Ni-HARD – a category of white cast iron alloy

Alloying elements such as Nickel (Ni) and Chromium (Cr) in Ni-Hard 4 impart a high hardness, extreme abrasion resistance and fracture resistance compared to the cast iron category of Ni-Hard1 and Ni-Hard 2 materials. The 4.5 – 7% Ni, 7-11% Cr and 2.5- 3.5% C in Grade 4 Ni-Hard provide abrasion resistance and toughness.
A small quantity of Ti & V is added to the carbon content to form titanium carbide and vanadium carbide. These formations impart high strength and improved properties, which provide specific and well-proven low-cost characteristics for specialised and often critical applications in both dry and wet media.

Process metallurgy

The melt charge usually consists of returned scrap from the foundry, iron scrap, steel scrap, and pigiron, along with alloying additions such as ferrosilicon. The melting unit most widely used is the electric induction furnace due to its simplicity, reliability, and flexibility. It is a costeffective furnace that can be superheated and is efficient regarding the quality of the charge materials used as some refining of undesirable elements (e.g. zinc, lead) can be done.
The new generation of coreless induction furnaces can achieve super-heated temperatures above 1550°C, due to high-performance electrical equipment with specific high power ratings. Improvements in quality mean the acid refractory lining can withstand high temperatures (high purity SiO2 materials, optimum crystalline structure, active agents addition). One application uses a SiC addition in a SiO2 – B2O3 mass, leading to an improved lining life (20–100%). This is especially effective when there are high rust levels in the metallic charge or even when employing oxygen in a furnace process.

Super-heated alloy

The melting temperature of Ni-Hard grades is around 1300° C. After tapping, the super-heated molten metal is transferred to the casting bay for cooling, shaping and solidification to convert into a final product. The temperature must be raised before the metal is poured into casts for cooling and setting. The pouring temperature must take into account heat loss caused during the transfer of the tapped metal through the ladles, in the distance between furnace and casting location, and also due to heat absorbed by the ladles.
The casting process can usually be done in permanent metal casts. The casts have a limited lifespan due to repeated exposure to high temperature molten metal, although they can be repurposed used for metals with low pouring temperature requirements. Therefore one of the main requirements of the casting process is refractoriness, or in other words, the capability of the cast to withstand the high temperatures of the molten metal without undergoing any changes in its physical properties and shape. This is a very important requirement in alloys with a high melting point such as steel on the cost point. However, this issue may be taken as secondary in alloys with lower melting points. Where alloys with a high melting point are being used, the moulds need to be lined with an insulating material with refractory properties so that the mould retains its shape and original characteristics.

Heat treatment & properties

It is essential to apply a heat treatment process of austenitising at 800-820°C for about 4 hours to achieve the required hardness, microstructure and other physical properties. This is followed by subsequent tempering at 450°C to relieve any residual stress generated in the casting process. Ni-Hard castings have a wear-resistant minimum of 550 Brinell hardness. The double annealing process (annealing at 550°C and 450°C) may be adopted for parts for which the hardness requirement is lower than 58 HRC, and is not suitable for the parts where hardness requirement is high than 60 HRC. In type ASTM A532 Class I, Cementite iron solidifies as austenite with Fe3C carbides.

Selection of types

Ni Hard 1: The most common and least expensive, popular for producing lowcost parts used to combat abrasion.
Ni Hard 2: May be exposed to high dynamic stress in areas like brick moulds, concrete/asphalt mixing. Type 2 in this group can replace Ni Hard 1 in the event of higher toughness requirements.
Ni Hard 4: The toughest and most corrosion resistant type. It is the closest stainless steel due to a higher Cr content combined with high Ni levels. Ni-Hard 4 is a high-strength material, a category of white cast iron having extreme abrasion resistance and fracture resistance compared to the cast iron category of Ni-Hard1 and Ni-Hard 2.

References:

1. NiDI
2. Observation in Foundry with IF melting