Knowledge

Among all the cast metals, gray cast iron is the most versatile. It has many desirable properties that no other casting alloy possesses, and it is the cheapest iron material that can be effectively utilized. Among iron alloys, gray cast iron has the lowest pouring temperature, which can be verified by its good fluidity and casting ability when pouring complex cavities. Due to its unique solidification mechanism, gray cast iron has very low or even no liquid or solid phase shrinkage, making it easy to obtain qualified castings. Compared with iron alloys of the same tensile strength, gray cast iron has better machinability and vibration damping capacity, higher thermal conductivity, and good corrosion resistance, oxidation resistance, and wear resistance.

 

For most applications, gray cast iron is often used in its as-cast state, and its mechanical properties are mainly determined by the chemical composition and cooling conditions in the mold. Through appropriate composition adjustment, the tensile strength of the castings can be increased from less than 140 MPa to over 400 MPa in the as-cast state, and the hardness can be increased from 100 HBV to 300 HBV. The performance of the castings can be further improved through subsequent heat treatment. After heat treatment, the morphology of the graphite does not change, although the volume of the graphite increases when it transforms from pearlite to ferrite, the precipitated graphite only adheres to the original graphite layers. The changes in the matrix due to heat treatment are basically the same as those in steel.

 

The following improvements can be achieved through heat treatment:

1. Release of internal stress

2. Improvement of machinability

3. Enhancement of strength

4. Increase in wear resistance

5. Improvement of performance uniformity in thick and large parts

 

1.1 Release of Internal Stress

The full release of internal stress can prevent problems such as warping, cracking, or dimensional deviations that may occur during subsequent processing or use. This is because the residual stress generated during the solidification of the casting is the main cause of dimensional deviations.

The stress relief heat treatment process involves slowly heating the casting to a sufficiently high temperature, typically between 500°C and 600°C, to release residual stress through plastic deformation. The casting is then held at this temperature for over an hour before being slowly cooled in the furnace to 200°C to 300°C and finally air-cooled to room temperature. This heat treatment process has an extremely minimal effect on the original strength and hardness of the casting.

The above heat treatment process can also be used to relieve stress generated during machining, cold working, or welding processes.

 

1.2 Improving Machinability:

The machinability of cast iron can be improved through annealing or normalizing. Both of these heat treatment processes require heating the castings to the austenite temperature range.

Annealing involves heating the castings to a critical temperature above 900°C to 940°C, holding for 1 hour, and increasing the holding time by 1 hour for every additional 25mm of casting wall thickness. Then, the castings are slowly cooled in the furnace to 300°C and then air-cooled to room temperature. Annealing softens the cast iron by transforming the microstructure into ferrite and free carbides.

Low-temperature or subcritical annealing is used for castings with a ferrite matrix and free carbides. The process involves heating the castings to 675°C to 725°C, holding for 1 hour, and increasing the holding time by 1 hour for every additional 25mm of casting wall thickness. Then, the castings are cooled in the furnace to 300°C and then air-cooled to room temperature. Subcritical annealing does not destroy the carbides.

Since carbon and silicon accelerate the decomposition of pearlite at annealing temperatures, the annealing time should be reduced when their content increases. On the other hand, elements that stabilize pearlite, such as antimony, tin, vanadium, chromium, copper, manganese, and phosphorus, will prolong the decomposition of pearlite.

Normalizing involves heating the castings above the critical point and holding for a certain period (the heating temperature and holding time are the same as annealing), and then directly air-cooling to room temperature. The normalizing process destroys the carbides but does not change the pearlite structure, and is suitable for producing high-strength and high-hardness cast iron parts.

Increasing the normalizing temperature will increase the carbon content in the austenite and the volume of cementite in the residual pearlite. The increase in cementite in the pearlite will increase the hardness and tensile strength of the cast iron parts.

Another factor affecting the hardness and tensile strength of gray cast iron parts is the pearlite lamellar spacing, which is determined by the cooling rate after austenitization and the alloy composition. The faster the cooling rate, the smaller the pearlite spacing, and the higher the hardness and tensile strength. However, an excessively high cooling rate can cause partial or complete martensitic transformation. Therefore, high normalizing temperature and high cooling rate will promote martensitic transformation.

 

1.3 Enhance Strength

The highest tensile strength can be achieved through quenching and tempering heat treatment. This process typically involves oil quenching followed by tempering at 400°C to 550°C.

This heat treatment process is limited to simple-shaped castings because oil quenching can cause warping or cracking tendencies in complex castings. Generally speaking, this method is better than increasing strength through alloying.

 

1.4 Improving Wear Resistance

Quenching and tempering treatment is also used to improve the wear resistance of cast iron parts. Quenching in oil or water above the critical point will inhibit the transformation of ferrite and pearlite and form a martensitic structure. Then tempering is carried out at different temperatures to obtain the desired hardness. Similarly, this treatment process can only be used for relatively simple-shaped castings.

Flame hardening or induction hardening is also often used to improve the wear resistance of the working surface of cast iron parts. A typical example is the convex surface of an automotive camshaft.

 

1.5 Homogenization

The properties of cast iron parts in their as-cast state may vary significantly at different locations, especially for complex parts and those with large variations in wall thickness. After heat treatment processes such as annealing and tempering, the properties of various parts of the castings become more uniform.

 

Vigor has more than 20 years of experience in producing castings, forging and do the related heat treatment processes. If any thing we can help or any parts need to be developed, please feel free to contact us at info@castings-forging.com