Train coupler pins - maraging or duplex steel?

[DrejzaVu]

We have had some train coupler pins failed over the past few years and I have been asked to do something about it. The pins fail at the top where an alignment plate is welded (weakly) and also in the middle where push/pull is applied. The failure mode for the latter are simple double/reverse bending fatigue at the center with origins on the surface where the pin gets all beat up from the pushing and pulling impacts that occur. So, ductility, fatigue resistance, impact toughness, hardness, and fracture toughness are all my considerations.

Researching metals to replace the standard carbon steel we use, I have come up with maraging steels and duplex steel (e.g., 2205) as possible improvements to the plain carbon steel we currently use. However, the costs of those aforementioned materials is much higher (like 10x to 18x higher).

Would such a material be a cost-beneficial investment in reliability/safety of our trains? Or would simply regular inspection/replacement of the carbon steel pin be a better course of action? Is there another material that is a nice compromise to both, such as 4140 or 4340?

Thanks in advance for your expert opinions.

Mark

 

[swall]

Would these be the North American Janney style couplers or the European link pin and buffer type?

 

[DrejzaVu]

Hmmm...don't know. I can tell you that the pin is approximately 2" diameter and 12" long. There is a flat rectangular plater welded to one end which keeps the pin oriented in one direction. The other end has a hole and a cotter pin so that it stay secure while coupled.

Mark

 

[Metalguy]

If you can weld the plate on BEFORE the pins are heat treated, 4340 hardened and tempered for 2 hr. min. at 1,000 deg F should do very well.

 

[rorschach]

2205 is going to have galling issues. All stainlesses will have that problem to one degree or another.

 

[TVP]

A quenched and tempered steel like 4140 or 4340 should be able to dramatically improve the performance over a low carbon steel. As Metalguy noted, you need to address the welding issue with respect to heat treatment.

Regarding the geometry, is it possible to redesign the pins and mating components so that impact is spread over a larger area?

 

[DrejzaVu]

So, how do I do this manufacturing Metalguy? I simply buy some 4340 rod and plate, weld it up, and then...??? Where would I send the finished piece(s) to have them heat treated? We do have a steel manufacturer right down the road. Who else will simply heat treat?

Mark

 

[Carburize]

If you use a lot of these then maybe a one piece forging in 4140 or 4340 would be a better option, removing the complication of welding on these somewhat difficult alloys. A small closed die forge shop could probably arrange to supply them fully heat treated ready for final machining.

 

[Metalguy]

DrejzaVu,
Most if not all large cities have heat treatment shops that can easily HT these pin/plate assemblies. 4340 steel is readily found and fairly easy to weld if you preheat it to 300 deg. F or so. *IF* you have a good welder (person, not the machine), you might get by with carbon steel plates--don't know how much load that weld sees and if a better weld on your orig. parts would have eliminated the weld cracking.

I would not use 4140 in this application. While far better than CS, you may find you have bought steel that is on the low side of the spec. for Mn, Cr and Mo, and it won't harden real well in 2"+ thickness. Stay with 4340-there shouldn't be any surprises, and heat treaters like it because it's so easy to HT.

The reason for welding before HT is to eliminate brittle HAZ'z.

 

[CryogenIndustries]

Can i suggest, to be more confident in the finished pin / plate assembly have it cryogenically treated (after the final welding and heat treatment). This should reduce the stress induced during the welding process and increase the wear resistance.

Something else to keep in mind is that 4340 not only is not only easy to heat treat but responds very well to cryogenic treatment.

Shannon Mafodda