Microstructural evolution in ultra-low-carbon steel weldments—Part I: Controlled thermal cycling and continuous cooling transformation diagram of the weld metal

The transformation behavior and microstructural evolution of the as-deposited weld metal from an ultra-low-carbon (ULC) weldment were characterized by dilatometry, optical microscopy, transmission electron microscopy, and microhardness measurements. These results were used to construct a continuous cooling transformation (CCT) diagram for this weld metal. The major microconstituents observed in this ULC weldment were (in order of decreasing cooling rate) coarse autotempered martensite, fine lath martensite, lath ferrite, and degenerate lath ferrite. No polygonal ferrite was observed. These results were also used to develop criteria to differentiate between the two predominant microstructures in these ULC steels, lath martensite, and lath ferrite, which can look quite similar but have very different properties.     

Stress-induced transformation in a carburized steel—Experiments and analysis

Stress-induced transformation of austenite to martensite was studied experimentally and numerically to gain an understanding of the increase in transformation strain with increasing tensile both and decreasing temperature. In addition, the anisotropy of the transformation strains was determined both experimentally and numerically. The material chosen for the study was a carburized 4320 steel with 35% retained austenite content. Monotonic and cyclic experiments were conducted in the range from -60 to 150°C. At 22°C, the volumetric transformation strain reached 0.006 at fracture in the uniaxial tensile test and 0.004 in the torsion test. Numerical calculations of the volumetric transformation strain and the anisotropy of the transformation strains were obtained with a modified Eshelby model where planes favorably oriented gradually transformed as stress was applied. The analysis predicted the experimentally observed transformation strains under both uniaxial and torsional loadings. The model also predicted the anisotropy of the transformation strains that was observed in experiments.     

Atom probe analysis of 2.25 Cr1Mo steel

Atom probe mass analysis was conducted on 2.25 Cr1Mo and 2.25 Cr steels containing a small amount of P and problems associated with this analysis were described. A reliable compositional analysis by the atom probe of these low alloying steels was demonstrated by comparing the compositions determined by the atom probe and by a chemical analysis. In order to apply the atom probe technique to the study of temper embrittlement of 2.25 Cr1 Mo steel, compositional variations with aging time at 773 K of alloy elements and P in ferrite phase were examined. It was found that Cr and P concentrations in ferrite increased within 10 h of the early stage of aging and then gradually decreased with further aging, while Mo concentration simply decreased with aging. It was suggested that P atoms in as tempered steel were concentrated in ferrite-cementite boundaries and they were dissolved into ferrite during a process in which M₃C was replaced by M₂C in the early stage of aging. These dissolved P atoms were then segregated to grain boundaries causing the temper embrittlement. It was found that the presence of Mo retarded the decrease of P concentration in ferrite with aging thus suppressing the grain boundary segregation of P.     

Cast-structure defects in the axial zone of 38XH3MΦA steel ingots and their behavior in upsetting

The structure and formation of defects in the axial zone of ingots with a different bottom configuration are considered. Computer simulation reveals the behavior of these defects during deformation. The structure is denser in the experimental ingots, because of the improved solidification conditions in the axial zone and the configuration of the shrinkage defects after upsetting. Only traces of the cracks remain.     

Rare Earth microalloied elements’ influence on the organization and capability of X65 pipeline steel

With the X65 pipeline steel as raw material,we researched the influence of Ce on the organization and the low-temperature impact toughness.The result of the experiment was that:Ce had big influence on the organization and mechanics capability of X65 pipeline steel,Ce could thin the solidification to improve the capability of the steel.1# sample that was X65 pipeline steel without RE had dimple and uneven,its dimple and grain size was biggish;1# sample with RE’s dimple was small and uniform.The low-temperature impact and impact energy of 2# sample with RE were both higher.     

