Cause Analysis and Treatment of Surface Cracks on Hydrogenation Heat Exchanger Overlay Test Plates

Literature Overview

This 2009 publication by Xue Xiaoqiang, Shi Jiqing, and Wang Qiang of Gansu Lanke Petrochemical High-Tech Equipment Co., Ltd. provides a detailed analysis of surface cracking observed on overlay welded test plates for hydrogenation heat exchangers. The work represents a valuable case study in defect analysis and remediation for critical pressure equipment, offering insights into the metallurgical and process factors that contribute to cracking in overlay welds.

Core Technical Content

Component Description

Hydrogenation heat exchangers are critical components in petrochemical and refinery processes, operating under high pressure (15–35 MPa) and temperature (300–450 °C) in the presence of hydrogen and hydrogen sulfide. The tube sheets and channel covers require overlay welding with nickel-based alloys (typically Inconel 625 or Hastelloy C276) to provide corrosion resistance.

Crack Characteristics

The observed cracks exhibited the following characteristics:

Location: Primarily in the top 1–2 mm of the overlay layer, near the surface.
Orientation: Random orientation, not aligned with welding direction.
Length: 5–30 mm, with some branching observed.
Depth: 0.5–2 mm, not penetrating through the overlay layer.
Morphology: Intergranular and transgranular features observed under metallographic examination.

Root Cause Analysis

Using a systematic approach (5W2H method and FMEA), the following potential causes were identified:

Factor	Evidence	Contribution
High residual stress	High stress levels measured by XRD	Major contributor
Hydrogen embrittlement	Hydrogen concentration measured in weld metal	Significant contributor
Microstructural instability	M-phase precipitation observed	Moderate contributor
Excessive heat input	Heat input above recommended range	Contributing factor
Material impurities	Sulfur and phosphorus above specification	Minor contributor

Detailed Analysis

Residual Stress

Residual stresses in overlay welds are primarily caused by:

Thermal contraction during cooling.
Phase transformations in the weld metal.
Constrained deformation due to the thick base material.

Typical residual stress levels in overlay welds can reach 300–500 MPa, approaching the yield strength of the overlay material. This creates conditions favorable for stress corrosion cracking and hydrogen-induced cracking.

Hydrogen Embrittlement

Hydrogen can enter the weld metal from:

Moisture in flux or shielding gas.
Contaminated base metal surface.
Decomposition of hydrogen-containing compounds in the arc atmosphere.

Hydrogen accumulation at grain boundaries and microstructural defects can cause delayed cracking, particularly in high-strength materials.

Microstructural Factors

Inconel 625 overlay welds can develop deleterious phases:

M-phase (Ni3Nb): Precipitates during slow cooling or post-weld heat treatment, reducing ductility.
δ-ferrite: Can form in weld metals with inadequate Nb content, affecting mechanical properties.
Laves phase: May form in weld metals with high Cr and Mo content, reducing toughness.

Treatment and Remediation

Immediate Remediation

For the specific case described:

Crack removal: Grind out all visible cracks to a smooth contour, extending 5–10 mm beyond the crack termination points.
Surface preparation: Clean the area thoroughly, removing all grinding debris and contamination.
Re-welding: Rebuild the overlay layer using qualified welding parameters, ensuring proper preheating and interpass temperature control.
Post-weld treatment: Apply stress relief heat treatment at 600–650 °C for 2–4 hours.
Inspection: Perform 100% non-destructive testing (RT or UT) to verify crack-free condition.

Process Optimization

To prevent recurrence, the following process improvements were recommended:

Parameter	Original	Optimized	Rationale
Preheat temperature	150 °C	250 °C	Reduce cooling rate, minimize residual stress
Interpass temperature	300 °C	250 °C	Reduce hydrogen diffusion, minimize phase precipitation
Heat input	35 kJ/cm	25–30 kJ/cm	Reduce thermal distortion, minimize phase formation
Post-weld treatment	None	600 °C for 4 hours	Relieve residual stress, stabilize microstructure
Shielding gas purity	99.5% Ar	99.99% Ar	Reduce hydrogen ingress

Quality Assurance Enhancements

Pre-weld material verification: Ensure base and overlay materials meet specification.
Weld procedure qualification: Qualify procedures with mechanical testing and hydrogen cracking tests.
In-process monitoring: Monitor welding parameters, interpass temperature, and gas purity.
Post-weld inspection: Perform 100% NDT plus hardness survey and metallographic examination.
Hydrogen testing: Conduct hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) tests per NACE MR0175.

Standards Compliance

The fabrication must comply with:

GB/T 150: Pressure vessel design and fabrication.
NB/T 47014: Welding procedure qualification.
API 934: Overlay welding of pressure vessels.
ASME VIII Div.1: If applicable.
NACE MR0175: Materials for H2S-containing environments.
JB/T 4730: Non-destructive testing.

Reflections and Implications

This case study illustrates the importance of systematic defect analysis in pressure vessel fabrication. The cracking observed on the hydrogenation heat exchanger test plates was not caused by a single factor but by the interaction of multiple contributing factors — residual stress, hydrogen embrittlement, microstructural instability, and process parameters.

A key insight is the value of post-weld heat treatment in overlay welding applications. While stress relief heat treatment is often considered optional for overlay welds, this case demonstrates its critical importance for preventing cracking in high-stress applications. The heat treatment relieves residual stresses, stabilizes the microstructure, and reduces hydrogen concentration.

The work also highlights the importance of process qualification and optimization. Standard welding procedures may be adequate for general applications but require careful optimization for critical components operating in harsh environments. The systematic approach to defect analysis — combining metallurgical examination, stress measurement, and process review — provides a robust framework for identifying root causes and implementing effective countermeasures.

This case study serves as a valuable reference for engineers involved in the fabrication of hydrogenation equipment, emphasizing the need for comprehensive quality assurance and process optimization to ensure long-term reliability in demanding service conditions.