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Analysis and Preventive Measures of Defects in Tube-Sheet Forgings
Introduction
Large free forging equipment is a symbol of industrial development, and the production of large forgings plays a vital role in national economic construction, defense, and cutting-edge scientific applications. As industries shift towards large-scale, high-standard equipment manufacturing, the demand for high-quality large forgings has increased. Among these, tube-sheet forgings are a key product but suffer from a high rejection rate, leading to significant economic losses. These forgings are widely used in heavy machinery, power generation, metallurgy, mining, petrochemical, and nuclear facilities. The steels used are typically heat-resistant and corrosion-resistant alloy grades.
The primary forming method for tube-sheet forgings is upsetting deformation. However, the upsetting process often results in dense inclusions due to the Rigid Sliding Tearing Effect (RST), causing a high rejection rate. Over 50% of rejected products are due to dense inclusions, either from internal (incomplete floating of inclusions during smelting) or external sources (refractory material contamination). To meet market demand and reduce these defects, forging processes for tube-sheet forgings need to control and prevent inclusions from becoming large flakes. Research has shown that reverse forging can effectively avoid the RST effect, improve hole-welding ability, shred and disperse large inclusions, and enhance forging efficiency.
Defects in Tube-Sheet Forgings and Their Causes
The manufacturing process of tube-sheet forgings generally involves: raw material (steel ingot) → heating → blanking → upsetting → forming → post-forging heat treatment.
1.1 Raw Material (Steel Ingot) Quality
The primary reason for rejection due to dense plastic inclusions is poor quality steel ingots. Defects such as excessive carbides, sulfides, segregation, secondary shrinkage, and high hydrogen content can lead to non-conformance during ultrasonic testing.
1.2 Blanking Process
- Forging tube sheets using electroslag remelting steel ingots without the drawing process and directly cutting the riser for upsetting can result in dense defects, causing rejection during ultrasonic testing.
- Misunderstanding the importance of drawing before blanking, some believe that meeting the upsetting ratio is sufficient for quality, neglecting the need for drawing to compact the material’s center, which can result in large inclusions and defects.
1.3 Heating Process
- Chemical changes on the billet surface due to the heating medium can lead to defects like oxidation, decarburization, and sulfide penetration.
- Failure to control the heating process properly can cause stress in the material, leading to cracking during upsetting or forging.
1.4 Upsetting Method
The upsetting method, while widely used for tube-sheet forgings due to its simplicity, can result in central plastic inclusion defects if not properly executed.
1.5 Forming Process
Upsetting followed by local upsetting, using methods such as rotational or stepped forging, can help reduce the formation of inclusion-induced cracks.
1.6 Post-Forging Heat Treatment
The post-forging heat treatment of alloy tube-sheet forgings is critical to prevent defects such as white spots (due to hydrogen) and internal cracks caused by uneven cooling.
RST Effect
The RST effect is a specific mechanical phenomenon leading to internal layered cracks in large free-forged disks and plates due to excessive deformation. This occurs when two rigid zones in the material meet, causing rigid sliding deformation and tearing. The RST effect can be mitigated by adjusting the upsetting process and using techniques like reverse forging.
Preventive Measures
3.1 High-Quality Raw Materials
Using high-quality steel ingots, produced through advanced smelting techniques and ensuring clean pouring systems, is essential to reduce inclusion defects.
3.2 Proper Heating Process
The heating process must consider the type of steel ingot, casting conditions, and whether the ingot has undergone annealing. Careful control of heating stress is crucial to avoid internal cracking.
3.3 Adequate Deformation Before Blanking
Adequate drawing before blanking is necessary to compact the center of the material and prevent defects like segregation and large inclusions.
3.4 Proper Upsetting and Forming Techniques
Upsetting should follow processes that avoid creating rigid zones that lead to the RST effect. Techniques such as reverse forging and adjusting the fiber direction of the material can help reduce defects.
3.5 Forming Process Control and Adjusting Allowances
Strict control of starting and final forging temperatures is necessary to prevent issues like coarse grain formation or mixed grain structure. Additionally, precise control of the temperature during final forging can prevent inclusion-induced defects.
3.6 Post-Forging Heat Treatment
Post-forging heat treatment should focus on improving mechanical properties, eliminating residual stress, refining grain structure, and ensuring uniformity in composition.
3.7 Avoiding RST Effect
Controlling the friction between the billet and the forging tools and maintaining appropriate deformation ratios can help prevent the RST effect.
Ideal vs. Real Situation
In ideal conditions, the rejection rate of tube-sheet forgings can be reduced from 40-50% to 80-90% passing rate. However, many manufacturers still focus on cost-cutting measures, using lower-grade materials and less sophisticated techniques, leading to higher rejection rates.
Conclusion
- Advanced techniques like FM (removing central tensile stress), WHF (wide die strong forging), and JST (central compaction) ensure better internal quality for tube-sheet forgings.
- Controlling the cutting allowance of steel ingots and repairing defects at high temperatures can reduce internal defects.
- Improving smelting and heat treatment processes is key to producing high-quality tube-sheet forgings.