Overlay Welding Process Research on 15CrMo Tube Sheets

Literature Overview

The literature under review investigates the overlay welding process applied to 15CrMo tube sheets, a critical component in high-temperature pressure vessels, heat exchangers, and hydrogenation reactors. 15CrMo steel (equivalent to ASTM A387 Gr.11 / EN 10025-4 16Mo3) is widely used in power generation and petrochemical equipment due to its excellent creep strength and oxidation resistance at elevated temperatures. However, tube sheets manufactured from 15CrMo often face severe corrosion and wear at the tube-to-tubesheet joint, particularly in sour service or high-temperature aqueous environments. The study explores various overlay welding processes including submerged arc welding (SAW), gas metal arc welding (GMAW), and plasma transferred arc (PTA) cladding to deposit corrosion-resistant layers on 15CrMo tube sheets, with emphasis on process parameter optimization, metallurgical bonding quality, and mechanical performance evaluation.

Core Technical Content and Process Parameters

The investigation centers on achieving a reliable metallurgical bond between the 15CrMo base metal and the overlay alloy while minimizing dilution and avoiding microcracking in the heat-affected zone (HAZ). The authors examined multiple overlay materials including 304 stainless steel, 316L stainless steel, and Inconel 625, selected based on the service environment. The following table summarizes the key process parameters studied:

Parameter	SAW Overlay	GMAW Overlay	PTA Cladding
Heat input	25–40 kJ/cm	8–15 kJ/cm	12–25 kJ/cm
Preheat temperature	150–250 °C	100–200 °C	150–250 °C
Interpass temperature	≤ 250 °C	≤ 200 °C	≤ 250 °C
Post-weld heat treatment	720–760 °C, 2–4 h	720–760 °C, 2–4 h	720–760 °C, 2–4 h
Typical overlay thickness	3–5 mm	2–4 mm	0.5–2 mm per pass
Dilution rate	15–25%	10–20%	5–15%

The study demonstrates that heat input control is the most critical factor in preventing microcracking in the 15CrMo HAZ. Excessive heat input above 40 kJ/cm leads to grain coarsening in the HAZ, reducing impact toughness and increasing susceptibility to temper embrittlement. Conversely, insufficient preheat below 100 °C results in hydrogen-induced cracking due to the high carbon equivalent (CEV ≈ 0.42) of 15CrMo steel.

Metallurgical Analysis and Defect Evaluation

Metallographic examination of the overlay welds reveals distinct microstructural zones: the base metal (15CrMo), the HAZ, the dilution zone (transition layer), and the overlay layer. The transition layer is particularly critical because it contains mixed microstructures of base metal and overlay alloy, often exhibiting reduced corrosion resistance due to dilution. The study reports that when using 316L stainless steel as the overlay material, the dilution zone can contain up to 20–25% carbon steel content, which may compromise the passivity of the overlay in chloride-containing environments.

Common defects identified during the investigation include:

Defect Type	Root Cause	Countermeasure
HAZ microcracking	Excessive heat input, insufficient preheat	Limit heat input to ≤ 35 kJ/cm, preheat to 200 °C
Overlay spalling	Poor metallurgical bond, high dilution	Increase number of overlay passes, reduce dilution
Dilution zone sensitization	Chromium depletion near grain boundaries	Apply PWHT at 720 °C for 4 h, consider Ni-based overlay
Undercut at tube holes	Uncontrolled arc wander	Use backing rings, optimize travel speed
Porosity	Moisture contamination of flux/wire	Dry flux at 300 °C for 2 h, clean base metal thoroughly

The study also highlights the importance of the final overlay pass quality. In PTA cladding, the final pass must achieve a dilution rate below 10% to ensure adequate corrosion resistance. This typically requires a minimum of three passes, with the first pass serving as a transition layer and subsequent passes progressively reducing dilution.

Engineering Practice Integration

From an engineering practice perspective, the study provides valuable guidance for the fabrication of clad tube sheets in heat exchangers and pressure vessels governed by NB/T 47002 and GB/T 150. The recommended procedure involves the following sequence:

Base tube sheet fabrication from 15CrMo plate, including full PWHT at 720–760 °C.
Surface preparation: grinding to expose sound metal, followed by solvent cleaning to remove oil and moisture.
Preheating to 200 °C uniformly across the overlay area, with infrared temperature monitoring.
Multi-pass overlay welding with controlled interpass temperature below 250 °C.
Final PWHT to relieve residual stresses and temper the overlay weld metal.
Non-destructive testing: magnetic particle testing (MT) for surface defects, ultrasonic testing (UT) for subsurface defects, and dye penetrant testing (PT) for tube hole edges.
Mechanical testing: hardness survey across the overlay interface, tensile bond strength testing, and intergranular corrosion testing per ASTM A263.

A practical case from the literature involves the repair of a hydrogenation reactor tube sheet that had developed intergranular corrosion in the dilution zone after 3 years of service. The repair strategy employed PTA cladding with Inconel 625 powder, achieving a dilution rate of 8% in the final pass. Post-repair testing confirmed that the overlay layer passed the intergranular corrosion test (ASTM A263 Method E) and the hardness profile showed a gradual transition from 240 HV (base metal) to 210 HV (overlay), well within the acceptable range specified by NB/T 47002.

Key Questions and Reflections

The literature raises several important questions that warrant further investigation. First, the long-term performance of the dilution zone under cyclic thermal loading remains uncertain. While the study confirms acceptable corrosion resistance at room temperature, the effect of thermal cycling on chromium depletion and sensitization in the transition layer is not fully addressed. Second, the study does not extensively discuss the residual stress distribution in the overlay weld, which is critical for fatigue life prediction in pressure vessels operating under cyclic conditions. Third, the economic comparison between different overlay processes is limited; PTA cladding offers superior metallurgical quality but at significantly higher equipment and consumable costs.

A noteworthy observation from the study is the strong correlation between preheat temperature and hydrogen content in the weld metal. The authors measured hydrogen levels using gas carrier dilution methods and found that preheating to 200 °C reduced weld metal hydrogen content to below 5 mL/100 g, which is well within the safe threshold for 15CrMo steel. This finding underscores the importance of rigorous preheat protocols in preventing delayed cracking.

Study Insights and Implications

The most valuable insight from this literature is the systematic approach to process parameter optimization using the combination of experimental welding trials and metallurgical analysis. The study demonstrates that achieving a reliable overlay on 15CrMo tube sheets requires a holistic approach that considers base metal chemistry, process selection, thermal management, and post-weld treatment as an integrated system rather than isolated variables. For engineers involved in the design and fabrication of bimetal pressure vessels, this study reinforces the principle that overlay welding is not merely a surface treatment but a metallurgical process that must be designed with the same rigor as base material selection and structural design. The practical recommendations provided—particularly regarding heat input limits, preheat protocols, and dilution control—can be directly applied to improve fabrication quality and service life in industrial practice.