1. Introduction
In marine, chemical, food processing and high-temperature mechanical fastening projects, stainless steel bolts (304/A2 and 316/A4) are the first choice for corrosion resistance. Yet many overseas maintenance teams encounter a recurring failure: the thread turns smoothly at first, then locks up suddenly during tightening, and no matter how hard the wrench is applied, it cannot be loosened. This metal-bonding phenomenon is called cold welding or galling. It happens most often in high-temperature working conditions, and forces operators into violent disassembly that strips threads and scrapes the fasteners.
In field failure statistics, more than 70% of scrapped stainless fasteners are not caused by corrosion, but by thread galling and wrong forced removal. The root cause is that stainless steel has poor friction and heat-dissipation performance. Once the passive film is broken, fresh metal bonds at the thread peaks, forming a self-locking deadlock. This article explains the cold welding principle of 304/316 stainless threads, sorts out the pitfalls of violent disassembly, and gives safe non-heat on-site removal methods for high-temperature seized bolts.
We are NINGBO TOPBOLT METALWORKS CO., LTD., and we support anti-seize treated stainless bolts for high-temperature equipment.
2. Cold Welding Principle of Stainless Steel Threads
2.1 What Is Thread Cold Welding (Galling)
Cold welding is not high-temperature welding. Stainless steel builds a thin chromium oxide passive film for corrosion protection. When the nut and bolt rotate under pressure, friction scrapes the film off the thread peaks. Fresh bare metal is exposed and bonds at the microscopic level. Once micro-welding points form, continued rotation tears the thread metal, and the contact area grows until the thread pair is fully seized.
2.2 304 VS 316 Susceptibility to Cold Welding
Material | Cold Welding Risk | Application Notes |
304 (A2) | High | General mechanical assembly; fast tightening under high pressure easily triggers galling |
316 (A4) | Medium-High | Marine and chemical use; molybdenum improves corrosion resistance but still seizes under heavy pressure and high temperature |
2.3 Why High-temperature Working Conditions Aggravate Seizure
Stainless steel has low thermal conductivity, so frictional heat stays on the thread contact points instead of dissipating. The local high temperature softens the thread surface, accelerates metal adhesion, and even burns off ordinary lubricants. Long-term thermal cycling in high-temperature equipment locks the thread even tighter, making disassembly far harder than at room temperature.
3. Common Wrong Operation: Violent Disassembly Risks
3.1 Typical On-site Mistakes
Most field teams increase wrench force or hammer the fastener directly when a bolt gets stuck. This is the biggest pitfall in stainless bolt maintenance and directly causes most scrapped fasteners.
3.2 Consequences of Forced Disassembly
1. Violent twisting tears the thread teeth, fractures the nut or bolt head, and makes the fastener completely unusable.
2. Broken bolt fragments remain inside the equipment hole and require complex hole repair.
3. Forced impact may damage surrounding precision parts, causing leakage or structural hidden danger.
4. Safe Non-heat On-site Removal Methods
4.1 Method Comparison Table
Method | Operation Steps | Applicable Seizure Level |
Vibration Loosening | Lightly tap the nut side repeatedly, then apply low torque and rotate forward and backward alternately | Mild cold welding, no heavy material transfer |
Penetrating Lubricant Soaking | Apply anti-seize agent fully, soak 15–30 minutes, then rotate slowly several times | Light thread bonding, clear thread profile |
Low-temperature Cooling | Spray low-temperature cooling agent on the nut to shrink it slightly, then perform low-torque rotation | High-temperature seized threads, chemical and oil & gas sites |
4.2 Vibration Loosening Method
Use a small hammer to tap the side of the nut or bolt head repeatedly. The vibration breaks the micro cold-welding bonding points between threads. While tapping, apply low torque and rotate forward and backward alternately. Do not apply large torque at once, so as to avoid tearing the thread.
4.3 Penetrating Lubricant Soaking Method
Apply penetrating anti-seize agent fully to the gap between the nut and bolt, and let it soak for 15–30 minutes to penetrate the bonded thread interface. Then rotate slowly in small angles. This method works best for slight cold welding without heavy thread material transfer.
4.4 Thermal Contraction Cooling Method
Spray low-temperature cooling agent on the nut. The nut shrinks slightly while the bolt dimension changes less, reducing thread contact pressure. After cooling, perform a low-torque rotation test. This method avoids flame heating and meets the safety requirements of flammable, explosion-proof and precision equipment sites.
5. Prevention Rules to Avoid Cold Welding
5.1 Assembly Standards Before Installation
1. Slow down the tightening speed and avoid impact wrenches for stainless thread pairs.
2. Apply high-temperature anti-seize compound for high-temperature working conditions.
3. Match bolts and nuts with different hardness if possible, and do not use identical stainless grades in heavy load rotating pairs.
4. Keep the threads clean. Burrs, grit and scratches on the thread surface raise the cold welding probability sharply.
5.2 High-temperature Working Condition Rules
For equipment running above 200°C, use molybdenum disulfide high-temperature anti-seize compound and replace fasteners regularly after thermal cycling. Do not reuse bolts that have already shown thread damage, because damaged threads re-seize easily in the next assembly.
6. FAQ
Q1: Is cold welding the same as rust seizure?
A1: No. Rust seizure comes from corrosion oxide jamming. Cold welding is microscopic metal bonding caused by friction pressure. Even brand-new clean stainless fasteners can suffer galling.
Q2: Can cold-welded bolts be reused after disassembly?
A2: Check the thread surface carefully. If there is obvious metal transfer, scratch or tooth deformation, replace the fastener. Damaged threads will re-seize easily in the next assembly.
Q3: Do fine thread stainless bolts have a higher cold welding risk?
A3: Yes. Fine thread has a smaller contact area and higher surface pressure, so galling happens more easily than with coarse thread under equal load.
Q4: How to disassemble high-temperature seized bolts safely?
A4: Use penetrating soaking or low-temperature cooling first, then vibration loosening. For repeated seizure cases, we at NINGBO TOPBOLT METALWORKS CO., LTD. recommend pre-coated anti-seize stainless fasteners.
