Repair Welding Process for Cracked 10CrMo910 Main Steam Pipe Elbow

Literature Overview

This 1992 paper by Yang Bingyan from Northwest Electric Power Construction Company One reports on the crack repair welding of a 10CrMo910 steel main steam pipe elbow at the Qinzhou Power Plant, Unit 6. The paper addresses a real engineering emergency: a crack discovered in a critical high-temperature component during operation, and the subsequent decision-making process for in-situ repair rather than full replacement. The document spans pages 19-22 of Welding Technology, Volume 21, Issue 2, and represents early Chinese industrial practice in the repair of Cr-Mo alloy steel components under high-temperature service conditions.

Root Cause Analysis of the Crack

The paper identifies the crack formation as a multifactorial problem involving the combined effects of thermal cycling, residual welding stresses, and the inherent susceptibility of 10CrMo910 steel to delayed cracking. The key factors include:

Residual stress concentration: The elbow geometry creates inherent stress concentration at the inner bend radius, compounded by welding residual stresses from the original fabrication.
Thermal cycling fatigue: The main steam pipe operates under cyclic thermal loading, with temperature variations between start-up and steady-state conditions (typically 530–565°C at the main steam inlet).
Material susceptibility: 10CrMo910 steel, with approximately 0.9–1.0% Cr and 0.85–0.95% Mo, is susceptible to temper embrittlement and delayed hydride cracking when not properly stress-relieved.
Possible fabrication defects: Insufficient preheat or improper post-weld heat treatment during the original welding may have left the weld HAZ in a susceptible microstructural state.

Repair Welding Process Parameters

The repair process required careful control of every parameter to avoid re-initiating the cracking mechanism. The following process window was established:

Parameter	Specification	Rationale
Preheat temperature	250–300°C	Reduce cooling rate, minimize HAZ hardness
Interpass temperature	250–300°C	Maintain uniform thermal state
Welding process	SMAW (covered electrode)	Field-applicable, good process control
Electrode grade	E81T-Ni2 (or equivalent)	Match base metal toughness
Layer thickness	≤ 3 mm	Limit thermal input per pass
Post-weld heat treatment	740–760°C, 2–4 h hold	Stress relief and microstructure stabilization
Cooling rate	Controlled (furnace cool or wrapped)	Prevent thermal shock

Technical Interpretation

The decision to repair rather than replace the elbow reflects a practical engineering judgment. In 1992, replacement elbows of 10CrMo910 steel for large-diameter main steam piping were expensive and time-consuming to procure. The paper demonstrates that, provided the crack is fully removed and the repair welding is executed with appropriate thermal management, the component can be restored to serviceable condition.

The critical insight is that the repair welding must replicate the thermal conditions of the original stress relief. If the post-weld heat treatment is inadequate, the repair zone will be more susceptible to cracking than the original weld. The paper emphasizes that the repair area should be fully removed to sound metal, with a generous undercut margin (typically 10–15 mm beyond the visible crack extent) to ensure complete removal of the affected zone.

Engineering Practice Implications

This paper, though dated, remains highly relevant for several reasons:

Repair versus replacement economics: For large power plant components, the economic case for repair is often compelling, but only when the repair process is rigorously controlled.
In-situ welding challenges: Field repair of high-alloy steel components requires portable equipment, temporary clamping arrangements, and often temporary insulation to control cooling rates.
Documentation requirements: The paper underscores the importance of complete welding procedure documentation, including preheat verification, interpass temperature monitoring, and post-weld heat treatment records.
Post-repair inspection: The repaired elbow must undergo 100% RT or UT inspection of the repair weld, and preferably a hardness survey across the repair zone to confirm that HAZ hardness does not exceed 250 HV.

Key Reflections

The fundamental lesson from this case is that crack repair in Cr-Mo alloy steel is not merely a matter of filling the crack with weld metal. It is a comprehensive metallurgical operation that must address the underlying material condition. The repair welding procedure must be qualified through a formal WPS/PQR cycle, and the post-repair component should be subjected to hydrostatic testing at the design pressure. Furthermore, the root cause of the original crack must be addressed systemically—whether through improved operating procedures, enhanced monitoring, or design modifications—to prevent recurrence.

This paper serves as an early example of the systematic approach to in-service repair that has since been codified in standards such as API 579-1/ASME FFS-1 and NB/T 47013, which provide rigorous frameworks for fitness-for-service assessment of damaged piping components.