Microstructure and Wear Resistance of Fe-Cr-C-B-N Surfacing Alloys

Literature Overview

This paper by Liu Zhengjun and colleagues, published in the Welding Journal (Vol. 38, No. 6, 2017, pp. 105-109), investigates the microstructure and abrasive wear performance of Fe-Cr-C-B-N system surfacing alloys deposited on low-carbon steel via open-arc surfacing. Funded by the Liaoning Provincial Department of Education General Research Fund (L2015075), this work was conducted at Shenyang University of Technology and Dalian Ocean University. The study focuses on the effect of nitrogen content on the microstructure evolution and wear behavior, addressing the increasingly important role of nitrogen as a microalloying element in surfacing alloy design.

Core Technical Findings

The surfacing alloys were deposited using flux-cored wire (药芯焊丝) via open-arc surfacing, and characterized using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), Rockwell hardness testing, and wet sand abrasion testing. The key results are summarized below:

Nitrogen Content (wt%)	Microstructure	Hardness (HRC)	Wear Loss (g)	Performance Assessment
0 (N-free reference)	Martensite + austenite + M23(C,B)6 + M3(C,B) + M2B	Lower	Higher	Baseline
0.17	Martensite + austenite + BN + M23(C,B)6 + M3(C,B) + M2B	62.7	0.0542	Optimal
Higher N	Excessive BN, reduced eutectic content	Higher but with degradation	Increased	Suboptimal

The microstructure of the surfacing layer consisted of martensite + austenite + BN + M23(C,B)6 + M3(C,B) + M2B. As nitrogen content increased, BN formed in the microstructure, primary austenite transformed to acicular martensite, and the quantity of interdendritic eutectic organization decreased. The hardness increased with nitrogen content, but the wear loss exhibited a non-monotonic trend, first decreasing and then increasing. At 0.17 wt% N, the matrix microstructure and hard phases achieved optimal matching, yielding a hardness of 62.7 HRC and a wear loss of only 0.0542 g.

Interpretation of Technical Points

The formation of BN (boron nitride) with increasing nitrogen content is a significant microstructural feature. BN is a hard phase with a hardness of approximately 2,500-3,000 HV, and its formation provides additional wear resistance to the surfacing alloy. However, the non-monotonic wear behavior indicates that there is an optimal nitrogen content beyond which the benefits of BN formation are offset by other microstructural changes.

The transformation of primary austenite to acicular martensite with increasing nitrogen is attributed to the nitrogen-induced shift in the austenite stability range. Nitrogen is a strong austenite stabilizer, but at higher concentrations, it can promote the formation of nitrogen-rich austenite that transforms to martensite during cooling. The acicular martensite morphology provides good hardness and wear resistance, but excessive amounts can lead to brittleness and cracking.

The reduction in interdendritic eutectic content with increasing nitrogen is explained by the nitrogen-induced change in solidification behavior. Nitrogen promotes the formation of BN and other nitrogen-containing phases that compete with the conventional eutectic phases (M23(C,B)6, M3(C,B)) for alloying elements. This competition reduces the volume fraction of the interdendritic eutectic, which can be beneficial for wear resistance but detrimental to toughness.

The optimal nitrogen content of 0.17 wt% represents a balance between hard phase formation and matrix toughness. At this composition, the BN, M23(C,B)6, M3(C,B), and M2B phases are distributed in a manner that provides effective wear resistance without creating excessive brittleness or cracking susceptibility. The hardness of 62.7 HRC (approximately 750 HV) is well above the typical wear resistance threshold of 50 HRC for severe abrasion applications.

Engineering Practice Implications

The findings have direct relevance to the design and specification of Fe-Cr-C-B-N surfacing alloys for industrial wear applications:

Nitrogen Content Control: The narrow optimal window of approximately 0.15-0.20 wt% N requires careful control of nitrogen pickup during welding. Factors such as flux composition, shielding gas purity, and welding speed can influence the nitrogen content in the final deposit.
Welding Process Selection: Open-arc surfacing with flux-cored wire was used, which provides adequate shielding and nitrogen control. Alternative processes such as submerged arc welding (SAW) or plasma surfacing may offer different nitrogen pickup characteristics that require separate optimization.
Application Selection: The Fe-Cr-C-B-N system is particularly suited for applications requiring high wear resistance in dry or wet abrasion environments, such as mining equipment, cement grinding components, and agricultural machinery.

Key Questions and Reflections

A significant question raised by this work is the long-term stability of the BN phase under service conditions. BN can undergo oxidation at elevated temperatures, and its thermal stability may be limited in high-temperature applications. The wear testing was performed under ambient conditions, and the behavior of the BN phase under thermal cycling or elevated temperature service should be evaluated.

Another important consideration is the effect of nitrogen on the weldability and cracking susceptibility of the surfacing alloy. Nitrogen can promote hydrogen-induced cracking, particularly in high-strength martensitic microstructures. The open-arc surfacing process may introduce additional hydrogen from moisture in the flux or shielding gas, and the combined effect of nitrogen and hydrogen on cracking should be investigated.

Furthermore, the study did not address the effect of post-weld heat treatment on the microstructure and wear performance. Tempering or solution treatment could potentially optimize the distribution of hard phases and improve toughness without sacrificing wear resistance.

Study Insights and Implications

This research demonstrates that nitrogen is an effective microalloying element for improving the wear resistance of Fe-Cr-C-B surfacing alloys, but only within a narrow composition window. The formation of BN as an additional hard phase provides a mechanism for wear resistance enhancement that is complementary to the conventional carbide and boride phases. For practitioners designing Fe-Cr-C-B-N surfacing alloys, the key takeaway is that nitrogen content must be carefully controlled to achieve the optimal balance between hard phase formation and matrix toughness. The non-monotonic wear behavior underscores the importance of systematic composition optimization rather than simply maximizing hard phase content.