Why Stainless Steel Bolts Are Hot Sellers
Corrosion is the main reason threaded fasteners fail in service, and stainless steel is the standard answer wherever moisture, chlorides, tunnel condensation or chemical atmospheres attack the fastener. On railway infrastructure, stainless bolts protect fishplate joints, tie plates, signalling equipment housings, cable troughs and station steelwork. The A2 and A4 grades defined by ISO 3506 cover nearly all of these duties, and because stainless bolts cost several times more than zinc-plated carbon steel, buyers need to choose both the right type and the right grade.
The Five Hot-Selling Types and Their Uses
| Type | Head and drive | Typical application | Rail-industry use |
|---|---|---|---|
| Hex head cap screw | Six-sided head, wrench driven | Structural fastening in construction and machinery | Fishplate bolts, tie plate fixings, brackets |
| Socket head cap screw | Cylindrical head, hex key drive | Precise fastening where access is limited | Signal equipment, instrument housings |
| Lag bolt | Hex head, wood thread | Securing heavy items to timber | Timber sleepers, timber bridges, platforms |
| Carriage bolt | Rounded head, square shoulder | Pull-through resistant joints in wood and steel | Timber bridge members, fencing, platform decking |
| Machine screw | Slotted or cross drive, uniform thread | Precision assemblies in electronics and machinery | Junction boxes, cable clamps, control cabinets |
Hex head cap screws are the most widely used stainless fastener because the wrench drive transmits high torque without rounding, which matters on fishplate bolts tightened to a defined torque. Socket head cap screws suit confined spaces and give better torque control for small-diameter connections. Lag bolts solve timber sleeper and bridge problems where a standard machine thread would strip. Carriage bolts resist pull-through because the square shoulder locks into the timber during tightening. Machine screws cover the light-duty precision work of track-side equipment.
Material Grades: A2 and A4
Stainless fastener grades follow ISO 3506. A2 (often called 304-type) is austenitic stainless steel for general atmospheric and fresh-water service; it is the default grade for indoor, urban and most outdoor railway applications. A4 (316-type) adds molybdenum and is specified for coastal, offshore and de-icing-salt environments where chlorides would pit A2 fasteners. The marking on the head tells the full story: A2-70 means grade A2 with a minimum tensile strength of 700 MPa, and A4-80 means grade A4 with a minimum tensile strength of 800 MPa. For rail fasteners, A2-70 hex head cap screws are the common choice for fishplate joints, while A4-80 is specified for coastal viaducts and tunnels where chloride levels are high. Never substitute carbon steel bolts of equal size for stainless grades without re-checking torque and galvanic compatibility.
Application Scenarios
Coastal railway lines and viaducts where salt spray reaches every exposed fastener: specify A4-80.
Metro tunnels with condensation and stray current: A2-70 with insulating washers to avoid galvanic couples with the rail.
Timber sleeper and bridge work: stainless lag bolts and carriage bolts that will never be painted again after a corrosion failure.
Signalling and telecommunication equipment: machine screws and socket head screws in A2 for humidity resistance.
Industrial sidings near chemical plants: A4 or higher-alloy stainless where process gases attack A2.
Selection and Procurement Points
When specifying stainless bolts, confirm five things. First, the grade: A2 or A4, and the property class such as A2-70 or A4-80. Second, the thread standard: metric coarse thread per ISO 68 and ISO 965 with tolerance 6g. Third, the coating: stainless needs no coating, but request passivation to restore the chromium oxide layer after machining. Fourth, galvanic compatibility: stainless bolted to carbon steel structures in a humid environment requires an isolating washer or coating on the carbon side. Fifth, traceability: require mill certificates showing chemical composition and mechanical tests for each heat, because counterfeit stainless fasteners with inadequate molybdenum are a real risk in some markets.
Frequently Asked Questions
Q1: Is A4 stainless always better than A2?
No. A4 resists chlorides better, but it costs more and is not needed for dry, indoor or fresh-water service. Select A4 only where chlorides or aggressive chemicals are present.
Q2: What does A2-70 mean on the bolt head?
It identifies the material as austenitic stainless grade A2 with a minimum tensile strength of 700 MPa. The marking system is defined by ISO 3506-1.
Q3: Can stainless steel bolts be used with aluminium components?
Yes, but the galvanic couple must be managed. Stainless against aluminium in a wet environment can corrode the aluminium; use isolating washers, sealants, or choose compatible grades after a corrosion review.
Q4: Do stainless bolts need the same torque as carbon steel bolts?
No. Austenitic stainless has different friction behaviour and a tendency to gall, so torque tables for stainless grades differ from carbon steel tables, and a lubricant such as molybdenum disulphide paste is often specified.
Q5: Why does a stainless bolt sometimes show rust spots?
Surface rust spots usually mean embedded free iron from machining or handling, or contamination by carbon steel tools. Passivation removes free iron and restores the protective chromium oxide film; this is why passivated stainless is standard for critical fasteners.