Effective Methods to Eliminate Internal Residual Stress in

Internal residual stress is an inevitable hidden defect during metal casting production.

Uneven cooling speed, inconsistent wall thickness and uneven mold restraint will

generate huge residual stress inside castings. Without proper stress relief treatment,

finished castings will warp, crack or suffer dimensional deformation during machining,

assembly and long-term storage, directly leading to high scrap rates and unstable

product precision. This article systematically sorts out mainstream industrial stress

elimination processes, from offline heat treatment to online casting process

optimization, to help manufacturers fundamentally control residual stress.

1. Thermal Stress Relief Annealing (Most Widely Used Standard Process)

Annealing is the most reliable way to eliminate casting residual stress, suitable for

steel, aluminum alloy, iron and copper castings. The core principle is to heat castings

to a specific temperature below the material recrystallization point, keep constant

temperature for a long time, then cool down slowly to release internal elastic strain

energy.

For gray iron and ductile iron castings: the standard holding temperature ranges from

500℃ to 600℃. Large thick-walled castings need 2–4 hours of heat preservation to

ensure uniform internal temperature. Slow furnace cooling below 200℃ can avoid

new secondary stress from rapid temperature drop.

For aluminum alloy die castings and sand castings: low-temperature stress relief

annealing at 200–300℃ for 1–2 hours can remove casting and machining stress

without damaging material hardness and surface finish.

This process can eliminate over 85% of residual stress, and is mandatory for

precision casting parts used in automotive, hydraulic and aerospace equipment.

2. Natural Aging & Artificial Aging (Low-Cost Auxiliary Stress Relief)

Natural aging means placing finished castings in a constant-temperature workshop

for several weeks to several months. Under natural temperature fluctuation, internal

stress is slowly released with slight material creep. Its advantages are low cost and

no equipment investment, but the cycle is extremely long and cannot meet mass

production delivery requirements.

Artificial aging (vibration aging) is a high-efficiency alternative to natural aging.

Professional vibration aging equipment transmits periodic mechanical vibration to castings for 20–60 minutes. Micro-plastic deformation occurs at stress concentration

areas, which balances internal stress distribution. Vibration aging saves 90% time

compared with natural aging, and costs far less than annealing furnaces. It is widely

used for medium and large structural steel castings that are inconvenient to put into

heating furnaces.

3. Front-End Casting Process Optimization (Prevent Stress from Source)

Stress elimination should not only rely on post-processing; reasonable casting design

and molding processes can greatly reduce residual stress generation at the source.

First, optimize casting wall thickness design: avoid abrupt thickness changes, use

smooth fillet transitions instead of sharp right-angle corners to reduce cooling speed

differences between thick and thin sections. Second, control cooling rate: slow down

the cooling speed of thick hot spots by setting chills or adjusting sand mold thermal

conductivity, to shrink the temperature difference inside the casting during

solidification.

In addition, reasonable gating and riser layout can realize sequential solidification,

lower the thermal gradient of castings, and effectively cut the residual stress caused

by uneven solidification shrinkage.

4. Stress Control After Machining

Cutting and grinding will introduce new machining residual stress on the casting

surface. For high-precision parts, a secondary low-temperature aging process is

required after rough machining: remove the stress generated by cutting before finish

machining, to guarantee stable dimensional tolerance after long-term use.

Conclusion

The treatment of casting internal residual stress adopts a combined logic of

“prevention first, post-treatment supplement”. Process optimization in casting design

can reduce stress source; vibration aging is suitable for mass low-cost production;

stress relief annealing is the ultimate solution for high-precision castings. Factories

can select matching stress elimination schemes according to material, part precision

requirement and production cycle, to fundamentally solve casting deformation and

cracking failures caused by residual stress.