The influence of pearlite on magneto-acoustic emission in plain carbon steels

The present study utilizes pulse height distribution analysis to investigate the relationship between magneto-acoustic emission (MAE) and pearlite (carbon) content in iron and plain carbon steel. All of the samples were found to display the same basic features in the pulse height distribution—(1) a primary peak, attributed to 90° domain wall motion, and (2) a secondary peak, which became more pronounced at higher fields and was believed to be associated with domain magnetization vector rotation. In iron, the rotation component of MAE was dominant even at low levels of magnetization. In contrast, a fully pearlitic sample exhibited MAE produced primarily by 90° domain wall motion. Distributions from samples having “intermediate” compositions (containing both ferrite and pearlite) resembled one another more than either of the two “pure” component samples, suggesting a strong interaction between 90° domain wall processes and the ferrite/pearlite boundary.     

Fracture of steels containing pearlite

The relative effects of pearlite and spherodite on ductile, cleavage, and fatigue failure are summarized. Neither the cleavage strength nor the fatigue endurance limit appear to depend directly on cementite contentper se. Spherodized steels cleave less readily than ferrite/pearlite steels. Ductile fracture resistance is lowered considerably by both types of cementite, pearlite being more deleterious. Ferrite/pearlite steels appear to exhibit slower fatigue crack growth rates at low stress intensity levels than high strength steels. At high stress intensity levels the behavior is reversed. Slip-incuded cracking of carbide lamellae appears easier than that of spherodized carbides. In ductile fracture situations the crack spreads progressively through a pearlite colony via preferential cracking of carbides and rupture of the intervening ferrite accompanied by large local shear strains. Fatigue fracture proceeds with formation of frequent branches, preferentially along the pearlite colony interface. This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

The role of hydrogen in the ductile fracture of plain carbon steels

In this report we consider the problem of hydrogen induced ductility losses in a plain carbon spheroidized steel. Specifically, the effect of internal hydrogen on the formation of voids from second phase (cementite) particles and their subsequent growth and coalescence was studied by careful microscopic inspection of uniaxially strained bars, both initially cylindrical and circumferentially notched, with and without hydrogen. Void initiation occurred with lower strains and stresses with hydrogen, although an equally important contribution to the ductility loss was from hydrogen accelerated void growth and coalescence. This latter process takes place by the propagation of voids along the grain, and possibly subgrain, boundaries which interlink the cementite spheroids. The results indicate that hydrogen facilitates interface separation, possibly by accumulating at the boundaries during hydrogenation of the specimen and lowering the cohesive strength, thereby making void initiation and growth along them easier.     

Hydrogen assisted fracture of spheroidized plain carbon steels

The role of hydrogen in the ductile fracture of spheroidized low carbon steels was studied. In addition, the relevant literature has been reviewed to develop a certain perspective on the problem. Both initially smooth and circumferentially notched tensile specimens were electrochemically charged with hydrogen at various cathodic current densities and then uniaxially strained various amounts. Comparisons with uncharged specimens showed that hydrogen promotes void initiation at cementite particles. Void growth and coalescence were also accelerated by hydrogen. Since a large proportion of void coalescence as well as the latter stages of void growth take place along grain, and possibly subgrain boundaries, hydrogen induced losses in interfacial cohesion may account for void initiation, growth and coalescence at lower stresses and strains. A partial transition in fracture mode was observed in steels with a low particle-matrix and grain boundary interfacial area per unit specimen volume. This quasicleavage mode was generally found to be associated with nonmetallic inclusions, suggesting that they act as hydrogen sinks during the charging process and then, during deformation, release a sufficiently large amount of hydrogen to cause quasicleavage fracture to occur. H. CIALONE, formerly with Brown University     

Austenite grain growth kinetics in Al-killed plain carbon steels

Austenite grain growth kinetics have been investigated in three Al-killed plain carbon steels. Experimental results have been validated using the statistical grain growth model by Abbruzzese and Lücke, which takes pinning by second-phase particles into account. It is shown that the pinning force is a function of the pre-heat-treatment schedule. Extrapolation to the conditions of a hot-strip mill indicates that grain growth occurs without pinning during conventional processing. Analytical relations are proposed to simulate austenite grain growth for Al-killed plain carbon steels for any thermal path in a hot-strip mill.     

