INTRO: Continuous casting of round billets has been combined with a Japanese head-hardening technique to enable the production in Pueblo of premium long-welded rails with a Brinell hardness of 390
BYLINE: Mark McLean
Manager, Rail ProductsCF&I Steel LP
IN MAY this year CF&I Steel LP uprated the specification of its premium head-hardened rail from DHH370 to DHH390, reflecting the achievement of an average surface hardness of 390 Brinell - which makes it the hardest rail available anywhere in the world. DHH390 is 14% harder than the AREA premium rail specification of 341 Brinell, whilst meeting all the metallurgical specifications.
The launch of the World Standard DHH390 rails marks the completion of a three-year upgrading of the Pueblo rail plant, following the takeover of CF&I by Oregon Steel Mills in 1993. The US$100m investment programme included conversion of the steel production plant from rectangular ingots to provide continuously-cast rounds for the rail mill, and the introduction of in-line head-hardening equipment using technology developed by Nippon Steel Corp in Japan. Further modifications to the Pueblo mill are now being drawn up to meet demands for tighter dimensional tolerances on finished rails.
The entire investment strategy was driven by the increased demand from US railroads for longer-lasting low-maintenance rails capable of handling increased loadings, higher speeds, and ever-changing track dynamics. Above all there is a constant drive for lower costs and improved quality from maintenance-of-way departments.
While calling for harder, cleaner rails and tighter dimensional tolerances, and for better performance by premium wear-resistant rails used on heavy-haul corridors and in sharp curves, the railroads also wanted quality assurance from the use of proven manufacturing and processing technologies.
The first major change at Pueblo came in February 1995, with a switch to 100% continuous cast rounds following modifications to the Mannesmann-Demag caster. The use of rounds up to 311mm diameter for rail manufacture offers three main advantages over conventional ingots. The round section offers improved control of cooling on the caster and re-heating in the rail mill furnace, which is particularly critical when producing premium rails.
Quality of the rail is improved by the elimination of the corners, which can lead to rail surface defects caused by corner cracking problems when using a square or rectangular cross-section. Similarly, a round section offers improved solidification patterns which reduce the risk of centre segregation and unacceptable macro-etch patterns inherent with square or rectangular castings.
At the same time a new twin-station ladle refining furnace and vacuum tank degasser were installed, allowing for adjustments in steel chemistry, precise temperature control and thermal homogeneity. Improved steel cleanliness improves the rail’s fatigue life and reduces shelling and detailed fractures, whilst precise control of the chemical content ensures consistent physical properties for both carbon steel and premium rails.
The Nippon Steel in-line head-hardening unit came on stream in July 1996, replacing an off-line hardening plant with an annual capacity of just 40000 tonnes. The Nippon unit has a theoretical capacity of 400000 tonnes a year. It makes use of the residual heat in the rails following rolling, with computer-controlled quenching for tight thermal management of the newly-rolled rails.
The process starts with an Italimpianti re-heat furnace to ensure precise control of bloom temperatures in the rolling mill. The 84-position walking beam furnace is natural gas fired, and computer controlled for maximum fuel efficiency. A closed-loop thermal model provides for self-adjustment to match the operating parameters of the mill.
Close control ensures minimum temperature variation along the length of a bloom, or between blooms. Multiple heating and quenching zones ensure a consistent temperature for blooms entering the mill, resulting in better section control. For example, the rear end of each bloom is made slightly hotter than the lead end, to compensate for natural air cooling which takes place as the bloom passes through the mill.
Following the rolling process, the rails pass directly to the head-hardening unit, where they are quench-cooled in a controlled manner to achieved the required hardness and metallurgical structure; close monitoring of entry and exit temparatures are critical to the process. Two separate heat-treating units are installed, each capable of handling rails up to 25 m long.
Unlike some other hardening techniques, the Nippon process does not require the addition of alloys to the steel. This means that in the event of any problems with the hardening plant, the rolled rails can easily be diverted to standard carbon steel stock - giving flexibility in the production process.
To meet a growing demand for long-welded rail strings ready to lay, CF&I has installed an in-line welding plant. This takes 25 m long head-hardened rails direct from the rolling mill and welds them into strings for dispatch directly to the railroads’ relaying sites. Final length is limited only by that of the carrying train, and is typically 490 m, with up to 40 strings per train.
Between July 1996 and May 1997, CF&I delivered over 60000 tonnes of DHH370 head-hardened rails to some of North America’s largest railroads: BNSF, UP, SP, NS, Montana Rail Link and several transit authorities. The rails have performed extremely well under a variety of traffic conditions.
CF&I’s original strategy was based on developing a premium rail comparable to that offered by Nippon Steel Corp. The system was designed to produce rails with a surface hardness of 370 Brinell and a minimum of 341 Brinell at a depth of 10mm. In practice, quality analysis of the finished head-hardened rails has showed consistently better results, with an average hardness of 390 Brinell almost 5% better than target. Constant attention to quality at all stages of the production process has paid off with the uprating of the rail to DHH390 as a new standard. o
CAPTION: CF&I’s plant is now able to produce over 400 000 tonnes of DHH390 rail a year
CAPTION: Microstructure of DHH 390 rails (1, 2) compared with NHH (3, 4) for 5 mm below the rail head surface (1, 3) and 25 mm below (2, 4)