Analysis of White Point Defects in Forged Components

Forged components are crucial structural parts used across various industries, including aerospace, automotive manufacturing, and energy. However, a common defect known as the white point defect in forged parts has raised significant concerns due to its impact on material performance and safety. Understanding the mechanisms behind white point formation and the factors that influence it is essential for optimizing production processes and improving product quality.

Causes of White Point Formation

White points primarily arise from the interaction between hydrogen and internal stresses within the forged material. Internal stress, or microstructural stress, occurs during the transformation of austenite into martensite or pearlite. White points typically form only when both a certain concentration of hydrogen and significant internal stress are present. For example, in single-phase austenite and ferrite forged materials, the absence of phase change-induced internal stress makes white points extremely rare. Thus, understanding the relationship between hydrogen and internal stress is key to studying white points.

Impact of Hydrogen

Hydrogen significantly affects the mechanical properties of forged components. It reduces the plasticity of steel and can lead to brittle fracture. When the hydrogen concentration in a forged part reaches a certain threshold, plasticity can drop sharply, resulting in hydrogen embrittlement. For instance, in 25Cr2Ni2Mo steel forgings, when hydrogen levels reach 14.5 cm³/100g, the elongation rate drops to 0.6%, and the reduction in area falls to zero after normalizing at 900°C and tempering at 600°C. This indicates a negative correlation between hydrogen content and the mechanical performance of the forged part.

Hydrogen Embrittlement

Hydrogen embrittlement not only affects plasticity but can also pose serious safety risks. Under prolonged stress, hydrogen can diffuse into stress concentration areas, leading to nearly zero local plasticity. When stress reaches a critical level, brittle fracture may occur. This phenomenon is particularly prominent in high-strength steels, necessitating careful control and management of hydrogen during design and use.

Hydrogen Precipitation

During forging, hydrogen absorbed by molten steel precipitates as the solubility decreases upon solidification, often remaining trapped in internal voids. When the forged part is heated before pressure processing, hydrogen can re-dissolve in the steel. During cooling, as austenite decomposes and temperature drops, hydrogen solubility decreases, causing hydrogen atoms to precipitate in microscopic voids within the forged material. These hydrogen atoms can combine to form molecular states, creating immense internal pressure, which can reach up to 1200 MPa under specific conditions.

Reaction with Carbon

Additionally, hydrogen can react with carbon in the steel to form methane (CH₄), further increasing the molecular pressure. This reaction can lead to decarburization observed on the surface of white points, reinforcing the understanding of hydrogen’s role in their formation.

Influence of Internal Stress

During cooling, internal stresses due to phase changes can become substantial. In steels with severe dendritic segregation, rapid cooling, and good hardenability, internal stress tends to be more pronounced. The combination of internal stress from hydrogen precipitation can lead to brittle fracture, resulting in white points. Additional stresses caused by uneven deformation during forging and thermal stresses during cooling can also contribute to the formation of white points.

Material Characteristics and White Point Formation

Different material characteristics significantly influence white point formation. Cast steel forgings, which typically have larger internal voids, are less sensitive to white point formation because hydrogen precipitation does not create significant internal stress. Similarly, ferritic and austenitic forgings, which undergo no phase change during cooling, also rarely exhibit white points. While bainitic steels may develop considerable internal stress during cooling, stable hydrides formed in these steels, along with complex carbides that hinder hydrogen precipitation, reduce the occurrence of white points.

Timing of White Point Appearance

White points usually become visible hours or even days after a forged part cools to room temperature. For example, a 160mm martensitic alloy structural steel forging showed no white points within 12, 24, or 48 hours after cooling, with the first appearance occurring at 72 hours. This characteristic highlights the importance of considering the cooling time in detecting and assessing white points during quality control.

Prevention and Control Measures

To effectively prevent and control the formation of white points in forgings, the following measures can be implemented.

Control Hydrogen Levels: During steelmaking, manage hydrogen absorption and take steps to minimize hydrogen dissolution, such as using vacuum degassing.
Optimize Heat Treatment: Implement appropriate heat treatment processes to reduce internal stress during phase changes and lower the risk of hydrogen precipitation.
Improve Material Selection: Choose materials with higher toughness and lower sensitivity to hydrogen embrittlement to enhance the hydrogen resistance of forgings.
Regular Inspection and Evaluation: Conduct timely white point inspections after cooling to ensure the quality and safety of forged components.

Conclusion

The formation of white points in forged components is a complex process influenced by factors such as hydrogen concentration, internal stress, and material characteristics. Gaining a deeper understanding of these mechanisms is crucial for improving forged component performance and reducing hydrogen embrittlement. By implementing effective heat treatment, controlling hydrogen levels, and optimizing forging processes, the risk of white point formation can be significantly reduced, enhancing the overall quality and reliability of forgings. Furthermore, regular inspections will help identify potential defects, ensuring the dependability of forged components in practical applications. Strengthening research and control of white points will contribute to the advancement of the forging industry and enhance product competitiveness.