CLADDING TECHNOLOGY SHANXI CO., LTDOptimization of Transition Layer Material for Low-Alloy Cast Steel Cladding
Literature Overview
This 2012 study by Bi Huan, Zhou Jie, Huang Liang, Liu Yang, Lu Shun, and Ding Yongfeng from Chongqing University's School of Materials Science and Engineering addresses a fundamental challenge in cladding technology: the selection and optimization of transition layer materials for low-alloy cast steel substrates. The research was published in "Hot Working Technology" and tackles the metallurgical compatibility problem that arises when depositing corrosion-resistant or wear-resistant overlays onto cast steel components.
Technical Challenge
Low-alloy cast steels, such as ZG20CrMo or ZG25CrMo, present unique challenges for cladding operations due to:
- Higher carbon content compared to equivalent wrought steels (typically 0.20–0.35% C)
- Higher hardenability resulting from the cast microstructure
- Potential presence of segregation and porosity in the substrate
- Higher residual stresses from the casting process
These factors combine to create a high susceptibility to cold cracking in the weld zone during overlay welding.
Substrate Comparison
| Property | Cast Steel (ZG20CrMo) | Wrought Steel (20CrMo) |
|---|---|---|
| Carbon content | 0.18–0.25% | 0.17–0.23% |
| Hardness (as-cast) | 200–250 HB | 180–220 HB |
| Hardenability | Higher | Moderate |
| Preheat requirement | 250–350°C | 150–250°C |
| Cracking susceptibility | High | Moderate |
Transition Layer Material Selection
The core contribution of this study is the systematic evaluation of transition layer materials to bridge the metallurgical gap between low-alloy cast steel substrates and the final overlay material.
Candidate Transition Materials Evaluated
| Material | Composition (wt%) | Hardness | Dilution Compatibility | Cracking Resistance |
|---|---|---|---|---|
| E309L (309L) | 23-27 Cr, 12-17 Ni, ≤0.03 C | 200-220 HB | Good | Excellent |
| E310 (310) | 24-26 Cr, 19-22 Ni, ≤0.08 C | 210-230 HB | Good | Good |
| Ni-Cr alloy | 15-20 Cr, balance Ni | 180-200 HB | Excellent | Excellent |
| E309Mo (309Mo) | 23-27 Cr, 12-17 Ni, 2-3 Mo | 200-225 HB | Good | Excellent |
| 316L | 16-18 Cr, 10-14 Ni, 2-3 Mo | 190-210 HB | Moderate | Good |
Optimization Criteria
The selection of the optimal transition layer material depends on multiple factors:
- Dilution tolerance: The transition layer must maintain its beneficial properties even at 20-30% substrate dilution.
- Cracking resistance: The material must accommodate the high carbon equivalent of the cast steel substrate.
- Thermal expansion compatibility: The coefficient of thermal expansion should be between that of the substrate and the final overlay.
- Cost considerations: Nickel-based materials offer superior performance but at significantly higher cost.
- Available welding processes: PTA, SAW, and GTAW each have different consumable options.
Recommended Transition Layer Strategies
Based on the study's findings, the following strategies are recommended for different overlay requirements:
Strategy 1: Stainless Steel Overlay on Cast Steel
- Substrate: ZG20CrMo (0.22% C)
- Transition layer: E309L, 1-2 passes, SAW or GTAW
- Final overlay: 316L, 2-3 passes, SAW or PTA
- Preheat: 300°C, maintain interpass temperature 250-300°C
- Post-weld treatment: 620°C × 2h stress relief
Strategy 2: Nickel-Based Overlay on Cast Steel
- Substrate: ZG20CrMo (0.22% C)
- Transition layer: Ni-Cr alloy (Incoloy 827 or equivalent), 1 pass, GTAW
- Final overlay: Inconel 625, 2-3 passes, PTA
- Preheat: 250-300°C, interpass temperature < 200°C
- Post-weld treatment: Not typically required for Ni-based systems
Process Optimization Parameters
| Process Parameter | GTAW Transition | SAW Overlay | PTA Overlay |
|---|---|---|---|
| Current | 120-180 A | 450-550 A | 400-500 A |
| Voltage | 18-22 V | 28-32 V | 25-28 V |
| Travel speed | 300-500 mm/min | 300-400 mm/min | 200-300 mm/min |
| Shielding gas | Ar | Flux (HJ431) | Ar |
| Heat input | 5-8 kJ/cm | 20-30 kJ/cm | 15-25 kJ/cm |
Defect Analysis and Prevention
| Defect | Root Cause | Prevention Method |
|---|---|---|
| Cold cracking | High carbon equivalent, hydrogen | Adequate preheat, low-hydrogen consumables |
| Hot cracking | Low melting point phases | Proper transition material selection |
| Poor bonding | Incomplete substrate melting | Increase first pass current |
| Excessive dilution | High heat input | Reduce current, increase travel speed |
| Porosity | Gas entrapment, flux issues | Proper shielding, dry consumables |
Study Insights and Engineering Implications
This research makes a significant contribution to the practical understanding of transition layer selection for cast steel cladding. Several key insights emerge:
- The 309L stainless steel transition layer offers the best balance of performance and cost for most applications involving stainless steel overlays on low-alloy cast steel.
- Nickel-based transition materials should be reserved for applications where maximum cracking resistance is required, such as when the substrate carbon content exceeds 0.25%.
- The number of transition passes is critical: one pass is typically insufficient to achieve adequate dilution buffering, while two or more passes provide a more robust metallurgical bridge.
- The study reinforces the principle that cladding process design must be substrate-specific, and that generic procedures developed for wrought steel may not be directly applicable to cast steel substrates.
The systematic approach presented in this paper aligns with modern quality management methodologies such as FMEA, where each potential failure mode in the cladding process is identified and addressed through appropriate transition layer design.