Myth 2. -“Harder rails are better” or even the converse “Softer rails are better”
- Both of these statements can be true at times, but they are then untrue at others. Confused? I'll try to explain.
There is no 'one rail fits all' solution. Knowledge of the operating conditions, traffic and previous degradation mechanisms (if possible), is vital if you are to optimise the choice of rail profile/grade. I could probably write a lengthy essay giving examples of what I believe is best to use for what circumstances but this isn’t the time or place and I briefly touched on grade selection in a previous article.
Whilst there is a general shift towards the use of harder rails in the rail industry, people should not make the mistake of thinking harder rails are better for every application. Indeed, it is at this point in the article I must make a slight detour to point out a very important distinction between hardness and wear resistance.
On a very general level, harder rails are generally more wear resistant, however people often assume they can be used interchangeably. They cannot. To use an automotive analogy, generally more expensive cars are more comfortable, but that doesn't mean a more expensive car is always more comfy than a cheaper one. Some of the most expensive cars are race cars and they aren't exactly an easy ride.
(more in original article link)
Back to the subject of hard vs. soft rails. If hard rails are generally more wear-resistant why would anyone ever want to use a softer rail? Well, rail wear is only one of the degradation mechanisms of rail. There are many others, however generally the two main types of degradation people usually recognise are wear and rolling contact fatigue.
Wear of the rail is fairly self explanatory, the surface of the rail is worn away by the passage of traffic. Eventually rails need replacing as too much material has been worn away.
Rolling Contact Fatigue (RCF) is also extremely important in determining rail life and maintenance requirements. Essentially under the passage of wheels, cracks may grow near or at the rail surface which can then continue to grow under the passage of more traffic. Eventually these cracks, if left to grow, can pose a threat to the integrity of the rail with disastrous consequences.
In reality both of these degradation types often happen in the rail at the same time, and the traffic conditions (and rail grade) determine which degradation is observed as both degradation types effectively race to degrade the rail.
If the wear rate is faster than the RCF, then you only ever see rail wear, as the cracks wear away faster than they can grow.
However if the wear rate is low, then RCF cracks initiate and grow faster than the surface is worn away and therefore the cracks become visible on the rails surface.
Both of these conditions require inspection and maintenance, but usually the only solution for a worn out rail is replacement. RCF may be controlled by rail grinding (removing the rail surface to remove the cracked surface).
If a network finds grinding difficult or impossible, or it is found more cost-effective to replace rails, then potentially a softer rail may be the most cost-effective solution in that case. This is however, generally the exception to the rule, which is why the industry is generally moving toward the use of more wear-resistant rails.
As a conclusion to this article I should also acknowledge my employer and plug the rail grades we have specifically designed to be resistant to rolling contact fatigue such as High Performance - HP335 and the Bainitic Low Fatigue - BLF320 and BLF360 grades. These depart from the conventional metallurgy of rail steels by offering improved levels of RCF-resistance above that considered normal for their hardness or wear-resistance. HP335 for example gives a great blend of wear-resistance and rolling contact fatigue-resistance, whereas BLF320 grade excels at RCF-resistance and has withstood 500 Million Gross Tonnes of traffic without any visible RCF crack growth. If you want to find out more about these then drop me a line.
I'll end by repeating the start of this article - There is no 'one rail fits all' solution! Understanding rail degradation is vital to allow you to pick the right option. Pick carefully, as it can make a huge difference to network performance.
