Metal treatment – Stock – Ferrous
Patent
1996-02-22
1997-07-08
Zimmerman, John
Metal treatment
Stock
Ferrous
238150, E01B 508, C21D 904
Patent
active
056456530
DESCRIPTION:
BRIEF SUMMARY
This application was filed under 35 USC 371 from PCT/GB94/01326 filed Jun. 20, 1994.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to rails and in particular to rails exhibiting improved strength, hardness and toughness.
2. Description of Related Art
The problems with making rails for railways are well known and may be summarised as the difficulty of providing both a hard running surface together with a tough rail which in this technology means having a resistance to fracture. Treatments of the head to make it hard are well known, but in general are found to have corresponding deleterious effects on the toughness. The rail must be able to resist the propagation of fatigue cracks.
Modern high performance rails are currently made by rolling steel of an appropriate composition and then cooling it. The rail may be cooled either directly after leaving the rolling mill, perhaps having been reheated, or after subsequent heat treatment. Cooling is controlled and the object is to create pearlite as the main component of the rail head. This pearlite has particular qualities of hardness and the cooling rate is in fact controlled to be below a particular rate for the steel composition in question so that it passes into what is known as the perlitic area on the continuous cooling transition (CCT) diagram for the steel. In some cases the cooling may be particularly controlled so that the path on the CCT diagram to passes through what is known as the "perlitic nose" when a pearlite of a fine inter lamellar spacing and consequently higher strength and hardness is produced. Unfortunately modern rail technology is now approaching the limits of hardness that can be achieved by a perlitic head because of the reductions in toughness brought about by the processing for increased hardness.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a rail having an improved fracture toughness impact resistance for a given hardness.
According to the present invention there is provided a rail for use in a railway having a head and a foot the head being a traffic carrying surface composed of low carbon martensite. The rail may be rolled from a low carbon steel, and the head, and optionally the foot, may be rapidly cooled by the application of water or water/air sprays. The carbon content of the rail may be between 0.1 and 0.4% and the rail may have alloying elements to improve the hardenability and may also contain titanium and niobium. The hardenability may fall into the ranges shown in Table 3 and the rail may be allowed to self temper by terminating the spray cooling and allowing the residual heat in the rail head to equalise under natural cooling.
The invention will now be described by way of example and with reference to the accompanying drawings
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a martensitic headed rail;
FIG. 2 is a representation of the Brinell hardness results for such a rail
FIG. 3 is a diagram of the relationship between wear rate and hardness for pearlitic and martensitic rails;
FIG. 4 is a diagram of the Jominy Hardenability data for a low carbon alloy steel;
FIG. 5 is a diagram of the variation of the Charpy V-notch impact energy for martensitic and pearlitic rails at varying temperatures;
FIG. 6 is a schematic diagram of one cooling arrangement for the production of rails;
FIG. 7 is a diagram of the hardenability bands for the production of martensitic rails; and
FIG. 8 is a schematic representation of the continuous cooling transformation diagram for a 0.8% carbon steel.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to FIG. 1 this shows a conventionally shaped flat bottomed railway rail 1. It has a foot 2 and head 3. The micro structure of the head in the shaded area 4 is martensite, while in region 5, where clearly the rate of cooling from external sprays is less it is a mixture of martensite and bainite. Where the foot has been cooled it is also largely martensite and the composition of the web 6 joining the foot and the hea
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Database WPI, Week 8433, Derwent Publications Ltd., London GB; AN 84-203941 & JP,A,59 116 321 (Nippon Steel Corp.) 5 Apr. 1984.
Jerath Vijay
Martin Ian W.
Price David J.
British Steel PLC
Zimmerman John
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