The common annealing methods and processes for Q420D high-strength steel plates are as follows:

 

Full Annealing: Heat the Q420D steel plate to 30-50°C above the temperature at which all ferrite transforms into austenite, i.e., 30-50°C above Ac3 (Ac3 is the temperature at which ferrite completely transforms into austenite when hypoeutectoid steel is heated). Hold it at this temperature for a period of time to fully austenitize and homogenize the steel structure, then slowly cool it along with the furnace. This annealing method can refine the steel structure, reduce hardness, improve plasticity, and enhance machinability.

Incomplete Annealing: Heat the steel plate to a temperature between Ac1 (the temperature at which pearlite transforms into austenite when eutectoid steel is heated) and Ac3 (for hypoeutectoid steel) or between Ac1 and Acm (the temperature at which secondary cementite completely dissolves into austenite when hypereutectoid steel is heated) (for hypereutectoid steel). After holding at this temperature, cool it slowly. This method can recrystallize pearlite, refine grains, reduce hardness, and eliminate internal stress. It is mainly used for forged and rolled parts of medium-carbon, high-carbon steels, and low-alloy structural steels, and can replace full annealing.

 

 

 

 

 

 

 

 

 

 

 

 

 

Isothermal Annealing: First, slowly heat the Q420D steel plate to 30-50°C above Ac3 (for hypoeutectoid steel) or Ac1 (for eutectoid and hypereutectoid steels), hold it at this temperature for a period of time to austenitize the steel, then quickly transfer it to another furnace with a temperature slightly lower than Ar1 (the temperature at which austenite transforms into pearlite). Keep it at this isothermal temperature until all austenite transforms into lamellar pearlite, and finally take it out of the furnace and cool it in air. Isothermal annealing can significantly shorten the production cycle and enable the workpiece to obtain a more uniform structure and performance. It is often used for alloy steels where the transformation of supercooled austenite in the pearlite phase transformation temperature range is relatively slow, and Q420D sometimes adopts this annealing method.

Stress Relief Annealing: Generally, heat the steel plate to a relatively low temperature, such as around 500-650°C, hold it at this temperature for a certain period of time, then cool it slowly, usually along with the furnace. Its purpose is to eliminate residual stresses generated in the steel plate during casting, forging, welding, or machining processes, stabilize the workpiece size, prevent deformation and cracking, while having little impact on the hardness, strength, and other mechanical properties of the material.

 

 

 

 

 

 

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