Low Heat Input Weld Overlay Repair of Large 40Cr Steel Shafts

Literature Overview

This 2000 publication by Kong Wei, Yang Jianli (Xinjiang Shihezi Thermal Power Plant) and Ou Bin (Xinjiang Equipment Installation Company Technical School) addresses the weld overlay repair of large-diameter 40Cr steel shafts using low heat input techniques. The paper was motivated by practical repair needs at thermal power stations, where large turbine and generator shafts occasionally require dimensional restoration at bearing journals, coupling hubs, or other critical interfaces.

Core Technical Content

Large 40Cr steel shafts in thermal power equipment typically have diameters ranging from 200 mm to over 500 mm. These shafts are subjected to:

High rotational speeds (3000 rpm for 50 Hz systems)
Significant bending and torsional stresses
Thermal cycling in turbine applications
Fatigue loading from cyclic operating conditions

The key challenge identified in this publication is the management of heat input during overlay welding on thick-section 40Cr shafts. 40Cr is a quenched and tempered medium-carbon alloy steel with a carbon equivalent (CE) that makes it susceptible to cold cracking when subjected to excessive welding heat input. The thick cross-section further exacerbates this issue due to high constraint and slow cooling rates from the bulk material.

Parameter	Typical Value	Rationale
Shaft material	40Cr, quenched and tempered	Base material specification
Base hardness	22-28 HRC	Typical tempered condition
Shaft diameter	200-500 mm	Large section
Preheat temperature	200-350°C	Reduces cooling rate and cracking risk
Interpass temperature	200-350°C	Maintains thermal continuity
Heat input	0.2-0.6 kJ/mm	Low heat input to control microstructure
Welding process	GTAW or low-current SMAW	Fine control of heat input
Weld deposit hardness	22-30 HRC	Match base metal condition
Post-weld heat treatment	Tempering at 550-620°C	Stress relief and microstructural uniformity

Technical Analysis of Low Heat Input Approach

The concept of low heat input welding for thick-section repair is fundamentally about controlling the thermal cycle to minimize adverse metallurgical transformations in the heat-affected zone. For 40Cr steel, the primary concerns are:

Martensite formation in the HAZ: Excessive heat input followed by rapid cooling (paradoxically possible in thick sections due to high thermal mass) can produce hard, brittle martensite in the HAZ, increasing susceptibility to cracking.
Softening of the base metal: Conversely, very high heat input with slow cooling can cause tempering softening of the base metal, reducing the strength and hardness in the region adjacent to the weld.
Residual stress accumulation: Thick sections develop high residual stresses from differential thermal contraction, which combined with any hard microstructural features, create conditions favorable for hydrogen-assisted cracking.

The low heat input approach addresses these concerns by limiting the thermal cycle severity. Using GTAW with consumable electrodes (such as ER50CrMo or ER55CrMo) at currents of 100-200 A, the heat input can be maintained below 0.5 kJ/mm while still achieving adequate penetration for each pass.

Welding Sequence and Thermal Management

For large-diameter shafts, the welding sequence is critical to managing distortion and residual stress:

Symmetric multi-pass approach: Welds are deposited in symmetric pairs around the shaft circumference to balance thermal distortion.
Progressive build-up: Multiple thin passes (1-2 mm each) are used to build the required overlay thickness, with each pass acting as a preheat for the subsequent pass.
Peening or tacking: In some cases, light peening of the weld surface during welding can introduce compressive residual stresses that offset tensile stresses.
Post-weld stress relief: Furnace stress relief at 550-620°C for a duration proportional to the shaft diameter (typically 1 hour per 25 mm of diameter) is essential.

Quality Verification and Defect Prevention

The quality assurance program for such repairs includes:

Hardness mapping: Cross-sectional Vickers hardness measurements from the shaft center through the HAZ and into the overlay to verify no unexpected hardening or softening.
Magnetic particle inspection: Full coverage MT of the repaired area to detect surface and near-surface cracks.
Ultrasonic testing: For subsurface defect detection in the HAZ and overlay region, particularly important for thick sections where surface methods have limited penetration.
Dimensional verification: Precision measurement of the restored bearing seat diameter, roundness, and taper after final machining.

Common defects and their countermeasures include:

Defect	Cause	Countermeasure
Cold cracking in HAZ	Excessive heat input, high restraint	Reduce heat input, increase preheat, use low-hydrogen consumable
Overlay cracking	High carbon content in deposit	Use Ni-base consumable with lower carbon, control cooling rate
Excessive dilution	Large electrode diameter, high current	Use smaller consumable, reduce current, increase travel speed
Surface porosity	Contaminated surface, inadequate shielding	Thorough surface preparation, ensure gas shielding
Residual stress cracking	Inadequate stress relief	Proper PWHT, consider in-situ stress relief

Study Insights

This publication is particularly valuable for its emphasis on the relationship between heat input control and metallurgical outcomes in thick-section repair. The authors' practical experience at a thermal power plant provides authentic insight into the real-world constraints of equipment repair—limited access, tight schedules, and the need for reliable, repeatable repair procedures. The low heat input philosophy advocated here has since been validated and refined in modern welding engineering, where computational thermal modeling can predict HAZ microstructure transformations with increasing accuracy. However, the empirical wisdom of limiting heat input for crack-sensitive materials remains a fundamental principle in repair welding practice.