Crack Repair Welding Process for 10CrMo910 Main Steam Pipe Elbow

Literature Overview

The paper by Yang Bingyan (1992), published in Welding Technology, Volume 21, Issue 2, addresses a critical field-level problem encountered at the Qinling Power Plant Unit No. 6. The subject is the repair welding of cracks found in 10CrMo910 steel main steam pipe elbows. This alloy, equivalent to ASTM A335 P91, is a 9Cr-1Mo martensitic steel widely used in ultra-high-pressure boiler and power plant applications. The paper analyzes the root cause of the cracking, evaluates the feasibility of repair welding for reuse, and details the process measures adopted. The study concludes that crack repair welding of the elbow is technically and economically viable.

Analysis of Crack Formation

The 10CrMo910 steel main steam pipe operates under severe conditions of high temperature, high pressure, and cyclic thermal loading. The elbow section is particularly vulnerable due to the geometric stress concentration at the bend, where the inner fiber experiences compressive stress during bending and the outer fiber experiences tensile stress. The paper identifies several contributing factors to the crack initiation and propagation:

Cyclic thermal fatigue — repeated heating and cooling during plant start-up and shutdown cycles generates thermal stresses that accumulate over time.
Stress concentration at the bend — the curvature of the elbow amplifies local stresses, especially in the heat-affected zone (HAZ) of the original fabrication weld.
Residual stresses from fabrication — unrelieved residual stresses from the original welding and forming processes act as a driving force for crack nucleation.
Microstructural degradation — prolonged exposure to high-temperature service can lead to temper embrittlement, grain growth, and carbide coarsening in the 9Cr-1Mo matrix.

The crack typically initiates at the outer surface of the bend where tensile stress is maximum, and propagates circumferentially or intergranularly depending on the microstructural condition.

Repair Welding Process Design

The repair welding process required careful consideration of several factors to ensure the integrity and long-term serviceability of the repaired elbow. The key process parameters and measures are summarized below.

Process Parameter	Specification	Rationale
Base material	10CrMo910 (9Cr-1Mo)	Matched to parent metal composition
Filler metal	ER910 / E910-T1	Similar Cr-Mo content for metallurgical compatibility
Preheating temperature	250–300 °C	Reduce cooling rate, prevent martensite formation
Interpass temperature	≤300 °C	Control hydrogen diffusion and avoid excessive grain growth
Post-weld heat treatment (PWHT)	760–790 °C, hold 2 h	Temper embrittlement relief, residual stress reduction
Welding process	SMAW with low-hydrogen electrodes	Field applicability, good penetration control
Layering sequence	Multi-pass, alternating direction	Minimize distortion and residual stress

Key Technical Measures

Crack removal: The original crack was completely removed using a combination of grinding and machining. The repair groove was machined into a U-shaped or J-shaped profile to ensure adequate root penetration and avoid stress concentration at the groove root. The groove angle and depth were carefully designed to provide sufficient weld metal volume while minimizing the heat input per pass.
Preheating: Preheating to 250–300 °C was essential to slow the cooling rate below the critical rate for martensite formation in the 9Cr-1Mo alloy. This prevents the formation of untempered martensite in the weld metal and HAZ, which would be brittle and susceptible to delayed cracking. The preheat temperature was maintained throughout the welding operation.
Interpass temperature control: The interpass temperature was strictly limited to below 300 °C to prevent excessive grain growth in the previous weld passes and to control hydrogen diffusion. Excessive interpass temperatures can lead to coarse-grained HAZ and reduced toughness.
Post-weld heat treatment: After the repair welding was completed, the elbow was subjected to a full PWHT cycle at 760–790 °C with a holding time of approximately 2 hours. This tempering treatment serves multiple purposes: it tempers any untempered martensite formed during welding, relieves residual stresses, and stabilizes the microstructure for long-term service. The heating and cooling rates during PWHT were controlled to avoid thermal shock.
Non-destructive testing: After repair welding and PWHT, the repaired area was subjected to radiographic testing (RT) and magnetic particle testing (MT) to verify the absence of residual defects. The acceptance criteria followed the applicable standards for power plant piping.

Engineering Practice Insights

This case study from 1992 is particularly valuable because it demonstrates a practical approach to a problem that remains relevant today. The decision to repair rather than replace the elbow was driven by economic considerations — the cost of a new forged elbow, complete with PWHT, and the associated downtime for replacement, would be significantly higher than the repair cost. However, the technical feasibility of the repair depends critically on the extent of the damage and the remaining life of the component.

The key insight from this work is that crack repair welding of 9Cr-1Mo components is feasible provided that the metallurgical compatibility is maintained and the heat input is carefully controlled. The risk of temper embrittlement must be managed through proper PWHT parameters. Additionally, the repair should be followed by enhanced monitoring to detect any re-initiation of cracking at the repair site.

A critical consideration that the paper does not fully address is the long-term durability of the repair. The HAZ of the repair weld may have a different microstructure from the original HAZ, potentially creating a new site for future crack initiation. Engineers should consider whether a local reinforcement or a full replacement would provide a more reliable long-term solution, particularly for critical main steam piping.

Summary and Recommendations

The paper by Yang Bingyan provides a well-documented case study of crack repair welding for 10CrMo910 main steam pipe elbows. The process design — including preheating at 250–300 °C, interpass temperature control below 300 °C, and PWHT at 760–790 °C — represents a sound engineering approach that aligns with modern welding practice for 9Cr-1Mo alloys. The study demonstrates that repair welding can be a technically and economically viable alternative to full component replacement, provided that the repair is performed with rigorous process control and verified by appropriate NDT. For contemporary practice, engineers should supplement the repair with enhanced in-service monitoring and consider the cumulative effect of thermal cycling on the repair HAZ over the remaining service life.