In technical terms Galling is a type of wear associated with the joining of two parts of material, and is actually precluded by another type of wear 'adhesion' which takes place before Galling can occur.
Best results for preventing galling are achieved when you;
Lubricate where possible before use with a solid type lubricant.
Keep torque within guidelines.
Use low speed applicators.
Clean, grit free product is best.
Select right quality and grade combination.
Adjust torque guidelines for lubrication.
Surface construction plays a large role and when greatly magnified as shown in the diagrams, we find the thread surface is in reality rough and irregular.
When two surfaces are brought into contact as with fasteners, the high points as seen in fig 3 take the initial load.
When pressure or static load is applied, these high points squash (deform) until the real contact area is increased to take the load.
If relative motion is introduced (ie spinning a nut on a bolt or tightening) then wear may occur due to the protective oxides rubbing off at the high points, exposing the base metals and causing them to weld together.
Adhesive wear occurs when the pressure/load is small and the weld is weak. A small amount of base material either transfers to the stronger side, or floats independently in the joint and is known as plastic deformation. This can be evidenced by spinning a nut on a bolt with your fingers, and noticing when the nut catches or sticks. When you push the nut over that point the transference of materials has occurred even though not seen by the eye.
Galling, also known as seizing, cold welding or pick up, occurs under higher stresses where stronger bonds or welds are formed between base metals, mainly because the contact surfaces being deformed are larger.
Generally the causes behind galling are due to high torque/tightening levels and fast application methods like speed/air guns.
In particular, stainless steel presents the majority of galling problems, mainly due to its Low Heat dissipation at the point of contact where the build up occurs (eg the high points) and what is recognised as a high co-efficient of friction, which basically means it heats up very quickly when rubbed together.
Solid type lubricants (Molybdenum Di-Sulphide) work as seen in fig 4, by creating layers over the materials that form a weaker bond than the base metals and forming a barrier that the high points cannot push through and therefore not allowing contact of the base materials.
- Sulphur to Sulphur atoms are weak and break easily.
- Sulphur to Molybdenum and base metals are strong.
Metals used in fastener manufacture are elastic materials which will stretch (elongate) under applied loads and return to their original shape when the load is removed.
However, if sufficient load is applied, the material will stretch beyond its yield point and enter a plastic zone, losing its elasticity and becoming permanently stretched.
Further increased load on the material will stretch it to its ultimate tensile strength at which point the material will fracture.
The materials of our particular concern are:
Steels - low tensile (mild steel)
- high tensile
- stainless steel
The major factor in determining the load a material can carry is its tensile strength, which is related to its hardness.
1. Tensile Strength - is an expression of the maximum capacity of a particular material to stretch under tension load, prior to failure.
It is normally expressed in: pounds/tons - imperial terms.
kilo newtons (kn) - metric terms.
2. Yield Stress (yield point) - is an expression of the theoretical point of stress (pressure) beyond which the material loses its elasticity and becomes permanently stretched; (realistically, a range rather than a single point).
Stress is load ÷ area the term will include a unit of area.
It is expressed as: lbf/in2 (PSI) - imperial terms.
N/mm2 (Mpa) - metric terms.
3. Proof Load Stress - is an expression of the minimum stress a material must achieve, prior to permanent elongation and, the stress which would be applied to test and remeasure a specific fastener to prove it had not permanently stretched and that it will carry the required load.
These terms will also include a unit of area, are approximately between 80% and 90% of the theoretical yield stress and are expressed in the same terms.
Proof load stresses also apply to nuts and are the point at which the nut is deemed to have failed; (= to the bolt UTS in a given diameter).
4. Ultimate Tensile Stress - is the theoretical minimum point at which the material will fracture. It is expressed in the same terms as yield stress and proof load stress.
These properties are used to calculate the proof load and breaking load for each diameter of each grade or class of product. (The calculated figures for each of these properties are listed by diameter in the relevant standards).
Proof loads and breaking loads are expressed as:-
imperial.............pounds force (lbf).
and are the units used by engineers in designing the elements of a joint.
The strength properties of an individual fastener are achieved by a combination of:-
- Appropriate base material selection
- Manufacturing processes.
Low carbon grades of steel are improved in hardness (strength) by cold working.
Medium carbon grades of steel are improved in hardness (strength), after cold working, by controlled heat treatment and quenching.
Austenitic grades in various strengths are improved in hardness by cold working.
Martensitic grades in various strengths are improved, after cold working, by controlled heat treatment and quenching.
Medium carbon grades of steel are improved in case hardness, after cold working, by heat treatment and quenching.
Fasteners carry loads in one of two ways:-
Where the load is acting to separate the fastened components along the shank length, it is referred to as a tensile load. Tensile loads try to elongate the fastener.
Where the load is acting to separate the fastened components across the shank diameter, it is referred to as a shear load. Shear loads try to cut the fastener in half.
The load carrying capability of a fastener is somewhat less in shear than in tensile and will further vary if the shear plane is across the threads rather than the plane shank.
Some applications could exert a combination of tensile and shear loads.
The strengths of a product group of fasteners are expressed:-
in the imperial system as grades
in the metric system as product class.
The approximate tensile strength comparison of steel grades and classes:-
Points to Note
Product markings are not uniform over all stainless fasteners.
Where A2 and A4 are used without property class, assume it is lowest strength grade unless supplied with a certificate.
A2 and A4 may be replaced with 304 or 316.
'M' used in Australia on non-stainless product to indicate metric is not consistently used on stainless.
MATERIALS, MECHANICAL PROPERTIES AND MARKINGS
The strengths of a product group of fasteners are expressed:-
|The approximate tensile strength comparison of steel grades and classes:-