Effect of Chromium on Fe-B Overlay Alloy Microstructure and Properties

Literature Overview

This paper by Wei Ruhuan, Han Mingzhen, Jing Rui, and Liu Qicong, published in Materials Protection in 2023, investigates the influence of chromium content on the microstructure, hardness, and wear resistance of Fe-B overlay alloys deposited by plasma arc welding onto Q235 steel substrates. The study was supported by a Jiamusi University student innovation and entrepreneurship training program (2021xj13), and it represents a systematic materials science investigation of alloy design for wear-resistant overlay applications.

Experimental Design and Methodology

The authors deposited Fe-B-Cr overlay alloys with varying chromium content (designated Cr-0, Cr-2, Cr-4, Cr-6, Cr-8, and Cr-10, referring to the chromium content in weight percent) onto Q235 steel substrates using plasma arc welding. The microstructure was characterized using optical metallography, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Mechanical properties were evaluated through Vickers hardness testing and pin-on-disk wear testing.

The experimental design follows a classical materials science approach: a single variable (chromium content) is systematically varied while all other parameters are held constant, allowing the effect of chromium to be isolated and quantified. This approach is particularly valuable for establishing design guidelines for overlay alloy development.

Microstructural Evolution with Chromium Content

The XRD analysis revealed that chromium dissolves into both the ferrite matrix and the iron boride phase, forming (Fe,Cr) and (Fe,Cr)₂B phases respectively. Importantly, the addition of chromium did not produce any new phases beyond those already present in the base Fe-B system. The microstructure of all compositions consisted of Fe₂B (or (Fe,Cr)₂B) and a eutectic mixture of ferrite and boride.

The SEM analysis showed that the boride phase became finer and more uniformly distributed as chromium content increased. This refinement is attributed to the solid solution strengthening effect of chromium, which increases the lattice strain and promotes the nucleation of finer boride particles during solidification. The finer boride dispersion provides more effective resistance to abrasive wear, as the hard boride particles are more uniformly distributed and less prone to being pulled out from the matrix during wear.

Hardness and Wear Resistance Correlation

The hardness increased monotonically with chromium content, from approximately 50 HRC for Cr-0 to 67 HRC for Cr-10. The wear resistance showed a similar trend, with the wear loss decreasing from 0.147 g for Cr-0 to 0.0275 g for Cr-10, representing a 375 percent improvement in relative wear resistance. The improvement in wear resistance is attributed to both the increased hardness and the refined boride dispersion.

However, it is important to note that the relationship between hardness and wear resistance is not strictly linear. The wear resistance improvement per unit hardness increase is greater at lower chromium contents and diminishes at higher chromium contents. This suggests that beyond a certain chromium level, the marginal benefit of additional chromium for wear resistance decreases, while the cost and potential brittleness increase.

Engineering Relevance and Alloy Design Guidelines

For practical overlay welding applications, this study provides valuable guidance on chromium content selection. For applications where moderate wear resistance is required and cost is a concern, a chromium content of 2 to 4 percent provides a good balance of performance and economy. For severe wear conditions, such as those encountered in mining equipment, cement mill liners, or pipe wear protection, a chromium content of 8 to 10 percent is recommended, as it provides significantly enhanced wear resistance.

The study also highlights the importance of understanding the microstructure-property relationship in overlay alloy design. Simply increasing hardness through alloying does not necessarily translate to proportional improvement in wear resistance. The microstructural refinement that accompanies chromium addition is equally important, as it provides a more uniform and stable wear-resistant structure.

Study Insights

The most significant contribution of this study is the demonstration that chromium addition to Fe-B overlay alloys achieves a dual benefit: solid solution strengthening of the ferrite matrix and refinement of the boride phase. This dual mechanism explains why the wear resistance improvement is greater than would be predicted from hardness alone. For engineers developing new overlay alloys, this finding suggests that alloying elements should be selected not only for their effect on hardness but also for their influence on microstructural refinement, as the latter often provides a disproportionate contribution to wear resistance.