Effects of matrix chromium-to-carbon ratio on high-stress abrasive wear behavior of high chromium white cast irons dual-reinforced by niobium carbides
Graphical Abstract
Introduction
In high-Cr white cast irons (WCI) for a chosen carbide volume fraction (CVF), increasing Cr:C ratio affects several aspects of the microstructure:
- (i)
increases the Cr content of the matrix, improving the corrosion resistance
- (ii)
increases hardenability, permitting air-hardening of castings;
- (iii)
decreases C content of the austenite, and thus C content and hardness of the martensite. Alloys with low Cr:C ratio have higher matrix hardness but poorer fracture toughness. If this is taken to extreme, the alloys can be too brittle to use;
- (iv)
increases the Cr:Fe ratio of the M7C3 and hence the hardness of these carbides; and
- (v)
can move the alloy into phase fields other than γ + M7C3, causing the formation of alternative carbides such M23C6 (at very high Cr:C) and M3C (at very low Cr:C) — both usually regarded as unfavorable for abrasion wear applications.
Therefore, selecting proper amounts of Cr and C (or preferably their ratio expressed as Cr:C) in the design of high-Cr WCIs for specific applications is of critical importance. These values should be so designed that in addition to desired chromium carbide volume fraction (Cr-CVF), the desired hardenability (or more specifically austenite composition) is achieved [1], [2], [3]. Fig. 1 schematically shows the effect of this alloy parameter on the different micro-constituents of NbC-reinforced high-Cr WCIs.
Few works have studied the effect of this key parameter on microstructural and mechanical characteristics of high-Cr WCIs. Wieczerzak et al. [4] stated that increasing Cr:C results in different types of Cr carbides, namely Cr23C6, C7C3 and Cr3C2. High-Cr WCIs generally contain other alloying elements such as Mn, Ni, Mo, Cu, etc. rather than only Cr. Some of these alloying elements along with Fe also partition into these carbides (indicated by M in their chemical formulas). Thus, it is more accurate to refer to Cr-rich carbides instead of Cr carbides, represented by M23C6, M7C3 and M3C. In their later work [5], they expressed that wear resistance of these alloys is affected by chemical composition, including Cr:C ratio, solidification conditions and ‘probably’ type, morphology and volume fraction of carbides. They did not report any wear performance data in this work. Also, Wieczerzak et al. believed that hypo-, on- and hyper-eutectic microstructures in high-Cr WCIs are formed as results of various Cr:C ratios of the alloy; this is not accurate, too.
Wiengmoon et al. [6] experimented the effect of Cr:C ratio on the microstructure, hardness and corrosion resistance of three high-Cr WCIs. They found that in all three 20Cr–3.0C, 27Cr–2.7C and 36Cr–2.1C, M7C3 carbides were formed. However, by increasing the Cr content (hence Cr:C ratio), the Cr content of the carbide increases whereas Fe content decreases, resulting in overall increase in Cr:Fe ratio of the carbide. The carbides micro-hardness is believed to increase by an increase in Cr:Fe ratio of the carbides [7], [8]. In the heat-treated samples of 20Cr and 27Cr irons, secondary carbides of M7C3 and M7C3 +M23C6 were observed, respectively. They believed that M23C6 seems to be the predominant secondary carbide in irons with Cr:C ratio of 7–10. They reported that volume fraction and size of secondary carbides in 20Cr iron were higher than those in 27Cr, presumably because of the lower Cr:C ratio of the former. As 36Cr alloy was not heat-treatable, probably due to its ferritic as-cast matrix, no secondary carbide was observed. Tabrett et al. [3] stated that in high-Cr WCIs with low Cr:C ratios, secondary carbides uniformly precipitate within the dendrites while adjacent to the eutectic carbides in high-Cr:C irons.
