Five major factors affecting the quenching deformation of parts

Quenching is often called steel’s “coming-of-age ceremony”—the leap that grants metal its essential hardness. But this process always carries a major risk: deformation. Deformation isn’t a mysterious occurrence; it is the direct result of the interaction among thermal, structural, and material properties. In essence, the battle for dimensional stability and preventing distortion plays out across five central factors. By grasping how these five operate, mastery over the quenching process and its risks becomes possible.

 

Deformation originates from steel’s chemical properties. Carbon content strongly affects the increase in martensite’s specific volume, leading to volume expansion as the iron lattice accommodates more carbon atoms. High-carbon steel resists plastic deformation and typically undergoes deformation due to thermal stress. In contrast, medium-carbon steels, with a higher Ms point, are more likely to experience structural stress, resulting in irregular warping.

  

More subtly, alloying elements have a double-edged sword effect. While elements like chromium, manganese, and nickel can improve hardenability, allowing for the use of milder cooling media and thus reducing thermal stress, they also lower the Ms point and increase the amount of retained austenite. This seems contradictory: while retained austenite can buffer some structural stress, its instability can trigger dimensional creep during subsequent use. Especially in high-alloy steels, such as high-speed steel or high-chromium die steel, if carbide segregation forms a banded pattern, it’s like burying an “expansion coefficient bomb” inside the metal. The expansion along the rolling direction far exceeds that in the vertical direction, leading to uncontrollable anisotropic deformation in the workpiece after quenching.

 

If material properties are internal factors, workpiece geometry is a crucial external factor. Sharp changes in cross-section and uneven thickness lead to stress concentration. When the cooling medium contacts the workpiece, thin-walled areas cool quickly, causing the surface to harden while the thick-walled core remains hot. This temperature gradient creates a significant thermal stress difference: surface tension and core compression.

 

This uneven cooling is vividly demonstrated on complex surfaces. For example, in shaft parts with keyways, the cooling rate at the groove opening is faster than the shaft diameter, causing the groove opening to shrink or even expand. In T-shaped parts or molds with sharp corners, rapid cooling at the edges creates “tensile stress black holes,” forcing the workpiece to bulge toward the faster-cooling side and even causing longitudinal cracks. Practice has shown that the more slender, thin-walled, and asymmetrical a workpiece is, the worse its rigidity against deformation. Bending, twisting, and even spiral deformation after quenching are almost inevitable consequences of physical laws. The so-called “dangerous dimensions” (e.g., 8-15mm for water quenching and 25-40mm for oil quenching of carbon steel) are precisely the critical points where the cross-sectional stress cannot be uniformly released. Once this range is entered, cracking and deformation will follow.

 

Quenching cooling is the “critical moment” of deformation. Choosing the cooling rate is a tightrope walk between “hardness” and “deformation.” The intensity of cooling directly determines the peak value of thermal stress. While water quenching ensures high hardness in high-carbon steel, rapid cooling during the vapor film rupture stage (a temperature difference of up to 700-800℃/s) generates tensile stresses exceeding 1000 MPa on the surface, often the culprit of surface cracking and overall warping. In contrast, oil cooling, though gentler, can lead to pearlite transformation in large cross-sections or in high-hardenability steels if the cooling rate is insufficient, resulting in an uneven microstructure and new deformations.

 

A deeper level of complexity lies in controlling the cooling stage. In the high-temperature region above the Ms point, slow cooling reduces thermal stress; however, in the low-temperature region below the Ms point, excessively rapid cooling dramatically increases structural stress (because the martensitic transformation is accompanied by a 4% volume expansion). Uneven cooling in the temperature range where martensitic transformation is most intense (200-300℃) creates a huge difference in structural stress within the workpiece, leading to a disordered “phase transformation sequence”—the parts that transform first expand and compress the parts that transform later, ultimately causing dimensional instability. Furthermore, the purity of the quenching medium (e.g., water content in oil), the stirring method, and the direction of immersion (vertical or oblique)—these minute operational differences, when magnified, can become the final straw that breaks the camel’s back in terms of dimensional accuracy.

 

Many believe deformation occurs only during cooling, but heating is also important. Heating temperature and holding time control the austenite structure. Higher temperatures create coarser grains, lowering toughness and increasing thermal expansion. Yield strength is low at high temperatures, so slender rods placed horizontally in the furnace can undergo bending from their own weight. Once plastic bending occurs, subsequent cooling stress typically worsens the deformation.

The heating rate also affects deformation. For high-alloy or large cross-section workpieces, inadequate preheating before placing them in a high-temperature furnace causes a significant surface-to-core temperature difference and elevated thermal stress. This adds to the structural stress created during cooling. Data shows that longer holding times are similar in effect to higher quenching temperatures, making austenite composition more uniform but also decreasing the Ms point and increasing retained austenite, causing inner holes to shrink and outer diameters to expand. This expansion is often irreversible.

 

Finally, the “previous life” state of the workpiece before quenching is often overlooked, but it plays a decisive role in the final deformation. The uniformity of the original structure is crucial. Lamellar pearlite (a layered arrangement of hard and soft phases) and spherical pearlite (globular arrangements) exhibit distinctly different behaviors during quenching: lamellar structures, due to their small and uneven specific volume, are prone to greater deformation. Tempered sorbite—a fine, uniform microstructure resulting from quenching and tempering or normalizing—has a uniform structure and a moderate specific volume, which can significantly reduce the volume change before and after quenching.

 

Residual stress from machining is a key factor. During cutting and grinding, the surface accumulates significant tensile or compressive stress. Without stress-relief annealing (usually at 500-700℃) before quenching, these stresses are released quickly during quenching, causing sudden twisting. In forged or rolled steel, severe banded segregation or unfavorable fiber orientation, such as fibers perpendicular to the force direction, causes inclusions to serve as crack sources or deformation pathways.

 

Ultimately, quenching deformation arises from the interaction of five major factors: material, structure, temperature, medium, and historical stress. Success in quenching requires proactive management of these factors throughout the entire process—from selecting materials with a “low-deformation gene” to balancing stress during heating and cooling to minimizing historical stresses. Only by managing all five can the workpiece achieve success in quenching. Success in quenching requires proactive management of these factors throughout the entire process—from selecting materials with a “low-deformation gene” to balancing stress during heating and cooling to minimizing historical stresses.