Changes in the dimensions of borided machine parts in the process of heating and cooling

1. The coefficient of thermal expansion of the boride case differs for different steels and is 1.5–2.0 times lower than that of the base metal. 2. The boride case is capable of elastic deformation under the influence of forces resulting from the expansion of the base metal during heating and the γ→α transformation, and also resulting from shrinkage of the base metal during the α→γ transformation and cooling, and has hardly any effect on the change in the volume of the base metal that occurs. 3. At temperatures below the γ→α transformation the reduction in transformation the reduction in the length of borided samples during cooling practically ceases, which may be due to precipitation of excess boron in the form of Fe2B from the α phase in the transition zone and the redistribution of stresses in the boride case. 4. To reduce deformation of borided parts it is expedient to cool them from boriding temperature at a rate suppressing the precipitation of excess boron from α phase in the transition zone and the unfavorable redistribution of stresses in the boride case.