CA1052599A - Wear resistant low alloy white cast iron - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

A low alloy white cast iron has been developed for wear resistance applications. The alloy consists essentially of about 2 to 4% carbon, 0.3 to 1.2% silicon, 0.5 to 1.5% manganese, 0.5 to 1.5% copper and 0.25 to 1% molybdenum, the remainder being sub-stantially iron except for incidental impurities commonly found in cast iron. The preferred alloy composition is 2.5 to 3% carbon, 0.6 to 0.9% silicon, about 1% manganese, about 1% copper and about 0.5% molybdenum, the rest being substantially iron. The process for manufacturing the above wear resistant alloy consists in melt-ing an alloy having the composition mentioned above, casting such alloy into moulds to produce a desired product such as grinding balls or slugs, shaking the product out of the moulds at a temper-ature of 750°C or higher, preferably about 900°C and cooling the product at a rate of 2 to 15°C/sec., preferably 5 to 10°C/sec.
The as-cast product is preferably heat treated at a temperature between 200 and 400°C, preferably about 260°C for a time of 1 to 8 hours, preferably about 4 hours to transform as much retained austenite as possible into martensite.

Description

105'~S~9 This invention relates to a low alloy white cast iron having high hardness and superior wear resistance.
In certain applications, such as ore grinding balls or slugs, performance is primarily determined by microstructure.
White cast irons contain several phases ~austenite, carbide, pearlite, bainite, and martensite) the relative amounts of which determine their overall hardness and toughness. The amount of each phase present in these materials is controlled by composi-tion, cooling rate from the pouring temperature to room tempera-ture and by heat treatment. In order to have a high overall hard-ness, substantial amounts of martensite and carbide must be pre-sent in the microstructure. These phases can be produced by pro-per alloying tailored to a given set of processing variables.
White cast irons previously used for grinding media were either unalloyed or alloyed with chromium alone or with combina-tions of nickel and chromium. However, these white c~ t irons suffered from a number of drawbacks. Unalloyed white cast irons and those containing chromium had a low hardness and therefore a poor wear resistance. White cast irons containing nickel and chromium had a superior wear resistance but were expensive to use due to the cost of the alloying constituents.
A low alloy white cast iron having high hardness and superior wear resistance has been disclosed in applicant's Cana-dian Patent No. 786,270 issued May 28, 1968 and in an article published by J.C.T. Farge, P. Chollet and J. Yernaux in the Found-ry Trade Journal, April 15, 1971, and entitled "Effect of Compos-ition, Cooling-rate and Heat-treatment on Properties of a new Wear-resistant White Iron". The alloying elements disclosed in the alloy were manganese, carbon, silicon, copper and molybdenum.

In Canadian Patent 786,270, manganese was disclosed as being in the range of 1.5 to 16%, preferably between 2.5 and 5%, carhon in the range of 2 to 4~, silicon in the range of 0 to 2%, copper in -1- ~

~(~5~55~9 the range of 0 to 2.5% and molybdenum in the range of 0 to 1%
with the total amount of copper plus molybdenum not less than 0.1%. In the above article, the combined effect of alloy con-tent, cooling rate from different shake-out temperatures, and heat treatment on the hardness and microstructure of sand-cast ore grinding balls containing about 3.2% carbon and 0.5% silicon was investigated over the range of 0.75 to 4.7% manganese, with a copper content of 0.5 and 1%, and a molybdenum content of 0.2%.
A serious problem was encountered during the production of grinding media having the composition described in Canadian Patent 786,270. White cast irons containing manganese in excess of 1.5% have a tendency, when in the molten state, to attack acid refractories normally used in melting furnaces such as cupo-las. Also, attempts to produce grinding balls on an industrial scale according to the procedure described in the above mentioned article were not completely successful, primarily because of the low molybdenum content of the alloy. The microstructure of the grinding balls contained substantial amounts of pearlite which resulted in low overall hardness. It was therefore necessary to develop a white cast iron which could be produced with no diffi-culties and which would display high hardness and superior wear resistance.
It has been found, in accordance with the present in-vention, that optimum hardness and superior wear resistance can be obtained with an alloy containing essentially of 2 to 4% car-bon, 0.3 to 1.2% silicon, 0.5 to 1.5% manganese, 0.5 to 1.5% cop-per and 0.25 to 1% molybdenum, the rest being iron except for in-cidental impurities normally found in cast iron. The alloy mi-crostructure consists essentially of carbide and martensite. The preferred alloy composition consists essentially of about 2.5 to 3% carbon, 0.6 to 0.9% silicon, about 1% manganese, about 1% cop-per and about 0.5% molybdenum, the rest being substantially iron.

