Effect of austenite-decomposition temperature on bainite morphology and properties of low-carbon steel after thermomechanical treatment

Peculiarities of the bainite structure formed in low-carbon steel 07G2NDMBT during isothermal austenite decomposition, namely, the sizes of crystallites, their mutual orientation, and the presence of cementite precipitates, are considered. The temperature range of the formation of bainite with the subgrain structure was determined. The size of the austenite grain and degree of hot deformation were found to affect the transformation of bainite that occurs upon subsequent cooling and the submicrocrystalline bainite structure. We studied the structure and mechanical properties of a rolled sheet 16 mm thick, which was subjected to thermomechanical treatment (TMT) under plant conditions in accordance with optimum regimes. It was shown that the high structure dispersion of the steel subjected to TMT is due to not only the formation of bainite with the subgrain structure, but also the partial transfer of crystal-structure defects from hot-rolled austenite to the final bainite structure.     

Rate range for formation of granular bainite in decomposition of austenite of steel 20Kh2NACh

The present work continues research on the special features of formation of granular bainite in steel 20Kh2NACh. A thermokinetic diagram of decomposition of austenite is plotted, and the range of cooling rates in which granular bainite is formed after rolling and after preliminary heating from various temperatures is determined. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 17–19, May, 1998.     

Study of the isothermal transformation of ductile iron with 0.5% Cu by electrical resistance measurement

A computer-controlled system for measuring electrical resistance has been developed and used to study the isothermal transformation of austenite in a ductile iron (3.31 % C, 3.12 % Si, 0.22 % Mn, 0.55 % Cu). The ability of the technique to follow the isothermal decomposition of austenite was established by measurements on an AISI4340 steel. The times at which the austenite decomposed to primary ferrite, pearlite, and bainite were accurately detected. In the ductile iron, the formation of pearlite and of bainite was easily detected, and an isothermal transformation diagram was constructed from the results. The temperature range for the formation of bainite is especially important in producing austempered ductile iron (ADI) and was mapped. An initial stage of decomposition of austenite to ferrite and high-carbon austenite is followed by a time delay; then the high-carbon austenite decomposes to bainite. The formation of ADI requires austempering to a structure of ferrite and high-carbon austenite, then quenching to retain this structure, thus avoiding the formation of bainite. This is achieved by isothermal transformation into the time-delay region. For the ductile iron studied here, this time region was about 2.6 h at 400 °C and increased to 277 h at 300 °C.     

Phase composition and fine structure of granular bainite in low-carbon low-alloy steel

The phase composition and special features of the structure of granular bainite in low-carbon low-alloy steels formed as a result of decomposition of austenite in the intermediate range under the conditions of continuous cooling are studied. The volume fractions of structural and phase components, the periods of crystal lattices of α-and γ-phases, the types of carbides, and defects of the crystal structure of the metal are determined. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 3–7, May, 2006.     

低碳Mn-Nb-V和Mn-Mo-Nb-V钢中的贝氏体转变

大森等和Habraken曾分别研究了低碳低合金钢中出现的B_Ⅰ型、B_Ⅱ型、B_Ⅲ型贝氏体和粒状贝氏体的特点和形态。我们对低碳Mn-Nb-V和Mn-Mo-Nb-V钢中的贝氏体进行光学显微镜和透射电子显微镜观察,研究贝氏体形态和转变。试验用钢用感应炉冶炼,锻成直径15 mm的圆棒,经退火加工成直径12mm厚2mm的片试样。其化学成分如下(wt-%):     

关于60Si2MnA钢奥氏体中温转变的研究

60Si2MnA钢过冷奥氏体的中温转变产物为贝氏体铁素体和残余奥氏体,贝氏体铁索体的形成与自催化效应密切相关。在回火过程中,贝氏体中的奥氏体以扩散转变方式分解为铁素体和渗碳体。从而说明奥氏体的等温转变与回火转变是本质不同的转变。     

低碳Si-Mn系TRIP钢的热处理工艺对组织的影响

低碳Si-Mn系TRIP钢有着复杂的显微组织，主要由多边形铁素体（F）＋无碳贝氏体（B）＋残留奥氏体（AR）组成。本试验采用了彩色金相法，并结合X－ray衍射、SEM和TEM等手段研究了低碳Si-Mn系TRIP钢显微组织与工艺的关系，发现随着两相区退火温度的升高，最终显微组织中铁素体基体体积分数变小，并且贝氏体量增多，残留奥氏体的稳定性呈起伏式变化；在贝氏体转变区的等温温度过高或过低，均使最终显微组织中残留奥氏体体积分数减少；在贝氏体转变区等温时，所形成贝氏体表现出粒状的特征。     

Features of Isothermal Formation of Carbide-Free Bainite in High-Carbon Manganese-Silicon Steel

Change in the stability of overcooled austenite of high-carbon manganese-silicon steel relative to decomposition via the intermediate stage upon isothermal holding and martensitic γ → α transformation upon subsequent cooling has been studied using dilatometry and X-ray diffraction. The kinetics of low-temperature bainite transformation has been studied. The tetragonality of the crystal lattice of bainite α-phase caused by the superequilibrium content of carbon was found.     

