CCT Diagram and Transformation Microstructure of Surface Cladding Layer

Literature Overview and Research Significance

This 1999 study by Wang Jianguo, Wang Gui, Liu Xiaogang, and Hou Chunqing from Baotou Iron and Steel Institute and Baotou Iron and Steel Group Corporation investigates the Continuous Cooling Transformation (CCT) diagram and transformation microstructure of surface cladding layers. Published in the Journal of Ordnance Materials Science and Engineering (兵器材料科学与工程), the research was supported by the Inner Mongolia Autonomous Region Key Project. The work addresses a fundamental gap in the understanding of the solidification and transformation behavior of cladding layers, which is critical for predicting and controlling the mechanical properties of overlay welds.

Core Technical Content

CCT Diagram Construction for Cladding Layers

The construction of a CCT diagram for a cladding layer involves determining the transformation start (Ts) and transformation finish (Tf) temperatures for various cooling rates. Unlike bulk materials, cladding layers experience rapid cooling due to the high thermal conductivity of the base metal, which significantly influences the transformation behavior.

Cooling Rate (°C/s)	Ts (°C)	Tf (°C)	Dominant Microstructure	Hardness (HV)
1 (furnace cooling)	780	650	Pearlite + Ferrite	250-300
5 (air cooling)	720	580	Fine Pearlite	350-400
20 (oil quenching)	680	520	Bainite	450-550
50 (brine quenching)	620	450	Martensite + Bainite	550-650
100 (self-quenching in cladding)	580	400	Martensite	600-700

Influence of Cooling Rate on Microstructure

The cooling rate in a cladding layer is typically much higher than in a bulk weld due to the thin cross-section of the deposited layer and the heat sink effect of the base metal. This results in:

Suppression of diffusional transformations (pearlite, ferrite)
Promotion of martensitic transformation
Formation of retained austenite at high carbon equivalent compositions
Development of residual stresses due to differential thermal contraction

Metallurgical Analysis and Property Correlation

Phase Transformation Mechanisms

The transformation behavior of the cladding layer is governed by several factors:

Chemical composition: The carbon and alloy element content determines the transformation temperatures and the type of phases formed.
Cooling rate: Higher cooling rates suppress diffusional transformations and promote martensite formation.
Base metal thermal mass: A thick, high-conductivity base metal increases the cooling rate of the overlay.
Overlay thickness: Thicker overlays cool more slowly, allowing more time for diffusional transformations.

Mechanical Property Relationships

Microstructure	Hardness (HV)	Tensile Strength (MPa)	Elongation (%)	Impact Energy (J)
Pearlite + Ferrite	250-300	500-600	15-20	40-60
Fine Pearlite	350-400	600-700	10-15	25-40
Bainite	450-550	700-850	5-10	15-25
Martensite	600-700	800-1000	2-5	5-15
Martensite + Retained Austenite	550-650	700-900	3-8	10-20

Engineering Practice and Process Control

Process Parameters Influencing Cooling Rate

Parameter	Effect on Cooling Rate	Control Strategy
Base metal thickness	Thicker base metal increases cooling rate	Preheat thick base metals
Overlay thickness	Thicker overlay reduces cooling rate	Multi-pass with adequate thickness
Interpass temperature	Higher interpass temperature reduces cooling rate	Control interpass temperature to 150-250°C
Welding speed	Higher speed reduces heat input, increases cooling rate	Optimize speed for desired properties
Electrode type	Low-hydrogen electrodes reduce hydrogen cracking risk	Select appropriate electrode for cooling rate

Residual Stress Management

The rapid cooling and phase transformation in cladding layers generate significant residual stresses. These stresses can lead to cracking, distortion, and reduced fatigue life. The study provides guidance on stress relief strategies:

Stress Relief Method	Temperature (°C)	Duration (h)	Effectiveness
Stress relief welding	-	-	Partial relief
Post-weld heat treatment (PWHT)	550-650	1-2 per 25 mm thickness	Effective
Mechanical peening	-	-	Compressive surface stresses
Shot peening	-	-	Compressive surface stresses, improved fatigue life

Study Reflections and Implications

The construction of CCT diagrams for cladding layers provides a fundamental tool for predicting and controlling the microstructure and properties of overlay welds. The study by Wang and colleagues demonstrates that the cooling rate, which is inherently high in cladding applications, significantly influences the transformation behavior and resulting mechanical properties.

One of the key insights is the recognition that the cooling rate in a cladding layer is not a fixed parameter but depends on multiple factors including base metal properties, overlay thickness, and welding parameters. This means that the same electrode and welding procedure can produce different microstructures and properties depending on the specific application conditions.

The study also highlights the importance of understanding the relationship between microstructure and properties for rational material selection and process design. For example, applications requiring high hardness and wear resistance may benefit from martensitic microstructures, while applications requiring toughness and fatigue resistance may require tempered martensite or bainitic structures.

From a quality assurance perspective, the CCT diagram provides a basis for establishing acceptance criteria for cladding layers. By measuring the hardness and microstructure of the overlay, engineers can infer the cooling rate and transformation behavior, and assess whether the overlay meets the required performance criteria.

This literature serves as a valuable reference for engineers and researchers working on cladding and overlay welding, providing both fundamental understanding and practical guidance for controlling the microstructure and properties of deposited layers through process parameter optimization.