The hazards of white spots and hydrogen embrittlement defects in large forgings are significant, so post-forging heat treatment is required to prevent white spots and hydrogen embrittlement, while also removing forging stress, reducing hardness, and refining grains. We have compiled our understanding of white spots related to post-forging heat treatment of large forgings into this document for reference.

1 Basic concepts related to white spots in steel

1.1 The hazards of white spots White spots are internal cracks caused by hydrogen in steel. When the hydrogen content in solid steel is too high, the steel becomes significantly brittle. Essentially, white spots also belong to brittle failure. The presence of white spots drastically reduces the mechanical properties of steel, especially the lateral plasticity and toughness, becoming the most dangerous source of fracture, severely affecting the performance and lifespan of parts. Therefore, once white spots are found in a forging, that forging must be scrapped.

1.2 The morphology of white spots On the longitudinal fracture surface of the forging, they appear as clearly defined circular or oval silvery-white spots; on transverse low-magnification samples, they appear as hair-like small cracks, a few millimeters long, with a maximum length of several tens of millimeters.

1.3 Conditions for the formation of white spots and formation temperature After forging, the hydrogen in the steel interacts with stress (deformation stress, thermal stress, organizational stress, especially organizational stress) to produce white spots. The formation temperature of white spots occurs when the forging blank cools to a relatively low temperature, approximately in the range of 250°C to room temperature.

1.4 The relationship between hydrogen and stress in the formation of white spots Literature [2] points out: "The presence of a sufficient amount of hydrogen in steel is a necessary condition for the formation of white spots, and the existence of internal stress promotes the formation of white spots. Therefore, the oversaturation of hydrogen content is the 'culprit' for the generation of white spots, while internal stress is the 'accomplice'." This thoroughly explains the mutual relationship between the two in the formation of white spots.

1.5 Three main characteristics of white spots 1) White spots never form on the surface of the forging. According to relevant data, their formation always occurs inside the forging, with the area containing white spots always a considerable distance from the surface, about 50 millimeters; 2) White spots form after forging when the forging cools to a relatively low temperature of 250°C to room temperature; 3) White spots do not form instantaneously but gradually. There is a period of time from the cessation of forging to the appearance of white spots, known as the incubation period or latent period for white spot formation. The length of the incubation period depends on the hydrogen content in the steel and also on the thickness of the forging.

2 Post-forging heat treatment of large forgings The heat treatment carried out immediately after forging is called post-forging heat treatment. The main purpose of post-forging heat treatment for large forgings is to relieve stress, recrystallize, refine grains, and simultaneously remove hydrogen.

2.1 Recrystallization treatment Large forgings undergo multiple recrystallization treatments to refine grains, improve structure, and enhance performance.

2.2 Dehydrogenation annealing Reduces the hydrogen in the forging to below the limit hydrogen content and ensures uniform distribution to eliminate the risks of white spots and hydrogen embrittlement. Due to the different solubility and diffusion coefficients of hydrogen in the 7 phase and alpha phase, especially the low solubility and high diffusion coefficient of hydrogen in the alpha phase, hydrogen continuously diffuses outward during the annealing process. Dehydrogenation annealing is often combined with recrystallization. The temperature for dehydrogenation annealing is usually around 650°C. After annealing, cooling should be as slow as possible to prevent the generation of new internal stress. Generally, cooling is divided into two stages: above 400°C, where the plasticity of steel is good, internal stress is not easily formed, so cooling can be faster; below 400°C, cooling speed should be slowed down. For forgings with many alloying elements and higher performance requirements, one or even multiple recrystallization heat treatments are required after forging to improve the structure and performance of the forgings.

3 Ways and measures to reduce hydrogen content in steel Different types of steel forgings have varying sensitivities to white spots. The main factor affecting the sensitivity of forgings is the hydrogen content in the steel; the higher the hydrogen content, the greater the sensitivity to white spots. Hydrogen in steel can exist in atomic, molecular, or compound states, with atomic hydrogen being decisive for the formation of white spots. The fundamental method to prevent the formation of white spots is to reduce the hydrogen content in steel, keeping the residual hydrogen in the steel below the limit hydrogen content where no white spots occur.

3.1 Main ways to reduce hydrogen content in steel (1) Steelmaking: Perform vacuum degassing on the molten steel or use vacuum casting methods. For certain forgings with higher requirements, the electroslag remelting process can be used to further improve the purity of the steel. (2) Post-forging heat treatment: Dehydrogenation annealing allows hydrogen in the steel to diffuse out.

3.2 Measures to prevent white spots Methods to prevent the formation of white spots in steel include two aspects: first, to try to minimize the hydrogen content in the molten steel and create conditions for hydrogen to diffuse out of the steel blank; second, to reduce the internal stress generated in the steel blank during cooling, especially organizational stress. To reduce the hydrogen content in steel, minimize or avoid using severely rusted scrap steel during smelting; use new roasted or heated lime; use heated ferroalloys; ensure that steelmaking tools are fully dried and baked, and the casting system is dry; the temperature of the molten steel should not be too high to avoid excessive hydrogen absorption, ensuring sufficient time and vigorous boiling of the molten steel to facilitate dehydrogenation. The slag should not be too thin, and the final slag should be appropriately viscous.

