Comparison of the characteristics of three annealing processes

Diffusion annealing (homogeneous annealing)

Practical range: alloy steel ingot and casting.

Objective: to eliminate the dendrite segregation produced by gold crystallization and to make the composition uniform, so it is also called uniform annealing.

Process: heat the ingot or casting to Ac1 and above, about 1000~2000 C, keep it warm for 10~15 hours, then cool it with the furnace.

Features: high temperature and long time heating.

The higher the content of alloying elements in steel is, the higher the heating temperature is, and the high temperature heating for a long time is another cause of tissue overheating, so it is necessary to conduct annealing or normalizing after diffusion annealing to eliminate overheating.

The spheroidizing annealing

Application: carbon steel and alloy steel with eutectoid or supereutectoid components.

Objective: to improve the machining performance of the spheroidized cementite by decreasing its hardness and preparing it for quenching.

Process: the supereutectoid steel is heated to about 20~40°C above AC1, and then cooled slowly to below 600°C for air cooling.

Process features: heating and slow cooling at low temperature.

When heating temperature more than AC1 line, cementite begins to dissolve, but does not dissolve, the lamellar cementite gradually broken into many small chains or dotted cementite, dispersion on the austenitic matrix, at the same time due to short time heating in low temperature, austenitic composition is uneven also, therefore in the process of slow cooling or after isothermal cooling, with the original dots or points of small deep carbon constitution as the core, to generate new core where austenitic enrichment, evenly to form granular cementite.

Stress relief annealing (low temperature annealing)

Purpose: to eliminate residual stresses in castings, forgings, welds, cold stamping and machined parts that may potentially deform or crack during later machining or use. Process: slowly heat the workpiece to 600~650°C, holding time, and then slowly cool it to 200°C with the furnace before air cooling.