Showing papers on "Bainite published in 2003"

Acceleration of Low-temperature Bainite

Abstract: Recent work has shown that bainitic ferrite plates produced by transformation at low temperatures can be as thin as 20 nm with a hardness in excess of 650 HV. However, it may take several days in order to achieve the required degree of transformation at low temperatures. In this work we report methods for accelerating the rate of reaction without compromising strength.

Development of Hard Bainite

Abstract: It is demonstrated that in a high-carbon steel where carbide precipitation is suppressed, bainite can be obtained by isothermal transformation at temperatures as low as 200°C. The time taken for nucleation at this temperature can be many days, but the transformation results in the growth of extremely thin platelets of bainite, so thin that the hardness of the resulting steel can be greater than 600 HV.

Decomposition of cementite in pearlitic steel due to plastic deformation

Abstract: The available experimental data and hypotheses concerning cementite decomposition during the cold work of pearlitic steels are reviewed. The results of studies performed using thermomagnetic analysis, Mossbauer spectroscopy, internal friction and APFIM are used to discuss the mechanism governing cementite decomposition. The following features of this phenomenon seem to be important: (i) the fraction of the decomposed cementite increases with the refining of the initial pearlitic structure, i.e. with the increase of the ferrite–cementite interfacial area; (ii) the decomposition effect saturates as strain increases; (iii) carbon–dislocation interaction in ferrite and MeC bonding in cementite have a strong influence on cementite decomposition. The conclusion is made that cementite decomposition is controlled by the transfer of carbon atoms from cementite to dislocations accumulated near the interface during deformation. This is because the binding enthalpy between carbon atoms and dislocations in ferrite exceeds the solution heat of cementite. Some relevant effects of cementite decomposition in practice are discussed.

The influence of chemical composition and microstructure of API linepipe steels on hydrogen induced cracking and sulfide stress corrosion cracking

Abstract: The chemical composition and microstructure are known to have a significant effect on the resistance to hydrogen induced cracking (HIC) and sulfide stress corrosion cracking (SSCC) of structural steels in wet H2S environments. In this paper, the influence of microstructure on HIC and SSCC behavior of two low C–Mn–Nb–Mo API linepipe steels has been investigated. Subjecting the steel to different thermomechanical processes modified the microstructure. The results showed that refined and homogeneous quenched and tempered bainite/martensite microstructures had the best performance with respect to both HIC and SSCC susceptibility.

Friction stir welding of DH36 steel

Abstract: Hot rolled DH36 carbon steel, 6.4 mm in thickness, was friction stir welded at speeds of 3.4 mm s-1 (8 in min-1), 5.1 mm s-1 (12 in min-1), and 7.6 mm s-1 (18 in min-1). Single pass welds free of volumetric defects were produced at each speed. The relationships between welding parameters and weld properties are discussed. Optical microscopy, microhardness testing, and transverse and longitudinal tensile tests have been performed. Bainite and martensite are found in the nugget region of the friction stir welds whereas the base material is comprised of ferrite and pearlite. The maximum hardness is observed in the weld nugget, and the hardness decreases gradually from the weld nugget, through the heat affected zone, to the base metal. Tensile testing also indicates overmatching of the weld metal relative to the base metal. Maximum hardness and longitudinal (all weld metal) tensile strengths increase with increasing welding speeds. Weld transverse tensile strengths are governed by the base metal prope...

