Nitrogen-Alloyed Overlay Alloy Reciprocating Friction Wear Behavior Study

Literature Overview

The paper published in Tribology (Chinese and English), Volume 40, Issue 5, 2020 (pages 586–592), by Li Jiaqi, Yang Ke, Wang Qiuyu, and colleagues from Hohai University, investigates the reciprocating friction and wear behavior of nitrogen-alloyed overlay alloys. The research introduces nitrogen into martensitic stainless steel through niobium and titanium fixation, and compares the tribological performance of nitrogen-added and non-nitrogen overlay alloys under different loads (5 N, 10 N, and 15 N) using a reciprocating friction-wear testing machine. This work is particularly relevant to engineers designing hardfaced components for applications involving cyclic loading and sliding contact, such as pump impellers, valve seats, and reciprocating tool inserts.

Core Technical Findings

The fundamental wear mechanisms identified in both alloy types include abrasive wear and surface fatigue wear. During reciprocating friction, the overlay surface endures periodic loading, producing distinct cutting traces and plastic deformation. The nitrogen-alloyed variant demonstrates significantly improved performance through two primary strengthening mechanisms: grain refinement and dispersion strengthening. Carbonitrides precipitate uniformly along the martensitic matrix boundaries and grain boundaries, increasing the resistance to plastic deformation and enhancing resistance to abrasive wear. The wear surface of the nitrogen-alloyed specimen exhibits fewer and shallower cutting traces, with markedly improved resistance to fatigue spalling.

Strengthening Mechanisms Comparison

Process and Metallurgical Analysis

The nitrogen fixation strategy using Nb and Ti is metallurgically sound. Both elements have extremely high affinities for nitrogen, forming thermodynamically stable carbonitrides (NbCN, TiCN) with lattice parameters closely matching austenite and martensite. This reduces lattice strain and minimizes the risk of cracking during solidification. The typical composition window for effective nitrogen fixation requires [Nb] and [Ti] to be maintained at levels ensuring nitrogen activity in the melt remains below the solubility limit of austenite, typically [N] < 0.15 wt% with sufficient carbide former content.

The martensitic stainless steel base provides inherent work-hardening capability, which synergizes with the dispersion-strengthened microstructure. Under reciprocating contact, the material experiences alternating compressive and tensile stresses at the surface. The carbonitride particles act as effective barriers to dislocation motion, delaying the onset of plastic instability that leads to fatigue spalling. This is consistent with the Hall-Petch relationship and Orowan strengthening theory, where the strengthening increment Δσ is proportional to the volume fraction and inversely proportional to the mean interparticle spacing of the dispersed phase.

Engineering Practice Implications

From a practical standpoint, this research directly informs the selection of overlay welding consumables for components subjected to reciprocating wear. In the oil and gas industry, downhole tools and reciprocating pump plungers experience exactly this type of cyclic loading. The findings suggest that nitrogen-alloyed overlay systems should be specified for applications where the combination of abrasive and fatigue wear dominates.

However, several practical considerations must be addressed:

• Nitrogen ingress control: During welding, excessive nitrogen pickup can lead to porosity. Shielding gas composition and arc stability must be carefully managed.
• Post-weld heat treatment: A tempering cycle at 550–600 °C may be required to relieve welding residual stresses while preserving the beneficial carbonitride dispersion.
• Dilution management: The base metal dilution affects the final nitrogen content in the overlay. Multi-pass welding with appropriate interpass temperature control is essential.

Key Questions and Reflections

A notable aspect of this study is the relatively low load range (5–15 N) used for reciprocating testing. While this provides excellent resolution for distinguishing microstructural differences, it does not directly correlate with heavy-duty industrial applications where contact pressures may reach several hundred MPa. Future work should extend the load range and incorporate lubrication conditions to better simulate real operating environments. Additionally, the study does not address the long-term stability of the carbonitride dispersion under prolonged thermal cycling, which is critical for components operating at elevated temperatures.

The choice of Nb and Ti as nitrogen fixers is well-established in metallurgy, but the optimal ratio between the two elements remains an open question. NbCN has a lower melting point than TiCN, which may affect solidification behavior and microsegregation patterns in multi-pass welds. This warrants further investigation for thick-section overlay applications.

Study Insights and Outlook

This research contributes valuable data to the growing body of knowledge on nitrogen-enhanced overlay welding systems. The dual strengthening mechanism — grain refinement combined with dispersion strengthening — provides a clear metallurgical rationale for the improved wear resistance. For engineers involved in overlay welding specification and consumable selection, the key takeaway is that nitrogen alloying, when properly controlled, offers a cost-effective route to enhance tribological performance without requiring expensive alloying elements such as cobalt or chromium in excessive quantities. The work underscores the importance of microstructural engineering in overlay welding, where the final performance is governed not merely by composition but by the size, distribution, and morphology of secondary phases within the weld metal.