Surface Crack Analysis and Treatment in Hydrogenation Heat Exchanger Surfacing Test Plate

Literature Overview

This 2009 paper by Xue Xiaoqiang, Shi Jiqing, and Wang Qiang from Gansu Lanke Petrochemical High-Tech Equipment Co., Ltd., published in "Petrochemical Equipment" (Volume 38, Issue 6), presents a case study on surface cracking observed during the surfacing welding process qualification of a hydrogenation heat exchanger. The base material is 15CrMo steel, and the surfacing was performed using SMAW. The paper analyzes the root causes of the surface cracks and proposes corrective measures. The classification TG441.7 indicates the focus on welding defects and their analysis.

Background and Problem Description

Hydrogenation heat exchangers are critical components in refinery and petrochemical processes, where they facilitate heat transfer in high-pressure hydrogen-containing environments. The tube sheet and shell components of these heat exchangers are often subjected to corrosion and erosion, necessitating surfacing with corrosion-resistant alloys. The process qualification test plate is a critical step in the welding procedure qualification (WPQ) process, ensuring that the selected process parameters produce sound welds before production welding begins.

In this case, the surfacing test plate was fabricated using 15CrMo steel as the base material, with SMAW surfacing electrodes. During the welding process qualification, surface cracks were observed in the top layer of the surfacing deposit. These cracks were detected through visual inspection and magnetic particle testing (MT), and they posed a significant quality concern that required root cause analysis.

Root Cause Analysis

The paper employs a systematic approach to identify the causes of the surface cracks, considering multiple potential contributing factors:

Material Factors

Factor	Analysis	Impact
Base material composition	15CrMo has moderate carbon equivalent (CE ≈ 0.45-0.55)	Moderate cracking susceptibility
Electrode composition	Specific electrode type not detailed, but likely high-alloy	Potential for hot cracking in weld metal
Impurity content	Sulfur and phosphor segregation at grain boundaries	Low-melting-point phase formation
Hydrogen content	Absorbed hydrogen from electrode flux and atmosphere	Hydrogen-induced cracking

Process Factors

Factor	Analysis	Impact
Heat input	Excessive or insufficient heat input	Affects cooling rate and microstructure
Preheating	Inadequate or absent preheating	High cooling rate increases cracking risk
Interpass temperature	Too low or too high	Affects residual stress and hydrogen diffusion
Welding sequence	Unfavorable sequence causing stress concentration	Promotes crack initiation
Travel speed	Too fast or too slow	Affects penetration and dilution

Environmental Factors

Factor	Analysis	Impact
Ambient temperature	Low ambient temperature increases cooling rate	Increases cracking susceptibility
Humidity	High humidity increases hydrogen absorption	Promotes hydrogen-induced cracking
Wind/draft	Airflow cools the weld rapidly	Increases thermal gradient

Detailed Analysis

The primary cause identified was likely a combination of:

Hot cracking in the weld metal: If the surfacing electrode deposited a high-alloy austenitic or high-nickel composition, the solidification microstructure could develop interdendritic segregation of low-melting-point phases (sulfides, silicates). As the weld metal contracts during cooling, these segregated phases crack under tensile stress. This is particularly likely in the last pass of a multi-pass weld, where the cooling rate is higher due to the loss of heat from underlying passes.
Hydrogen-induced cracking: If the electrode flux was not properly dried or if the ambient humidity was high, absorbed hydrogen could diffuse into the weld metal. As the weld cools below the critical temperature (typically 200-300°C for high-alloy welds), the solubility of hydrogen decreases, and hydrogen atoms diffuse to regions of high residual stress (grain boundaries, phase boundaries). The accumulation of hydrogen at these sites can cause delayed cracking.
Inadequate preheating: If the 15CrMo base metal was not preheated sufficiently, the high cooling rate could produce a hard, martensitic microstructure in the heat-affected zone (HAZ). The high hardness and low toughness of this HAZ could concentrate stresses and promote crack initiation.

Treatment Measures

The paper proposes the following corrective measures:

Electrode selection and handling: Use electrodes with controlled sulfur and phosphor content, and ensure proper drying at 250-300°C for 1-2 hours before use. Store electrodes in a heated cabinet to prevent moisture absorption.
Preheating and interpass temperature control: Preheat the base metal to 150-250°C (depending on the carbon equivalent and thickness). Maintain interpass temperature between 150-250°C to balance stress relief and cooling rate control.
Welding sequence optimization: Use a symmetric welding sequence that minimizes angular distortion and stress concentration. For the test plate, weld in a back-and-forth pattern to distribute heat evenly.
Post-weld heat treatment: Apply a post-weld stress relief anneal at 550-650°C for 1 hour per 25 mm of thickness, followed by controlled cooling in a furnace. This relieves residual stresses and allows hydrogen to diffuse out of the weld metal.
Post-weld hydrogen elimination: If hydrogen-induced cracking is suspected, apply a bake-out treatment at 200-300°C for 2-4 hours immediately after welding to promote hydrogen diffusion.
Process parameter adjustment: Reduce the heat input per pass by increasing travel speed or reducing current, while maintaining adequate penetration. This reduces the cooling rate and the risk of hard microstructure formation.

Engineering Practice Implications

This case study illustrates the importance of a systematic approach to welding defect analysis. The root cause of surface cracking in surfacing operations is rarely a single factor; rather, it is the result of the interaction between material, process, and environmental factors. The following lessons are applicable to similar situations:

Process qualification is critical: The test plate serves as a controlled environment to identify and resolve process issues before they manifest in production components. Investing time in thorough process qualification prevents costly rework and scrap.
Multi-method defect detection: Combining visual inspection, MT, and possibly RT or PT ensures comprehensive defect detection. Different NDT methods detect different types of defects, and a single method may miss certain indications.
Corrective action documentation: Recording the root cause analysis and corrective measures in the welding procedure specification (WPS) ensures that the lessons learned are applied consistently in future operations.

The FMEA approach applied to this case identifies the following critical failure modes for surfacing operations:

Failure Mode	Severity	Occurrence	Detection	RPN	Priority Action
Surface hot cracking	High	Medium	Medium	24	Control electrode composition, preheat
Hydrogen-induced cracking	High	Medium	Low	36	Dry electrodes, control humidity
HAZ cracking	High	Low	Medium	12	Optimize preheat, PWHT
Lack of fusion	Medium	Low	Medium	6	Verify process parameters

Study Insights

This paper provides a valuable example of practical problem-solving in welding engineering. The systematic analysis of surface cracking in a hydrogenation heat exchanger surfacing test plate demonstrates the importance of understanding the interplay between material chemistry, process parameters, and environmental conditions. The proposed corrective measures are straightforward and well-established in welding practice, but their effective implementation requires discipline and attention to detail. For engineers involved in the fabrication of high-pressure equipment, this case study reinforces the principle that process qualification is not merely a regulatory requirement but a practical tool for ensuring quality and safety.