A new steel

Abstract

 A corrosion and abrasion resitant steel contains in percentages by mass: C 0,24, Cr 7,08, Ni 3,2, S 0 ,007, P 0,012, Si 0,027, Mn 0,02 and Mo 0,01, the balance being iron and incidental impurities. The steel was hot rolled in seven passes after being heated to 1100°C. The rolled steel was heated to 1110°C and subjected to a half hour oil quench at 238°C. The heat treated steel was both hard and tough.

Description

[0001] This invention relates to a new steel.

[0002] Wear is one of most commonly encountered problems in equipment used during mining operations. There is an increasing trend towards mechanical systems in mining. Their success to a large extent will depend on overcoming the problem of severe abrasive wear in South Africa associated with the mining and handling of extremely hard abrasive quartzitic ore. To warrant the expense of manufacturing such systems their components should have as extended a wear life as possible.

[0003] In a mining environment wear takes places due to abrasion and also due to corrosion by mine waters. Thus conventional stainless steel lasts much longer in a mining environment, in the absence of extreme abrasion, than mild steel due to the corrosion resistance of stainless steel.

[0004] It is an object of the invention to provide a new steel which is both corrosion and abrasion resistant to a greater extent then commercially available steels.

SUMMARY OF THE INVENTION

[0005] A steel according to the invention comprises, apart from iron and incidental impurities, 0,22 to 0,27% carbon 7,0 to 8,5% chromium and 2,8 to 3,2% nickel, all percentages being by mass. A nominal 8% of Cr should be aimed at.

[0006] Further according to the invention, if the following incidental impurities are present they should not exceed the percentages stated below:

[0007] Sulphur 0,008, phosphorous 0,02, copper 0,2, tin 0,03, titanium 0,001, manganese 0,5, molybdenum 0,06 and aluminium 0,05.

[0008] In the as rolled condition this steel after 7 passes shows a fairly high Vickers hardness, but a relatively low toughness on the Charpy scale. To improve the latter the steel should be heated to a temperature of the order of 1100°C and oil quenched. Tempering at a temperature of between 200 and 250°C improves the toughness still further. A further heat treatment does not improve the toughness, but at some sacrifice of toughness a slight increase in hardness may be obtained.

DESCRIPTION OF AN EMBODIMENT

[0009] In an example of the invention a steel was made which in addition to iron contained the following elements in the given percentages by mass: C 0,24, Cr 7,08, Ni 3,2, S0,007, P 0,012, Si 0,027, Mn 0,02 and Mo 0,01.

[0010] A 5 kg ingot 54mm square was rolled to a 12 mm flat bar in about seven passes. At the start of each pass the temperature was 1100°C. The finishing temperature was not measured.

[0011] Samples of the finished bar were cut and polished. The Vickers hardness and the toughness of a sample as rolled, and four samples subjected to different heat treatments were determined. The results appear in the following table.

[0012] The microstructure of the heat treated product was a fine-grained low carbon martensite with retained austenite.

[0013] Four tons of the above described steel were made by a local steelmaker and made into components for testing. The components were heat treated as in sample A3. Parts were welded together before heat treatment.

[0014] The components were characterised by having high strength and good ductility. The steel also showed a high hardness with good toughness. Usually hardness and toughness are mutually exclusive.

[0015] As indicated above, the limits for many impurities are fairly high so that standard steel making practices and equipment can be used.

[0016] The steel was found to be readily machinable and weldable so that special fabrication practices are not called for. Due to its hardness and toughness the steel is only 40% as machinable an En9 steel.

[0017] In situations where the product was exposed to the synergistic effects of corrosion and abrasion, the steel performed exceptionally well, better than commercial abrasion resistant steels. Commercial stainless steel are more corrosion resistant, but have higher coefficients of friction which makes them unsuitable for applications involving sliding abrasion, such as shaker conveyers.

Claims

1. A method of making a steel by forming a melt comprising, apart from iron and incidental impurities, 0,22 to 0,27% carbon, 7,0 to 8,5% chromium and 2,8 to 3,2% nickel, all percentages being by mass, and casting the melt to form an ingot.

2. The method claimed in claim 1 in which in the melt, if the following impurities are present, they do not exceed the percentages stated below:
sulphur 0,008, phosphorous 0,02, copper 0,2 tin 0,03, titanium 0,001, manganese 0,5, molybdenum 0,06 and aluminium 0,05.

3. The method claimed in either one of the above claims followed by the steps of rolling the ingot in a plurality of passes to form a steel product.

4. The method claimed in claim 3 in which the steel product is heated to a temperature of the order of 1100°C and is then oil quenched.

5. The method claimed in claim 4 in which the quenched product is tempered at a temperature between 200 and 250°C.

6. The method claimed in claim 5 followed by a further heat treatment of heating the product to a temperature of the order of 1100°C and oil quenching it after which it is tempered at a temperature between 200 and 250°C.