What is the difference between overheating and overburning in heat treatment of cast steel parts? How to avoid overheating?

1、 What is the difference between overheating and overburning in cast steel heat treatment?

Definition and Essence Overheating: Refers to the phenomenon of significantly coarse austenite grains in cast steel due to excessively high heating temperature or long holding time during heat treatment. At this time, there is no oxidation or melting inside the material, only abnormal grain structure. Overheating: It is a more serious defect than overheating, where the heating temperature exceeds the solidus line of the steel, causing local melting or oxidation of grain boundaries and breaking the bonding force between grains. Macroscopic and microscopic characteristics of overheating: Macroscopic: There is no significant change on the surface of the steel, and the fracture surface may exhibit coarse grain luster (such as a "sugar like" fracture surface). Microscopic: Austenitic grains are severely coarse, possibly accompanied by abnormal structures such as Weibull structure. Overheating: Macroscopic: There may be oxide scale, bulges or cracks on the surface, and the fracture surface is rough and lacks metallic luster. Microscopic: There are melting marks, oxide inclusions, and even grain boundary cracking at the grain boundaries. The impact on performance is overheating: it can cause a slight decrease in steel strength and hardness, a significant decrease in plasticity and toughness, and a significant decrease in impact toughness. -Overheating: Severe deterioration of the mechanical properties of steel, almost complete loss of strength and plasticity, material becoming brittle and unable to be repaired through subsequent heat treatment. Repackable overheating: By reheating (within the normal temperature range) and normalizing or annealing treatment, the grain size can be refined and some properties can be restored. Overheating: It is an irreversible defect that cannot be repaired once it occurs, and the material can only be scrapped.

2、 Preventive measures for overheating phenomenon during heat treatment of low carbon steel and medium carbon steel

Low carbon cast steel (carbon content ≤ 0.25%) and medium carbon cast steel (carbon content 0.25%~0.60%) have slightly different sensitivity to overheating during heat treatment due to compositional differences. However, the core idea of preventing overheating is the same, and specific measures are as follows:

1、 Strictly control the heating temperature and holding time to accurately set the temperature range: Low carbon cast steel: The austenitizing temperature is usually 850~920 ℃ (avoid exceeding 950 ℃ to prevent excessive dissolution of ferrite and coarse grain size). Medium carbon cast steel: The austenitization temperature is usually 820~880 ℃ (too high can cause complete dissolution of pearlite and rapid grain growth). In practical operation, it is necessary to accurately monitor the furnace temperature through thermocouples based on the thickness of the castings and the amount of furnace loading, in order to avoid local overheating. Reasonably control the insulation time: Based on the principle of "complete austenitization and no coarsening of grains", the insulation time is calculated according to the effective thickness of the casting (generally every 10mm thickness insulation for 30-60 minutes). When loading into the furnace, avoid excessive accumulation, ensure uniform heating, and reduce local insulation for too long.

2、 The optimized heating process adopts a stepped heating method: for large or complex castings, preheat at a lower temperature (such as 600-700 ℃) first, and then slowly raise to the austenitization temperature to reduce temperature difference stress while avoiding local overheating caused by rapid heating. Avoid repeated heating: Multiple heating can accumulate the risk of coarse grains. Try to minimize the number of repeated heat treatments for repaired parts, and if necessary, lower the secondary heating temperature (10-20 ℃ lower than the first time).

3、 Adjusting process details based on cast steel composition for low-carbon cast steel: Due to poor hardenability, normalizing treatment is often used to refine grains, and the heating temperature should be strictly controlled at 30-50 ℃ above Ac3 (Ac3 is the critical temperature for austenitization, while low-carbon steel is about 830-900 ℃) to avoid blindly raising the temperature in pursuit of hardenability. Medium carbon cast steel: prone to Weibull structure (ferrite precipitation along austenite grain boundaries) after overheating, precise control of insulation time is required to ensure complete dissolution of carbides, without blindly prolonging the time; If quenching and tempering treatment is carried out, the quenching heating temperature should avoid the "dangerous zone" of rapid grain growth (usually 10-30 ℃ lower than the normalizing temperature).

4、 Improve heating equipment and furnace installation methods to ensure uniform furnace temperature: regularly calibrate the temperature control system of the heating furnace, check the distribution of heating elements inside the furnace, and avoid local hot spots; When heating large items, anti radiation plates can be used for isolation, or flow guiding devices can be installed inside the furnace to ensure a uniform temperature field. Reasonable furnace installation: Reserve sufficient gap between castings (generally not less than 1/3 of the casting thickness) to avoid stacking and obstruction; Slender and thin-walled components are supported vertically or horizontally to reduce local heat concentration.

5、 Strengthen process monitoring and real-time monitoring of detection: For mass-produced castings, the first piece needs to undergo heat treatment, and the rationality of the process is confirmed through metallographic examination (observing grain size); Regular sampling during production to ensure that the grain grade meets the requirements (generally controlled at level 5 or above, with finer grains resulting in higher grades). Recording and tracing: Detailed recording of heating temperature, insulation time, furnace loading volume and other parameters for each furnace. In case of abnormalities, the cause can be quickly traced and the process can be adjusted in a timely manner.

Through the above measures, overheating defects during heat treatment of low-carbon and medium carbon cast steel can be effectively prevented, ensuring that the mechanical properties (such as toughness and strength) of the castings meet the design requirements.