Effect of Annealing Temperature on the Microstructure and Properties of NM360 Wear-Resistant Steel Cladding Layer

Research Background and Motivation

NM360 is a widely used wear-resistant steel grade characterized by a martensitic microstructure with hardness in the range of 360 HV, designed for applications requiring moderate wear resistance combined with good toughness and weldability. When NM360 is used as a cladding layer on carbon steel or low-alloy steel substrates, the annealing treatment applied after welding becomes a critical process variable that determines the final microstructure, mechanical properties, and long-term service performance. The referenced literature systematically investigates the effect of annealing temperature on the NM360 cladding layer, providing essential data for post-weld heat treatment (PWHT) specification.

Experimental Methodology and Test Matrix

The study employed NM360 cladding layers deposited by submerged arc welding (SAW) onto Q345B carbon steel substrates, with cladding thicknesses of 6 mm and 10 mm. The as-welded cladding layers were subjected to annealing at five temperature levels: 500 °C, 550 °C, 600 °C, 650 °C, and 700 °C, with a holding time of 2 hours per 25 mm of thickness, followed by furnace cooling. The following table presents the comprehensive test matrix and key results:

Annealing Temperature (°C)	Hardness (HV30)	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)	Impact Energy (J, -20°C)	Retained Austenite (%)
As-welded	410–430	780–820	950–1000	12–15	28–35	8–12
500	395–415	750–790	920–970	13–16	32–38	6–9
550	375–395	700–740	880–930	14–17	38–45	4–7
600	355–375	650–690	840–890	15–18	45–55	2–5
650	340–360	600–640	800–850	16–20	50–60	1–3
700	320–340	550–590	760–810	17–21	55–65	0–1

Microstructural Evolution with Annealing Temperature

The annealing treatment induced a systematic transformation of the as-welded microstructure. The as-welded NM360 cladding layer consisted of tempered martensite with retained austenite islands and dispersed carbides. As the annealing temperature increased, the following microstructural changes occurred:

500 °C annealing: Partial tempering of the martensite occurred, with fine carbide precipitation within the martensite laths. The retained austenite remained largely stable, and the overall microstructure retained its high-density dislocation structure. This condition provided the best balance between hardness and toughness.
550 °C annealing: Further carbide coarsening and spheroidization began, with carbide size increasing from 50–100 nm to 100–200 nm. The martensite lath boundaries became more distinct, and the dislocation density decreased significantly. Retained austenite began to decompose, transforming to ferrite and cementite.
600 °C annealing: The microstructure transitioned to a tempered martensite with well-defined carbide networks. The retained austenite was largely decomposed, and the matrix hardness dropped below the NM360 nominal specification of 360 HV. The microstructure approached a sorbite-like appearance.
650 °C annealing: Significant grain growth and carbide coarsening occurred. The microstructure evolved toward a coarse pearlite-like structure with lamellar carbides. The hardness dropped substantially, and the material lost its wear-resistant character.
700 °C annealing: Full spheroidization of carbides and significant grain growth produced a soft ferrite-pearlite microstructure. This condition was unsuitable for wear-resistant service as the hardness fell below 340 HV.

Mechanical Property Analysis

The mechanical property evolution revealed several important trends. Hardness decreased monotonically with increasing annealing temperature, with a rate of approximately 3–4 HV per 50 °C increase. This linear relationship provides a practical tool for predicting hardness from annealing temperature. Yield strength and tensile strength followed similar trends, with the tensile-to-yield ratio remaining relatively constant at 1.20–1.25, indicating consistent ductile-to-brittle transition behavior.

The most significant finding was the improvement in impact energy with increasing annealing temperature. The -20 °C Charpy impact energy increased from 28–35 J in the as-welded condition to 55–65 J at 700 °C, representing a nearly doubling of toughness. This improvement was attributed to the reduction of retained austenite (which can transform to brittle martensite under impact loading) and the tempering of the martensitic matrix.

Weldability and Residual Stress Considerations

The annealing temperature also significantly influenced the residual stress state and weldability of the NM360 cladding system. As-welded residual stresses in the cladding layer typically reached 300–450 MPa, which posed a risk of stress corrosion cracking and fatigue failure. Annealing at 550–600 °C reduced residual stresses by 40–60%, while annealing at 650–700 °C achieved 70–85% stress relief.

However, excessive annealing temperatures introduced new concerns. At temperatures above 620 °C, the base metal (Q345B) began to undergo stress relief and microstructural changes, potentially reducing the base metal strength. Additionally, the heat-affected zone (HAZ) between the cladding and base metal experienced differential thermal expansion during cooling, which could introduce new residual stresses if the annealing temperature was too high or the cooling rate too rapid.

Optimal Annealing Temperature Determination

Based on the comprehensive data, the optimal annealing temperature for NM360 cladding layers was identified as 550–600 °C. This range provides:

Hardness within the NM360 specification range (355–395 HV)
Impact energy exceeding 38 J at -20 °C, meeting most industrial toughness requirements
Residual stress reduction of 50–65%, significantly reducing cracking risk
Minimal degradation of the base metal properties
Retained austenite content below 7%, reducing the risk of delayed cracking

For applications requiring higher toughness (such as cryogenic service or high-impact loading), annealing at 600–650 °C may be acceptable, provided that the resulting hardness reduction is acceptable for the specific wear service conditions.

Engineering Practice and Process Recommendations

The following process recommendations are derived from the study findings:

Standard annealing specification: 550–600 °C for 2 hours per 25 mm thickness, furnace cooling to below 300 °C before air cooling.
High-toughness specification: 600–650 °C for 2 hours per 25 mm thickness, furnace cooling to below 200 °C before air cooling.
Pre-annealing inspection: Hardness testing of the as-welded cladding layer should be performed before annealing to establish a baseline and verify cladding quality.
Post-annealing verification: Hardness, impact testing, and non-destructive examination (MT or PT) should be performed after annealing to verify the final properties.
Temperature uniformity: The furnace temperature uniformity should be maintained within ±15 °C across the workpiece to prevent differential microstructural evolution.

Key Technical Insights and Reflections

The most important insight from this study is that the annealing temperature for NM360 cladding layers is not merely a stress relief parameter but a microstructure-tuning tool that directly determines the final wear resistance and toughness of the cladding. The study challenges the common industry practice of using a single PWHT temperature for all cladding applications, demonstrating that the optimal temperature must be selected based on the specific balance of wear resistance and toughness required by the application.

Another critical finding is the role of retained austenite as a double-edged factor. While retained austenite contributes to the as-welded toughness through transformation toughening, it also poses a risk of delayed martensitic transformation under service loading, which can cause cracking. The annealing treatment effectively manages this risk by converting unstable retained austenite to stable ferrite-carbide structures.

Summary

This study provides a comprehensive understanding of how annealing temperature governs the microstructure and mechanical properties of NM360 wear-resistant steel cladding layers. The optimal annealing range of 550–600 °C achieves the best balance between maintaining wear-resistant hardness (355–395 HV) and achieving acceptable toughness (impact energy >38 J at -20 °C) while significantly reducing residual stresses. For engineers specifying post-weld heat treatment for NM360 cladding applications, this study provides quantitative data to support temperature selection based on the specific mechanical property requirements of the service environment. The systematic approach to annealing temperature optimization demonstrated in this study should be adopted as standard practice in cladding quality control procedures.