4130 surface hardening/welding

[larue]

I need to come up with a plan to build up material on a 4130 cog which feeds anchor chain(the cog teeth are very worn). I would like to deposit weld metal and then have the excess removed, but unfortunately the cog is huge and cannot be moved making it near immpossible to achieve the high temperatures(stress relieveing at 1200F) required to get the desired hardness of 28-30Rc after welding. I was thinking of using another surface treatment after welding to get the desired hardness....liquid nitriding, carburizing or laser hardening???
Anyone have any creative ideas as to how I can solve this problem?
Thanks for your input

 

[metengr]

Based on your description of the problem, if you need to restore worn teeth on a low speed cog you might want to look at a technique we have used to perform weld restoration without the need for post weld heat treatment. The technique is called "temper bead" welding.

The approach is to deposit a butter layer using stringer beads and then remain on top of the butter layer for the remaining weld passes until you reached the required thickness of the weld deposit. You must remain on top of the butter layer at all times for this method to be effective. The temper bead technique, if performed correctly, provides some level of tempering to the previous weld passes and the heat affected zone of the base metal.

Considering the hardenability of 4130 steel, I would try this method on a test coupon to simulate field conditions and to optimze your welding technique. Section the coupon and perform some hardness testing of the weld deposit and base metal HAZ.

I would apply a preheat of 450 deg F and use the GTAW and/or SMAW processes. The GTAW can be used for the butter layer and subsequent layers (about 3 layers)to provide for grain refinement in the base metal HAZ. After the deposition of 3 layers, you could switch to SMAW to provide for increased heat input (use a larger diameter weld rod) and subsequent tempering affects.

As far as filler metal, I would use an ER80s B2 (GTAW) and E8018 B2 (SMAW). This filler material would work well for this application and in the as-welded condition could provide the hardness range for the wear surface without the need for any surface treatment.

 

[GRoberts]

You might consider building up the cog with a ductile & tough material first. This could be a low alloy electrode like 8018-C3 or even austenitic stainless steel so that there is less risk of hydrogen cracking in the HAZ of the weld. The overlay to the final dimension with a hardfacing wire that can achieve your desired hardness in one or two layers. There are hundreds of them to choose from. A manufacturer could probably suggest something that has been used for similar applications. As Metengr alludes to, the main concern might not be getting the proper hardness in the outer layer, but getting a HAZ in the 4130 base material that won't crack on you. A high preheat would also be recommended. You could use a HAZ cracking test such as the controlled thermal severity (CTS) test to determine a preheat temperature that will suffice for your particular application. The Batelle underbead cracking test might also work and is a bit simpler.

 

[Metalguy]

Agree with the use of high preheat during welding. Given a large thickness (over ~1" where you'd be welding), you won't have to worry about a hard, martensitic HAZ--the cooling rate down thru the critical temp. of ~1,200 deg. F will be slow enough to avoid problems.

You should easily be able to build up the cog teeth with something that will be FAR more resistant to wear than 4130 at Rc 30!