Kinetics of martensite decomposition in Ti /6Al /4V /xH alloys

Abstract: The kinetics of martensite decomposition in hydrogenated Ti–6Al–4V alloy samples was studied. To produce the martensite structure, the samples containing 0, 10, 20 and 30 at.% H were annealed in the β phase field and water quenched. Optical microscopy, transmission electron microscopy, X-ray diffraction analysis and microhardness testing techniques were utilized to study the phases and phase transformations and to obtain time–temperature-transformation (TTT) diagrams for the martensite decomposition. The martensite structure of the hydrogenated samples consisted of a mixture of hexagonal close packed (hcp) α′ and orthorhombic α″ martensites. The amount of the orthorhombic α″ martensite increased from 0 to ∼80 vol.% when the hydrogen content of the alloy was increased from 0 to 30 at.%. During aging at temperatures below the β transus temperature and above the martensite start temperature (Ms), the martensite structure transformed into a mixture of hexagonal close-packed (hcp) α and body-centered-cubic (bcc) β phases. At aging temperatures below the Ms, on the other hand, the martensite first transformed partially into a metastable β phase, and then equilibrium α and β phases were formed. On quenching after aging, in both these cases, the β transformed into martensite plus residual β, with the amount of the latter increasing with an increase in the hydrogen concentration and a decrease in the aging temperature. Hydrogen additions lowered the Ms of the Ti–6Al–4V alloy, and for samples containing 30 at.% H the Ms was below 500 °C. In the alloys containing 20 and 30 at.% H, a hydride phase was also detected. Complete decomposition of the martensite structure in the samples containing 30 at.% H and aged at 530 °C resulted in a fine and homogenous equiaxed microstructure consisting of a mixture of α, β and hydride phases. The nose temperature for the start of the martensite decomposition decreased from 800 to 625 °C when the hydrogen concentration increased from 0 to 30 at.%. The nose time for the start of the martensite decomposition increased from 6 s to 10 min when the hydrogen concentration increased from 0 to 10 at.% and did not change significantly with further increase in the hydrogen concentration.

Low temperature bainite

Abstract: It is demonstrated that bainite can be obtained by isothermal transformation at a temperature which is so low that the calculated diffusion distance of an iron atom, over the time scale of the experiment, is many orders of magnitude less than an interatomic spacing; and yet, it is possible to obtain plates of bainite which are a few micrometers in length. Bainite has been obtained by isothermal transformation at 400 K. The time taken for nucleation at this temperature can be many days, but the transformation results in the growth of extremely thin platelets of bainite, so thin that the hardness of the resulting steel can be greater than 600HV.

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Abstract: Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.

Low cycle fatigue behavior of a multiphase microalloyed medium carbon steel: comparison between ferrite–pearlite and quenched and tempered microstructures

Abstract: In an attempt to improve fatigue and fracture resistance, a multiphase (ferrite–bainite–martensite) microstructure was developed in a V-bearing medium carbon microalloyed steel using a two-step cooling and annealing (TSCA) treatment following finish forging. The monotonic, cyclic stress–strain and low cycle fatigue behavior of this steel are reported. These results are compared with those of ferrite–pearlite and tempered martensite microstructures obtained by air cooling (AC) and quenching and tempering (Q&T), respectively. The tensile properties of the multiphase microstructure are superior to those of the ferrite–pearlite and the Q&T microstructures. Under cyclic loading, the ferrite–pearlite microstructure showed hardening at higher total strain amplitudes (≥0.7%) and softening at lower total strain amplitudes (<0.7%). The quenched and tempered and the ferrite–bainite–martensite (TSCA) microstructures displayed cyclic softening at all total strain amplitudes employed. Despite the cyclic softening, the ferrite–bainite–martensite structure was cyclically stronger than the ferrite–pearlite and the Q&T microstructures. Bilinearity in the Coffin–Manson plots was observed in Q&T and the multiphase TSCA conditions. An analysis of fracture surface provided evidence for predominantly ductile crack growth (microvoid coalescence and growth) in the ferrite–pearlite microstructure and mixed mode (ductile and brittle) crack growth in Q&T and the multiphase TSCA microstructures.

Analysis of effect of alloying elements on martensite start temperature of steels

Abstract: The stabilisation of austenite, a phenomenon that frequently occurs, renders the transformation from austenite to martensite difficult. The straightforward method of analysing the effect of a specific factor on the stabilisation of austenite is through its influence on the martensite start temperature M s. The present work outlines the use of an artificial neural network to model the M s of engineering steels based on their chemical composition and austenite grain size. The results are focused on elucidating the role in the stabilisation of austenite of alloying elements in steels, including less common elements such as vanadium and niobium, as well as the austenite grain size. Moreover, a physical interpretation of the results is presented.

