Detection of Ni–Sn phases in temper embrittled Ni–Cr steels
A GENERIC problem in ferritic steels is the reduction in the grain boundary cohesive strength produced by the segregation of trace impurities of Groups IVB and VB of the Periodic Table. Such segregation results in a deterioration in mechanical properties produced, for example, by reversible temper embittlement1 and it also promotes enhanced susceptibility to a range of environmental enhanced cracking processes2–4. It has long been recognised that the embrittlement of steels is enhanced by the presence of certain alloy elements, that is Ni, Cr or Mn, and measurements using Auger electron spectroscopy (AES)5–7 have demonstrated that specific combinations of alloy and impurity elements cosegregate to the boundaries, for example, Ni+Sn, Ni+Sb, Cr+P. These observations can be rationalised using Guttmann's phenomenological theory8 which attributes the increased driving force for segregation to a positive interaction between alloy and impurity solute atoms. Previous investigations1,9–11 have failed to detect the presence of any three dimensional phase using conventional transmission electron microscopy and it is generally considered that the alloy element–trace impurity atoms form two dimensional phases at the boundaries. The only direct evidence for such alloy–impurity interactions and phase formation has been obtained using Mössbauer spectroscopy12,13 but it is not possible using this technique to draw any positive conclusions regarding the location of such compounds. Measurements on a pure Fe-3.5 wt% Ni alloy doped with 119Sn12 showed that Ni atoms associate preferentially with Sn atoms during tempering at 650°C and the marked change in the spectrum observed on ageing at 500°C, within the embrittling temperature range, is consistent with the formation of a Ni–Sn compound (Ni3Sn2 or Ni3Sn4). We report here some initial results from a detailed electron microscopic analysis of the microstructures of a commercial (En30A) Ni–Cr steel and a special laboratory made steel (BE 10) having similar Ni and Cr contents, but doped specifically with tin. The composition of the two steels is presented in Table 1. Both steels were oil quenched from 950°C, tempered for 1 h at 650°C and aged at 500°C for times up to 5,000 h. Carbon extraction replicas, taken from polished surfaces were examined in a Philips EM400 microscope fitted with an energy dispersive X-ray system. Selected individual precipitate particles were analysed using convergent beam microdiffraction and X-ray analysis. Attention was focused on the particles which delineated the lath packet and previous austenite boundaries, that is, high-angle boundaries.
Kinetics of non-equilibrium grain-boundary segregation
I. During the isothermal holding at a constant temperature after a sample is cooled quickly enough from a higher temperature a critical time exists, at which a maximum level of NGS will occur. This is the most characteristic aspect of NGS isotherm. The formula of critical time was given in Section 2 to predict the time at which maximum level of NGS will occur during the isotherm. 2. A kinetic model of NGS was suggested from Section 3 to Section 5 of this paper, which is along the same thought structure as McLean's model of EGS. A thermodynamic equation for NGS was first established describing the maximum concentration of NGS that can occur due to cooling between any two temperatures, and then an isothermal kinetic equation was established by solving Fick's equation of diffusion based on the thermodynamic equation. Based on the isothermal kinetic equation the grain boundary segregation level during cooling was calculated using effective time method and correction factor method. 3. The NGS can occur during cooling. The different cooling rates can produce different levels of NGS. There is a certain cooling rate for a kind of material, at which when the material is cooled a maximum level of NGS will appear. This cooling rate is called the critical cooling rate and is another characteristic aspect of NGS. 4. In Section 6, we suggested such a new model to describe the interactions between different kinds of solute and solute atom with vacancy for NGS: In a binary solution of 2 in solvent 3, if solute 2 has a binding energy with a vacancy beyond a suitable range for NGS to occur, then little or no NGS of solute 2 would occur in the binary. If solute 1 has a binding energy with a vacancy in a suitable range for NGS to occur in solvent 3, and if a attraction between the solute pairs 1-2 is stronger than that between pairs 1-3 or 2-3 (α' > 0), then NGS of solute 2 would also occur along with NGS of solute 1 in the ternary alloy when solute 1 is added to the 2-3 binary alloy. This is called NGCS. 5. A model for the effects of applied stress on grain boundary segregation/dilution of solute has been suggested and a set of kinetic equations have been established in Section 7. The simulation using the equations is in good accordance with the observations of sulfur and phosphorus grain-boundary segregation in steels. It is based on the following assumptions: (1) grain boundaries work as free sources of vacancies when a compressive stress is exerted on them and as free sinks when a tension stress is exerted. (2) Oversaturated vacancies induced by applied stress will be combined with the solute atoms to form complexes. (3) A flux of complexes will produce a concomitant but reverse flux of solute atoms between the grain boundary and the center. (4) The effects of applied stress, NGS/NGD, are controlled all by the balance between these two diffusion fluxes. As a result of this balance, they have a most characteristic aspect, critical time, in a manner to be similar to the NGS induced by quenching. 6. For some solute elements, which have NGS characteristic when a sample is quenched from a quenching temperature with a medium cooling rate, the resultant segregation at grain boundaries will be the sum of EGS produced at the quenching temperature and NGS produced during the quenching. Such a resultant segregation level will vary with the holding temperature when quenching rate is constant.
Preparation and performance evaluation of swine manure bio-oil modified rubber asphalt binder
Abstract In order to study the effect of the preparation process of swine manure bio-oil modified rubber asphalt binder on the rheological properties of the binder, the matrix asphalt and asphalt rubber were used as the control group, and swine manure bio-oil modified rubber asphalt binders were obtained by three preparation processes. Then the rheological properties of five binders were evaluated by rotational viscosity, DSR, MSCR and BBR tests. The results show that the addition of swine manure bio-oil to asphalt rubber is beneficial to the workability, but cause negative effect on its high temperature performance, fatigue performance and low temperature crack resistance. The preparation method of premixing bio-oil and rubber powder can reduce the degradation of high-temperature performance of asphalt rubber caused by addition of swine manure bio-oil to a certain extent. The fatigue properties of the three bio-binders are close, and the critical temperature is less than 25 °C. The process of premixing bio-oil and rubber powder will increase the creep stiffness and reduce the low-temperature performance. However, the low-temperature performance of the binder obtained by this process is still better than that of the matrix asphalt and can meet the requirements of use. Therefore, it is recommended to use the preparation process of premixing bio-oil and rubber powder to prepare swine manure bio-oil modified asphalt rubber.