Views: 0 Author: Site Editor Publish Time: 2026-06-15 Origin: Site
Large journal shafts are core transmission components in heavy machinery, marine engines, steam turbines, and large compressors. They are subjected to immense alternating loads, wear, and fatigue. If a journal shaft develops cracks, dimensional deviations, or surface defects, it could result in the scrapping of the entire machine or even a major safety incident.
Unlike ordinary parts, which can undergo concentrated inspection at a specific stage, large journals are high-value components with lengthy manufacturing processes (typically spanning weeks or even months). Any loss of control at any stage of the process can result in irreversible damage. Therefore, it is essential to establish a comprehensive quality control system covering the entire process from forging to CNC machining—not merely “post-production inspection,” but “process control.”
Large journal shafts are typically formed using open-die forging or die forging, and the materials used are mostly alloy steels (such as 42CrMo, 34CrNiMo6, and 40CrNiMoA) or stainless steel.
Key Control Points
Control items | requirements and methods | common defects |
Chemical Composition | Re-testing by batch number, spectroscopic analysis; C, S, P, and alloying elements must comply with standards | Composition segregation, excessive levels of harmful elements |
Forging Ratio | ≥3 (to ensure thorough forging and grain refinement) | Insufficient forging ratio → Coarse grain structure, poor mechanical properties |
Forging Temperature | Initial forging 1150–1200°C, final forging ≥850°C (to avoid overheating and overburning) | Overheating → coarse grain; overburning → intergranular oxidation, irreparable |
Surface Quality | Visual inspection + magnetic particle testing; folds and cracks are not permitted | Folds and cracks will carry over to subsequent processing |
Quality Control Measures
Each forging is accompanied by a Material Certificate and a Forging Process Card.
Immediately after forging, conduct a visual inspection and ultrasonic testing (UT). If internal cracks or inclusions are detected, the part is immediately rejected to prevent it from entering subsequent costly processes.
Large journal shafts typically require preliminary heat treatment (normalizing/annealing) and final heat treatment (quenching and tempering: quenching + high-temperature tempering) to achieve good overall mechanical properties.
Key Control Points
process | control parameters | inspection and verification |
Normalizing/Annealing | Heating temperature, holding time (calculated based on the largest cross-section), cooling method (air cooling or furnace cooling) | Grain size rating (required: Grade 5–8), hardness testing (HB) |
Quenching | Austenitizing temperature (840–880°C), holding time, quenching medium (water, oil, or polymer) | Surface hardness, hardened layer depth (determined via end-quench test or hardness gradient) |
Tempering | Temperature (550–650°C), holding time, cooling | Final hardness (HB or HRC), tensile/impact test specimens (in-furnace test bars provided with each batch) |
Common Issues and Solutions
Quenching Cracks: Large, complex-shaped parts are prone to cracking at sharp corners → Design with fillet radii of R ≥ 5 mm and pre-cool before quenching.
Hardness Variations: Improper loading or insufficient cooling → Use specialized fixtures to ensure proper spacing and increase agitation.
Temper brittleness: Can be mitigated in Mo-containing steels; rapid cooling after tempering.
Quality Control Deliverables: Heat treatment curve records (traceable temperature-time data), hardness test reports, metallographic reports.
Before proceeding to machining, the forgings must undergo comprehensive non-destructive testing.
inspection method | inspection object | acceptance criteria | defect treatment |
Ultrasonic Testing (UT) | Internal defects (porosity, inclusions, cracks, white spots) | In accordance with ASTM A388 or GB/T 6402; typically requires ≤ Φ2 mm equivalent | Defects exceeding the limit are rejected or downgraded for use |
Magnetic Particle Testing (MT) | Surface and near-surface cracks, folds | No linear indications permitted | May be ground out; wall thickness must be remeasured after grinding, and the remaining wall thickness must meet design requirements |
Key Point: UT of large journal sections requires zoned scanning (different sensitivities for journal sections, fillet sections, and flange sections) and must be performed and evaluated by certified Level II or higher personnel.
The primary purpose of rough machining is to quickly remove scale from the forged surface and most of the excess material, while exposing any potential defects.
Key Quality Control Points
Machining Allowance Control: Leave a semi-finishing allowance of 3–5 mm (in the diameter direction) according to the process. An excessive allowance can cause work hardening, while an insufficient allowance will prevent the decarburized layer from being removed.
Rough Geometric Alignment: Drill center holes and use the center holes at both ends as reference points for machining the outer diameter and end faces. The concentricity of the center holes at both ends must be ≤0.03 mm.
Stress Relief: After rough machining, stress-relief annealing (held at 500–550°C, followed by furnace cooling) or natural aging (left flat for 48 hours) is recommended to minimize deformation during subsequent finishing operations.
Inspection Items
Outer diameter and length dimensions after rough machining (measured with a tape measure or large caliper).
Check for runout of the center holes.
The objective of semi-finishing is to achieve precise geometric shapes and ensure uniform finishing allowances (typically 0.5–1.0 mm in the diameter direction).
