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 cementite morphology on pearlitic wire drawing deformation

A comparative study was conducted on the effects of lamellar cementites and globular cementites on the cold drawing process and the mechanical properties of pearlitic wire steel, with the help of metallographic microscope, scanning electron microscope, transmission electron microscope, tensile tester and hardness tester. The lamellar cementites showed the deformation capacity to some extent during the cold drawing process. As the drawing strain increased, the pearlitic wire with globular cementites evolved into the fibrous form gradually and no obvious defects were found in the microstructure. The globular cementites turned to the drawing direction without any deformation of itself during the deformation process. And micro- cracks occurred in the cementite/ferrite interface due to stress concentration caused by pinning dislocations in spherical cementites. The strength and hardness of both pearlitic wires gradually increased as the drawing strain rose. And the pearlitic wire with lamellar cementites had a higher drawing hardening rate. The ferrite <110> texture formed in both pearlitic wires during the cold drawing process. Compared with the globular pearlite, the pearlitic wire with lamellar cementites had higher ferrite <110> texture intensity. And the difference of their ferrite <110> texture intensity became bigger and bigger as the drawing strain increased.     

Cleavage fracture of steels at low temperatures

Model of cleavage fracture: deformation twin creates microcrack in cementite lamella; critical event in fracture process is extension of crack into ferrite under the action of twin and applied stresses. Stress analysis of the model yields fracture stress. Mechnical tests with tension, compression and notched bend specimens at temperatures 4 K < T < 100 K. Evaluation of local fracture stress by finite element analysis. Comparison of theoretical and experimental results.     

Microstructures of aluminum compressed at various rates and temperatures

Specimens of 1100 aluminum were compressed uniformly to a logarithmic strain of 0.7 (50 pct reduction) at constant true strain rates in a Cam Plastometer. Tests were conducted at strain rates between 220 and 0.1 sec^?1 and between 500°C and 20°C. Exmination of the microstructures by electron microscopy indicated that the deformed structure consisted of dislocation cells or subgrains. As the temperature of testing was raised or the strain rate lowered, the resulting subgrains were larger, and the dislocations in the subboundaries were arranged in more orderly arrays. The cell sizes are related quantitatively to the flow parameters. The dependence of dynamic recovery on temperature and strain rate is compared to the dependence observed in extrusion of a similar alloy. At high strain rates at 400° and 500°C, partial recrystallization occurred while the specimens were cooling to room temperature.     

Cyclic deformation of superduplex stainless steels at intermediate temperatures

Cyclic deformation of a superduplex stainless steel at temperatures ranging from room temperature to 475 °C was evaluated for two different strain amplitudes. Cyclic hardening-softening response and the corresponding substructural features within each constituent phase of the alloy were characterized. Experimental evidence, such as abnormal cyclic hardening, inverse strain rate sensitivity (SRS), and serrated flow, reveals the existence of dynamic strain aging (DSA) in the studied temperature range. Substructural evolution suggests that DSA induces changes in the distribution of plastic strain between austenite and ferrite. In the case of tests performed at 475°C, there exists a significant influence of thermal embrittlement too. A. GIRONèS, formerly Doctor, with the Department of Materials Science and Metallurgical Engineering Universitat Politècnica de Catalunya, 08028 Barcelona, Spain.     

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.     

The deformation and fracture of TiAl at elevated temperatures

The tensile properties of the intermetallic compound TiAl have been determined at several temperatures in the range 25 to 1000°C. Additional variables studied were the influence of strain rate and the effect of exposure to oxidizing conditions prior to testing. The modes of deformation under the various testing conditions were studied in the electron microscope, the modes of fracture were studied in the scanning electron microscope, and these data were correlated with the mechanical properties. The results indicate that the ductilebrittle transition behavior of TiAl at about 700°C is controlled by the trailinga/6 [112] partial dislocation components of thea [011] superdislocations overcoming their pinning barriers. It was also shown that prior exposure to oxidizing conditions does not markedly influence the mechanical properties of TiAl.     

