Mechanical and Physical Properties of Differently Alloyed Sintered Steels as a Function of the Sintering Temperature

In this paper, the effect of processes occurring during the sintering of four powder metallurgy steel grades on the resulting properties were investigated. This included three grades prepared from plain iron powder with admixed graphite, one grade alloyed also with elemental copper and another with Fe-Mn-Si masteralloy. One further grade was prepared from Cr-Mo pre-alloyed powder with admixed graphite. The effect of the sintering processes was examined in the temperature range of 700–1300 °C in an inert atmosphere (Ar). In order to study oxygen removal, DTA/TG runs linked with mass spectrometry (MS) as well as C/O elemental analysis were performed. Charpy impact tests and fractography studies were performed to study the effect of the temperature on the formation and growth of sintering contacts. Characterization also included metallography, dimensional change, sintered density, and hardness measurements to describe the dissolution of carbon and alloying elements during the process. Physical properties that were measured were electrical conductivity and coercive force. The results showed that, in all steels, the reduction of oxides that occur during the heating stage plays a key role in the formation and growth of the sintering contacts as well as in the completion of alloying processes. In the chromium alloy steel, the presence of the stable chromium oxides delays these processes up to higher temperatures, while in the other steels that are based on plain iron powder, these processes take place earlier in the heating stage, at lower temperatures. Compared to the standard Fe-C and Fe-Cu-C grades, the Cr-Mo steel requires more sophisticated sintering to ensure oxygen removal, but on the other hand it offers the best properties. The masteralloy variant, finally, can be regarded as a highly attractive compromise between manufacturing requirements, alloy element content, and product properties.

Effect of groundwater forced seepage on heat transfer characteristics of borehole heat exchangers

A system of borehole heat exchangers (BHEs) combined with pumping–injection wells is established in areas where the groundwater is shallow and the seepage velocity is weak. The pumping and injection wells are set on both sides of the BHEs. According to the three-dimensional unsteady-state heat transfer model in the aquifer, the convection–dispersion analytical solution of excess temperature is derived that considers groundwater-forced seepage and thermal dispersion effects and axial effect of the BHEs. Then, we use the dimensional analysis method and similarity criteria to build a controllable forced seepage sandbox. The software FEFLOW 7.1 is adopted and the simulation results are validated by the theoretical analysis and the indoor experiment test. On this basis, the numerical simulation is used to explore the influence of different pumping–injection flow volume on the Darcy flow velocity of the aquifer where the BHEs are located, as well as the average heat transfer efficiency and the heat transfer rates with borehole depth. The results show that when the pumping flow volume increases from 200 m3 day−1 to 1200 m3 day−1, the Darcy velocity correspondingly increases to about 10 times. The average heat efficiency coefficient of the BHEs is increased by 11.5% in cooling stage, and by 7.5% in heating stage. When the pumping–injection flow volume is 400–600 m3 day−1, the increment of heat transfer rates of the BHEs reaches 12.8–17.9 W m−1 and 3.6–4.2 W m−1 per unit of borehole depth during the cooling stage and heating stage, respectively, and then decreases as the flow volume increases gradually.

Effect of groundwater forced seepage on heat transfer characteristics of borehole heat exchangers

A borehole heat exchangers (BHEs) combined with pumping-injection well is established in areas where the groundwater is shallow and the seepage velocity is weak. The pumping and injection wells are set on both sides of the BHEs. According to the three-dimensional unsteady heat transfer model in aquifer, the convection-dispersion analytical solution of excess temperature is derived that considers groundwater forced seepage and thermal dispersion effects and axial effect of the BHEs. Then, the dimensional analysis method and similarity criteria we used to build a controllable forced seepage sandbox. The software FEFLOW 7.1 is adopted and the simulation results are validated by the theoretical analysis and the indoor experiment test. On this basis, the numerical simulation calculation is used to explore the influence of different pumping-injection flow volume on the Darcy flow velocity of the aquifer where the BHEs are located, the average heat transfer efficiency and the heat transfer rates with borehole depth. The results show that when the pumping flow volume increases from 200 m3∙d-1 to 1200 m3∙d-1, the Darcy velocity correspondingly increases to about 10 times. The average heat efficiency coefficient of the BHEs is increased by 11.5% in cooling stage, and by 7.5% in heating stage. When the pumping-injection flow volume is 400~600 m3∙d-1, the increment of heat transfer rates of the BHEs reaches 12.8~17.9 W∙m-1 and 3.6~4.2 W∙m-1 per unit of borehole depth during the cooling stage and heating stage respectively, and then decreases as the flow volume increases gradually.

Effect of groundwater forced seepage on heat transfer characteristics of borehole heat exchangers

A borehole heat exchangers (BHEs) combined with pumping-injection well is established in areas where groundwater is shallow but the seepage velocity is weak, which sets up pumping and injection wells on both sides of the BHEs. According to the three-dimensional unsteady state heat transfer model in aquifer, we derive the convection-dispersion analytical solution of excess temperature in aquifer that considers groundwater forced seepage and thermal dispersion effects in aquifer and the axial effect of the BHEs. Then, we use dimensional analysis method and similarity criteria to build a controllable forced seepage sandbox. The theoretical analysis is combined with the indoor experiment test to verify the correctness and accuracy of the numerical simulation software FEFLOW7.1. On this basis, we perform the numerical simulation calculation to explore the effects of different pumping-injection flow volume on the Darcy flow velocity of the aquifer where the BHEs are located, the average heat transfer efficiency and the heat transfer rates per unit borehole depth of the BHEs. The results show that when the pumping flow volume is increased from 200 m3∙d-1 to 1200 m3∙d-1, the Darcy velocity correspondingly increases to about 10 times the previous velocity. The average heat efficiency coefficient of the BHEs increases by 11.5% in cooling stage, and by 7.5% in heating stage. When the pumping-injection flow volume is 400~600 m3∙d-1, the increment of heat transfer rates per unit borehole depth of the BHEs reaches 12.8~17.9 W∙m-1 and 3.6~4.2 W∙m-1 during the cooling stage and heating stage respectively, and then decreases as the flow volume increases gradually.

Temperature-Programmed Oxidation Experiments on Typical Bituminous Coal Under Inert Conditions

In this study, an experimental investigation was presented on the oxidation behaviors of bituminous coal for different inert gases (N2 and CO2) at different concentrations (oxygen concentration indexes 21%, 18.4%, 15.8%, and 13.1%) using a temperature-programmed experimental device. The purpose of this research was to examine the oxidation patterns of bituminous coal under different inert conditions. The results showed that: (1) the oxidative heating of the coal underwent two stages: an initial slow heating stage and a fast heating stage. The injection of both inert gases would result in a delay in the crossing point temperature (CPT) of the coal, but the injection of N2 resulted in greater delays in the CPT of the coal; (2) the injection of both N2 and CO2 inhibited the concentrations of CO and alkane/olefin gases produced from the oxidative heating of the coal, with CO2 displaying higher inhibition efficiencies than that of N2; (3) Under a non-inerting environment, the C2H4 and C2H6 generation temperatures were 110 °C and 100 °C. Under an inerting environment, when N2 was injected, the higher the N2 concentration, the higher the initial C2H4 and C2H6 generation temperatures; when CO2 was injected, the higher the CO2 concentration, the lower the initial C2H4 and C2H6 generation temperatures; and (4) under a non-inerting environment, the C3H8 generation temperature was 90 °C; and when an inert gas was injected, there was a hysteresis in the C3H8 generation temperature for all concentrations. The above research results can be used to predict the spontaneous combustion of residual coal in an inert environment and prevent fires.