Microstructure and Microhardness of 45 Steel Cladding Metal

Literature Overview

This 2009 study published in the "Journal of Xihua University (Natural Science Edition)" by Zhang Youyi, Qu Jinshan, and Yang Yue from Sichuan Vocational and Technical College of Engineering and Xihua University investigates the microstructure and microhardness distribution of cladding metal deposited on 45 steel (a medium-carbon steel equivalent to AISI 1045). The study focuses on understanding the metallurgical characteristics of the overlay layer produced by welding processes on this widely used structural steel substrate, which is common in mechanical components, shafts, gears, and general engineering applications requiring surface hardening or wear resistance.

Core Technical Analysis

Microstructural Characteristics of the Cladding Layer

The microstructure of the cladding metal deposited on 45 steel is determined by the interaction between the filler metal composition, the welding heat input, the cooling rate, and the dilution from the base metal. For typical carbon steel or low-alloy steel cladding on 45 steel, the microstructure evolution follows the following pattern:

The study reveals that the hardness gradient from the cladding surface to the base metal is a direct consequence of the varying dilution levels and cooling rates experienced by each successive layer. The first deposited layer experiences the highest dilution from the 45 steel substrate (approximately 0.45% C), resulting in a higher carbon equivalent and consequently harder microstructure. Subsequent layers experience progressively lower dilution as the previous cladding layers provide a more compatible substrate.

Heat Input and Cooling Rate Effects

The welding heat input is the primary process parameter controlling the cladding microstructure. Higher heat input leads to:

• Slower cooling rates and coarser microstructures
• Greater dilution from the base metal
• Potentially softer cladding layers with lower hardness but improved toughness
• Reduced residual stresses but increased risk of softening in the HAZ

Conversely, lower heat input produces:

• Faster cooling rates and finer microstructures
• Harder cladding layers but with increased brittleness
• Higher residual stresses that may cause cracking
• Reduced dilution but potentially incomplete fusion

For 45 steel cladding applications, a heat input range of 0.5–1.5 kJ/mm is typically optimal, balancing hardness requirements with crack resistance and bond strength.

Microhardness Distribution and Engineering Significance

The microhardness profile across the cladding layer is critical for predicting wear resistance and fatigue performance. The study demonstrates that the maximum hardness is typically found in the first pass region where the carbon dilution from the 45 steel substrate is highest, creating a martensitic microstructure. This region also represents the most vulnerable zone for cracking due to high residual stresses and the presence of retained austenite that may transform under subsequent loading or thermal cycling.

Key Reflections and Practical Implications

The findings from this study have direct relevance to the repair and hardfacing of 45 steel components in industrial settings. Engineers must understand that the cladding process on medium-carbon steels creates a complex microstructural gradient that requires careful control to achieve the desired balance of hardness, toughness, and durability. The use of preheating (typically 200–300 °C for 45 steel) and controlled interpass temperatures (maintained below 300 °C) are essential practices to minimize cracking risks in the high-carbon-dilution regions.

From a standards perspective, the qualification of cladding procedures on 45 steel substrates must consider the higher carbon equivalent of this material compared to low-carbon steels. The dilution effect must be accounted for in the procedure qualification, and the final hardness and microstructure of the bond line region must be verified through metallographic examination and microhardness testing in accordance with applicable standards such as NB/T 47014 or ASME IX.

This study provides valuable baseline data for engineers working with carbon steel cladding systems and highlights the importance of understanding the metallurgical interactions at the cladding-base metal interface for reliable component performance.