Observations on the effect of cementite particles on the fracture toughness of spheroidized carbon steels

The results of experimental studies of the influence of cementite particles on the fracture toughness of a number of spheroidized carbon steels at low temperatures were analyzed in terms of current theories of crack-tip behavior. The fracture toughness parameterK _IC was evaluated by using circumferentially notched and fatigue-cracked cylindrical specimens. The conclusions are summarized as follows: 1) In general,K _IC decreases with increasing volume fraction and increasing size of the carbide particles. 2) Crack initiation occurs at the carbide particles. 3) Crack propagation occurs by cleavage if the stress conditions satisfy the Ritchie, Knott and Rice criterion that a critical cleavage stress is achieved over a minimum microstructural size scale. The critical stress is that required to propagate a crack from a particle and the minimum size scale is of the order of 1 to 2 grain sizes. 4) Crack propagation occurs initially by fibrous rupture if the stress intensification is insufficient to attain the critical cleavage stress. P. Rawal was formerly affiliated.     

Relation between formability and precipitates in Ti-IF enamelling steels

The influence of steel composition and processing conditions on the nature of precipitates in Ti interstitial free (IF) steels for enamelling were investigated in detail. Steels designed for enamelling applications require a higher amount of precipitates than conventional deep drawing IF steels, e.g. for automotive applications. Formable IF grades for enamelling applications have high Ti, S and C contents and the properties have hitherto not been investigated in detail. This work focused on the correlation between precipitate nature, texture development and deep drawability. It was found that a low reheating temperature, a high coiling temperature and a C/S ratio in the range of 0.4–0.5, favoured the beneficial precipitation of Ti₄C₂S₂. Furthermore, it is shown that for a steel with a given C/S ratio, a high Ti₄C₂S₂/Ti₄S₅-ratio resulted in improved deep drawability. The deep drawability decreases with increasing C/S ratio. This is due to the decrease in γ-fibre intensity, caused by an increased amount of TiC-precipitates.     

Bainite formation in low carbon Cr-Ni steels

A low carbon Cr-Ni steel has been used to investigate the formation of upper bainite. Experimental results indicate that the start temperatures of the three morphologies of upper bainite in this steel,i.e., carbide-free bainite, bainite with carbide between and within ferrite laths, are about 600°, 500δ, and 425 °C, respectively; the habit plane of bainitic ferrite in this steel is close to (1 7 11)_α, which is 13.3 deg away from (0 ll)_α; and the orientation relationship between cementite and ferrite is consistent with Bagaryatskii’s. By means of the superelement approach, a thermodynamic treatment which applies to Fe-C alloys is extended into that suitable for low alloy steels, and calculation shows that the driving force for bainite formation at B_S temperatures is insufficient to compensate for shear strain energy. Formerly Graduate Student, Department of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, People’s Republic of China.     

Microstructures of deformation and fracture of cementite in pearlitic carbon steels strained at various temperatures

In order to make clear the effect of temperature on deformation and fracture of cementite in steels, observations by transmission electron microscopy were made on cementite in carbon steels strained in tension at various temperatures ranging from -196 to 700°C. Hardly any plastic deformation of cementite was detected at-196 and-78°C. It was confirmed that above room temperature cementite in steel can deform due to dislocation slip and that the deformation becomes easier as the temperature increases. Slip in cementite at room temperature and at 300°C seems to be confined to only (100) or (001). Dislocations observed at room temperature and at 300°C were mostly isolated and straight. Above 400°C (100), (010), (001), and some {110} planes are all operative slip planes. Dislocation loops, dipoles, cusps, and networks were frequently found. These observations indicate that double slipping and the interaction of dislocations can occur and that the deformability of cementite above 400°C is very large. An appreciable degree of dynamic recovery was detected above 500°C. The fracture of cementite at -196 and-78°C occurred in a cleavage manner along some crystallographic planes such as (110), (100) or (210). Above room temperature fracture occurred along an activated slip plane and was preceded by some amount of slip on that plane. Above 400°C cementite fracture, caused by joining of voids, formed along an activated slip plane was frequently observed.     

Solute carbon and nitrogen in the ferrite after continuous annealing of low carbon low aluminium steels

Low N_sol levels not only require reduced N contents (≤ 20 ppm) in steel but also Al/N ratios higher than 5 and C contents higher than 0.025% because of the synergic effect between N and C precipitations; scavenging of Al on nitrogen is far from complete even in steels coiled at 750°C. Low C_sol levels are achieved either at very lowC contents (C ≤ 0.003%) or at C contents higher than 0.025%. Due to high C_sol, carbon aging is expected in steels coiled at 750°C. Softer, bake hardenable steels with good aging resistance will be obtained in the range 0.025–0.030 % C independently of the coiling temperature. C supersaturation of the ferrite is reduced at higher cooling rates after continuous annealing. Changes in C and N supersaturation according to the steel composition and the continuous annealing process are explained, taking into account the carbides morphology and the annealing conditions.     

