Koz wrote:
What seems to be happening is related to the number
of cycles. Eventually, either the plug weld breaks off or in the case
of the press fits, things just eventually work to loosen the hole in the
rod so the pin eventually works itself out. Typical life is about 2
years and in that time, they see about 2 million cycles (quick calculation)
On the fatigue failure of the welds, I'm thinking of two causes:
- Excess Martensite in the weld since the dowel has a high carbon
surface. The carbon enriches the molten weld, cools quick, martensite
forms. Add high cycles and the brittle martensite fails easily.
- Thermal stresses in the weld, since the cold CRS rod is a heat sink.
Both of these would be helped by a pre-heat. Need to have the
surrounding metal just after welding to be above the Martensite Start
temp to prevent M formation. This is pretty hard to estimate since the
molten pool has unknown % carbon. And it will depend on how much of
the dowel is melted in the pool. If the pool is ~1050 steel, M-start
is about 600F. I'd talk to a welding expert for a rec on preheat temp.
1/2" dia wouldn't take long to get to temp, and that is good since the
exposed pin will lose some carbon and hardness if it is preheated too
long. (May need to protect the exposed pin during preheat with a high
carbon cover). Perhaps a post-weld stress relief as well, though that
is probably more cost than you want.
Make sure both parts and the filler rod have lowest possible alloy
content (Cr, Mo, Mn...) for lowest hardenability. Oh!-- if you are
currently using an alloy steel dowel, changing to a case-hardened
carbon steel dowel would result in much less M w/o any preheat. And
have nearly the same surface hardness. And save $.
Yet another idea: Make everything w/ low carbon steel, then carburize
the welded ass'y in large batches. No martensite in the weld
guaranteed. $ per part depends on how big your parts and oven are.
Good, simple, explanation of Martensite in welds:
http://www.esabna.com/EUWeb/SA_handbook/585sa3_1.htm
Also are you plug welding on one or both sides of the CRS? Two welds
would lower the stresses, perhaps double the life, slow the cool, too.
David Malicky
www.malicky.com/davidm