Effects of repeated thermal cyclings on the transformation temperature, microstructure and mechanical properties have been investigated in a cobalt-free tungsten-bearing maraging steel(W-250). The alloy has a composition of Fe-19Ni-4.5W-1.2Ti-0.1Al and is strengthened by $Ni_3W$ and $\eta-Ni_3Ti$ precipitates in a bcc martensitic matrix. The $M_s$ temperature of the alloy increased significantly, while the $A_s$ temperature decreased with repeated thermal cyclings between room temperature and above $A_f$. The shifts of the transformation temperatures were attributed to the local enrichment and depletion of solute atoms around fine precipitates formed during heating. The degree of heterogeneity in solute distribution increased with increasing number of heating and cooling cycles. The presence of more solute-enriched area in martensite by thermal cyclings leads to lowering $A_s$ temperature during heating. The austenite to martensite transformation during cooling starts at the solute poor area of austenite. Therefore, the more solute-depleted area produced by the thermal cycling gives rise to a higher $M_s$ temperature. The decrease in $A_s$ temperature and the increase in $M_s$ temperature vary also with the thermal cycling conditions. The faster the heating rate, the smaller variations in transformation temperatures occur, because the microsegregation due to precipitation in each cycle is inhibited. When the cycling temperature is either low or the holding time is short, the variations in transformation temperatures are large due to less chance to homogenize. In the extreme case, some austenite is so enriched in solute that its $M_s$ temperature falls down below room temperature, thereby the austenite remaining untransformed to martensite at room temperature.
The austenite grains just transformed from martensite during heating inherit the prior austenite grain boundaries unless recrystallization occurs. When the alloy is continuously heated with heating rate of 100℃/min, the austenite grain size does not change up to 870℃, which is 140℃ higher than $A_f$ temperature. Small recrystallized grains were observed at 870℃, and found to form preferentially at the prior austenite grain boundaries. When the temperature exceeded 870℃, the recrystallization proceeded into the remaining unrecrystallized region inside the grain. The recrystallization temperature becomes higher with faster heating rate. With a holding time of one four, the recrystallization was completed at 850℃ and the grain size reduced from 220㎛ to 70㎛.
Since the recrystallization occurring during heating is based on the lattice defects produced by martensite to austenite transformation, the repeated thermal cyclings produce refined structure in the alloy. If the cycling temperature, however, was low below 775℃, the thermal cyclings were ineffective due to the retardation in recrystallization. On the other hand, a cycling temperature of higher than 875℃ resulted in a grain growth after recrystallization. The optimum temperature in grain refinement was found to be 850℃ for the holding time of 30 min. Using 850℃ as a cycling temperature, the grain size was reduced to 15㎛ at 6 cycles and it remained about constant with further cycles. With this condition, no austenite phase was found to be retained at room temperature.
The refined structure by thermal cyclings endowed, as a whole, an improvement in mechanical properties including tensile properties and fracture toughness. The increase of strength and fracture toughness depended on the refinement of the alloy. That is, a more refined structure showed higher values of both strength and toughness. After four thermal cyclings between room temperature and 85℃ the yield strength increased from 1806 to 1885MPa and fracture toughness also from 73.7 to 76.8 MPam$^{1/2}$ for the aged condition of 480℃ for 3 hrs.