Austenite stabilization from direct cementite conversion in low-alloy steels

Tr ansformation i nduced p lasticity (TRIP) effects associated with austenite dispersions in low alloy Fe-Mn-Si steels can be enhanced by austenite stabilisation. Austenite which forms during conventional intercritical annealing does not possess the required stability in order to exhibit TRIP effects. In this work, thermodynamic calculations indicated that it is feasible to form austenite by a cementite to austenite conversion which occurs under paraequilibrium conditions, i.e with partition of carbon but with no partition of substitutional alloying elements. In this way the austenite inherits the manganese content of cementite and is chemically stabilised. A treatment consisting of a two-step annealing has been examined. In the first step, soft annealing, an Mn-enriched cementite dispersion in ferrite is formed. In the second step, intercritical annealing, austenite nucleates on the cementite particles, which are consumed to form austenite. It was experimentally determined that this austenite has been enriched in manganese and carbon and, therefore, is stabilised. The conversion reaction is followed by the conventional austenite nucleation at ferrite grain boundaries. This austenite is lean in manganese and is not stable. The net effect of the two-step annealing treatment is a significant austenite stabilisation relative to simple intercritical annealing, indicating a potential for enhanced TRIP effects in this class of steels.     

Fundamental studies on the deoxidation of low carbon steels with ferrosilicon

Laboratory size (4.6 kg) low carbon-iron melts were deoxidized using plain ferrosilicon, ferrosilicon with calcium aluminate flux, and ferrosilicon with calcium silicate flux. The dissolved oxygen and total oxygen contents in these heats were measured as a function of time and temperature using oxygen probes and quenched pin samples taken from the melts. The dissolved oxygen values, as measured by the oxygen probes, indicated that the iron-silicon deoxidation reaches equilibrium within five minutes of reaction time. However, the total oxygen (dissolved oxygen+oxygen in oxide inclusions), as measured from the quenched pin samples, took almost twenty minutes to reach a steady state. The two aforementioned features were common to all the experimental heats. Below 1978 K the ferrosilicon heats with calcium silicate or calcium aluminate flux had lower steady state total oxygen values as compared to the plain ferrosilicon heats; the difference was more significant at lower quenching temperatures. Also, below 1978 K, the dissolved oxygen-temperature relationship of flux heats was identical to their steady state total oxygen-temperature relationship, which indicated that the addition of fluxes leads to elimination of oxide inclusions. The performance of calcium aluminate and the calcium silicate fluxes in removing the oxide inclusions was found to be identical in these laboratorysize heats. U. B. PAL, formerly Senior Metallurgist, Melting and Primary Operations, Allegheny Ludlum Corporation, Technical Center, Brackenridge, PA 15014     

Influence of boron on the decomposition of austenite in low carbon alloyed steels

The influence of boron on the isothermal decomposition of Fe-Ni₆-C_0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C_0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe₂₃(B, C)₆ (FCC structure with a ≈ 10.6?). The grain boundary Fe₂₃(B, C)₆ were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe₂₃(B, C)₆ precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.     

Influence of boron on the decomposition of austenite in low carbon alloyed steels

The influence of boron on the isothermal decomposition of Fe-Ni₆-C_0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C_0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe₂₃(B, C)₆ (FCC structure with a ≈ 10.6Å). The grain boundary Fe₂₃(B, C)₆ were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe₂₃(B, C)₆ precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.     

The tempering of low carbon steels containing tungsten

The tempering behavior of three steels each containing 0.20 pct C and having tungsten contents of 2.1, 3.9 and 5.9 pct has been followed by thermomagnetic analysis and electron microscopy. Using a Sucksmith Balance, the proportions of autotempered carbide and retained austenite in as-quenched specimens were estimated, and the amount of cementite precipitated upon subsequent tempering measured accurately. Solution of tungsten in cementite during tempering was monitored by observing changes in Curie temperature. The magnetic nature of alloy carbides precipitating at high temperatures allowed tentative identification and this directed and assisted the electron microscopy study which provided new information on the morphology of tungsten carbides. J. B. LUPTON, formerly Research Student, Metallurgy Department, University of Sheffield, England, S. MURPHY, formerly Research Fellow, Metallurgy Department, University of Sheffield,     

