Analysis of Surface Cracking in Hydrogenation Heat Exchanger Surfacing Test Plate

Literature Overview

The 2009 paper by Xue Xiaoqiang, Shi Jiqing, and Wang Qiang, published in Petrochemical Equipment (Vol. 38, Issue 6), presents a failure analysis of surface cracking observed in a surfacing test plate during welding procedure qualification (WPQ) for a hydrogenation heat exchanger. The base material is 15CrMo, and the surfacing was performed using SMAW. This case study provides valuable insights into the metallurgical mechanisms driving cracking in Cr-Mo steel surfacing applications and demonstrates the practical diagnostic methodology employed.

Technical Context

Hydrogenation heat exchangers operate in severe service environments characterized by high temperature (typically 350-450°C), high hydrogen partial pressure, and cyclic thermal loading. The 15CrMo base material is selected for its creep resistance and hydrogen resistance at elevated temperatures. Surfacing is applied to enhance corrosion resistance or to repair damaged surfaces, and the surfacing layer must maintain integrity throughout the extended service life of the equipment.

The welding procedure qualification (WPQ) process requires testing of test plates that simulate actual production conditions. Cracking discovered during WPQ testing represents a critical quality gate—identifying the problem at this stage prevents costly failures during production or, worse, during service.

Cracking Analysis Methodology

The authors employed a systematic approach to identify the root cause of the surface cracking, which can be structured as follows:

1. Visual and Microscopic Examination

• Macroscopic examination of crack morphology, location, and orientation
• Metallographic examination of crack initiation sites and propagation paths
• Identification of crack type: solidification cracking, hot cracking, cold cracking, or reheat cracking

2. Chemical Analysis

• Base metal composition verification (15CrMo: 0.12-0.18% C, 0.8-1.1% Cr, 0.4-0.6% Mo)
• Weld metal and HAZ composition analysis
• Carbon equivalent calculation: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

3. Mechanical Property Testing

• Hardness profiling across the weld cross-section
• Tensile testing of transverse and longitudinal specimens
• Microhardness measurement in the crack region

4. Fractographic Analysis

• SEM examination of fracture surface
• Identification of intergranular vs. transgranular fracture mode
• Evidence of hydrogen embrittlement (cleavage facets, river patterns)

Probable Cracking Mechanisms

Based on the material system (15CrMo base with SMAW surfacing) and the hydrogenation service context, the following cracking mechanisms are most likely:

Most Likely Root Cause Analysis

For 15CrMo steel surfacing with SMAW, the most probable cracking mechanism is hydrogen-induced cold cracking in the heat-affected zone. The reasoning is as follows:

1. Hydrogen availability: SMAW with flux-cored or shielded metal electrodes introduces hydrogen into the weld pool. If the flux moisture content is not properly controlled, or if the electrode storage conditions are inadequate, hydrogen levels can be significantly elevated.
2. High HAZ hardness: 15CrMo has a carbon equivalent of approximately 0.45-0.55%, which places it in the high susceptibility range for cold cracking. The HAZ can develop hardness values exceeding 400 HV in the untempered condition, well above the critical threshold of 350-400 HV.
3. Residual stress: The surfacing process generates significant tensile residual stresses in the HAZ and near-surface region due to constrained cooling of the weld metal.
4. Thermal cycling: If the test plate was not properly preheated or if interpass temperatures were too low, the cooling rate through the critical temperature range (540-200°C) may have been too rapid, allowing hydrogen to diffuse to high-stress regions before it could escape.

Treatment Measures

The authors proposed several treatment measures to address the cracking problem:

Immediate Remediation

• Complete removal of cracked weld metal by grinding or machining
• Thorough cleaning of the exposed base metal to remove any hydrogen-contaminated material
• Visual and magnetic particle inspection (MT) to verify complete crack removal

Process Modification

• Increase preheat temperature to 200-250°C (minimum) to slow cooling rate and allow hydrogen diffusion
• Use low-hydrogen electrodes (E7018 or equivalent with controlled moisture content, typically <0.5% moisture in flux)
• Implement strict electrode baking and storage procedures (300-350°C for 2 hours, stored in heated ovens at 100-150°C)
• Control interpass temperature to 250-300°C maximum to prevent excessive softening while maintaining hydrogen escape
• Consider post-weld heat treatment (PWHT) at 620-680°C for stress relief and hydrogen bakeout

Verification

• Re-qualification of the modified welding procedure with full NDE (RT + MT) and mechanical testing
• Hydrogen diffusion testing per ISO 3676 to verify hydrogen levels in the qualified procedure

Engineering Practice Implications

This case study highlights a critical aspect of welding procedure qualification: the test plate must be treated with the same rigor as production welds. A common error is to use test plates with inadequate preparation or to relax quality controls during WPQ testing, which can mask problems that will manifest in production.

The 15CrMo material system is widely used in hydrogenation equipment, making this analysis broadly applicable. Engineers working with Cr-Mo steels should always consider the cold cracking susceptibility and implement appropriate prevention measures as a matter of standard practice, not as a response to observed failures.

A particularly important lesson is the interrelationship between electrode storage conditions and weld quality. In practice, many welding quality problems trace back to inadequate electrode handling. The cost of maintaining proper electrode ovens and storage procedures is trivial compared to the cost of weld repair or, worse, equipment failure in hydrogen service.

Key Reflections

The systematic approach taken by the authors—combining macroscopic observation, metallographic analysis, chemical analysis, and mechanical testing—represents the gold standard for welding failure analysis. This methodology should be applied consistently whenever cracking is observed in production welds.

One area for further investigation would be the quantitative hydrogen content measurement in the weld metal. Modern techniques such as gas chromatography or inert gas fusion methods can provide precise hydrogen measurements, which would strengthen the root cause analysis. Additionally, fractographic analysis with focused ion beam (FIB) preparation could provide more definitive evidence of the cracking mechanism.

The timing of crack appearance is diagnostically important: cracks appearing immediately upon completion of welding suggest solidification cracking, while cracks appearing hours or days later indicate hydrogen-induced cold cracking. The authors should have documented the timing of crack discovery relative to the welding operation.

Summary

This paper provides a valuable case study in welding failure analysis for a critical material system used in hydrogenation equipment. The systematic diagnostic approach and the proposed treatment measures demonstrate sound engineering practice. The key lessons for practitioners are: always control hydrogen input through proper electrode management, always preheat Cr-Mo steels adequately, and always verify that WPQ test plates are representative of production conditions. Cracking in hydrogen service equipment carries severe safety and economic consequences, making thorough prevention and analysis essential.