"Soft" Threads on Hardened Parts 2

[atlarson]

I have seen a note on drawings of items such as shafts with threaded ends to harden the part but leave the threads soft. What is the purpose of this, and how is it accomplished?

Andy Larson

 

[TVP]

The shaft is probably induction hardened so that only the surface (usually less than 5 mm deep) microstructure is modified. The higher hardness on the surface imparts better wear resistance, and an improvement in bending strength, without the need to harden the entire shaft. This lowers the overall cost because you do not need a steel alloy capable of being through-hardened (SAE 1035-1045 grades are quite common), and the heat treating cycle is very fast-- the time from beginning of induction hardening to the quench can be 20 seconds or less. Tens of millions of piston rods/shaft for automotive struts and dampers (shock absorbers) are processed in this manner. The threaded ends are designed to work with a nominal property class 5.8 strength level, not the more common 8.8 to 10.9 that is obtained from a quenched and tempered structure.

 

[butelja]

The sharp endges on threads can lead to extreme brittleness and quench cracking during the heat treating. This is because the quench rate of the thread, which acts like a fin, is much greater than of the shaft it is on.