The internal quality of flange forgings directly affects the operational safety of piping systems. In actual production, however, various forging defects occur from time to time, affecting product yield at best and leading to sudden in‑service failures at worst. A thorough understanding of the typical modes and root causes of defects in flange forgings is of great practical significance for optimising forging processes and improving product quality.
Improper control of finishing forging temperature is one common cause of flange cracking. In one case, a heat exchanger channel cover flange cracked during a low‑pressure hydrotest. Comprehensive testing—including visual examination, surface hardness, chemical composition, mechanical properties, metallography, and stress analysis—revealed that the hardness of the material at the large‑end corner of the flange neck was significantly higher than normal, with coarse and uneven grains, not typical of a normalised structure. Analysis confirmed that a low finishing forging temperature and non‑standard normalising practice were the main causes of cracking. When the finishing forging temperature is too low, the metal’s plasticity drops markedly and deformation resistance rises sharply, making crack initiation highly likely at stress concentration zones.
Forging overheating and burning are another common process defect. In one case, a flange sleeve fitting for an oil pipeline developed circumferential cracks at the flange edge after hot forging and quenching‑and‑tempering treatment. Chemical composition analysis and metallographic examination showed that the workpiece had been heated to excessive temperatures for too long during forging, causing overheating and burning with intergranular oxidation. The intergranular bonding strength was thereby significantly reduced, leading to embrittlement and eventual cracking during subsequent hot forging.
Insufficient or non‑uniform forging ratio can also cause quality problems. The content and distribution of non‑metallic inclusions in flange forgings directly affect mechanical properties. When the forging ratio is insufficient, the dendrites and segregation in the as‑cast structure cannot be adequately broken down and homogenised, and inclusions tend to align in bands along the forging direction, severely reducing transverse mechanical properties. For high‑alloy steel flanges such as F92 steel used in ultra‑supercritical thermal power units, traditional piercing and mandrel‑drawing processes frequently result in cracks and external concavities in the forged bore, with product yield historically as low as about one‑quarter.
Folding defects are surface or near‑surface defects formed during forging that may propagate further during subsequent processing or service. In one case, a crack was found near the circumferential weld on the tapered neck of an F316L forged flange. Analysis confirmed that a forging fold was the primary cause of cracking; during subsequent assembly welding, welding stresses and assembly constraint stresses caused the fold to propagate.
Micro‑manufacturing defects should not be overlooked. In one case, cracks appeared on the outer surface of a 304/304L stainless steel flange base material. Analysis indicated that the crack origin was related to micro‑defects introduced during the forging and piercing processes, and that cracks propagated along grain boundaries under stress, ultimately leading to failure.
The above cases demonstrate that flange forging defects often involve interactions across multiple process steps. Negligence in any one aspect—temperature control, deformation parameters, die design, or cooling regime—can leave hidden defects in the final product. Strengthening whole‑process quality control and optimising key process parameters are effective paths to fundamentally reducing forging defects.
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