Rail steels-developments, processing and use

ASTM STP 644

The book contains twenty papers, and in some cases, their discussion, given at an ASTM Symposium in Denver, Color- ado, USA, 1 7 - 1 8 November 1976. Most of the Papers are based on North American railways' operation, but three are from the UK, two from Australia, and one from Japan. European comments on variations in practice and techniques also appear in the discussions.

It would be difficult to find aspects of the problems concerned with the manufacture and use of rail steels which have not been commented upon some- where in the text but, because it is a collection of Papers, some problems are discussed at length by more than one author. The book of 476 pages is not, therefore, as condensed as it could have been, had an editor set out to identify and describe the problems. Nonetheless, the problems embrace so wide a range that no really short version could hope to cover all the information in this volume.

A history of railway development is portrayed, showing the continual uprating due to increasing weight and capacity of rolling stock, from an aver- age truck weight of 30 tons (30.5 tonnes) in 1860 to present-day aver- ages of over 100 tons (101.6 tonnes), and on the locomotive side, from 35 tons (35.6 tonnes) to 4 0 0 - 5 0 0 tons ( 4 0 5 - 5 0 8 tonnes) for articulated steam, electric, and diesel-electric types, all dictated by industrial growth, competition and market forces. Speeds of up to 125 mile/h (200 km/h) are a reality or an aim for some lines in the near future. A modern train may comprise 10 000 tons, (~10 k tonnes), and lines in dense traffic areas can carry 70 million tons (~70 M tonnes) per annum. The development of the tee- section rail has gone from a weight of 1001bs per yard (49.5 kg/m) in 1900, to 155 lbs per yard (76.9 kg/m) in 1955, but corresponding load/wheel diameter ratios have continually increased and 900 lbs/

inch (16 Mg/m) of diameter and above have proved highly detrimental.

The papers are grouped into four sections - 'An Introduction to Raft Steels', 'Effects of Alloy Additions and Special Processing on Rail Steels', 'Strength and Fracture of Raft Steels', and 'Fatigue in Rail Steels', but there is much overlap, some Papers giving wide coverage of many aspects of me proDtems.

Standardization increased rapidly after the American Civil War, and technical advice has fallen to two organisafions - the Association of American Rail Roads and the Ameri- can Railway Engineering Association - but it is considered by some that these bodies should have much greater powers, as, even today, they cannot make their advice mandatory on materials, processing, testing, or section dimensions. Overall, the picture

painted is a gloomy one on the ability of North American Railways, in par- ticular, to cope satisfactorily and safely with the increasing demands of the traffic.

In many Papers the pattern of break- downs is detailed. Rail steel is essenti- ally medium-to-high carbon steel, but not a great deal has been published on the problems of its use as rail line. Early transverse cracks were often initiated at sites with high hydrogen content. How the problem was allevia- ted by controlled cooling is discussed along with detailed commentary on raising the general quality of rail steels. In 1976, however, only one rail steel mill in North America had vacum degassing facilities, the modern effec- tive method of reducing overall gas content. Full heat treatment of the steel can add about fifty per cent to the rail strength, giving increased resistance to plastic flow, wear, fatigue, and shelling, the principal failure modes. Shelling first appeared with the wide introduction of heavier rolling stock in the 1940's and its mechanism is deformation of the steel beyond its elastic limit causing

sub-surface shear. This is a highly des- tructive damage mechanism which is encountered most frequently in the outer rail of curves. The severity of damage can be reduced by lubrication of the rails, but the reduction is at t h e expense of increased contact fatigue, so the subject of lubrication remains controversial.

The special features of failures arising at rail joints, again mostly in conditions of overloading are con- sidered. The chief results of the stresses of the joints are load-to-web separation, cracks from the ends of bolt-holes, and end-face metal flow giving rise to build-up. Like the rails in general, the limit has been reached in the ability to increase section thickness and give more generous fillets. Quarter-mile (0.4 kin) welded lengths of rail with bolted joints, the bolt-holes of which are farther from the ends, and hardened end faces, have been used to counter frequent joint failures.

The methods of welding rail lines are surveyed, and the processes dis- cussed include thermite, gas, flash, and arc welding. Porosity, voids, and non-metallic inclusions are the chief inherent faults of the thermite process, but, as in other forms of welding where excess metal has to be removed by grinding, martensitic areas have resul- ted in brittle cracks. Sounder welds are obtained by flash welding but, with sufficient upset, there are rail end straightening problems. Gas- pressure welding is similar in that hot abutting faces are forced together, but this process is relatively slow, Eleetroslag, submerged and enclosed arc procedures are all used, but to a lesser extent than the foregoing.

Metallurgical work is described in the study of the effect of pearlite lamellar refinement, grain boundary condition and prior austenitic grain size on strength and toughness. Even in the heat-treated condition, carbon rail steels are failing to cope with some traffic conditions, and

44 INT. J. FATIGUE January 1980

further grain refinement with both hardness and ductility increases has been achieved by the use of small additions of vanadium. Australian laboratory investigations of the effects of vanadium, chromium, niobium (USA-columbium) and molyb- denum have shown that, although much stronger as well more ductile rail steels can be manufactured, there are accom- panying disadvantages - particularly welding problems and brittle zones from microstructural irregularities. A European patented compound rail is described where the rail head consists of a hard 1% chromium steel on a softer, more ductile web and base, while British Steel Corporation's continuous casting techniques are claimed to provide superior internal and external properties, reduced inclusion content, more uniformity, and controlled gas content.

Optical and electron microscopic

techniques and fractography have been used to study the effects of microstructure, defects, lack of uni- formiw, and inclusion content on vari- ous mechanisms of rail destruction, such as checks (surface cracks associa- ted with inclusions), gross plastic deformation, abrasive wear, corruga- tions, fatigue, and shelling. There are many high-quality photographs, photomicrographs and electron micrographs in the book.

Accelerated laboratory tests, stress analysis and fracture mechanics have been used to study fatigue in rail steel but ambivalent views have arisen, some authors opining that varying com- position and structure had little effect, while others thought strict micro- structural control was essential to reduce crack growth rate. Further attempts at simulating, in the labora- tory, realistic applications of service stress conditions will, no doubt, have

to be continually expanded. The book presents the situation

( 1976 ), particularly in North America, of the attempted effort to satisfactorily maintain railway lines as a continuing challenge, heightened by economics and constricted by market fo~c~-s. With- out further research into the optimiza- tion and commercial application of alloy steels or reduction of traffic weight and level, general deterioration is likely to escalate, with increasing frequency of rag fagures, accidents, and spiralling mamte~c'e costs.

The contents of the book are well presented, with clear type and good lay- out, making it pleasant to peruse or con- sult. Engineers and metallurgists, as well senior administrative staff concerned with rag systems, should find it most useful and interesting.

J. B lackwe l l

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