-2-A

~05'~S~9 The process for manufacturing the above wear resistant low alloy white cast iron comprises the steps of melting the above alloy in a suitable furnace such as a cupola or an electric furnace, casting the alloy into moulds to produce a desired pro-duct, such as grinding balls or slugs, shaking the product out of the moulds at a temperature of 750C or higher, preferably about 900C, and cooling the article at a rate of 2 to 15C/sec., pre-ferably 5 to 10C/sec. The various cooling media may be water, quenching oils and aqueous solutions of quenchants such as the one identified by the Trade Mark "Aqua-Quench".
The product is preferably heat treated at a temperature of 200 to 400C, preferably about 260C for a time period of 1 to 8 hours, preferably 4 hours to transform as much retained aus-tenite as possible into martensite.
Optimum hardness was obtained in water spray cooled slugs containing 3% carbon, 0.9% silicon, 1% manganese, 1% copper and 0.5% molybdenum having a microstructure consisting primarily of carbide and martensite.
An example of the procedure followed will now be disclo-sed with reference to Figure 1 which shows the relationship of alloy hardness versus percentage of molybdenum.
A number of experiments were carried out to investigate the effect of variations in carbon, silicon, manganese, copper and molybdenum. These experiments were made to establish the opera-ting range for each addition element. The following alloys were prepared and cast into lz-in. slugs:
1) White cast iron nominally containing 0.9~ Si + 1%
Mn + 1% Cu + 0.5% Mo and either 2.0, 2.5, 3.0, 3.5, or 4.0% C.
2) White cast iron nominally containing 3% C + 1% Mn +
1% Cu + 0.5% Mo and either 0.3, 0.6, 0.9, 1.2 or 1.5% Si.

3) White cast iron nominally containing 3% C + 0.9% Si + 1% Cu + 0.5% Mo and either 0.5, 1.0 or 1.5% Mn.

4) White cast iron nominally containing 3% C + 0.9%

~05'~S99 Si + 1% Mn + 0.5% Mo and either 0.5, 1.0 or 1.5%Cu.

5) White cast iron nominally containing 3% C + 0.9%
Si + 1% Mn + 1% Cu and either 0~ 0;25, 0.5 or 1.0~ Mo.
Alloy charges consisted typically of the following com-ponents:
Pig iron, steel scrap, ferro-manganese, ferro-silicon, ferro-molybdenum and copper scrap. The various materials were melted in a coreless induction furnace equipped with an alumina crucible. The molten metal was deslagged and poured into a pre-heated, movable clay graphite tundish located above a castingstand. The casting stand comprised cast iron moulds each cont-aining a number of l~-in. slug cavities and two cooling tanks one for water spraying and one for liquid quenching. The molten metal was poured intc, the tundish, and flowed into the moulds through suitable orifices. The moulds were preheated to 120 C
and were coated with graphite. The slugs were shaken out of the - moulds at about 900C and were either water spray cooled or quen-ched in water containing 20~ Aqua-Quench~ The corresponding coo-ling rates were established using thermocouples inserted into slug cavities while the metal was still molten and connected to a recording instrument. Recording of the temperature was start-ed from the time of shake-out (900C) and continued until the slug temperature reached 150 C. It was found that the rate of cooling varied from 5 to 10C/sec. depending on the cooling med-ium. The as-cast slugs were then subjected to a heat treatment of 4 hours at 260 C. The hardness and the microstructure of the as-cast and heat treated slugs are given in the following Tables I and II.

1(~5'~S99 TABLE I

PROPERTIES OF 1-2-in. ~IITE CAST IRON GRINDING SLUGS
CONTAINING 1% Mn + 1% Cu + 0.5% Mo _ . _ _ . .

... . . .
Nominal TypeAs-Cast Heat treated for Composition of_ _ _ _,4h at 260C
_ %Cooling' Hardness Microstructure** Hardness Micro-C Si B.H.N. B.H.N. structure**
_ _ . .___ . _ __ 2.0 0.9 s.c. 495 RA + M + C 570 M* + C
_ _ a.q. 460 RA + M + C 555 M* + C
2.5 0.9 s.c. 705 M + C + RA 690 M* + C
a.q. 705 M + C + RA 670 M* + C
3.0 0.9 s.c. 710 C + M + RA 700 C + M*
_ a.q. 690 C + M + RA 690 C + M*
3.5 0.9 s.c. 655 C + M + RA + G 635 C + M* + G
a.q. 655 C + M + RA + G 615 C + M* + G
~ 4.0 0.9 s.c. 670 C + M + RA + G 655 C + M* + G
; aOqQ 595 C + M + RA + G 635 C + M* + G
. ~ _ ~ I ._ . . . . . .
l 3.0 0.3 s.c~ 655 C + M + RA 705 C + M*
I a.q. 635 C + M + RA 705 C + M*
3.0 0.6 s.c. 690 C + M + RA 705 C + M*
a.q. 635 C + M + RA 690 C + M*
. . _ 3.0 1.2 s.c. 670 C + M + RA + (G) 690 C + M* + (G) a.q. 720 C + M + RA + (G) 705 C + M* + (G) 3.0 1.5 s.c. 535 M + G + C + RA 445 M* + G + C
a.qO 560 C + M + RA + G 495 C + M* + G
.. .
* sOc. Water spray cooled a.q. Quenched in water containing 20% Aqua-Quench~
** M Martensi~e C Carbide RA Retained Austenite M* Complex phase consisting of tempered martensite, retained austenite, bainite and fresh martensite G Graphite ( ) Traces ~5;~599 TABLE II

