CCT Diagrams and Transformation Microstructure of Surface Hardfacing Metals

Research Significance and Methodology

The study by Wang Jianguo and colleagues from Baotou Steel Institute and Baotou Steel Group, published in "Ordnance Materials and Science and Engineering" in 1999, investigates the continuous cooling transformation behavior of hardfacing metals using a Formastor-Digital automatic phase transformation recorder. This research was supported by the Inner Mongolia Autonomous Region Key Research Program. The use of CCT diagrams for hardfacing metals is of fundamental importance because the cooling rates experienced during hardfacing are typically much faster than those encountered in conventional welding, and the resulting microstructures can differ dramatically from equilibrium expectations.

CCT Diagram Characteristics of Hardfacing Metals

The study reveals several critical findings about the transformation behavior of the hardfacing metal under simulated hardfacing thermal cycles:

Characteristic	Finding	Engineering Implication
Hardenability	Very high	Base metal dilution significantly affects hardfacing properties
Austenite to pearlite transformation	Strongly suppressed	Pearlite rarely forms in normal hardfacing conditions
Austenite to bainite transformation	Significantly delayed	Bainite formation requires slower cooling than typical hardfacing rates
Critical quenching rate	Approximately 300 degrees Celsius per minute	Cooling rates below this threshold allow diffusional transformations

The extremely high hardenability of the hardfacing metal means that even moderate cooling rates produce fully hardened structures, typically martensite or martensite-like phases. This is both advantageous for wear resistance and potentially problematic for toughness and residual stress management.

Microstructural Non-Uniformity Under Hardfacing Thermal Cycles

A particularly important finding is the severe microstructural non-uniformity that develops when cooling rates vary slightly within the hardfacing deposit. In a multi-pass hardfacing operation, each pass experiences a different thermal history: the first pass cools primarily into the base metal, while subsequent passes cool into previously deposited layers that act as preheated substrates. This creates a gradient of microstructures across the build-up:

First pass (bottom layer): Highest cooling rate, finest martensitic structure, highest hardness, highest residual stress.
Intermediate passes: Moderate cooling rate, mixed martensite and bainite, moderate hardness.
Final pass (surface layer): Lowest cooling rate due to preheating by underlying layers, potential for bainite or even tempered martensite, lower hardness but improved toughness.

This non-uniformity has direct implications for the service performance of hardfaced components. A component designed for uniform wear resistance may experience uneven wear patterns if the surface hardness varies significantly across the build-up depth.

Process Implications and Control Strategies

Understanding the CCT behavior of hardfacing metals enables engineers to develop more effective process control strategies:

Interpass temperature management: Maintaining interpass temperatures within a controlled range (typically 150 to 350 degrees Celsius for most hardfacing applications) ensures that each subsequent pass experiences a similar thermal cycle, promoting microstructural uniformity.
Preheating strategy: Preheating the base metal to an appropriate temperature reduces the cooling rate of the first pass, narrowing the microstructural gradient between the bottom and surface layers.
Post-weld tempering: For hardfacing metals with extremely high hardenability, post-weld tempering is often necessary to reduce residual stresses and improve toughness, but this must be carefully controlled to avoid softening the hardfacing deposit below acceptable wear resistance levels.
Welding parameter optimization: Travel speed, current, and voltage directly influence cooling rates. Faster travel speeds increase cooling rates and promote finer, harder structures, while slower speeds allow more complete transformations.

Reflections on Fundamental Welding Metallurgy

This study, though published in 1999, remains highly relevant to contemporary hardfacing practice. The fundamental metallurgical principles it describes have not changed, even as welding technology has advanced significantly. Modern engineers working with laser cladding, plasma transfer arc hardfacing, or cold spray technologies still must confront the same challenges of microstructural control under rapid solidification and cooling conditions.

The study also highlights the importance of CCT diagrams as a design tool. Unlike TTT diagrams, which require isothermal conditions that are rarely achieved in welding, CCT diagrams directly represent the non-isothermal conditions of real welding processes. For hardfacing applications where cooling rates can range from 10 to over 1000 degrees Celsius per minute depending on the process, the CCT diagram provides an indispensable map for predicting and controlling microstructure.