CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of the Interface Layer in Cladding on 35CrMo Steel
Introduction
This 2022 study by Xu Yali, Xue Lintao, and Guan Xin investigates the metallurgical characteristics of the interface (bond) layer formed during weld overlay cladding on 35CrMo steel. The work was supported by a vocational education research project (XJZJKT-2021Y34) and represents a focused materials science examination of a widely used medium-carbon alloy steel substrate in pressure vessel and heavy equipment fabrication.
35CrMo is a quenched and tempered low-alloy steel containing approximately 0.35% carbon, 0.8–1.1% chromium, and 0.15–0.30% molybdenum. It offers good strength, toughness, and temper stability, making it suitable for pressure vessel shells, flanges, and structural components. However, its relatively high carbon equivalent (CE ≈ 0.45–0.55) makes it susceptible to cracking during welding, which directly impacts the integrity of the cladding interface.
Interface Layer Formation Mechanism
The interface layer in weld overlay cladding is the critical zone where the overlay material meets the base metal. Its microstructure is determined by the complex interplay of thermal cycling, solidification behavior, and elemental diffusion during the welding process.
During cladding, the base metal is partially melted and mixed with the deposited overlay material. The resulting interface layer composition is a function of:
- Dilution rate: The fraction of base metal incorporated into the weld. For a single pass, dilution typically ranges from 15% to 30% depending on heat input and substrate thickness.
- Solidification rate: Determined by cooling rate, which is influenced by heat input, plate thickness, and preheat temperature.
- Phase transformations: The interface layer undergoes austenitization during welding and may transform to martensite, bainite, or tempered products upon cooling.
The microstructure evolution at the interface typically follows this sequence:
- Weld metal: The overlay material solidifies first, forming a dendritic microstructure with interdendritic phases.
- Bond layer (interface): A narrow zone of mixed composition where base metal and overlay material intermix. This zone may exhibit coarse grain growth, carbide precipitation, and phase segregation.
- Heat-affected zone (HAZ): The base metal region that was heated above the Ac1 temperature but not melted. In 35CrMo, this zone may develop martensite or bainite depending on cooling rate.
Microstructural Analysis
The study characterizes the interface layer using optical microscopy (OM) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). The following microstructural features are observed:
| Zone | Microstructure | Key Phases | Composition Trend |
|---|---|---|---|
| Overlay weld metal | Dendritic with interdendritic ferrite/cementite | Austenite + ferrite + carbides | Rich in overlay alloying elements |
| Bond layer | Coarse-grained, mixed phase | Martensite + retained austenite + carbides | Intermediate composition between base and overlay |
| HAZ (near interface) | Fine-grained martensite or bainite | Martensite, carbide precipitation | Base metal composition, possibly with slight alloying element pickup |
| HAZ (far from interface) | Tempered martensite or bainite | Tempered martensite | Base metal composition, unaffected by dilution |
The bond layer is the most critical zone for mechanical performance because it is subject to compositional inhomogeneity and may contain brittle phases. In the study, the bond layer exhibits a hardness peak due to the formation of martensite, which can create a region of high hardness but reduced toughness.
Mechanical Properties
The mechanical properties of the cladding interface are evaluated through microhardness testing, tensile testing, and bend testing:
| Property | Overlay Layer | Bond Layer | Base Metal (35CrMo Q+T) |
|---|---|---|---|
| Hardness (HV30) | 250–350 | 350–450 | 280–320 |
| Tensile strength (MPa) | 550–650 | 600–700 | 620–720 |
| Elongation (%) | 15–25 | 10–15 | 18–22 |
| Impact energy (J, 20 °C) | 40–60 | 25–40 | 50–70 |
The bond layer hardness is consistently higher than both the overlay and base metal due to martensitic transformation during rapid cooling. This hardness peak can be beneficial for wear resistance but detrimental to toughness and crack resistance.
Factors Influencing Interface Quality
| Factor | Effect on Interface | Control Strategy |
|---|---|---|
| Heat input | Higher heat input increases dilution and grain coarsening | Use moderate heat input (0.8–1.5 kJ/mm) |
| Preheat temperature | Higher preheat reduces cooling rate and cracking risk | Preheat to 200–300 °C for 35CrMo |
| Interpass temperature | Excessive interpass temperature promotes grain growth | Maintain ≤ 250 °C |
| Overlay material selection | Higher alloy content reduces dilution effect | Use austenitic or nickel-based overlay for better compatibility |
| Number of passes | More passes reduce dilution per pass but increase thermal cycles | Use 2–3 passes with controlled dilution |
| Post-weld heat treatment (PWHT) | Tempering reduces bond layer hardness and improves toughness | Temper at 600–650 °C for 2 h per 25 mm thickness |
Engineering Practice Considerations
In pressure vessel fabrication, the cladding interface layer must satisfy acceptance criteria defined by standards such as GB/T 150 and ASME VIII. Key acceptance requirements include:
- Bond strength: The overlay must remain bonded to the base metal under mechanical loading. A common test involves applying a shear or tensile load to a coupon and verifying that failure occurs in the base metal or overlay, not at the interface.
- Crack resistance: The interface must be free of cracks, particularly in the HAZ and bond layer. This is verified by magnetic particle testing (MT) or dye penetrant testing (PT) of the surface, and ultrasonic testing (UT) for subsurface defects.
- Corrosion resistance: The overlay material must provide adequate corrosion resistance, and the interface must not be a preferential site for galvanic corrosion.
For 35CrMo substrates, the high carbon equivalent necessitates careful welding procedure design. The authors recommend using low-hydrogen consumables, controlling hydrogen pickup, and implementing post-weld heat treatment to temper the interface layer and reduce residual stress.
Study Insights
This study provides valuable insight into the metallurgical behavior of the cladding interface on 35CrMo steel, a material widely used in pressure vessel construction. The key finding is that the bond layer is a zone of metallurgical complexity where dilution, phase transformation, and compositional segregation interact to produce a microstructure that is distinct from both the overlay and base metal. Understanding this behavior is essential for predicting cladding performance and designing appropriate welding procedures. The study also underscores the importance of post-weld heat treatment in tempering the hard, brittle bond layer and restoring ductility. For engineers developing cladding procedures for medium-carbon alloy steels, the findings reinforce that the interface layer is not a passive zone but an active region that must be deliberately engineered through process parameter control.