CCT Diagrams and Transformation Microstructure of Surface Cladding Layer Metals

Literature Overview

This 1999 publication by Wang Jianguo, Wang Gui, Liu Xiaogang, and Hou Chunqing from Baotou Steel Institute and Baotou Steel Group Corporation, supported by an Inner Mongolia Autonomous Region key research project, presents a systematic investigation into the continuous cooling transformation (CCT) diagrams of surface cladding layer metals and their resulting transformation microstructures and mechanical properties. The work addresses a fundamental gap in cladding technology: the lack of comprehensive CCT data for overlay alloys, which is essential for predicting the microstructure and properties of cladding layers under different cooling conditions encountered during practical welding operations.

Core Technical Content

The study employs dilatometry and isothermal transformation techniques to construct CCT diagrams for several representative cladding alloys, including austenitic stainless steels, martensitic stainless steels, and nickel-based superalloys. The CCT diagrams reveal the critical cooling rates required to achieve specific microstructural outcomes, such as fully martensitic, mixed austenite-ferrite, or fully austenitic structures, depending on the alloy composition and cooling conditions.

Key Findings from CCT Analysis

Dilatometry Study Methodology

The dilatometry experiments were conducted on cylindrical specimens with a diameter of 10 mm and a height of 30 mm, heated to temperatures of 1100-1300 °C and then cooled at various rates ranging from 1 °C/s to 500 °C/s. The linear expansion and contraction of the specimens during cooling were recorded to identify transformation temperatures, including the austenite start temperature (As), austenite finish temperature (Af), martensite start temperature (Ms), and martensite finish temperature (Mf).

The CCT diagrams were constructed by plotting the transformation start and finish times against temperature for each cooling rate, identifying the critical cooling rate (CCR) below which transformation occurs to a specific microstructure. For martensitic alloys, the CCR is defined as the minimum cooling rate required to suppress diffusion-controlled transformations and achieve full martensitic transformation.

Microstructural Characterization

Metallographic examination of the cladding layers produced under different cooling conditions revealed distinct microstructural features that correlate directly with the CCT diagram predictions. At slow cooling rates (1-10 °C/s), the martensitic cladding alloys exhibited mixed structures containing carbide precipitates, ferrite, and retained austenite, resulting in reduced hardness but improved toughness. At intermediate cooling rates (20-50 °C/s), the microstructure consisted primarily of martensite with fine carbide distribution, achieving an optimal balance of hardness and toughness. At high cooling rates (> 100 °C/s), fully martensitic structures with minimal carbide precipitation were observed, yielding the highest hardness values but increased susceptibility to cracking.

Engineering Practice Implications

The CCT data presented in this study provides critical input for the design and optimization of cladding processes. By understanding the relationship between cooling rate and resulting microstructure, process engineers can select welding parameters that achieve the desired mechanical properties in the overlay layer. For example, when a high-hardness surface is required for wear resistance, parameters should be selected to ensure cooling rates above the CCR, while for applications requiring good toughness, parameters should be adjusted to allow slower cooling and the formation of tempered or mixed microstructures.

The study also emphasizes the importance of considering the actual cooling conditions in practical cladding operations, which are influenced by factors such as base material thermal conductivity, preheat temperature, interpass temperature, and ambient conditions. The CCT diagrams serve as a predictive tool that can be used in conjunction with thermal modeling to optimize process parameters for specific application requirements.

This foundational research work from 1999 remains highly relevant to contemporary cladding technology, as the fundamental metallurgical principles governing microstructure formation are independent of the specific welding process employed.