Large forgings, such as wind turbine main shafts, marine crankshafts, and nuclear power rotors, serve as the "backbone" for critical national sectors like energy, heavy industry, and shipping. They are typically complex in structure and operate under severe service conditions. Their quality directly impacts the lifespan and safety of major equipment. However, throughout the lengthy journey from ingot to finished product, various "hidden flaws" can quietly develop. This article systematically analyzes the most common internal defects in large forgings, their causes, and how modern manufacturing anticipates and prevents them.
I. Internal Cavity-Type Defects: The "Congenital Deficiencies" and "Acquired Disorders" of the Material
These defects primarily stem from issues with the metal's density.
1. Porosity and Shrinkage Cavities
Appearance and Characteristics: Like tiny pores in a sponge, porosity consists of non-dense areas formed during the solidification of the steel ingot. As the top (riser) solidifies last and shrinks in volume, it lacks sufficient liquid metal feed. These defects are mostly found in the center of the ingot and below the hot top.
Causes:
Melting and Pouring: High gas content in the molten steel, improper pouring temperature, ineffective feeding by the riser.
Improper Forging Process: Insufficient forging ratio (upsetting, stretching ratios), failing to effectively weld shut these original cavities.
Preventive Measures:
Optimize Melting: Use advanced technologies like vacuum refining and electroslag remelting to reduce gases and impurities in the steel.
Improve Ingot Casting: Design proper ingot molds and insulating risers to enhance directional solidification and feeding capability.
Adequate Forging: Apply sufficient forging ratios through large deformation upsetting and stretching to completely weld shut cavities under high temperature and triaxial compressive stress.
2. Inclusions
Appearance and Characteristics: Non-metallic substances, such as oxides, sulfides, and silicates, become embedded in the metal matrix. Like "grit," they disrupt the continuity of the matrix and act as stress concentrators and initiation sites for fatigue cracks.
Causes:
Endogenous Inclusions: Deoxidation and desulfurization products from the melting process not fully floating out and remaining in the molten steel.
Exogenous Inclusions: Foreign objects like refractory materials or slag mixing into the molten steel during pouring.
Preventive Measures:
Clean Steel Melting: Enhance ladle refining to promote the flotation and separation of inclusions.
Process Cleanliness: Use high-quality refractories and ensure the cleanliness of the pouring system.
Deformation and Fragmentation: Use proper forging processes to break down large, continuous inclusions into fine, dispersed particles, reducing their harmfulness.
II. Crack-Type Defects: The "Ruthless Tearing" of Stress
Cracks are the most dangerous defects in forgings, directly related to temperature and stress.
1. Forging Cracks
Appearance and Characteristics: Cracks that occur on the surface or inside the forging during the forging process. Surface cracks often appear as craze cracks or straight lines, while internal cracks are difficult to detect.
Causes:
Overheating and Burning: Heating temperature too high causes coarse grains (overheating) or even oxidation and melting of grain boundaries (burning), drastically reducing metal plasticity, causing cracking upon forging.
Thermal and Transformation Stresses: Excessively rapid heating or cooling creates large temperature differences between the surface and core, generating significant thermal stress.
Improper Deformation: Excessive hammering, too fast deformation speed, or不合理 distribution of deformation leading to local stress exceeding the material's limit.
Preventive Measures:
Precise Temperature Control: Strictly adhere to heating specifications, use computer control systems to prevent burning and overheating.
Slow Preheating: For large high-alloy steel forgings, step heating and thorough soaking are essential.
Optimize Forging Process: Control the amount and speed of deformation, avoid large deformations in the low-temperature range.
2. Flakes (Hydrogen-Induced Cracking)
Appearance and Characteristics: Silvery-white, round, or oval spots on the longitudinal fracture of a forging, appearing as fine hairline cracks on the transverse section. This is a particularly lethal defect specific to large forgings.
Causes:
The Culprit - Hydrogen: Excessively high hydrogen content in the steel is the direct cause.
Internal Stress: During post-forging cooling, hydrogen accumulates at micro-defects, creating enormous pressure which, combined with transformation stress and thermal stress, leads to internal cracking.
Preventive Measures:
Hydrogen-Removal Melting: Use vacuum pouring, the most fundamental and effective measure to prevent flakes.
Slow Cooling Annealing: Post-forging "flake prevention annealing" is mandatory, involving prolonged holding at temperatures where hydrogen has high diffusivity (around 600-650°C) to allow hydrogen to slowly diffuse out.
3. Cooling Cracks
Appearance and Characteristics: Cracks that occur during post-forging heat treatment or cooling, often related to transformation stresses.
Causes: Phase transformations (e.g., to martensite) cause volume expansion, generating significant transformation stress. When this stress叠加 with thermal stress exceeds the material's strength, cracking occurs.
Preventive Measures: Establish appropriate heat treatment cycles, especially controlling quenching cooling rates, or use advanced processes like austempering or marquenching.
III. Microstructure and Property Inhomogeneity: "Disorder" in the Micro World
Even without macroscopic defects, inhomogeneous microstructure severely affects forging performance.
1. Coarse Grains
Causes: Excessively high starting forging temperature, excessively high finishing forging temperature, deformation falling into the "critical deformation range."
Harm: Reduces the toughness and strength of the forging, increasing "brittleness."
Prevention: Control the forging temperature range and deformation amount, and refine grains through subsequent heat treatments like normalizing or annealing.
2. Banded Structure
Causes: Segregation of elements like phosphorus and sulfur in the steel is elongated into band-like distributions during hot working.
Harm: Causes anisotropy in material properties, with transverse properties (like toughness and ductility) significantly lower than longitudinal ones.
Prevention: Improve steel cleanliness, use multiple upsetting-stretching combination processes to break up segregation bands.
Conclusion: Building a "Great Wall" of Quality Towards "Zero Defects"
The manufacturing of large forgings is a systematic engineering battle against defects. Modern forging companies build a robust quality defense through a full-process quality control system of "Refined Material, Refined Melting, Refined Forging, Refined Heat Treatment":
Digital Simulation: Use computer simulation of the forging process to predict metal flow, temperature fields, and stress fields, optimizing the process plan.
Non-Destructive Testing (NDT): Widely use technologies like ultrasonic testing and radiography, akin to giving forgings a "CT scan," ensuring defects have nowhere to hide.
Full-Process Monitoring: Every key parameter, from melting to heat treatment, is precisely recorded and controlled, achieving traceability.
It is the continuous advancement of these technologies, combined with meticulous craftsmanship, that ensures the strength and reliability of every "Pillar of Industry," enabling them to bear the weight of their era under extreme operating conditions.
