Large free forging equipment is a hallmark of a country’s industrial development level, and the large forgings produced play a vital role in national economic construction, defense equipment, and central installations in modern cutting-edge science. As various industries gradually shift towards larger and higher-standard equipment manufacturing, the internal quality requirements for large forgings are also increasing.
Challenges with Tube Sheet Forgings
Tube sheet forgings are among the primary products of large forgings, and these products have a high defect rate, resulting in significant economic losses. They are widely used in critical equipment or key components in industries such as heavy machinery, power generation, metallurgy, mining, petrochemicals, and nuclear facilities. The steel materials used are mostly heat-resistant and corrosion-resistant alloy steels
Tube Sheet
Forming Methods and Defect Analysis
The forming method for tube sheet forgings primarily involves upset forging. However, during the upsetting forming process, a significant number of defects arise due to the dense inclusions of plasticity, leading to a high scrap rate caused by the RST effect (Rigid Slide Tearing Effect). A comprehensive analysis reveals that over 50% of scrap from tube sheet forgings is attributed to dense inclusions, which include internal inclusions (inadequately floated inclusions resulting from the smelting process) and external inclusions (foreign materials mixed in).
Focus on Inclusion Control
To meet the market demand for large tube sheet forgings, controlling and reducing the formation of flaky inclusions is a key focus of forging process research. Based on literature and production experience, a careful comparative analysis of forging methods for tube sheet forgings suggests that using reverse forging during the upset process can effectively avoid the formation of the RST effect, improve the bonding ability of voids, fragment and disperse large inclusions, and enhance forging efficiency.
Defects and Causes of Tube Sheet Forgings
The forming process for tube sheet forgings generally includes: raw materials (steel ingots) → heating → blanking → upsetting → forming → post-forging heat treatment.
Quality of Raw Materials (Steel Ingots)
The high scrap rate of large forgings due to dense plastic inclusions is often attributed to poor quality of the steel ingots. Issues such as excessive carbides, sulfides, severe segregation, secondary shrinkage cavities, and high hydrogen content contribute to this.
Blanking Process
Direct Upsetting of Steel Ingots:
When a batch of tube sheets is forged from electric arc remelted steel ingots without an elongation step, and the ingots are sawed and directly upset, a forging ratio of 2.6 can lead to ultrasonic testing failures due to the presence of dense defects.
Misunderstandings About Elongation:
Some forging technicians and operators mistakenly believe that meeting the specified upsetting ratio guarantees quality, neglecting the importance of the elongation ratio before blanking. This results in internal voids that fail to bond completely and inclusions that are not fragmented or dispersed, leading to scrap.
Additionally, failure to control the amount of cutoff from the ingot’s head and bottom can cause defects from sulfur and phosphorus inclusions at the head and heavy metal deposits at the bottom, resulting in non-compliance.
Heating Process
Defects caused by changes in the chemical state of the blank’s surface (such as oxidation, decarburization, carburization, and penetration of sulfur or copper) and uneven temperature distribution within the blank can lead to excessive internal stress (including thermal stress and microstructural stress) and cracking.
Ignoring the cracks that inclusions have not fully separated can prevent effective filling of voids during the upset forming process, leading to scrap after rough machining due to dense plastic inclusions.
Anomalous changes in internal microstructure, such as overheating, overburning, and insufficient heating, can occur when process technicians fail to control the final forging temperature and deformation, leading to RST effects and mixed crystals.
Upsetting Method
Upsetting is the most commonly used forging method for tube sheet forgings due to its simplicity and versatility. However, direct upsetting of tube sheet products often results in the formation of core plastic inclusion defects.
Forming Process
After the overall upsetting process, tube sheet forgings enter the local upsetting process. Currently, the most commonly used methods for local upsetting include “rotating into the anvil” and “displacing the anvil,” as well as “arc anvil” and “flower-shaped anvil methods.” The “rotating into the anvil” method is practical for tube sheet forgings, as it produces fewer inclusion-related cracks and is easily promoted. In specific applications, the pre-upsetting deformation can be appropriately reduced to further control inclusion-related cracks.
