High chrome alloy is a range of erosion-corrosion resistant white cast irons developed for use in slurry pumps subjected to severe wear in corrosive environments. These are multi-phase materials consisting of hard carbides (for erosion and wear resistance) and a stainless ferrous matrix (for corrosion resistance). The properties of the carbide phase and the composition of the ferrous matrix can be selected to optimise the performance of the alloy for a wide range of slurries.
High Chrome A05 Alloy
The microstructure of High chrome alloy A05 (27 Cr) shown in Figure 1(a) consists of hard eutectic chromium carbides in a fully hardened martensitic matrix. The material is covered by a number of different Internationally recognised standards including: ISO 21988 Grade 25Cr, AS 2027 Grade 27Cr; BS 4844 Grade 3D and ASTM A532 Grade IIIA.
The wear resistant material 27Cr was developed and patented in 1917 and was one of the first white cast irons. This material significantly outperforms the other commercially available white irons (i.e. the Ni-Hards and the Chromium/Molybdenum White irons such as 15Cr-3Mo) in slurry pump applications where erosive and corrosive wear predominates.
High chrome alloy A05 is Tobee’s standard erosion resistant alloy for slurry pump castings.
The success of high chrome alloy A05 in slurry pump applications is due to the relatively high chromium content of the alloy. As previously shown, the ferrous matrix of white cast irons exhibit good corrosion resistance when the chromium content is in excess of 12% by weight, a level that is readily achieved in 27Cr but not in 15Cr-3Mo.
High Chrome A49 Alloy
High chrome A49 is essentially a high chromium-low carbon white cast iron. The microstructure (shown in Figure 1(b)) is hypoeutectic and consists of eutectic chromium carbides in an austenite/martensite matrix.
The carbon content in High chrome A49 is lower than High chrome A05 resulting in less chromium in the carbide phase and more chromium in the matrix. Thus the ferrous matrix of High chrome A49 is richer in dissolved chromium than the ferrous matrix of High chrome A05. As a consequence, High chrome A49 offers greater corrosion resistance than High chrome A05 in mildly acidic environments.
High chrome A49 alloy finds use in moderately corrosive conditions where erosion resistance is also important such as some Flue Gas Desulphurisation (FGD) applications.
Table 1 - Typical erosion and corrosion test results for High chrome alloys
|Alloy||Carbide type||Carbide volume|
|Matrix type||Matrix Cr|
High Chrome Slurry Pump Applications:
High chrome A05 can be applied in a wide range of slurries that are mildly corrosive (generally down to pH of 4) and where the slurries are particularly erosive (high solids concentration). Other applications where A05 performs well include sea water slurries (in dredging) and where hypersaline water from bores may be used for plant water in remote locations.
High chrome A49 is designed for slurries containing weak sulphuric acid and/or nitric acid solutions. The most widespread use is in FGD applications with chloride levels up to 50,000 ppm depending on pH. Other applications include mine dewatering where dissolved salts can cause severe corrosion. Solids concentrations are generally less than 20% by weight.
Hebei Tobee Pump Co.,Limited