4 Large forgings can avoid hydrogen removal annealing after forging if the hydrogen content is controlled. It is widely recognized in both domestic and international industries (forging, heat treatment) that when the hydrogen content in forgings is below (2-3)×10, white spots can be avoided. Therefore, 1) for general large forgings, the hydrogen content should be ≤(2)×10; 2) for important large forgings, the hydrogen content should be (1～1.5)×10. At this point, the hydrogen content in the steel has dropped below the limit that would cause white spots, so hydrogen removal annealing after forging is not necessary, but recrystallization must be performed. Hydrogen removal annealing after forging is a product of outdated and low-level smelting processes. (2) Current purity levels of special steel in China. The purity of the steel melt is a key factor in ensuring the quality of forgings. Currently, major special steel enterprises in China have significantly improved their production processes and technology levels through the introduction of production equipment and technological transformation. The commonly used smelting method is: electric furnace rough smelting + secondary refining. The methods for secondary refining typically include powder spraying, vacuum carbon deoxidation (VCD), ladle refining (VAD, CF), atmospheric oxygen-argon decarburization, and electroslag remelting. Through secondary refining, hydrogen, oxygen, and nitrogen in the steel can be controlled at very low levels. Currently, purity can reach ~IJEH]≤(2)×10～, [O]≤(15)×10, [N]≤(50)×10; the purity has reached an internationally advanced level. When the hydrogen content in forgings is at (3～7 or higher)×10, there is no doubt that heat treatment after forging must be carried out immediately after forging to avoid hydrogen removal annealing.

5 Examples of large forgings avoiding hydrogen removal annealing after forging (1) Shandong Jinan Yilait Heavy Industry Co., Ltd. Shandong Jinan Yilait Heavy Industry Co., Ltd. is a large forging enterprise jointly invested by the Spanish Gestamp Group, strictly following the processes of "blanking, forging, heat treatment, machining, and flaw detection" to provide processing for wind power generation gear ring forgings. Yilait has strict regulations on the hydrogen content of the steel used, with hydrogen content ≤(1～1.5)×10, which can completely avoid the occurrence of white spots. Therefore, hydrogen removal annealing after forging is not performed. The company has many foreign orders, and to date, there has not been a single case of white spots in forgings due to the absence of hydrogen removal annealing after forging, nor have there been any products scrapped due to the lack of hydrogen removal annealing. (2) Hebei Huigong Machinery Equipment Co., Ltd. Hebei Huigong Machinery Equipment Co., Ltd. specializes in producing imported mining vehicle wheel edge reducer assemblies, engine transfer case assemblies (PTO), transmission transfer case assemblies (PTO), and heavy-duty gears for construction machinery. Drawing on the advanced experience of the American gear industry in heat treatment of large forgings, our company clearly stated the requirements for steel purity when signing technical agreements with domestic special steel enterprises for steel procurement: [H]≤(2)×10, [O]≤(15)×10, [N]≤(50)×10. For large forgings, the hydrogen content is further controlled between ≤(2)×10 and ≤(1～1.5)×10, which can completely avoid the occurrence of white spots. Therefore, there is no longer a requirement for the forging plant to perform hydrogen removal annealing, but only one to two recrystallization heat treatments are conducted. To date, there have been no incidents of white spots in forgings due to the absence of hydrogen removal annealing, nor any serious accidents resulting in product scrapping. Additionally, the gears produced from these forgings have been functioning normally over long-term use. (3) FAW and Dongfeng Automotive Bridge Factory (Company). As far as I know, the gears produced by the FAW and Dongfeng automotive bridge factories (companies) in China have very strict requirements for the purity of gear steel, thus their forgings also avoid hydrogen removal annealing after forging.

6 Precautions in heat treatment of large forgings after forging (1) Procurement and storage of steel. When procuring steel, it should be selected according to the seasons of spring, summer, autumn, and winter to ensure that the steel meets purity standards, avoiding the impact of the annual rainy season on the steel. There should be a certain amount of stock to ensure quality for urgent needs. (2) Sensitivity to white spots. During the cooling process of forgings, hydrogen on the surface easily diffuses out, while hydrogen in the core is not easily expelled. Therefore, the larger the cross-section of the forgings, the more likely white spots are to occur. As the cross-section of the forgings increases, the degree of segregation in the steel also becomes more severe, further promoting the formation of white spots. The larger the surface area of the forgings, the easier it is for hydrogen to precipitate. Therefore, hollow forgings (such as gear rings) are less sensitive to white spots than solid forgings (such as gear shafts). Low carbon steel with a carbon content of less than 0.3 has very low sensitivity to white spots. As the carbon content increases, the sensitivity of the steel to white spots increases. The faster the cooling speed after forging, the greater the sensitivity to white spots. 1) Steel that does not produce white spots: such as austenitic steel, ferritic steel, austenitic-ferritic steel, and duplex steel. Generally, carbon steel with a carbon content of less than 0.39% and a manganese content of less than 0.6% under normal production conditions will not form white spots. Therefore, there is no need for hydrogen removal annealing after forging for this type of steel; 2) Steel sensitive to white spots: pearlitic steel has low sensitivity to white spots, martensitic-pearlitic steel has higher sensitivity, and martensitic steel has the highest sensitivity to white spots. For example: 20Cr2Ni4, 18Cr2Ni4W, 20CrNi2MO, 40CrNi2MO, 40CrNiMO, etc. The limit hydrogen content for martensitic steel is ≤(1.8)×10. (3) Reworking large forgings that have already produced white spots and are scrapped. Forgings that have already produced white spots can be processed into smaller-sized blank components through reworking and other thermal processing methods. The smaller-sized steel blanks after reworking need to undergo slow cooling and annealing, which can recover some of the scrapped steel. (4) Provide quality inspection reports when delivering forgings. The forging plant must provide a quality inspection report upon delivery of the forgings, which must include: 1) purity levels of [H], [N], [O]; 2) chemical composition and mechanical properties; 3) forging ratio; 4) ultrasonic flaw detection and other related content.