Influence of sulfide inclusions and pearlite content on the mechanical properties of hot-rolled carbon steels

Sulfur content and sulfide shape are known to have a marked influence on the tensile ductility and notch toughness of plate steels. To investigate the initiation and growth of fractures at inclusions during plastic straining, a detailed study was conducted with a series of 0.1 and 0.2 pct carbon, 1.0 pct manganese steels containing either 0.004 or 0.013 pct sulfur with and without rare-earth additions. This paper describes the results of this study and evaluates the influence of sulfur content and sulfide shape on the anisotropy in tensile ductility and notch toughness in the steels and assesses the influence of other factors, such as pearlite content, affecting the ductility and toughness. Both globular and stringered sulfide inclusions had a detrimental effect on reduction of area, shelf energy, and transition temperature, which was particularly evident in deterioration of through-thickness properties and which was much more severe for stringered inclusions than for globular inclusions. Increased pearlite content was more detrimental to reduction of area and transition temperature when stringered inclusions were also present, whereas its effect on shelf energy appeared to be about the same regardless of the presence of inclusions or their morphology.     

Dilatation measurements of plain carbon steels and their thermodynamic analysis

Dilatation of the low-carbon steels with small additions of mass contents of Mn (up to 1.50%), Si (up to 0.347%), Nb (up to 0.053%), and V (up to 0.082%), was measured at a heating rate of 3°C/min, and the experimental results were compared with calculations based on thermodynamic models. The differences between experiments and calculations were analysed. It was found that the thermal expansion of pearlite and of austenite in the steels exhibits almost linear temperature dependencies, and these dependencies are described very well by the present calculations. During the transformation of pearlite to austenite, contraction of the steels may occur due to the dissolution of cementite within a narrow temperature range. The dilatation of the steels during the transformation of ferrite to austenite depends on the competition between the thermal expansion and the transformation process, and it finally leads to an increase in the length change to a maximum followed by a decrease down to the temperature at which the transformation is completed. For some steels, however, a certain amount of ferrite may remain in the samples during heating even at temperatures well above that of the minimum dilatation. This will affect the determination of the A₃ temperature, and makes the expansion of the steels deviate from the true expansion behaviour of austenite.     

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.     

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.     

Microstructural modification of plain carbon steels irradiated by high-energy electron beam

The main objective of the present study is to analyze the microstructural modification of the surface hardened by the irradiation of high-energy electron beam in 0.18 pct C and 0.38 pct C plain carbon steels. Steel samples were irradiated using an electron accelerator (1.4 MeV), and the detailed microstructures of the irradiated surface were examined. Upon irradiation, the ferrite-pearlite structure near the sample surface was changed to the dual-phase structure, i.e., ferrite and martensite, and fine particles or needlelike lamellae were observed in the ferrite/martensite interface. In order to investigate these complex microstructures as well as the martensitic transformation mechanism, the simulation test, including thermal cycles of abrupt heating and quenching, was carried out. The test results indicated that the irradiated surface was heated up to about 1100°C and then quenched to room temperature, which was enough to obtain the surface hardening through martensitic transformation. Thermal analysis of the irradiated surface was also carried out for systematic understanding of the microstructural modification in terms of the irradiation parameters such as beam travel speed.     

An effect of a strong magnetic field on the phase transformation in plain carbon steels

The effect of a magnetic field on the Gibbs free energy of a material depends on its magnetization behaviors. To investigate the change in the Fe-Fe₃C phase diagram caused by a high external magnetic field, the magnetic Gibbs free energies of the phases such as austenite, ferrite, and cementite are calculated on the basis of the molecular field theory. Using the calculated Gibbs free energy as a function of weight percent carbon and temperature at a particular magnetic field, a phase diagram of the Fe-Fe₃C system is drawn. The phase diagram is shifted upward so that the Ac ₁ and Ac ₃ temperatures increase as the magnetic field is applied, but the Ac _m temperature change is almost independent of applied magnetic field value. The increase of eutectoid temperature and composition and its application to microstructural control are discussed.     

Effect of deformation on the austenite-to-ferrite transformation in a plain carbon and two microalloyed steels

Isothermal compression tests were carried out on three steels: (i) a plain C, (ii) a Mo, and (iii) a Mo-Nb-V microalloyed grade in order to study the effect of deformation on the austenite-to-ferrite transformation. Dynamic TTT (DTTT) curves were determined, which show clearly the extent to which deformation accelerates the decomposition of austenite. The latter effect is diminished by the addition of the alloying elements Mo, Nb, and V and is further reduced as the temperature is increased. The substitutional elements Mo, Nb, and V appear to reduce the nucleation rate through reduction of the austenite grain boundary energy. The growth rate is also reduced by these elements, apparently through the solute drag-like effect. The microstructural results indicate that the ferrite formed under dynamic conditions becomes more homogeneous and finer when the strain rate or the temperature is increased. Under static conditions, increasing the prestrain or the prestraining strain rate accelerates the γ-to-α transformation and reduces the mean grain size of the ferrite, although the highest transformation rate is still associated with the dynamic case.     