Effects on the cementite distribution in low carbon enamelling steels

To enamel modern LC‐steels it is necessary to provide a sufficient amount of hydrogen recombination sites as well as hydrogen traps within the materials microstructure to keep the hydrogen inside the steel. Hence surface defects like fish scaling, which are related to the effusion of hydrogen, can be avoided. Therefore it is necessary to produce internal surfaces inside the steel In form of hard and brittle particles which can be fractured during cold rolling and produce voids. For LC‐steels these particles could be formed by iron carbides. Aiming at an increased amount of cementite particles inside the steel, the carbon content could be raised or the parameters of the thermomechanical treatment (TMT) could be adjusted for a given carbon content to form coarse cementite particles. In this investigation the TMT‐parameters were systematically varied in hot compression tests and the results were evaluated by quantitative metallography. The focus was laid on the variation of the final deformation temperature, the coiling temperature and the cooling rate after coiling.     

Influence of hot deformation on the transformation behaviour and structure of pearlitic steels

A study was made of the influence of hot deformation on the transformation behaviour, the structure and the mechanical properties of a pearlitic steel containing 0.65% C. The production parameters of a modern hot strip mill were taken as a basis for the deformation schedule and cooling performed with the aid of a hot deformation simulator (Wumsi). Parameters to be pointed out as significantly influencing the transformation behaviour are, in particular, the finishing temperature and the cooling rate after hot deformation. By exploiting the possibility of raising the cooling rate after deformation in the same measure as is attainable on a hot strip mill, a yield strength increase of at least 150 MPa is achievable.     

Tensile failure behavior of plain carbon steels at elevated temperatures

The onset of tensile instability and the occurrence of fracture in plain carbon steels containing up to 1.89C has been examined in the temperature range 500 to 1300 °C and the strain-rate range 6 X lO^-6 to 2 × 10⁻² s⁻¹. In the ferrite-plus-pearlite mixtures at temperatures below the eutectoid temperature, the work-hardening exponent decreases with increasing amount of pearlite, and there is a corresponding decrease in the Considére strain. However, the onset of necking is delayed to well beyond the Considére strain, and these mixtures are inherently ductile even at the eutectoid composition. In the austenite region, the general intrusion of dynamic recrystallization compctes with intergranular embrittlement at temperatures below about 1050 °C. The embrittlement is related to precipitation which takes place either during cooling (MnS) or at the deformation temperature [AIN, Nb (CN),etc.]. In hypereutectoid steels, the ductility of austenite-plus-cementite and pearlite-plus-cementite mixtures diminishes drastically with decreasing temperature and increasing amount of cementite. The areas of possible fracture modes are mapped in temperature-strain rate and temperature-carbon content space.     

The deformation and fracture of Ti3Al at elevated temperatures

The tensile properties of the intermetallic compound Ti₃Al have been determined in air at several temperatures within the range of 25 to 900 °C. The dislocation structures produced by the various testing conditions were studied in the electron microscope and the fracture modes were studied in the scanning electron microscope. These microstructural observations were correlated with the mechanical properties. The results indicate that Ti₃Al has only limited ductility even at 900 °C. The apparent ductile-brittle transition which occurs above 600 °C is due to increasing amounts of intergranular cracking. Some increase in ductility above 600 °C is due to the onset of dislocation cross slipping. The fracture mode up to 600 °C is entirely cleavage. Above 600 °C the fracture shows increasing evidence of plasticity; however, cleavage remains the main fracture mode up to 900 °C. Formerly with the Materials Laboratory Formerly in the Processing and High Temperature Materials Branch     

Influence of deformation substructure on flow and fracture of fully pearlitic steel

Tensile stress-strain data over the whole strain range were obtained for a range of pearlites from very coarse to relatively fine (interlamellar spacings 0.53 and 0.13 μm, respectively). Transmission electron microscopy (TEM) for pearlite subjected to various amounts of strain was performed. Coupling mechanical data with TEM examination provided a detailed picture of how pearlite yields, deforms, work hardens, and fails under uniaxial tension. It is shown that yielding and work hardening of pearlite are largely controlled by processes occurring in ferrite. The role of a cementite plate at low stresses is mainly to limit the slip distance in ferrite. It is found that the tensile fracture is determined by processes in the colonies with lamellae parallel to the tensile axis and that the stress necessary to break a cementite plate corresponds to the true U.T.S. The influence of interlamellar spacing on the yield strength, flow stress, and the true U.T.S. is quantitatively explained.     

