GB2072702A - White cast iron - Google Patents

Abstract

A white cast iron alloy is produced with a sub-critical heat treatment to give hardness values in excess of 750 H.V. The alloy comprises, by weight, Carbon 2.8% - 3.5% Silicon 0.6% - 2.0% Manganese 0.05% - 0.5% Sulphur 0.05% - 0.25% Phosphorus 0.5% - 1.5% Nickel 3.5% - 5.0% Chromium 2.5% - 4.5% Molybdenum 0.2% - 0.7% With the balance being iron and incidental impurities. The alloy may contaln up to 0.01% bismuth by weight to ensure against possible graphite formation particularly in heavy section castings.

Description

SPECIFICATION High hardness cast irons This invention relates to white or carbidic cast irons alloyed with nickel and chromium to give very good abrasion resistance. Such cast irons have well established uses in the metallurgical, mining, power, cement, ceramic, paint, coal, foundry and other industries and have proved to be an economic replacement both for ordinary white iron and manganese steel because, although they do not have the impact resistance of the latter alloy, this is not required for many abrasive resistance applications.

The micro-structure of known nickel-chromium irons consists of about fifty per cent of eutectic carbides in a matrix of martensite, bainite and retained austenite. The irons can be used in the as cast condition but it is usually recommended that a sub-critical heat treatment, such as four to twelve hours at a temperature in the range 200"C. to 2750C. be applied. Such heat treatment increases the toughness and may increase the hardness, although more marked improvement in the latter property can be achieved by a doubie treatment at 450"C. and 275"C.; however this more expensive technique is generally not justified.

The hardness of nickel-chromium martensitic cast irons ranges from approximately 580 H.V. to as high as 750 H.V. and the best abrasion resistance is generally found with the highest hardness. Many standard specifications designate a minimum hardness in nickel-chromium martensitic irons of 538 H.V. to 585 H.V.

and it will be appreciated by those in the art that this is an unacceptably low requirement. For example hardened steel gives values of at least 800 H.V. and the reason for the relatively low hardness values for nickel-chromium martensitic irons is associated with two considerations viz.

1. The presence of significant amounts of retained austenite is normal with the compositions presently used and can only be eliminated to a significant extent either by a very prolonged sub-critical treatment or a high temperature heat treatment.

2. High temperature treatments which involve re-heating into the austenite range followed by air cooling, and which could lead to better hardness values, are not usually desirable because the relative brittleness of the irons makes the castings susceptible to cracking.

It may therefore be stated that although the known nickel-chromium martensitic irons perform quite well there is a need to develop higher hardness values. Thus in recent years more expensive materials having intrinsically higher hardness values have started to replace the nickel-chromium martensitic irons in industrial use.

A known composition range for a cast nickel-chromium martensitic white iron is as follows, the percentages being by weight, Total Carbon 2.8% - 3.5% Silicon 0.3%-0.5% Manganese 0.3%- 0.7% Sulphur 0.15 maximum Phosphorous 0.3% maximum Nickel 3.5%-4.5% Chromium 1.5%-2.5% Molybdenum 0 -0.4% Balance iron and incidental impurities In particular it is generally stated that the silicon content should not exceed approximately 0.5% because, with irons of established composition, graphite will form during solidification of the casting so that hardness is reduced.Also it is generally recommended that the manganese content should not normally fall below 0.3% because the hardenability of the iron could be reduced by pearliteformation. Normally this has hitherto been regarded as being undesirable because the hardness of the iron would again be lowered.

As a typical example of present practice an iron of the following composition has been produced, the percentages being by weight, 3.65% carbon; 0.62% silicon; 0.57% manganese; 0.062% sulphur; 0.106% phosphorous; 3.99% nickel; 2.25% chromium; with the balance being iron and incidental impurities.

Such an iron had a matrix structure which was approximately 40% transformed to martensite and the hardness was 626H.V. in the as cast state. After heating for 16 hours at 275"C. the hardness increased to 647 H.V. whilst a treatment for 16 hours at 2000C resulted in a hardness of 655 H.V. Such a cast of course readily achieves the minimum standard specification but, as stated above, it has now become desirable to provide a higher hardness value in the cast.

It is the object of the present invention to provide a white cast iron alloy having improved hardness in comparison with irons already known.

Thus in accordance with the present invention there is provided a white cast iron alloy comprising, by weight, Carbon 2.8% - 3.5% Silicon 0.6% - 2.0% Manganese 0.05% - 0.5% Sulphur 0.05% - 0.25% Phosphorous 0.5% - 1.5% Nickel 3.5% - 5.0% Chromium 2.5% -4.5% Molybdenum 0.2% - 0.7% With the balance being iron and incidental impurities The composition of the alloy defined in the preceding paragraph may be modified by the deliberate introduction of up to 0.01% by weight of bismuth.

It will be appreciated that the alloying additions having a significant effect on the characteristics of the cast.

