Effect of Normalizing Temperature on Microstructure and Properties of NM360 Wear-Resistant Steel Overlay

Literature Overview

This paper by Deng Hanzhong and Meng Xiangfeng, published in Materials in Mechanical Engineering (Vol. 39, No. 4, 2015), investigates the effect of normalizing temperature on the microstructure and mechanical properties of overlay welds deposited on NM360 wear-resistant steel using D507MoNb electrodes. The study examines microstructure evolution, hardness, impact toughness, and abrasive wear resistance across a range of normalizing temperatures, providing practical guidance for post-weld heat treatment of repaired wear-resistant components.

Core Technical Content

Background: NM360 Wear-Resistant Steel and Overlay Repair

NM360 is a high-strength, wear-resistant steel with typical properties:

• Hardness: 360–400 HV
• Tensile strength: ≥1300 MPa
• Yield strength: ≥1000 MPa
• Impact energy (20°C): ≥34 J
• Composition: C 0.25–0.35%, Mn 1.5–2.0%, Si 0.5–0.8%, Mo 0.5–0.8%, Nb 0.03–0.05%

When NM360 components suffer wear damage, overlay welding repair is a common restoration method. However, the welding process introduces a heat-affected zone (HAZ) with modified microstructure and potentially degraded properties. Post-weld normalizing treatment is used to restore mechanical properties.

Welding Process Parameters

Microstructure Evolution with Normalizing Temperature

Mechanical Property Results

Analysis of Results

Why 900°C Normalizing is Optimal

The 900°C normalizing temperature produces the best comprehensive properties for the following reasons:

1. Adequate austenitization: At 900°C, the microstructure is fully austenitized (Ac₃ for NM360 is approximately 850–870°C), ensuring complete transformation during subsequent cooling.
2. Controlled grain growth: The austenite grain size at 900°C is moderate (9–11 grade), providing a good balance between toughness (fine grains) and strength (adequate grain size for precipitation).
3. Optimal carbide distribution: At 900°C, carbides are partially dissolved and redistributed during the transformation, producing a uniform distribution of fine carbides in the ferrite-pearlite matrix.
4. Avoidance of Widmanstätten: At temperatures above 1000°C, the austenite grain size becomes excessive, and upon cooling, Widmanstätten ferrite forms—coarse, plate-like ferrite that degrades toughness and wear resistance.

Widmanstätten Formation Mechanism

Widmanstätten ferrite forms when:

• Austenite grain size is coarse (>13 grade)
• Cooling rate is moderate (1–10°C/s)
• Alloying elements (Mo, Nb) promote ferrite formation

The Widmanstätten plates are typically 1–5 μm thick and appear as dark, needle-like features in micrographs. They act as stress concentrators and crack initiation sites, significantly reducing impact toughness.

Engineering Practice Integration

Practical Guidelines for NM360 Overlay Repair

Based on the findings of this study and my engineering experience, I recommend the following procedure for NM360 overlay repair:

1. Pre-weld preparation:

• Grind damaged area to remove all worn material
• Bevel the repair area to facilitate proper fusion
• Preheat to 100–150°C to reduce hydrogen-induced cracking risk

1. Welding execution:

• Use D507MoNb or equivalent low-hydrogen electrode
• Maintain interpass temperature below 150°C
• Use short arcs and avoid excessive heat input
• Keep bead width to length ratio ≤1:3

1. Post-weld heat treatment:

• Normalize at 900°C ± 10°C
• Hold for 30–60 minutes (depending on component thickness)
• Air cool to room temperature
• Optionally temper at 550–600°C for 2 hours if toughness is critical

1. Post-treatment inspection:

• Visual examination for cracks
• Magnetic particle testing (MT) for surface and near-surface defects
• Hardness testing (minimum 380 HV)
• Impact testing on coupon if required by specification

Comparison with Other Post-Weld Treatments

Key Insights and Reflections

This study provides clear, actionable guidance for the repair of NM360 components. The key insight is that post-weld normalizing is not merely a "nice to have" treatment but an essential step for achieving acceptable mechanical properties in overlay repairs of wear-resistant steels.

Several practical considerations deserve emphasis:

1. The narrow optimal window: The difference between 900°C (optimal) and 1050°C (poor) is only 150°C, yet the property degradation is dramatic. This underscores the importance of precise temperature control during normalizing.
2. Widmanstätten as a red flag: The appearance of Widmanstätten ferrite at 1050°C serves as a visual indicator that the normalizing temperature has exceeded the optimal range. Engineers should be trained to recognize this microstructure during quality inspection.
3. Property trade-offs: No single normalizing temperature optimizes all properties simultaneously. The 900°C treatment provides the best balance, but if maximum hardness is required (at the expense of toughness), a lower temperature (850°C) or no normalizing may be appropriate for non-structural applications.
4. Scale-up considerations: The study was conducted on laboratory coupons. For large components, thermal gradients during normalizing can cause differential microstructure and residual stress. For thick sections (>50 mm), controlled cooling rates and possibly isothermal transformation treatments should be considered.

The systematic approach of testing multiple normalizing temperatures and correlating microstructure with properties exemplifies good metallurgical engineering practice. This methodology should be applied whenever developing repair procedures for critical wear-resistant components, as the optimal parameters may vary with base material composition, component geometry, and service conditions.