Post-Weld Tempering Effects on Hardfacing Deposits on 45 Steel Substrate

Literature Overview

Published in Metal Heat Treatment in 2013 by researchers from Chongqing University, this study investigates the effects of post-weld tempering treatment on the microstructure, hardness, and wear resistance of hardfacing deposits applied to 45 steel (medium carbon steel) substrate. The comparison between air-cooled (as-welded) and tempered conditions provides critical guidance for post-weld heat treatment procedures in hardfacing repair operations.

Experimental Conditions and Results

The study compared two post-weld conditions: direct air cooling (as-welded) and tempering at 500–550°C. The hardfacing was applied to 45 steel, which is widely used for molds, shafts, and mechanical components requiring moderate strength and good machinability.

Microstructural Comparison

Condition	Microstructure	Characterization
Air-cooled (as-welded)	Coarse lenticular martensite + abundant retained austenite	High hardness, low toughness
Tempered at 500–550°C	Fine troostite + small amount of retained austenite	Balanced hardness and toughness

Performance Comparison

Property	Air-Cooled	Tempered (500–550°C)	Change
Deposit hardness	Baseline	10% lower than as-welded	Decrease
Base metal hardness near fusion zone	Lower	Increased	Improvement
Hardness gradient at fusion zone	Steep	Reduced	Smoother transition
Wear resistance	Baseline	35% higher	Significant improvement

Metallurgical Analysis

As-Welded Condition

The coarse lenticular martensite with abundant retained austenite is typical of rapid cooling conditions in hardfacing. The high carbon equivalent of the hardfacing deposit combined with rapid solidification from the molten pool produces a highly stressed microstructure. While this condition provides maximum hardness, the retained austenite is metastable and may transform during service, leading to dimensional instability and potential cracking.

Tempered Condition

Tempering at 500–550°C transforms the martensite to troostite (a lamellar structure of ferrite and cementite), which provides a more thermodynamically stable microstructure. The 10% reduction in hardness is acceptable when considered alongside the 35% improvement in wear resistance. This apparent paradox is explained by the fact that wear resistance depends not only on hardness but also on microstructural stability, toughness, and the ability to resist plastic deformation during sliding contact.

Fusion Zone Hardness Gradient

The reduction in hardness gradient at the fusion zone after tempering is particularly significant from a structural integrity perspective. A steep hardness gradient creates stress concentrations that can initiate cracking under thermal or mechanical cycling. The tempered condition produces a more gradual transition, which distributes stresses more evenly and reduces the risk of interfacial cracking.

Engineering Practice Guidelines

Based on this study, the following practical recommendations emerge for hardfacing repair operations on medium carbon steel substrates:

Post-weld tempering is strongly recommended for hardfacing deposits on 45 steel and similar substrates. The 35% wear resistance improvement far outweighs the 10% hardness reduction.
Tempering temperature of 500–550°C provides optimal balance between hardness retention and toughness improvement for martensitic hardfacing deposits.
Fusion zone integrity is significantly improved by tempering, reducing the risk of cracking during service.
The tempering treatment also benefits the base metal near the fusion zone by relieving welding residual stresses and improving the local toughness.

Process Planning Considerations

In a FMEA (Failure Mode and Effects Analysis) framework for hardfacing repairs:

Failure Mode	Cause	Effect	Detection	Prevention
Hardfacing deposit cracking	High residual stress, brittle martensite	Loss of overlay, component failure	Visual inspection, MT	Post-weld tempering
Fusion zone cracking	Hardness gradient, thermal stresses	Through-thickness cracking	UT, PT	Tempering + controlled welding parameters
Dimensional instability	Retained austenite transformation	Component out of tolerance	Dimensional checking	Tempering to stabilize microstructure
Premature wear failure	Insufficient wear resistance	Frequent repair cycles	Service life tracking	Optimized tempering parameters

Study Reflections

This study provides a clear demonstration that maximum hardness does not necessarily equate to optimal service performance. The 35% wear resistance improvement from tempering, despite a 10% hardness reduction, challenges the common misconception that harder always means better for hardfacing applications. The microstructural refinement from coarse martensite to fine troostite creates a more cohesive and fatigue-resistant microstructure that better withstands the complex loading conditions encountered in service.

For practitioners involved in hardfacing repair operations, this work reinforces the importance of post-weld heat treatment as an integral part of the hardfacing process, not as an optional afterthought. The tempering treatment should be specified in the welding procedure specification (WPS) for all hardfacing applications on medium carbon steel substrates, with temperature ranges of 500–550°C providing the optimal balance of properties. The improvement in fusion zone integrity is particularly important for critical applications where cracking at the overlay-substrate interface would lead to complete failure of the repair.