Fastener Thread Galling: Root Causes & Prevention Solutions For Overseas Projects

Thread galling, also known as thread seizing, is a frequent field failure in overseas fastener projects. It occurs mostly on SUS304, SUS316 stainless‑steel and high‑strength alloy‑steel bolts. During tightening, threads jam suddenly, neither further tightening nor smooth disassembly is possible due to cold welding adhesion, and fasteners become scrap. It may cause equipment shutdown and construction delay. Many overseas buyers misjudge it as thread tolerance defect, while in most cases it is caused by assembly condition and material matching.

The root cause is metallic cold welding. Friction during tightening generates instant high temperature. Micro protrusions on thread surfaces melt and stick together. Stainless steel has poor thermal conductivity and retains frictional heat, making it more vulnerable to galling. Alloy‑steel fasteners also suffer seizing under high‑speed dry‑assembly conditions.

Main triggering factors: Excessive tightening speed builds up massive frictional heat, especially on automatic production lines. Over‑torque deforms thread contact surfaces. Burrs and foreign particles trapped between threads raise local stress. Too‑small hardness difference between bolt and nut accelerates adhesion. Lack of lubrication leads to direct metal‑to‑metal friction.

Practical countermeasures for international projects: Reduce tightening speed for stainless‑steel fasteners. Apply torque strictly according to specified preload instead of manual experience. Remove chips and dirt from threads before assembly. Match bolt‑nut with proper hardness difference, with nut slightly softer than bolt. Adopt anti‑galling coating or assembly grease such as molybdenum‑disulfide or PTFE coating.

Surface finish reference: PTFE or MoS₂ passivation provides built‑in friction reduction for stainless‑steel fasteners. Ordinary bright passivation offers no anti‑galling property. Clearly specify anti‑gall coating requirement on export drawings instead of simple “natural passivation”.

Sampling suggestion: Perform assembly validation with real‑site tool, speed and torque during prototype phase. Visual inspection and salt‑spray test are not enough. Manual lab assembly may work well, while mass‑production high‑speed tightening triggers mass galling.

On‑site operation reminder: Stop tightening immediately once abnormal thread resistance appears. Forced rotation will worsen cold welding and result in permanent seizure.

Thread galling comes from multiple coupled factors rather than single‑side product defect. For exported machinery and engineering projects, specify speed limit, torque range, lubrication and surface finish within fastener specification to minimize on‑site losses.