US3290185A - Alloy steel - Google Patents

Description

United States Patent No Drawing. Filed Mar. 21, 1962, Ser. No. 181,450 1 Claim. (Cl. 14836) This invention relate to alloy steels, and provides alloy steels which can be made without the aid of costly alloying ingredients or difficult and costly manufacturing techniques, and which have valuable properties,

An alloy steel according to the invention contains as essential alloying ingredients 0.6 to 1.0% carbon, 0.2 to 0.5% (preferably 0.2 to 0.35%) aluminum and 0.2 to 0.5 (preferably 0.2 to 0.35%) silicon, and optionally, carbide stabilizers such as manganese chromium and molybdenum up to a total at which graphitisation is not substantially hindered (preferably not more than 0.3% manganese and not more than 0.5% other carbide stabilizers), and any other non-carbide-forming elements up to a total of 20%, the balance being iron and impurities.

The aluminum and the silicon constitute graphitising agents; that is to say, they have the property of transforming the carbon content from the dissolved or combine (e.g. iron carbide) form in which it imparts hardness to the steel, to a graphitic form in which it is present in the steel as graphite particles, the steel in the latter form having properties of softness and ductility akin to those of essentially ferritic steels. Indeed, those properties are in some cases even better than those of ferritic steels, apparently due to the graphite particles acting as an internal lubricant to facilitate deformation of the grain structure,

Aluminum is a better graphitising agent than silicon, but it is not practicable to increase the percentage of aluminum above the stated upper limit in order to obtain the desired degree of graphitisation since this would weaken the steel. Although silicon is a less effective graphitising agent that aluminum it is used to bring the total content of grap'hitising agents up to the necessary level without the weakening that would be experienced if aluminium alone were used for this purpose.

The said impurities may include, for example, phosphorus, sulphur, and nitrogen. Where sulphur is present the alloy should also contain at least the stoichiometric amount of manganese required to combine with the sulphur, but the alloy may contain more manganese than this up to the above mentioned upper limit. The phos- .phorus content should not exceed 0.1%, and up to that amount may even be useful for increasing the hardness of the steel when in the ferritic or pearlitic states and the hardenability when in the martensitic state.

The other non-carbide-forming elements mentioned include, for example, copper and nickel which have the elTect of increasing the strength of the steel and also increasing its resistance to corrosion. For ordinary purposes, therefore, the use of such additional elements is unnecessary in that the steel without them has adequate properties for most ordinary purposes, and their use would increase the cost of the product.

The steel can exist at ordinary temperature in three different states, namely, a graphitised state, a martensitic state and a pearlitic state.

In the graphitised state, all or most of the carbon content of the steel is in the form of globular graphite. This state is quite characteristic of the steel according to the invention. It distinguishes this steel from all known kinds of steel for the double reason that a similar state does not exist for other kinds of steel, and in this state the steel according to the invention can be worked like Patented Dec. 6, 1966 low-carbon mild steel by plastic deformation, in spite of its considerably higher carbon content.

A graphitised state has been known for cast iron. Moreover, there exists a so-called high-carbon steel con taining about 1.5% of carbon which is difficult to classify. That material, however, is neither steel nor cast iron and can be used for very limited purposes only.

If the steel according to the invention is heated to a sufliciently high temperature, it becomes au'stenitic, and by being quench-hardened it is converted into martensitic steel. This latter can be tempered to any desired degree of hardness by being re-heated in a conventional manner and for a conventional, relatively short time, whereby the steel is converted into the sorbitic state. The graphitised state can be produced it the steel in either the martensitic of the pearlitic state is heated for a long time at a moderate temperature in air and allowed to cool gradually. This treatment first produces globular carbides which subsequently decompose and form globular graphite. The heat-treatment is successfully terminated when the steel has assumed the structure of ferrite grains with globular graphite at the boundaries of the grains. The graphitisation needs to be carried out in the presence of oxygen if it is to proceed quickly.

The maximum amount of carbide stabilizers which can be tolerated varies with their nature and with the nature and proportions of the other constituents of the alloy. However, in any particular case one can ascertain whether the percentage of carbide stabilizers has been exceeded by testing a sample which ha undergone the heat treatment intended to produce the graphitised state for the presence of free graphite, e.g, by microscopy or chemical dissolution followed by filtration.

The expression carbide stabilizers is well known in the steel art. However, for purpose of clarity some further explanation may be in order. It is, of course, well known that metals form carbides. However, to be a carbide stabilizer for steel, the heat of reaction of the respective carbide must be higher than that of the iron carbide. A mixture of several metals such as tungsten, chrome and the like will from complex carbides in the steel. Generally the higher the heat of reaction of the metal carbide, the greater its stability. Typical well known carbide stabilizers include chromium, molybdenum, vanadum and titanium. The quantity of carbide stabilzer present n an alloy will vary. considerably. Thus it is known that the lower the heat of reaction of the carbide the greater may be the quantity of stabilizer present without adverse elfect. Similarly the greater the quantity of carbon, silicon and aluminium in the alloy, the greater the quantity of stabilizer permitted.

The following mechanical properties are typicalfor the steel according to the invention in graphitised, pearlitic and martensitic states:

Graph- Peral- Martenitised itic sitic Tensile strength, kgJmm. 35-40 240 Proof stress, kg./mm. 12-30 90 240 Elongation on 50 mm gauge length,

percent 30-40 15 Re'luction of area, perce 35-50 20 Brindell hardness -120 270 685 with mild steel by a mere heat-hardening operation and are comparable to those for the achievement of which a case-hardening operation is usually required.

