Introduction: Why Is Quality Control of Forged Journals Critical?

Heavy-duty paper machine rolls are the core components in the pressing section of paper machines. The journal – as the critical structure connecting the roll body to the bearings – continuously withstands radial compressive loads, alternating bending stresses, and torsional fatigue loads under high-speed rotation. Any quality issue with the journal – whether internal shrinkage cavities, inclusions, cracks, or insufficient surface hardness, or dimensional deviations – can lead to premature bearing failure, roll runout, or even catastrophic equipment accidents.

42CrMo (AISI 4140/4142) alloy steel, with its excellent hardenability, high strength, and good toughness, has become the mainstream material for heavy-duty paper machine roll journals. After a complete quenching and tempering heat treatment, 42CrMo alloy structural steel can achieve tensile strength above 900MPa, excellent hardenability, uniform mechanical properties across the wall thickness, and significantly improved alternating fatigue resistance, compression resistance, and bending resistance. However, the superiority of the material can only be transformed into product reliability through strict full-process quality control.

This article follows the typical process flow for forged journals – raw material → forging → heat treatment → rough machining → non-destructive testing → finishing → final inspection – systematically explaining the quality control points at each stage.

Part 1: Raw Material Quality Control – The Source of Quality

1.1 Ingot Selection and Chemical Composition Control

The 42CrMo steel ingot used for paper machine roll journals should be vacuum-degassed refined billet to ensure internal density and low gas content.

Each batch of ingot shall undergo spectrometric re-testing upon receipt, strictly limiting harmful impurities such as sulphur and phosphorus, and screening out ingots with inherent defects in advance.

1.2 Cutting and Forging Ratio Calculation

Technical personnel calculate the cutting dimensions based on the final journal outer diameter and overall length, reserving adequate forging ratio and machining allowance. The overall forging ratio for conventional journal forgings is controlled at 4 or above; for heavy-duty applications, the forging ratio is increased to 4.5–6.

Sufficient plastic deformation is the key to breaking down coarse as-cast grains, welding internal micro-porosities, and bringing the matrix density close to the theoretical density.

Part 2: Forging Process Quality Control – Ensuring Densification

2.1 Stepwise Heating – The First Step in Preventing Cracking and Eccentricity

Alloy steel has low thermal conductivity. Rapid heating causes excessive temperature differences between the outer and inner layers of the billet, leading to inconsistent deformation during forging and resulting in eccentricity and uneven wall thickness.

Key heating process points:

• Low-temperature stage: Heating rate not exceeding 50°C/h below 600°C – fully release internal thermal stress

• Medium-temperature stage: Ramp steadily to 850°C and hold for uniform soaking

• High-temperature stage: Final heating to 1150°C–1200°C for the starting forging temperature

• Soaking time: Calculated based on cross-section thickness – 1.5 hours per 100mm wall thickness

• Final forging temperature: Strictly maintain the 820°C minimum

Infrared temperature monitoring throughout ensures the temperature difference between the outer and inner layers is controlled within 20°C, preventing both surface overheating and core under-heating.

2.2 Multi-Heat Progressive Forging – Layer-by-Layer Densification

The core principle of journal forging is multi-heat, small reductions, and progressive forming – rejecting the single-pass large-deformation forging mode.

Key forging sequence points:

During forging, concentricity must be checked and calibrated continuously. Immediately after final forging, roundness straightening is performed to correct any minor eccentricity generated during forging.

2.3 Common Defect Prevention During Forging

Part 3: Heat Treatment Quality Control – The Key to Performance

After forging, the journal still contains forging residual stress, coarse and uneven grain structure, and low surface hardness. Forging alone cannot meet the demands of long-term continuous production. Heat treatment is the critical process for adjusting the internal metallurgical structure, balancing strength and toughness, and extending journal service life.

3.1 Quenching and Tempering (Quenching + High-Temperature Tempering)

42CrMo journals typically undergo quenching + high-temperature tempering (quenching and tempering).

Quenching process points:

Water quenching is not suitable for thick-walled 42CrMo journals – the severe temperature gradient can easily induce micro-cracks and distortion. Interrupted oil cooling produces a fine martensitic structure on the surface to improve hardness and wear resistance, while retaining adequate toughness in the core, achieving a balance between wear resistance and bending resistance.