中碳珠光体型高速车轮钢的韧化机理

 韧性是影响高速车轮运行安全的关键性能指标。为了阐明中碳珠光体型高速车轮钢的韧化机理,对夹杂物改性和组织韧化两方面进行了深入研究。研究结果表明,硫化物包裹氧化物的夹杂物改性提高了车轮钢的韧性。车轮钢中的氧化物夹杂容易在夹杂物与基体界面处产生裂纹,并向周围基体扩展;当氧化物被硫化物包裹后,裂纹仅在夹杂物本身产生,保护了周围基体。在w（Mn）＝0.75%的成分体系下,当硫的质量分数提高到0.006%及以上时,硫化物在固相线温度以上析出,可以实现对氧化物的较好包裹,改善车轮钢的韧性;硫化物在车轮热加工过程中会发生回溶与再析出,破坏复合夹杂物的包裹效果,提高硫质量分数或降低热加工温度,可以提高复合夹杂物的热稳定性。奥氏体晶粒尺寸和先共析铁素体体积分数是车轮钢组织韧化的关键控制因素。细化奥氏体晶粒尺寸、提高铁素体体积分数,断口中解理面尺寸减小,韧性撕裂区增多。     

Tensile behavior and deformation mechanism of quenching and partitioning treated steels at different deforming temperatures

The effects of deforming temperatures on the tensile behaviors of quenching and partitioning treated steels were investigated.It was found that the ultimate tensile strength of the steel decreased with the increasing temperature from 25 to 100°C,reached the maximum value at 300°C,and then declined by a significant extent when the temperature further reached 400°C.The total elongations at 100,200 and 300°C are at about the same level.The steel achieved optimal mechanical properties at 300°C due to the proper transformation behavior of retained austenite since the stability of retained austenite is largely dependent on the deforming temperature.When tested at 100 and 200°C,the retained austenite was reluctant to transform,while at the other temperatures,about 10 vol.% of retained austenite transformed during the tensile tests.The relationship between the stability of retained austenite and the work hardening behavior of quenching and partitioning treated steels at different deforming temperatures was also studied and discussed in detail.In order to obtain excellent mechanical properties,the stability of retained austenite should be carefully controlled so that the effect of transformation-induced plasticity could take place continuously during plastic deformation.     

Effect of carbon content on the plastic flow of plain carbon steels at elevated temperatures

The elevated-temperature plastic-flow behavior of plain carbon steels with a base composition of 0.8 Mn and 0.25 Si was examined as a function of carbon content in the range 0.005 to 1.54 wt pct at strain rates from 6 x 10^-6 to 2 x 10^-2 sec^-1. Beyond 0.05 C the flow stress at a strain of 0.1 decreased with increasing carbon content at the rate of 13 MPa per pct carbon. However, the degree of softening depended on the strain level at which the flow stress was measured, because the increasing carbon content also decreased the rate of work hardening. The inferred increase in recovery processes with increasing carbon content is in agreement with the effects of carbon on diffusivity, elastic modulus, and lattice spacing, as well as the observed increase in grain growth with increasing carbon content. In the range 850 to 1300 °C (1562 to 2372 °F), the temperature dependence of the flow stress can be represented by σ= A exp (-BT) whereA depends on carbon content and strain, andB depends primarily on strain rate. Extrapolation to higher temperatures yields the carbon-content dependence of the flow stress at the austenite solidus.     

High strain rate deformation and failure of A533B steel at various temperatures

A study of the response of A533B steel of high strain rate loading in shear is presented. Experiments have been conducted using a torsional split Hopkinson bar technique at strain rates of 800 s⁻¹ and 5000 s⁻¹, and various temperatures ranging from −150° to 300°C. The results show a sensitivity of the material to both the strain rate and he temperature at which it deforms. The failure process of the material at temperatures of 5° and −150°C is closely examined. The results show a ductile failure at both temperatures even though the stress-strain curves from tests at −150°C show no, or very little, homogeneous plastic deformation beyond the observed yield point. The failure process at both temperatures is associated with nucleation, growth and coalescence of voids.     

Formation of pearlitic banded structures in ferritic-pearlitic steels

The conditions for ferrite and pearlite banding in strip and plate made of structural steels were investigated. Factors found to influence the formation of banded structures were the cooling rate during the γ/α-transformation, the former austenite grain size, and the work-hardened condition of the former austenite. Analyses with the aid of an electron beam microprobe made it possible to demonstrate that the carbon-rich bands correspond locally with banded manganese enrichments, yet that they do not form before the course of the γ/α-transformation as a result of secondary segregation. It was possible to explain the mechanism of action of the influencing factors on the basis of this model.