Key Concept: Raw materials are the “genes” of forging quality.

The forging process can be likened to “nurture,” while raw materials determine “innate genes.” If the raw materials have fatal flaws, no matter how advanced the forging technology and heat treatment processes are, they cannot fundamentally correct the problem and will often only increase costs. Therefore, paying attention to raw materials is the first and most critical line of defense for ensuring high quality, high reliability, and high consistency in forgings.

Specific Impacts of Key Raw Material Indicators on Forging Quality

1. Chemical Composition

l Impact: Chemical composition directly determines the material’s ultimate basic mechanical properties (strength, hardness, toughness, and plasticity), physical properties (heat resistance and corrosion resistance), and processability (forgeability, hardenability, and weldability).

l Consequences:

Composition deviations may result in forgings failing to meet design specifications through heat treatment. For example, low carbon content results in insufficient strength and hardness, while inaccurate alloying element content affects hardenability, resulting in substandard core properties.

Impurity elements: Sulfur (S), phosphorus (P), hydrogen (H), and oxygen (O) are harmful.

High levels of S and P can form low-melting-point compounds at grain boundaries, causing hot brittleness (cracking during forging) and cold brittleness (a sharp drop in low-temperature impact toughness).

Hydrogen (H): During the cooling process after forging, precipitated atomic hydrogen combines with molecular hydrogen at internal defects, generating enormous pressure and causing white spots (cracking). This is an extremely dangerous internal defect that can lead to sudden brittle fracture.

2. Metallurgical Quality

This reflects the “innate health” of the raw material, and its impact is fatal and irreversible.

l Inclusions

Impact: They disrupt the continuity of the metal matrix, acting as internal “microcracks.” Under stress, inclusion tips generate stress concentrations, becoming the initiation point for fatigue cracks, significantly reducing the fatigue strength, toughness, and ductility of the forging.

Types:Plastic inclusions (such as sulfides): During forging, they extend along the deformation direction, forming flow lines and leading to anisotropic mechanical properties.

Brittle inclusions (such as oxides and silicates): During forging, they may be broken into chain-like distributions, severely fragmenting the matrix.

l Segregation

Impact: During ingot solidification, uneven element distribution occurs, resulting in localized chemical compositions that differ from the overall composition.

Consequences: Forging is difficult to completely eliminate segregation. Severe segregation can lead to uneven microstructure and properties, and abnormal microstructures (such as martensite bands) may appear in localized areas, causing quenching distortion, cracking, or premature failure during service.

l Porosity & Shrinkage

Impact: Solidification defects in the center of the ingot.

Consequences: If the forging ratio (forging pressure ratio) is insufficient and these defects are not welded together, these defects will remain in the forging, becoming stress concentration sources and crack propagation channels, severely reducing the load-bearing capacity, especially the life under dynamic loads.

3. Surface Quality

l Impact: Surface defects such as cracks, folds, scars, and coarse scratches on the raw material.

l Consequences: These surface defects will further expand and deepen during the heating and deformation process during forging. Even if they can be removed through subsequent machining allowances, this will increase costs and the risk of scrap. Failure to remove them can lead to fatigue fracture.

4. Internal Macrostructure

l Impact: Raw material grain size, dendrite morphology, and other factors.

l Consequences: Coarse original grains or dendrites will be inherited by the forging. Even after forging, this can lead to uneven structures such as mixed or coarse grains, deteriorating the strength and toughness of the forging.

How to Value Raw Materials: Quality Control Process

To ensure forging quality, strict control of raw materials throughout the entire process is essential:

1. Source Selection:

Select reputable steel companies with advanced processes (such as external refining and vacuum degassing) as suppliers.

Select raw materials of appropriate quality grades (e.g., high-quality steel, advanced high-quality steel, and special high-quality steel) based on the service conditions of the forgings (e.g., aviation, nuclear power, wind power, etc.).

2. Incoming Inspection:

l Chemical Composition Analysis: Rapid and accurate verification using spectrometers and other instruments.

l Macrostructural Inspection: Low-magnification acid immersion testing to detect defects such as segregation, porosity, and residual shrinkage cavities.

l Microstructural Inspection: Metallographic examination to determine the type, grade, and distribution of non-metallic inclusions.

l Ultrasonic Testing: Used for rapid, large-scale internal defect detection of bars and billets, effectively detecting internal cracks, inclusions, and white spots.

l Surface Inspection: Visual inspection, magnetic particle testing (MT), or penetrant testing (PT) to inspect surface defects.

3. Process Traceability:

Establish a comprehensive raw material-forging traceability system to ensure that each batch of forgings can be traced back to the raw material heat and batch number, enabling precise identification and accountability of quality issues.

Conclusion: Considering Both Economic and Safety Benefits

l From an economic perspective, using substandard raw materials may appear to reduce procurement costs, but this can lead to increased scrap rates during the forging process, scrapping due to problems discovered during subsequent processing (greater losses), and post-sales compensation for failures, resulting in a sharp increase in total costs.

l From a safety perspective, even minor defects in raw materials can escalate during service in critical load-bearing components, leading to catastrophic fractures and significant loss of life and property.

Therefore, attaching great importance to raw material quality is an inevitable choice for forging companies to mitigate risks, maintain their reputation, enhance core competitiveness, and move towards high-end manufacturing. It does not increase costs, but rather represents a crucial and cost-effective investment in superior product performance, safety, and reliability.