These properties. render the steel according to the invention especially suitable as a material for making tools, screws and many other articles that are produced by plastic deformation followed by a hardening operation, since the hardening can be achieved by a mere heat-treatment.

For example, if the steel according to the invention is heated for a short time, e.g. about minutes for a cross section of 25 mm. diameter, to temperatures from 750 to 850 C., preferably about 800 C., it assumes the austenitic state in which all the carbon ha dissolved in the austenite. Subsequent quench-hardening produces the martensitic state of the steel in which the carbon is still dissolved as a supersaturated solution, The martensitic steel is converted into the graphitised state by a heat-treatment at temperatures in the range from about 600 to about 720C., preferably about 650 C., for a long time, being of the order of 25 to 50 hours, preferably about 40 hours, for a cross section of 25 mm. diameter. This heat treatment is preferably performed in a partly oxidising atmosphere, e.g. in air. After this heat treatment the steel is allowed to cool gradually. It is now in the graphitised state in which it can be subjected to plastic deformation without recrystallsation.

Work-pieces made from the graphitised steel can be hardened by being heated to a temperature from 800 to 900 C. preferably about 850 C., for a relatively short time, e.g. between 15 and 30 minutes, in air or in a reducing or a neutral atmosphere, which heat-treatment is followed by quenching and tempering. The temperature at which steel in the rnartensitic state is converted into steel in the graphitic state is known in the art as the lower critical transformation temperature. The temperaure at which the steel in the graphitic state is converted into the austenitic form is known as the upper critical transformation temperature.

In some cases the steel according to the invention which has been hardened by heat treatment exhibits greater hardness than a heat treated carbon steel of the same carbon content.

To summarise, the present invention provides an alloy steel which need not contain high cost alloying elements, which has properties of workability and mechinability comparable to those of mild steel, yet which by simple heat treatment can be transformed so as to have mechanical properties similar to those of a straight carbon steel of the same carbon content,

A specific example of a steel according to the invention is as follows:

Percent Carbon 0.8 Aluminium 0.3 Undissolved aluminum 0.02 Silicon 0.3 Manganese 0.25 Sulphur 0.05 Phosphorus 0.05 Total other carbide stabilizers 0.05

Balance, iron and impurities.

After casting, homogenization, and rolling this steel showed, in its structure, elongated manganese sulphide of the constituents in the austenite, an austenitisation temperature 70 C. higher than the Ac point 830 C. is recommended.

Water quenching from this temperature resulted in a fully martensitic structure entirely free of carbides. In this martensitic state the steel should be subjected to graphitization treatment at 650 C. (in a 6 mm. dia. specimen the reaction set in after approx. 2 hours and ended after approximately 20 hours). The graphitization treatment should be carried out in an oxidising atmosphere. In the fully graphitised state the steel had a Brinell hardness of approximately 100 and was very suitable for manufacturing processes involving heavy plastic deformation as well as for machining at a high rate of metal removal.

The employment of an oxygen atmosphere is desirable because oxidation within the steel stnucture catalyses graphitization. More precisely, the oxygen diffuses into the steel and oxidizes the silicon and/ or aluminium. This oxidation acts to remove carbon from solution because the carbon precipitates out as graphite on the newly formed oxides.

The finished product could be austentised by conventional methods and quench-hardened to obtain a hardness of approximately 700' Brinell.

In technological tests it was established that the hardenability of the steel was superior to that of its straight carbon counterpart of similar carbon content. Its susceptibility to quenching cracks was substantially lower than that of a similar straight carbon steel. Its mechanical properties were superior to those of similar straight carbon steel. In the fully graphitised state it was softer than dead mild steel, and in the fully martensitic state it was both harder and stronger than its 0.8% straight carbon counterpart while possessing equal thermal stability at moderate temperatures. Even in the pearlitic state this steel was stronger than its straight carbon steel counterpart.

This steel could be employed for parts having to undergo heavy cold deformation in their manufacture, such as hobbed plastic dies or Allen bolts. After the final forming process the parts may be subjected to the conventional hardening, tempering or nitriding processes according to individual requirements. The steel can be supplied by the producer in the fully graphitised state with a Brinell hardness of the finished part to 680, with a corfacturer can, after forming and machining, increase the Brinell hardness of the finished part to 680, with a corresponding tensile strength of 240 kg./mm. by tempering it at 200 C. for 30 minutes after the hardening operation. This procedure is especially recommended for essentially cheap parts, which have to undergo heavy cold deformation during manufacture and whose final bulk properties are of major importance.

What we claim is:

An alloy steel in the substantially full graphitized state containing by weight about 0.6 to 1.0% carbon; 0.2 to 0.5% aluminium, 0.2 to 0.35% silicon and the remainder all iron and impurities, andbeing substantially free from carbide stabilizers.

References Cited by the Examiner UNITED STATES PATENTS 1,850,953 3/1932 Armstrong -124 1,939,390 12/1933 Cox 75124 2,087,764 7/1937 Bonte 148-12 2,413,602 12/1946 Bonte 75123 2,478,723 8/ 1949' Trantin 75124 2,749,238 6/1956 Millis et a1. 75123 DAVID L. RECK, Primary Examiner.

N. F. MARKVA, P. WEINSTEIN, Assistant Examiners.