In the field of metal processing, the quality of forgings is one of the key factors in measuring product performance. Among these, the presence of non-metallic inclusions has a significant impact on forgings. This article will explore in depth the sources of non-metallic inclusions, their effects on forging performance, and how to effectively control their content and distribution, to help relevant practitioners better understand and address this issue.
Non-metallic inclusions are a common internal defect in steel, and their formation is closely related to the steelmaking and casting processes. During steelmaking, sulfides, silicates, and other oxides in the raw materials are the main components of inclusions. In alkaline steelmaking, deoxidizers such as silicon, manganese, and aluminum are added to steel, forming manganese silicate, aluminate, aluminum oxide, and manganese sulfide. These substances react further with calcium oxide in the slag, producing new compounds that constitute non-metallic inclusions.
In addition to chemical reactions during steelmaking, non-metallic inclusions can also be introduced during metal melting and casting. For example, foreign substances such as refractory materials or sand that fall into molten steel can react chemically with the steel to form inclusions. When molten steel flows from the tapping ladle into the steel teeming ladle, fine inclusions may mix into the steel. If the tapping ladle walls or steel teeming ladle walls are not clean, non-metallic inclusions can also form. Furthermore, the risers of various types of steel ingots are usually made of refractory materials, which are also an important source of non-metallic inclusions.
Shortly after steel tapping and final deoxidation, the metal will contain uniformly distributed suspended particles, about 90% of which are non-metallic inclusions of 5 μm in size. The distribution of these inclusions in ingots is not uniform, often concentrating towards the bottom of the ingot and near the ingot centerline. This distribution characteristic makes non-metallic inclusions more likely to affect forging performance during subsequent processes.
Before discussing how to effectively control non-metallic inclusions, we first need to understand their specific impact on forging performance. Although the content of non-metallic inclusions in steel is relatively small, their presence can have a profound effect on the mechanical properties, machinability, and service life of forgings.
The content, type, size, and distribution of non-metallic inclusions all affect forging performance, with size and distribution being the most significant factors. Analysis of fracture surfaces of forgings shows that non-metallic inclusions often exist in chains or clusters, which severely affects the usability of steel.
Different inclusion compositions have varying impacts on steel properties. Aluminum oxide, spinel, and calcium aluminate inclusions in various oxide inclusions in steel are not easily deformed during pressure processing, and microcracks often appear around them. Among them, calcium aluminate is the most harmful, followed by aluminum oxide, then spinel and titanium nitride, while silicates and iron oxides have relatively smaller impacts. Certain silicates and small amounts of sulfides do not cause significant adverse effects on steel forgings because they have good plasticity and can deform plastically during processing without producing cracks. In particular, when oxides are present in steel, sulfides can even reduce the harmful effects of oxides.
However, oxide inclusions or oxide-silicate inclusions are brittle and prone to fracture during machining, leaving small, sharp pits on the steel surface. Under harsh operating conditions, the presence of these inclusions may lead to continuous fractures. For example, during forging, if there are too many low-melting-point inclusions such as excessive sulfides along grain boundaries in the steel ingot, hot brittleness may occur during forging, causing cracks to expand continuously and reducing forging performance. Sometimes, non-metallic inclusions are also present in martensite, generally formed under compressive stress. As austenite transforms and expands in volume, non-metallic inclusions are subjected to tensile stress, and some even consider these inclusions as nucleation sites for austenite transformation.
In open-die forging, if the anvil-to-stock width ratio and stock width ratio are not appropriate, cracks may form around non-metallic inclusions. When these ratios are not controlled properly, axial and transverse tensile stresses occur in the deformed material, forming new crack sources at grain boundaries or weak areas. After one forging cycle and a 90° rotation, during the second cycle of elongation along the forging length, if the stock width ratio is much greater than 1 and the anvil width ratio is improperly controlled, severe shear deformation may occur near the horizontal symmetrical plane of the deformation zone, generating layered transverse crack sources. Longitudinal cracks from the previous cycle and layered transverse cracks after 90° rotation may occur at the same position and direction within the blank, and after repeated cycles, internal cracks may form in the forging.