In an attempt to correlate the microstructural and mechanical properties of two high-Cr WCIs with chemical compositions and heat treatment regimes, Chen et al. [9] found that the iron with higher Cr:C (lower C-Si content) showed hypo-eutectic as-cast microstructure while the alloy with lower Cr:C (higher C-Si content) is hyper-eutectic. Again, rather than Cr:C ratio of the alloy, it is more appropriate to use the location of alloy on the Fe-Cr-C ternary phase diagram to determine its as-cast microstructure. In addition, C-Si content or carbon equivalent (CE = C + ⅓Si) is less common in WCIs although few researchers have used it. In graphitic cast irons, however, Si as a graphite promoter can be used (along with C in CE equation) to predict the final microstructure, whether CE is greater, equal or less than 4.3. Chen et al. also reported that alloy with higher Cr:C ratio has a harder as-cast matrix, plausibly due to its lower C in austenite, higher Ms temperature and hence higher fraction of transformed martensite. However, after heat treatment (destabilization at 940 °C and quench to RT), the hardness difference was insignificant.
Baik and Loper [10] reported the effect of Cr:C ratio on the partitioning coefficient of Cr (KCr) to the austenite. They observed that KCr decreased linearly with a decrease in Cr:C ratio, decreasing the hardenability as the consequence. The iron with Cr:C ratio of 4.7 showed pearlite in as-cast condition whereas 5.9 Cr:C resulted in austenitic as-cast matrix. It is now well-established [1], [2], [11], [12] that hardenability of these irons increases with increase in Cr at a given C or decrease in C at a given Cr (overall increase in Cr:C ratio).
Davis [13] used Cr:C ratio of the high-Cr-Mo WCIs to show its effect on hardenability (represented by critical diameter for air hardening, mm) at different Mo levels. Dodd et al. [2] based on their experimental data developed a linear regression equation for hardenability of high-Cr irons as a function of Cr:C ratio and alloying elements such as Ni, Cu, Mo and Mn. Hardenability is defined as the time to reach the pearlite nose in CCT diagram. Although their equation showed stronger effects of alloying elements compared to Cr:C ratio, later examinations revealed that this effect was not as powerful as the effect that model predicts. Moreover, addition of those alloying elements is restricted compared to Cr, due to economical and metallurgical considerations. Laird et al. [14] showed that optimum destabilization temperature and maximum achievable hardness at this temperature depend on Cr:C ratio. At higher Cr:C ratios, the hardness peak shifts to higher destabilization temperatures and the peak hardness decreases. The formation of low-C martensite after heat-treatment at high Cr:C ratios is the reason for such phenomenon.
In the range of 0.5–4 wt%, Mo partitions into matrix, Cr-rich carbides and molybdenum carbides. Depending on Cr:C of the iron, various types of Mo-carbides form through eutectic transformation along grain boundaries between metallic phase and Cr-rich carbides. Bouhamla et al. [15] found that at low Cr:C ratios (e.g. 6), the formation of Mo2C is promoted. At high C contents of the alloy, or more specifically at low Cr:C ratios, small patches of Mo2C solidify during the eutectic reaction. At higher Cr:C ratios (>10), Mo6C solidifies [16].
Including above works, the literature in this field have mainly studied the effect of Cr:C on: (a) hardenability and austenite stability [1], [2], [10], [11], [12], [13]; (b) types of Cr-rich carbide [4], [6], [16], [17], [18]; (c) types of secondary carbides after destabilization [6], [19], [20], [21], [22], [23]; and (d) possibility of formation and type of Mo-carbides [3], [7], [15], [24]. The effects of Cr:C on the abrasion performance of these irons have been investigated in very few works.
Çetinkaya [25] found that Cr:C ratio of high-Cr WCIs has an influence on abrasion resistance measured by pin-on-disk apparatus. He reported that wear resistance decreases with an increase in Cr:C from 4.6 in alloy 5 (3.3C–15 Cr–2.5Mo) to 5.7 in alloy 2 (2.9C–17 Cr–1.2Mo). In his both abrasion tests with SiC and Al2O3, the ranking of the alloys were the same. But further examination of his alloys compositions reveals that in addition to Cr:C ratio, Cr-CVF and Mo also vary. As the alloy parameters in this work were not systematically designed, any conclusions must be drawn with care, bearing in mind that the other confounding variables were not kept constant. A good example is alloy 1 with Cr:C ratio of 3.3 (2.7C–9 Cr–0.4Mo). According to Çetinkaya, due to its lowest Cr:C ratio, this alloy is expected to show the best wear resistance, but using both abrasive papers, it only shows moderate performance with the rank 3 out of 5.