High-temperature tempering process points:

After quenching and tempering, the overall hardness of the journal stabilises at HB280–330, with tensile strength, yield strength, and impact toughness reaching a balanced state. Some studies indicate that a 860°C quenching + 640°C tempering heat treatment scheme can achieve the optimal combination of high strength and high toughness.

3.2 Induction Hardening (Optional – for High Wear Resistance Requirements)

For orders requiring higher surface wear resistance, surface induction hardening of the outer diameter can be added on top of the overall quenching and tempering treatment:

• Hardened layer depth: Up to 6mm

• Surface hardness: Up to 55 HRC

• Structural effect: Achieving "hard shell, tough core" – wear-resistant surface, impact-resistant core

3.3 Common Heat Treatment Defect Prevention

Part 4: Non-Destructive Testing – The "Vigilant Eye" for Quality

Non-destructive testing is an indispensable step in the forged journal quality control system. It enables detection of internal defects before machining, preventing hidden defects from being carried into subsequent operations.

4.1 Ultrasonic Testing (UT)

Ultrasonic testing is the primary method for detecting internal defects (shrinkage cavities, inclusions, porosity, cracks) in forged journals.

EN 10228-3 specifies the techniques for manual pulse-echo ultrasonic testing of ferritic or martensitic steel forgings.

4.2 Magnetic Particle Testing (MT)

Magnetic particle testing is used to detect surface and near-surface defects (cracks, laps, seams, etc.) on the journal.

For journal forgings, 100% segmented multi-directional magnetisation should be performed after tempering to inspect for circumferential and longitudinal cracks.

4.3 Macrostructure and Microcleanliness

In accordance with standards such as ASTM A649, Macrotech examination and Microcleanliness evaluation should also be performed.

Part 5: Machining and Dimensional Control – Precision Assurance

5.1 Rough Machining

The main purpose of rough machining is to remove the forged surface scale and most of the allowance, while exposing any potential defects.

• Machining allowance: Reserved per procedure – typically 3–6mm per step

• Datum establishment: Machining is performed using the journal centreline as the reference

• Stress relief: Stress relief treatment after rough machining

5.2 Finishing

Finishing is the final assurance of journal dimensional accuracy and surface quality.

Key dimensional control:

The surface roughness and runout of the finished journal directly affect bearing life and operational stability – excessive runout will cause vibration and accelerate bearing wear.

5.3 Hardness Re-Check

After finishing, hardness re-check is performed on the journal surface to verify the effectiveness of the heat treatment and ensure no surface damage was caused by the machining process. The hardness of quenched and tempered 42CrMo journals is typically controlled in the HB260–320 range.

Part 6: Final Inspection and Documentation

6.1 Final Inspection Items

6.2 Delivery Documentation

Complete delivery documentation is a reflection of professionalism:

• Material certificate (chemical composition, mechanical properties)

• Heat treatment curve records

• NDT reports (UT/MT)

• Dimensional inspection report

• Hardness test report

• 3.1/3.2 certification (per customer requirements)

Full-Process Quality Control Summary

Conclusion: Forged Shaft Quality = Full-Process Systems Engineering

Quality control for heavy-duty paper machine roll 42CrMo journals cannot be guaranteed by a single stage of "high standards." It is a full-process systems engineering covering raw material, forging, heat treatment, non-destructive testing, and finishing.

Core principles:

• Source control: Vacuum-degassed ingot + strict chemical composition control

• Process control: Stepwise heating, multi-heat forging, precise quenching and tempering – every step executed to standard

• Inspection verification: Dual NDT (UT + MT) to ensure both internal and surface quality

• Precision assurance: Finishing with strict control of dimensions, roughness, and runout

When these five stages form a closed loop, the quality of 42CrMo forged journals ceases to be a gamble – it becomes a predictable, repeatable, and verifiable engineering capability. For high-value, high-risk rotating components such as paper machine rolls, this is the fundamental guarantee of long-term reliable equipment operation.

"The quality of a forged shaft begins with the ingot, is shaped by forging, refined by heat treatment, verified by NDT, and completed by finishing – every step is indispensable."