Microstructural characterization of continuous casting rollers with overlay welded layer

In continuous casting (CC) of steel, the solidifying slab is guided by multiple caster rollers. Since the core of the slab remains molten, high thermal and mechanical loadings arise for the surface regions of the caster rollers. Overlay welding is a common method of giving surfaces high resistance against aforementioned loadings, whereas a high-quality weld is essential for increased roller lifetime and decreased facility downtime. Within this work, several methods were used to characterize the overlay welds before and after service.
The chemical composition of the welds was determined via optical emission spectroscopy (OES) and energy dispersive X-ray analysis (EDX). Significant deviations in the elemental composition between the as-welded state and the state after service raise concerns about the reliability of comparing the samples. The microstructure was characterized using optical microscopy (OM), synchrotron X-ray diffraction (XRD), and scanning electron microscopy (SEM)-based methods.
The microstructure of all the examined samples mainly comprises lath martensite. No precipitates were detected using synchrotron XRD investigation and Rietveld analysis, OM, and scanning transmission electron microscopy (STEM). The presence of austenite was confirmed for all analyzed samples before and after service. The as-welded volume fraction of austenite was 2%-6%. Since the post-service samples exhibited significantly higher volume fractions of austenite, the possibility of austenite reversion was discussed. The formation of σ phase was excluded for all examined welds; however, since the emergence of δ ferrite in the microstructure is detrimental to the overlay welds and δ ferrite occurs non-uniformly in the microstructure, elaborate investigation was required. In the investigated welds, δ ferrite was encountered in the form of clusters in the material. Conversely, δ ferrite tends to form network-like structures surrounding the martensitic laths. Clusters of δ ferrite were encountered significantly more frequently in the post-service samples.
Throughout the investigation, the existence of near-surface layers evidently prone to macroscopic cracking was deduced. The encountered feature corresponds to a Cr-enriched zone of reduced hardness at a scale of several mm in length and about 1 mm in depth, which exhibits increased volume fractions of austenite and δ ferrite. Different surface regions of the same roller sample show comparable irregularities. Given the evident role of the Cr-enriched regions in roller fatigue, an extended investigation of the encountered microstructural irregularities in post-service samples is advocated.
A decrease in hardness was determined for the post-service samples (450-460 HV1 compared to 480 HV1 in the as-welded state). The decreased hardness corresponds to the increased austenite volume fractions addressed above.
The investigation reveals that the microstructure of the roller surfaces significantly influences roller fatigue. Higher volume fractions of δ ferrite, which are associated with a Cr-rich matrix, increase the susceptibility to crack propagation. Conversely, higher volume fractions of austenite are detrimental since they are linked to a hardness reduction. Overall, achieving a homogeneous microstructure free from major anomalies is crucial to maintaining the integrity of caster roller surfaces over extended periods of use.