PROPERTI~S OF l-l-in. WHITE CAST IRON GRINDING SLUGS
CONTAINING 3~

Nominal T~pe As-Cast Heat Treated for Compositionof ~ ~ 4h. at 260C
% C~oling~ Hardness Microstructure** Hardness Micro-_ B.H.N. B.H.N. structure**
I Mn Mo ~u __ ._ . _ _ 0.5 0.5 l.O s.c. 655 C + M + RA + (P) 670 C + M* + (P) a.q. 705 C + M + RA 705 C + M*
_ _. .
1.0 0.5 1.0 s.c. 710 C + M + RA 700 C + M*
a.q. 690 C + M + RA 690 C + M*
.
1.5 0.5 1.0 s.c. 685 C + M + RA 690 C + M*
a.q. 670 C + M + RA 690 C + M*
_._ _ . .
1.0 0.5 0.5 a~q. 655 C + M + RA + (P) 670 C + M + (P) _ _ _ 1.0 0.5 1.5 s.c. 685 C + M + RA 710 C + M*
_ = _ a.q. 655 C + M + RA 670 , C + M*

1.00.25 1.0 s.c. 615 C + M + RA + P 635 C + M* + P
a.q. 635 C + M + RA + P 670 C + M* + P
_ 1.0 1.0 1 0 s.c. 655 C + M + RA 710 C + M*
. a.q. 685 C + M + RA 720 C + M*
_ .

* s.cO Water spray cooled a.q. Quenched in water containing 20% Aqua-Quench~
** M Martensite C Carbide RA Retained Austenite M* Co~plex phase consisting of tempered martensite,retained austenite, bainite and fresh martensite ( ) Traces P Pearlite 105'~S99 Thc following observations can be made from these results:
1) The risk af graphite flakes formation in white cast iron containing o.9% silicon increases as the carbon content in-creases beyond 3%. The overall hardness of cast iron normally decreases as the amount of graphite flakes increases.
2) The risk of graphite flakes formation in white cast iron containing 3% carbon + 1% manganese + 1% copper and 0.5%
molybdenum increases as the silicon content increases beyond 0.9%.
It has generally been recognized that silicon contents of less than 0.6% adversely affect the fluidity of molten iron, while the present results show that silicon contents higher than 0.9% in-crease the tendency for graphite flakes formation. Thus the silicon content of the new alloy should preferably fall within the limits of 0.6 to 0.9%.
3) Traces of pearlite are present in white cast iron containing 3% carbon + 0.9% silicon + 1% copper + 0.5% molybdenum and 0.5% manganese. This indicates that more than 0.5% manganese must be present in order to avoid pearlite formation. On the other hand, manganese contents greater than 1.5% are detrimental to furnace refractories.
4) Traces of pearlite are present in white cast iron containing 3% carbon + 0.9% silicon + 1% manganese + 0.5% molyb-denum and 0.5% copper. This indicates that more than 0.5% copper must be present in order to avoid pearlite formation. With 1%
copper, no pearlite is present in the microstructure. A further increase from 1.0 to 1.5% copper did not result in additional hardness improvement.
5) Increasing the molybdenum content from 0.25 to 0.5%
significantly increases the hardness of white cast iron containing 3% carbon + 0.9% silicon + 1% manganese + 1% copper. Figure 1 illustrates the effect of increasin~ the molybdenum content on 105;~S99 the hardness of the alloy. It can be seen that increasing the molybdenum content from 0.25 to 0.5% increases the hardness from 615 B.H.N. to 710 ~.H.N. when the slugs are water spray cooled and from 635 to 690 B.H.N. when the slugs are quenched in water containing 20~ ~qua-Quench. Increasing the molybdenum content from 0.5 to 1% lowers the hardness of the water spray cooled slugs and does not significantly affect the hardness of the slugs quenched in water containing 20% Aqua-Quench. It does however increase the hardness of the slugs which were subsequently heat treated.
On the basis of the above considerations, it is clear that the optimum alloy composition to avoid the formation of graphite flakes and to obtain a high hardness and a microstruc-ture primarily consisting of carbide and martensite should be as follows:
2.5 to 3% carbon 0.6 to 0.9% silicon 1% manganese 1% copper , 0.5% molybdenum Full scale foundry tests have shown that the new white cast iron of the present invention may be melted and cast using ordinary foundry practice and casting methods. The melting equipment used so far in these full scale tests has been a chan-nel-type induction furnace. However, other melting equipment such as cupolas or various types of electric furnaces could also be used. Tests to date have been made on ll-inch grinding balls cast in permanent moulds. Sand casting could also be used provided that the products are shaken out of the mould at a temperature of 750C or higher.
Laboratory wear tests and ore grinding field tests were carried out on l'-inch diameter grinding balls CdSt from the pre-~szsg9 ferred alloy and subjected to the preferred pr~ce~sing schedule.
The results are given in Table III.
T~BLE III

