N9T stainless steel in a mold, with subsequent hot rolling of the three-layer sheet. Such samples have a structure in which Kh18N9T stainless steel appears at the outer edge on both sides, while the core consists of St3 structural steel. Analysis of the boundary between the steels confirms the absence of defects: it is continuous and of high quality. The microstructure of the junction is investigated by optical, scanning-probe, and electron microscopy. Three structural components are observed from the pearlitic to the austenitic steel: a weakened section of the ferritic layer; a strengthened section of the ferritic layer; and a dark-etching layer at the austenitic steel. The following results are obtained by scanning-probe microscopy—in particular, the constantforce contact method—and optical metallography: on approaching the boundary from the St3 steel, a carbon- free layer with purely ferritic structure is observed, rather than the usual structure for low-carbon steel, which consists of a ferrite matrix with pearlite colonies. On approaching the boundary from the Kh18N9T steel, a carburized layer is observed. In addition, the boundary includes an intermediate carbide layer (depth up to 50 μm). The change in microhardness in the region where the St3 structural steel meets the external layer of Kh18N9T stainless steel indicates considerable increase in strength of the materials. Elemental microanalysis of the St3 steel–Kh18N9T steel boundary reveals the change in concentration of the alloying elements on approaching the boundary. The presence of chromium in St3 steel and the increase in carbon concentration in Kh18N9T stainless steel confirm that two opposing diffusional fluxes are formed: the diffusion of carbon from the St3 steel; and the diffusion of alloying elements from Kh18N9T steel. The resulting carbides explain the increased hardness of both steels close to the boundary.     

Enhancement of the Mechanical Properties of a V–Ti–N Microalloyed Steel Treated by a Novel Precipitation-Quenching & Partitioning Process

In this study, a novel precipitation-quenching & partitioning (P-Q&P) process was proposed by combining a proper intermediate holding treatment with the Q&P process, which successfully increased the strength of a V–Ti–N microalloyed steel without sacrificing the plasticity. However, the impact toughness of the P-Q&P samples is lower than that of the Q&P sample. Compared to the Q&P sample, the P-Q&P samples have more retained austenite. In addition, coarser substructures of martensite and bainite were formed in the P-Q&P samples. All the P-Q&P and Q&P samples contain two types of carbonitrides, which are the large-size particles (enriched in Ti) formed or undissolved in austenite and the small-size particles (enriched in V) formed in martensite and bainite. The P-Q&P samples have a smaller size and larger volume fraction of the large-size particles than the Q&P sample. The increase in the strength of the P-Q&P samples is attributed to the precipitation strengthening of the carbonitrides formed in austenite during the intermediate holding treatment. And the maintained elongation is mainly caused by the higher austenite content in the P-Q&P samples. The poor toughness of the P-Q&P samples is mainly resulted from the coarser substructures.

     

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.     

The Effect of Cold Rolling on Microstructure and Mechanical Properties of a New Cr–Mn Austenitic Stainless Steel in Comparison with AISI 316 Stainless Steel

In the present study the effect of room temperature rolling on microstructure and mechanical properties of a new Cr–Mn austenitic stainless steel (containing 12 %Cr, 23 %Mn and 0.13 %C) and AISI 316 steel was investigated. The specimens of these steels were cold rolled at various thickness reductions of 0, 12, 25, 37 and 50 %. Microstructural investigations were carried out using optical microscopy, magnetic field test and X-ray diffraction technique. Hardness and tensile test methods were also done to evaluate the mechanical properties. Results showed that some of austenite phase transformed to martensite during cold rolling in the 316 steel, while there was no strain induced transformation in the Cr–Mn steel. It was also found that the newly developed steel had higher strength and higher specific strength than those of the 316 steel, while its ductility was the same as that of the 316.     

Early Martensitic Transformation in a 0.74C–1.15Mn–1.08Cr High Carbon Steel

Metallurgical and Materials Transactions A - The martensitic transformation in a high carbon steel was studied by a new experimental approach focusing on the nucleation and growth as well as the...     

Resistance Spot Welding of Quenching and Partitioning (Q&P) Third-Generation Advanced High-Strength Steel: Process–Microstructure–Performance

Metallurgical and Materials Transactions A - This paper investigates process–microstructure–performance relationships in Q&P980 third-generation advanced high-strength steel...     

Multilayer Mg–Stainless Steel Sheets,Twinning and Texture Evolution

In the present study, different combinations of multilayer sheets were prepared from 1 and 2 mm Mg AZ31 along with 0.25, 0.5, and 1 mm 304 L stainless steel. The texture and microstructure of the elongated samples (20 and 30 pct strain) were studied. It was found that the transversal stress plays an important role in both texture evolution and twinning in these composites. The obtained pole figures revealed an axial texture tilt with increasing steel layer volume fraction (V _f). It was found that this is a direct effect of transverse stress, which becomes more significant upon reducing Mg V _f. This extra stress component tilts the basal planes away from the original normal direction in monolithic samples. Moreover, our results indicate that with decreasing Mg V _f, twinning activity was increased in the 20 pct deformed samples but reduced in the samples with 30 pct elongation. It is known that at high strains where sufficient transverse stress is generated, the activity of prismatic slip is significantly enhanced, which promotes the motion of dislocations and reduces the necessity of twinning. With decreasing Mg V _f, stronger transversal stress is generated and Mg reaches the critical threshold of prismatic activity at lower strains.

