Effect of Chromium Content on Microstructure and Abrasive Wear Resistance of Fe-Cr-B-C Overlay Alloys After Heat Treatment

Literature Overview

This 2015 study by Wang Shenglin and colleagues, published in Transactions of Materials and Heat Treatment, investigates the influence of chromium content on the microstructure and abrasive wear resistance of Fe-Cr-B-C system overlay alloys fabricated using flux-cored wire (FCW) overlay welding on Q235 steel. The research was funded by the Beijing Municipal Education Commission Science and Technology Project (Grant No. PXM2014_014204_07_000040). The study is particularly relevant for engineers designing wear-resistant overlay systems for mining, cement, and material handling equipment where abrasive wear is the dominant failure mode.

Core Technical Findings

Four chromium levels (12%, 14%, 16%, and 18% by mass) were investigated in the Fe-Cr-B-C overlay alloy system. The overlay alloys were subjected to 600 degrees Celsius heat treatment, and both as-welded and heat-treated conditions were evaluated.

The primary hard phase identified is (Fe,Cr)23(B,C)6, a complex borocarbide that provides the wear resistance of the overlay layer. At 14% Cr, this hard phase remains stable at elevated temperatures, which is a critical finding for applications involving thermal cycling. The as-welded microstructure consists of martensite and retained austenite, which transforms to tempered sorbite after 600 degrees Celsius heat treatment.

Wear Performance Analysis

The study's most striking finding concerns the dramatic impact of chromium content on post-heat-treatment wear resistance. The 12% Cr alloy showed a relative wear resistance that dropped from 10.65 in the as-welded condition to 2.08 after heat treatment, representing only 19.5% of the as-welded value. In contrast, the 16% Cr alloy maintained a relative wear resistance of 9.08 after heat treatment, demonstrating excellent wear performance.

This dramatic difference is attributed to the stability of the (Fe,Cr)23(B,C)6 hard phase. At 12% Cr, the hard phase is not thermally stable and dissolves or transforms during heat treatment, leading to a substantial loss of wear resistance. At 16% Cr, the hard phase remains stable, preserving the wear resistance.

Process and Standards Analysis

The flux-cored wire overlay welding process was used to deposit the alloys on Q235 steel, which is a common base material for structural applications. The 600 degrees Celsius heat treatment temperature was selected to simulate service conditions and to evaluate the thermal stability of the microstructure. From a standards perspective, this work relates to ISO 14716 for overlay welding of wear-resistant surfaces and various ASTM specifications for hardfacing alloys.

The engineering significance of the chromium threshold at 14% is substantial. It provides a clear design guideline: for applications requiring wear resistance after heat treatment or thermal exposure, the chromium content should be at least 14% to ensure the stability of the hard borocarbide phase.

Key Questions and Reflections

An important question is whether the 18% Cr alloy offers any advantage over the 16% Cr alloy. The study does not explicitly compare these two, but metallurgical reasoning suggests that beyond a certain chromium level, additional chromium may not provide proportional improvements in wear resistance and could potentially increase brittleness or reduce weldability. Future work should investigate the optimal chromium window and the trade-offs between wear resistance, toughness, and weldability.

Another practical consideration is the effect of the Q235 base steel on the overlay properties. Dilution from the base metal could reduce the effective chromium content at the interface, potentially creating a zone of reduced wear resistance. This is a common challenge in overlay welding that must be addressed through multi-layer deposition strategies.

Summary

This study provides a clear and actionable design guideline for Fe-Cr-B-C overlay alloys: chromium content must be at least 14% to ensure the thermal stability of the (Fe,Cr)23(B,C)6 hard phase and maintain wear resistance after heat treatment. The findings have direct implications for the selection of overlay welding consumables and process design in applications where thermal stability of wear resistance is critical.