Standard fasteners are widely used general mechanical components, including bolts, screws, studs, nuts, washers, pins, rivets, circlips, keys, expansion bolts, rigging hardware, special-shaped fasteners and other categories. They are extensively applied in equipment manufacturing, steel structures, engineering machinery, shipbuilding, aviation and construction industries for connecting and fixing mechanical parts. These fasteners can achieve permanent fastening or be disassembled and reassembled to facilitate equipment maintenance and component replacement, serving as indispensable basic components in industrial manufacturing.
1. Bolt Heat Treatment Process
Bolt heat treatment is also known as hardening treatment. Ordinary iron and carbon steel bolts feature low hardness and require heat treatment to improve strength and toughness. In contrast, stainless steel bolts possess excellent comprehensive mechanical properties and generally do not require hardening. According to material characteristics and application scenarios, different quenching and tempering processes are adopted to meet the requirements of bolt strength, plasticity, toughness and wear resistance.
1.1 Principles of Heat Treatment for Different Steels
Structural quenched and tempered steel adopts quenching followed by high-temperature tempering (500~650℃). Spring steel adopts quenching followed by medium-temperature tempering (420~520℃). Carburized alloy steel adopts carburizing and quenching followed by low-temperature tempering (150~250℃). After low-carbon steel and alloy carbon steel are quenched to form martensite, the strength decreases while plasticity and toughness increase with the rise of tempering temperature. Targeted heat treatment methods are selected according to carbon content to obtain fastener materials with customized performance.
Low-carbon alloy steel can be quenched and tempered at a low temperature below 250℃ to form high-strength low-carbon martensite. Surface carburizing is applied to improve surface wear resistance, which is suitable for wear-resistant structural bolts.
Medium carbon steel adopts quenching plus high-temperature tempering at 500~650℃, namely quenching and tempering treatment. It maintains sufficient strength while retaining good plasticity, which is the mainstream process for high-strength structural bolts. To pursue ultra-high strength with acceptable reduction in plasticity and toughness, low-temperature tempering can be adopted to produce ultra-high-strength bolts.
Medium and high carbon steels (60#, 70#, 80#, 90#) are mainly used for spring elastic fasteners. Quenching and medium-temperature tempering are applied to ensure high elastic limit, yield limit and fatigue resistance.
1.2 Standard Heat Treatment Operation Procedure
(1) Heat Treatment for Quenched and Tempered Steel
Pre-treatment: Normalizing, annealing or high-temperature tempering is performed to refine grains, eliminate banded structures, uniform hardness and improve machinability, forming uniform equiaxed fine grain structures.
Quenching: Workpieces are heated to approximately 850℃. Water quenching, oil quenching or air quenching is selected according to steel hardenability and workpiece size. Quenched steel features high hardness, poor plasticity and large internal residual stress, which requires timely tempering for stress relief.
Tempering: High-temperature tempering at 400~500℃ improves steel plasticity and toughness with moderate strength. Steels sensitive to temper brittleness require rapid cooling after tempering to avoid brittleness defects. For ultra-high strength requirements, low-temperature tempering at around 200℃ is adopted to form high-hardness tempered martensite.
(2) Heat Treatment for Spring Steel
Quenching is carried out at 830~870℃ with oil cooling, followed by medium-temperature tempering at 420~520℃ to form stable tempered troostite structure and ensure excellent elasticity and fatigue resistance.
(3) Heat Treatment for Carburized Steel
Carburizing: A chemical heat treatment process that infiltrates carbon elements into the steel surface in an active medium at high temperature. The procedure includes preheating at 850℃, carburizing at 890℃ and diffusion at 840℃ to increase surface carbon content.
Quenching: Carbon and low-alloy carburized steel adopts direct quenching or single quenching process.
Tempering: Low-temperature tempering is applied to eliminate quenching internal stress and improve the strength, toughness and wear resistance of the carburized layer.
2. Identification Methods for Stainless Steel, Galvanized and Nickel-Plated Bolts
2.1 Appearance Color Identification (Most Intuitive Method)
Stainless steel bolts present a natural matte raw metal color with uniform surface tone without obvious coating reflection. Galvanized bolts are available in white zinc, color zinc and black zinc finishes, showing white, iridescent or black appearances. Nickel-plated bolts feature uniform and bright coatings with a high-gloss silver metallic appearance and strong reflectivity.
2.2 Magnetic Identification with Magnet
Most stainless steel bolts are non-magnetic and cannot be attracted by magnets. Galvanized and nickel-plated bolts are based on carbon steel substrates with magnetism and can be normally attracted by magnets.
2.3 Oxidation and Passivation Characteristics
Nickel-plated bolts have excellent passivation performance. A dense protective passivation film rapidly forms on the nickel coating after electroplating with outstanding oxidation resistance. Galvanized bolts are covered with pure zinc coating without high-strength passivation protection and are prone to oxidation and discoloration. Stainless steel bolts rely on internal chromium and nickel alloy elements for corrosion resistance with no additional surface coating.
2.4 Acid and Alkali Corrosion Resistance
With chromium and nickel alloy elements, stainless steel bolts possess optimal resistance to strong acid and alkali corrosion. The passivation film on nickel-plated bolts effectively isolates corrosive media with slow corrosion rate. The zinc coating of galvanized bolts has high chemical activity and is corroded and consumed rapidly in acid and alkali environments.
3. Summary
For rapid daily identification, appearance color is the primary criterion: natural matte metal indicates stainless steel bolts; white, iridescent or black surfaces indicate galvanized bolts; high-gloss silver surfaces indicate nickel-plated bolts. For accurate identification, comprehensive judgment combining magnetism, oxidation resistance and acid-base corrosion resistance is recommended to quickly confirm bolt materials and surface treatments for optimal working condition matching.