Microstructural evolution in a 17-4 PH stainless steel after aging at 400 °C

The microstructure of 17-4 PH stainless steel at various stages of heat treatment, i.e., after solution heat treatment, tempering at 580 °C, and long-term aging at 400 °C, have been studied by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). The solution-treated specimen consists largely of martensite with a small fraction of δ-ferrite. No precipitates are present in the martensite phase, while spherical fcc-Cu particles are present in the δ-ferrite. After tempering for 4 hours at 580 °C, coherent Cu particles precipitate in the martensite phase. At this stage, the Cr concentration in the martensite phase is still uniform. After 5000 hours aging at 400 °C, the martensite spinodaly decomposes into Fe-rich α and Cr-enriched α′. In addition, fine particles of the G-phase (structure type D8 _a , space group Fm m) enriched in Si, Ni, and Mn have been found in intimate contact with the Cu precipitates. Following spinodal decomposition of the martensite phase, G-phase precipitation occurs after long-term aging.     

Metal interlayers to prevent ‘hard zone’ formation in dissimilar weldments of Cr-Mo steels — A comparison between Cu,Co and Ni

High temperature exposure of dissimilar ferritic steel weldments either during service or during post weld heat treatment has been found to result in the formation of deleterious ‘hard’ and ‘soft’ zones near the weld interface. Activity gradient driven carbon diffusion was found to be responsible for the formation of the zones. One of the methods suggested for suppression of the zone formation is by use of carbon diffusion barriers. The effectiveness of this method was investigated in this study using numerical simulations based on finite difference method. Diffusion barriers like copper, cobalt and nickel were evaluated for this purpose due to their positive interaction parameter, ? _C ^M with carbon. It was found that these interlayers if introduced between 9Cr-1Mo and 2¼Cr-1Mo ferritic steels reduce the propensity for formation of hard and soft zones. It was found by computation that their efficiency correlates well with the differences in interaction parameter with carbon.     

Microstructural aspects of strain localization in AlMg alloys

The literature contains a number of continuum plasticity models describing the onset of strain localization but little definitive work which describes the microstructural transitions which accompany localization. In the present study a range of metallographic methods have been used in order to observe the progression of localization from events within single grains to the spatial organization of these events across the entire sample. The deformation mode used was cold rolling and observations were made using a variety of orientations relative to the rolling direction. It is concluded that in the AlMg system, localization begins by a structural instability in the accumulated dislocation substructure and later becomes organized into macroscopic bands due to local stress concentrations. The structure of the macroscopic bands is complex. They contain some high angle boundaries suggesting that they form due to the rapid and cooperative action of a number of slip systems over distances of the order of 0.2 μm. The bands cross grain boundaries and are organized in a cooperative sense because they represent local softening events. Thus, the shear bands in AlMg appear to form without texture softening or the need for the precursor of a lamellar structure. They involve a dramatic local change in the process of dislocation accumulation which is essentially a form a local dynamic recovery. The events become spatially organized to form macroscopic bands inclined at approx. 35° to the rolling plane as required by continuum plasticity.     

Relationship between tension toughness and fracture toughness of low carbon low alloy steel北大核心CSCD

Basic mechanical properties for four kinds of low carbon and low alloy steels, including S355, S275, Q345D, Q345E, were examined by means of tension toughness and plane strain fracture toughness tests at room temperature to -100 °C Test results of yield strength, tensile strengh, tension toughness, fracture toughness and ductile-brittle transition temperature for each steel were obtained for the sake of identifying the relationship between tension toughness and fracture toughness. More than 200 groups of samples were tested and analysed. It shows that there is a linear relationship between tension toughness Uk and fracture toughness J0.2BL for each steel at a tempearture which is greater than its own ductile-brittle transition temperature respectively. And this sastis-fied the requirement of the linear equation.     

Laws of nitriding of alloy steel in a plasma–arc remelting furnace

Metal drop samples are taken during plasma–arc remelting of 12Kh25N16G7AR steel billets. An analysis of the drops shows that the fraction of steel nitriding in the drops accounts for 20–40% of the total steel nitriding at a nitrogen content of 0.06% in a billet. An equation is derived to estimate the nitriding of a liquid Cr–Ni–Mn steel from the nitrogen concentration in the billet to be remelted.     

Material expenditures in deep desulfurization of steel in the ladle–furnace unit

The desulfurization of steel in a 160-t casting ladle is investigated. On that basis, a technology for reducing the sulfur content of low-carbon and low-alloy pipe steel to 0.001–0.003% is developed. The material expenditures in deep desulfurization of steel in a 160-t ladle–furnace unit are assessed.     

