Influência da adição de nióbio no comportamento tribológico de ferro fundido branco alto cromo ensaio pino-sobre-discocontrapinodemetalduro

Abstract

This work investigated the effect of niobium additions (0, 0.5 and 1 wt %) on the friction co- efficient and wear-track morphology of high-chromium white cast iron (HCWCI, ASTM A532 class III A) tested in a pin-on-disk configuration (ASTM G99). The counter-body was a tungsten carbide (WC-Co) pin. For each of the three alloys we measured Rockwell C hardness, pre-test Ra surface roughness, the complete time series of the coefficient of friction (COF) during ten minutes of sliding, and forty-three images of the wear track. The images were segmented in Im- ageJ with the Trainable Weka Segmentation plug-in into two classes — fragments (FRAG) and continuous track (PISTA). The COF in both the running-in (0–15 s) and steady-state (15–600 s) regimes increased uniform with Nb (μsteady = 0.494 → 0.565 → 0.627), with statistically signifi- cant pairwise differences (one-way ANOVA, p < 10−3). Hardness remained essentially constant (about 51–52 HRC). The fragment area fraction roughly doubled when 0.5 wt % Nb was added (15 % → 29 %) and the continuous-track area fraction dropped from 55 % to 43 %. A linear re- gression μsteady = 0.133 · wt% Nb + 0.495 (R2 = 0.999; p = 0.022) summarised the trend. The Pearson correlation between μsteady and the continuous-track area fraction was −0.82. Com- bining the microstructural evidence reported by MELO (2018) and the tribological evidence obtained here, we propose that the dominant wear mechanism shifts from adhesive/abrasive to delamination with third-body formation, which explains the increase in friction without a cor- responding hardness change. Keywords: Sliding wear. High-chromium white cast iron. Niobium. Pin-on-disk. Image seg- mentation.