Bursting is a serious defect in metal forging, manifesting as localized cracking, tearing, or rupture. It refers to cracks forming on the surface or within the interior of the workpiece during heating, forging, cooling, or subsequent heat treatment. Fundamentally, bursting occurs when localized tensile stress exceeds the metal's strength or ductility; instead of deforming smoothly under compression, the material undergoes tensile failure.

The causes of bursting can generally be categorized into temperature issues, improper deformation, raw material defects, and die design flaws:

1. Improper Forging Temperature

1) Overheating (excessively high temperature): Premature cracking often stems from an excessively high forging temperature; this specific condition is frequently referred to as "overburning."

2) Insufficient heating (excessively low temperature): Processing steel at an improper temperature reduces its ductility, making it prone to cracking or rupture under forging pressure.

3) Uneven heating: Non-uniform or insufficient heating creates severe temperature gradients within the metal, often triggering internal cracks and bursting.

4) Internal ruptures caused by processing steel at these improper temperatures are metallurgically termed "forging bursts."

2. Excessive Deformation and Process Errors

1) Drastic cross-section reduction: If the cross-sectional area is reduced too rapidly or by too large a margin in a single forging pass, forging bursts are highly likely to occur.

2) Improper hammering: Insufficient forging penetration is a common defect—often caused by blows that are too light or too rapid—resulting in a failure to effectively weld or consolidate the core of the metal part.

3) Internal rupture occurs when the stress applied to the metal during forging is excessive.

3. Raw Material Defects

1) Inherent defects: Pre-existing surface defects or internal voids in the raw material can lead to bursting during the intense deformation of the forging process.

2) Inclusions: The center of the raw stock may contain high concentrations of non-metallic inclusions; these severely weaken the core strength and can lead to central cracking during hot forging. 

3) Low ductility in the core region of the workpiece material can also trigger center cracking during deformation.

4. Die Design and Tooling Issues

1) Improper die design, unreasonable processing parameters, and insufficient heating are the most common causes of forging defects.

2) Forging dies featuring sharp corners, inadequate radii, or abrupt changes in cross-section can impede smooth metal flow and generate localized tensile stresses, leading to cracking.

3) In specialized processes such as tube hydroforming, insufficient internal pressure during axial feeding can result in global buckling instability and material rupture.

Prevention Strategies

To prevent cracking, metallurgists and forging engineers must strictly control heating temperatures to ensure uniform material plasticity.

Key measures include:

1) Limiting the cross-sectional reduction per forging pass and using high-quality raw materials with minimal inclusions or porosity.

2) Optimizing die design—incorporating smooth transitions and generous radii—to ensure the metal remains under compressive stress, thereby effectively eliminating cracking caused by tensile stress.

3) Surface cracks can typically be detected at an early stage using non-destructive testing methods, such as magnetic particle inspection.