Localized Surface Hardening 3

[nilcram]

I am in search of a surface hardening process that can be contained to specific areas of a part. I am designing a system that is being driven into the subsurface with a hydraulic powered percussion hammer.
The design calls for a series of tubular rods that are threaded end to end with a threaded connection. The thread is similar to a buttress or rope thread. I need to keep the core of the threaded part around 40-45 HRC for fatigue resistance and toughness. On the other hand, I need a higher hardness in the threads to keep them from wearing out. The threads themselves are approximately .03"-.05" deep. Each part consists of a male or pin thread, and a mating female or box thread.
Induction hardening has been tried, but it does not work to harden the internal female threads.

Various gas nitriding process have been used, but the root of the thread cannot have a high surface hardness. Otherwise premature failure will occur.

This is similar to hardening the teeth on a gear. I either need the face of the thread harder, or the entire thread itself harder. This is true for both the male and female thread. The only trick is, I have to leave the root of the thread alone. It must be in the 40-45HRC range.

Material has historically been 4130 and 4140, but that is open to change as well.

I would like to think there is someway I can mask or protect the root surface of the thread and harden the rest of the thread or thread face.

 

[Carburize]

With such small threads masking of the roots is going to be difficult - have you considered surface hardening the whole thread followed by a post hardeneing grinding pass that only touches in the root of the thread taking off any case and leaving the flanks hard?

 

[rede]

nilcram does not specify the size of the thread (Diameter) only the tooth size range.
What is happening now?
Can the thread be deeper/larger?

The threads can be shot peened which will introduce a compressive stress layer to the parent material. The tricky part if it were neccessary is in masking the thread roots to avoid the peening effect, but it can be done.

 

[nilcram]

Good question. The diameters range from 1.25" up to 4.25". The thread is pretty much as deep as it can be without major design changes.

The threads are currently peened. Althought the advantange is minimal due to the fact that when the thread erodes away from the percussion, the compressive stresses go away with it. We do get some benefit by having the root of the thread peened, since that is the highest stress point of the part.

Thank you for your reply.

 

[metman]

You can mask your threads by Copper plating. Now grind your tap to remove the root cutting action only then retap to remove Copper on the thread flanks. Nitride and you have the friction zones of threads very wear resistant but with tough roots.

Jesus is THE life,
Leonard

 

[nilcram]

metman, thank you for your post.

Copper plating is something I will need to learn more about. It sounds promising. I won't be able to retap the threads after Copper plating, because this thread has to be single pointed on a CNC. It has a taper (1-2 degrees) and multiple thread starts. However, I think I may be able to use the Copper plating to my advantage anyway.

Thanks.