离心铸造工艺优化

Some Tips for ensuring consistent quality centrifugal castings

from short air-cooled dies

Introduction:

For Ni-Resist groove insert manufacture the ideal system is to have carousel type centrifugal casting machines with dies as long as possible, and equipped with automatic control of rpm, temperature and cycle time. The large volume reputed manufacturers generally use 2-meter dies. Timed water-cooling is adopted to ensure uniform temperature of the die from end to end.

In producing captive castings or moving to low cost suppliers, the centrifugal casting machines generally used have short dies of 300mm length without water-cooling. The control on microstructure becomes more difficult, particularly at the rear end of the pots due to more rapid solidification. The front of the pot stays hot longer, being closer to the hot metal poured in at this end.

In order to secure high quality and consistency of microstructure of the inserts produced from such pots, from one end to the other, some simple principles need to be rigidly followed in the production process.

Melt Preparation:

Input raw materials:

The high percentage of Nickel in this material requires and efficient melting and mixing furnace of adequate capacity. The best suited for this purpose is a Mains frequency Induction furnace of minimum 500-Kg capacity and preferably of 1000-Kg capacity.

The input raw materials need to be of reasonable purity. Carbide forming elements like Mo, Va, Ti and W should be kept to traces. Caution: Many commercial pig irons contain high Mo and Ti. It is desirable that in the final insert these elements should be controlled to within the following limits:

Mo > 0.05%

Ti > 0.04%

W > 0.02%

Va > 0.02%

Steel scrap is a source of tramp elements that can cause unpredictable problems in Ni-resist materials. The steel scrap should be limited to max 10% of charge as far as possible and of a known source and composition.

Chips and borings should also be kept as low as possible, if they cannot be avoided altogether. It is desirable to limit these to a max of 60% and should be dry and clean. Mixing chips uniformly in the melt is difficult. The losses of certain elements from the chips are high and there is a risk of air and hydrogen trapping in the chips, both of which promote bad graphite and coarse/clustered carbides.

Sulfur in the melt is necessary in a narrow range. It should be kept around 0.06% for getting the desired hardness. Higher Sulfur causes problems to the microstructure.

Carbon Equivalent:

The Carbon Equivalent ( CE ) is known to be a critical parameter for cast irons. This factor plays a significant role in the type of microstructure developed in the casting. It is closely inter-related to the rate of solidification, which in turn is influenced by the cross-sections of the castings and temperatures.

The general convention is to compute the CE value according to the Volume/Surface (V/S) ratio of a casting. For 3-dimensional castings, which are of varying cross-sections, this is very difficult. The centrifugal pots, though 3-dimensional, are however of symmetrical section throughout the length and hence V/S can be calculated for these as shown in the following formula:

Volume / Surface Area Ratio V/S

V/S = ( D 2 - d 2 ) H / 4 ( DH + dH + ( D 2 - d 2 ) / 2 ) Where, D = Outer Diameter ( OD ) of the Centrifugal Cast Pot

d = Inner Diameter ( I.D. ) of th

e Centrifugal Cast Pot H = Height o

f the Centrifugal Cast Pot A better factor for the centrifugal cast pot would be the Degree of Saturation, which is given by the formula:

D EGRE

E O

F S ATURATION S c :

Si Ni 0055,0Ni 047,0Si 33,03,4C S c ⋅⋅+⋅-⋅-=;

where, C, Si and Ni are the actual values in weight-%

For our applications the desirable value of S c = 0,80 - 0,95

Installing a CE meter at the furnace will enable a tighter and more immediate control over the Carbon and Silicon content in the metal before casting the pots.

Recommended range of C, Si & Mn after inoculation:

Carbon: 2.70 – 2.80%

Silicon: 2.10 – 2.20%

Manganese: 1.20 – 1.30%