It is required to understand the metamorphic structure which is one of characteristics of rail in order to realize the feature of rail brazing.
1. Rail is made by carbon steel
Rail is one of categories in carbon steel with carbon content approx..0.8 to 1.0 percent.
The structure of carbon steel changes depending on temperature.
In the condition of room temperature, it’s structure calls Perlite distributing to Cementite of hard structure in laminated condition.
This Perlite structure has tough and strong properties and change to another structure called Austenite on the condition of exceeding metamorphic temperature 723 degree C.
Austenite structure has characteristic of elongation nature. We can image it in forming to some knife or sword in the condition of red color very high heated steel material and stretching by hammer at workshop.
The temperature staying in the condition of Austenite structure shall be cooling down slowly for return to former structure of Perlite which is available at the lower temperature condition of Perlite structure.
In case of rapid cool down, Cementite structure can not be in precipitation to the laminated condition and transfer to Martensite in condition of spreading over the structure entirely. The nature of Martensite has very hard and fragile which is suitable for making knife and Japanese sword.
Metamorphic structure of carbon steel
Structural condition graph of carbon steel
2. Rails dislike heating
It seems to be destroy dangerous to rail at the part of Martensite structure if there is this one somewhere in the rail.
In the past time, actually such accident happened to rail with this Martensite structure.
In this situation, scope of application for heating train rail works must be limited and to be carried out with strictly careful attention. Otherwise it is too risky for works under the condition of exceeding the temperature of changing metamorphic structure.
There are various kind of rail:
It depends on the size: 60k rail, 50k rail( standard is variation), 40k rail, 37k rail,
30k rail, etc. (for instance, 60k rail shows 60kg weight per one meter length)
Further, there is heat treated rail. This is hardened the rail top surface by heat treatment. It aims to reduce wear and tear of rail.
The possibility of bad affection by heating will be more increased to the heat treated rail and small size rail (less than 40k rail)