MORPHOLOGY AND PHASE TRANSFORMATION OF GRANULAR BAINITE AND GRANULAR STRUCTURE

本文研究了低碳合金钢奥氏体在连续冷却或等温转变过程中形成的组织形态和转变机理。结果表明在中温贝氏体区得到“粒状贝氏体”组织,在先共析转变区得到“粒状组织”。两者都有“铁素体基体+小岛”形貌。但前者有表面浮凸现象,铁素体呈长条状,与母相维持K-S关系,惯习面为{111}_γ,小岛也多星长条状平行排列。后者无表面浮凸,铁素体呈无规则形状,与母相无严格位向关系,小岛亦呈无规则排列。然而这两种组织都是由扩散型相变机制形成的。最后,提出了形成这两种组织的相变模型。     

PHASE AND STRUCTURE TRANSFORMATION OF BAINITE IN 3Cr3Mo3VNb STEEL DURING TEMPERING

本文研究了3Cr3Mo3VNb钢等温淬火后贝氏体回火时的相及组织转变规律。实验结果表明,贝氏体的二次硬化机制和马氏体一样,主要是由Mo_2C,V_4C_3,NbC等细小弥散的合金碳化物析出造成的。回火时,贝氏体中粗大的渗碳体溶解较慢,减慢了合金碳化物的析出速率。贝氏体中的残余奥氏体分解和转变的行为与马氏体中的不同。贝氏体中位错密度较低,不利于合金碳化物的聚集粗化。以上诸因素的共同作用,使贝氏体具有比直接淬火所获得的马氏体更高的二次硬化效应、热强性和组织稳定性。     

In situ observation and electron backscattered diffraction analysis of granular bainite in simulated heat-affected zone of high-strength low-alloy steel

In this study, the formation of granular bainite (GB) which may form in the heat-affected zone of double-sided double arc welding joint was in situ observed. The crystallographic characteristics of GB were also compared with those of microstructure mainly comprising lath bainite (LB). The results show that bainite packets exist at the initial stage of GB transformation and can be distinguished by the distribution of retained austenite and martensite–austenite constituents. The bainite blocks of GB are larger than those of LB which makes both the length and fraction of high-angle grain boundaries in GB less than those of LB which partly lead to the brittleness of granular bainite.     

Nucleation Analysis of Variant Transformed from Austenite with Σ_3 Boundary in High-Strength Low-Alloy Steel

We elucidate here the effects of annealing twins on phase transformation products based on electron back-scatter diffraction analysis and corresponding microstructure visualization and quantification methods.Martensite and bainite were obtained by rapid continuous cooling and isothermal processing at different temperatures,respectively,which were designed to study variants formed in austenite with Σ_3 boundary.The isothermal transformation near martensite start(M_s) temperature was most conducive in obtaining the highest content of twin-related V_1/V_2 variant pair.Based on the classical nucleation theory of martensite and bainite,respectively,the role of austenite Σ_3 boundary in martensite and bainite transformation is illustrated.     

低碳钢中晶界铁素体/原奥氏体界面对贝氏体转变的影响

采用电子背散射衍射 (EBSD) 研究了低碳Fe--C--Mn--Si钢中晶界铁素体/原奥氏体界面对贝氏体形核的影响. 通过两阶段等温热 处理, 获得了晶界铁素体和贝氏体的混合组织. 结合金相观察和取向测量, 发现晶界铁素体与贝氏铁素体之间的界面分为两种, 一种界面不清晰, 一种界面清晰. 分析表明, 在晶界铁素体/贝氏体界面不清晰一侧, 晶界铁素体与原奥氏体保持取向关系, 贝氏体在这类界面形 核生长, 且取向与晶界铁素体保持一致; 在晶界铁素体/贝氏体界面清晰一侧, 晶界铁素体与原奥氏体无取向关系, 且贝氏体与晶界铁素体之间取向差较大.     

Acoustic emission in bearing steel during isothermal formation of midrib

The first attempt of an acoustic emission (AE) application for the control of isothermal transformation, at the temperature near the martensite start (M _S ) in 100CrMnSi6-4 bearing steel, is made herein. A unique method for recording the signals of the AE was developed. The Short Time Fourier Transform algorithm was used for displaying the signal values on the spectrogram graphs. Acoustic emission showed the nucleation and growth of the isothermal martensite in the residual austenite. The maximum power spectral density of signals was determined using the relative intensity of the RMS values. The observed energy of separated signals during the second stage of residual austenite decomposition is comparable to the energy of the initial stage at lower temperatures. It was proposed as a new process concept under paraequilibrium that involves the midrib formation and swing back in the kinetics of lower bainite.     

