Post Heat Treatment Standard for Gray Cast Iron Casting
Heat treatment is an important process in the manufacture of iron and steel. It provides the necessary conditions for changing microstructure, improving the mechanical properties of material, eliminating internal stress and improving dimensional accuracy. This document specifies the heat treatment process and acceptance criteria of gray cast iron casting.
1. Scope
This standard applies to the heat treatment of gray cast iron castings, including general heat treatment and stress relief heat treatment.
2. Definitions
2.1 Gray cast iron: Generally, the pearlite and cementite in gray cast iron appear in the form of flake graphite, and its supporting phase is ferrite.
2.2 Normalizing: The heat treatment process of gray cast iron in which the workpiece is heated and cooled in the furnace to achieve the purpose of uniform organization and approximately the same components and improving mechanical properties.
2.3 Annealing: The heat treatment process of gray cast iron in which the workpiece is heated to a temperature above the upper critical point Ac1, kept for a certain period of time, cooled to 550℃, and then cooled.
2.4 Stress relief annealing: The heat treatment process of gray cast iron in which the workpiece is heated to a temperature below the upper critical point Ac1, and kept for a certain period of time to eliminate internal stress.
3. Heat Treatment Process
3.1 Normalizing
3.1.1 Temperature
The normalizing temperature of various gray cast iron castings should be determined according to the characteristic of the castings and verify with experiment methods. Generally, the normalizing temperature should be in the range of 900-920℃.
3.1.2 Holding time
Usually, the time should be twice as much as the time of heating.
3.1.3 Cooling method
According with the size of the casting, the natural cooling in the furnace or the forced cooling by air or water can be adopted in the process of normalizing.
3.2 Annealing
3.2.1 Temperature
According to the characteristic of the castings and verify with experiment methods, the annealing temperature should be usually in the range of 600-750℃.
3.2.2 Holding time
When the temperature is higher than 650℃, the holding time should be twice as much as the time of heating. And when the temperature is 600℃ or lower, the holding time should not be less than 6h.
3.2.3 Cooling method
The cooling method should be usually natural cooling in the furnace or cooling in the sand.
3.3 Stress relief annealing
3.3.1 Temperature
According with the characteristic of the castings and verify with experiment methods, the stress relief annealing temperature should be usually in the range of 400-550℃.
3.3.2 Holding time
Usually, the time should be twice as much as the time of heating.
3.3.3 Cooling method
The cooling method should be usually natural cooling in the furnace or cooling in the sand.
4. Acceptance Criteria
The acceptance criteria of gray cast iron castings after heat treatment should meet the design requirements of the castings and should be tested with an impact hammer. The elongation of impact specimen should be greater than 5%. The non-metallic inclusions in the heat treated specimen should not be more than the standard requirements.
5. Records
The records of heat treatment should include material grade, furnace temperature, furnace temperature control, holding time, cooling method and righting method and other conditions.
6. Others
The heat treatment of gray cast iron castings should be carried out strictly according to the process specified in this standard and relevant standards and specifications, and the inspection and acceptance of the heat treated castings should be conducted according to the specified requirements.