Post-Forging Heat Treatment
Large tube sheet forgings are often made from heat-resistant and corrosion-resistant alloy steels. Sometimes, due to production conditions, these forgings cannot undergo timely post-forging heat treatment. If not performed promptly, hydrogen gas cannot escape, leading to white spots. Uneven cooling during rapid cooling can create internal stress, resulting in subsurface cracking and fine cracks in the center, which can lead to scrap during testing.
RST Effect
Large round and plate-type free forgings develop internal layered crack defects due to significant deformation during the forging process, caused by a special mechanical effect known as the RST effect. The characteristic is that when the dimensions of the forging tools (such as the anvil) in two directions (length and width) vastly exceed the height of the blank, the upper and lower rigid zones within the blank meet. Subsequently, under continued pressure, a layered rigid sliding deformation occurs within the rigid zone, causing tearing. In reality, the metal within the rigid zone is generally not entirely in a rigid state; instead, it exhibits a certain gradient of small strain rates starting from the surface to the center of the blank. The RST effect does not occur suddenly but develops over time.
Targeted Prevention Measures
Use of High-Quality Raw Materials (Steel Ingots)
For large tube sheet forgings, it is essential to employ advanced smelting techniques, carefully control the smelting and pouring process, and maintain a clean and dry pouring system. The elements contained must meet relevant technical standards without negligence. Typically, the raw materials for large tube sheet forgings are steel ingots, often electric arc remelted and treated for cleanliness. Various types of inclusions inevitably exist within large steel ingots. For external inclusions, careful selection of raw materials and maintaining a clean pouring system are crucial. For internal inclusions, improvement can be effectively achieved during the smelting and pouring processes.
Reasonable Arrangement of Heating Process
- Due to the sequential solidification of molten steel in the ingot mold, defects in the center of the steel ingot (such as porosity, segregation, inclusions, bubbles, etc.) are generally more severe. Early removal from the mold, short insulation times, and inadequate stress relief (no annealing after pouring) exacerbate these issues, which must be considered in heating.
- To repair potential visible inclusion cracks from previous processes, maintaining high temperatures can effectively fill voids in internal cracks that have not been fully separated by inclusions, allowing for effective filling and ensuring that the required deformation for final forging can compact any remaining micro-voids.
- During the final forging deformation, controlling the final forging temperature and deformation amount is crucial. At this stage, the deformation amount for tube sheet forgings is small; thus, to avoid coarse crystals and mixed crystals, as well as the RST effect, heating temperature and uniformity must be controlled to ensure internal quality.
Sufficient Deformation Process Before Blanking
Blanking for tube sheet forgings is necessary to create conditions for subsequent upsetting.
Direct Upsetting After Sawing:
Directly upsetting the head of the steel ingot after sawing can lead to core plastic inclusion defects. Controlling the amount cut from both ends of the ingot can stabilize the elemental composition of tube sheet forgings. The internal cast structure, severe segregation, secondary shrinkage cavities, and excessive carbides and sulfides need to be fragmented and dispersed to avoid scrap. After undergoing high-temperature insulation and secondary forging, most products undergo ultrasonic testing; however, this process generates significant resource and economic waste.
Pre-forging Blanking Process:
Due to the small amount of deformation, the central compaction capability is insufficient. The pre-forging process should employ a large deformation ratio with a width-to-height ratio greater than 1.3:1, using advanced technologies such as FM (to eliminate central tensile stress), WHF (wide anvil strong forging), and JST (central compaction) to meet and control the requirements of anvil width ratio W/H > 0.6 and single-sided deformation > 15%. Suppose the elongation ratio is not significant, and the subsequent upsetting ratio is also small. In that case, it indicates insufficient deformation, making it difficult for the deformation force to penetrate the core area of the forging.
Reasonable Selection of Upsetting and Forming Methods
After the elongation process, tube sheet forgings undergo upsetting. For tube sheet forgings with special requirements, methods such as spherical and conical upsetting plates, as well as changing fiber directions, are used. Once a specific range (aspect ratio > 1.1) or pressure value is reached, local upsetting is employed until the final forging stage is achieved.
