A model for predicting austenite to bainite phase transformation in producing dual phase steels

Based on the theory of free energy change between austenite and bainite phases during the cooling period, a material model is developed to predict the incubation time of the transformation from austenite to bainite for 50CrNiMo deformed at different strain levels and austenite states. Further constitutive equations are developed to model the growth of the nucleated bainite phase. These enable the bainite volume fraction to be predicted for different material cooling conditions after hot forming. To determine the material constants within the phase transformation model, an objective function is formulated to quantify the relative errors between experimental and computed data. The phase transformation model and the objective functions are implemented into an Evolutionary Programming (EP) based software package through a user defined subroutine for the optimisation. The experimental data of 50CrNiMo is used to evaluate the capability of the model in prediction of TTT diagrams and bainite phase growth for pre-plastic deformations of 0, 25 and 50%.     

Metal dusting of low alloy steels

The metal dusting of two low alloy steels was investigated at 475°C in flowing CO-H₂-H₂O mixtures at atmospheric pressure and a_C > 1. The reaction sequence comprises: (1) oversaturation with C, formation of cementite und its decomposition to metal particles and carbon, and (2) additional carbon deposition on the metal particles from the atmosphere. The metal wastage rate r₁ was determined by analysis of the corrosion product after exposures, this rate is constant with time und virtually independent of the environment. The carbon deposition from the atmosphere was determined by thermogravimetry, its rate r₂ increases linearly with time, which can be explained by the catalytic action of the metal particles – periodic changes are superposed. The rate of carbon deposition r₂ is proportional to the carbon activity in the atmosphere. The metal dusting could not be suppressed by increasing the oxygen activity or preoxidation, even if magnetite should be stable. Addition of H₂S, however, effectively suppresses the attack.     

低碳低合金高强钢的连续转变行为及其相变模型

用DIL805型热膨胀仪研究了低碳低合金高强钢(HSLA)奥氏体连续冷却过程的相变规律,用膨胀法结合金相法建立了实验钢奥氏体的连续冷却转变曲线(CCT),随着冷却速度的增加,实验钢的组织分别发生了铁素体转变、珠光体转变、贝氏体转变和马氏体转变。建立了相变点-冷却速度以及相变量-冷却速度之间的数学方程,并回归计算了拟合度较高的相变模型。结果表明,计算值与试验值之间能很好的吻合,证明了该相变模型的可行性。     

微量Nb对低碳低合金管线钢相变的影响

采用Gleeble-1500D型热模拟机,测定了不同Nb含量的低碳微合金管线钢在不同冷却速度下,过冷奥氏体连续冷却相变点,分析观察了显微组织,测定了其显微硬度.结果表明,在同一冷速下,随着Nb含量的增加,过冷奥氏体连续冷却相变点降低,组织中容易出现板条状贝氏体铁素体,显微硬度提高;在相同成分下,随着冷速的增加,含铌钢中铁素体越来越细小,由等轴大块铁素体组织向板条状贝氏体铁素体转变,显微硬度提高.     

The susceptibility to sulphide stress cracking of low alloy steels

The susceptibility to sulphide stress cracking (SSC) of low alloy Mn-V and Cr-Mo-Nb steels in standard NACE solution was determined. The critical stress (S_c) and threshold stress (σ_th) of steels were determined in both as-received and heat treated condition. In the as-received state the steel shows a highly susceptibility to SSC dominantly due to its microstucture. By quenching and tempering of steels at high temperature the SSC resistance was increased. It was found that the susceptibility to SSC for Cr-Mo-Nb steel is lower than that of Mn-V steel at a comparable yield strength. It was explained by the presence of different second phases in the ferrite matrix and spheroidized carbide particles resulting from the higher tempering temperature. In addition to Fe₃C, the presence of Cr₃C₂, Mo₂C and NbC in the ferrite matrix of Cr-Mo-Nb steel play an important role in their resistance to SSC. In all cases tested was S_c>σ_th which suggests of the importance of plastic deformation during the test. The microfractographic analysis of specimens before and after SSC testing have been done. Occurrence of cracks in the near outer surface of non-failed tensile specimens during 720 hours test suggested that SSC could be explained as a combination of hydrogen-induced cracking (HIC) and stress corrosion cracking (SCC).     

Cu-0.1Ag-0.1Fe合金的相变动力学

通过对Cu-0.1Ag-0.1Fe合金恒温时效过程中的导电率与析出的新相体积间的关系的测定与分析,研究了该合金的时效动力学,并由此确定了不同温度下描述新相转变比率与时效时间关系的Avrami经验方程和计算导电率随时效时间变化的导电率方程,从而描绘出不同温度时效时的相变动力学"S"曲线以及等温转变"C"曲线。     

Determining phase volume fraction in steels by electron backscattered diffraction

The relative sharpness of electron backscattered diffraction (EBSD) patterns is used to quantify the volume fraction of ferrite in low-carbon steels. EBSD pattern quality (PQ) maps are shown to improve imaging of martensite and ferrite over optical and secondary electron imaging. Ferrite volume fraction in isothermally reacted Fe–0.12C–3.28Ni are quantified using PQ maps by manual point counting and image thresholding.     