Twin roll strip casting of low carbon steels

Abstract

Strip casting of low carbon steels has been investigated using a laboratory twin roll machine with copper rolls. The following grades have been studied: a deep drawing low carbon (LC) steel, a LC steel with titanium addition, and a high strength low alloy (HSLA) grade. The casting behaviour of these steels has been examined, and the observed surface defects classified. Defects are most pronounced for the LC grade and significantly less for the HSLA steel and the LC steel with titanium addition. The as cast structure has been analysed. It can be modified by post-cast treatment, e.g. by normalisation or in line rolling. The mechanical properties of cold rolled and annealed strip materials and their textures are presented. Satisfactory sheet properties can be achieved both as hot band and as cold rolled sheet when adequate treatment steps are applied. Consequences for strip casting applications and future research are discussed.     

共析钢中片层状渗碳体冷轧后的形态变化

The pearlitic transformation and the deformation behavior of lamellar cementite after cold rolling in eutectoid steels Fe-0. 76%C-0. 137%Mn (mass fraction) were studied by means of Formastor-F (Full Automatic Transformation Testing Instrument) and field emission scanning electronic microscopy (FESEM) observation. Fine and coarse pearlite were obtained in the eutectoid steels austenitized at 900℃ for 15min, then hold at 620℃ for 90 s and 690℃ for 7 h, respectively. Thedeformation behavior of cold rolled lamellar cementite could be classified as: cleavage fracture, inhomogeneous slip, fragmentation, thinning or necking, and homogeneous bending. The cementite lamellae with the thickness of more than 100 um could be deformed plastically.     

Microstructures controlling the ductile crack growth resistance of low carbon steels

Microstructures controlling the ductile crack growth resistance in the ductile-brittle fracture transition region have been investigated with three low carbon low alloy steels, which showed characteristic differences in the R curves. The crack growth resistance is related to both the primary dimple morphology and the total length of local shear zones appearing on the fracture surface; the latter contribution predominates over the former. The heterogeneity of the microstructures, which constrains slip propagation at the grain boundaries, supplies sites for easy void nucleation and induces local shear and the resulting surface roughness.     

The dissolution of cementite in a low carbon steel during isothermal annealing at 700°C

An experimental and theoretical investigation of the dissolution of cementite in ferrite at 700 °C has been performed. Special attention has been paid to the variation of the matrix carbon content with annealing time and prior heat treatments. A fine structure of spherical cementite particles yields a more rapid increase in carbon dissolved at short times whereas a coarse structure of grain boundary cementite yields a more rapid increase at rather long times. A lower temperature annealing extended over several hundred hours yields a Mn enrichment in the cementite and displaces the subsequent dissolution at 700 °C toward longer times. The effect is not so pronounced in the case of the coarse structure due to a lower degree of enrichment. Formerly Graduate Student of The University of Tokyo     

低碳和超低碳含铝系列钢硅含量的控制

采取合理控制含铝系列钢出钢碳含量、使用硅含量低的合金、改进脱氧合金化制度、规范钢包和真空室的管理、使用镁质涂料中包和规范大包换包操作等措施,杜绝含铝系列钢因硅含量出格而改判.     

Effects of copper on proeutectoid cementite precipitation

The effect of copper on proeutectoid cementite precipitation was investigated by examining the isothermal transformation characteristics of Fe-C and Fe-C-Cu alloys that had comparable carbon contents. The TTT diagrams generated for the Fe-1.43 wt pct C and the Fe-1.49 wt pct C-4.90 wt pct Cu alloys showed that the kinetics of proeutectoid cementite precipitation were not significantly affected by copper. The morphology of the proeutectoid cementite, as seen in the optical microscope, was also substantially the same in both alloys. However, transmission electron microscopy revealed the presence of small epsilon-copper precipitates within the proeutectoid cementite of the copper containing steel. It was concluded that this precipitation of ε-Cu took place at the cementite : austenite interphase boundaries, and that the transport of copper to the precipitates was accomplished by interphase boundary diffusion. The small influence of copper on the kinetics of proeutectoid cementite precipitation is discussed in terms of diffusional growth theories and the structure of the cementite : austenite interphase boundary.     

