sludge gear pump

Understanding effects of Cr content on the slurry erosion behavior of high-Cr cast irons through local property mapping and computational analysis

Abstract

High-Cr cast irons (HCCIs) have demonstrated excellent performance during various industrial processes involving mining, oil sand slurry handling, and manufacturing. Attempts have been made to extend concentration ranges of carbon and chromium up to a higher level, e.g., 6%C and 35%Cr or higher. However, it is not clear how the Cr content influences properties of (F e,C r)7 C 3, and the ferrous matrix, and thus the performance of HCCIs during wear processes, e.g., slurry erosion for oil sand transport. In this study, we investigated how the chromium content, in the range of 5–35%Cr, affected the performance of HCCIs with 5%C during erosion tests in slurries at two pH levels and four velocities. In particular, local mechanical properties and electron work function which is a measure of electrochemical stability were analyzed for both carbides and the ferrous matrix in the HCCIs using a micro-indenter and a multi-mode atomic force microscope. First-principles calculation was conducted to understand mechanisms responsible for the observed phenomena.

Introduction

High Chromium Cast Irons (HCCIs) are widely used in mining, slurry pumping and manufacturing industries, where high resistance to wear, erosion and erosion-corrosion is required [1]. The excellent performance of HCCIs results from the combination of hard carbides, e.g., M 7 C 3, and a ferrous matrix [2], [3], [4]. The matrix helps absorb impact energy and enhance toughness of the material, while the hard carbides play a crucial role in withstanding the wearing stress as the reinforcement [2]. Since Cr is a strong carbide-forming element, increasing Cr content helps form M 7 C 3 carbides. Commercially used HCCIs have their chromium content in the range of 23–30 wt.% Cr, while in recent years efforts have been made to push both contents of carbon and chromium to higher levels in order to obtain higher strength and corrosion resistance for applications involving aggressive chemical conditions, such as dredging [4]. It appears that alloys with higher chromium concentrations exhibit higher resistance to erosion-corrosion. However, whether this trend is applicable to all HCCIs needs to be investigated, since the Cr content may affect compositions of carbides and matrix, leading to variations in their properties.

When concerning properties of M 7 C 3 carbide, it is noticed that more Cr does not necessarily produce harder carbides. For example, the calculation by Zhang et al. shows that F e 4 C r 3 C 3 has the largest elastic modulus and hardness among all (F e,C r)7 C 3 carbides with different Cr/Fe ratios [5]. Xiao et al. [6] also computationally demonstrate that the elastic modulus of F e 16 C r 12 C 6 is larger than that of C r 7 C 3. Thus, it is worth investigating and determining the optimal Cr content for maximized benefits from the high-Cr cast irons.

Beside its influence on properties of carbide, alloyed chromium also affects the mechanical properties of the matrix through solid-solution strengthening mechanism. Furthermore, Cr renders the ferrous alloy resistant to corrosion, leading to elevated resistance to erosion-corrosion. Thus, it is also of importance to understand such effects in order to maximize the benefits from alloyed chromium.

In this study, we investigated effects of chromium content, in the range of 5–35%Cr, on the performance of HCCIs containing 5%C during erosion tests in slurries with two pH values and four velocities. In particular, local mechanical properties and electron work function which is a measure of electrochemical stability were analyzed for both the ferrous matrix and (F e,C r)7 C 3 carbides using a micro-indenter and a multi-mode atomic force microscope. First-principle calculation was conducted to analyze (F e,C r)7 C 3 carbides and the matrix to understand mechanisms responsible for observed phenomena.

Conclusions

The following conclusions are drawn from this study:

1. As Cr/Fe ratio increases, the strength of carbide, (F e,C r)7 C 3, in high-Cr cast irons initially increases, reaching the maximum at about C r/F e≈4/3, and then decreases as Cr/Fe continuously increases.

2. The strength of the ferrous matrix increases with an increase in the content of alloyed Cr.

3. The higher the electron work function, the higher the mechanical strength and stability. This relationship is applicable to both the metallic matrix and (F e,C r