Recursos de la Unidad Económica de Convivencia, si están integrados en alguna (Cuadro 8/P.N.C.)

The final extent of inter-dendritic segregation that is observed at any point in a steel ingot is the product of three main influences:

u "Th~ cOOling rate

9 The type of crystal growth 9 The composition of the steel.

ADISHESHA ON ASTM A-295 52100 BEARING STEEL 2 9

Increasing the cooling rate decreases the segregation, whereas with alloying additions, particularly carbon, segregation increases. The work of Doherty and Melford [4] has shown that segregation is characteristically higher for equiaxed crystal growth rather than for columnar growth in the same region of the ingot. Most alloying elements in steel have distribution coefficients between the solid and liquid phases of less than one. Consequently, when liquid alloy freezes, according to theory of differential or selective solidification, metal of high purity solidifies first.

The solute enriched liquid, i.e. segregate (principally carbon, phosphorus and sulphur), diffuses inwards at finite rates, but solidification also progresses at a finite rate that decreases with distance from the surface. Hence, segregation does not extend far into the liquid, but is restricted b311 a narrow layer of liquid metal immediately adjacent to the solid/liquid interface in the "mushy" zone. If liquid solidifies at this point, micro- segregation will result.

Micro-segregation of chromium is found to be decreased by both silicon and manganese additions [5]. In the case of silicon this is due to a smaller solidification interval, whereas manganese increases the partition coefficient for chromium between austenite and liquid. Therefore, in high carbon alloy steels in addition to segregation of alloying elements, various types of carbides will be present.

Origin of banded structure

A handed structure can be described as a segregated structure of approximately parallel bands of two different phases, e.g. ferrite and pearlite, aligned in the direction of working. With the advent of modern Metallographic techniques it is now well established that banded microstructure in wrought steels are manifestations of the heterogeneous distribution of alloying elements that result from dendritic or small scale segregation during solidification of an ingot or a bloom. These include, elements like nickel, chromium, molybdenum, titanium, manganese, etc., used as alloying additions and phosphorus, sulphur, arsenic, tin, copper, etc., present as residuals.

The essential steps through which a banded structure in steel develops are: -

9 Micro-segregation of alloying elements during ingot solidification and subsequent alignment by mechanical working.

9 Carbon re-distribution into banded layers on cooling from austenitizing temperature. Some elements in steel segregate more readily than others. [6]. Carbon diffuses very rapidly whereas elements such as manganese, nickel, chromium, molybdenum, tin, copper, etc., diffuse very slowly at temperatures normally used for rolling or forging, so the alloy segregation persists throughout processing. During mechanical working, the cast structure is broken down and after a large reduction in cross section, the network of the segregated pattern is formed into distinct bands. The alloy rich and alloy-depleted bands have different transformation characteristics and, thus, on cooling a laminated microstructure are produced. The alloy-depleted bands transforming at a relatively high temperature will have lower carbon, whereas, alloy rich bands enriched with carbon will transform into a carbon-rich phase.

Mechanism to reduce segregation and structural banding

Some o f the proposed mechanisms for reducing the severity of carbide segregation in ball bearing steels are:

9 Prolonged heating prior to rolling 9 Reduced finish rolling temperature

9 Intensive cooling after rolling including quenching prior to annealing 9 More prolonged annealing (spberoidization)

9 Thermo-mechanical treatment.

An increase in the rate of cooling ingots during solidification increase the rate of crystallization [7] and the zone of directional columnar dendrite is reduced and carbides are expected to be refined and uniformly distributed. Prolonged soaking of the cast ingots or blooms before rolling is supposed to homogenize the ingot [8]. Stepped heating at lfigh temperature 1160 ~ C, 1200 ~ C, 1280 ~ C and 1180~ is expected to reduce structural banding significantly and improve bearing life [9]. Similarly reduced finish rolling temperature and intensive cooling after rolling are also reported to reduce severity of

carbide banding

[10].

An attempt has been made to study the effects of these factors on carbide banding in commercially produced ingots/products of ASTM A 295- 52100 beating steel, with the objective of arriving at optimum process parameters to reduce degree of carbide banding.

Experimental Procedure

Melts of ASTM A295-52100 steel were made in a commercial 45 ton Electric Arc Furnace at Mahindra Ugine Steel Company Limited (MUSCO), aluminum killed, ladle refined and vacuum degassed. Molten steel was homogenized by purging inert gas and then cast into ingots of 3ton weight having an average cross section of 450x450mm, by up-hill teeming. Continuous casting was done in a three strand, 9/16m radius, closed pouring caster having facilities for electro-magnetic stirring and auto mould level control and mould size 250X200 ram. Solidified ingots/blooms were subsequently rolled in a 2- high, 860ram reversible blooming mill, and cooled under controlled conditions, surface conditioned and subsequently rolled to different sizes in a 550ram 3-high, 4-stand bar mill. Samples were selected from the rolled products for evaluation of banding.

To study the effects of super heat on banding, teeming temperatures were varied from

1500~ to 1560~ (liquidus temperature 1435~ and other parameters were kept the

same. Similarly, for studying the effect of reduction ratio ingots from the same heat were rolled into different sizes. For studying the effects of soaking time ingots and blooms were soaked at high temperature for a prolonged period prior to hot rolling and samples were selected from the rolled products.

For studying the effect of ingot size on banding, ingots of average cross section 370X370mm, 395X395mm, 450X450mm and 500X500mm were cast in the same heat and were rolled to different sizes so that super heat and reduction ratio were constant.

The effect of heat treatment on the carbide banding was studied on this steel. The heat treatment was conducted in a muffle furnace on samples selected from spheroidized annealed bars. Approximately, 15-ram thick slices were austenitized at different temperatures by soaking for 20 min and then quenched in oil. The quenched samples were polished and etched and examined for carbide banding. Similarly, samples were austenitized at 850 ~ C and soaked for different periods at the same temperature and then quenched in oil.

For the examination of banding, samples with full cross section of approximately 15mm thickness were cut from the rolled products on an abrasive cut-off machine and oil quenched from 850 ~ C (soaking time 25 min). Quenched samples were polished on series

ADISHESHA ON ASTM A-295 52100 BEARING STEEL 31

of micro-polishing papers, finished with 1 micron and 0.5 micron alumina powders and etched with 10% nital. Etched samples were examined using an optical microscope. Staid Eisen Prufolatt 1520-78 chart was used for rating the banding. Series 7 was used as reference.

Since there was a high degree o f scatter in the data for the banding index over the cross section of a sample and also from sample to sample, in a particular melt, each melt was assigned an average Carbide Banding Index (CBI). This was assessed by examining several fields across the sample for about 10 samples per melt, instead of evaluating by the conventional worst field rating. Whenever, several heats were examined graphs have been plotted showing them as separate series and a typical trend line has been drawn to show the correlation.

Results and Discussions.