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Why are positioning pins prone to deformation?
The core reason why the mold part positioning pin is prone to deformation during heat treatment is the imbalance in the distribution of internal stresses, coupled with the amplifying effect of its own structural characteristics, ultimately leading to shape changes. This can be analyzed in three specific aspects:
1. "Clash" of stresses: Dual impact of thermal stress and structural stress
The role of thermal stress: During heating, the surface of the positioning pin heats up and expands first, while the internal temperature is low and expands slowly, causing the surface to be subjected to inward compressive stress. During cooling, the surface contracts first, while the interior is still in an expanded state, causing the surface to be subjected to tensile stress. This "expansion and contraction" temperature difference change, in slender or stepped positioning pins, easily leads to bending due to excessive internal and external stress differences.
The trouble caused by structural stress: During quenching, the transformation of austenite to martensite is accompanied by a volume expansion of about 3%. If the cooling rate is uneven (for example, the edges and corners cool quickly, while the center cools slowly), the surface layer that transforms first will restrict the internal expansion, resulting in the surface layer being compressed and the interior being pulled, forming a twisting force.
1. Structural "problems": Asymmetric shapes amplify the risk of deformation
The structural characteristics of the positioning pin make it a "disaster area" for deformation:
Excessive slenderness ratio: When the slenderness ratio of the positioning pin is greater than 5:1 (such as 100mm in length and 20mm in diameter), it is like a thin rod, and its rigidity is greatly reduced.
Local structural mutations: Positioning pins with steps, through holes or threads, due to sudden changes in cross-section (such as the diameter suddenly reducing from 30mm to 15mm at the step), will lead to uneven cooling and contraction, and easily produce twisting at the transition.
Uneven cross-section: For positioning pins with asymmetric cross-sections (such as square positioning pins), the cooling rate difference at the edges and corners will cause warping.
1. Process "adding fuel to the fire": improper operation exacerbates deformation
Uneven heating: Stacking of workpieces or being close to the furnace wall leads to local overheating and increased thermal expansion differences.
"Eccentric" cooling: When the workpiece is tilted into water during quenching, one side cools and contracts first, while the other side is still in a high-temperature expansion state, directly causing bending.
Insufficient pre-treatment: If the forged positioning pin is not subjected to spheroidizing annealing, there are network carbides inside, and the uneven structural transformation during heat treatment will produce additional stress.
Summary: Deformation is the "combined force" of multiple factors
The easy deformation of the positioning pin is the result of the combined action of internal stresses (thermal stress + structural stress), its own structure (slender, special-shaped) and process operations. Among them, stress imbalance is the root cause, and the structural characteristics make this imbalance more likely to appear as macroscopic deformation. To control deformation, it is necessary to reduce stress differences, optimize structural design and standardize processes, and break the "stress-structure-deformation" chain reaction.
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