Influence of thermo-mechanical processing and different post-cooling techniques on structure and properties of an ultra low carbon Cu bearing HSLA forging

Abstract: An ultra low carbon Cu bearing HSLA steel, micro alloyed with Nb and Ti was forged in two stages and subsequently cooled at different cooling rates. Variation in microstructures and the mechanical properties at different cooling rates have been studied. Volume fraction of bainitic phase has been found to vary with different cooling rates. Maximum strength was achieved in this steel at an intermediate cooling due to precipitation hardening of very fine microalloying carbides, carbonitrides and e-Cu precipitates. Finer polygonal ferrite and granular bainite structure obtained at the slowest cooling rate resulted in high impact toughness values at ambient and sub ambient temperatures.

Method for producing steel with retained austenite

Abstract: The present invention relates to a process for producing steel with retained austenite. In one embodiment, the process comprises the steps of heating a steel alloy to produce austenite, quenching the steel to produce martensite, and carbon partitioning to transfer carbon from the martensite to the austenite.

Relationship between nanohardness and microstructures in high-purity Fe-C as-quenched and quench-tempered martensite

Abstract: The relationship between the nanohardness and the microstructures in the Fe–C martensite was studied to understand the contributions of the matrix and the grain boundary to the macroscopic strength. As-quenched martensite was examined for five kinds of Fe–C alloys with various carbon contents in the range of 0.1–0.8 mass%, while quench-tempered martensite was investigated for an Fe–0.4% C alloy. The ratio of the nanohardness to the macrohardness Hn/Hv was much smaller for the Fe–C martensite than those for the single crystals, indicating that there is a significant grain-boundary effect for the martensite. The ratio Hn/Hv of the as-quenched martensite decreased with an increase in the carbon content since the size of the block structure decreased with increasing carbon content. For the quench-tempered specimens, a significant reduction of the grain-boundary effect occured at the tempering temperature of 723 K. It is mainly due to the depression of the locking parameter caused by the disappearance of the film-like carbides on the boundaries.

Effect of cooling rate on the as-quenched microstructure and mechanical properties of HSLA-100 steel plates

Abstract: The effect of cooling rate on the as-quenched microstructure and mechanical properties of a 14-mm-thick HSLA-100 steel using various cooling media such as brine, water, oil, air, and furnace has been studied. While quenching in brine, water, and oil resulted in lath martensite structures, the granular bainite and martensite-austenite (M-A) constituents were found in air- or furnace-cooled specimens. The average lath spacing increased slightly on decreasing the cooling rate (300 nm in brine-quenched specimen to 400 nm in oil-quenched specimen). The precipitates of Cu and Nb(C, N) were observed in all the quenching conditions except in the brine-quenched specimen. The as-quenched strength and toughness of the brine-, water-, and oil-quenched specimens were higher (yield strength: 894 to 997 MPa, ultimate tensile strength: 1119 to 1153 MPa, and Charpy V-notch energies: 65 to 70 J at −85 °C) than those of air- and furnace-cooled specimens (yield strength: 640 to 670 MPa, ultimate tensile strength: 944 to 1001 MPa, and Charpy V-notch energies: 10 to 20 J at −85 °C). For industrial production of HSLA-100 steel plates, oil or water quenching is recommended in lower thickness plates (<25 mm). For production of thicker plates, however, water quenching is more suitable.

On the nature of internal interfaces in tempered martensite ferritic steels

Abstract: This paper presents results on the evolution of microstructure in 12% Cr tempered martensite ferritic steels during heat treatment and creep. Prior austenite grain size and micro grain size measurements were performed using optical microscopy, and scanning and transmission electron microscopy. Moreover, orientations of micro grains are determined using TEM and SEM. Prior austenite grain sizes, micro grain dimensions and orientation relationships between micro grains are interpreted in terms of processes associated with the formation of martensite and ferrite during heat treatment and creep. The elongated micro grains (in the interior of prior austenite grains) can have high- or low-angle boundaries. The hierarchical evolution of internal interfaces is described. Micro grain boundaries most often represent (i) variant boundaries separating individual former martensite variants and (ii) subgrain boundaries (associated with recovery processes during tempering and creep). The present results also sho...

