Some cast alloys have a smaller coefficient of thermal expansion and better inherent dimensional stability, which helps them achieve less distortion during heat treatment.

1. Cast Irons

• Gray cast iron has a high damping capacity and lower thermal expansion than steel, which contributes to its dimensional stability under thermal cycling. Stress relief annealing between 550–650°C is common to reduce internal stresses from casting without sacrificing strength and hardness. The common gray cast iron grades are GG-15, GG-20, and GG-25.
• Nodular (ductile) cast iron can also offer good stability. Furthermore, low thermal expansion ductile cast iron series (e.g., B1, B2, B3) with high nickel and cobalt contents show dimension change rates of only 0.005–026% and 0–0.044% along the transverse and longitudinal axes, which is substantially lower than regular ductile cast iron (0.086% and 0.485%) or stainless steel (0.188% and 0.682%).

2. Cast Steels

While distortion is generally higher in cast steels due to phase transformation, certain low-distortion flame-hardened cast steels have been developed. Flame hardening is used because the lower transformation temperature of these specific alloys helps avoid significant distortion and prevents quench cracking.

3. Aluminum Alloys

• Aluminum alloys require careful process control due to their high thermal conductivity, low melting point (~660°C), and tendency for sagging under gravity at elevated temperatures.
• ADC12 (Al-Si-Cu) is a die-casting alloy that offers good dimensional stability. Its heat treatment must be controlled because it causes expansion strain from the precipitation of elements like silicon.
• A356 and A365 are alloys with good castability and heat treatability. Their distortion can be minimized through optimized heating and quenching processes to prevent cracking and maintain precision.

4. Low-Expansion Alloys

These special alloys are designed for applications demanding near-zero thermal expansion and high dimensional stability across a wide temperature range.

• Invar 36 (Fe-36% Ni) has an extremely low coefficient of thermal expansion (CTE), which maintains near-constant dimensions from cryogenic temperatures up to 260°
• Super Invar (Fe-32% Ni-5% Co) offers an even lower thermal expansion coefficient than standard Invar for the most demanding applications.
• Kovar (Fe-Ni-Co) is another low-CTE alloy that maintains minimal dimensional changes with temperature fluctuations.

5. Other Alloys

• Zamak (Zn-Al) alloy has a low melting temperature (approx. 390°C), which minimizes thermal stress on molds and tools, contributing to long-term dimensional accuracy.
• Some superalloys like GH242 (Ni-Mo-Cr) inherently possess low thermal expansion coefficients combined with high-temperature strength, ensuring stability.

General Principles for Low-Distortion Heat Treatment

In addition to material selection, there are general principles you should follow in your process to minimize distortion during heat treatment:

• Always perform a stress-relief annealing step before any high-temperature heat treatment to remove residual casting stresses.
• Optimize alloy composition to lower the Ms point for a more gradual transformation and reduce volume change and stress during quenching.
• Control quenching speed to prevent high thermal gradients (e.g., by choosing air cooling when applicable), as rapid cooling is a primary cause of distortion.