What is Ni-Hard Steel?
Ni-Hard is a white cast iron, alloyed with nickel and chromium suitable for low impact, sliding abrasion for both wet and dry applications. Ni-Hard is an extremely wear-resistant material, cast in forms and shapes which are ideal for use in abrasive and wear environments and applications. The use of this type of material generally began with Rod Mills and Ball Mills, where impacts were considered low enough for this brittle yet highly abrasive resistant wear material to perform well. However, it is now considered obsolete in light of the use of high chrome irons and chrome-moly white iron. Ni-Hard castings are produced with a wear-resistant minimum of 550 Brinell hardness, hard white cast iron containing 4% Ni and 2% chrome, used for abrasive resistant and wear-resistant applications in the following industries:
- Earth Handling
- Cement mills
Ni-hard steel standard is ASTM A532 Type 1, Type 2, and Type 4.
For mill liners, our foundry uses ASTM A532 Type 4 to cast.
Ni-Hard Mill Liners Material Chemical Composition
The role of different chemical elements in Ni-hard mill liners:
Carbon: most of them exist in carbide in the form of the compound, and the content of carbon dissolved in the matrix is relatively low. In order to make the alloy have a certain toughness, the carbon content is selected in the range of Hypoeutectic. The higher the carbon content is, the more carbides there are, the lower the hardenability is, and the toughness is very low after quenching; if the carbon content is too low and the carbide content is too small, the alloy can not be hardened, and the alloy composition deviates from the eutectic component, which is easy to appear shrinkage cavity and porosity. The carbon content in the alloy not only determines the number of carbides and eutectic carbides, but also the carbon dissolved in the matrix also has a very important impact on the subsequent heat treatment of the alloy. With the increase of carbon content in the matrix, the martensite transformation point in the alloy decreases, resulting in an increase of the residual austenite volume, and the matrix may not be hardened enough.
Chromium: chromium is a strong carbide forming element. Adding appropriate chromium can ensure the existence of a certain amount of M7C3 type carbide, which will improve the wear resistance of the material.
Silicon: Silicon is an element promoting graphitization, mainly exists in the matrix to strengthen the matrix, when the content is high, pearlite is easy to appear. In addition, when the alloy has enough hardenability, adding appropriate silicon can reduce retained austenite and improve wear resistance.
Nickel: nickel is a stabilizing element of austenite, which can greatly improve the hardenability of the alloy. Due to the formation of a large number of carbides in the alloy, the enrichment degree of nickel in the matrix is significantly increased and the hardenability can be fully exerted. When the content of nickel is 4% ~ 6%, martensite structure can be obtained, which can improve the wear resistance of the material.
Manganese: it can eliminate the harmful effect of sulfur, stabilize carbides, and inhibit the formation of pearlite. Manganese is a strong stable austenite element in martensitic white cast iron. However, if the content is too high, the retained austenite will be increased and the strength will be reduced.
|Chemical Composition of Ni-Hard Mill Liners|
Ni-Hard Mill Liners Heat Treatment
The main purpose of heat treatment is to obtain the required hardness and ideal microstructure. In the heat treatment process, the austenitizing temperature is the most important. In addition, the control of holding time and the cooling rate has different effects. The following heat treatment systems can be selected for wear-resistant parts of hard nickel cast iron IV material:
- Two low-temperature temperings at 550 ℃ and 450 ℃ is adopted.
- The annealing temperature is determined according to the actual composition of the parts, Annealing at 750 ℃ ~ 850 ℃.
In the process of heat treatment, the heating rate and cooling rate should be strictly controlled to ensure uniform heating and cooling of parts, so as to avoid cracking caused by thermal stress.
Relevant process parameters
- Process scale: referring to relevant foreign data, laboratory test data, and production practice, the scale should be 1.5% – 2.0%.
- Machining allowance: because the hardness of the material after heat treatment reaches above 60HRC, it is very difficult to process. Therefore, the machining allowance should be as small as possible. In principle, the machining allowance should be sufficient, generally 2-3mm.
- Pouring temperature: in order to ensure the internal structure of the casting is compact, the pouring temperature should be controlled at a lower temperature, usually no more than 1300 ℃.
- Boxing time: due to the large cracking tendency of the material, the boxing time should be strictly controlled according to the season after pouring. Generally, the box can be opened one week after casting.
- Design of gating and riser system: since the hardness of nickel hard cast iron is more than 50HRC, it is easy to crack after being subjected to rapid heat and cooling. Therefore, gas cutting or arc gouging can not be used for water risers, and only mechanical methods can be used. In order to facilitate the removal of water riser, when designing the water riser, the riser seat should be about 15mm higher than the live surface, and under the condition of sufficient feeding, a “neck” is designed at the root of the riser. As for the number of risers, the principle is to ensure the internal dense structure; in the gating system, there are one straight gate, one transverse gate, and four internal nozzles, which belong to open gating system.
- Cleaning and grinding: after heat treatment of mill liners, the water and riser root shall be cleaned and polished. During grinding, local overheating shall not be generated to avoid cracks.