Heat treatment of tool steel is a metallurgical process that alters the microstructure of a metal to achieve specific properties. For mold steel, this process involves heating the steel to a specific temperature, holding it there for a certain time, and then cooling it at a controlled rate. The goal is to enhance properties such as hardness, strength, toughness, and wear resistance, which are crucial for mold components.
The commonly used heat treatment methods for steel are annealing, normalizing, quenching, and tempering.
Annealing
Annealing is a heat treatment process where steel is heated to a specific temperature, held at that temperature for a certain period, and then allowed to cool slowly in the furnace. The resulting microstructure is typically pearlite and ferrite.
The purpose of annealing is to eliminate structural defects, improve the microstructure for uniform composition, refine grain size, enhance the mechanical properties of the steel, and reduce residual stress. At the same time, annealing can decrease hardness, increase ductility and toughness, and improve machinability.
Since annealing eliminates and improves the structural defects and internal stresses left over from previous processes and prepares for subsequent processes, it is considered a semi-finished product heat treatment, also known as preparatory heat treatment.
Normalizing
Normalizing is a heat treatment process where steel is heated to a temperature above its critical point, causing it to transform completely into a uniform austenite structure. Subsequently, it is allowed to cool naturally in the air. The resulting microstructure is typically sorbite. This process eliminates network carbides in hypereutectoid steels and refines grain size in hypereutectoid steels, improving overall mechanical properties. For components with less stringent requirements, normalizing can be a more economical alternative to annealing.
The primary distinction between normalizing and annealing lies in the cooling rate. Normalizing involves a slightly faster cooling rate than annealing, resulting in a finer microstructure and enhanced mechanical properties. Additionally, normalizing does not require the use of a furnace for cooling, leading to higher productivity. Consequently, normalizing is often preferred over annealing in manufacturing processes.
Primary Applications of Normalizing Heat Treatment:
- Low-Carbon Steel: Normalizing results in slightly higher hardness and better toughness compared to annealing, making it suitable as a pre-treatment for machining.
- Medium-Carbon Steel: It can replace quenching and tempering as the final heat treatment.
- Tool Steel, Bearing Steel, and Carburized Steel: Normalizing can reduce or suppress the formation of network carbides, resulting in a fine-grained structure required for spheroidizing annealing.
- Cast Steel: It refines the as-cast structure, improving machinability.
- Large Forgings: Normalizing can be the final heat treatment, reducing the risk of cracking during quenching.
- Nodular Cast Iron: It increases hardness, strength, and wear resistance, making it ideal for components like crankshafts and connecting rods in automobiles, tractors, and diesel engines.
Quenching
Quenching is a heat treatment process where steel is heated to a specific temperature, held at that temperature for a certain period, and then rapidly cooled in a quenching medium. This rapid cooling, at a rate greater than the critical cooling rate, results in a non-equilibrium microstructure primarily consisting of martensite or lower bainite. Quenching enhances the strength and hardness of steel but reduces its ductility. Common quenching media include water, oil, alkaline solutions, and salt solutions.
Quenching is widely used for tools, molds, gauges, and components requiring surface wear resistance, such as gears, rollers, and carburized parts, due to its ability to increase metal workpieces’ hardness and wear resistance. Additionally, quenching can impart specific physical and chemical properties to certain steels, such as increasing the ferromagnetism of permanent magnets and improving the corrosion resistance of stainless steel.
Tempering
Tempering is a heat treatment process where steel that has already been hardened is reheated to a specific temperature and then cooled using a particular method. Tempering aims to eliminate the internal stresses caused by quenching, reduce hardness and brittleness, and achieve the desired mechanical properties. Tempering is often used in conjunction with quenching and normalizing. Based on the tempering temperature, it can be classified into low-temperature tempering, medium-temperature tempering, and high-temperature tempering.
Low-temperature tempering (150-250°C):
- Produces tempered martensite.
- Aims to maintain high hardness and wear resistance while reducing internal stresses and brittleness.
- Commonly used for high-carbon cutting tools, gauges, dies, rolling bearings, and carburized parts.
- Results in a hardness of around 60HBC.
Medium-temperature tempering (350-500°C):
- Aims to achieve high yield strength, elastic limit, and toughness.
- Primarily used for springs and hot working dies.
- Results in a hardness of around 45HBC.
High-temperature tempering (500-650°C):
- Often referred to as quenching and tempering or “quench and temper.”
- Provides a good balance of strength, hardness, ductility, and toughness.
- Widely used for critical structural components in automobiles, tractors, and machine tools, such as connecting rods, bolts, gears, and shafts.
- Results in a hardness of around 260HBW.
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