Non-metallic inclusions are unevenly distributed in steel ingots, with the most hazardous areas being segregated regions near the bottom of the ingot (negative segregation zones) and the interface between the ingot and riser, about 300–500 mm from the edge of the riser. For example, in generator rotors, inclusions such as some carbides and discontinuous defects can produce high stress concentration, causing rotor fracture. Moreover, non-metallic inclusions often coexist with segregation, and higher inclusion content worsens segregation. To minimize the impact of segregation and inclusions, it is common to trim the top and bottom of the ingot, removing 30% from the top (equivalent to the riser plus about 25 mm of ingot) and trimming the bottom according to ingot size, with the bottom trim mass reaching up to 15% of the ingot mass.
The presence of laminated structures in steel greatly affects machinability. Severe laminated structures result in low plasticity and impact toughness. The formation of laminated fracture surfaces is closely related to non-metallic inclusions, depending not only on their quantity but also on composition, morphology, and distribution. The tendency of inclusions to form laminated fractures and affect tangential properties varies: the most severe are elongated sulfides (FeS, MnS) and elongated manganese silicates (2MnO·SiO₂), followed by elliptical FeO, round SiO₂, 2MnO·SiO₂, 2FeO·SiO₂, and less severe are brittle inclusions like Al₂O₃, 3Al₂O₃·2SiO₂. Among inclusions affecting plasticity and impact toughness, excessive sulfide content is the most detrimental, so the sulfur content in the original billet steel must be strictly controlled, generally not exceeding 0.025%.
The fundamental way to reduce non-metallic inclusions is to control the source processes. First, efforts should be made to minimize inclusion sources during steelmaking and casting. This includes ensuring the quality of raw materials to avoid excessive impurities entering the steelmaking process. At the same time, chemical reaction conditions during steelmaking should be strictly controlled to reduce unnecessary inclusion formation. During casting, the tapping ladle and teeming ladle should be kept clean to prevent foreign materials such as refractory particles or sand from entering the molten steel.
Second, inclusions already formed in the steel should be allowed to float to the riser zone as much as possible. This can be achieved by optimizing the pouring process and controlling the flow speed and direction of molten steel to facilitate inclusion floatation. For example, using appropriate pouring temperatures and rates can promote inclusion flotation. At the same time, properly designing the riser shape and size ensures that floating inclusions are accommodated effectively, preventing them from reentering the ingot.
During forging, although inclusions cannot be completely eliminated, proper forging processes can reduce the size of coarse inclusions and disperse dense inclusions. Specific control methods include:
High-temperature diffusion annealing of steel ingots is an effective method. During annealing, the ingot's internal structure becomes balanced, reducing segregation. Inclusions can diffuse and migrate at high temperatures, resulting in a more uniform distribution. This helps reduce local accumulation of inclusions, mitigating their adverse effects on forging performance.
The riser end should be trimmed sufficiently. Since the riser zone is prone to inclusion accumulation, trimming removes most inclusions. The top should be trimmed 30%, equivalent to the riser plus about 25 mm of ingot. This effectively reduces inclusion content in the forging, improving quality.
Forge with large reductions and upsetting ratios. Using full anvil feed and large reductions causes inclusions in the ingot center to deform and weld voids. Increasing the upsetting ratio, preferably to 2, allows inclusions to be fully broken and dispersed, reducing their impact on forging performance. Large reductions apply greater pressure to inclusions, breaking them into smaller particles, while a larger upsetting ratio distributes inclusions more evenly along the ingot length, preventing local accumulation.
Non-metallic inclusions are common internal defects in steel that have important impacts on forging performance. Understanding their sources, distribution characteristics, and effects on forgings helps implement effective control measures. By minimizing inclusion formation during steelmaking and casting, allowing formed inclusions to float to risers, and using proper forging processes such as high-temperature diffusion annealing, riser trimming, and forging with large reductions and increased upsetting ratios, the content of non-metallic inclusions can be reduced and their distribution improved, thereby enhancing forging quality and performance. In practical production, relevant practitioners should comprehensively apply these methods according to specific conditions to ensure the forging quality meets usage requirements.
