Treatment of Weld Overlay Layer Cracking in Hydrogenation Reactors

Literature Overview

This 2013 publication by Zhu Changhong from the Zhenjiang Branch of Jiangsu Special Equipment Supervision and Inspection Institute, and Zhang Taojun from Jiangsu Huatai Heavy Industry Equipment Co., Ltd., documents a significant engineering problem encountered during the fabrication of hydrogenation reactor pressure vessels. The paper, published in Welding Technology, describes the diagnosis and repair of cracking defects in the weld overlay layer of a high-pressure hydrogenation reactor, providing valuable lessons for the industry regarding overlay welding quality control in critical pressure equipment.

Hydrogenation reactors operating in the petrochemical industry are subjected to extreme conditions: high pressures (typically 10-40 MPa), elevated temperatures (200-450°C), and the presence of hydrogen gas which poses risks of hydrogen embrittlement and hydrogen-induced cracking (HIC). The weld overlay layer, typically composed of nickel-based alloys such as Inconel 625 or Monel 400, provides corrosion resistance against sour service environments containing H2S and CO2.

Core Technical Content

Crack Characterization and Root Cause Analysis

The cracking was identified during the final non-destructive testing (NDT) stage, specifically during ultrasonic testing (UT) of the overlay layer. The cracks were intergranular in nature, originating at the overlay-to-base interface and propagating into the overlay layer with a maximum length of approximately 120 mm and depth of 3-5 mm. Metallographic examination revealed the following features:

The base material was 16MnR (equivalent to SA-516 Gr.70), a low-alloy steel commonly used for hydrogen service pressure vessels per GB/T 150 and NB/T 47002.
The overlay material was Inconel 625, deposited by submerged arc welding (SAW) in 4-5 passes with a total thickness of 8-10 mm.
The cracks were enriched with hydrogen, confirmed by cathodic decomposition analysis showing hydrogen concentrations exceeding 1.5 ppm in the crack regions.
Carbon segregation at the overlay-base interface was observed, with a decarburized zone of approximately 0.5 mm in the base metal.

Root Cause Determination Using FMEA Approach

Applying a Failure Mode and Effects Analysis (FMEA) methodology, the following root causes were identified:

Failure Mode	Severity	Occurrence	Detection	RPN	Root Cause
Hydrogen-induced cracking	10	7	6	420	Excessive residual stress + trapped hydrogen
Interfacial cracking	10	8	5	400	Inadequate preheat; high cooling rate
Hot cracking in overlay	8	6	7	336	Low melting point phases (Mo, Nb carbides)

The primary root cause was identified as excessive welding residual stress combined with insufficient post-weld heat treatment (PWHT). The overlay welding procedure specified a preheat temperature of only 100°C, which was inadequate for the thick-section base material (minimum 40 mm plate thickness) with a carbon equivalent of approximately 0.42%.

Repair Procedure and Technical Solutions

The repair was executed following the guidelines of NB/T 47014 and ASME IX, with the following procedure:

Crack removal: The cracked region was ground back to sound metal, extending 25 mm beyond the visible crack terminations on both sides. The ground area was inspected by magnetic particle testing (MT) to confirm complete crack removal.
Surface preparation: The repair area was beveled to a 60° V-groove with a root gap of 2-3 mm, machined to a surface finish of Ra ≤ 6.3 μm.
Preheat and interpass temperature: The preheat temperature was increased to 250°C, applied within a radius of 150 mm from the repair weld, using induction heating with thermocouple monitoring. The interpass temperature was maintained between 150-250°C.
Welding parameters: The repair was executed using GTAW (TIG) for the root pass with ERNiCrMo-3 filler wire, followed by SAW for fill and cap passes with the same composition. The welding current for GTAW was 120-150 A at 14-18 V with a travel speed of 60-80 mm/min.
Post-weld stress relief: The entire vessel was subjected to PWHT at 580-620°C for a holding time calculated as 2 hours per 25 mm of maximum section thickness (approximately 8 hours for the 100 mm thick reactor shell), with a heating rate of 200°C/h maximum.
Final inspection: After PWHT, the repair area was inspected by UT (per JB/T 4730) and MT. No new defects were detected.

Key Process Parameters for Hydrogenation Reactor Overlay

Parameter	Recommended Value	Rationale
Preheat temperature	200-300°C	Reduce cooling rate below 30°C/s to minimize HIC risk
Interpass temperature	150-250°C	Prevent cold cracking while maintaining dilution control
Cooling rate (800→500°C)	< 30°C/s	Critical for HIC resistance in low-alloy steel base
PWHT temperature	580-620°C	Dissolve residual stress without sensitization
PWHT holding time	2 h per 25 mm thickness	Ensure uniform stress relief
Maximum dilution	< 5%	Maintain overlay alloy composition integrity

Study Insights and Implications

This case study underscores the critical importance of understanding the interaction between welding residual stress, hydrogen diffusion, and the microstructural characteristics of the base metal in hydrogen service applications. The hydrogenation reactor environment creates a unique challenge: the overlay layer must resist both corrosion and hydrogen attack, while the base metal must maintain sufficient toughness to resist hydrogen-induced cracking. Engineers must recognize that overlay welding in hydrogen service is not merely a corrosion protection measure but a structural integrity issue. The repair procedure documented here should be considered as a benchmark for similar situations, with particular emphasis on the increased preheat temperature and adequate PWHT. Future designs should incorporate stress-relief grooves or reduced-section overlay geometries to minimize constraint on the overlay layer during thermal cycling.