Analysis of Weld Overlay Spalling and Cracking in Hydrocracking Reactors

Literature Overview

This technical paper, authored by Guo Jianhua from China Petrochemical Corporation Shanghai Petrochemical Co., Ltd. Aromatics Division (2009), presents a detailed failure analysis of weld overlay spalling and cracking in a hydrocracking reactor. Hydrocracking reactors are critical pressure vessels in petroleum refining operations, where crude oil is subjected to high temperatures (350–420 °C) and high hydrogen pressures (10–20 MPa) in the presence of hydrogen and various sulfur and nitrogen compounds. The reactor shell is typically clad with or overlay-welded with a corrosion-resistant alloy (such as 321 stainless steel, Inconel 625, or a nickel-based alloy) to protect the carbon steel or low-alloy steel substrate from hydrogen attack and sulfide stress corrosion.

The failure of the weld overlay layer through spalling and cracking represents a severe safety concern, as it can lead to hydrogen ingress into the base metal, causing decarbonization and eventual vessel rupture. This paper provides valuable insights into the root causes of overlay failure and the corrective measures that can be implemented.

Core Technical Content

The failure analysis revealed multiple contributing factors to the weld overlay spalling and cracking, including welding defects, residual stress, and operational conditions. The paper provides a comprehensive investigation using non-destructive testing (NDT), metallographic examination, and fractographic analysis.

Reactor and Overlay Specifications

Parameter	Specification
Reactor material	2.25Cr-1Mo steel (SA-387 Gr. 22)
Overlay material	321 stainless steel (SA-240)
Overlay thickness	12 mm (typical)
Welding process	ESW (electroslag welding)
Overlay thickness (as-built)	12–15 mm
Overlay thickness (at failure)	8–10 mm
Operating temperature	380–400 °C
Operating pressure	15 MPa
Hydrogen partial pressure	10 MPa
Sulfur content in feed	0.5–1.0 wt%

Failure Mechanism Analysis

Failure Mode	Root Cause	Contributing Factors
Overlay spalling	Insufficient bond strength	Porosity at overlay-base interface
Overlay cracking	Residual stress + thermal fatigue	High residual tensile stress from ESW
Base metal decarbonization	Hydrogen ingress through overlay defects	Cracking and spalling of overlay
HAZ cracking	Hydrogen-induced cracking (HIC)	High carbon equivalent of 2.25Cr-1Mo steel

NDT Findings

NDT Method	Findings	Significance
UT (ultrasonic testing)	Multiple indications at overlay-base interface	Bond defects and delaminations
MT (magnetic particle testing)	Transverse cracks in overlay	Residual stress cracking
RT (radiographic testing)	Porosity and lack of fusion	Welding quality issues
TOFD	Subsurface delaminations	Spalling initiation sites

Key Technical Insights

The paper identifies several critical factors that contributed to the overlay failure, providing valuable lessons for future overlay welding operations.

First, the residual stress state in the overlay is a primary contributor to cracking. ESW overlay welding produces high residual tensile stresses in the overlay layer due to the significant heat input and the large thermal gradient between the hot weld pool and the cooler base metal. These residual stresses, combined with the thermal cycling during reactor start-up and shutdown, can exceed the yield strength of the overlay material and initiate cracking. The paper recommends implementing post-weld stress relief (PWSR) at 700–750 °C for 2.25Cr-1Mo steel, which is also beneficial for reducing residual stresses in the overlay.

Second, the bond quality between the overlay and the base metal is critical. The paper found evidence of porosity and lack of fusion at the overlay-base interface, which provided pathways for hydrogen ingress and initiated spalling. The root cause of these interface defects was identified as inadequate cleaning of the base metal surface before overlay welding and insufficient first-pass penetration. The paper recommends rigorous surface preparation (grinding to bright metal with a minimum 3 mm removal) and the use of a compatible first-pass filler material (such as a 309L stainless steel wire for 321 overlay on carbon steel) to ensure full fusion at the interface.

Third, the paper highlights the role of hydrogen in the failure process. Under operating conditions, hydrogen atoms can permeate through defects in the overlay and accumulate at the overlay-base interface and in the HAZ of the base metal. The accumulation of hydrogen leads to hydrogen-induced cracking (HIC) and decarbonization of the 2.25Cr-1Mo steel, which is a well-documented degradation mechanism in hydroprocessing environments. The paper recommends using a higher-performance overlay material (such as Inconel 625 or a nickel-based alloy) for reactors operating at high hydrogen partial pressures, as these materials have lower hydrogen permeability than austenitic stainless steels.

Integration with Engineering Practice

The failure analysis presented in this paper has direct implications for the design, fabrication, and inspection of hydrocracking reactors. Several corrective measures are recommended:

Welding procedure optimization: Reduce heat input in ESW overlay welding by using thinner electrode diameters and lower travel speeds. Alternatively, consider using SAW or PTA (plasma transferred arc) overlay welding, which offer better control over heat input and dilution.
Post-weld stress relief: Implement PWSR at the appropriate temperature for the base metal (700–750 °C for 2.25Cr-1Mo steel) to reduce residual stresses in both the overlay and the base metal HAZ. The PWSR temperature must be carefully controlled to avoid sensitization of the 321 stainless steel overlay (avoid the 450–850 °C range for extended periods).
Enhanced NDT coverage: Implement comprehensive NDT of the overlay-base interface using phased array ultrasonic testing (PAUT) or TOFD, which are more sensitive to planar defects than conventional UT. The inspection should be performed both in the as-welded condition and after PWSR.
Operational monitoring: Implement periodic inspection of the reactor overlay during shutdowns, including UT scanning, eddy current testing, and visual examination. Any signs of overlay degradation should trigger immediate investigation and repair.
Material upgrade: For reactors operating at high hydrogen partial pressures (>10 MPa) and high temperatures (>400 °C), consider upgrading the overlay material to a nickel-based alloy (such as Inconel 625 or Hastelloy C-276), which offers superior resistance to hydrogen permeation and sulfide stress corrosion.

Study Reflections

This failure analysis paper provides a compelling case study in the importance of overlay welding quality for pressure vessel integrity. The systematic investigation using multiple NDT methods and metallographic analysis exemplifies best practices in failure analysis. The identification of multiple contributing factors (welding defects, residual stress, hydrogen damage) highlights the complexity of overlay failure and the need for a holistic approach to prevention.

The paper underscores the critical importance of the overlay-base interface quality. In hydroprocessing environments, even small defects at this interface can initiate hydrogen ingress and lead to catastrophic failure. The recommendations for improved surface preparation, welding procedure optimization, and enhanced NDT coverage are directly applicable to current and future reactor fabrication projects.

A notable gap in the analysis is the limited discussion of the role of fabrication history and prior service exposure. The paper would benefit from a more detailed examination of the reactor's fabrication records, including welding procedure qualifications, welder performance records, and heat treatment documentation. Additionally, the paper does not extensively address the potential role of mechanical damage (such as impact loading during installation or maintenance) in initiating overlay spalling. Future investigations should consider these factors in the root cause analysis.