Microstructural characteristics of quasicrystalline phases in alloys of AlCuFeCo

An investigation of the structural characteristics of quasicrystalline phases obtained in quaternary alloys is carried out. The coexistence between the quasicrystalline phases phases is analyzed. HREM images and diffraction patterns which correspond to the decagonal and icosahedral phases show pronouned deviations from the perfect decagonal and icosahedral symmetries. These effects are more pronounced when the specimen is annealed. Both kind of quasicrystalline phases show planar faults and dislocation-type of defects. Planar faults show the same image contrast features as in the crystalline case. Evidence is presented based on image contrast characteristics of two different types of planar faults. Dislocations in these phases show no evidence of spliting into partials.     

Rolling-contact-fatigue behavior in backup roll of 5 ％ Cr forged steel

With a focus on the backup roll,a rolling-contact-fatigue experiment was performed on samples of 5％ Cr forged steel.The P-S-N fatigue curves were determined and the fatigue strength was calculated.The emergence of cracks on the test-sample surfaces was observed at different fatigue cycles.A micro-hardness tester was used to measure the hardness of the subsurface fatigue layer.The microstructures were analyzed at various magnifications with an optical microscope,scanning electron microscope,and transmission electron microscope.Based on these tests,the rolling-contact-fatigue mechanism of the large forged steel backup roll was also considered.The results showed that the contact-fatigue strength of the tested backup roll steel was 1 249 MPa;the surface fatigue crack lengthened continuously as the number of cycles increased and followed an S-shaped curve;the subsurface fatigue hardness reached its highest value at about 90 (HV) increment from the matrix hardness of 540 (HV) in the backup roll;the subsurface martensite/bainite microstructure was crushed and the dislocation density was greatly increased.Under alternating contact stresses,the surface/subsurface material was damaged and exhibited many microdefects.At the least,the surface fatigue layer on backup rolls should be fully removed before the microcracks enter a period of rapid propagation.     

Early-stage widmanstatten growth of the γ phase in a duplex steel

The present work studies the Widmanstatten γ-phase morphology and crystallographic orientation relationships with ferrite during its initial-stage growth in a duplex steel. Transmission electron microscopy (TEM) and convergent-beam electron diffraction (CBED) analyses have been applied to determine the orientation relationships with high precision. When the secondary γ phase and the α phase have special orientation relationships close to the K-S (Kurdjumov-Sachs) relation, the preferential growth involves γ phase, protruding into α phase and this leads, finally, to the formation of Widmanstatten needles. The needles grow nearly along the invariant-line direction, but they do not satisfy exactly the K-S relation, and a slight angular deviation (ϑ _[110]α ) is always present. This deviation is explained by a compromise between the well-known invariant-line condition, which offers the easiest way for directional growth of the γ phase, and a good atomic matching of the close-packed (111) _γ and (110) _α planes of both phases. A matching minimum at ϑ _[110]α = 2.3 deg is obtained when only three pairs of the nearest-neighbor atoms are considered in an atomic misfit calculation.     

Characteristics analysis of inclusion of 60Si2Mn–Cr spring steel via experiments and thermodynamic calculations

A plant trial of the production of 60Si2Mn–Cr spring steel using silicon–manganese combined with aluminium to deoxidise was performed, and the characteristics of inclusions during ladle furnace refining, calcium treatment and in billets were investigated by scanning electron microscope–energy dispersive spectroscopy and thermodynamic calculations. The formation mechanisms of oxide and CaS inclusions are discussed. The experimental observation and thermodynamic analysis showed that calcium treatment cannot entirely modify large-size MgO·Al₂O₃ spinel inclusions into homogeneous CaO–MgO–Al₂O₃ inclusions, but formed a liquid xCaO·yAl₂O₃ layer on its surface. When the Al content was 0.05 mass%, [Mg], [Ca] and [O] in molten steel could be controlled at 2.7～5 ppm, 2.5～8 ppm and 4.1～5.2 ppm, respectively, to achieve inclusions in the low melting point region. A large amount of CaS was generated in the present process due to a higher sulphur concentration in the molten steel and an excessive amount of Ca–Si wire. To avoid/reduce its formation, the sulphur concentration should be controlled to below 70 ppm.