How To Stop Thread Galling On Stainless Fasteners
By Stainless Steel Expert – Randolph Smith: www.pwrmfg.com
A few times each year we receive calls from Distributors and End User’s who are in conflict with their customer or installation crew over the quality of stainless steel bolts and nuts. The end user’s complaint is that during installation the bolts are twisting off and/or the bolt’s threads are seizing to the nut’s thread. The frustration of the supplier is that all required inspections of the fasteners indicate they are acceptable, but the fact remains that they are not working.
This problem is called “thread galling.” According to the IFI – Industrial Fastener Institute’s 6th Edition Standards Book (page B-28),
Thread galling seems to be the most prevalent with fasteners made of stainless steel, aluminum, titanium, and other alloys which self-generate an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing – the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out.
Carpenter Technologies, the fastener industry’s largest supplier of stainless steel raw material, refers to this type of galling in their technical guide as “cold welding.” Anyone who has seen a bolt and nut with this problem understands the graphic nature of this description.
The IFI and Carpenter Technologies give three suggestions for dealing with the problem of thread galling in the use of stainless steel fasteners:
1. Slowing down the installation RPM speed will frequently reduce, or sometimes solve completely, the problem. As the installation RPM increases, the heat generated during tightening increases. As the heat increases, so does the tendency for the occurrence of thread galling.
Another factor affecting thread galling in stainless steel fastener applications is thread roughness. The rougher the thread flanks, the greater the likelihood galling will occur. In an application where the bolt is galling with the internal thread, the bolt is usually presumed to be at fault, because it is the breaking component. Generally, it is the internal thread that is causing the problem instead of the bolt. This is because most bolt threads are smoother than most nut threads. Bolt threads are generally roll threaded, therefore, their thread flanks are relatively smooth. Internal threads are always cut, producing rougher thread flanks than those of the bolts they are mating with. The reason galling problems are inconsistent is probably due largely to the inconsistencies in the tapping operation.
Rougher than normal internal threads may be the result of the use of dull taps or the tapping may have been done at an inappropriately high RPM.
Fortunately, stainless steel bolt and nut galling problems do not occur everyday, but when they do, it usually creates a customer crisis. Knowledge of why this occurs and how to remedy it, can save the supplier or end user much grief and the cost savings of the additional labor.
Below are the questions that should be asked and the suggestions that should be made immediately when you are confronted with a customer’s or end user’s complaint about thread galling:
1. Are you using the same driver RPM you have used in the past to install these stainless fasteners?
2. Are the bolts and/or internal threads lubricated?
In applications where galling is a repetitive problem, it is advisable to supply the fasteners with pre-applied lubrication to eliminate future problems
3. Are you using the same grade of stainless steel for the bolts and nuts?
Be sure the suggested grade meets their corrosion needs and changing the material does not cause a procurement problem.
When thread galling occurs in stainless steel bolt and nut applications, don’t panic. Try the suggestions listed above. One, or a combination of these, will probably resolve the problem immediately.