Heat treatment for die casting usually includes these：

• Annealing
• Normalizing
• Quenching
• Tempering

These steps improve the mechanical properties of die castings, such as mechanical strength, fatigue strength, ductility and impact strength, as well as dimensional stability.

Heat treatment for die casting

The heat treatment of die castings is a very critical part of their manufacturing process.

Which directly affects the performance and quality of die castings.

Heat treatment mainly includes annealing, normalizing, quenching, and tempering processes, which are “four fires” in die casting.

Heat treatment for die casting——Annealing

The purpose of annealing is to reduce the hardness of the material and improve the plasticity and toughness.

Soft annealing is usually employed, heated to 850°C, then cooled to 650°C at a rate of 10°C per hour, and finally air-cooled.  

Heat treatment for die casting——Normalizing

Normalizing is to heat the steel to 30~50 °C above AC3, keep it warm and cool it in the air to obtain a finer pearlite structure.

The cooling rate after normalizing affects the amount of ferrite precipitated and thus the hardness.

Heat treatment for die casting——Quenching

Quenching is to heat the steel above the phase change temperature (AC1 or ACm).

And after heat preservation, at a fast cooling rate (such as water cooling)

That is greater than the critical cooling rate of the steel to obtain a martensitic structure with high hardness.  

Preheating temperature

600-900°C, usually preheated in two stages, the first stage is at 600-650°C, and the second stage is at 820-850°C.

Austenitization temperature: 1000-1030°C.

Quenching medium: high-speed gas/circulating atmosphere, vacuum quenching, salt bath furnace or flow pellet furnace, etc.

Heat treatment for die casting——Tempering

Tempering is a heat treatment process that reheats quenched parts to a temperature below the AC1 line, with enough time to stay warm, and then cools down to room temperature.

Temperature

Die-casting die tempered three times, hot forging die and extrusion die tempered twice to achieve the required hardness

Holding time

Lasts for two hours and guarantees the tempering temperature to avoid tempering brittleness.

The role of heat treatment 

Through heat treatment technology, the stress and deformation in the die casting can be effectively eliminated.

• Annealing treatment
• Stress-relieving annealing
• Isothermal quenching
• Solution treatment and aging treatment
• ABP treatment
• High-temperature annealing
• Thermodynamic simulation

Annealing treatment

Heat the casting to an appropriate temperature (such as 550 °C ~ 650 °C), and then keep it warm for a period of time.

So that the temperature of all parts of the casting and the surface and the inside is uniform, and the residual stress is relaxed and stabilized under this heating temperature.

After that, it is cooled to room temperature at a slow cooling rate (such as 20°C/h~40°C/h), which can basically eliminate the internal stress of the casting.

Stress-relieving annealing

For steel castings, stress-relieving annealing is usually used to eliminate the internal stress of the casting and prevent cracking.

This treatment generally includes high-temperature diffusion annealing, complete annealing or incomplete annealing.

Isothermal quenching

The study shows that during the isothermal quenching process, the surface hot compressive stress decreases with the increase of isothermal temperature.

Therefore, it is necessary to control the magnitude of residual stress and adjust the temperature of isothermal quenching.

Solution treatment and aging treatment

For aluminum alloys, solution treatment (e.g., 555°C, 2 hours) and aging treatment (e.g., 170°C, 7 hours) can effectively reduce residual stress.

However, it is important to note that quenching may increase residual stresses.

So careful selection of the appropriate heat treatment process is important.

ABP treatment

In the die casting mould, after a certain number of times each die casting,reduces residual tensile stress on the mold surface, thereby increasing mold life.

High-temperature annealing

Annealing of gray iron castings at high temperatures, eliminating the white mouth structure and reducing the hardness.

Above eutectic temperatures, cementite decomposes into graphite, which relieves casting stresses.

Thermodynamic simulation

The thermodynamic simulation of the casting using finite element software can predict the stress distribution of the casting at different temperatures.

And optimize the heat treatment process accordingly to achieve the best stress relief effect.

Heat Treatment Process of Die Casting

The thermal treatment process for die casting is structured around four sequential steps.

Each step plays an important part in stabilizing the size & crystalline structure of the parts.

Enhancing their mechanical properties, improving their machining performance, and removing any internal stress.

• Casting Solidification
• Solution Annealing
• Quenching
• Artificial Aging

Casting Solidification

Injecting molten metal into the mold cavity at high speed and rapidly cooling the metal down is the initial step of die casting.

This complete process is done with die casting machines.

Fast cavity filling and fast cooling removes any chance of filling gaps in the mold or disturbed structural stability of the parts.

Solution Annealing

Annealing helps with stabilizing the shape and size of die casting parts.

It’s an important step which also removes the casting stress of the parts caused by machining.

Annealing of die casting parts is done at 450-490 ℃ temperature for a brief time.

Quenching

Die casting parts undergo solution treatment, also known as cold treatment, to maximize the dissolution of reinforcing elements.

This process involves annealing or heating the parts to around 500°C for several hours, followed by rapid quenching in water at 60-100°C.

Artificial Aging

Aging encompasses a low-temperature tempering technique that includes both natural and artificial aging.

Natural aging refers to the aging of parts at ambient room temperature.

While artificial aging involves subjecting them to temperatures ranging from 120 to 280℃ for extended durations.

Incomplete artificial aging

It is performed at ~120℃ temperature for 3-4 hours.

Incomplete artificial aging of die casting parts increases their tensile strength, toughness, and plasticity at the cost of lower corrosion resistance.

Complete artificial aging

It is performed at ~150-180℃ temperature for >5 hours.

It sufficiently increases the tensile strength of parts at the cost of lower elongation.

Over-aging

It is performed at ~280℃ temperature for >3 hours.

It strengthens the parts by increasing their stress resistance and corrosion resistance.

Haichen‘s Heat Treatment Specifications

Equipment requirements

The annealing furnace should be closed, insulated, and equipped with an exhaust device; Local air supply and cooling devices should be installed at the job post.  

During heat treatment, especially when heated to high temperatures without protection, this can easily lead to scrapping of the mold or burning the surface layer.

For this reason, Haichen usually uses equipment such as vacuum furnaces, salt bath furnaces or inert gas furnaces for heat treatment.  

Operation specification

Haichen requires the operator to handle the material as gently as possible when loading and unloading to reduce the noise caused by the collision of the casting.