The pitting corrosion of low alloy and mild steels

Electrochemical investigations and microscopical examinations of pitting corrosion of low alloy (weathering) steels, copper bearing steel and plain carbon steel have been performed. Higher inclination to pit formation in steels containing alloying elements were ascertained in corrosive environments containing chlorides. Scanning electron microscopy enabled the determination of differences in morphology of pits formed on the steels under study.     

Effects of indium addition to CuAu alloy on phase transformation behaviors

The effects of indium addition to CuAu on the alloy phase transformation behavior were investigated by hardness tests, electrical resistivity measurements, X-ray diffraction and transmission electron microscopy. These established that indium additions lower the critical temperature and stabilize the CuAu II phase at room temperature. Formation of the long period ordered phase produces an age-hardening; although this age-hardening is retarded, the degree of hardening is comparable to that in CuAu. It was noted that the axial ratio increases and the antiphase domain size decreases.     

Corrosion behaviour of copper containing low alloy steels in sulphuric acid

The influence of the addition of Cu on the corrosion resistance of low alloy steel in sulphuric acid was investigated using AC and DC electrochemical methods and a weight loss test, which took place in 10 wt% H₂SO₄ solution at room temperature. The electrochemical measurements indicated that the corrosion rate is suppressed by the addition of Cu due to a higher hydrogen overpotential and prevention of the active dissolution. Surface analyses (XPS, EPMA and SEM) of the corroded surfaces conducted after the immersion test indicated that the rust layer formed on the Cu containing steels was enriched with Cu compounds.     

The effects of low frequency load cycles on crack initiation in low alloy steels

The stress corrosion behaviour of low alloy steels was investigated with low frequency cyclic loading in 2 M (NH₄)₂CO₃ at 70°C. The tests were carried out using triangular and sawtooth load cycles with a constant critical strain-rate prevailing within periods of increasing stress irrespective of frequency. Since frequency changes with stress amplitude when using equal ?-values, the effects of frequency were also determined. The experiments were concentrated on crack initiation and initial crack propagation. The results show that the threshold stress which causes stress corrosion cracking (SCC) in ammonium carbonate solution is considerably reduced for all steels tested during low-frequency load cycles. In contrast to tests with constant load, propagating stress corrosion cracks can be initiated below the lower yield strength. The effects of stress/strain amplitude and of the magnitude of deformation on crack propagation were determined.     

Influence of the alloy element on corrosion morphology of the low alloy steels exposed to the atmospheric environments

Morphology of the corroded surface of low alloy steels beneath rust after long-term exposure test in the atmospheric environment was analyzed. The form of the corroded surface was measured with the laser displacement sensor scanning the surface. The resultant height map was divided by the mesh and the maximum corrosion depth was calculated in each cell. The maximum depth was arranged by the extreme value analysis. From this analysis two kinds of corrosion patterns were distinguished; i.e., uniform corrosion and local corrosion. Electrolytic iron shows the only uniform corrosion pattern. The addition of Cu, Ni and Cr changed the form of the corroded surface from the uniform corrosion to the combined pattern (uniform corrosion + local corrosion). The addition of Cr has a marked effect in changing the corrosion pattern.     

Thermophysical property measurements on low alloy high strength carbon steels

The solidus/liquidus temperatures, heat of fusion, specific heat capacity, and thermal expansion for AISI 43 10 and 4320 steels were measured in this study. Both steels have similar heat of fusion values and solidification temperature ranges but differ in the absolute solidus/liquidus temperatures. The specific heat capacity vs. temperature curves for both steels show similar trends with difference in the peak value at austenite phase transformation temperatures.

Both steels demonstrate similar thermal expansion trends and absolute values. The thermal expansion vs. temperature curve shows considerable difference between the heating part and the subsequent cooling part which reflects the different solid state phase changes experienced during the heating and cooling of these two heat-treatable steels. This phenomenon should be taken into account during the modeling of welding, casting and other processing of these steels. The relative density as a function of temperature for these steels can be estimated based on the thermal expansion values.     

2000 MPa grade low alloy,ultrahigh strength multiphase steels

Abstract

The benefit of steels with a combination of high strength and good ductility has attracted generations of researchers to work in this area. The strength of current Chinese structural steels typically falls in the range 400–1000 MPa. Higher strengths are desired to achieve the aims of saving energy and raw materials and protecting the environment, through reducing the weight of steel parts, or of improving safety standards. Thus, there is an urgent need to develop ultrahigh strength steel.     

Modeling of phase transformation behavior in hot-deformed and continuously cooled C-Mn steels

Computer models of phase transformation from austenite to ferrite, austenite to pearlite, and austenite to bainite in hot-deformed carbon-manganese steels during continuous cooling were established on the basis of Cahn’s transformation theory, thermal-dilatometric experiments, and thermodynamic calculations. These models showed good agreement with results measured from pilot hot rolling experiments.