The structural mechanism of transformation during heating of steel

Conclusions

• 1. The precipitation of carbides and impurities in the boundaries of martensite platelets during heating to Ac₁ favors retention of the subgrain boundaries with prolonged tempering at high temperature.

• 2. The better mechanical properties after tempering at high temperature (improvement), as compared with annealing to spheroidal or lamellar pearlite, is due to retention of the numerous subgrain boundaries pinned by particles of second phase.

• 3. The reason for the formation of pseudomonocrystalline or restored (in size, shape, and crystallographic orientation) austenite grains in slow heating of quenched steel after the α→γ transformation is the preferential nucleation of austenite in the low-angle boundaries of the subgrains. In this case the growing sections of austenite retain the lamellar form and low-angle boundaries until they encounter each other on completion of the α→γ transformation.

• 4. The incomplete “restoration” of the grains and reduction of the recrystallization temperature at medium heating rates are due to the partial formation of spherical sections of austenite in carbide phase boundaries, which have a random orientation and higher angle boundaries. The reason for the formation of these sections is evidently the unrelaxed stresses occurring around the particles due to the difference in the expansion coefficients of the matrix and the carbides.