Galling in large diameter steel threads 1

[NickA]

Hi, I don't know if anyone here will be able to help me but I have a question regarding similar materials galling in a threaded application.

First a little background,

My company produces heavy load hydraulic equipment. We normally design hydraulic cylinders with the end caps threaded into the tube (10-24" diameter threads 6 threads per inch). The material we normally use is ASTM A516 Gr. 70 (70KSI Tensile) for the end caps and ASTM A106 gr. C (70 KSI Tensile) for the tube.

Now for the challenge at hand,

We currently have a contract to deliver some equipment to a very cold environment (-40C ambient temperature). We have chosen materials that will have good fracture toughness because of the low temperature. (We have some pretty strict Charpy V-notch test requirements at -40C). The materials we chose to use are ASTM A514 (110 KSI Tensile)for the end caps and a low allow centrifugally cast steel for the tube (112.7 KSI Tensile). When we try to thread the endcap onto the tube the end cap threads tend to cut into the tube threads and then material starts to gall. Most of the damage occurs on the tube.

We realize that the metals are fairly similar in hardness and therefore not an ideal match. However the materials we normally use are even more similar and don't give us a problem!

If anyone has any comments or suggestions I would love to hear them. I am not a metallurgist just a lowly mechanical engineer, so if you have any knowledge that may help me or if you can point me in the direction of someone who can I would greatly appreciate it.

-Nick Atallah

 

[EdDanzer]

We have machined a lot of A514 plate, it is similar to 4140HT. I will agree the steady rest only is a not adequate support. We made some 12” bore cylinders with 14” OD and 4 TPI that have a 5” thick A514 barrel nut with a ring of bolt holes for mounting. We have built a live center to chuck the ID of tubes so the barrel can be faced to length, ID chamfered, OD turned to size, chamfered and threaded in 1 setup. For the long tubes it is necessary to turn an area for a steady as close to the threaded area as possible to reduce chatter.
When threading A514 plate you should limit the boring bar to 2D stick out, this means a 2” boring bar cannot stick out from the tool holder more than 4”, so you may need a larger diameter bar to reduce chatter created by a limp bar, or do creative programming to reduce insert contact.
One problem we found when using a bull nose live center to prep barrels for a steady rest was that the area for the steady was never very round or concentric to the bore, and caused thread and assembly problems.