Views: 49 Author: Elena Publish Time: 2026-06-12 Origin: www.fuchun-casting.com
In the field of metal manufacturing, casting is one of the most fundamental and widely used processes. However, during and after solidification, castings inevitably develop internal forces known as stress in casting. Understanding the nature of this stress, its origins, and its consequences is critical for producing high-quality metal components. This article explores the concept of casting stress, the role of phase transformation, the definition and significance of residual stress, and the detrimental effects of stress on castings.
Stress in casting refers to the internal forces that arise within a solidified or solidifying metal component due to uneven temperature distribution, volume changes during phase transformation, and mechanical constraints. These stresses can be temporary (during cooling) or permanent (remaining at room temperature). Casting stress is primarily categorized into thermal stress and transformation stress, both of which can lead to residual stress in the final product.
When a casting solidifies, with the decrease of temperature, the casting will become solid. In this process, phase transformation in metals and alloys occurs. As the casting enters the solid phase, transformation will also cause shrinkage or expansion depending on the specific alloy system. For example, when austenite transforms into ferrite or martensite in steel, a significant volume expansion takes place. This phase change does not happen uniformly across the casting.
Due to the existence of a thicker outer wall of the casting, the temperature of the outer layer drops rapidly, but the temperature of the inner layer will drop very slowly. Therefore, the thicker the wall thickness is, the longer the cooling time will be. This phenomenon results in different shrinkage rates of the inner and outer layers of a complete casting and different thickness sections. The outer layer, cooling faster, tends to solidify and contract earlier, while the inner, hotter core remains more ductile and contracts later. This phenomenon will affect the mutual restraint between the inner and outer layers of the casting, resulting in tensile stress (on the surface) or irreversible deformation (warping, cracking) of the casting. Thermal stress is exactly this phenomenon: the thickness of the casting wall is not uniform, and in the same unit time the degree of shrinkage is different, leading to internal force imbalance.
To fully grasp the effects, one must understand residual stress definition.Residual stress in steel(or any material) is the stress that remains inside a component after the original cause of the stress (such as thermal gradients or external forces) has been removed. In other words, it is the locked-in stress that exists in a casting at room temperature, without any applied load. Residual stresses can be tensile (pulling apart) or compressive (pushing together). They arise from non-uniform plastic deformation, phase transformations, or temperature gradients during manufacturing. In steel castings, residual stress is especially common due to the complex phase transformations between austenite, ferrite, pearlite, and martensite.
The effects of stress, particularly residual stress, are largely detrimental to casting quality and performance. The remnants of casting stress have a very bad effect on the quality of a casting. Residual stress is an important factor that causes deformation during the production, storage, and even service life of castings. It will reduce some properties of castings after long-term accumulation, including fatigue strength, dimensional stability, and corrosion resistance.
One of the most immediate effects is distortion. When the internal residual stress exceeds the elastic limit of the material at a given temperature, the casting will warp or twist to relieve the stress. For example, a long, thin casting with uneven cooling may become banana-shaped. In large-scale production processes, if the castings are deformed, the castings cannot be put into the pre-prepared mold or fixture, which leads to the failure of the production line. This increases scrap rates and production costs.
If the direction of residual stress obtained by a casting and its direction of working stress (stress applied during use) are duplicated, then during the casting’s working time, the two forces will produce mutual reinforcement. For instance, a tensile residual stress combined with an external tensile load can quickly exceed the ultimate tensile strength of the casting material, resulting in deformation or even fracture. This is particularly dangerous for safety-critical components such as turbine blades, railway wheels, and structural supports.
For some castings with high precision requirements, such as engine blocks, gearboxes, or aerospace housings, even minute deformations are unacceptable. If the residual stress in the castings causes even a 0.1 mm deviation from the design, it will greatly affect the accuracy of mating parts, leading to assembly difficulties, leakage, vibration, or premature wear. This also causes economic losses through rework, rejection, or field failures.
Residual tensile stress in steel, when combined with a corrosive environment, can initiate stress corrosion cracking. This is a slow but catastrophic failure mode that occurs far below the normal yield strength. Many industrial accidents have been traced back to untreated residual stress in cast components.
To minimize the negative effects, foundries employ several strategies: proper gating and riser design to promote uniform cooling, controlled cooling rates, heat treatments (such as stress relieving annealing or normalizing), and mechanical methods (like vibratory stress relief). Understanding the root causes—temperature gradients, phase transformation in metals and alloys, and geometric constraints—is the first step toward producing stress-free or low-stress castings.
In conclusion, stress in casting is an unavoidable phenomenon arising from uneven cooling and phase changes.Residual stress definition en compasses all locked-in forces that persist after manufacturing.Residual stress in steel is particularly problematic due to complex phase transformations. The effects of stress include deformation, cracking, loss of precision, and economic losses. Therefore, controlling and relieving casting stress is essential for reliable, high-quality metal products.
