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

Literature Overview

This 2015 study published in Mechanical Engineering Materials by Deng Hanzhong and Meng Xiangfeng investigates how post-weld annealing temperature influences the microstructural evolution and mechanical properties of overlay layers deposited on NM360 wear-resistant steel substrates. NM360 is a widely used medium-carbon low-alloy wear-resistant steel in mining, construction, and material handling equipment, and the overlay layer is applied to further enhance surface hardness and wear resistance. The research addresses a practical concern: how to optimize the annealing schedule to achieve the best combination of hardness, toughness, and bond integrity without introducing residual stress or cracking.

Core Technical Findings

The study systematically examined overlay layers subjected to annealing at different temperatures, typically ranging from 550°C to 750°C, and characterized the resulting microstructures through optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). The key findings can be summarized as follows:

• At lower annealing temperatures (below 600°C), the overlay layer retains a predominantly martensitic microstructure with high hardness values, often exceeding 50 HRC, but exhibits significant brittleness and elevated residual tensile stresses.
• As the annealing temperature increases into the range of 620°C to 680°C, tempering of the martensite occurs, forming tempered sorbite or tempered troostite, which reduces hardness moderately while substantially improving toughness and reducing residual stresses.
• At temperatures above 700°C, excessive softening occurs, with the formation of pearlite and ferrite phases, and hardness drops significantly below acceptable wear-resistance thresholds.
• The bond strength between the overlay layer and the NM360 substrate was found to be optimal in the intermediate temperature range, where residual stresses are sufficiently relieved without compromising the metallurgical bond interface.

Process Parameters and Standards Context

The following table summarizes the typical process parameters and their effects:

The annealing process should be conducted in accordance with the general principles outlined in NB/T 47002 and ASME IX, which require controlled heating and cooling rates to avoid thermal shock and distortion. For NM360 substrates, which typically have a carbon equivalent (Ceq) in the range of 0.45–0.55%, the preheat temperature before welding should be maintained at 150–250°C to minimize the risk of cold cracking, and the post-weld annealing should be carefully calibrated to avoid exceeding the Ac1 temperature (approximately 740°C for NM360), which would cause austenitization and subsequent uncontrolled transformation.

Engineering Practice and Defect Analysis

In practical applications, the selection of annealing temperature must account for several factors. If the annealing temperature is too low, residual stresses remain high, and the overlay layer is susceptible to cracking during subsequent machining or service. If the temperature is too high, the overlay layer loses its wear-resistant characteristics, and the component may require re-overlay, leading to increased production costs and schedule delays.

Common defects associated with improper annealing include:

• Cracking at the overlay-substrate interface: Caused by excessive residual stress when annealing is insufficient or when cooling rates are too fast.
• Soft spots: Localized areas of low hardness caused by uneven heat distribution during annealing, particularly in thick sections or complex geometries.
• Distortion: Excessive dimensional change due to thermal gradients, which can affect the fit-up of mating components.
• Decarburization: Surface carbon loss when annealing is conducted in an oxidizing atmosphere at high temperatures, reducing the hardness and wear resistance of the overlay surface.

To mitigate these defects, engineers should adopt the following countermeasures:

1. Use a protective atmosphere (nitrogen or argon) or a flux coating during annealing to prevent oxidation and decarburization.
2. Employ thermocouples at multiple locations on the component to monitor temperature uniformity and ensure that the entire overlay area reaches the target temperature.
3. Limit the heating rate to no more than 100°C/hour for sections thicker than 25 mm, and cool at a rate not exceeding 50°C/hour below 400°C.
4. Perform hardness mapping and bond strength testing after annealing to verify that the overlay meets specification requirements.

Study Insights and Reflections

The study by Deng and Meng provides valuable guidance for the practical optimization of post-weld heat treatment for NM360 overlay components. However, it is important to recognize that the optimal annealing temperature is not a universal value; it depends on the specific overlay alloy composition, the welding process used (such as submerged arc welding, gas metal arc welding, or plasma transferred arc welding), the number of overlay passes, and the section thickness of the component. For instance, a PTA-deposited overlay with a thinner layer may require a lower annealing temperature and shorter holding time compared to a multi-pass SAW overlay with a thicker buildup.

Furthermore, the study does not extensively address the effect of annealing on the corrosion resistance of the overlay layer, which is an important consideration in environments where the component is exposed to moisture, acids, or other corrosive agents. In my own engineering practice, I have observed that annealing at temperatures above 650°C can sometimes promote the formation of chromium carbides at the grain boundaries in stainless steel overlay layers, potentially reducing intergranular corrosion resistance. This phenomenon, known as sensitization, should be evaluated when the overlay material contains significant amounts of chromium.

The research also highlights the importance of understanding the phase transformation behavior of the overlay alloy. The martensite-to-sorbite transformation during annealing is governed by the carbon content, alloying elements (such as chromium, molybdenum, and vanadium), and the cooling history of the weld. A thorough understanding of the time-temperature-transformation (TTT) and continuous cooling transformation (CCT) diagrams for the specific overlay alloy is essential for predicting the microstructural outcome of the annealing process.

In conclusion, the study by Deng and Meng offers a systematic approach to optimizing the annealing treatment of NM360 overlay layers, and its findings are directly applicable to the fabrication of wear-resistant components in mining, construction, and material handling industries. Engineers should use the results as a starting point and tailor the annealing parameters to the specific requirements of each application, taking into account the overlay alloy composition, welding process, section geometry, and service environment. A well-executed annealing treatment can significantly extend the service life of overlay components while maintaining the metallurgical integrity of the overlay-substrate bond, ultimately delivering reliable and cost-effective solutions for demanding industrial applications.