Effect of Normalizing Temperature on NM360 Wear-Resistant Steel Surfacing Layer

Literature Overview

This 2015 study by Deng Hanzhong and Meng Xiangfeng investigates the influence of normalizing temperature on the microstructure and mechanical properties of surfacing layers deposited on NM360 wear-resistant steel using D507MoNb electrode. The research examines how post-weld heat treatment (normalizing) at different temperatures affects microstructure evolution, hardness, impact toughness, and abrasive wear resistance of the surfacing layer.

Core Technical Content

Base Material and Surfacing Configuration

NM360 is a high-strength wear-resistant steel with a base hardness of approximately 360 HV, containing elements such as Cr, Mo, Nb, and V that promote fine carbide formation. The D507MoNb electrode is a low-hydrogen basic electrode designed for hardfacing applications, depositing a layer rich in Mo and Nb carbides.

Microstructural Evolution with Normalizing Temperature

The as-welded surfacing layer consists of martensite matrix with dispersed carbide particles (Mo₂C, NbC, and mixed carbides). After normalizing treatment, the microstructure transforms as follows:

Normalizing Temperature	Microstructure	Grain Size	Hardness (HV)	Impact Energy (J)	Wear Rate (mg/1000m)
As-welded	Martensite + carbides	Fine	650–700	15–20	25–35
800 °C	Ferrite + pearlite + carbides	Fine	500–550	40–50	20–30
850 °C	Ferrite + pearlite + carbides	Medium	480–520	45–55	18–28
900 °C	Ferrite + pearlite + carbides	Medium	450–500	50–60	15–25
950 °C	Ferrite + pearlite + carbides	Coarse	420–460	45–55	20–30
1000 °C	Ferrite + pearlite + carbides	Coarse	380–420	40–50	25–35
1050 °C	Ferrite + pearlite + carbides + Widmanstätten	Very coarse	350–400	30–40	30–40

Optimal Normalizing Temperature

The study identifies 900 °C as the optimal normalizing temperature for NM360 surfacing layers, providing the best balance between hardness, toughness, and wear resistance. At this temperature:

The martensite fully transforms to ferrite and pearlite, eliminating residual stresses
Carbide particles maintain their dispersed distribution without excessive coarsening
The grain structure is refined to an optimal size that supports both strength and toughness
The Widmanstätten structure has not yet formed, which would degrade toughness

Heat Treatment Process Analysis

Normalizing Mechanism in Surfacing Layers

Normalizing of surfacing layers differs from normalizing of homogeneous weldments due to several factors:

Composition gradient: The surfacing layer has a different composition from the base metal, creating a heterogeneous system
Thermal history: The surfacing layer has already experienced rapid cooling during welding, creating a metastable microstructure
Carbide stability: Refractory carbides (NbC, Mo₂C) remain stable during normalizing and act as nucleation sites for grain refinement
Residual stress relief: Normalizing effectively relieves welding residual stresses that could cause delayed cracking or distortion

Critical Temperature Considerations

Temperature Range	Metallurgical Event	Effect on Properties
< 727 °C	Subcritical annealing	Minimal change, stress relief only
727–850 °C	Pearlite formation begins	Gradual softening
850–950 °C	Complete austenitization, fine grain	Optimal property balance
950–1050 °C	Grain coarsening, Widmanstätten formation	Degraded toughness
> 1050 °C	Excessive grain growth, carbide coarsening	Severe property degradation

Engineering Practice Recommendations

Welding Procedure for NM360 Surfacing

Preheating: 150–200 °C to reduce base metal thermal gradient and prevent base metal cracking
Welding parameters: Current 200–250 A, arc voltage 28–32 V, travel speed 300–400 mm/min
Interpass temperature: Maintain below 250 °C
Number of passes: 2–3 passes for typical 3–5 mm overlay thickness
Post-weld treatment: Normalize at 900 °C, hold for 30–60 min per 25 mm thickness, air cool

Quality Control Points

Verify hardness uniformity across the surfacing layer (acceptable range: 450–500 HV at 900 °C normalized condition)
Conduct impact testing on representative samples (minimum 50 J at 20 °C)
Perform metallographic examination to confirm absence of Widmanstätten structure
Measure carbide distribution and size (maximum individual carbide size < 50 μm)

Study Insights and Practical Implications

This research provides clear guidance for engineers working on repair and hardfacing applications for wear-resistant steel components. The key insight is that the as-welded condition, while offering the highest hardness, is not necessarily optimal for service performance due to excessive brittleness and residual stress. The 900 °C normalizing treatment represents a practical compromise that maintains adequate hardness while significantly improving toughness and reducing cracking susceptibility.

The observation that Widmanstätten structure forms at 1050 °C is particularly important for quality control. In production settings, temperature control during normalizing is critical, and engineers should ensure that furnace temperature uniformity is maintained within ±10 °C to prevent localized Widmanstätten formation.

For maintenance and repair operations where post-weld heat treatment may not be available, the as-welded condition can be used provided the application does not involve impact loading or cyclic stress. In such cases, additional passes with reduced heat input and careful interpass temperature control can partially mitigate the brittleness issue.