Mecanical properties of cementite and fabrication of artificial pearlite

Abstract: The deformation behavior of cementite was studied using eutectoid steels with pearlitic and spheroidite structures and fine grained bulk cementite prepared by sintering the mechanically alloyed powders. Cementite lamellae in deformed pearlite exhibited inhomogeneous slip, thinning by deck-of-cards slip, homogeneous bending, fragmentation, cleavage fracture and cracking by shear bands. Complete dissolution of cementite lamellae and spheroidal cementite was also observed in specimens deformed at high strain rates. The polycrystalline bulk cementite is brittle and fractures within elastic limit when deformed below 573 K. However, large plastic deformation was observed when compressed at high temperatures (over 773 K) and low strain rates (10 -5 ~ 10 -3 s -1 ). The artificial pearlite was successfully fabricated by hot compression starting from the

Structural change and straining in Fe–Pd polycrystals by magnetic field

Abstract: The effect of a magnetic field on martensite structures in Fe–30 and –30.5at.% Pd has been studied using optical and electron (TEM) microscopy. No appreciable change to the martensite structure is observed with optical microscopy when a magnetic field of ∼8×105 A m−1 is applied to a sample in the fully martensite state. The fractions of elementary martensite units (Bain correspondence variants—BCVs), existing as alternately stacked twins in a martensite plate and observed by electron microscopy, remain almost the same whether a TEM foil is under practically no magnetic field or under a field of more than ∼8×105 A m−1. Fe–Pd is more difficult to magnetically saturate in the martensite state than in the austenite state, even though the saturation magnetization is only slightly smaller in the former state. Hysteresis in a magnetization–magnetic field relationship is small. From these observations, it is concluded that the magnetization in the martensite state is mostly achieved by rotation of the magnetization vector, with hardly any contribution from the movement of magnetic domain walls. This is reasoned using the argument that the interfaces of BCVs offer large resistance to the movement of magnetic domain walls.

Process for producing high tensile hot-dip zinc-coated steel sheet of excellent ductility and antifatigue properties

Abstract: A hot rolled plate of a composition comprising 0.05 to 0.20 mass% of C, 0.3 to 1.8 mass% of Si, 1.0 to 3.0 mass% of Mn and the remainder consisting of Fe and inevitable impurities, having a composite texture consisting of ferrite main phase and second phase wherein the second phase comprises 80 vol.% or more of bainite and the remainder consisting of any one or two or more of martensite, retained austenite and pearlite and wherein the ratio of plate-thickness-direction average length to average length of the second phase is 0.7 or more, as a starting material, is subjected to cold rolling, given heat treatment, hot dip zinc coating and cooling operations. As a result, there can be obtained a high tensile hot-dip zinc-coated steel sheet exhibiting a ductility that is satisfactory for use as a material of automobile parts and also excellent antifatigue properties.

High-strength thin steel sheet superior in hydrogen embrittlement resistance, weldability, hole-expandability, and ductility and manufacturing method therefor

Abstract: PROBLEM TO BE SOLVED: To simultaneously improve hydrogen embrittlement resistance, weldability, hole-expandability, and ductility of a high-strength steel sheet having a tensile strength of 800 MPa or higher. SOLUTION: This steel sheet comprises, by mass%, 0.05-0.3% C, 0.01-3.0% Si, 0.01-4.0% Mn, 0.0001-0.020% P, 0.0001-0.020% S, 0.01-3.0% Al, 0.0001-0.01% N, and one or more elements of 0.001-5.5% Ni, 0.001-3.0% Cu, 0.001-5.0% Cr and 0.005-5% Mo, and the balance being iron with unavoidable impurities; has a microstructure comprising the main phase consisting of one or both of bainite and bainitic ferrite, the secondary phase consisting of austenite which occupies an area rate V γ of 3 to 30%, and the balance being ferrite and/or martensite; has a tensile strength TS of 800 MPa or higher; and satisfies the following expressions (1-1) and (1-2): 0≤0.8×{2Cu+20Mo+3Ni+Cr}-{0.1-3.5×10 7 ×(TS) -3.1 }-0.3V γ --- (1-1) and 0≤Si+Al+7.67C-1.78 --- (1-2). COPYRIGHT: (C)2005,JPO&NCIPI