The right technology in the right place

Materials World magazine
,
1 Sep 2017

New technology is set to enhance plant and equipment, reduce environmental footprint and improve product capability, as John Wilkinson and Gari Harris argue. 

In the metals production industry, continuous investment in technology to enhance the processes and products is essential. To achieve this, British Steel, UK, has developed a solution based on a rail steel product that resists degradation, increasing the life of the rail while decreasing its maintenance requirements. Preventing and reducing rolling contact fatigue (RCF) was the main driver for the development of a patented HP335 steel grade, previously known as HPrail.  

Traditionally, increased alloying and heat treatment are employed to improve RCF performance, but these techniques can only do so to a limited extent. HP335 is an as-rolled product that has been metallurgically engineered to be wear and RCF resistant. Selective additions of carbon, silicon and vanadium can:

  • increase the proportion of cementite in the pearlite
  • refine the interlamellar spacing
  • strengthen the pearlitic ferrite
  • locally impede dislocation movement and crack growth 

Introducing new and fundamentally critical steel to the rail industry, which is traditionally rather risk averse, is challenging. However, working in close partnership with Network Rail, UK, and other partners, HP335 has reduced costs by using the right rail in the right place.

After two years of commencing track testing, HP335 achieved full product approval and, in less than five years, it has been incorporated into Network Rail and European rail standards and installed on more than 1,000km of UK networks, tramways, London Underground and Network Rail. 

Developing a new steel

The new product development was done in a number of stages. The first stage of proving the concept ranged from laboratory tests carried out on small scale samples of around 50kg of steel, through to producing a pilot-scale seven tonne cast of material from which to make full rails, and then trialling these rails using the British Steel Scunthorpe internal network of rail lines where they are subject to axle loads up to 46 tonnes. There were no unmanageable risks associated with the steel and therefore the next stage of testing could begin – introducing rails into the public network.

A number of suitable trial sites were selected in collaboration with rail customers to obtain performance-monitoring data. These sites were monitored intensively over a number of years to evaluate the real life performance of the HP335, as well as the performance of the standard grade rail at the same site. Regular site monitoring by teams from Network Rail and British Steel, and the sharing of all the data gathered, ensured transparency of material performance. Rail life spans were commonly at least doubled and grinding requirements were reduced by a factor of three at some sites.

From the vast quantities of site monitoring data, where HP335 rails had accumulated more than 700 million tonnes of traffic, there were improvements in performance for major rail degradation mechanisms. Wear rates were reduced and RCF was slowed. Under such close monitoring, the track testing led to full product approval two years after the first rail was installed for evaluation. 

Zinc for no corrosion 

Aggressive rail environments such as coastal tracks, wet tunnels and level crossings can cause excessive corrosion. This called for the development of a superior cathodic protection system called Zinoco, which enhances durability and improves tolerance to damage compared with current coating technologies, such as glass flake epoxies, polyester and aluminium metal spray. 

Zinoco, which derives from the words ‘zinc for no corrosion', offers a dual line of defence against corrosion, employing barrier and cathodic protection methods to enhance rail life. Cathodic protection prevents corrosion through converting all of the anodic (active) sites on the steel rail surface to cathodic (passive) sites by supplying electrical current (or free electrons) from the Zinoco coating. As long as the current-free electrons arrive at the cathode (steel) faster than oxygen is arriving, no corrosion will occur. Therefore, with Zinoco coated rail, the zinc-rich coating forms an anode, which corrodes in preference to the steel rail (cathode), therefore preventing the rail from corroding. The zinc oxide that forms creates a strong barrier to further corrosion.

To produce the coated rails, a new Zinoco plant has been built at British Steel’s Scunthorpe works with the automated facility coating rails more efficiently and consistently. The coating is a thermally sprayed zinc-rich alloy that is applied to rails of any grade at lengths of up to 216m. The coating is applied to the web and around the foot of the rail. The ends are usually left uncoated to facilitate installation welding. Its performance in the laboratory and during trials has led to Zinoco receiving product approval from Network Rail, London Underground and Régie Autonome des Transports Parisiens (RATP) across the UK, Ireland and France.

Energy savings 

The aluminium smelting industry consumes extensive quantities of electrical energy. The industry uses the Hall–Héroult process – a proven method for aluminium production using steel cathode collector bars. 

Typical low carbon steel in contact with the graphite cathode at normal cell operating temperatures will, over time in operation, pick up carbon by diffusion from the graphite. This causes a reduction in electrical efficiency of the collector bar – increasing energy costs and potentially leading to its replacement. By introducing a specific chemistry to minimise this issue, Condumax cathode collector bars aim to prolong life in service to reduce electricity during operation. To understand this further, a more detailed look at the metallurgy of iron and carbon in the steel bar is required.

In aluminium smelting, aluminium oxide is dissolved in molten cryolite and electrolysis of the molten salt bath produces aluminium metal. The Hall–Héroult process applied at industrial scale happens at 940–980°C and produces 99.5–99.8% pure aluminium. The electrodes in the cell are formed of carbon and the circuit between cells is completed by a steel collector bar either with or without a copper insert. This equalises the current density across the bar and reduces electrical short circuits.

Ferrite is a body-centered cubic form of iron. It is this crystalline structure, stable below 727°C, which gives steel and cast iron magnetic properties – known as the ferromagnetic material. Carbon in the microstructure is usually present as pearlite – a two-phased, lamellar or layered structure consisting of alternating layers of ferrite and cementite (Fe3C).

Above 912°C, the face-centered cubic allotrope of iron, austenite is stable. At operating temperatures within the cell and at the standard chemistry levels of commercially available ultra-low carbon (ULC) collector bars of 0.06% carbon, both phases can exist. Below 0.022% of carbon the ferrite remains stable at cell operating temperatures. The relative affinity for carbon displayed by the ferrite phase compared to the typical austenite phase, which exists in a standard collector bar, is the metallurgy that is of interest.

Only a very small amount of carbon can be dissolved in ferrite – the maximum solubility is approximately 0.02% weight at 723°C. Above 760°C and 0.022% carbon, a phase transition from body-centered cubic to the face centered cubic austenite takes place. This phase can dissolve considerably more carbon, equalling 2.03% by mass. The austenite phase at operating temperature of typical steel ULC collector bar then leads to carbon diffusion from the graphite cathode into the collector bar, reducing the electrical efficiency. This technology can produce Condumax, which takes advantage of the metallurgy to prolong the collector bar life in-service and offer significant electricity savings during operation. The lower resistance of the Condumax bar over a prolonged service life generates energy cost savings up to 20% and significantly improves conductivity by up to 40%. 

The metallurgy suggests that producing a collector bar with a carbon content <0.02% will significantly reduce this carbon pick up in-service and prolong service life and efficiency. For the steel manufacturer, such a low carbon content is not without its challenges. 

This new steel takes advantage of the metallurgy for the basic oxygen steelmaking producer by aggressively blowing the vessel with oxygen to drive out the carbon. But without careful control of the technology, refractory damage in the vessel is a real possibility. Following trials, a steel was developed to meet these process and product requirements. 

John Wilkinson is IPR & Training Manager and Gari Harris is Research and Development Manager at British Steel, UK.