US1908889A - Martensitic cracking still tube - Google Patents

Description

Patented May 16, 1933' I in,"

WERE 1B. BROWNE, F BRACKENRIDGE, PENNSYLVANIA.

. rmsrrrc CRACKING srrnr. TUBE No Drawing.

This invention relates to cracking still tubes used in the production of gasoline and other hydrocarbon products from mineral oil, and relates more particularly to a new martensitic alloy material from which these tubes can be made.

In the production of gasoline and other hydrocarbon products from mineral oil, most of the processes involve relatively high tem peratures and high pressures. In the construction of equipment for hydrogenation processes, as well as for the'ordinarycrack ing still processes, care must beexercised in selecting the proper material, and in many' cases development of the process has been retarded due to the fact that there was no 7 metal available which had all the characteristics necessary to make the process successful.

Ordinary steel tubes have been extensively used in' these various processes, but-because they corrode rapidlyand oxidize at relatively low temperatures, and. because steel has very little strength at the temperatures which are used, it has been necessary to use tubes of extremely heavy Wall section, but even with this precaution the life of the equipment has been extremely short, and the industry is always confronted with a fear .of the disastrous explosion which would sacrifice many lives, to say nothing of the de-. struction of very costly equipment.

Ameasure of success has been-obtained by the application to this use of an alloy steel tube with a composition of 18% chromium and 8% nickel. This alloy almost fulfillsmost of the requirements of the service. It resists the corroding influences of the process and does not oxidizewithin the temperature range of theservice. It is an austenitic alloy, and therefore has great strength at high temperatures. Unfortunately, all ans-- tenitic alloys which are solid solutions of the various constituents are unstable and susceptible to dissociation if heated between the Application filed September 1, 1931. Serial No. 560,652.

range of temperature of 800. F. and 1400 F. This dissociation precipitates a constituent assumed to be carbide between the grain boundaries, and when this dissociation has taken place, the material loses its resistance to shock andunder certain conditions of serviceis liable to rupture suddenly.

In the operation of cracking still processes, a steel tube when subjected to any unusual condition of pressure and temperature will, because of its ductility, bulge before it actually ruptures. This characteristic makes it possible for the operator to detectthe unusual condition and stop the operation before a disastrous explosion takes place. When austenitic alloy tubes are used, even though theyare far superior to ordinary steel from the standpoint of corrosion and other factors involved in the process, they become hot short due to the dissociation of carbide, and when an unusual condition of pressure and temperature exists they sud-1 ,denly rupture without any. warning such as is indicated by steel tubes. This undesirable property of the austenitic alloys makes their use extremelydangerous under such condi tlons.

One. of the objects of the present invention is to provide a cracking still tube which will obviate all the disadvantages and objections of the prior art cracking still tubes and'which will provide advantages peculiar to its own constitution. a

Another 'obj ect of the invention is the provision of a cracking still tube which is composed of a metal which is capable of resisting the corroding influence of high sulphur oils and which has a stable structure when operated at temperatures up to 1400 F.

A further object of the invention is the provision of a cracking still tube which has relatively great strength at temperatures up to 1400 F.; which resists oxidization within the above temperature range and which is characterized by high resistance to shock when heated up to 1400 F.

A still further object of the invention is the provision of a cracking still tube composed of a metal that does not become emrittled or cold short after long exposure at temperatures within the above range, and which has a high ductility in order to avoid sudden rupture during the process.

A still further object is the provision of a martensitic cracking still tube which is composed of an alloy containing about 12% chromium and about 3% tungsten and which will bulge when it is subjected to stress greater I than its creep limit, thus giving the necessary warning to avoid an explosion.

The ideal metal for the purpose should be capable of resisting the corroding influence of high sulphur oils, should have a stable structure when'operated at a temperature up to 1400 F., and should have relatively high strength at temperatures up to 1400 F.; it should also resist oxidization within the above temperature range, and further, be characterized by high resistance to shock when heated up to 1400 F. The metal should not become embrittled or cold short after long exposure to temperature within the above range and should have a high ductility to avoid sudden rupture during the process.

I have discovered that I can obtain all these ideal conditions by means of a martensitic cracking still tube composed of the ingredients hereinafter designated. I have found, for exam le that analloytube containing in the neigh orhood of 12% chromium and 3% tungsten fulfills all these requirements for the particular type of service, and material of within the temperature range of the process and under conditions of service which produce a stress greater than its creep limit the metal will bulge, giving the necessary warning to avoid an ex losion.

I have found t at the alloy most suitable for the purposes above specified comes within the followin range of composition carbon up to about 1 o, chromium from about 8% to about 16%, and tungsten from about 0.5% to 5%. A typical alloy according to the present invention contains a maximum of about 0.12% of carbon, about 12% to 14% of chromlum, and about 2% to 4% of tungsten. A

tube within the above range of composition.

will be characterized by great strength at high temperatures, resistance to corrosion and oxidization, and will be ductile and have great resistance to shock at all temperatures within the operating range of the cracking still process.

It is tobe understood that the above range and typical analysis are purely illustrative and I am not limited thereto, as I may vary them within considerable limits without however departing from the spirit and scope of the present invention. It may, for example, be desirable to vary the composition or the ingredients for special applications where exceptional properties are required. Such variations, modifications, substitutions or omissions are within the spirit and scope of the present invention, particularly insofar as covered by the subjoined claims.

What I claim as new'and desire to secure by Letters Patent is: I

1. As a new article of manufacture, a martensitic alloy steel cracking still tube characterized by high resistance to corrosion from high sulphur oils; by stability of structure, relatively great strength and resistance to oxidation attemperatures up to and as high as 1400 F.; by retention of unembrittlement even after long exposure to temperatures up to and as high as 1400 F.; and by bulging rather than bursting when subjected to stress greater than its creep limit, said tube comprising in combination the following elements: carbon from a very small fraction up to about 1%; chromium from about 8% to about 16%; tungsten from about 0.5% to about 5%; the balance being substantially only iron.

2. As a new article of manufacture, a martensitic-alloysteel cracking still tube characterized by high resistance to corrosion from high sulphur oils; by stability of structure, relatively great strength and resistance to oxidation at temperatures up to and as high as 1400 F.; by retention of unembrittlement even after long exposure to temperatures up to and as high as 1400 F.; and by bulging rather than bursting when subjected to stress greater than its creep limit, said tube com prising in combination the following elements: carbon in the neighborhood of about 0.12%; chromium in the neighborhood of about 12-14%; tungsten in the neighborhood of about 24% the balance being substantially only iron.

3. As a new article of manufacture, a martensltic alloy steel cracking still tube characterlzed by high resistance to corrosion from high sulphur oils; by stability of structure,

relatively great strength and resistance to oxidation at temperatures up to and as high as 1400 F.; by retention of unembrittlement even after long exposure to temperatures up to and as high as 1400 F.; and by bulging rather than bursting when subjected to stress greater than its creep limit, said tube COII1- prising in combination the following elements: carbon from a very small fraction up to about 1%; chromium in the neighborhood of about 12%; tungsten in the neighborhood of about 3%; the balance being substantially only iron. I

In testimony whereof, I have hereunto subscribed my name this 28th day of August,

VERE B. BROWN E.