Tempering to reduce eliminate Martensite on Rail 2

[MechEng1977]

We apply electrical wire by thermite weld to subway rail.
The thermite weld creates a btittle martensite area which has been known to crack.

I was wondering if I can temper the metal on site (at the subway track). Is this possible and if so how?

Thanks in advance

 

[metengr]

MechEng1977;
If this is a small "tack weld" by the thermite process to attach wires, you can try to locally post weld heat treat this weld and rail base metal in the field using a neutral flame gas torch with Tempstiks to verify your PWHT (or tempering) temperature. This local PWHT method is conducive to field application. The duration of heat could be as little as 15 minutes at PWHT temperature, while carefully monitoring the metal temperature of the rail and weld using Tempstiks so the region is not overheated.

The temperature for local PWHT is selected based on the chemical composition of the rail steel and the original heat treatment specification. If the rail was supplied in a tempered condition, you need to remain below this original tempering temperature. For a gas torch application of PWHT, I would remain 100 deg F below the original tempering temperature for the rail.

 

[MikeMet]

If you are working on standards straight rail, it is typically high carbon steel but can also be significant amounts of manganese. Frogs, cross overs and switch points are typically a manganese steel for added ducitlity. A typical post weld heat treatment for high carbon steel rail is 700 deg. F followed by air cool, but shielded from wind, rain etc. The time at temp is dependent on the thickness of the weld. In your case for a low penetration surface weld, 5 -10 min. should suffice.

If the track is true manganese steel, no PWHT is required. If you are getting cracking at your thermite welds, I'm willing to bet you have high carbon steel track and will require a PWHT.

 

[MechEng1977]

Thanks for the responses Metengr and MikeMet. The rail is tangent (straight) therefore it is standard rail 300 HB (Brinell Hardness). The following is the chemical composition (weight percent):

Carbon 0.74 - 0.84
Maganese 0.80 - 1.1
Phosphorus 0.035
Sulfur 0.037
Silicon 0.01 - 0.060
Nickel 0.25 (max)
Chromium 0.25 - 0.50
Molybdenum 0.10 (max)
Vanadium 0.03 - 0.05

How do I find the PWHT for this rail?

 

[metengr]

MechEng1977;
You need to locate the specification for this rail. I have seen AAR rail specifications that in addition to rail steel composition, the specification provides information regarding heat treatment. This is the information you need to know to optimize your PWHT temperature to avoid affecting the bulk properties of the rail upon exposure to local PWHT temperature.