RES~LTS OF WEAR RESISTANCE TESTS

_ . .. ... _ . . .
Material Size and Laboratory Test * Field test~**
Shape of Weight loss, mg Consumption __ Grinding media ~ __ lb/ton of ore New white ~ast iron ll-in, diameter 59.9 2.13 balls . _. ... _._ . . .
Forged steel balls 6302 2.38 . . ___ White cast iron l-~-in. slugs 73.5 3.18 ~ ~ .. .. .___._ * Pin Test: A cylindrical pin (0.25-in. diameter X l-in. long) is machined from the grinding media to be tested. The pin moves back and forth in a nonoverlapping pattern across a fresh abrasive cloth of 180-mesh alumina while under a load of 15 lb. As it travels, it also rotates around its axis at 20 rpm. After 7 minutes the test is stopped and the weight loss of the pin is determined. ~
** Tests conducted in industrial size ball mills for several months.
;

~OSZ~ig9 It can be seen that the new white cast iron displays a better wear resistance than chromium-bearing white cast iron and forged steel grinding media. At the present time, forged steel is the most widely used material for ore grinding in North America and its wear resistance is equivalent to that of white cast irons containing nickel and chromium.
In the above description, the alloy compositions are given in weight percent.
Although the invention has been disclosed with reference to a preferred example, it is to be understood that other alloy compositions are also envisaged within the broad range disclosed and that the various steps for making the alloy including the coo-ling rates from various shake-out temperatures and the temperature of the heat treatment may be varied within the limits defined in the accompanying claims.

30 .

._

Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A wear resistant low alloy white cast iron consis-ting essentially in weight percent of about 2 to 4% carbon, 0.3 to 1.2% silicon, 0.5 to 1.5% manganese, 0.5 to 1.5% copper and 0.25 to 1% molybdenum, the remainder being substantially all iron ex-cept for incidental impurities commonly found in cast iron, said alloy having a microstructure consisting essentially of carbide and martensite.

2. A wear resistant low alloy white cast iron as defi-ned in claim 1, wherein the composition is between 2.5 and 3% car-bon, 0.6 and 0.9% silicon, about 1% manganese, about 1% copper and about 0.5% molybdenum.

3. A process for manufacturing a wear resistant low alloy white cast iron comprising:
a) melting an alloy consisting essentially in weight percent of about 2 to 4% carbon, 0.3 to 1.2% silicon, 0.5 to 1.5%
manganese, 0.5 to 1.5% copper and 0.25 to 1% molybdenum, the re-mainder being substantially all iron except for incidental impuri-ties commonly found in cast iron, b) casting said alloy into moulds to produce the desi-red product;
c) shaking said product out of the moulds at a tempe-rature of 750°C or higher; and d) cooling the as-cast product at a rate of 2 to 15°C/sec.

4. A process as defined in claim 3, further comprising the step of heat treating the alloy at a temperature between 200 and 400°C for a time of 1 to 8 hours.

5. A process as defined in claim 4, wherein the alloy composition is essentially about 2.5 to 3% carbon, 0.6 to 0.9%
silicon, about 1% manganese, about 1% copper and about 0.5% mo-lybdenum.

6. A process as defined in claim 5, wherein the shake-out temperature is about 900°C.

7. A process as defined in claim 6, wherein cooling is done by water spraying.

8. A process as defined in claim 6, wherein cooling is done by quenching in water containing various amounts of quen-chants.

9. A process as defined in claim 7 or 8, wherein the cooling rate is between 5 and 10°C/sec.

10. A process as defined in claim 4, wherein the alloy is heat treated at a temperature of about 260°C for about 4 hours.