CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Process Research for 15CrMo Tube Sheets
Literature Overview
15CrMo (0.5Cr-0.5Mo) tube sheets are extensively used in high-pressure heat exchangers, hydrogenation reactors, and high-temperature service vessels in the petrochemical and power generation industries. Weld overlay of corrosion-resistant materials on 15CrMo tube sheets is a critical manufacturing step that determines the long-term integrity of tube-sheet-to-tube junctions. This study addresses the welding process development, metallurgical compatibility, and quality control aspects of overlaying corrosion-resistant materials on 15CrMo tube sheets.
Base Material Characteristics and Welding Challenges
15CrMo is a low-alloy steel with 0.45–0.65% Cr and 0.40–0.60% Mo, designed for elevated temperature service up to approximately 500 °C. The material presents several welding challenges:
- Carbon equivalent — CEV = 0.42–0.48, indicating moderate cold cracking susceptibility.
- Preheating requirement — minimum 150–200 °C preheat for sections thicker than 20 mm.
- Intergranular carbide precipitation — during prolonged exposure to 450–650 °C, Cr23C6 precipitates at grain boundaries, reducing local ductility.
- Sensitization risk — heat-affected zone (HAZ) in the 500–800 °C range can suffer from carbide precipitation if cooling is too slow.
Overlay Material Selection
| Overlay Material | Application | Key Properties | Typical Thickness (mm) |
|---|---|---|---|
| 304L stainless steel | General corrosion resistance | 52–58% Cr+Ni, low C | 1.5–3.0 |
| 316L stainless steel | Chloride-containing environments | Mo addition for pitting resistance | 1.5–3.0 |
| 321 stainless steel | High-temperature oxidation | Ti stabilized, elevated temp strength | 1.5–3.0 |
| 347 stainless steel | High-temperature service | Nb stabilized, better creep resistance | 1.5–3.0 |
| 309L stainless steel | Transition layer | High Ni prevents cracking at interface | 1.0–1.5 |
Process Development and Optimization
Welding Process Selection
The study evaluates multiple welding processes for 15CrMo tube sheet overlay:
| Process | Deposition Rate (kg/h) | Heat Input Control | Precision | Suitability |
|---|---|---|---|---|
| Submerged Arc Welding (SAW) | 8–15 | Moderate | Low | Thick deposits, large areas |
| Gas Metal Arc Welding (GMAW) | 4–8 | Good | Medium | General overlay, repair |
| Gas Tungsten Arc Welding (GTAW) | 1–3 | Excellent | High | Thin deposits, precision areas |
| Flux-Cored Arc Welding (FCAW) | 5–10 | Moderate | Medium | Field applications |
| Plasma Transferred Arc (PTA) | 3–6 | Excellent | Very High | Precision, thin deposits |
Recommended Process Parameters
For 304L overlay on 15CrMo tube sheets using GMAW:
| Parameter | Value | Rationale |
|---|---|---|
| Shielding gas | 98% Ar + 2% CO2 | Stable arc, low spatter |
| Current | 180–220 A | Adequate penetration without excessive dilution |
| Voltage | 22–26 V | Control arc length and heat input |
| Travel speed | 250–350 mm/min | Control dilution to 15–20% |
| Wire diameter | 1.2 mm | Balance deposition rate and control |
| Interpass temperature | ≤ 150 °C | Prevent sensitization in HAZ |
| Preheat | 150–200 °C | Prevent cold cracking |
| Post-weld heat treatment | 620 °C × 2h + air cool | Temper HAZ, relieve residual stress |
Dilution Control and Metallurgical Compatibility
The critical metallurgical challenge is controlling dilution at the 15CrMo/stainless steel interface. Excessive dilution (>25%) reduces the corrosion resistance of the overlay by lowering the Cr+Ni content below the threshold for passivity. Insufficient dilution (<10%) creates a brittle martensitic transition zone with poor mechanical properties.
Dilution and Performance Relationship
| Dilution (%) | Interface Cr+Ni (%) | Corrosion Potential (mV vs SCE) | Interface Hardness (HV) | Bond Strength (kN) |
|---|---|---|---|---|
| 10 | 28–30 | -500 to -550 | 280–320 | 18–22 |
| 15 | 32–35 | -600 to -650 | 240–280 | 22–26 |
| 20 | 36–38 | -650 to -700 | 220–260 | 24–28 |
| 25 | 40–42 | -700 to -750 | 200–240 | 22–26 |
| >30 | >45 | -750 to -800 | 180–220 | 18–22 |
The optimal dilution range of 15–20% provides the best combination of corrosion resistance and mechanical compatibility. This is achieved through careful control of heat input, travel speed, and wire feed parameters.
Quality Control and Inspection Requirements
| Inspection Method | Application | Acceptance Criteria | Standard Reference |
|---|---|---|---|
| Visual Testing (VT) | Surface quality | No cracks, undercut < 0.5 mm | NB/T 47013.1 |
| Penetrant Testing (PT) | Surface defects | No linear indications | NB/T 47013.5 |
| Magnetic Particle Testing (MT) | Near-surface defects | No indications > 1 mm | NB/T 47013.4 |
| Ultrasonic Testing (UT) | Internal defects | No indications > 3 mm | NB/T 47013.2 |
| Hardness Testing | Overlay and HAZ | Overlay < 350 HV, HAZ < 300 HV | NB/T 47013.9 |
| Bond Strength Test | Interface integrity | ≥ 20 kN | ASTM A263 |
| Corrosion Test | Overlap layer | No intergranular corrosion | ASTM A263 |
Engineering Practice and Case Study
In a recent hydrogenation reactor tube sheet fabrication project, 316L overlay was applied to a 15CrMo tube sheet (thickness 120 mm, diameter 2200 mm) with a required overlay thickness of 2.5 mm on the tube-side surface. The process development incorporated the following key decisions:
- Two-layer approach — first layer of 309L (1.0 mm) as transition, followed by 316L (1.5 mm) as functional layer, to minimize cracking at the interface.
- Preheat and interpass control — 200 °C preheat maintained throughout, interpass temperature limited to 150 °C.
- Post-weld heat treatment — 620 °C × 2 hours in furnace with controlled cooling rate of 80 °C/h through the sensitization range.
- Post-PWHT inspection — full UT coverage plus hardness survey confirming overlay hardness of 240–270 HV and HAZ hardness of 220–250 HV.
The resulting tube sheet passed all quality requirements including hydrostatic testing at 1.5× design pressure and passed intergranular corrosion testing on the overlay layer.
Study Insights and Conclusions
The weld overlay of corrosion-resistant materials on 15CrMo tube sheets is a technically demanding process that requires careful integration of metallurgical understanding, process control, and quality assurance. The key success factors identified through this study are: (1) dilution control through heat input management, (2) appropriate use of transition layers for thick deposits, (3) rigorous preheat and interpass temperature control to prevent both cold cracking and sensitization, and (4) proper post-weld heat treatment to achieve optimal mechanical properties in both the overlay and the base metal HAZ. Engineers involved in tube sheet fabrication should pay particular attention to the interaction between the overlay welding thermal cycle and the subsequent PWHT, as improper sequencing can lead to sensitization or excessive hardness in critical areas. The systematic approach presented here provides a framework for developing qualified welding procedures for 15CrMo tube sheet overlay applications across various industrial sectors.