     

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.     

Erosion and Corrosion of Mild Steel and Stainless Steel in Sodium Carbonate／Bicarbonate Solution Bearing Alumina Particle

With a rotating cylinder electrode apparatus, the polarization behaviors of the mildsteel and the stainless steel 0Crl8Ni9 in NaHCO3 (0.5M) NazCO3 (0.5M) solution with andwithout erodent particles were investigated and compared. The results show that the rotationspeed of cylinder hardly affects the polarization behavior of sample in solution without particlesbut exerts a great influence on that with particles. Increasing rotation speed, the free corrosionpotential shifts to positive direction and the oxygen limiting current density increases. Both themild steel and stainless steel 0Crl8Ni9 experience a significant increase of the mass loss by in-creasing erosion, and erosive wear was dominated by severe micro-plowing. The insufficientmechanical strength of both materials leads to a low resistance to particle removal. Increasingperipheral velocities of the rotating cylinder enhances the corrosion rate of the mild steel. Thestainless steel 0Crl8Ni9, due to a high erosive wear, also suffers from similar erosion-corrosiondamage, despite that its corrosion resistance is much higher than that of the mild steel.     

Effects of Cerium Addition on Inclusions and Solidification Structure of a Low-Nickel Si–Mn-Killed Stainless Steel

The effect of cerium on inclusions and solidification structure of a low-nickel Si–Mn-killed stainless steel is studied using laboratory experiments. When the cerium content in steel increased from 0 to 250 ppm, modification sequence of inclusions is Si–Mn(–Al)–O and MnS → Ce–Si–Mn–O–S → Ce(–Si)–O–S → CeS and CeC₂. The number density and area fraction of inclusion first decrease with the increase in the cerium content and then increase due to the formation of CeC₂ inclusions when the cerium content is bigger than 150 ppm, which is precipitated in solid steel during solidification. When the cerium content increases from 0 to 250 ppm, the fraction of equiaxed grain zones of steel ingot first increases and reaches a maximum value when the cerium content is 54 ppm; then the fraction of equiaxed grain zones decreases with the increase of the cerium content. 2D lattice misfit calculations are performed and it is found that there are no heterogeneous nucleation cores in the steel without cerium during solidification. For the steel with cerium, Ce_4.67Si₃O₁₃, Ce₂O₂S, and CeS inclusions act as heterogeneous nucleation cores, increasing the fraction of the equiaxed grain zone. Bigger effective heterogeneous nucleation cores number density leads to a larger fraction of the equiaxed grain zone.     

Production Conditions and Properties of Layered Materials Type of Composite Based on a Stainless Steel ― Steel

Fabrication conditions and some properties of wear-resistant and corrosion-resistant layered materials type of composite based on a stainless steel steel were investigated. The materials were produced by joint pressing and sintering of the layers. The working layer was based on stainless steel Kh18N15 with additions of Cr₃C₂ and MoS₂, and the substrates were unalloyed iron, low-alloy chromium steel, and stainless steel. The quality of layer bonding was evaluated by metallographic investigation and electron-probe microanalysis of the interface between layers, and also determination of the strength properties. It was established that the most favorable conditions for structure and property formation in the layered composites are obtained by using a stainless steel substrate and a working layer with an intermediate sublayer.     

Structure Evolution and Solidification Behavior of Austenitic Stainless Steel in Pulsed Magnetic Field

 To understand the solidification behavior of austenitic stainless steel in pulsed magnetic field, the solidification process is investigated by means of the self made high voltage pulse power source and the solidification tester. The results show that the solidification structure of austenitic stainless steel can be remarkably refined in pulsed magnetic field, yet the grains become coarse again when the magnetic intensity is exceedingly large, indicating that an optimal intensity range existed for structure refinement. The solidification temperature can be enhanced with an increase in the magnetic intensity. The solidification time is shortened obviously, but the shortening degree is reduced with the increase of the magnetic intensity.     

Fatigue Strength Improvement of Welded Joint by Ultrasonic Peening in Ultra-Fine Grain Steel

Thefatiguestrengthisthemostimportantser vicepropertyofaweldedjoint .Duetotheexistenceoftensileresidualstressesand geometricaldisconti nuityacrosstheweld ,thefailurenormallyoccursattheweldedjoint .Hence ,itiscommonforimportantstructurestoapplyastressrelievingheattreatmentafterwelding .Thisis ,however ,prohibitedforweldedjointsinultra finegrainsteels .Thegrainre finementistheonlystrengtheningmethodthaten hancesbothstrengthandtoughness .IntheNationalKeyFundamentalResearchDevelopmentProject“th…