Key Control Points
Coaxiality Control: Using the center holes at both ends as a reference, complete the semi-finishing of the shaft neck’s outer diameter, fillet, and shoulder in a single setup.
Fillet Transition: The fillet radius (R) between the shaft neck and the flange must comply with the drawing specifications to prevent stress concentration. Check using a circular template.
Hardness Retest: Perform a hardness test (using a Leeb hardness tester) on the semi-finished surfaces to verify the heat treatment results.
Common Issues
Wear of the center holes leading to loss of reference → Grind the center holes before finishing to restore accuracy.
Black skin (incomplete removal of scale) found after semi-finishing → Perform rework and check whether the roughing allowance was set too small.
Finishing is typically performed using large CNC horizontal lathes or CNC grinding machines to ensure the dimensional tolerances (typically IT6–IT7), roundness, cylindricity, and surface roughness (Ra 0.4–0.8 μm) of the journal.
Key Points for Finishing Turning (for HRC 30–40 quenched and tempered steel)
parameter | recommendation | notes |
Cutting Tool | CBN or ceramic inserts | Hard turning instead of grinding for higher efficiency |
Cutting Speed | 100–150 m/min | To prevent vibration |
Feed Rate | 0.05–0.10 mm/rev | To ensure surface roughness |
Depth of Cut | 0.1–0.3 mm | For final finishing |
Key Points for Grinding Finishing (for high-precision requirements, such as Ra ≤ 0.2 μm or roundness ≤ 0.005 mm)
Use a CNC external cylindrical grinder with grinding wheel grit sizes: 60#–80# (rough grinding) → 120# (finishing) → superfinishing.
Employ in-process measurement (using a measuring instrument to control dimensions); the tool retracts automatically once the tolerance is achieved.
oolant: High-flow water-based grinding fluid to prevent burning.
Mandatory Inspection Items After Finishing
Diameter: Micrometer or pneumatic measuring instrument; multi-point measurement (at least 3 cross-sections, 2 directions per cross-section).
Roundness/Cylindricity: Roundness gauge or CMM.
Surface roughness: Portable roughness tester (measured along the axial direction).
Runout: Radial runout of the journal relative to the center holes at both ends ≤ 0.01 mm.
Once finishing is complete, non-destructive testing must be performed again, as the machining process may have introduced grinding cracks or surface burn marks.
Cleaning and Rust Prevention:
Use a cleaning agent to remove oil and metal shavings.
Coat the journal surfaces with rust-preventive oil and wrap them in vapor-phase rust-preventive paper.
Do not touch the finished surfaces directly with your hands to prevent corrosion caused by sweat.
The final output of end-to-end quality control is a comprehensive quality dossier, which is delivered to the customer along with the product. The dossier should include:
Raw material certificates (melt number, chemical composition)
Forging process sheets and inspection records
Heat treatment curves and hardness reports
Ultrasonic testing reports
Process inspection records (dimensions, runout, and surface roughness for each process)
Final non-destructive testing reports
Dimensional inspection reports (including CMM data)
Each large journal must have a unique serial number, and all records must be linked to that serial number. If the customer encounters issues during use, the specific process and operator can be quickly traced.
Background: A paper machine journal diameter of φ320 mm, a length of 1800 mm, made of 42CrMo steel, with a quenched and tempered hardness of HB 280–320, a required journal roundness of 0.008 mm, and a surface roughness of Ra 0.4 μm.
Key Implementation Points:
Post-forging UT inspection detected a φ2.5mm inclusion. While this is acceptable according to standards, the location was marked and avoided during subsequent machining.
Heat treatment was performed using a pit-type furnace with vertical hoisting to ensure hardness uniformity (circumferential hardness variation ≤15HB).
Stress-relief annealing was performed after rough turning, followed by 72 hours of natural aging.
After semi-finishing, perform precision grinding: rough grinding with a CBN wheel, fine grinding with an alumina wheel, and final superfinishing with an oilstone.
Post-grinding MT inspection revealed no cracks, and Barkhausen testing showed no burn marks.
Results: Final roundness 0.006 mm, surface roughness Ra 0.32 μm, passed inspection on the first attempt. After delivery, the component operated for 5,000 hours without any abnormalities.
The value of large journal shafts means they cannot rely on “final inspection” alone to ensure quality. True quality assurance comes from maintaining control at every stage, from forging to CNC machining.
Summary of Key Points for the Entire Process:
Forging: Control temperature and aspect ratio to eliminate internal defects.
Heat Treatment: Achieve uniform microstructure and hardness.
Non-Destructive Testing: Detect every crack.
Machining: Remove material in stages guided by precise reference points.
Final Inspection: Conduct comprehensive measurements and establish traceable records.
When these eight stages form a closed-loop system, and each stage has clear control standards, inspection methods, and designated responsible parties, the “quality of large journal shafts” is no longer a matter of luck, but a reproducible engineering capability.
We hope this article provides a practical reference framework for engineers engaged in the manufacturing of large shaft components. Starting with your next project, review each item against this checklist, and you will find that your scrap rate drops significantly and customer trust increases markedly.
