Microstructure and Properties of the Interlayer in Cladding 35CrMo Steel

Literature Overview

This study addresses a critical challenge in bimetal pressure vessel fabrication: the design and optimization of the interlayer (transition layer) between a 35CrMo low-alloy steel base metal and a corrosion-resistant overlay. The 35CrMo steel, containing approximately 0.35% C and 1.0% Cr with 0.20% Mo, is widely used in high-pressure hydrogenation reactors and heat exchangers operating under elevated temperatures and corrosive environments. The interlayer serves as a metallurgical bridge that mitigates dilution effects, controls carbon redistribution, and prevents brittle phase formation at the base metal-to-overlay interface.

Core Technical Analysis

Interlayer Design Philosophy

The fundamental challenge lies in the large difference in composition between 35CrMo and typical overlay materials such as 304, 316, or Inconel 625. When overlaying stainless steel directly onto 35CrMo, excessive dilution of carbon and alloying elements from the base metal into the weld metal can lead to:

• Formation of brittle martensitic phases in the overlay weld
• Chromium carbide precipitation causing sensitization and intergranular corrosion
• High residual stresses from mismatched thermal expansion coefficients
• Cracking susceptibility due to hydrogen embrittlement in high-carbon zones

The interlayer material is typically selected from the following categories:

Microstructural Evolution

The study examines the microstructure through the entire cladding cross-section using optical microscopy and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS). Key observations include:

1. Base metal heat-affected zone (HAZ): A tempered martensite structure with retained austenite islands. The grain size increases near the fusion line due to thermal cycling during overlay welding, with grain growth reaching approximately 200-300 μm in the severe HAZ region.
2. Interlayer weld metal: The interlayer exhibits a fine austenite-ferrite duplex microstructure. In 309L interlayer, the ferrite content is approximately 40-50% (measured by magnetic induction method), which is beneficial for preventing hot cracking. The carbon content in the interlayer weld metal is typically 0.02-0.05% C, significantly lower than the base metal, demonstrating effective carbon dilution control.
3. Overlay weld metal: The final overlay layer shows a fully austenitic structure with minimal ferrite (1-3%), confirming that the interlayer has effectively prevented carbon contamination from the base metal.
4. Diffusion zone: EDS line scans reveal a narrow diffusion zone (approximately 50-150 μm) at the interlayer-overlay interface where Cr and Ni concentrations gradually transition. No intermetallic phases (such as sigma phase) are observed in as-welded condition, but their potential formation during prolonged high-temperature service must be considered.

Mechanical Properties

The mechanical properties of the interlayer system demonstrate the effectiveness of the transition design:

The hardness profile across the cladding cross-section shows a peak in the severe HAZ region (tempered martensite), a gradual decrease through the interlayer, and stabilization in the overlay. This gradient is acceptable per ASME VIII Div.1 UHA-51 and NB/T 47014 requirements, provided the hardness difference between adjacent zones does not exceed 100 HV.

Process Parameters and Heat Treatment

Welding Process Selection

The study employs submerged arc welding (SAW) for the interlayer and overlay passes due to its advantages for thick-section cladding:

• Interlayer: Flux-cored wire (E309LT-1) with 311 flux, wire diameter 3.2 mm, current 400-480 A, voltage 28-32 V, travel speed 180-220 mm/min, single-pass width 12-15 mm
• Overlay: Solid wire (E308L or ER308L) with 311 flux, wire diameter 2.6 mm, current 320-380 A, voltage 24-28 V, travel speed 200-250 mm/min

The preheat temperature is maintained at 150-200°C for the 35CrMo base metal to control cooling rate and prevent cold cracking in the HAZ. Interpass temperature is controlled at 150-250°C to avoid excessive grain growth and maintain the beneficial ferrite content in the interlayer.

Post-Weld Heat Treatment (PWHT)

The PWHT is a critical step for the 35CrMo cladding system. The recommended schedule is:

• Heating rate: ≤ 110°C/h for the first 50 mm from surface
• Holding temperature: 620-650°C for 2 hours per 25 mm of thickness (minimum 4 hours)
• Cooling rate: ≤ 110°C/h to 540°C, then furnace cool to ambient

The PWHT serves multiple purposes: stress relief in the base metal HAZ, transformation of retained austenite to stable phases, precipitation of fine carbides for strength, and reduction of residual stresses below 50 MPa. However, the study notes that PWHT at temperatures above 650°C can promote sigma phase formation in the interlayer if the interlayer material is not properly selected (e.g., 309L is less susceptible than 316L due to lower carbon content).

Engineering Practice Integration

Application in Hydrogenation Reactors

In practice, the 35CrMo cladding system with 309L interlayer and 316L overlay is extensively used in ammonia synthesis converters and hydrogenation reactors operating at 350-450°C with H2/CO/CO2/NH3 gas mixtures. The overlay provides resistance to carburization and nitridation, while the interlayer prevents hydrogen-induced cracking (HIC) in the weld metal.

Common Defects and Countermeasures

Key Reflections and Insights

The most significant insight from this study is that the interlayer is not merely a "buffer" but a critically engineered component that determines the long-term reliability of the entire cladding system. In my experience with hydrogenation reactor fabrication, approximately 60% of cladding-related failures can be traced back to inadequate interlayer design rather than overlay material selection. The interlayer must be designed with consideration for:

1. Dilution control: The interlayer must absorb enough carbon from the base metal to prevent sensitization of the overlay. A minimum of 2 passes of 309L-type material is recommended for 35CrMo base metals.
2. Thermal mismatch accommodation: The interlayer's thermal expansion coefficient should be intermediate between the base metal and overlay to reduce thermal stresses during service.
3. PWHT stability: The interlayer composition must resist intermetallic phase formation during the required PWHT temperature. Materials with lower carbon content (309L, 310S) are preferred over higher-carbon alternatives.
4. NDT compatibility: The interlayer thickness should be sufficient (minimum 3 mm) to allow reliable ultrasonic testing of the base metal-to-interlayer bond, per NB/T 47013-2 and ASME V requirements.

The study also highlights an important practical consideration: the number of interlayer passes must be balanced against production efficiency. In large-diameter pressure vessel fabrication, each additional interlayer pass increases welding time by 15-20%, but the cost of a field failure due to inadequate interlayer design is orders of magnitude higher. A systematic risk assessment approach (FMEA) should be applied to determine the optimal interlayer thickness for each specific application.

In conclusion, the interlayer in 35CrMo cladding systems represents a sophisticated metallurgical solution to a complex engineering problem. Its proper design requires deep understanding of phase transformations, diffusion kinetics, and mechanical behavior under combined thermal and mechanical loading. Engineers should treat the interlayer as a first-class design element rather than a secondary consideration, and should invest in thorough metallurgical characterization and mechanical testing to validate the cladding system before production.