CLADDING TECHNOLOGY SHANXI CO., LTDCorrosion-Resistant Hardfacing of 20MnMo Heads
Literature Overview and Background
20MnMo is a low-alloy steel commonly used for pressure vessel heads and covers due to its good strength and weldability. However, it lacks inherent corrosion resistance, making it vulnerable in aggressive chemical environments such as those found in hydrogenation reactors, sulfuric acid processing, and chlorinated service. This literature presents a corrosion-resistant hardfacing technology for 20MnMo heads, covering alloy selection, welding procedure, quality assurance, and service performance. The study is particularly relevant to engineers working in the chemical and petrochemical industries.
Core Technical Content
The hardfacing alloy system is based on austenitic stainless steel with controlled chromium and nickel content to ensure full austenitic microstructure and excellent resistance to pitting and crevice corrosion. Two primary alloy compositions are evaluated: a 316L-type alloy (Cr 16–18%, Ni 10–14%, Mo 2–3%) for general corrosion service, and a 316L with tungsten addition (Cr 18–20%, Ni 10–14%, Mo 2–3%, W 0.5–1.0%) for enhanced pitting resistance.
| Parameter | 316L-Type Overlay | 316L-W-Type Overlay |
|---|---|---|
| Cr content (%) | 16–18 | 18–20 |
| Ni content (%) | 10–14 | 10–14 |
| Mo content (%) | 2–3 | 2–3 |
| W content (%) | — | 0.5–1.0 |
| Target hardness (HB) | 150–200 | 160–210 |
| Overlay thickness | 3–6 mm | 3–6 mm |
| Dilution control | < 25% | < 20% |
| Welding process | SMAW / SAW / GTAW | SMAW / SAW / GTAW |
The welding procedure involves a multi-pass approach with a nickel-base transition layer (e.g., ENi-CrFe or E309L) to control dilution and prevent carbon pickup from the base metal. The transition layer is typically 1.5–3.0 mm thick, followed by 2–4 passes of the stainless steel overlay.
Process Parameters and Weld Procedure Specification
The following welding parameters are specified for the hardfacing operation:
| Pass | Process | Consumable | Current (A) | Voltage (V) | Speed (mm/min) |
|---|---|---|---|---|---|
| Transition layer | SMAW | E309L-16 | 180–220 | 24–28 | 200–300 |
| Overlay pass 1 | SMAW | E316L-16 | 160–200 | 22–26 | 180–280 |
| Overlay pass 2 | SMAW | E316L-16 | 160–200 | 22–26 | 180–280 |
| Overlay pass 3 | SAW (optional) | A213 flux + ER316L wire | 400–600 | 28–34 | 300–500 |
Preheating at 150–250 °C is recommended to reduce residual stress and minimize the risk of hydrogen-induced cracking in the base metal. The interpass temperature should not exceed 250 °C to prevent excessive grain growth in the overlay. Post-weld stress relief at 550–620 °C for 2 hours per 25 mm of thickness is recommended for critical applications.
Quality Assurance and Corrosion Testing
The quality assurance program includes the following tests:
| Test | Method | Acceptance Criterion |
|---|---|---|
| Dilution analysis | Optical emission spectroscopy | Base metal dilution < 25% |
| Intergranular corrosion | ASTM A923 Practice E (65% HNO3) | No intergranular attack |
| Pitting corrosion | ASTM G48 (NaCl solution) | No pits > 50 μm depth |
| Hardness | Rockwell B scale | Within specified range |
| MT inspection | AC magnetic particle | No surface cracks or indications |
| UT inspection | A-scan | No subsurface defects |
The intergranular corrosion test is particularly critical because 20MnMo base metal can contribute carbon to the overlay during welding, potentially causing sensitization of the austenitic stainless steel. The nickel-base transition layer effectively prevents this carbon pickup, ensuring the overlay maintains full resistance to intergranular corrosion.
Engineering Practice and Service Experience
Field experience with 20MnMo heads hardfaced using this technology shows excellent corrosion resistance in sulfuric acid solutions (up to 20% concentration at 60 °C), hydrochloric acid solutions (up to 5% at 40 °C), and chloride-containing environments. The overlay thickness loss after 5 years of continuous service in a sulfuric acid environment was measured at less than 0.1 mm, demonstrating excellent long-term durability.
A key lesson from field experience is that the overlay must be applied to the entire internal surface of the head, not just the bottom or low-point areas. Partial coverage leads to crevice corrosion at the overlay edge, which can initiate rapid failure. Additionally, the overlay edge should be ground smooth to avoid stress concentration and crevice formation.
Study Insights and Implications
This literature provides a comprehensive, well-validated hardfacing technology for corrosion protection of 20MnMo pressure vessel heads. The key insight is that the nickel-base transition layer is essential for controlling dilution and preventing sensitization of the stainless steel overlay. For engineers designing corrosion-resistant hardfacing solutions for pressure vessels, the literature reinforces the importance of full-surface coverage, proper dilution control, and rigorous corrosion testing. The technology is particularly suitable for retrofit applications where replacing the entire vessel with a clad or lined design is not economically feasible. The systematic approach to process development, quality assurance, and field validation provides a reliable framework for implementing this technology in industrial practice.