Recirculation furnace for the heat treatment of large semiproducts and investigation of its thermal operation

Conclusions

• 1. We worked out an arrangement for the recirculation of the combustion products in continuous furnaces for the in-line heat treatment of long stock with circular cross section, and also of pipes in a horizontal position. Uniform heating of the metal is ensured: at the stage of technological holding the temperature fluctuations amount to \+-4\2-5\dgC at t_f=860\dgC and to 6\2-7\dgC at t_f=560\dgC. The largest temperature gradient along the stock at the stage of heating to the specified temperature amounted to 50\2-60\dgC at t_f=560\dgC and to 20\2-30\dgC at t_f=860\dgC.

• 2. We worked out a method of directly determining the frequency of recirculation in the hot furnace during the treatment of metal; it consists in measuring the temperature of fresh combustion products, spent gases, and their mixture, with subsequent substitution of these temperatures into the found dependence

  $$K = \frac{{t_{fr} - t_{spt} }}{{t_{mx} - t_{spt} }}.$$

  The frequency of recirculation, determined by this method, was 6–8, at t_f=560°C and 2–3 at t_f=860°C.

• 3. The newly designed continuous recirculation furnace is recommended for various heat-treatment regimes in the temperature range 500\2-900\dgC: normalization, heating for hardening, annealing, tempering, simultaneous treatment in two rows of the furnace, each row having a different temperature regime, treatment of long stock with different temperature specification along it, combined regimes including stepped annealing, normalization and tempering or annealing, normalization and slow cooling on line, etc.