After the overall upsetting process, tube sheet forgings enter the local upsetting process. The most commonly used methods for local upsetting include “rotating into the anvil” and “displacing the anvil,” as well as “arc anvil” and “flower-shaped anvil methods.” The “reverse method” utilizes a step created by rotating into the anvil method, which is no taller than one-third of the upper anvil’s arc.
After flipping the blank 180° at high temperatures, the displacing method applies pressure from the center to the edges, rotating at each angle to break and disperse any inclusions toward the circumference. Although the local deformation in the displacing method is substantial, flipping the blank helps effectively eliminate the RST effect. Many organizations are researching forging methods for tube sheet products to mitigate dense plastic inclusion defects and the RST effect, thereby improving internal quality.
Control of Forming Process and Adjustment of Allowances
During the forging of tube sheet forgings, it is vital to control both the initial and final forging temperatures. To repair potential visible plastic inclusion cracks from prior forging processes, it is necessary to enhance temperature and deformation control in the final forging stage. When heating before the last forging, the allowance is small, only 8% to 15% of the forming size, making temperature control critical. To prevent excessive heating that causes grain growth and overheating without deformation, and to avoid uneven heating that leads to mixed crystals, strict temperature control is essential, ensuring uniformity and determining appropriate insulation times.
Ensuring Proper Post-Forging Heat Treatment
The overall goal of post-forging heat treatment for large tube sheet forgings is to:
- Improve mechanical properties, refine grain structure, and homogenize composition.
- Eliminate and reduce residual stresses, and adjust overheating and coarse structures formed during forging, thereby reducing internal non-uniformity.
- Ensure that no white spots appear in the forgings, thus guaranteeing and enhancing various properties (such as heat resistance, wear resistance, and corrosion resistance) for subsequent processing.
During rapid cooling, as the temperature decreases, the solubility of hydrogen in the steel decreases, causing hydrogen to accumulate in the microscopic pores and form molecular states, which complicates diffusion. This creates significant local pressure, leading to internal fractures when the material’s breaking strength is exceeded, resulting in white spots.
The diffusion coefficient of hydrogen in steel is highest between 600°C and 320°C; thus, hydrogen remaining below 300°C can cause cracks. Additionally, rapid cooling can create internal stresses due to uneven thermal expansion or contraction, leading to subsurface cracking and internal stress fractures.
Avoiding RST Effect: Process Guidelines
The RST effect is primarily related to the height of the rigid body inside the blank caused by surface friction during the forging process. Its mechanism develops through three stages: “elastic compression deformation,” “rigid shear deformation,” and “rigid sliding tearing.”
The reverse method for tube sheet forgings is suitable for forging at high temperatures during secondary heating. By reasonably controlling the width-to-height ratio and deformation amount during the forging process, the rigid bodies within the blank can be kept from encountering each other, effectively avoiding the RST effect and the resulting “layered crack” defects.
Ideal Conditions and Reality of Tube Sheet Forgings
In an ideal state, the qualification rate for tube sheet forgings should improve from the current 40% to 50% defect rate to an 80% to 90% qualification rate. In reality, production conditions are far from satisfactory. The intense competition in the market economy has led many domestic manufacturers to remain in a crude steel model, not seeking advanced forming technologies but primarily competing on sales prices.
When there is no price advantage, manufacturers seek relatively low-cost raw materials (electric furnace steel ingots), use lower-cost (direct heating) methods, and directly forge after sawing the ingot heads. They either do not control the cooling speed or perform heat treatment (tempering) after rapid cooling.
Steel Tube Sheet
Conclusion
By employing a large deformation ratio with a width-to-height ratio greater than 1.3:1 (relative deformation amount of 15% to 20%) during the elongation process, and using advanced techniques such as FM (to eliminate central tensile stress), WHF (wide anvil strong forging), and JST (central compaction), while ensuring and controlling the anvil width ratio W/H > 0.6 and single-sided deformation > 15%, the internal quality of tube sheet forgings can be effectively guaranteed.
Controlling the amount cut from both ends of the steel ingot can stabilize the composition of tube sheet forgings; repairing internal cracks in the forgings at high temperatures meets the requirements for repairing and compacting microscopic voids. Effectively improve the level of smelting and heat treatment technology.