原位观察铌对高碳钢珠光体相变的影响

 为了研究微合金元素铌(Nb)对高碳钢中珠光体相变的影响,在高温激光共聚焦显微镜下原位观察了不含铌和含铌高碳钢连续冷却过程中珠光体动态形核和长大行为。结果表明,在高碳钢中添加铌增加了珠光体形核点的数量,这是因为铌提高珠光体单位面积形核数量。同时,铌元素减慢珠光体长大速率是由于铌显著阻碍珠光体长大,但当铌质量分数超过0.014%后,阻碍珠光体长大速率的效果不再进一步增加。从以上结果可知,在高碳钢中添加铌促进珠光体形核,但是减慢珠光体长大速率。所以,为了更加准确地研究铌元素对珠光体相变的影响,选用不含铌及铌质量分数为0.027%的两种高碳钢,在Gleeble-3500热模拟试验机上进行与高温原位观察试验相同试验工艺的热膨胀试验。通过热膨胀试验发现,铌的添加增大过冷度,导致降低了珠光体相变温度区间,但是铌显著阻碍碳在奥氏体中的扩散系数,所以铌减慢珠光体长大速率。另外,铌减慢连续冷却条件下的珠光体相变动力学,推迟珠光体相变,从而降低珠光体相变速率,表明铌对珠光体长大的阻碍作用强于其对珠光体形核的促进作用。因此,在高碳钢中,铌元素的添加推迟珠光体相变。此外,铌的添加增大过冷度,使含铌高碳钢的珠光体片层细化,提高了含铌高碳钢的硬度,但在铌质量分数超过0.014%后,细化效果不再进一步增强。     

Effect of deformation on ferrite nucleation and growth in a plain carbon and two microalloyed steels

Isothermal compression tests were carried out on plain C, Mo, and Mo−Nb−V microalloyed steels in order to study the effect of austenite deformation on the ferrite nucleation and growth rates. The nucleation rate increases with deformation and the degree of supersaturation, Ae₃−T; it appears to be reduced by the substitutional elements Mo, Nb, and V through reduction of the austenite grain boundary energy. The growth rate increases with the degree of supersaturation and is also reduced by these elements, apparently through the solute drag-like effect. Under static conditions, increasing the prestraining strain rate increases the nucleation rate, but this increase is small compared to the effect of concurrent deformation. The growth rate under static conditions decreases as the deformation or the strain rate is increased. E. E, formerly with the Department of Metallurgical Engineering, McGill University     

低碳钢中伪珠光体分析

 利用光学显微镜检测了抗拉强度超过标准上限的低碳钢的金相组织,试样中伪珠光体体积分数达到50%以上,明显高于正常组织中的珠光体体积分数,因此具有显著的珠光体强化效果;并且伪珠光体存在明显的退化现象。对比分析生产工艺后认为,层流冷却过程中对先共析铁素体析出的抑制以及终冷温度低于空冷的[Ar1]是产生这种退化显著的伪珠光体组织的必要条件。结合生产实践分析了影响先共析铁素体析出的因素。     

Effect of composition and initial grain size on the dynamic recrystallization of austenite in plain carbon steels

The effect of composition on the dynamic recrystallization behavior of plain carbon steels has been examined in the temperature range 850 to 1300 °C and the strain-rate range 6 × 10^-6 to 2 × 10^-2 s^-1. With increasing solute content, the onset of dynamic recrystallization is delayed and the rate of recrystallization is decreased. At 1000 °C the elements can be ranked in order of increasing effectiveness as C, Ni, Mn, Si, P. At higher temperatures the effect of composition is complicated by its effect on grain coarsening, which itself postpones the onset and slows the rate of recrystallization. Similarly, for steels in which precipitation may occur, a fine-grain structure can promote the earlier onset and faster rate of recrystallization. Thus, out of an examination of the effect of composition comes an appreciation of the influence of initial grain size. Another factor which can complicate any explanation of the compositional dependence of dynamic recrystallization is grain-boundary segregation, which is probably responsible for the strong retarding influence of phosphorus.