Deformation and fracture in martensitic carbon steels tempered at low temperatures

This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described. The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering. Dr. George Krauss is currently University Emeritus Professor at the Colorado School of Mines. He received the B.S. in Metallurgical Engineering from Lehigh University in 1955 and the M.S. and Sc.D. degrees in Metallurgy from the Massachusetts Institute of Technology in 1958 and 1961, respectively, after working at the Superior Tube Company as a Development Engineer in 1956. In 1962–63, he was an NSF Postdoctoral Fellow at the Max-Planck-Institut für Eisenforshung (Düsseldorf, Germany). He served at Lehigh University as Assistant Professor, Associate Professor, and Professor of Metallurgy and Materials Science from 1963 to 1975 and, in 1975, joined the faculty of the Colorado School of Mines as the AMAX Professor of Physical Metallurgy. He was the John Henry Moore Professor of Metallurgical and Materials Engineering at the time of his retirement from the Colorado School of Mines in 1997. In 1984, Dr. Krauss was a principal in the establishment of the Advanced Steel Processing and Products Research Center, an NSF industry-university cooperative research center at the Colorado School of Mines, and served as its first director until 1993. He has authored the book Steels: Heat Treatment and Processing Principles, ASM International, 1990, coauthored the book Tool Steels, Fifth Edition, ASM International, 1998, and edited or coedited several conference volumes on topics including tempering of steel, carburizing, zinc-based coatings on steel, and microalloyed forging steels. He has published over 280 papers and lectured widely at technical conferences, universities, corporations, and ASM chapters, including a number of keynote, invited, and honorary lectures. Dr. Krauss has served as the President of the International Federation of Heat Treatment and Surface Modification, 1989–91, and as President of ASM International, 1996–97. He is a Fellow of ASM International and has received the Adolf Martens Medal of the German Society for Heat Treatment and Materials Technology, the Charles S. Barrett Silver Medal of the Rocky Mountain Chapter ASM, the George Brown Gold Medal of the Colorado School of Mines, and several other professional and teaching awards, including the ASM Albert Easton White Distinguished Teacher Award in 1999. He is an Honorary Member of the Iron and Steel Institute of Japan and a Distinguished Member of the Iron and Steel Society of AIME.     

Deformation and fracture in martensitic carbon steels tempered at low temperatures

This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described. The Edward DeMille Campbell Memorial Lecture was established in 1926 as an annual lecture in memory of and in recognition of the outstanding scientific contributions to the metallurgical profession by a distinguished educator who was blind for all but two years of his professional life. It recognizes demonstrated ability in metallurgical science and engineering. Dr. George Krauss is currently University Emeritus Professor at the Colorado School of Mines. He received the B.S. in Metallurgical Engineering from Lehigh University in 1955 and the M.S. and Sc.D. degrees in Metallurgy from the Massachusetts Institute of Technology in 1958 and 1961, respectively, after working at the Superior Tube Company as a Development Engineer in 1956. In 1962–63, he was an NSF Postdoctoral Fellow at the Max-Planck-Institut für Eisenforshung (Düsseldorf, Germany). He served at Lehigh University as Assistant Professor, Associate Professor, and Professor of Metallurgy and Materials Science from 1963 to 1975 and, in 1975, joined the faculty of the Colorado School of Mines as the AMAX Professor of Physical Metallurgy. He was the John Henry Moore Professor of Metallurgical and Materials Engineering at the time of his retirement from the Colorado School of Mines in 1997. In 1984, Dr. Krauss was a principal in the establishment of the Advanced Steel Processing and Products Research Center, an NSF industry-university cooperative research center at the Colorado School of Mines, and served as its first director until 1993. He has authored the book Steels: Heat Treatment and Processing Principles, ASM International, 1990, coauthored the book Tool Steels, Fifth Edition, ASM International, 1998, and edited or coedited several conference volumes on topics including tempering of steel, carburizing, zinc-based coatings on steel, and microalloyed forging steels. He has published over 280 papers and lectured widely at technical conferences, universities, corporations, and ASM chapters, including a number of keynote, invited, and honorary lectures. Dr. Krauss has served as the President of the International Federation of Heat Treatment and Surface Modification, 1989–91, and as President of ASM International, 1996–97. He is a Fellow of ASM International and has received the Adolf Martens Medal of the German Society for Heat Treatment and Materials Technology, the Charles S. Barrett Silver Medal of the Rocky Mountain Chapter ASM, the George Brown Gold Medal of the Colorado School of Mines, and several other professional and teaching awards, including the ASM Albert Easton White Distinguished Teacher Award in 1999. He is an Honorary Member of the Iron and Steel Institute of Japan and a Distinguished Member of the Iron and Steel Society of AIME.     

Effect of thermomechanical processing on structure and properties of low carbon steels

The microstructure of one plain carbon and two microalloyed steels has been refined by two different thermomechanical processing schedules in a laboratory rolling mill. The factors controlling the structural refinement have been investigated. The results indicate that a combination of single/two stage rolling prior to reaustenitization treatment and finish rolling in the (α + γ) region followed by controlled cooling improves the mechanical properties by refining the ferrite grain size.