Thus it is known that retained carbon in the matrix leads to retained austenite which is undesirable, whereas it is most desirable, from the hardness aspect, to effect the maximum possible transformation to martensite.

It is also known that silicon and nickel favour graphite formation which is undesirable in the matrix as this leads to a reduction in hardness, whereas it is desirable to cause the formation of carbides.

The alloy in accordance with the invention conveniently has a hardness in excess of 750 H.V.

Thus the cast iron in accordance with the invention has been developed from the following premises.

1. A high silicon vague will lead to a harder matrix but will also promote the formation of graphite especially in large slowly cooled castings. On the other hand high silicon and phosphorous values would be advantageous to remove carbon from solution and thus reduce the retained austenite.

2. A high manganese value lowers the transformation temperature at which martensite is formed and it is therefore desirable to lower the manganese value. By keeping a relatively high sulphur level, the sulphur will combine with the manganese effectively to remove it.

3. Nickel favours graphite formation and can be restricted, with the introduction of molybdenum, to promote hardenability. However if nickel is too low a pearlitic matrix will be formed but if relatively high chromium values are present to take the carbon into a carbide phase, the molybdenum levels can be lowered and the nickel retained. It is economically desirable to lower the molybdenum level as molybdenum is a far more expensive element than nickel.

4. Additionally to ensure against possible graphite formation in heavy section castings, advantage may be taken of the well known carbide stabilising effect of small bismuth additions eg. up to 0.01% by weight.

Thus in accordance with the invention a white cast iron of high hardness can be produced wherein the composition, by weight, comprises the following ranges of elements: Carbon 2.8% - 3.5% Silicon 0.6% -2.0% Manganese 0.05% - 0.5% Sulphur 0.05% - 0.25% Phosphorous 0.5% -1.5% Nickel 3.5% -5.0% Chromium 2.5% -4.5% Molybdenum 0.2% -0.7% With the balance being iron and incidental impurities By way of specific example a white iron was sand cast and was analysed to the following composition by weight, 3.12% Carbon; 1.23% silicon; 0.31% manganese; 0.11% sulphur; 0.52% phosphorous; 4.13% nickel; 3.23% chromium; 0.29% molybdenum; and 0.0056% bismuth with the balance being iron and incidental impurities.

Keel blocks and transverse rupture test bars were cast and the following results obtained: Sample cut from Hardness Keel Block Head H. V. 30 63.5 mm.x 60.3 mm x 57.1 mm. 785 Section average The rupture bars were heat treated for eight hours at 275"C and the hardness was found to be 754 HV.30 whilst the following properties were obtained: Transverse Rupture Deflection in mm.

strength KNlmm2) 375 6.3 375 6.8 384 7.6 Also 50 mm. diameter balls were cast, heat treated for eight hours at 275"C and then tested on a fatigue drop machine. The results obtained were as follows: New Grade of Conventional lron in accordance Ni-hard lron with the invention 1. 6,721 1. 3.838* 2. 9,528 2. 5,848 3. 10,569 3. 1,917 Drops to fracture 4. 4,379 Drops to fracture *Spalled Afurtherexperimental castwas made to produce 35 mm. diameter bars to the following composition by weight: 2.89% Carbon; 1.50% silicon; 0.31% manganese; 0.074% sulphur; 1.13% phosphorous; 3.93% nickel; 2.81% chromium; 0.39% molybdenum and 0.005% bismuth with the balance being iron and incidental impurities.

The following hardness results were obtained: As Cast Heat Treated 8 Hours at 2750C 2 Hours at8000C air cooled air cooled 819 HV.30 804 HV.30 870.HV.30 It will be seen that these values compare favourably to the normal 630 HV.30 hardness obtained from conventional Ni-hard Type 2 iron.

Claims (5)

1. Awhite cast iron alloy comprising, by weight, Carbon 2.8% -3.5% Silicon 0.6% - 2.0% Manganese 0.05% - 0.5% Sulphur 0.05% - 0.25% Phosphorous 0.5% - 1.5% Nickel 3.5% - 5.0% Chromium 2.5% -4.5% Molybdenum 0.2% -0.7% With the balance being iron and incidental impurities

2. Awhite cast iron alloy comprising, by weight, 3.12% carbon; 1.23% silicon; 0.31% manganese; 0.11% sulphur; 0.52% phosphorous; 4.13% nickel; 3.23% chromium; 0.29% molybdenum with the balance being iron and incidental impurities.

3. A white cast iron alloy comprising, by weight, 2.89% carbon; 1.50% silicon; 0.31% manganese; 0.074% sulphur; 1.13% phosphorous; 3.93% nickel; 2.81% chromium; 0.39% molybdenum with the balance being iron and incidental impurities.

4. An alloy as claimed in any one of Claims 1 to 3 including up to 0.01% by weight of bismuth.

5. An alloy as claimed in any one of the preceding claims having a hardness in excess of 750 H.V.