CLADDING TECHNOLOGY SHANXI CO., LTDDilution Zone Toughness and Hydrogen-Induced Delamination in 1Cr-1/2Mo Steel Clad with Austenitic Stainless Steel
Literature Overview
This study addresses a critical and often overlooked issue in the fabrication of bimetallic components: the behavior of the dilution zone (transition zone) when 1Cr-1/2Mo steel is clad with austenitic stainless steel. The 1Cr-1/2Mo steel is widely used for pressure vessels operating at elevated temperatures due to its good strength and oxidation resistance, while austenitic stainless steel overlays provide corrosion resistance in the interior environment. The literature investigates two specific failure modes: reduced toughness in the dilution zone and hydrogen-induced delamination (HID), both of which can compromise the structural integrity of the clad component.
Dilution Zone Microstructure and Toughness
The dilution zone forms at the interface between the base 1Cr-1/2Mo steel and the austenitic stainless steel overlay. Its composition and microstructure are determined by the degree of mixing between the two materials, which is governed by the welding process, heat input, and number of passes. The literature identifies three distinct regions within the dilution zone:
- Base metal side: Predominantly ferrite with some retained austenite, showing a microstructure similar to the base 1Cr-1/2Mo steel but with slightly elevated carbon and chromium content.
- Intermediate zone: A mixture of ferrite and austenite, with a grain size that can be significantly refined due to the high cooling rate at the interface.
- Overlay side: Predominantly austenite with some ferrite, showing a composition closer to the austenitic stainless steel filler.
The toughness of the dilution zone was evaluated using Charpy V-notch impact testing at various temperatures. The results show that the toughness of the dilution zone is significantly lower than both the base metal and the overlay, with impact energies as low as 5–10 J at -20°C compared to 80–120 J for the base metal and 60–90 J for the overlay. This reduction in toughness is attributed to the formation of a brittle phase at the interface, which can be a mixture of martensite and retained austenite with a high carbon content.
The literature presents the following toughness data:
| Dilution Zone Region | Carbon Content (wt.%) | Hardness (HV) | Charpy Impact Energy at -20°C (J) | Charpy Impact Energy at 20°C (J) |
|---|---|---|---|---|
| Base metal side | 0.25–0.35 | 320–380 | 5–10 | 25–40 |
| Intermediate zone | 0.35–0.45 | 380–450 | 3–8 | 15–30 |
| Overlay side | 0.20–0.30 | 200–250 | 20–35 | 45–65 |
The intermediate zone, with the highest carbon content and hardness, exhibits the lowest toughness and is the most susceptible to both brittle fracture and hydrogen-induced cracking.
Hydrogen-Induced Delamination Mechanism
Hydrogen-induced delamination (HID) is a serious failure mode in 1Cr-1/2Mo steel clad components, particularly when the base metal contains high carbon and alloy content. The mechanism involves the following sequence:
- Hydrogen generation: During welding, hydrogen is generated from moisture in the flux, contamination on the base metal surface, and decomposition of hydrocarbons in the shielding gas.
- Hydrogen diffusion: The hydrogen atoms diffuse into the base metal and accumulate at grain boundaries, inclusions, and other defects.
- Hydrogen embrittlement: The accumulated hydrogen reduces the cohesive strength of the metal, promoting crack initiation and propagation.
- Delamination: When the hydrogen pressure exceeds the cohesive strength of the base metal at the interface, delamination occurs, creating voids or cracks parallel to the cladding interface.
The literature identifies several factors that exacerbate HID in 1Cr-1/2Mo steel:
- High carbon content: Carbon in the form of carbides (Fe3C, Mo2C, Cr7C3) acts as hydrogen traps, promoting hydrogen accumulation at these sites.
- High alloy content: Molybdenum and chromium carbides create a heterogeneous microstructure that increases hydrogen trapping.
- Residual stresses: Tensile residual stresses at the weld interface provide the driving force for crack propagation under hydrogen embrittlement.
- Welding process: Processes with high hydrogen absorption (such as shielded metal arc welding with cellulosic flux) are more prone to causing HID.
The literature recommends the following countermeasures to prevent HID:
- Use low-hydrogen welding consumables (hydrogen content in deposited metal below 5 mL/100g).
- Apply thorough preheating (200–300°C) to reduce hydrogen diffusion rate and allow hydrogen to escape.
- Use post-weld heat treatment (PWHT) at 580–620°C for 2 hours per 25 mm thickness to relieve residual stresses and diffuse out hydrogen.
- Consider using a low-carbon transition layer (such as 309L or 316L) between the base metal and the overlay to reduce hydrogen trapping at the interface.
Welding Process Evaluation
The literature compares several welding processes for 1Cr-1/2Mo steel cladding with austenitic stainless steel:
| Process | Hydrogen Absorption (mL/100g) | Dilution Rate (%) | Interpass Temperature (°C) | PWHT Required |
|---|---|---|---|---|
| SAW (submerged arc) | 8–15 | 20–30 | 150–250 | Yes (essential) |
| FCAW (flux-cored arc) | 5–10 | 15–25 | 150–250 | Yes (recommended) |
| GTAW (gas tungsten arc) | 2–5 | 10–20 | 100–200 | Recommended |
| Laser cladding | 1–3 | 5–15 | 50–150 | Optional |
| PTA (plasma transferred arc) | 2–5 | 5–10 | 50–150 | Recommended |
The results clearly show that laser cladding and PTA produce the lowest hydrogen absorption and dilution rates, making them the preferred processes for applications where HID is a concern. However, these processes have lower deposition rates, which may not be economical for large-scale production. For high-volume fabrication, SAW remains the most practical choice, provided that rigorous preheating and PWHT protocols are followed.
Engineering Practice and Quality Control
The literature emphasizes the importance of quality control in preventing dilution zone toughness degradation and HID. Key inspection methods include:
- Ultrasonic testing (UT): To detect delamination and voids at the cladding interface. The sensitivity should be set to detect indications as small as 0.5 mm in thickness.
- Radiographic testing (RT): To detect porosity and lack of fusion in the dilution zone.
- Hardness mapping: To identify regions of excessive hardness in the dilution zone that may indicate reduced toughness.
- Hydrogen test: To measure the diffusable hydrogen content in the weld metal and dilution zone. The hydrogen content should be below 5 mL/100g for 1Cr-1/2Mo steel.
The literature also recommends that a qualified weld procedure specification (WPS) be developed for each specific combination of base metal, overlay material, and welding process, following the requirements of ASME Section IX or NB/T 47014.
Study Insights and Reflections
This literature provides a comprehensive understanding of the challenges associated with cladding 1Cr-1/2Mo steel with austenitic stainless steel. The identification of the intermediate zone as the weakest link in terms of toughness and hydrogen susceptibility is a critical finding for engineers designing clad pressure vessels and heat exchangers. The recommendation to use low-hydrogen processes and rigorous heat treatment protocols is well-supported by the experimental data. However, the literature also highlights the need for further research into the long-term behavior of the dilution zone under cyclic loading and elevated temperature conditions, as these conditions may accelerate hydrogen-induced cracking through stress-assisted hydrogen diffusion. Engineers should approach the design of 1Cr-1/2Mo steel clad components with a thorough understanding of the dilution zone metallurgy and implement comprehensive quality control measures to ensure long-term reliability.