What are the key points of heat treatment process for cold pier molds

The heat treatment of cold pier molds is the core link that determines their final mechanical properties (hardness, toughness, wear resistance) and service life. It is necessary to balance "high strength and high hardness" with "sufficient toughness", while strictly controlling the risks of deformation and cracking. The key points of its process can be centered around four core stages: pretreatment, quenching, tempering, and deformation control, as follows:
1、 Preprocessing: laying the organizational foundation for subsequent heat treatment
The core purpose of pre-processing is to eliminate forging stress, refine grain size, improve processing performance, and provide a uniform original structure (such as spheroidized pearlite) for final heat treatment, avoiding cracking or uneven performance during subsequent quenching.
Spheroidization annealing (for high carbon and high alloy die steel)
Applicable materials: Cr12MoV, DC53, SKD11, etc. (with a carbon content of ≥ 1.0%), require spheroidization annealing to transform the network carbides into uniformly distributed spherical carbides, reducing the risk of quenching cracks.
Key points of the process:
Heating temperature: 780-820 ℃ (slightly higher than the Ac1 line to ensure partial dissolution of carbides);
Insulation time: 3-6 hours (depending on the mold size, ensure sufficient spheroidization of carbides);
Cooling method: Slow cooling (air cooling after furnace cooling to below 500 ℃) to avoid the precipitation of secondary network carbides.
Inspection standard: After annealing, the hardness is HB200-250, and the carbides in the metallographic structure are spherical and evenly distributed (without network or band segregation).
Stress relief annealing (for processed molds)
Applicable scenarios: After rough machining of molds (such as milling and grinding), eliminate machining stress and avoid cracking due to stress accumulation during quenching.
Process key points: Heat to 550-650 ℃, hold for 2-4 hours, slowly cool in the furnace to below 200 ℃, and remove from the furnace.
2、 Quenching: the key to determining the hardness and wear resistance of molds
The goal of quenching is to obtain a martensitic structure (ensuring high hardness) while minimizing residual austenite and quenching stress. Accurate control of heating temperature, insulation time, and cooling rate is required based on the composition of the mold material.
Heating temperature: Avoid overheating or underheating
Low alloy cold work die steel (such as Cr12MoV): 950-1000 ℃ (too high can lead to coarse grains and decreased toughness; too low can result in insufficient carbide dissolution and hardness);
High speed steel (such as W6Mo5Cr4V2): 1200-1240 ℃ (higher temperature is required to fully dissolve the alloy elements and ensure wear resistance);
Powder metallurgy steel (such as ASP-60): 1180-1220 ℃ (uniform distribution of carbides, narrow heating temperature range, requiring precise control).
Insulation time: Ensure that the carbides are fully dissolved and the grains are not coarse
Principle: "Thoroughly burn, avoid excessive length". The insulation time is related to the effective thickness of the mold (usually 1-3 hours), for example:
Small mold (thickness<20mm): insulation for 1-1.5 hours;
Large molds (thickness>50mm): insulated for 2-3 hours.
Attention: The insulation time for high-speed steel should be shorter (to avoid grain growth), usually 0.5-1 hour.
Cooling method: Control the cooling rate to reduce stress and deformation
Slow cooling rate: prone to formation of pearlite and insufficient hardness; Too fast cooling rate: excessive stress, easy to crack.
Common cooling methods:
Oil cooling: suitable for Cr12MoV, DC53, etc. (with moderate cooling rate to reduce the risk of cracking), with oil temperature controlled at 50-80 ℃ (to avoid sudden drops in cooling rate);
Graded quenching: After removing the mold from the quenching temperature, it is first placed in a 200-300 ℃ nitrate bath for insulation (10-20 minutes), and then air cooled. Can reduce the temperature difference between inside and outside, reduce deformation (suitable for complex shaped molds);
Air cooling: Only suitable for high-speed steel (such as W6Mo5Cr4V2), use air to slowly cool to avoid cracking.
Key taboos
Avoid rapid cooling of the mold in the 600-400 ℃ range (martensitic transformation zone, which can easily generate significant tissue stress);
After quenching, it is necessary to immediately temper (usually ≤ 2 hours) to prevent cracking caused by unstable residual austenite.