Heat Treatment for Auto Fasteners

As the automotive industry shifts toward lightweight and high-performance design, fasteners directly impact vehicle safety and reliability. Grade 8.8, 9.8, 10.9, and 12.9 bolts are widely used in engines, chassis, body structures, and other critical areas. Achieving the required tensile strength, yield ratio, and other core specifications depends on the heat treatment process.

Material Composition

Automotive high-strength bolts are typically made of alloy structural steels such as 40Cr and 20MnTiB. Carbon content must be precisely controlled. For grade 12.9 bolts, carbon content is controlled between 0.38% and 0.45%. Manganese and titanium refine grain size and improve hardenability. Sulfur and phosphorus must be limited to below 0.035%.

Heating Parameters

For grade 8.8 bolts made of 40Cr, heating temperature is controlled between 850°C and 870°C. For grade 12.9 bolts, heating temperature is raised to 880-900°C. For bolts below 10 mm diameter, holding time is 30-40 minutes. For bolts above 20 mm, holding time is 60-90 minutes.

Cooling Rate and Medium

For grade 8.8 and 9.8 bolts, oil cooling is acceptable. For grade 10.9 and 12.9 bolts, water or brine cooling is required. Cooling time must be strictly controlled to prevent stress concentration.

Tempering

For grade 8.8 bolts, tempering temperature is approximately 500-550°C. For grade 12.9 bolts, tempering temperature is controlled between 200-250°C. Holding time is typically 60-120 minutes.

Surface and Loading

Oxide scale and oil contamination must be removed by pickling or sandblasting before heat treatment. Avoid stacking bolts too densely in the furnace. Use dedicated fixtures to separate bolts for mass production.