Root Cause Analysis and Improvement of Cracking in Stellite Alloy Cladding Layer of Domestic Steam-Driven Pump Balance Disc

Literature Overview

This case study presents a detailed root cause analysis of cracking in the Stellite alloy weld overlay cladding layer of a domestic steam-driven pump balance disc, along with the engineering improvements implemented to resolve the issue. Balance discs in steam-driven pumps are critical components that maintain axial balance of the rotor assembly and are subjected to extreme operating conditions including high temperatures, high pressures, and cyclic thermal and mechanical loading. Stellite alloy (typically Stellite 6, a cobalt-chromium-tungsten alloy) is commonly selected for the balance disc cladding due to its excellent wear resistance, high-temperature strength, and corrosion resistance. However, cracking in the Stellite cladding layer can lead to catastrophic pump failure, making root cause analysis and preventive improvement essential.

Failure Investigation and Root Cause Identification

The investigation followed a systematic approach combining macroscopic examination, metallographic analysis, fracture surface analysis (SEM), and process review. The following root causes were identified:

Primary root cause: Excessive residual stress combined with hydrogen embrittlement.

The cracking pattern observed on the balance disc cladding layer was characterized by:

Transverse cracks perpendicular to the cladding direction
Cracks originating at the cladding boundaries and propagating inward
Intergranular and transgranular mixed cracking morphology

The metallographic analysis revealed:

A coarse-grained microstructure in the HAZ adjacent to the cladding layer, indicating excessive heat input during welding
Chromium carbide precipitation at grain boundaries in the HAZ, leading to localized embrittlement
Porosity and lack of fusion at the cladding/substrate interface in certain regions

Investigation Method	Finding	Significance
Macroscopic examination	Transverse cracks at cladding boundaries	Indicates tensile residual stress concentration
Metallography (optical)	Coarse HAZ grains, carbide precipitation	Indicates excessive heat input and sensitization
SEM fracture surface	Mixed intergranular/transgranular morphology	Indicates combined stress and embrittlement mechanisms
Hardness mapping	Hardness variation from 450 HV (cladding) to 300 HV (HAZ) to 200 HV (base)	Indicates microstructural heterogeneity
Process review	Interpass temperature exceeded 250°C	Indicates excessive cumulative heat input
Material certification	Base metal carbon content at upper limit	Indicates susceptibility to HAZ embrittlement

Secondary contributing factors:

The base metal (a low-alloy steel) had a carbon equivalent (CE) at the upper limit of the specified range, increasing its susceptibility to HAZ cracking.
The welding procedure did not specify interpass temperature limits, leading to excessive heat input accumulation during multi-pass cladding.
The preheating temperature was insufficient for the carbon equivalent of the base metal, leading to rapid cooling and martensitic transformation in the HAZ.
No post-weld stress relief was performed, leaving high residual stresses in the component.

Improvement Measures and Verification

Based on the root cause analysis, the following improvement measures were implemented:

Welding procedure revision:

Interpass temperature limited to 150°C maximum
Preheating temperature increased to 200°C (based on CE calculation)
Heat input reduced by 30% through increased travel speed and reduced arc current
Post-weld stress relief specified at 580°C for 2 hours (within the tempering range for the HAZ)

Material specification revision:

Base metal carbon content specified at the lower half of the acceptable range (≤ 0.15% C)
Stellite filler material specified with controlled sulfur content (≤ 0.02% S) to minimize hot cracking susceptibility

Welding sequence optimization:

Multi-pass cladding with alternating travel directions to distribute residual stress
Back-step welding employed to reduce directional stress concentration
Each pass thickness limited to 1.5–2.0 mm to control individual pass heat input

Enhanced inspection protocol:

Magnetic particle testing (MT) of all cladding surfaces and boundaries
Ultrasonic testing (UT) of the cladding/substrate interface for lack of fusion
Hardness profiling across the cladding thickness and HAZ
Metallographic examination of representative sections for microstructural verification

Verification Results

After implementing the improvement measures, a new batch of balance discs was manufactured and tested. The verification results confirmed the effectiveness of the improvements:

Parameter	Before Improvement	After Improvement
Cladding hardness (HV)	450–500	420–480
HAZ hardness (HV)	300–350	250–300
Residual stress (MPa)	400–500	150–250
MT indication	Cracks detected	No indications
UT indication	Lack of fusion detected	No indications
Service life (months)	3–6	> 24 (ongoing)

The residual stress reduction from 400–500 MPa to 150–250 MPa, achieved through the combination of improved welding parameters and post-weld stress relief, was the most significant factor in eliminating cracking. The hardness reduction in both the cladding layer and the HAZ, while modest, contributed to improved toughness and crack resistance.

Study Insights and Engineering Lessons

This case study provides several valuable lessons for engineers working with Stellite alloy cladding in high-temperature, high-pressure applications:

The HAZ is often the weakest link in cladded components. While the Stellite cladding layer itself may have excellent mechanical properties, the adjacent HAZ can be significantly embrittled by excessive heat input, carbide precipitation, and martensitic transformation. Engineers must pay equal attention to the HAZ as to the cladding layer itself.
Process discipline is critical. The failure was not caused by an inherent limitation of the Stellite cladding technology but by deviations from optimal process parameters. Strict adherence to welding procedure specifications, including interpass temperature limits and preheating requirements, is essential for producing reliable cladding.
A systematic root cause analysis approach is indispensable. The combination of macroscopic examination, metallographic analysis, fracture surface analysis, and process review provided a comprehensive understanding of the failure mechanism and enabled the development of targeted improvement measures. Without this systematic approach, the root cause might have been misidentified, leading to ineffective or inappropriate corrective actions.
Post-weld stress relief should be considered as a standard practice for critical cladded components. The residual stress reduction achieved through stress relief was the single most effective measure in eliminating cracking. For components subjected to cyclic loading and thermal cycling, stress relief should be specified as a mandatory post-weld operation.

This case study serves as a reminder that even well-established technologies like Stellite cladding can fail when process control is inadequate. The systematic investigation and improvement approach demonstrated here provides a model for addressing similar failures in other cladded component applications across the energy, chemical, and power generation industries.