QT500 Casting Bearing House End-to-End Quality Control

Introduction: QT500 Bearing Housings—Why Are the Quality Requirements So Stringent?

QT500 (common grades include QT500-7 and QT500-10) is a ferritic-pearlitic hybrid matrix ductile iron that offers a balance of strength and toughness. Taking QT500-7 as an example, it has a tensile strength of ≥500 MPa, a yield strength of ≥320 MPa, an elongation of ≥7%, and a hardness of 170–230 HB.

Bearing housings manufactured from QT500 serve as core load-bearing components in critical parts of wind turbines, heavy machinery, and crushing equipment. The quality of the bearing housing directly determines the installation accuracy of the bearings, as well as the operational stability and service life of the equipment. A shrinkage cavity, slag inclusion, or poor spheroidization that goes undetected during the casting process may lead to catastrophic failure after several years of equipment operation.

Therefore, QT500 bearing housings cannot rely solely on “final inspection” for quality assurance; an end-to-end quality control system must be established, covering the entire process from raw materials to final product delivery. This article analyzes the key control points at each stage in the sequence of the manufacturing process.

I. Raw Materials and Smelting Control—The Genetic Engineering of Quality

The quality of QT500 bearing housings begins with the molten iron. The purity and composition of the raw materials directly determine the spheroidization effect and the final mechanical properties.

1.1 Key Composition Control

The typical composition control ranges for QT500-7 are as follows:

1.2 Smelting Process Control

Smelting Temperature: Must reach above the alloy’s liquidus line (ductile iron ≥1450℃) to ensure uniform composition.

Deoxidation and Degassing: Add deoxidizers such as ferrosilicon and aluminum to remove oxides and gases from the melt, preventing porosity and slag inclusions in the castings.

Composition Adjustment: Take samples during the late stage of melting for spectral analysis and add alloying elements based on the results.

Spheroidization and Inoculation Treatment: Use FeSiMg alloy as a spheroidizing agent via the injection method, combined with CaBaSi inoculant, to improve the elongation of the castings.

II. Control of Spheroidization Rate—The “Key Indicator” for QT500

The spheroidization rate is the most critical indicator for measuring the quality of ductile iron. The spheroidization rate requirement for QT500-7 is ≥80% (spheroidization grade ≥ Grade 2).

Key Control Measures

Rapid Testing at the Furnace: Use metallographic or thermal analyzers at the furnace to quickly assess the spheroidization effect before pouring.

Prevention of Spheroidization Decay: The time between the completion of the spheroidization treatment and the end of pouring must be strictly controlled (typically ≤10 minutes) to prevent spheroidization decay.

Pouring Ladle Management: Use pouring ladles specifically designed for ductile iron to strengthen process control during spheroidization treatment.

Direct Consequences of Poor Spheroidization: The graphite morphology changes from spherical to flake-like or worm-like, tensile strength drops sharply, and bearing housings may suffer brittle fracture during service.

III. Casting Process Design—Eliminating Shrinkage Cavities, Shrinkage Porosity, and Inclusion

Bearing housing structures typically have significant variations in wall thickness, and hot spots (such as bosses and wall thickness transitions) are prone to shrinkage cavities and shrinkage porosity.

3.1 Gating System Design

Gate Location: Introduce molten metal from thick-walled areas or the center to avoid direct impact on the sand mold (to prevent sand blowout); use bottom-pour or stepped gates to reduce turbulence and oxidation.

Sprues and Chills: Install sprues at hot spots to compensate for shrinkage, or place chills to accelerate cooling and achieve sequential solidification.

Process Optimization: By using casting simulation software (such as AnyCasting) to optimize pouring temperature, pouring speed, and chill placement, the probability of shrinkage cavities can be reduced to below 0.09%.

Pouring Temperature: The typical range is 1370–1430°C.

3.2 Selection of Molding Process

QT500 bearing housings typically use the resin sand molding process, which achieves dimensional accuracy of CT8 to CT10 and high surface quality, making it suitable for castings with complex internal cavities.

3.3 Cooling Control

The unboxing temperature for ductile iron castings should be ≤600°C. Large castings must be cooled with the furnace or slowly cooled in a sand pile to prevent deformation or cracking caused by rapid cooling.

Cooling schemes are optimized based on the casting’s solidification temperature distribution by appropriately setting the size and position of chill blocks.

