Microstructure and Microhardness of 45 Steel Overlay Metal

Literature Overview

This study by Zhang Youyi, Qu Jinshan, and Yang Yue, published in 2009 in the Journal of Xihua University (Natural Science Edition), investigates the microstructure and microhardness of 45 steel overlay metal. The research was conducted by authors affiliated with Sichuan Engineering Vocational and Technical College and the School of Materials Science and Engineering at Xihua University. The study provides fundamental understanding of how welding parameters and material composition influence the microstructure and hardness profile of overlay layers deposited on medium carbon steel substrates.

Core Technical Points

45 steel, a medium carbon steel with approximately 0.42–0.50 percent carbon content, is one of the most widely used structural steels in China. The overlay of additional metal layers onto 45 steel substrates is performed for various purposes, including surface hardening, wear resistance improvement, and dimensional restoration. Understanding the microstructure and hardness distribution of the overlay layer is essential for predicting service performance.

Microstructural Characteristics of the Overlay Layer

The microstructure of the overlay layer deposited on 45 steel is influenced by several factors including the welding process, consumable composition, heat input, and cooling rate. Typical microstructural features include:

Microstructural Feature	Location	Formation Mechanism	Hardness Contribution
Martensite	Overlay weld metal	Rapid cooling from austenite	High (400–600 HV)
Bainite	Overlay weld metal	Moderate cooling rate	Moderate (300–450 HV)
Ferrite-pearlite	Overlay weld metal	Slow cooling	Low (150–250 HV)
Columnar grains	Near fusion line	Directional solidification	Variable
Equiaxed grains	Center of weld	Equiaxed nucleation	Variable

Hardness Distribution Profile

The hardness distribution across the overlay layer and into the base metal is a critical parameter for assessing the effectiveness of the overlay process. A typical hardness profile for a multi-pass overlay on 45 steel shows:

Overlay surface: Highest hardness due to rapid cooling and possible carbide formation, typically 400–550 HV for martensitic microstructures.
Overlay interior: Moderate hardness with a mix of martensite and bainite, typically 300–450 HV.
Overlay-base metal interface: Transition zone with variable hardness depending on dilution and thermal cycling, typically 250–400 HV.
Base metal heat-affected zone: Softened zone due to tempering of existing martensite or reduced hardness from grain growth, typically 150–250 HV.
Base metal: Original hardness of 45 steel, typically 200–250 HV in the normalized condition.

Influence of Welding Parameters on Microstructure

The welding parameters exert significant control over the resulting microstructure and hardness profile:

Heat input: Higher heat input promotes grain growth and reduces cooling rates, leading to softer microstructures with more bainite and ferrite. Lower heat input promotes martensite formation and higher hardness.
Interpass temperature: Controlling the interpass temperature between passes is critical for preventing excessive grain growth and maintaining the desired hardness profile. Typical interpass temperatures for overlay welding on 45 steel are maintained below 250 degrees Celsius.
Welding current and voltage: These parameters directly influence the heat input and weld pool geometry, which in turn affect the solidification rate and microstructure.
Travel speed: Higher travel speeds reduce heat input and promote finer microstructures, while lower travel speeds increase heat input and can lead to coarser grains.

Engineering Practice Considerations

For engineers specifying overlay welding on 45 steel components, several practical considerations must be addressed:

Preheating: Preheating of 45 steel to 150–250 degrees Celsius before overlay welding helps reduce hydrogen-induced cracking risk and moderates the cooling rate.
Post-weld heat treatment: Stress relief annealing at 550–650 degrees Celsius can reduce residual stresses and improve toughness without significantly reducing hardness.
Dilution control: The dilution of overlay material with base metal should be controlled to maintain the desired surface properties. Multi-pass techniques with increasing dilution ratios can be employed to optimize the hardness gradient.
Inspection: Visual and magnetic particle inspection of the overlay surface is essential to detect surface cracks and lack of fusion that could compromise the integrity of the overlay layer.

Study Insights and Reflections

The study by Zhang Youyi and colleagues provides valuable insights into the microstructure-hardness relationships in 45 steel overlay layers, which are fundamental to understanding the performance of overlay-treated components. The systematic investigation of microstructural evolution and hardness distribution across the overlay layer offers engineers a basis for predicting the mechanical behavior of overlay-treated parts under service conditions.

However, it is important to recognize that hardness alone does not fully characterize the mechanical performance of an overlay layer. Properties such as toughness, fatigue resistance, and residual stress state also play critical roles in determining service life. Engineers should complement hardness measurements with other mechanical property tests and non-destructive evaluation techniques to obtain a comprehensive assessment of overlay layer quality.