Decomposition of supercooled austenite of powder phosphorus steels

The structure of powder phosphorus steels is investigated. New models of isothermal decomposition of super-cooled austenite are suggested which make it possible to predict the kinetics of the γ→α transformation in the pearlite and bainite temperature ranges. It is shown that mechanical alloying has a favorable effect on the formation of structure in steels. Grain disintegration accelerates the decomposition of austenite mainly due to the growth of the specific surface. At the same time, the contribution of 0.65–1.1% phosphorus additives to the process is low. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 3–7, May, 1999.     

Effect of silicon and prior deformation of austenite on isothermal transformation in low carbon steels

Isothermal transformation (TTT) behavior of the low carbon steels with two Si con-tents (0.50 wt pct and 1.35 wt pct) was investigated with and without the prior deformation. The results show that Si and the prior deformation of the austenite have significant effects on the transformation of the ferrite and bainite. The addition of Si refines the ferrite grains, accelerates the polygonal ferrite transformation and the formation of M/A constituents, leading to the improvement of the strength. The ferrite grains formed under the prior deformation of the austenite become more ho-mogeneous and refined. However, the influence of deformation on the tensile strength of both steels is dependent on the isothermal temperatures. Thermodynamic calcu-lation indicates that Si and prior deformation reduce the incubation time of both ferrite and bainite transformation, but the effect is weakened by the decrease of the isothermal temperatures.     

Nb对中低碳超级贝氏体钢组织与性能的影响

针对无Nb和0.05wt%Nb两种中低碳钢,研究了Nb对0.25wt%C超级贝氏体钢组织与性能的影响。结果表明,对两种试验钢采用轧后先快冷后缓冷的等温工艺,均可获得贝氏体组织。300 ℃等温8 h,含Nb钢贝氏体含量达到80%,屈服强度提高12% (109 MPa),冲击吸收能量达到52 J。通过Thermal-Calc软件计算并结合TEM观察发现,含Nb钢中Nb元素与Mo等元素形成复杂碳化物,析出的细小碳化物钉扎板条边界,细化贝氏体板条,抑制板条合并与粗化,提高板条的稳定性。等温8 h后,含Nb钢的贝氏体铁素体板条尺寸在150~200 nm之间。利用背散射电子和EBSD分析发现,Nb元素通过促进碳化物的析出,降低过冷奥氏体稳定性,促进贝氏体转变,抑制马氏体转变,细化残留奥氏体,提高了组织的均匀性和稳定性,是性能提高的主要机制。     

Some microstructural observations of bainite stars in a eutectoid steel

In some steels, when austenite decomposes isothermally to bainite, it forms in the morphology of “stars” that nucleate inside the austenite grains. The microstructural features of stars formed upon isothermal transformation in a high-purity eutectoid steel containing 0.32% Mo have been examined in detail using scanning electron microscopy. The stars consist of sets of parallel branches within which exist long parallel carbide rods in ferrite needles, and short carbide rods or particles between these regions. Serial sectioning revealed that the branches are needle-shaped, and not plate-like. There was evidence that a contribution to the growth of the stars is made from the nucleation of bainite regions ahead of the advancing bainite star interface.     

Numerical prediction of microstructure and hardness in multicycle simulations

Thermal-microstructural predictions are made and compared to physical simulations of heat-affected zones in multipass and weaved welds. The microstructural prediction algorithm includes reaustenitization kinetics, grain growth, austenite decomposition kinetics, hardness, and tempering. Microstructural simulation of weaved welds requires that the algorithm include transient reaustenitization, austenite decomposition for arbitrary thermal cycles including during reheating, and tempering. Material properties for each of these phenomena are taken from the best available literature. The numerical predictions are compared with the results of physical simulations made at the Metals Technology Laboratory, CANMET, on a Gleeble 1500 simulator. Thermal histories used in the physical simulations included single-pass welds, isothermal tempering, two-cycle, and three-cycle welds. The two-and three-cycle welds include temper-bead and weaved-weld simulations. A recurring theme in the analysis is the significant variation found in the material properties for the same grade of steel. This affected all the material properties used including those governing reaustenitization, austenite grain growth, austenite decomposition, and hardness. Hardness measurements taken from the literature show a variation of ±5 to 30 HV on the same sample. Alloy differences within the allowable range also led to hardness variations of ±30 HV for the heat-affected zone of multipass welds. The predicted hardnesses agree extremely well with those taken from the physical simulations. Some differences due to problems with the austenite decomposition properties were noted in that bainite formation was predicted to occur somewhat more rapidly than was found experimentally. Reaustenitization values predicted during the rapid excursions to intercritical temperatures were also in good qualitative agreement with those measured experimentally.