Sabet et al. [26] cited that excellent abrasion resistance in Fe–Cr–C hard facings can be obtained in the Cr:C ratio range of 5–8. This range results in high proportion of Cr-rich carbides to be of M7C3-type in all hypo-, on- and hyper-eutectic compositions. However, Sabet et al. did not study this effect and just used Cr:C ratio of 6 in their work. Abdel-Aziz et al. [27] attempted to predict the abrasive wear rate of high-Cr WCIs using artificial neural network. They used Cr:C ratio, tempering temperature and load as the input variables for their multi-layer perceptron network. They stated that alloys with higher Cr-CVF showed lower weight loss. They also reported that wear rate “significantly” increased with an increase in Cr:C ratio. However, the sensitivity analysis showed that Cr:C ratio had small effect on the wear rate. In both experimental and modeling conclusions, Abdel-Aziz et al. seem to overlook the influence of other confounding variables, particularly Cr-CVF (varying in a wide range of 16.5–41.7%).
Despite the significant effects of matrix Cr:C ratio on the microstructural and tribological properties of high-Cr WCIs, the literature lacks systematic studies on the effect of this key alloy parameter, particularly on abrasion resistance. The effect of this alloy parameter on abrasive wear behavior of NbC-bearing high-Cr WCIs has never been investigated. In almost all of the works in this field Cr:C ratio of the alloy (or bulk) has been used. Whereas reference to the bulk Cr:C ratio is only a shorthand way of indicating the position of an alloy between the phase boundaries on either side of the γ + M7C3 field, and hence indicating the austenite composition. Cr:C ratio across the field is also affected by Cr-CVF (i.e. proximity to the eutectic). Therefore, it is more appropriate to refer to matrix Cr:C ratio in both plain-Cr and dual-reinforced high-Cr WCIs.
The present study aims to address the above knowledge gaps, evaluating the effect of Cr:C ratio on high-stress abrasion (HSA) of NbC-reinforced high-Cr WCIs, quantitatively and qualitatively. Eight Nb-containing hypo-eutectic WCIs with 22% Cr-CVF, 6% Nb-CVF and covering a wide range of Cr:C ratios were prepared. The ball mill abrasion test (BMAT) was employed to quantify the HSA performance of the alloys in less competent (basalt) and competent (quartzite) abrasives. Comprehensive SEM examinations of the worn surfaces were conducted to understand the interactions of the microstructure with abrasion components.
Section snippets
Niobium carbide volume fraction
As a strong carbide former, Nb is stronger than other conventional carbide-forming elements in WCIs such as Cr, Mo, Fe, and Mn. Therefore, Nb tends to combine stoichiometrically with the required amount of C to form NbC in the early stages of the solidification. The remaining C in the liquid combines with Cr and other carbide-forming elements in further cooling to precipitate Cr-rich carbides. Some C remains in solution in the austenite. Hence, “bulk” alloy of Nb-containing WCI consists of two
Hardness
Vickers hardness of heat-treated samples of Series CC (Cr:C ratio) are presented in Fig. 4, as functions of the Cr:C ratios of the host alloy (a) and matrix (b). This series shows very large variation in hardness values; 190 HV between maximum and minimum. This demonstrates that Cr:C can have a powerful effect on bulk properties. This wide range of values is due to the low hardness produced by either very-low or very-high Cr:C ratios. For intermediate Cr:C, the hardness is less sensitive to
Conclusions
In order to study the effects of Cr:C ratio on high-stress abrasion behavior, an alloy series of NbC-reinforced high-Cr white cast irons were designed and cast with nominally constant chromium carbide volume fraction (Cr-CVF) and niobium carbide volume fraction (Nb-CVF). Using the ball mill abrasion test, heat-treated samples were tested in basalt (less competent) and quartzite (competent) abrasives. The following are concluded after assessing hardness, microstructure, abrasion performance, and
CRediT authorship contribution statement
Hamid Pourasiabi: Conceptualization, Methodology, Validation, Investigation, Resources, Writing – original draft, Writing – review & editing, Visualization, Project administration. J.D. Gates: Conceptualization, Resources, Writing – review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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