IV. Heat Treatment—Stress Relief and Microstructural Stabilization

QT500 bearing housing castings must undergo stress-relief annealing after cooling:

Heating temperature: 550–650°C

Holding time: 2–4 hours (adjust according to wall thickness)

Cooling method: In-furnace cooling

Why is annealing necessary? If casting stresses are not eliminated, they will be released during subsequent CNC machining, causing deformation of the bearing bore and dimensional deviations. For bearing housings requiring high precision, a secondary stress-relief annealing may be performed after rough machining.

For defective microstructures exhibiting eutectic carbides, a normalizing + tempering heat treatment must be performed to achieve a balanced microstructure consisting of pearlite and 30%–60% ferrite.

V. CNC Machining—Dual Assurance of Precision and Surface Quality

5.1 Reference Surface Selection and Machining Processes

Rough Reference Surface: Use the bearing housing mounting surface (bottom surface or flange surface) as the rough reference surface; machine the mounting surface first.

Fine Reference Surface: Use the mounting surface as the fine reference surface to machine the bearing bore, ensuring the perpendicularity and coaxiality between the bore and the mounting surface.

Typical Processes: Rough Machining → Semi-finishing → (Second Annealing) → Finishing.

5.2 Machining Equipment and Cutting Tools

High-precision hole patterns are machined using machining centers or CNC lathes, in conjunction with fixtures for positioning (two pins on one face), to ensure a hole spacing tolerance of ≤±0.05 mm.

Carbide cutting tools are used, and cutting parameters are controlled (for machining holes in cast iron: v = 50–80 m/min, f = 0.1–0.3 mm/r) to minimize thermal deformation.

VI. Quality Inspection System—Full-Process Control

Quality inspection of QT500 bearing housings must cover three stages: casting, machining, and finished products.

6.1 Casting Stage Inspection

6.2 In-Process Inspection

Process Inspection: Verify critical dimensions after each processing step.

Final Inspection: Conduct a comprehensive inspection of the bearing bore’s diameter, roundness, concentricity, and perpendicularity using a coordinate measuring machine (CMM).

Surface Hardness Test: The hardened layer depth on the contact surface must be ≥1.2 mm.

VII. Cleaning, Rust Prevention, and Packaging—The Final Line of Defense Before Delivery

7.1 Cleaning and Deburring

High-pressure spraying or ultrasonic cleaning is used to remove oil and metal shavings.

All sharp edges resulting from machining must be deburred; bearing bores and mounting surfaces must be free of any burrs detectable by touch.

The parts must be thoroughly dried after cleaning before rust prevention treatment is applied.

All machined surfaces must be coated with a rust inhibitor; wooden pallets must not come into direct contact with metal surfaces.

7.2 Packaging Requirements

Wooden crates or pallets must be reinforced at all four corners to prevent shifting during transport.

For export, heat-treated or fumigated wooden packaging (bearing the IPPC mark) must be used.

Affix “Fragile” and “Keep Dry” labels.

VIII. Documentation and Traceability

The documentation accompanying the shipment reflects our professionalism:

• Material Certificate (batch number, chemical composition, mechanical properties)

• Dimensional Inspection Report (records of critical dimensions)

• Non-Destructive Testing Report (UT/MT/PT)

• Heat Treatment Curve Records

• Certificate of Conformity

• Rust Prevention Validity Period Statement

Each product must have a unique serial number, and all inspection records must be linked to that serial number to ensure full traceability throughout the entire process.

Conclusion: The Quality of QT500 Bearing Housings = End-to-End System Capability

End-to-end quality control for QT500 ductile iron bearing housings is not merely a matter of “high standards” at any single stage, but rather a systematic process built upon the cumulative control of every step—from melting to delivery.

From the moment a QT500 bearing housing is cast until it reaches the customer’s hands, it undergoes chemical composition control, spheroidization testing, casting simulation optimization, stress-relief annealing, precision machining, coordinate measuring machine (CMM) inspection, non-destructive testing, rust-proof packaging, and comprehensive documentation for traceability—with clear standards, verifiable data, and designated responsible parties at every stage. As a result, product quality is no longer a matter of “luck,” but rather a reproducible engineering capability.

What customers receive is not just a bearing housing, but a commitment to quality that stands the test of time.