Manufacturing process and performance guarantee of railway bolts
- What are the forging process differences between 8.8 and 10.9 grade bolts?
8.8 grade bolts use medium - carbon steel (e.g., 45 steel), forged at 850 - 950℃ with single - step upsetting, offering simplicity and low cost. 10.9 grade bolts adopt alloy structural steel (e.g., 35CrMo), requiring two - step forging: rough forging at 1050 - 1100℃ for shaping and finish forging at 850 - 900℃ for grain refinement. 10.9 grade forging consumes 20% more energy but boosts tensile strength from 800MPa to 1000MPa, suitable for critical high - speed and heavy - haul railway applications.
- How do "thread rolling" and "cutting" processes for bolt threads compare?
Thread rolling forms threads by roller extrusion, compacting surface grains to increase strength by 15% - 20% compared to cutting. It eliminates machining marks, enhancing fatigue resistance, and is widely used for railway bolts. Cutting processes remove material with tools, offering high efficiency but leaving tool marks that can initiate cracks. A railway project switched from cut - thread to rolled - thread bolts after fatigue fractures caused by vibration.
- What are the key parameter controls for bolt "quenching and tempering treatment"?
Quenching and tempering involves quenching (820 - 860℃, oil - cooled) and tempering (550 - 650℃). Excessive quenching temperature coarsens grains, reducing toughness; low tempering (<500℃) fails to relieve stress, while high tempering (>700℃) decreases strength. A batch of bolts with 480℃ tempering had 250MPa residual stress and brittle fractures; adjusting to 600℃ reduced stress to 80MPa, meeting standards.
- What are the anti - corrosion differences between bolt "blackening treatment" and "Dacromet treatment"?
Blackening creates a 0.5 - 1μm Fe₃O₄ film through chemical oxidation, offering low cost but limited corrosion resistance (48 - hour salt spray test), suitable for dry environments. Dacromet treatment immerses bolts in a zinc - chromate coating and sinters at 300℃ to form a 6 - 8μm dense film, free from hydrogen embrittlement and passing 1000 - hour salt spray tests, ideal for humid and salt - fog areas. A coastal railway reduced 60% one - year rusting rate by switching from blackening to Dacromet treatment.
- How does the "metallographic structure" of bolts affect mechanical properties?
An ideal bolt features tempered sorbite, balancing strength and toughness. Coarse pearlite or bainite reduces toughness, while excessive ferrite weakens strength. Controlling heat treatment ensures uniform tempered sorbite. A factory's bolts with insufficient quenching cooling had bainite - rich structures, achieving only 60% of standard impact toughness; after process adjustment, the toughness returned to normal.